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THE MOLECULAR BEAM EPITAXIAL GROWTH OF HIGH QUALITY GALLIUM-ARSENIDE - ALUMINUM-GALLIUM - ARSENIDE HETEROSTRUCTURES FOR MICROWAVE DEVICE APPLICATIONS (QUANTUM WELLS, IMPURITY, PHOTOLUMINESCENCE, MODULATION DOPING)

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8607233
Maki, Paul A rth u r
THE MOLECULAR BEAM EPITAXIAL GROWTH OF HIGH QUALITY GALLIUMARSENIDE-ALUMINUM GALLIUM-ARSENIDE HETEROSTRUCTURES FOR
MICROWAVE DEVICE APPLICATIONS
Cornell University
University
Microfilms
International
Ph.D.
1986
300 N. Zeeb Road, Ann Arbor, Ml 48106
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
THE MOLECULAR BEAM EPITAXIAL GROWTH OF HIGH QUALITY GaAs-AlGaAs
HETEROSTRUCTURES FOR MICROWAVE DEVICE APPLICATIONS
A T hesis
P re s e n te d to th e F a c u lty of th e G raduate School
of C ornell U niversity
i n P a r t i a l F u l f i l l m e n t o f th e R eq u irem en t f o r t h e Degree of
D octor of P h ilo so p h y
by
P a u l A. Maki
J a n u a r y 1986
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(c)
P a u l A r t h u r Maki 1986
ALL RIGHTS RESERVED
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
BIOGRAPHICAL SKETCH
The a u t h o r was b o r n i n New H a v e n , C o n n e c t i c u t i n
1956.
H i s r e l a t i v e l y c a r e f r e e y o u t h was s p e n t i n
M i n n e s o t a an d l a t e r A r i z o n a .
He g r a d u a t e d f ro m C a m e lb a c k
High S c h o o l i n P h o e n ix i n 1974.
He r e c e i v e d h i s B a c h e l o r
o f S c i e n c e i n E l e c t r i c a l E n g i n e e r i n g i n J u n e o f 1978 f r o m
t h e M a s s a c h u s e tts I n s t i t u t e o f T echnology i n Cam bridge,
MA.
H i s t h e s i s work was on t h e p r e p a r a t i o n o f G r a p h i t e -
F erric
C h l o r i d e from h i g h l y o r i e n t e d p y r o l y t i c g r a p h i t e
u n d e r t h e d i r e c t i o n o f P r o f e s s o r M.S. D r e s s e l h a u s .
From A u g u s t o f 1978 t o A u g u s t o f 1 9 8 1 , h e was a
member o f t h e t e c h n i c a l s t a f f
in th e S o lid S t a t e D iv isio n
o f W a t k i n s - J o h n s o n Company i n S c o t t s V a l l e y , CA, w h e r e he
worked on f r e q u e n c y a g i l e v a r a c t o r tu n e d microwave
o scillato rs.
H i s work i n c l u d e d d e v e l o p m e n t o f f a s t
tuning, f a s t s e ttlin g
u l t r a l i n e a r S - b a n d an d X-ban d
o s c i l l a t o r s , a n d medium b a n d w i d t h lo w n o i s e o s c i l l a t o r s
c o v e r i n g t h e f r e q u e n c y r a n g e 2 GHz t o 18 GHz.
I n S e p t e m b e r o f 19 81 he b e g a n g r a d u a t e s t u d y i n
E l e c t r i c a l E n g in eerin g a t C o rn e ll U n iv e r s ity under th e
d i r e c t i o n o f P r o f e s s o r L .F . Eastm an.
Since th e F a l l of
1984 h e h a s w orked p a r t t i m e a t t h e G e n e r a l E l e c t r i c
E l e c t r o n i c s L a b o r a t o r y a t S y r a c u s e , New Y o r k , w h i l e
c o m p l e t i n g h i s g r a d u a t e work a t C o r n e l l .
ii
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D r . Maki r e c e i v e d t h e 1983 b e s t s t u d e n t p a p e r awar d
fo r the Jo u rn a l of E le c tro n ic M a te ria ls .
iii
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To My P a r e n t s ,
S ister
an d B r o t h e r s
iv
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ACKNOWLEDGEMENTS
The a u t h o r w i s h e s t o e x p r e s s h i s s i n c e r e s t g r a t i t u d e
t o h i s com m ittee c h a irm a n , P r o f e s s o r L e s t e r F. Eastman,
f o r h i s s u p p o r t and g u i d a n c e d u r i n g t h i s w o r k .
H is
e n t h u s i a s m and f r e s h i n s i g h t s h a v e p r o v i d e d c o n s t a n t
encouragem ent.
I a l s o w i s h t o t h a n k my o t h e r c o m m i t t e e
m em b er s, P r o f e s s o r J . M . B a l l a n t y n e and P r o f e s s o r J.W.
Mayer f o r t h e i r t i m e a nd e f f o r t s s p e n t on my b e h a l f .
A ll
h a v e b e e n d e m a n d in g and i n t e r e s t e d c o m m i t t e e member s,
s i g n i f i c a n t l y s h a p i n g my a c a d e m i c p r o g r a m a nd r e s e a r c h .
The s t r u c t u r e o f t h e g r a d u a t e p r o g r a m c o m b in e d w i t h t h e
r e s e a r c h o p p o r t u n i t i e s have p ro v id e d a unique environm ent
w h i c h I h a v e f o u n d t o be p a r t i c u l a r l y s a t i s f y i n g .
T h a n k s g o e s t o A.R . Ca law a f o r h i s g e n e r o u s a d v i c e
and g u id a n c e i n t h i s work.
S p e c i a l t h a n k s go t o D r .
S u s a n P a l m a t e e r f o r h e r s u p p o r t d u r i n g my g r a d u a t e w o r k .
I w i s h t o a c k n o w l e d g e t h e p e o p l e w i t h whom I h a v e
e n jo y ed c l o s e c o l l a b o r a t i o n d u r in g t h i s work.
D r . U.
M i s h r a , f o r work on t h e VET, a nd J . W endt, h i s s u c c e s s o r ,
fo r diode p r o c e s s in g .
D r . L. C a m n i t z , f o r e n j o y a b l e
e f f o r t s on t h e s i n g l e q uantum w e l l MODFET.
D r. H.Q. L e ,
f o r many i l l u m i n a t i n g d i s c u s s i o n s on t h e p h y s i c s o f
qu an tu m w e l l s t r u c t u r e s .
D r . M. H o l l i s ,
e x p e r t i s e i n SIMS i n t e r p r e t a t i o n .
for his
D r . W. S c h a f f , f o r
c o l l a b o r a t i o n on s i l i c o n r e d i s t r i b u t i o n i n m o d u l a t i o n
v
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d o p e d s t r u c t u r e s and e l e c t r i c a l m e a s u r e m e n t s .
G. S m i t h
o f G . E . C o r p o r a t e R e s e a r c h and D e v e l o p m e n t f o r SIMS
m e a s u r e m e n t s . B.
DeCooman, f o r TEM m e a s u r e m e n t s and
d iscu ssio n s.
I w i s h t o t h a n k t h o s e who h a v e p r o v i d e d s u p p o r t and
a ssistan c e.
D r . G. W icks f o r o r g a n i z a t i o n o f t h e MBE
m a c h i n e a nd t r a i n i n g
in photolum inescence.
Jack Berry,
f o r f a c i l i t y m an ag em en t an d a s s i s t a n c e i n MBE
m a i n t a i n e n c e , a s w e l l a s humor i n t h e f a c e o f a d v e r s i t y .
D r . D. W oodar d, f o r a d v i c e i n d e v i c e p r o c e s s i n g .
Dr. P.
T a s k e r f o r e n j o y a b l e d i s c u s s i o n s and i m p r e s s i v e e f f o r t s
i n t h e d e v e lo p m e n t o f microwave m easurem ent c a p a b i l i t i e s .
I a l s o w i s h t o a c k n o w l e d g e D r . J . C . M . Hwang a nd D r .
T a n Hua Yu f o r a d v i c e a nd p r o v i d i n g t h e u s e o f t h e G .E .
E l e c t r o n i c s L a b o r a t o r y f a c i l i t i e s , w h i c h a l l o w e d me t o do
much o f t h e MBE work w h i c h wou ld h a v e b e e n d i f f i c u l t t o
pursue a t C o rn ell.
I a l s o w ish t o acknowledge t h e g r a d u a t e s t u d e n t s
p a s t a nd p r e s e n t w i t h whom I h a v e h ad t h e o p p o r t u n i t y t o
work.
I n p a r t i c u l a r , W.T. B e a r d , D r . A. Brown, D r . P.
K i r c h n e r , D r . H. L e e , D r . L. L u n a r d i , D r . T. O h a s h i a nd
a l l t h e g r a d u a t e s t u d e n t s i n v o l v e d i n MBE on t h e GEN I I .
I a l s o w i s h t o e x p r e s s ray g r a t i t u d e t o L. B r o w n r i d g e
a n d E. Weaver f o r t h e i r f r i e n d s h i p and a s s i s t a n c e i n t h e
p r e p a r a t i o n o f r e p o r t s an d p a p e r s d u r i n g my t i m e a t
C ornell.
S p e c i a l t h a n k s go t o Elma Weaver f o r h e r
vi
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p a t i e n c e and u n t i r i n g a s s i s t a n c e i n t h e p r e p a r a t i o n o f
th is
th esis.
Jane Jo rgensen r e c e iv e s thanks fo r her
e x c e lle n t artw ork.
The work i n t h i s t h e s i s h a s b e e n s u p p o r t e d f ro m
se v e ra l sources.
I w i s h t o t h a n k t h e s u p p o r t o f DARPA f o r
t h e n - n h e t e r o j u n c t i o n a nd VET w o r k .
The work on q u an tu m
w e l l s t r u c t u r e s was s u p p o r t e d by t h e A i r F o r c e O f f i c e o f
S c i e n t i f i c R e s e a r c h a nd t h e O f f i c e o f N a v a l R e s e a r c h .
F in ally ,
I w i s h t o t h a n k a l l my c l o s e f r i e n d s f o r
t h e i r v a l u e d s u p p o r t a nd e n c o u r a g e m e n t , w h i c h h a s h e l p e d
me t h r o u g h o u t t h e y e a r s .
v ii
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TABLE OF CONTENTS
CHAPTER
1
TITLE
PAGE
TECHNICAL BACKGROUND
1
1.1
2
H e t e r o ju n c tio n DeviceR equirem ents
1 .1 .1
n - n AlGaAs-GaAs H e t e r o j u n c t i o n s
1 .1 .2
Undoped a n d M o d u l a t i o n Doped
3
S i n g l e Quantum W e l l s on T h i c k
AlGaAs B u f f e r s
1.2
1.3
2
6
G e n e r a l B a c k g r o u n d on t h e G row th o f
GaAs and AlGaAs by MBE
13
1 .2 .1
G allium A rsen id e
15
1 .2 .2
Aluminum-GalliumA r s e n id e
18
S co pe o f T h i s T h e s i s
19
MBE GROWTH OF THICK GaAs ANDAlGaAs
22
2.1
E x p e r i m e n t a l M e th o d s
22
2 .1 .1
MBE M a c h i n e s
23
2 .1 .2
S ubstrate P reparation
24
2 .1 .3
M easurement o f S u b s t r a t e
Tem peratures
2 .1 .4
Beam P r e s s u r e M e a s u r e m e n t s
Group I I I
27
2 .1 .5
Grou p V Beam P r e s s u r e s
28
2 .1 .6
R eflectio n E lectro n D iffra c tio n
P attern s
2.2
25
Flux R a tio s
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33
39
2.3
2.4
2.5
3
Beam P r e s s u r e s A lo n g t h e M e t a l R i c h
Edge o£ GaAs
57
2 .3 .1
E xperim ental
58
2 .3 .2
R esults
58
C o m p a r i s o n w i t h AlGaAs Gr owth
64
2 .4 .1
E xperim ental
66
2 .4 .2
R e s u l t s a nd D i s c u s s i o n
66
GaAs a nd AlGaAs Growth
75
2 .5 .1
GaAs Growth
75
2 .5 .2
AlGaAs Growth
81
N-N HETEROJUNCTION GROWTH
87
3.1
In tro d u ctio n
87
3.2
n-n H etero ju n ctio n E l e c tr i c a l
C h aracteristics
89
3.3
MBE Gro wth C o n d i t i o n s
93
3.4
I m p u r itie s in n-n H e te r o ju n c tio n s
98
3 .4 .1
E xperim ental
99
3.4 .2
C apacitance-V oltage P r o f ilin g
101
3 .4 .3
Photolum inescence
104
3 .4 .4
S e c o n d a r y I o n Mass S p e c t r o s c o p y
105
3.4 .5
Deep L e v e l T r a n s i e n t S p e c t r o s c o p y
121
3.5
Summary o f n - n H e t e r o j u n c t i o n R e s u l t s
3.6
D i s t r i b u t i o n o f I m p u r i t i e s i n an
E le c tro s ta tic P otential
4
GaAs-AlGaAs
4.1
INTERFACE GROWTH
G row th o f I n t e r f a c e s a t Low F l u x R a t i o s
ix
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121
123
127
127
4 .1 .1
AlGaAs Grown on GaAs
129
4 .1 .2
GaAs G rowth on AlGaAs
131
4.2
P r a c t i c a l C o n s id e ra tio n s o f Flux R a tio s
137
4.3
Growth o f Undoped Quantum W e l l S t r u c t u r e s
141
4 .3 .1
Experim ental
141
4 .3 .2
R esults
145
4 .3 .3
D iscussion
151
4.4
F a c to rs L im itin g L inew idth
158
4 .4 .1
Experim ental
160
4 .4 .2
D iscussion
164
4 .4 .3
F i n i t e W e l l Model f o r I n t e r f a c e
a nd B a r r i e r A l l o y F l u c t u a t i o n s
4.5
4.6
S i R e d i s t r i b u t i o n i n M o d u l a t i o n Doped
S tructures
173
4 .5 .1
M a t e r i a l s G row th and H a l l E f f e c t
174
4 .5 .2
SIMS S e c o n d a r y I o n Mass
S pectroscopy
174
4 .5 .3
Photolum inescence
176
4 .5 .4
TEM M e a s u r e m e n t s
181
4 .5 .5
D iscussion
186
S ilicon R ed istrib u tio n
4 .6 .1
4 .6 .2
187
S ilic o n Surface Segregation
on AlGaAs
4.7
168
S ilic o n D iffusion
189
195
E l e c t r i c a l P r o p e r t i e s o f M o d u l a t i o n Doped
S i n g l e Quantum W e l l s
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200
4.8
4 .7 .1
E xperim ental
201
4 .7 .2
H a l l M easurement R e s u l t s
204
4 .7 .3
Light S e n s it i v i t y
207
4 .7 .4
S e n s itiv ity to Im p u rities
207
4 .7 .5
Summary o f H a l l M e a s u r e m e n t s
209
Photolum inescence L in esh ap es of
M o d u l a t i o n Doped S t r u c t u r e s
4 .8 .1
211
L ocalized S t a t e C o n trib u tio n to
Lineshape
4 .8 .2
213
C o n trib u tio n of a D egenerate
I m p u r i t y Band t o t h e D e n s i t y
of S ta te s
5
CONCLUSIONS
APPENDIX A
AND FUTURE WORK
237
BARRIER HEIGHT DEPENDENCE OF AN N-N
HETEROJUNCTION OF APPLIEDVOLTAGE
APPENDIX C
230
ELIMINATION OF FLUX TRANSIENTS IN
MOLECULAR BEAM EPITAXY
APPENDIX B
221
252
VERTICAL ELECTRON TRANSISTOR (VET)
IN GaAs WITH A HETEROJUNCTION
(AlGaAs-GaAs) CATHODE
APPENDIX D
SUB-MICROMETER QUANTUM WELL HEMT WITH
AN A1q 3Ga0>?As BUFFERLAYER
APPENDIX E
256
262
LIST OF PRESENTATIONS AND
PUBLICATIONS
REFERENCES
269
273
xi
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LIST OF TABLES
TABLE
TITLE
PAGE
2.1
H a ll M easurem ents.
77
2.2
A1q 3GaQ 7 M e a s u r e m e n t s .
85
3.1
S u b s t r a t e Type.
101
4 .1
Growth C o n d i t i o n s .
144
4.2
E^h p e a k e n e r g i e s , E^ h FWHM
l i n e w i d t h s a nd s p a c i n g s o f e x t r i n s i c
4.3
peaks r e l a t i v e to
150
El h ^ Lz^ l i n e w i d t h s .
164
xii
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LIST OF ILLUSTRATIONS
1.1
PAGE
TITLE
FIGURE
Layer s t r u c t u r e o f an n-AlGaAsf
4
n-GaAs h e t e r o j u n c t i o n .
1.2
C o n d u c t i o n ban d d i a g r a m o f an n - A lG aA s,
4
n-GaAs d i o d e .
1.3
Layer d iag ram o f a m o d u la tio n doped
s t r u c t u r e , a) r e g u l a r s t r u c t u r e , AlGaAs:
S i grown on GaAs; b) i n v e r t e d s t r u c t u r e ,
8
GaAs gr ow n o n A l G a A s t S i .
1.4
E x c i t o n l i n e w i d t h d e p e n d e n c e on s u b s t r a t e
t e m p e r a t u r e f o r a m u l t i p l e quantum w e l l
11
stru ctu re.
2.1
Beam e q u i v a l e n t p r e s s u r e s o f Ga # 3 , Ga # 4 ,
a nd A1 #5 a s a f u n c t i o n o f i n v e r s e c e l l
29
t e m p e r a t u r e f o r t h e G.E. m achine.
2.2
Beam e q u i v a l e n t p r e s s u r e b e h a v i o r o f A s 4
a s a f u n c t i o n o f t h e Ga p r e s s u r e ,
a)
T r a n s i e n t b e h a v i o r o f As4 a s a f u n c t i o n
o f t i m e a t Ga p r e s s u r e t e r m i n a t i o n ,
b)
S t e a d y s t a t e and t r a n s i e n t As4 p r e s s u r e
a s a f u n c t i o n o f t h e Ga p r e s s u r e .
32
2.3
A rsenic r ic h
36
2.4
Gallium
(110)
2x4,
(or a r s e n i c )
(100) GaAs.
stab le
(110)
3x1,
36
(100) GaAs.
xiii
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2.5
G allium r i c h
(110)
4x2,
(100) GaAs.
37
2.6
M etal s t a b l e
(110)
3x1,
(100) AlGaAs.
37
2.7
T r a n s i t i o n s o f t h e s u r f a c e s t r u c t u r e as
a f u n c t i o n o f t h e Ga a r r i v a l r a t e s and
the s u b s tra te tem peratures.
38
2.8
G e n e r a l i z e d f l u x e s on s u r f a c e .
42
2.9
Schem atic a p p l i c a t i o n of th e s o li d u s
f i e l d d i a g r a m t o MBE g r o w t h .
2.10
55
As^ BEP a s a f u n c t i o n o f t h e Ga BEP
along th e g a lliu m r i c h edge of th e s o l i d u s
f i e l d p a ra m e te riz e d w ith th e s u b s t r a t e
t e m p e r a t u r e , Ts u b *
2.11
60
A s 4 BEP a s a f u n c t i o n o f Ga BEP a l o n g
t h e g a lliu m r i c h edge of t h e s o l i d u s
f i e l d p a ra m e te riz e d w ith th e s u b s t r a t e
t e m p e r a t u r e Ts u b .
2.12
61
P l o t o f PQ and a a s a f u n c t i o n o f
in v erse s u b s tr a te tem perature.
2.13
62
As^ BEP a s a f u n c t i o n o f Ga BEP f o r
GaAs and Alg 20Ga0 80As a l o n 9
th e m etal r i c h edge of s o l i d u s f i e l d .
2.14
67
V ap o r p r e s s u r e o f As^ a s a f u n c t i o n
o f A1 c o m p o s i t i o n
(x) a s s u m i n g R a o l t ' s
law .
71
xiv
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.15
P l o t o f t h e o r e t i c a l a nd e x p e r i m e n t a l 30K
an d 77K m o b i l i t y v e r s u s c a r r i e r
c o n c e n t r a t i o n f o r n - t y p e S i - d o p e d GaAs.
2.16
78
Photolum inescence s p e c tr a of l i g h t l y Si
d o p e d n " l a y e r (1534) a) B a n d e d g e and
a c c e p to r reg io n as a f u n c tio n of
tem perature.b)
B an d e d g e e x c i t o n
r e g i o n a t 4K.
2.17
80
4K FWHM l i n e w i d t h o f S i d o ped
AIq 3 0Ga0 7 QAs a s a f u n c t i o n o f
H all c a r r i e r c o n c e n tra tio n .
2.18
83
PL d a t a f o r d o p e d s a m p l e s shown i n
F i g u r e 2 . 1 7 a nd T a b l e 2 . 2 .
2.19
F u l l w i d t h maximum l i n e w i d t h
84
(<*) a s
a f u n c t i o n o f al um inum c o m p o s i t i o n .
86
3.1
Layer s t r u c t u r e of th e n-n h e t e r o ju n c tio n .
94
3.2
300K I - V c h a r a c t e r i s t i c o f n - n h e t e r o ­
j u n c t i o n a) m e a s u r e d c h a r a c t e r i s t i c s ;
b) c a l c u l a t e d c h a r a c t e r i s t i c s .
3.3
95
77K I - V c h a r a c t e r i s t i c s o f n - n h e t e r o ­
j u n c t i o n a) m e a s u r e d c h a r a c t e r i s t i c s ;
b) c a l c u l a t e d c h a r a c t e r i s t i c s .
3.4
96
Layer s t r u c t u r e fo r n-n h e t e r o ju n c tio n
used in s u b s t r a t e im p u rity s tu d y .
xv
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100
log
v s . d i s t a n c e f o r l a y e r s grown
o n LEC s u b s t r a t e s a s d e t e r m i n e d u s i n g
C-V p r o f i l i n g ,
a) L a y e r 1127N;
b) l a y e r 1127A.
3.6
Log
102
v s . d i s t a n c e f o r l a y e r grown
on n+ s u b s t r a t e s a s d e t e r i n e d u s i n g
C-V p r o f i l i n g ,
a) L a y e r 1128N;
b) l a y e r 1128A.
3.7
103
AlGaAs PL s p e c t r a on t h e n - n h e t e r o ­
j u n c t i o n grown o n LEC s u b s t r a t e s ,
a) 1127N;
3.8
b) 1127A.
106
AlGaAs PL s p e c t r a o f t h e n - n h e t e r o ­
j u n c t i o n grown o n n+ s u b s t r a t e s ,
a) 11 28N, b) 1128A.
3.9
107
Slow s p u t t e r i n g SIMS s c a n s u s i n g C+
s
beam o f n - n h e t e r o j u n c t i o n s grown on
LEC s u b s t r a t e s ,
3.10
a) 1127A, b)
1127N.
110
Slow s p u t t e r i n g SIMS u s i n g C+
beam o f n - n h e t e r o j u n c t i o n s grown on
n+ s u b s t r a t e s ,
3.11
a) 1128A, b) 1128N.
111
F a s t s p u t t e r i n g SIMS s c a n s u s i n g C+
s
beam o f n - n h e t e r o j u n c t i o n s grown on
LEC s u b s t r a t e s ,
3.12
a) 1127A, b) 1127N.
113
F a s t s p u t t e r i n g s c a n s u s i n g C+
o
beam o f n - n h e t e r o j u n c t i o n s grown on n +
su b strates,
a) 1128N, b) 1128A.
xvi
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114
3.13
Slow s p u t t e r i n g SIMS s c a n s u s i n g Oij
beam o f n - n h e t e r o j u n c t i o n s grown o n LEC
su b strates,
3.14
a) 1127N,
b) 1127A.
115
Slow s p u t t e r i n g SIMS s c a n s u s i n g 0+
beam o f n - n h e t e r o j u n c t i o n s grown o n n +
su b strate s,
3.15
a)
1128N, b) 1128A.
116 —
F a s t s p u t t e r i n g s c a n s u s i n g Ojj
beam o f n - n h e t e r o j u n c t i o n s grown o n LEC
su b strates,
3.16
a) 1127N,
b) 1127A.
118
F a s t s p u t t e r i n g s c a n s u s i n g Ojj
beam o f n - n h e t e r o j u n c t i o n s grown o n n+
su b strate s,
3.17
a) 1128N,
b) 1128A.
119
DLTS s c a n o f AlGaAs b a r r i e r o f t h e n - n
h e t e r o j u n c t i o n grown on a n o n a n n e a l e d
LEC s u b s t r a t e
4.1
(1127N).
122
T r a n s i t i o n t i m e f ro m m e t a l s t a b l e t o
m etal r ic h c o n d itio n as a fu n c tio n of
log
(BEPa ) a nd l o g (BEP r a t i o ) a t
as4
680°C f o r a GaAs g r o w t h r a t e o f 0 . 9 urn/
h r f o r a 1 0 _20g a 0 80A s .
4.2
132
C o n v entional m e t a l l u r g i c a l form at fo r th e
Ga-As b i n a r y p h a s e d i a g r a m , w i t h a r s e n i c
f r a c t i o n XAs a s a l i n e a r a b s c i s s a
139
scale.
S i n g l e q u an tu m w e l l l a y e r s t r u c t u r e
s t u d i e d i n t h i s work.
xvii
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143
4.4
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW
and o f GaAs b u f f e r o f s a m p l e s
(a)
6 3 9 (N),
146
(b) 638 (N).
4.5
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW i n
s a m p l e 5 3 9 (N) a t pump p o w e r s o f
(a) 0.1W/cm2 ,
4.6
147
(b) 0.01W/cm 2 .
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW i n
s a m p l e 6 3 8 (N) a t pump p o w e r s o f
(a) 0.1W/cm2 ,
4.7
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW i n
s a m p l e 639(A) a t pump p o w e r s o f
0.1W/cm2 ,
4.8
(a)
148
(b) 0.01W/cm2 ,
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW i n
s a m p l e 638(A) a t pump p o w e r s o f
0 . lW/cm2 ,
4.9
147
(b) 0.01W/cm2 .
(a)
(b) 0.01W/cm2 .
V a r ia tio n of im p u rity lum inescence a c ro s s
w a f e r f o r a 1008 q u a n t u m w e l l .
4.10
148
157
4K s p e c t r a o f t h e h e a v y h o l e e x c i t o n f o r
SQW thickness (L ) from 200S
z
to 1008.
4.11
;6;
4K PL s p e c t r a o f SQW s a m p l e s s h o w i n g
c l a d d i n g l a y e r l u m i n e s c e n c e and SQW
lum inescence.
4.12
162
4K PL s p e c t r a o f SQW s a m p l e s s h o w i n g
c l a d d i n g l a y e r l u m i n e s c e n c e and SQW
lum inescence.
xviii
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163
4.13
P l o t o f h e a v y h o l e e x c i t o n FWHM l i n e w i d t h
166
as a fu n c tio n of w ell th ic k n e ss .
4.14
PL s p e c t r a o f a w id e q u a n tu m w e l l a s a
167
fu n c tio n of tem perature.
4.15
M o d u la tio n doped l a y e r s t r u c t u r e s used
s i l i c o n r e d i s tr ib u tio n experim ents.
4.16
SIMS s c a n o f s a m p l e w i t h s i l i c o n
i n t r o d u c e d a f t e r quantum w e l l g r o w t h .
4.17
180
(a) Dar k f i e l d
image o f t h e s a m p l e shown
in Fig. 4 .1 5 (a ).
(b) L a t t i c e im ag e o f
182
h etero in terface.
4.20
(a) Dar k f i e l d
in F ig.
image o f t h e s a m p l e shown
4.1 5 (b ).
(b) L a t t i c e image
183
of h etero in te rface.
4.21
178
4K PL s c a n o f m o d u l a t i o n d o p e d SQW shown
in F ig . 4.15.
4.19
177
SIMS s c a n o f s a m p l e w i t h s i l i c o n
i n t r o d u c e d p r i o r t o quantum w e l l g r o w th .
4.18
175
(a) D ar k f i e l d
image o f a 30&
m o d u l a t i o n d o p e d w e l l q u a n tu m s t r u c t u r e
s h o w i n g p y r a m i d s on t h e s u r f a c e .
(b)
L a t t i c e image o f t h e s u r f a c e p y r a m i d
185
stru ctu re.
4.22
L a y e r s t r u c t u r e f o r r u n s 333 and 334 f o r
c o n t i n u o u s and g r o w t h i n t e r r u p t i o n
192
experim ents.
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4.23
SIMS s c a n s o f l a y e r 333 w i t h g r o w t h
193
in terru p tio n .
4.24
SIMS s c a n o f l a y e r 334 w i t h o u t g r o w t h
194
in terru p tio n .
4.25
Schem atic c r o s s s e c t io n of a s in g le
202
q u a n t u m w e l l HEMT.
4.26
C o n d u c t i o n ban d d i a g r a m f o r a s i n g l e
203
q u a n t u m w e l l HEMT.
4.27
D e t a i l e d c o n d u c t i o n ban d d i a g r a m a t
q u an tu m w e l l w i t h g r o u n d s t a t e e n e r g y EQ,
wave f u n c t i o n ^ , a nd F e r m i
203
l e v e l Ep .
4.28
H a ll e le c tr o n m o b ility as a fu n c tio n of
205
spacer thickness.
4.29
77K H a l l e l e c t r o n m o b i l i t y a s a f u n c t i o n
of sh e e t ch arg e, N , param eterized
s
by s p a c e r t h i c k n e s s .
4.30
206
77K H a l l e l e c t r o n m o b i l i t y a t a s t e p
e tc h e d sample as a f u n c t i o n o f H a ll s h e e t
208
charge.
4.31
77K H a l l m o b i l i t i e s a s a f u n c t i o n o f
i n t e n t i o n a l w ell im p u ritie s .
4.32
210
2K PL l i n e s h a p e on a 2D e l e c t r o n g a s a s
a f u n c t i o n o f e n e r g y o f b o t h l i n e a r and
log i n t e n s i t y s c a le s .
xx
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212
4.33
Photolum inescence s p e c tra of a H all
sam ple.
4.34
214
P h o t o l u m i n e s c e n c e s p e c t r a o f an
e tc h e d sam ple.
4.35
PL l i n e s h a p e o f a 2D e l e c t r o n g a s a s
214
a
fu n c tio n of energy for s e v e ra l c r y s t a l
t e m p e r a t u r e s fro m 4K t o 44K.
4.36
215
N orm alized l i n e s h a p e as a f u n c t i o n
o f e n e r g y b e lo w t h e g r o u n d s t a t e o f a
q u a n t u m w e l l f o r a n u n s c r e e n e d co u lo m b
p o ten tial.
4.37
220
N orm alized l i n e s h a p e as a f u n c t i o n of
e n e r g y b e lo w t h e g r o u n d s t a t e o f a
q u a n t u m w e l l f o r t h e s c r e e n e d c o lo u m b
p o ten tial.
4.38
222
C a lc u la te d norm alized lin e s h a p e L (E ), as
a
f u n c t i o n o f e n e r g y b elow t h e f i r s t
c o n d u c t i o n s u b b a n d f o r a q u a n tu m w e l l
w ith a d e g e n e ra te donor im p u rity band.
4.39
C a lc u la te d norm alized lin e s h a p e , L (£ ), as
226
.
f u n c t i o n o f e n e r g y below t h e f i r s t
c o n d u c t i o n s u b b a n d f o r a q u an tu m w e l l
w ith a l i g h t donor d e n s i t y .
A1
D i a g r a m o f t h e new c r u c i b l e a s s e m b l y .
A2
C o m p a r i s o n o f t h e g a l l i u m c e l l BEP
tran sien t.
227
241
244
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A3
D oping c o n c e n t r a t i o n a s a f u n c t i o n o f
d e p t h f o r a u n i f o r m l y d o p e d GaAs f i l m .
A4
245
U n i f o r m i t y map o f t h e f u l l c h a n n e l o f
1x400 urn g a t e FETS f a b r i c a t e d on a
GaAs f i l m grown u s i n g t h e two c r u c i b l e
246
cell.
A5
G eom etrical c o n s tr u c tio n fo r a c o n s ta n t
m elt a re a as seen a t th e s u b s t r a te
249
p o sitio n .
xxii
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CHAPTER 1
TECHNICAL BACKGROUND
I n t h e c o n t e x t of sem ico n d u c to r d e v i c e s , t h e
r e a l i z a t i o n of
h e t e r o j u n c t i o n s a s a d e v i c e s t r u c t u r e m ust
rank as one of t h e f u n d a m e n ta l advancem ents a lo n g w ith
b i p o l a r d e v i c e s and t h e m a j o r i t y c a r r i e r
tran sisto r
(F ET).
fie ld effect
The h e t e r o j u n c t i o n h a s p r o v i d e d
p e rfo rm a n c e im provem ents i n such d e v i c e s as
heterostructure la se rs
a s w e l l as new d e v i c e s s u c h a s t h e
m o d u l a t i o n d o ped f i e l d e f f e c t t r a n s i s t o r
SDHT) . 1
(MODFET, HEMT,
A t t h e w r i t i n g o f t h i s t h e s i s , d e v i c e s s u c h as
t h e MODFET which r e q u i r e a b r u p t c o m p o s i t i o n a l
h ave been grown m o s t s u c c e s s f u l l y by m o l e c u l a r
epitaxy
(MBE) , a l t h o u g h o t h e r e p i t a x i a l
have e n jo y e d c o n tin u e d
in terfaces
beam
growth te c h n iq u e s
p ro g re s s tow ards a b r u p t
in terf aces.
The d e v e l o p m e n t a nd g r a d u a l
beam e p i t a x y
i m p ro v e m e n t o f m o l e c u l a r
(MBE) h a s p r o v i d e d an u n i q u e t o o l f o r t h e
r e a l i z a t i o n of p r e c i s e s t r u c t u r e s
for device a p p lic a tio n s
and f u n d a m e n t a l m a t e r i a l s t u d i e s .
The a d v a n c e d s t a t e of
MBE t e c h n o l o g y t o d a y r e f l e c t s
t h e e f f o r t o f many
r e s e a r c h e r s and many l a b o r a t o r i e s .
E s p e c ia lly rapid
d e v e l o p m e n t o c c u r r e d e a r l y on t h r o u g h t h e d e d i c a t e d
e f f o r t s o f t h e h a n d f u l of r e s e a r c h e r s whose s t u d i e s
1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2
o u t l i n e d t h e b a s i c u n d e r s t a n d i n g o f t h e p r o c e s s of
m o l e c u l a r beam e p i t a x y .
Cho and A r t h u r 2 s t i l l
A r e v i e w o f t h e e a r l y p a p e r s by
p r o v i d e s an i m p o r t a n t b a s i s f o r t h e
u n d e r s t a n d i n g o f MBE.
T his ch ap ter serv es
to o u tlin e the b asic
h e t e r o j u n c t i o n m a t e r i a l problem s i n v e s t i g a t e d i n t h i s
t h e s i s and r e v i e w s t h e " s t a t e o f t h e a r t "
i n MBE
r e g a r d i n g t h e g r o w t h o f t h i c k GaAs a nd AlGaA s.
It
begins
w ith a d i s c u s s i o n of th e h e t e r o j u n c t i o n m a t e r i a l
r e q u i r e m e n t s i n t h e c o n t e x t of d e v i c e s .
1.1
H e te ro j u n c tio n D evice R equirem ents
As MBE i s a p p l i e d t o h i g h s p e e d d e v i c e s , t h e new
co n strain ts
p l a c e d on t h e m a t e r i a l s c a n n o t be a v o i d e d .
Of p a r t i c u l a r i m p o r t a n c e i n m ic r o w a v e d e v i c e s a r e
m ateria ls ex h ib itin g ex cellen t electro n tra n sp o rt
p r o p e r t i e s a n d low d e e p l e v e l c o n c e n t r a t i o n s .
In su latin g
b u f f e r s of GaAs and AlGaAs a r e r e q u i r e d f o r PET
a p p l i c a t i o n s t o m inim ize s u b s t r a t e e f f e c t s such as
p a r a s i t i c b u f f e r c u r r e n t s and b a c k g a t i n g e f f e c t s .
Many
d e v i c e s t r u c t u r e s i n t h e AlGaAs/GaAs s y s t e m e m p h a s i z e
h e tero ju n ctio n in te rfa c e p ro p e rtie s .
D e v i c e s s u c h as
quantum w e l l and h e t e r o s t r u c t u r e l a s e r s and h i g h e l e c t r o n
m o b i l i t y f i e l d e f f e c t t r a n s i s t o r s w i t h AlGaAs b u f f e r
l a y e r s r e q u i r e t h a t d e v i c e q u a l i t y GaAs be grown on
AlGaAs and t h a t t h e i n t e r f a c e
be s m o o t h , a b r u p t , a nd
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3
f r e e of recom bination c e n t e r s .
O th e r h e t e r o j u n c t i o n d e v ic e s t r u c t u r e s , su c h as n-n
hetero ju n ctio n s for
e le c tr o n cath o d es in v e r t i c a l F E T 's, 3
e m p h a s i z e t h e b u l k p r o p e r t i e s o f AlGaAs i n b a r r i e r s of
diodes.
I n t h i s s e c t i o n , t h e p r e v i o u s w ork i n MBE on t h e
g r o w t h o f n - n h e t e r o j u n c t i o n s , i n v e r t e d m o d u l a t i o n doped
stru ctu res
(GaAs grown o n A l G a A s : S i ) , and un doped s i n g l e
quan tum w e l l s on t h i c k AlGaAs b u f f e r s i s d i s c u s s e d .
1 .1 .1
n - n AlGaAs-GaAs H e t e r o j u nct i o n s
An AlGaAs-GaAs n - n h e t e r o j u n c t i o n i s f o r m e d a t t h e
m e ta llu rg ic a l contact
b e t w e e n an n - t y p e GaAs l a y e r and an
n - t y p e AlGaAs l a y e r .
A t y p i c a l l a y e r s t r u c t u r e and band
d i a g r a m a r e shown i n F i g s . 1 . 1
and 1 . 2 r e s p e c t i v e l y .
The e a r l i e s t e x p e r i m e n t s and t h e o r y o f
h e t e r o j u n c t i o n s a r e due t o A n d e r s o n ^ i n t h e s y s t e m o f GeGaAs.
The p r i n c i p l e f e a t u r e o f s u c h s t r u c t u r e s i s t h e
p r e s e n c e o f c o n d u c t i o n and v a l a n c e band d i s c o n t i n u i t e s a t
t h e i n t e r f a c e w h ic h c o n t r o l u n i p o l a r t r a n s p o r t .
The e a r l i e s t s t u d i e s on n - G a A s , n- A lG aA s
h e t e r o j u n c t i o n s s p a r k e d some c o n t r o v e r s y , f o r w h i l e
e n e r g y b a r r i e r s wer e known t o e x i s t from t h e o p t i c a l
C
C
e x p e r i m e n t s o f D i n g l e 3 a nd Chang , no e v i d e n c e o f e n e r g y
b a r r i e r s c o u l d be f o u n d i n e l e c t r o n t r a n s p o r t
m easurem ents.
S p e c u l a t i o n on t h e l a c k o f r e c t i f i c a t i o n
c e n t e r e d on i n t e r f a c e c o m p o s i t i o n a l g r a d i n g w h ic h
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
V
”^+
n+ Tapered
rr
n“
I
GaAs \ ~ 2000A
o
AlGaAs I ~ 5 0 0 Ag
A1GoAs~7 ^ 1000 A
GaAs \ ^ 7 5 0 0 A
GaAs /
F igure 1 .1.
substrate
L a y e r s t r u c t u r e o f a n n-AlGaAs
n-GaAs h e t e r o j u n c t i o n .
GaAs
^ * 0 .3 ^ a 0 . 7 ^ s
— Near
flat band
— Zero b ias
AE,
T— K— H— H— X— K— * =
F igure 1.2.
C o n d u ctio n band d iag ram o f an
n - A l G a A s , n-GaAs d i o d e .
AE i s t h e
c
c o n d u c tio n band d i s c o n t i n u i t y between
t h e tw o m a t e r i a l s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5
e f f e c t i v e l y l o w e r s t h e b a r r i e r p e a k , o r some c u r r e n t
t r a n s p o r t m e c h a n is m a r o u n d t h e i n t e r f a c e ,
such as along a
7
A u g e r p r o f i l i n g s t u d i e s by G a r n e r e t a l .
surface.
d e m o n s t r a t e d t h a t t y p i c a l LPE j u n c t i o n s c o u l d a c h i e v e
i n t e r f a c e g r a d i n g o f < 150 8 , w h i c h was t h o u g h t t o be
s u f f i c i e n t l y sm all to provide a r e c t i f y i n g b a r r i e r .
P a r t i c u l a r l y c u r i o u s i s t h a t n o n - r e c t i f y i n g b e h a v i o r was
a l s o r e p o r t e d f o r MBE grown h e t e r o j u n c t i o n s w h i c h a r e
O
p r e s u m e d t o be c o m p o s i t i o n a l l y a b r u p t .
The d i f f i c u l t y was r e s o l v e d by t h e work o f C h a n d r a
9
i n LPE grow n h e t e r o j u n c t i o n s w h i c h showed e x c e l l e n t
r e c t i f y i n g c h a r a c t e r i s t i c s which a g r e e d w e l l w i t h t h e o r y .
The e f f e c t o f i n t e r f a c e g r a d i n g was t r e a t e d r a t h e r
c o m p l e t e l y f o r t h e t h e r m i o n i c e m i s s i o n c a s e by L ee a n d
P e a r s o n , ^ where t h e i n f l u e n c e o f doping c o n c e n t r a t i o n
a n d c o m p o s i t i o n a l g r a d i n g on b a r r i e r s was q u a n t i f i e d .
The y o b t a i n e d t h e o r e t i c a l p r e d i c t i o n s i n good a g r e e m e n t
w ith th e e x p e rim e n ta l r e s u l t s of Chandra.
A fte r C h a n d ra 's s u c c e ss , l i t t l e
found.
work i n MBE c a n be
G o s s a r d 11, d e m o n s t r a t e d r e c t i f y i n g j u n c t i o n s fro m
stru ctu res
which were c o m p o s i t i o n a l l y g r a d e d , w i t h a
peak c o m p o sitio n of x = 0 .4 0 .
A comment w h i c h c a n be
made a b o u t t h e s e s t r u c t u r e s h o w e v e r , i s t h a t t h e m e a s u r e d
b a r r i e r h e i g h t s a r e somewhat h i g h e r t h a n t h a t p r e d i c t e d
from t h e p r e s e n t l y a c c e p t e d c o n d u c t i o n band
d iscontinuity.
12
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6
The i n t e r e s t i n n - n h e t e r o j u n c t i o n s w hich m o t i v a t e d
t h e work r e p o r t e d h e r e a r o s e f r o m t h e p o s s i b i l i t y of u s i n g
t h e c o n d u c t i o n band d i s c o n t i n u i t y a t t h e h e t e r o i n t e r f a c e
3
to a c c e le r a te e le c tro n s in a very s h o rt d ista n c e .
H igher e l e c t r o n v e l o c i t i e s a r e e x p ected as th e e l e c t r o n
c a n be e x c i t e d i n a d i s t a n c e c o n s i d e r a b l y l e s s t h a n a
mean f r e e p a t h , w h i c h i s
average ste a d y s ta t e
t h e n o r m a l l i m i t a t i o n on t h e
electron v e lo c ity .
o f i n t e r e s t i s on t h e o r d e r o f
T he e n e r g y r a n g e
0 . 2 5 eV, h e n c e aluminum
c o m p o s i t i o n s i n t h e r a n g e of 0 . 2 t o 0 . 3 3 a r e r e q u i r e d .
In th is
t h e s i s , th e grow th c o n d i t i o n s fo r n-n
h etero ju n ctio n s e x h ib itin g
presented.
excellent re c tific a tio n
are
I n c o n t r a s t t o p r e v i o u s work i n MBE, h i g h
g r o w t h t e m p e r a t u r e s a r e us ed t o o p t i m i z e m a t e r i a l q u a l i t y
in the b a r r i e r .
1 .1 .2
Undoped and M o d u l a t i o n Doped S i n g l e Quantum
W e l l s o n T h i c k AlGaAs B u f f e r s
T h i s d i s c u s s i o n of quantum w e l l s t r u c t u r e s i s
g r o u p e d t o g e t h e r , a s t h e i r p r o b l e m s hav e h i s t o r i c a l l y
been l i n k e d t o g e t h e r i n t h e l i t e r a t u r e .
was i d e n t i f i e d
Th e m a i n p r o b le m
t o be t h e p r o b le m o f t h e g r o w t h o f
h ig h
q u a l i t y GaAs o n AlGaAs and a p p e a r e d i n t h e e l e c t r o n
t r a n s p o r t p r o p e r t i e s of m o d u l a t i o n d o p e d s t r u c t u r e s and
i n t h e o p t i c a l p r o p e r t i e s of undoped quantum w e l l
stru ctu res.
The m a in d e v i c e a p p l i c a t i o n s
f o r t h e GaAs
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
7
grow n on AlGaAs s t r u c t u r e s
FETs a n d MODFET.
i s i n AlGaAs b u f f e r e d GaAs
The p r e s e n c e o f t h e AlGaAs b u f f e r
pro v id es a b a r r i e r to e le c tr o n i n j e c t i o n in to th e b u ffe r
and a low er s a t u r a t i o n v e l o c i t y f o r e l e c t r o n s i n th e
b u f f e r , b o t h c h a r a c t e r i s t i c s w h ic h r e d u c e p a r a s i t i c
buffer cu rren ts.
1^
B u f f e r c u r r e n t s have been invoked as
a d e g r a d i n g f a c t o r i n s h o r t g a t e FET s t r u c t u r e s .
13 14
'
The d i f f e r e n c e i n e l e c t r o n t r a n s p o r t b e t w e e n t h e two
ty p e s of m o d u la tio n doped s t r u c t u r e , i l l u s t r a t e d
1 . 3 , where t h e s o - c a l l e d r e g u l a r s t r u c t u r e
g r o w n on GaAs) a nd t h e i n v e r t e d s t r u c t u r e
AlGaAssSi)
are in d icated .
in F ig.
(A lG a A s:S i
(GaAs grown on
While t h e r e g u l a r s t r u c t u r e
t y p i c a l l y e x h i b i t s e l e c t r o n m o b i l i t i e s w e l l o v e r 1 0 0 ,0 0 0
2
cm / V . s , t h e i n v e r t e d s t r u c t u r e h a s o n l y shown a maximum
m o b i l i t y o f 30,000 t o 40,000 c m ^ /V .s.
The f a i l u r e o f
t h i s s t r u c t u r e was a t t r i b u t e d t o b o t h l a t t i c e
strain at
the i n t e r f a c e , or m e ta llu r g ic a l i n te r f a c e roughness.
15
As
a s m a l l l a t t i c e m i s m a t c h e x i s t s b e t w e e n GaAs a n d AlGaAs,
i t was p r o p o s e d t h a t t h i s w o u ld r e s u l t i n s t r a i n i n t h e
GaAs a nd i n c r e a s e d d e f e c t and d i s l o c a t i o n d e n s i t i e s w h i c h
would c a u s e poor m o b i l i t i e s f o r e l e c t r o n s a t t h e
in terface.
A t n e a r l y t h e same t i m e , i n i t i a l s t u d i e s o f s i n g l e
qu an tu m w e l l s grown o n t h i c k AlGaAs e x h i b i t e d p o o r
i n t r i n s i c l u m i n e s c e n c e a nd l a r g e e x t r i n s i c l u m i n e s c e n c e .
I n t r i n s i c lu m in e sc e n c e i s d e f in e d as re c o m b in a tio n of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
MODULATION DOPED STRUCTURES
REGULAR
AlGaAs
( 1 77 > 1 0 0 ,0 0 0
-------------------------------- 2 DEG
Ga As
SUBSTRATE
INVERTED
J l 77 - 3 1 , 0 0 0
Ga As
2 DEG
Al Ga As
SUBSTRATE
F ig u re 1.3
Layer d ia g ra m o f a m o d u la tio n doped
stru ctu re,
a) r e g u l a r s t r u c t u r e , AlGaAs:
S i grown on GaAs; b) i n v e r t e d s t r u c t u r e ,
GaAs gr ow n on A l G a A s : S i .
i s the
e l e c t r o n m o b i l i t y a t 77K.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
9
excitons
i n t h e g r o u n d s t a t e o f t h e quantum w e l l
1
stru ctu re.
The l i n e w i d t h of t h e e x c i t o n i c t r a n s i t i o n
h as b e e n s u g g e s t e d t o be r e l a t e d t o t h e a v e r a g e i n t e r f a c e
s m o o t h n e s s o f t h e quantum w e l l . 1 7 , 1 8
recom bination energy r e f l e c t s
T ^e e x c i t o n
the ground s t a t e
in th e
l o c a l n e i g h b o r h o o d o f t h e e x c i t o n s , and t h e o v e r a l l
l i n e w i d t h as a f u n c t i o n of a l l p o s s i b l e l o c a l
n e ig h b o rh o o d s along t h e w e l l .
The i n i t i a l s t u d i e s sh ow ed
i n t r i n s i c r e c o m b i n a t i o n w h ic h was weak a n d b r o a d ,
s u g g e s t i n g a l a r g e d e n s i t y of n o n - r a d i a t i v e c e n t e r s o r
com peting r e c o m b in a tio n mechanisms, and rough w ell
IQ
in terfaces.
E x t r i n s i c lum inescence i s im p u rity r e l a t e d
an d i n d i c a t e s
th e p re se n c e of
im p u rities.
As c a r b o n c a n s o m e t i m e s be t h e d o m i n a n t b a c k g r o u n d
i m p u r i t y i n MBE m a c h i n e s , M i l l e r e t a l . 2 0 a t t r i b u t e d
the
p o o r o p t i c a l p r o p e r t i e s o f s i n g l e qua ntum w e l l s t o t h e
p r e s e n c e of c a rb o n .
They p o s t u l a t e d
th a t carbon f l o a t s
on t h e s u r f a c e o f AlGaAs d u r i n g g r o w t h and i n h i b i t s
p la n a r growth r e s u l t i n g i n m e t a l l u r g i c a l
surface.
r o u g h n e s s of t h e
As GaAs g r o w th b e g i n s , t h e s u r f a c e r o u g h n e s s
was t h o u g h t t o be f r o z e n i n , a n d an e n h a n c e d
i n c o r p o r a t i o n of c arb o n c o n t r i b u t e d t o t h e e x t r i n s i c
lum inescence c o n te n t.
Gossard e t a l .
21
s u b s e q u e n t l y sh ow ed t h a t t h e
o p t i c a l p r o p e r t i e s o f s i n g l e w e l l s c o u l d be im p r o v e d i f
t h e y w e r e grown on a m u l t i l a y e r AlGaAs-GaAs b u f f e r r a t h e r
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
10
t h a n a c o n s t a n t c o m p o s i t i o n AlGaAs b u f f e r .
The
i m p r o v e m e n t i n t h e quantum w e l l p r o p e r t i e s was a t t r i b u t e d
t o t h e t r a p p i n g of
im purities
in the b u ffe r,
relie f
of
l a t t i c e m i s m a t c h s t r e s s e s , 15 o r a s m o o t h i n g c a p a b i l i t y o f
the m u ltila y e r b u ffe r.
22
A d d i t i o n a l work by W e i s b u c h e t a l . 1 ^ o n t h e
i n f l u e n c e o f s u b s t r a t e t e m p e r a t u r e on t h e e x c i t a t i o n
lin e w id th of
a m u l t i p l e quantum w e l l s t r u c t u r e
d e m o n s t r a t e d t h a t a minimum i n l i n e w i d t h o c c u r r e d f o r a
s u b s t r a t e te m p e r a t u r e of 680°C.
B ecause of t h e p roposed
r e l a t i o n s h i p b e t w e e n l i n e w i d t h an d i n t e r f a c e q u a l i t y ,
they id e n tifie d
th is
te m p e r a tu re as a c r i t i c a l
t e m p e r a t u r e f o r minimum i n t e r f a c e d i s o r d e r .
B oth a b o v e
a n d b e lo w 680°C t h e i n t e r f a c e was s u g g e s t e d t o be more
disordered.
T heir d ata is reproduced i n F ig . 1 .4 .
I t i s i n t e r e s t i n g fro m a h i s t o r i c a l p e r s p e c t i v e t h a t
t h e f a i l u r e o f t h e i n v e r t e d m o d u l a t i o n do p ed s t r u c t u r e
a nd t h e p o o r o p t i c a l p r o p e r t i e s o f s i n g l e quantum w e l l s
w e r e l i n k e d t o g e t h e r and a t t r i b u t e d t o t h e same e f f e c t s .
I n f a c t , t h e s e a r e two d i f f e r e n t p r o b l e m s , n a m e l y t h e
g r o w t h o f GaAs o n A l G a A s : S i and t h e g r o w t h o f GaAs on
u n d o p e d AlG aA s.
In the f i r s t
p la c e , both s t r u c t u r e s a re i n a sense a
p r o b l e m i n AlGaAs b u f f e r g r o w t h , a s t h e s m o o t h n e s s and
q u a l i t y of t h e u n d e r l y in g l a y e r d u r in g grow th i s a major
f a c to r d eterm ining th e q u a lity of
the m a t e r i a l s
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(meV)
11
EXCITATION
SPECTRUM
LINEWIDTH
GaAs-Ga,
5
Al As MQW
X
200 £ BARRIERS AND WELLS
1
}
4
\
3
i
I
2
1
0
570
610
650
690
730
SUBSTRATE TEMPERATURE DURING GROWTH T (°C)
s
F ig u re 1.4.
E x c i t o n l i n e w i d t h d e p e n d e n c e on s u b s t r a t e
t e m p e r a t u r e f o r a m u l t i p l e q u a n tu m w e l l
stru ctu re.
A f t e r r e f e r e n c e 17.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
12
s u b s e q u e n t l y gro wn on t o p o f t h e b u f f e r .
The s u c c e s s o f
t h e r e g u l a r m o d u la tio n doped s t r u c t u r e i s
in a la rg e p a r t
d u e t o t h e h i g h q u a l i t y GaAs t h a t c a n be grown by MBE.
T h u s t h e o p t i m i z a t i o n o f AlGaAs b u f f e r q u a l i t y i s an
i m p o r t a n t a s p e c t o f t h e p r o b l e m o f GaAs g r o w t h on und o p ed
AlGaAs.
The a d d i t i o n a l c o m p l i c a t i o n i n t h e i n v e r t e d
m o d u l a t i o n doped s t r u c t u r e i s t h e p r e s e n c e o f t h e s i l i c o n
d o p a n t i n t h e AlGaAs j u s t b e n e a t h t h e i n t e r f a c e .
I n t h i s t h e s i s , t h e g r o w t h o f h i g h q u a l i t y GaAs
s i n g l e q u an tu m w e l l s o n t h i c k AlGaAs i s d e m o n s t r a t e d .
T h i s i s a c h i e v e d by t h e c a r e f u l c o n t r o l o f g r o w t h
c o n d i t i o n s w h i c h y i e l d s a h i g h q u a l i t y AlGaAs b u f f e r a nd
h i g h q u a l i t y GaAs q u a n tu m w e l l s i m u l t a n e o u s l y .
related
S ubstrate
i m p u r itie s a re i d e n t i f i e d as a f a c t o r c o n t r o l l i n g
e x t r i n s i c lum inescence.
Quantum w e l l l u m i n e s c e n c e i s
r e p o r t e d w h ic h s i m u l t a n e o u s l y e x h i b i t s t h e l o w e s t
i m p u r i t y c o n t e n t and n a r r o w e s t e x c i t o n l i n e w i d t h s e v e r
reported fo r s in g le w ells.
Using t h e s e growth
t e c h n i q u e s , t h e r e g u l a r m o d u l a t i o n d o p e d s t r u c t u r e and
t h e i n v e r t e d m o d u l a t i o n s t r u c t u r e a r e s t u d i e d , and t h e
f a i l u r e o f t h e i n v e r t e d s t r u c t u r e i s shown t o r e s u l t from
Si s u r f a c e s e g r e g a t i o n d u r in g growth r a t h e r th a n
m e t a l l u r g i c a l roughness a t th e i n t e r f a c e .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
13
1.2
G e n e r a l B a c k g r o u n d on t h e Growth o f GaAs and AlGaAs
b v MBE
The f o l l o w i n g s e c t i o n s t r e a t t h e g r o w t h o f b u l k
m a t e r i a l , w ith d i s c u s s i o n o f growth p a r a m e te r s .
An
u n d e r s ta n d in g o f t h i c k growth w i l l p r o v id e a n a t u r a l
i n t r o d u c t i o n t o some c o n s i d e r a t i o n s o f i n t e r f a c e g r o w t h
and a l l o w a d i v i s i o n o f t h e p r o b l e m o f i n t e r f a c e g r o w t h
into n atu ral sectio n s.
The u n i q u e p r o p e r t y o f MBE g r o w t h f ro m t h e b r o a d e s t
p o i n t o f view i s t h a t i t
i s a t e c h n i q u e w h ic h a l l o w s r e a l
t i m e i n d e p e n d e n t c o n t r o l and a n a l y s i s o f t h e b a s i c
e x t e r n a l thermodynamic p a r a m e t e r s which g o v e rn c r y s t a l
grow th.
These a r e th e r a t e of d e p o s i t i o n of th e
c o n s t i t u e n t e l e m e n t s , t h e c r y s t a l t e m p e r a t u r e , and t h e
s u rfa c e sto ich io m etry .
The u s e o f g l a n c i n g a n g l e
r e f l e c t i o n e le c tr o n d i f f r a c t i o n p a tte r n s enables r e a l
t i m e i n s i t u a n a l y s i s o f t h e s u r f a c e s t o i c h i o m e t r y and
g e n e ra l c r y s t a l l i n e q u a l i t y w ith o u t in flu e n c in g the
grow th p r o c e s s , a t l e a s t to f i r s t o r d e r .
The f l e x i b i l i t y
o f i n d e p e n d e n t l y a d j u s t i n g and c o n s t a n t l y a n a l y z i n g t h e s e
p a r a m e te rs i s u n p a r a l l e l e d in c r y s t a l growth tec h n o lo g y
a nd i s a v a i l a b l e o n s e m i - p r o d u c t i o n m a c h i n e s c u r r e n t l y
a v a i l a b l e c o m m e r c i a l l y a nd i s by no m i a n s l i m i t e d t o t h e
research laboratory.
Because of th e a d d i t i o n a l c o m p le x ity th e s e
cap ab ilities
i n t r o d u c e i n t o t h e problem o f c r y s t a l growth
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
14
it
is
im portant to id e n tif y se p a ra te p ie c e s of the la rg e r
p r o b l e m o f g r o w i n g a goo d f i l m .
The f i r s t c o n s i d e r a t i o n
i s t h e g r o w t h o f f i l m s w i t h good i n t r i n s i c q u a l i t y i n t h e
absence of i n t e n t i o n a l dopants or u n in te n tio n a l
im purities.
A p o o r c h o i c e o f g r o w t h c o n d i t i o n s h a s by
f a r th e g r e a t e s t p o t e n t i a l to produce a f ilm o f poor
s t r u c t u r a l a nd e l e c t r i c a l q u a l i t y .
A " p o o r c h o i c e " means
an in c o m p a tib le c o m b in a tio n o f growth r a t e ,
and s u b s t r a t e t e m p e r a t u r e .
flux r a tio ,
This d e s c r i p t i o n a n t i c i p a t e s
t h e f a c t t h a t good c r y s t a l g r o w t h c a n o c c u r o v e r a w id e
ran g e of grow th c o n d i t i o n s .
H aving i d e n t i f i e d t h e grow th c o n d i t i o n s which
p r o d u c e good q u a l i t y m a t e r i a l , t h e n e x t c o n s i d e r a t i o n i s
the presence of u n in te n tio n a l im p u ritie s . A ctu ally
i d e n t i f y i n g t h e s o u r c e s o f t h e s e i m p u r i t i e s and
c o n t r o l l i n g them h a s p r o v e d t o be a p a r t i c u l a r l y a c t i v e
a r e a o f r e s e a r c h as m a t e r i a l q u a l i t y has im proved.
In p r i n c i p l e , a f t e r having u n dersto od th e b e s t
growth c o n d i t i o n s f o r th e i n t r i n s i c m a t e r i a l ,
the
a d d i t i o n o f d o p a n t s i n s m a l l c o n c e n t r a t i o n s w o u ld seem t o
be a r e l a t i v e l y s i m p l e m a t t e r .
g e n e r a l l y n o t been t r u e .
I n GaAs, t h i s h a s
C o n siderable re se a rc h e f f o r t
h a s b e e n s p e n t i n i d e n t i f y i n g s u i t a b l e n a nd p t y p e
d o p a n t s a nd h a s a c h i e v e d d o p a n t s w h i c h c a n be u t i l i z e d
a d e q u a t e l y , w ith due c o n s i d e r a t i o n of norm al d i f f u s i o n
p r o c e s s e s and a t r e a s o n a b l e c o n c e n t r a t i o n s .
In th e case
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
15
o f AlG aA s, t h e p r o b le m i s e v e n w o r s e .
The p o o r d o p a n t
b e h a v i o r o f m o s t n - t y p e d o p a n t s by a l l c r y s t a l g r o w t h
te c h n iq u e s rem ains th e fundam ental l i m i t i n g f a c t o r
u s e o f AlG aA s.
in the
We b e g i n o u r d i s c u s s i o n o f g r o w t h
c o n d i t i o n w i t h G aA s, a s t h i s s y s t e m i s f a i r l y w e l l
understood.
T h e n , t h e s e same c o n c e p t s w i l l - b e d i s c u s s e d
i n t h e c o n t e x t of A lG aA s , a l t h o u g h t h i s c a n b e s t be
d e s c r i b e d a s an e x t r a p o l a t i o n .
1 .2 .1
G allium A rse n id e
GaAs i s t h e m o st s t u d i e d and b e s t u n d e r s t o o d o f a l l
p o s s i b l e I I I - V compound s e m i c o n d u c t o r s grown by m o l e c u l a r
beam e p i t a x y .
There a r e s e v e r a l
a r t i c l e s on
w h ich p r o v i d e b o t h a h i s t o r i c a l
i n t r o d u c t i o n and a t h e o r e t i c a l
grow th.
e x c e l l e n t review
A very la rg e
u n d e r s t a n d i n g o f MBE
body o f e x p e r i m e n t a l work c a n be
f o u n d on t h e t h e r m o d y n a m i c s a nd k i n e t i c s
of MBE g r o w t h
f ro m t h e s e a r t i c l e s .
As MBE has been a p p l i e d t o d e v i c e s , new p r o b l e m s
r e l a t e d t o m a t e r i a l t e c h n o l o g y h a v e been e n c o u n t e r e d .
A t t e n t i o n has been f o c u s e d on t h e e l e c t r i c a l
p r o p e r t i e s o f MBE m a t e r i a l and i t s
and o p t i c a l
i n f l u e n c e on d e v i c e s .
W hile t h e f o l l o w i n g d i s c u s s i o n i s i n c o m p l e t e , i t does
illu strate
g e n e r a l g r o w t h c o n d i t i o n s w h i c h r e s u l t i n good
q u ality m aterial.
Deep l e v e l s a r e p r o b a b l y t h e m o s t u n s e e n y e t
in flu en tial
p r o p e r tie s of m a t e r i a l s .
U sually
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16
u n o b s e r v a b l e a t low f r e q u e n c i e s and lo w f i e l d s , t h e y
a r i s e t o i n f l u e n c e t h e high f i e l d
and h i g h f r e q u e n c y
b eh av io r of d e v ic e s i n a d e t r im e n t a l f a s h i o n .
I n MBE,
t h i s has been s t u d i e d a s a f u n c t i o n of growth c o n d i t i o n s .
Seme e a r l y wo rk r e d u c i n g d e e p l e v e l s i n MBE was d o n e by
oc
S tall,
w h e r e t h e i n c r e a s e o f s u b s t r a t e t e m p e r a t u r e was
f o u n d t o im p ro v e d e e p l e v e l
behavior.
A n o th e r s t u d y which i s l i k e l y r e l a t e d
i s t h e work by
M etz e e t a l . ^ ^ , w h e r e i t was shown t h a t m a t e r i a l
e x h i b i t i n g good e l e c t r i c a l and o p t i c a l p r o p e r t i e s c o u l d
be grown o v e r a w i d e s u b s t r a t e t e m p e r a t u r e r a n g e by
s u i t a b l y a d j u s t i n g t h e growth r a t e .
go od m a t e r i a l c o u l d be grown a t
T h e y showed t h a t
both high te m p e r a tu re s
a n d f a s t g r o w t h r a t e s and a l s o a t v e r y lo w t e m p e r a t u r e s
and s l o w g r o w t h r a t e s .
They a t t r i b u t e d
th is in ter­
r e l a t i o n s h i p between c h a n g e s i n t h e s u r f a c e m o b i l i t y o f
Ga a d a t o m s w i t h t e m p e r a t u r e and t h e t i m e r e q u i r e d f o r t h e '
adatom s t o f i n d t h e p ro p er s i t e f o r t h e growth of high
q u ality m ateria l.
They showed f o r t h e f i r s t
tim e th e
i m p o r t a n t k i n e t i c c o n s i d e r a t i o n s i n t h e growth of high
q uality m a te ria l.
I t i s t h e i n t r i n s i c k i n e t i c s o f MBE
growth which have t h e g r e a t e s t p o t e n t i a l t o i n t r o d u c e
d e f e c t s i n m a t e r i a l s , a n d m u s t be g i v e n p r i m a r y
c o n s i d e r a t i o n i n c h o o s in g growth c o n d i t i o n s .
The f i n a l
im p u rities.
a s p e c t o f MBE_growth i s
th e p re se n c e of
T h e s e a r i s e f ro m tw o s o u r c e s .
F irst,
is
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17
m achine and s e c o n d , s o u r c e m a t e r i a l q u a l i t y .
A t t h e p r e s e n t t i m e , t h e s t a t e o f t h e a r t f o r MBE
m a t e r i a l i s p ty p e , < 101 4 , p re d o m in a te ly carb o n .
i s t h o u g h t t o be f ro m t h e m a c h i n e a m b i e n t .
p a p e r , h o w e v e r , Skromme e t a l .
27
This
In a re c e n t
dem onstrated t h a t
e l e m e n t a l As s o u r c e m a t e r i a l c a n be a s i g n i f i c a n t s o u r c e
o f b o t h s u l f u r a nd c a r b o n .
Thus a h i g h p u r i t y m a c h i n e i s
n a t u r a l l y d e p e n d e n t on h i g h p u r i t y s o u r c e m a t e r i a l .
A s s u m in g t h a t s u c h a m a c h i n e i s a v a i l a b l e ,
the next
l i m i t i n g f a c t o r i s t h e r e d i s t r i b u t i o n o f i m p u r i t i e s a nd
d e f e c t s f ro m s u b s t r a t e m a t e r i a l s .
W h i l e known t o be
i m p o r t a n t i n b u l k m a t e r i a l s and g r o w t h t e c h n i q u e s s u c h
as vapor phase e p ita x y
(VPE) a n d l i q u i d p h a s e e p i t a x y
( L P E ) , t h i s h a s o n l y r e c e n t l y b e e n f o u n d t o be a n
i m p o r t a n t f a c t o r i n MBE g r o w t h .
An e x t e n s i v e r e v i e w and
s t u d y o f t h i s f a c t o r was d o n e by P a l m a t e e r
28
.
An
i m p o r t a n t f i n d i n g o f t h i s work was t h a t s u b s t r a t e
a n n e a l i n g a nd r e p o l i s h i n g was e f f e c t i v e i n r e d u c i n g t h e s e
i m p u r i t i e s i n MBE g r o w t h .
A s e c o n d f i n d i n g was t h a t t h e
r e d i s t r i b u t i o n o f some i m p u r i t i e s c o u l d be r e d u c e d
t h r o u g h lo w V : I I I f l u x r a t i o s .
In g e n e ra l term s, t h i s
c o u l d be s t a t e d a s g r o w i n g a t n e a r s t o i c h i o m e t r i c
co n d itio n s.
T h is r e d u c e s i n t r i n s i c d e f e c t s and can
r e d u c e t h e d i f f u s i o n c o e f f i c i e n t s o f i m p u r i t i e s which
d i f f u s e through d e f e c t r e a c t i o n s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
18
I n g e n e r a l , GaAs q u a l i t y i n MBE g r o w t h i s
improved
by g r o w t h a t n e a r s t o i c h i o m e t r i c c o n d i t i o n s , a t g r o w t h
r a t e s a nd s u b s t r a t e t e m p e r a t u r e s a p p r o p r i a t e from k i n e t i c
co n sid e ratio n s.
1 .2 .2
A lu m in u m - G a l liu m A r s e n i d e G r o w th
GaAs m o d e l s o f g r o w t h c a n be e x t r a p o l a t e d t o AlGaAs
grow th.
I t h a s been shown t h a t low f l u x r a t i o s
and h i g h
s u b s t r a t e te m p e r a tu r e s g e n e r a l l y improve t h e o p t i c a l
p r o p e r t i e s o f AlGaA s3^ ”3*-.
In a d d i t i o n , th e k i n e t i c
r e l a t i o n s h i p be tw e e n g r o w t h r a t e and s u b s t r a t e
t e m p e r a t u r e has been d e m o n s t r a t e d .
32
I n a d d i t i o n t o c a r b o n , o x y g e n i s t h o u g h t t o be an
i m p o r t a n t b a c k g r o u n d c o n t a m i n a n t o f AlGaAs i n MBE g r o w t h .
T h i s c a n be c o n s i d e r e d a m a c h i n e p r o p e r t y and c a n be
m inim ized thro ugh p ro p e r t e c h n i q u e s .
S u b stra te im purity
r e d i s t r i b u t i o n h a s been o b s e r v e d b u t i s l e s s w e l l
docum ented.
A c u r i o u s p r o b le m i n AlGaAs g r o w t h i s p o o r n - t y p e
dopant behavior i
G e n e r a l l y , n - t y p e d o p a n t s i n AlGaAs
e x h i b i t a deepening of
i o n i z a t i o n e n e r g y and a two s i t e
o c c u p a n c y , s h a l l o w d o n o r s and t h e so c a l l e d D e e p Donor/DX
c e n t e r . 3 3 ' 34
The b e h a v i o r w o r s e n s w i t h i n c r e a s i n g A1
c o m p o s i t i o n , r e a c h i n g a peak n e a r x = 0 . 5 0 and t h e n
im proving as x + 1 .0 .
T h i s p r o b le m has been o b s e r v e d and
s t u d i e d i n e v e r y c r y s t a l growth t e c h n i q u e f o r
all
n-type
dopants.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19
A n o t h e r a s p e c t o f AlGaAs g r o w t h i s
poor s u r f a c e
m o r p h o l o g y a t seme c o m b i n a t i o n s o f g r o w t h r a t e s ,
s u b s t r a t e t e m p e r a t u r e s , a nd f l u x r a t i o s .
t h o u g h t t o be r e l a t e d
to d iffe re n c e s
b e t w e e n Ga a n d A1 a d a t o m s .
oc_o 7
This i s
in surface m ob ility
T h i s p r o b l e m i s an i m p o r t a n t
a s p e c t o f i n t e r f a c e g r o w t h , a s t h e s m o o t h n e s s of t h e
growing s u r f a c e i n l a r g e p a r t d e t e r m in e s t h e sm oothness
of the h e te r o in te r f a c e .
more l e n g t h i n C h a p te r
1.3
T h i s p r o b le m i s d i s c u s s e d a t
4.
Scope o f T h i s T h e s i s
T h is t h e s i s i s c o n c e rn e d w ith t h e grow th of high
q u a l i t y h e t e r o i n t e r f a c e s w i t h an e m p h a s i s on h i g h q u a l i t y
m a t e r i a l on b o t h s i d e s of t h e i n t e r f a c e .
The t h r e e m ain
h e t e r o s t r u c t u r e s s t u d i e d a r e t h e n-n h e t e r o j u n c t i o n ,
u ndop ed s i n g l e quantum w e l l s t r u c t u r e , a n d t h e m o d u l a t i o n
do ped s i n g l e quantum w e l l .
C hapter 2 d e t a i l s
the e x p e rim e n ta l
techniques
r e l e v a n t t o t h e g r o w t h of h i g h q u a l i t y t h i c k GaAs and
AlGaAs by MBE.
A f l u x mode l i s d e s c r i b e d w h i c h p r o v i d e s
a b a s i s f o r t h e d i s c u s s i o n of t h e meaning of t h e f lu x
r a t i o and s u r f a c e s t o i c h i o m e t r y w h ic h o c c u r s i n
subsequent ch apters.
the o p t i c a l
E xperim ental d a ta is
and e l e c t r i c a l
p r e s e n t e d on
p r o p e r t i e s of GaAs and AlGaAs
grown by t h e a u t h o r .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20
In C h a p te r 3 t h e growth o f n -n h e t e r o j u n c t i o n s i s
c o n s i d e r e d fro m t h e p o i n t o f v i e w o f o p t i m i z i n g t h e
AlGaAs q u a l i t y i n t h e b a r r i e r r e g i o n .
re c tify in g ju n ctions are presented.
High q u a l i t y
The i n f l u e n c e o f
s u b s t r a t e i m p u r i t i e s and r e d i s t r i b u t i o n o f i m p u r i t i e s i n
the e l e c t r o s t a t i c f ie ld s
is considered.
The l i m i t i n g f a c t o r i n t h e d e v i c e p e r f o r m a n c e i s
i d e n t i f i e d a s t h e DX c e n t e r b e h a v i o r o f A l xGa1- x A s : S i .
C h ap ter 4 d i s c u s s e s th e growth o f h igh q u a l i t y
h e te r o in te r f a c e s , p a r t i c u l a r l y the tr a n s ie n t
sto ic h io m e try a t the in t e r f a c e .
A s i n g l e q u an tu m w e l l i s
u s e d t o d e m o n s t r a t e t h a t h i g h q u a l i t y GaAs c a n be grown
on t h i c k AlGaAs w i t h s m o o t h w e l l i n t e r f a c e s .
affecting
The f a c t o r s
t h e o b s e r v e d l i n e w i d t h a r e i d e n t i f i e d and a l l o w
a c o n n e c t i o n w i t h t h e work d i s c u s s e d i n C h a p t e r 1 .
t h e problem o f Si r e d i s t r i b u t i o n
s i n g l e q u a n tu m w e l l s t r u c t u r e .
Then
is dem onstrated in the
Si surface segregation is
i d e n t i f i e d as th e f a i l u r e of th e in v e r te d m odulation
doped s t r u c t u r e a t h ig h grow th t e m p e r a t u r e s .
A general
d i s c u s s i o n on t h e d i f f e r e n c e s i n s e g r e g a t i o n b e h a v i o r o f
S i on GaAs a nd AlGaAs f o l l o w s .
The e l e c t r i c a l and o p t i c a l p r o p e r t i e s o f r e g u l a r
m o d u l a t i o n d o ped s i n g l e q u a n tu m w e l l s d i s c u s s e d .
A
p h e n o m e n o l o g i c a l mode l o f t h e l u m i n e s c e n c e l i n e s h a p e o f a
t w o - d i m e n s i o n a l e l e c t r o n g a s i n a quantum w a l l i s
f o r m u l a t e d a nd co m p ar ed t o e x p e r i m e n t .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
21
I n C h a p t e r 5/ c o n c l u s i o n s o f t h e t h e s i s a r e m a d e ,
and p o s s i b l e f u t u r e work i s d i s c u s s e d .
references
In a d d itio n ,
t o t h e a p p l i c a t i o n o f t h i s work t o m icro w av e
d e v i c e s , and s t u d i e s o f t h e p r o p e r t i e s o f quantum w e l l
s tr u c tu r e s are noted.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 2
MBE GROWTH OF THICK GaAs and AlGaAs
The p u r p o s e o f t h i s c h a p t e r i s t o d e s c r i b e t h e
ex p erim en tal tech n iq u es r e le v a n t to th e fo llo w in g
c h a p t e r s a n d d e m o n s t r a t e t h e g r o w t h o f h i g h q u a l i t y GaAs
a n d AlG aA s.
W hile th e grow th c o n d i t i o n s f o r h ig h q u a l i t y
m a t e r i a l i s w e l l known, a s d i s c u s s e d i n C h a p t e r 1 , MBE
r e q u i r e s a c o n s i d e r a b l e am o u n t o f e x p e r i m e n t a l
still
sk ill.
As r e p o r t e d g r o w t h c o n d i t i o n s c a n v a r y
c o n s i d e r a b l y f r o m l a b t o l a b , t h e m a t e r i a l q u a l i t y and
grow th c o n d i t i o n s r e p o r t e d h e re p ro v id e a r e f e r e n c e fo r
th e c o n d itio n s rep o rted in the subsequent c h a p te rs.
An i m p o r t a n t p o i n t t h r o u g h o u t t h e p r e s e n t a t i o n i n
th is
c h a p t e r i s g r o w t h n e a r t h e m e t a l r i c h e d g e a t low
V :III
flu x r a tio s .
To u n d e r s t a n d w h a t t h i s m e a n s , a f l u x
model i s f o rm u la te d i n o r d e r t o d i s c u s s t h e n a t u r e of
s u r f a c e s t o i c h i o m e t r y and t h e c o m p o n e n t s o f t h e f l u x
ratio .
T h i s i s c o m p a r e d t o e x p e r i m e n t a l d a t a f o r GaAs
a n d A lG a A s , a n d i m p o r t a n t d i f f e r e n c e s a r e h i g h l i g h t e d ,
which a r e r e l e v a n t t o th e growth of h e t e r o s t r u c t u r e s a t
low f l u x r a t i o s .
2.1
E x p e r i m e n t a l M eth o d s
The b u l k o f t h e work was p e r f o r m e d on t h e m a c h i n e a t
C o r n e l l U n i v e r s i t y w h i l e some o f t h e l a t e r work
22
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
23
(chronologically),
p a r t i c u l a r l y th e flu x m easurem ents,
w e r e p e r f o r m e d on t h e m a c h i n e a t t h e E l e c t r o n i c s
L a b o r a t o r y o f t h e G e n e r a l E l e c t r i c Company, S y r a c u s e , NY.
A d e s c r i p t i o n o f t h e m a c h i n e s t r u c t u r e c a n be f o u n d i n
*3 0
t h e V a r i a n u s e r s m a n u a l -30 o r t h e t h e s i s
2 .1 .1
by P a l m a t e e r
28
.
MBE M a c h i n e s
The work r e p o r t e d i n t h i s t h e s i s was d o n e on two
m achines,
b o t h o f t h e V a r i a n GEN I I t y p e .
B o th s y s t e m s
a r e e q u i p p e d w i t h a l o a d l o c k , p r e p a r a t i o n c h a m b e r and
main grow th cham ber.
The g r o w t h c h a m b e r c a r o u s e l i s
f i t t e d w ith a 2 inch diam eter r o t a t i n g s u b s t r a te holder
an d a beam f l u x m o n i t o r w h ic h c a n be r o t a t e d i n t o t h e
g r o w t h p o s i t i o n f o r a c c u r a t e beam i n t e n s i t y m e a s u r e m e n t s .
B o th m a c h i n e s a r e e q u i p p e d w i t h o p t i c a l p y r o m e t e r s a t t h e
s o u r c e f l a n g e , which a llo w a m o n i t o r i n g of t h e s u b s t r a t e
t e m p e r a t u r e w h ic h i s i n d e p e n d e n t o f t h e s u b s t r a t e
therm ocouple w ith o u t i n t e r r u p t i o n o f th e g row th.
G lancing angle e le c tr o n r e f l e c t i o n d i f f r a c t i o n
i s used t o
m onitor th e s u b s t r a t e s u rf a c e q u a l i t y d u rin g grow th.
B oth s y s t e m s h a v e a q u a d r u p o l e m as s s p e c t r o m e t e r f o r t h e
a n a l y s i s o f r e s i d u a l background g a s e s .
E lem ental sources
a r e m a in ta in e d in p y r o l y t i c boron n i t r i d e
(PBN) c r u c i b l e s
w i t h i n f u r n a c e s c o n s t r u c t e d p r e d o m i n a t e l y o f t a n t a l u m and
PBN.
T y p i c a l b a s e p r e s s u r e s i n t h e g r o w t h and
p r e p a r a t i o n c h a m b e r s r e a c h low l O ” ^® T o r r i n t h e a b s e n c e
o f b a c k g r o u n d As p r e s s u r e s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
24
B o t h m a c h i n e s u t i l i z e LN2 c o o l i n g o f t h e s o u r c e
flan g es.
The GE m a c h i n e e n j o y s c o n t i n u o u s LN2 and i s
e q u ip p e d w i t h a p h a s e s e p a r a t o r , w h i l e t h e machine a t
C o r n e l l i s o n l y c o o l e d d u r i n g g r o w t h and t h e s o u r c e
s h r o u d an d c r y o p a n e l s a r e a l l o w e d t o warm up b e t w e e n
grow ths.
S p e c i f i c d e t a i l s o f m a c h i n e s e t up f o r t h e GE
s y s t e m c a n b e f o u n d i n t h e t h e s i s by P a l m a t e e r
28
.
U n f o r tu n a te ly th e C o r n e ll m a c h in e 's o p e r a t i o n has
f o l l o w e d more o f a n o r a l t r a d i t i o n .
V a r i a n MBE m a n u a l
•DO
In i t s absence, the
s u ff ic e s ra th e r w ell fo r a
d e s c r i p t i o n o f s u c c e s s f u l machine o p e r a t i o n .
The m a c h i n e a t GE i s a h i g h p u r i t y m a c h i n e a s t h i s
i s a s p e c i f i e d r e q u i r e m e n t f o r i t s o p e r a t i o n due t o t h e
s t r i n g e n t deman ds on t h e m a t e r i a l i t p r o d u c e s .
b a c k g r o u n d s a r e low 10
cm
T ypical
P -type or s e m i- in s u la tin g
w i t h n - e l e c t r o n m o b i l i t i e s r e g u l a r l y a b o v e 1 0 0 , 0 0 0 cm / V s.
T h e C o r n e l l m a c h i n e h a s e x h i b i t e d somewhat more
varied p u rity le v e l.
h i g h a s 10
15
cm
-3
T y p i c a l back g ro u n d s have ran g ed as
p - t y p e and a s low a s 10
14
-3
cm
p-type.
The h i g h e s t e l e c t r o n 77K m o b i l i t y o b t a i n e d f o r GaAs i s
101 ,0 0 0 cm^/V.s
2 .1 .2
( r u n 1538) a t 3 x 1 0 ^
cm
S u b stra te P reparation
E x c ep t where n o t e d , most s u b s t r a t e s used i n t h i s
w ork a r e u n d o p e d and p r e p o l i s h e d , grown by t h e L i q u i d
E n c a p s u l a t e d C z o c h r a l s k i m eth o d
(LEC).
S ubstrates are
p r e p a r e d f o r g r o w t h by a n u l t r a s o n i c s o l v e n t r i n s e ;
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
25
a c e t o n e , m e t h a n o l , DI w a t e r .
Then t h e s u b s t r a t e i s
im m ers ed i n c o n c e n t r a t e d s u l f u r i c a c i d f o r two m i n u t e s t o
remove r e s i d u a l o r g a n i c s a nd w a t e r .
The s u b s t r a t e s a r e
e t c h e d f o r 4 m i n u t e s i n c o o l e d 7 : 1 : 1 H2S0 4 :H2 0 2 :H20
removing a p p r o x i m a te ly 3 p
of surface.
The e t c h e d
s u b s t r a t e i s f l u s h e d w i t h DI w a t e r and a l l o w e d t o s i t f o r
s e v e r a l m i n u t e s i m m ers ed i n DI w a t e r t o grow an o x i d e .
The s u b s t r a t e s a r e m o u n ted on molybdenum s u b s t r a t e h o l d e r
blocks u sing high p u r i t y In .
The s u b s t r a t e h o l d e r i s
l o a d e d i n t o t h e m a c h i n e and p l a c e d u n d e r vac uu m.
P rio r
to grow th, th e s u b s t r a t e h o ld er i s o u tg assed a t
a p p r o x i m a t e l y 400°C u n t i l t h e b a s e p r e s s u r e i n t h e
p r e p a r a t i o n chamber i s ap p ro a c h e d
(~ 2 x l O - *^ T o r r ) .
The s u b s t r a t e h o l d e r i s l o a d e d i n t o t h e g r o w t h c h a m b e r
after
t h e p r e p a r a t i o n c h am ber p r e s s u r e d r o p s t o t h e b a s e
pressure.
The beam p r e s s u r e s a r e m e a s u r e d a nd t h e
s u b s t r a t e h o ld e r i s r o t a t e d i n t o th e growth p o s i t i o n .
The s u b s t r a t e i s b r o u g h t up i n t e m p e r a t u r e u n t i l t h e
o x i d e d e s o r p t i o n i s o b s e r v e d u s i n g t h e RED p a t t e r n s .
The
s u b s t r a t e i s a l l o w e d t o s i t f o r s e v e r a l m i n u t e s u n d e r an
a r s e n i c f l u x t o e n s u re th e rem oval o f a l l s u r f a c e o x id e s .
The s u b s t r a t e i s now r e a d y f o r t e m p e r a t u r e c a l i b r a t i o n
and g r o w t h .
2 .1 .3
Measurement o f S u b s t r a t e T e m p e ra tu re s
The c o n v e n t i o n a l method o f m o n i t o r i n g s u b s t r a t e
t e m p e r a t u r e i s by a t h e r m o c o u p l e i n i n t i m a t e t h e r m a l
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
26
c o n ta c t w ith the s u b s tr a te h o ld e r.
equipped w ith a r o t a t i n g
B e c a u s e t h e GEN I I
s u b s tra te h o ld er, in tim ate
c o n t a c t c a n n o t be m a i n t a i n e d a s t h e t h e r m o c o u p l e
i t s e l f does not r o t a t e .
is
(TC)
The d e s i g n r e l i e s on a TC w e l d e d
t o a t a n t a l u m d i s c w h ic h c o l l e c t s r a d i a t i v e h e a t w i t h i n a
w e l l b o r e d i n t o t h e back o f t h e molybdenum s u b s t r a t e
h older.
The s p a c i n g b e t w e e n t h e two a s s e m b l i e s o f a b o u t
1 mm i s m a i n t a i n e d by a s a p p h i r e b e a d w h i c h a c t s a s a
b e a r in g s u r f a c e f o r th e s p r i n g lo ad ed therm ocouple
a g a in s t the s u b s tr a te h o ld er.
For various reasons
r e l a t e d t o s t r a y r a d i a t i o n f ro m t h e h e a t e r f i l a m e n t s a nd
d i f f e r e n c e s or changing block e m i s s i v i t y , th e
th erm o c o u p le f a i l s to p r o v id e an a c c u r a t e te m p e ra tu re
re a d in g fo r th e c lo se d loop s u b s t r a t e h e a te r c o n t r o l
system .
So t h a t a c c u r a t e t e m p e r a t u r e c o n t r o l i s
a t t a i n e d , an o p t i c a l pyrom eter i s used t o m easure th e
s u b s t r a t e t e m p e r a t u r e t h r o u g h a window f l a n g e o f
a p p r o p r ia te t r a n s m i s s i v i t y lo c a te d a t th e c e n t r a l p o r t of
th e source fla n g e .
The p y r o m e t e r i t s e l f
does n o t p r o v id e
an a b s o l u t e t e m p e r a t u r e m easu rem en t, so a c a l i b r a t i o n
p r o c e d u r e i s u s e d f o r e a c h g r o w t h , a s s u g g e s t e d by
Calawa.
39
The s u b s t r a t e xs b r o u g h t up t o t h e g r o w t h
t e m p e r a t u r e r a n g e an d t h e o x i d e d e s o r b e d u n d e r a n a r s e n i c
flu x .
G e n e ra lly , a t the oxide d e s o rp tio n tem perature a
( 3 x 1 ) s u r f a c e r e c o n s t r u c t i o n i s o b s e r v e d i n t h e RED
p a t t e r n i n t h e a b s e n c e o f any i n c i d e n t f l u x e s i f
a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
27
reco n stru ctio n p a tte rn is p resen t.
f lu x , the s u b s tra te
40°C.
Under an a r s e n i c
tem perature is r a is e d approxim ately
In th e absence of in c id e n t flu x e s a
(4x2) m e t a l
r i c h p a t t e r n s h o u l d be o b s e r v e d a f t e r a t r a n s i t i o n t i m e
o f a f ew s e c o n d s .
The t e m p e r a t u r e i s l o w e r e d i n s m a l l i n c r e m e n t s < 5°C
a nd a l l o w e d t o s t a b i l i z e u n t i l i n t h e a b s e n c e o f i n c i d e n t
f l u x e s t h e t r a n s i t i o n t im e from t h e
(3x1) p a t t e r n t o t h e
through the
(4x2) p a t t e r n i s > 10 s e c s .
t i m e t o move f r o m (2x4)
second.
(2x4)
to
The
(3x 1) s h o u l d be l e s s t h a n 1
The a u t h o r d e s i g n a t e s t h i s
t e m p e r a t u r e a s 640°C
and t h e n m e a s u re s o f f s e t s from t h i s p o i n t u s in g t h e
o p t i c a l pyrom eter.
2 .1 .4
Beam P r e s s u r e M e a s u r e m e n t s
Group I I I E lem en ts
As t h e g r o u p I I I
e l e m e n t s a r e t y p i c a l l y an o r d e r o f
m a g n i t u d e l o w e r t h a n t h e a r s e n i c p r e s s u r e s , and t h e
a r s e n i c b a c k g r o u n d r a n g e s f ro m 10
—8
t o 10
—7
Torr the
m e a s u r e m e n t o f t h e g a l l i u m a nd alum in um p r e s s u r e s c a n be
i n f l u e n c e d by t h e b a c k g r o u n d As p r e s s u r e .
A ccurate
m e a s u r e m e n t s c a n be o b t a i n e d by t a k i n g t h e d i f f e r e n c e
betw een p r e s s u r e w ith th e group I I I
stead y s t a t e
c e l l o p e n and t h e
background p r e s s u r e a f t e r
it
is closed.
U n f o r t u n a t e l y , A1 p r e s s u r e s a r e t y p i c a l l y on t h e o r d e r o f
t h e As b a c k g r o u n d w h i l e t h e c e l l s a r e o n , a nd l a r g e
e r r o r s a r e i n t r o d u c e d by t h i s m e t h o d .
A more a c c u r a t e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
28
m e th o d i s t o p l o t a c u r v e o f beam p r e s s u r e s f o r Ga an d A1
v s . c e l l t e m p e r a t u r e w h i l e t h e As c e l l s a r e a t i d l e an d
t h e background i s low.
Pig.
An e x a m p l e o f t h i s
i s shown i n
2 .1.
In th e a u t h o r 's e x p e r ie n c e , th e grow th r a t e i s
r e a s o n a b l y s t a b l e o v e r p e r i o d s o f a t l e a s t a w eek,
a l t h o u g h t h i s d e p e n d s on c e l l c o n f i g u r a t i o n
A) a n d m a c h i n e u s a g e .
(see Appendix
To d e t e r m i n e a beam p r e s s u r e , t h e
c e l l t e m p e r a t u r e u s e d i s l o o k e d up ( o r c a l c u l a t e d )
curve.
on th e
T h i s m e th o d a s s u r e s c o n s i s t e n c y i n c a l c u l a t i n g
f l u x r a t i o s an d aluminum c o m p o s i t i o n s .
The c o m p o s i t i o n
o f AlGaAs c a n b e e x p r e s s e d i n t e r m s o f t h e beam
e q u i v a l e n t p r e s s u r e s fro m t h e f o l l o w i n g e q u a t i o n
(derived
below ):
f o a = K(| _1}
*A1
(2.1)
x
w h e r e x i s t h e alumin um more f r a c t i o n an d K i s an
e x p e r i m e n t a l l y d e te r m in e d p a r a m e t e r which t y p i c a l l y h as a
v a lu e of 1.7 < K < 2 .0 .
I t i s g e n e ra lly a c c u ra te i f the
s u b s t r a t e t e m p e r a t u r e i s c o n s t a n t f o r t h e c a l i b r a t e d an d
c a lc u la te d grow ths.
At h i g h e r t e m p e r a t u r e s , Ga
d e s o r b t i o n i s i m p o r t a n t , and t h e K v a l u e b e g i n s t o r i s e .
2 .1 .5
G rou p V Beam P r e s s u r e s
An a c c u r a t e m e a s u r e m e n t o f t h e g r o u p V e l e m e n t s i s
com plicated
by t h e r a p i d t i m e d e p e n d e n c e o f t h e As
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
29
.-5
10
Beam Equivalent Pressure of
Ga Cell vs Inverse Cell
Tem perature
10
—AH/kT
P = P 0 exp
_ * . "P0 = 3.359 E4Torr
Ga 4
_A H=3.l4eV
50
'O mV
.-6
Ga 3
> = 2 .0 0 4 5 E 4 Torr
AH =3.15922 eV
10
>>
‘0
Id6
22.00
mV
21.50
O
»0 .“
20.50
c-7
20.00
mV
10
10
o
\
Al # 5
P0 = l.46036E 4T orr
A H=3.68281 eV
10
6.0
l\
6.5
7.0
7.5
10
x I0”4 ( l / T )
Figure 2 .1 .
Beam e q u i v a l e n t p r e s s u r e s o f Ga # 3 , Ga # 4 ,
a n d A l #5 a s a f u n c t i o n o f i n v e r s e c e l l
t e m p e r a t u r e f o r t h e G.E. m a c h in e .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
30
b a c k g r o u n d when t h e c e l l s h u t t e r i s o p e n e d , w h i c h r e s u l t s
b e c a u s e t h e As i s p o o r l y pumped by t h e MBE m a c h i n e .
S e v e r a l schemes have been u sed t o a c c o u n t f o r t h e tim e
dep en d en ce, b u t most f a i l t o meet th e im p o rta n t c r i t e r i o n
of r e p e a ta b ility .
The m o s t d e s i r a b l e s i t u a t i o n
s te a d y s t a t e m easurem ent.
Calawa
39
is a
A t e c h n i q u e s u g g e s t e d by
i s th e measurement o f t h e a r s e n i c f l u x w ith a
c o n co m itan t group I I I f l u x .
A steady s t a t e co n d itio n
w hich i s o n l y d e p e n d e n t on c e l l t r a n s i e n t s
repeatably a tta in e d .
i s r a p i d l y and
T his technique a ls o a c c u r a te ly
m e a su re s t h e t r u e a r s e n i c p r e s s u r e d u r in g grow th as t h e
p u m p in g e f f e c t p r o v i d e d t h e g r o u p I I I
flu x is included.
T h is measurement g e n e r a l l y p r o v id e s a low er r e a d in g th a n
in th e absence of a group I I I f lu x .
An a d d i t i o n a l r e f i n e m e n t o f t h i s
technique involves
th e o b s e rv a tio n t h a t th e p resence of a group I I I
in c re a se the p ressu re read in g .
f lu x can
As t h e m e a s u r e d g r o u p I I I
p r e s s u r e s a r e g e n e r a l l y a f a c t o r o f 7 t o 25 l o w e r , t h e y
can c o n tr ib u te a s e v e ra l p e rc e n t in c re a s e in th e re a d in g .
To rem ove t h i s e f f e c t t h e a u t h o r m e a s u r e s t h e beam
p r e s s u r e which i s o b s e r v e d a t t h e i n s t a n t t h e g ro u p I I I
e l e m e n t (s)
are s h u tte re d a f te r a steady s t a t e co n d itio n
is attain ed .
The beam p r e s s u r e e x h i b i t s two t y p e s o f
b e h a v i o r , w h ic h a r e d e p e n d e n t upon t h e r e l a t i v e m a g n i t u d e
of th e group I I I
elem ent p r e s s u r e ( s ) .
The f i r s t
behavior
o c c u r s when t h e g r o u p I I I p r e s s u r e o n l y l o w e r s t h e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
31
arsen ic
b a c k g r o u n d w h i l e n o t i n c r e a s i n g t h e beam
p re ss u re w ith i t s p re ss u re .
At th e i n s t a n t of
s h u t t e r i n g , t h e p r e s s u r e i m m e d i a t e l y b e g i n s t o r i s e f ro m
the stea d y s t a t e value.
The s e c o n d b e h a v i o r i s o b s e r v e d
when t h e g r o u p I I I e l e m e n t i s a s i g n i f i c a n t c o m p o n e n t o f
t h e i n c i d e n t p r e s s u r e a s w e l l a s pu m p in g t h e As
background.
At th e i n s t a n t th e group I I I
elem ent i s
s h u tte r e d , th e p re ssu re read in g drops in s ta n ta n e o u s ly ,
and t h e r e a d i n g i s t a k e n .
Th e a u t h o r f e e l s t h a t t h i s
is
an a c c u r a t e r e a d i n g a s t h e pumping e f f e c t i s s t i l l
r e t a i n e d w h i l e t h e e r r o r i n t r o d u c e d by t h e p r e s e n c e o f
th e group I I I
removed.
e lem en t on t h e d i r e c t m easurem ent i s
A d ia g ra m a tic d e s c r i p t i o n of th e s te a d y s t a t e
v s t r a n s i e n t b e h a v i o r i s shown i n F i g . 2 . 2 ( a )
an d an
e x a m p l e o f t h e d a t a o b t a i n e d by t h i s m e th o d i s shown i n
F i g . 2 . 2 ( b ) where th e v a r i o u s m easurem ents o b t a i n e d a r e
p lo tte d as a fu n c tio n of th e g alliu m p re s s u re .
The
e f f e c t o f i n c r e a s e d p r e s s u r e r e a d i n g due t o t h e g a l l i u m
p r e s s u r e c a n be s e e n m o s t c l e a r l y a t t h e lo w a r s e n i c
p r e s s u r e s where th e g a l l i u m p r e s s u r e i s p a r t i c u l a r l y
high.
W hile i t
is u su ally tru e th a t th ese s itu a tio n s
g e n e r a l l y r e p r e s e n t g ro w th c o n d i t i o n s where t h e s u r f a c e
be com es m e t a l r i c h , t h e m o re m o d e s t i n f l u e n c e s a r e
i m p o r t a n t , a n d t h i s m e th o d p r o v i d e s good c h e c k f o r t h i s
effect.
T h is ap p ro ach i s m ost im p o r t a n t where th e
e f f e c t s o f f l u x r a t i o a r e b e i n g q u a n t i f i e d and i s u sed i n
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
32
— PB=As only steady state
(As,As+Ga) vs R.
— PB=As + Ga steady state
— P b just after termination
of Ga flux
O'
As 0+ As 7
/
—o
As 0
<J
oo
—o
As 7
R n (x I0"7 Torr)
Behavior
CLIMB *c
BE
PB
steady
DIP»D
'A^'Ga
steady
state
v_y
time
^termination
t steady
state
Ga
F igure 2.2.
Beam e q u i v a l e n t p r e s s u r e b e h a v i o r o f As^
a s a f u n c t i o n o f t h e Ga p r e s s u r e ,
a)
T r a n s i e n t b e h a v i o r o f As^ a s a f u n c t i o n o f
t i m e a t Ga p r e s s u r e t e r m i n a t i o n ,
b) S t e a d y
s t a t e a n d t r a n s i e n t As^ p r e s s u r e a s a
f u n c t i o n o f t h e Ga p r e s s u r e .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
33
t h e f l u x model m easurem ent below.
2 .1 .6
R eflectio n E lectro n D iffra c tio n P attern s
The c a p a b i l i t y o f i n s i t u r e a l t i m e m o n i t o r i n g o f
th e s u rfa c e c r y s t a l s t r u c t u r e through r e f l e c t i o n e le c tr o n
d iffractio n
(RED) i s o n e o f t h e m a in a d v a n t a g e s o f MBE.
The r e s u l t i n g d i f f r a c t i o n p a t t e r n s a l l o w a n e v a l u a t i o n o f
s e v e ra l im portant p r o p e rtie s of the s u rfa c e .
F irst,
s u c c e s s f u l o x id e rem oval i s i n d i c a t e d from t h e a p p e a r a n c e
o f a r e c o n s t r u c t r e d p a t t e r n f r o m an i n i t i a l l y
p attern .
am orphous
Secondly th e o b s e r v a tio n o f s u rf a c e
r e c o n s t r u c t i o n s as a f u n c tio n of s u b s t r a t e te m p e ra tu re
a llo w s an a c c u r a t e c a l i b r a t i o n of th e a c t u a l s u b s t r a t e
tem perature.
F i n a l l y , d u rin g grow th, in fo rm a tio n i s
o b t a i n e d c o n c e r n i n g s u r f a c e s t o i c h i o m e t r y an d s u r f a c e
cry stallin ity .
The d i f f r a c t i o n g e o m e t r y c o n s i s t s o f a w e l l
c o l l i m a t e d ~ 10 KeV e l e c t r o n beam i n c i d e n t a t a n a n g l e o f
~ 6 degrees w ith re s p e c t to the s u b s tr a te s u rfa c e .
The
d i f f r a c t e d e l e c t r o n s a r e o b s e r v e d on a p h o s p h o r s c r e e n
situ ated
in the p o s itio n near sp ecu lar r e f le c tio n .
The o b s e r v e d p a t t e r n f r o m Von L a u e d i f f r a c t i o n
t h e o r y w o u ld i n v o l v e t h e r e f l e c t i o n s a l l o w e d by t h e
i n t e r s e c t i o n of th e r e c i p r o c a l l a t t i c e of the s u rfa c e
w i t h t h e s u r f a c e d e s c r i b e d by t h e m a g n i t u d e o f t h e
i n c i d e n t e l e c t r o n momentum v e c t o r
(Ewald c o n s t r u c t i o n )
along w ith a p p r o p ria te s tr u c t u r e f a c t o r s .
40
N orm ally,
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
34
f o r a two d i m e n s i o n a l l a t t i c e w h i c h d e s c r i b e s a s u r f a c e ,
the r e c ip ro c a l l a t t i c e
i s a fam ily of l i n e s
d ir e c tio n p e rp e n d ic u la r to th e s u rfa c e .
of t h i s
This i s
in the
The i n t e r s e c t i o n
l a t t i c e w i t h t h e Ewald s p h e r e r e s u l t s
p o ssible re fle c tio n s .
(rods)
i n many
i n f a c t what i s o b serv ed
i n Low E n e r g y E l e c t r o n D i f f r a c t i o n
(LEED) , w h i c h h a s a n
i n c i d e n t a n g l e which i s n e a r l y p e r p e n d i c u l a r t o t h e
s u r f a c e . 41
I n g l a n c i n g i n c i d e n c e o f RED, w h a t i s
observed i s rods in th e d i f f r a c t i o n p a t t e r n , n o t s p o ts ,
w h i c h i s u n c h a r a c t e r i s t i c f o r a two d i m e n s i o n a l l a t t i c e .
Th e a p p e a r a n c e o f r o d s i s a t t r i b u t e d t o e x p e r i m e n t a l
l i m i t a t i o n s an d l o n g r a n g e s u r f a c e d i s o r d e r b e t w e e n
r e g i o n s o f l o c a l o r d e r w h ic h t h i c k e n t h e s u r f a c e o f t h e
Ew ald s p h e r e an d b r o a d e n t h e r e c i p r o c a l l a t t i c e
into c y lin d e rs, re sp e c tiv e ly .
lines
Both o f t h e s e e f f e c t s
w o u ld i n c r e a s e t h e r e g i o n o v e r w h i c h r e f l e c t i o n s a r e
a l l o w e d . 42,43
Th e m a i n u s e f u l n e s s o f d i f f r a c t i o n p a t t e r n s
is th a t
t h e o b s e r v e d s u r f a c e r e c o n s t r u c t i o n c a n be c o r r e l a t e d
q u a l i t a t i v e l y w i t h t h e s u r f a c e s t o i c h i o m e t r y w h ic h i s
c o n t r o l l e d by t h e i n c i d e n t f l u x r a t i o .
T h u s RED p r o v i d e s
a n i m p o r t a n t mean s o f e v a l u a t i n g g r o w t h s t o i c h i o m e t r y ,
which i s an i m p o r t a n t f a c t o r i n c r y s t a l q u a l i t y .
The t h r e e m o s t i m p o r t a n t p a t t e r n s a r e t h e a r s e n i c
stab le
(As r i c h )
(001) - 2 x 4 , t h e g a l l i u m s t a b l e o r m e t a l
stab le
(001) - 3 x 1 , an d t h e g a l l i u m r i c h
(001) - 4 x 2 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
35
P h o t o g r a p h s o f t h e s e t h r e e p a t t e r n s a r e shown i n P i g s .
2 . 3 , 2 . 4 , a n d 2 . 5 f o r GaAs.
The (001)
AlGaAs i s shown i n F i g . 2 . 6 .
- 3x1 p a t t e r n f o r
The 3x1 s u r f a c e
r e c o n s t r u c t i o n s a r e t h e c o n d i t i o n s under which t h e b e s t
q u a l i t y AlGaAs i s g r o w n , an d i s i m p o r t a n t i n
h e t e r o j u n c t io n grow th.
The Ga s t a b l e p a t t e r n c o r r e s p o n d s
t o n e a r s t o i c h i o m e t r i c grow th.
A t 600 °C a n d b e l o w , t h i s
p a t t e r n o n ly e x i s t s f o r a very narrow ran g e o f f lu x
ratio s.
At h i g h e r te m p e r a t u r e s ~ 680°C, t h e ran g e o f
f l u x r a t i o s o v e r which t h i s p a t t e r n i s o b s e rv e d b ro a d e n s
considerab ly.
A lo n g t h e
(llO )
d i r e c t io n another s e t of
p a t t e r n s i s o b s e rv e d , b u t in p r a c t i c e th ey a re n o t as
u s e f u l fo r e v a lu a tin g s u r f a c e s to ic h io m e tr y as th e y la c k
d i s t i n c t f e a tu r e s a t higher te m p e ra tu re s.
There i s a w o n d erfu l m enagerie o f r e c o n s t r u c t i o n
p a t t e r n s v i s i b l e o v e r t h e s u b s t r a t e r a n g e o f 540°C t o
600°C f o r t h e v a r i o u s c r y s t a l o r i e n t a t i o n s , more nu m e r o u s
A O
t h a n n e e d be m e n t i o n e d h e r e .
Cho
has d iscu ssed th e se
p a t t e r n s as a f u n c t i o n o f s u b s t r a t e t e m p e r a t u r e and f l u x
r a t i o , a summary o f w h i c h i s shown i n F i g .
s u b s t r a t e t e m p e r a t u r e s up t o 6 0 0 ° C .
2.7 fo r
The i m p o r t a n t p o i n t
t o be b r o u g h t o u t h e r e i s t h a t a t h i g h e r t e m p e r a t u r e s ,
m ost o f th e r e c o n s t r u c t i o n l i n e s a r e c o n s id e r a b l y reduced
i n i n t e n s i t y d u e t o s c a t t e r i n g by t h e t h e r m a l m o t i o n s o f
t h e a d a to m s, and a g e n e r a l l x l p a t t e r n a p p e a r s a lo n g th e
<110>.
N o n e th e le ss, c a r e f u l o b s e rv a tio n can s t i l l
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
36
F igure 2.3.
A rsenic r ic h
F igure 2.4.
G allium
(110)
2x4,
(or a rsen ic)
(100) GaAs.
stab le
(110)
3x1,
(100) GaAs.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
F igure 2 . 5 .
Gallium r i c h
(110)
4x2,
(100) GaAs.
F ig u re 2.6.
M etal s t a b l e
(110)
3x1,
(100) AlGaAs.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
38
590°C
533°C
485°C
101
10
O B S E R V E D IN T H E ( T T Q ) A Z I M U T H
A -FROM 1/ 4 ORDER T O 1/ 3 ORDER
O - F R O M 1 / 3 O R D E R T O 1 12 O R D E R
X - F R O M 1/2 ORDER T O 1/3 ORDER
♦ -FROM 1/ 3 ORDER TO 1/4 ORDER
1
\\
SEC
Xo
■w w
ORDER
101
ARRI VAL
RATE
#/CM
X CD
ORDER
GALLIUM
X
OO.
XX X
10
ORDER
ORDER
10
1
120
128
132
140
136
1 Q0 0 / T ° K
F igure 2.7.
T ra n sitio n s of th e surface s tru c tu re as a
f u n c t i o n o f t h e Ga a r r i v a l r a t e s a n d t h e
s u b s tr a te tem peratures.
Since th e r e i s
h y s te r e s i s o f th e t r a n s i t i o n s as th e subt r a t e tem perature i s v a rie d ,
two s e t s o f
c u r v e s , one f o r i n c r e a s e i n t e m p e r a t u r e and
th e o th e r f o r decrease in tem perature are
presented.
A f t e r r e f e r e n c e 43.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
39
p r o v i d e a n i n d i c a t i o n o f how c l o s e t h e s u r f a c e
s t o i c h i o m e t r y i s t o th e m e ta l r i c h edge o f th e s o l i d u s
fie ld .
As t h i s c o n d i t i o n i s a p p r o a c h e d , e l e m e n t s o f t h e
1 / 6 o r d e r r e c o n s t r u c t i o n c a n j u s t be o b s e r v e d a l o n g t h e
<110> a x i s ,
i f o n e knows w h a t t o l o o k f o r .
T h i s c a n be
u s e f u l b e c a u s e o f t h e wide r a n g e o v e r which t h e
(3x1)
r e c o n s t r u c t i o n a p p e a r s a l o n g t h e < 110 > d i r e c t i o n .
2.2
Beam E q u i v a l e n t P r e s s u r e R a t i o
The beam e q u i v a l e n t p r e s s u r e
(BEP) r a t i o i s d e f i n e d
a s t h e r a t i o o f t h e m e a s u r e d beam p r e s s u r e s s p e c i f i e d
above.
p
^
It
- ?BBAs
EP
"
PBE
( 2 .2)
Ga
i s a l s o s p e c i f i e d g e n e r a l l y as th e group V to group
I I I pressure ra tio .
The p r e s s u r e r a t i o i s a m e a s u r e o f t h e s t o i c h i o m e t r y
o f t h e i n c i d e n t e l e m e n t a l beam.
Some g e n e r a l r a n g e s a r e
a minimum p r e s s u r e r a t i o o f a p p r o x i m a t e l y 7 : 1 a t 600°C t o
a s h i g h a s 3 0 : 1 a t v e r y h i g h g r o w t h t e m p e r a t u r e s > 700°C
in o rd e r to m ain tain a m etal s t a b l e s u rfa c e sto ic h io m e try
n e a r t h e m e t a l r i c h edge o f t h e s o l i d u s f i e l d .
A s p e c i f i c a t i o n of the p r e s s u r e r a t i o alone i s
i n s u f f i c i e n t t o i n d i c a t e th e s u r f a c e s t o i c h i o m e t r y , as
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
40
a t a g i v e n t e m p e r a t u r e , a n o v e r p r e s s u r e o f one o f t h e
beam s o u r c e s i s r e q u i r e d t o m a i n t a i n a c r y s t a l s u r f a c e
r e c o n s t r u c t i o n i n t h e a b s e n c e o f t h e s e c o n d beam s o u r c e .
As a d d i t i o n a l a t o m s f ro m t h e beams a r e a d d e d t o t h e
s u r f a c e , a r a t i o o f a d d i t i o n a l amounts i s u s u a l l y
required.
P h e n o m e n o l o g i c a l l y , t h e r e l a t i o n s h i p m i g h t be
s p e c ifie d as:
P
= P
BAs4
+ a P
°
(2.3)
Ga
PQ i s t h e o v e r p r e s s u r e t e r m , w h i c h w i l l be shown t o be
t h e d e s o r b i n g p r e s s u r e s f orm t h e s u r f a c e .
The
c o e f f i c i e n t a is the r a t i o of a d d itio n a l p ressu res
r e q u i r e d f o r growth.
The beam e q u i v a l e n t p r e s s u r e r a t i o
g i v e n by t h i s e x p r e s s i o n i s :
PB
W . _ i!i . Ja. + „
Ga
(2-4)
BGa
D e p e n d i n g on t h e m a g n i t u d e o f P Q , i t
meaning of t h e p r e s s u r e r a t i o i s
i s c le a r thac the
incom plete.
In the
f o l l o w i n g s e c t i o n , a model i s d i s c u s s e d i n o r d e r t o
j u s t i f y th e above e q u a tio n .
The m o d el w i l l p r o v i d e a b a s i s t o d i s c u s s t h e
c o n n e c t i o n b e t w e e n t h e i n c i d e n t beam s t o i c h i o m e t r y and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
41
the su rfa c e s to ic h io m e try .
As s i t e o c c u p a n c y and
im p u rity in c o rp o ra tio n i s o fte n lin k e d to the c o n tro l of
i n t r i n s i c d e f e c t s , t h e r e l a t i o n s h i p between th e b u lk
d e f e c t c o n c e n t r a t i o n and t h e e x t e r n a l l y c o n t r o l l e d
param eters
im portant.
( t s u |3 f g r o w t h r a t e , V : I I I BEP r a t i o )
is quite
The s u r f a c e s t o i c h i o m e t r y i s n o t n e c e s s a r i l y
d i r e c t l y r e l a t e d t o t h e i n c i d e n t beam s t o i c h i o m e t r y .
F i n a l l y , t h e m o d el i s u s e d t o s u g g e s t s i g n i f i c a n t
d ifferences
b e t w e e n AlGaAs a n d GaAs s u r f a c e p r o c e s s e s .
The s u r f a c e w i l l be t r e a t e d a s a s e p a r a t e r e g i o n
i n t o w h i c h s u r f a c e a d a t o m s e n t e r f ro m t h e i n c i d e n t beams
a nd l e a v e e i t h e r t h r o u g h i n c o r p o r a t i o n o r d e s o r p t i o n .
F o r t h e moment t h e p r o c e s s by w h i c h As o n t h e s u r f a c e i s
o b t a i n e d f ro m As^ o r t h e f orm by w h i c h i t l e a v e s ,
p r e d o m i n a t e l y AS2 w i l l be n e g l e c t e d .
The m o d e l w i l l be
f o rm u la te d i n term s of th e f lu x e s f i r s t ,
w ith the
r e s u ltin g ex p ressio n s converted in to p ressu res for
com parison w ith e x p erim en t.
flu x es are d e ta ile d
The g e n e r a l i z e d s u r f a c e and
in th e F igure 2 ,8 .
The s t e a d y s t a t e c o n d i t i o n s a r e
w h e r e N, t h e s u r f a c e c o n c e n t r a t i o n o f t h e r e s p e c t i v e
a d a t o m s , i s r e l a t e d t o t h e i n c i d e n t f l u x e s by
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
42
Vacuum
Surface
Bulk
F igure 2.8.
G e n e r a l i z e d f l u x e s o n s u r f a c e . J.
and
B
As,
a r e i n c i d e n t A s . a n d Ga beam f l d x e s .
Ga
4
Jn
and J n
a r e d e s o r b i n g A s 9 a n d Ga
uA s2
Ga
£
f l u x e s from t h e s u r f a c e .
J.
a n d J . __
c As
1
a r e t h e As a n d Ga a t o m s i n c o r p o r a t e d i n t o
th e c r y s t a l d u r in g grow th.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
43
- J
3t
BGa
(2 . 6)
- j
Ga
i n c Ga
3NAs
^
Jn
(2.7)
= J BA s “
\
s
“
J i - As
i s m e a n t t o be t h e a t o m i c As c o n c e n t r a t i o n t h a t
BA s
r e s u l t s f r o m t h e d i s a s s o c i a t i o n o f a n As^ m o l e c u l e fro m
t h e i n c i d e n t beam.
The p r o c e s s by w h ic h t h a t o c c u r s w i l l
be lum pe d i n t o a c o n s t a n t e v e n t u a l l y .
JB
Ga
JB
As
= JD
Ga
= JD
As
+ J ine
+ J inc
So
<2 * 8 >
GA
( 2 *9)
As
An a p p r o x i m a t e r e l a t i o n s h i p e x i s t s b e t w e e n J .
and
Ga
J.
i n c As
J inc
Ga
~ J inc
As
^ neglecting in tr in s ic c ry s ta l
d e f e c t s a nd i m p u r i t i e s
(2 . 10)
Thus
- J
+ JB
= J,
B
D
As
As
Ga
Gel
( 2 . 1 1 a)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
44
A t t h i s p o i n t t h e components o f t h e p h e n o m e n o lo g ic a l Eq.
(2.3)
are apparent.
In term s of th e f l a x e s th e e q u a tio n
ca n be s t a t e d as
J B * J o + J Ga
(2 -l l b )
where
J o = J DAc! - J n
AS
(2.11c)
GA
J Q i s th e excess f lu x te rm , i n analogy w ith the
o v e r p r e s s u r e t e r m PQ, w h i l e t h e Ga f l u x c o e f f i c i e n t i s 1 ,
a s t h e m o l e c u l a r fo rm o f t h e f l u x e s h a s n o t y e t b e e n
taken in to account.
(over p r e s s u r e )
term i s th e d i f f e r e n c e between th e
desorbing flu x e s
ratio
I t c a n be s e e n t h a t t h e e x c e s s f l u x
(pressures).
The a n a l o g y t o t h e BEP
i s th e f lu x r a t i o , d e f in e d as
„
\ s
F
^
J d A3~ \ a
Ga
^
.
.
(2 .1 2 )
Ga
Above 6 4 0 ° C , J n
> Jn
, and a t 6 8 0 ° C , J n
^As
Ga
As
dom inates,
'
so t h a t th e c o n s t a n t term i s
j
* J
°
(2.13)
AS
T h u s t h e f l u x r a t i o d e p e n d s on t h e m a g n i t u d e o f b o t h J n
As
and J n
Ga
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
45
In order to r e la te
t h i s e q u a t i o n t o t h e beam
p r e s s u r e s , t h e s u r f a c e c h e m i s t r y m u s t be c o n s i d e r e d .
The
p r e d o m i n a t e i n c i d e n t f l u x f r o m e l e m e n t a l As s o u r c e s i s
A s 4 , 44 t h u s
Xa = C<3a Xa
Xs
= I b A s / As4
(2'14>
= Ifts ' PbA s 4 ' ° A s 4
( 2 ’ 15)
w h e r e C ^ , t h e c o n v e r s i o n f ro m f l u x p r e s s u r e i s g i v e n by
i
8 kT. 1 / 2
Af
iri
ci ■ «irsr>
<5^
i g
i
<2a6)
a n d I As i s a number w h ic h d e s c r i b e s o n a v e r a g e how many
As a t o m s a r e o b t a i n e d f r o m e a c h As t e t r a m e r i n c i d e n t on
the s u rfa c e .
T h i s number d e p e n d s on t h e s u b s t r a t e
t e m p e r a t u r e and o t h e r f a c t o r s ,
coverage.
i n c l u d i n g t h e Ga s u r f a c e
T^ a n d m^ a r e t h e c e l l t e m p e r a t u r e and
m o l e c u l a r m ass r e s p e c t i v e l y , Tg i s t h e g a u g e t e m p e r a t u r e ,
a n d H^ i s t h e r e l a t i v e
h . = 0.4 + 0 .6
1
i o n i z a t i o n e f f i c i e n c y g i v e n by
Zi
h-j)
Z. =
# of e le c tro n s , 0
p er m olecule
L/)
So t h e e q u a t i o n becomes
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
46
As,
■BA s,
'AS,
= [Jr
- J D, ]
Ga
As
+ c,GaPB
(2.18)
Ga
So
BA s .
4
[Jr
XA s . CA s .
"As
4
4
- J
D
Ga
] +
CGaPB
Ga
(2.19)
XAs . CA s .
4
4
t h e p r e d o m i n a t e fo rm o f t h e d e s o r b i n g f l u x o f As fro m
GaAs i s A s 244 t h u s
Jn
= 2Jn
= 2C
Pn
As
DA s2
s A s2 DA s2
(2 . 20)
(2 . 21)
Xa =XaXa
where
C
=
1
si
(2irmi kTi ) 1 / 2
(2.22)
a s t h e f l u x now i s o t r o p i c a l l y e m i t s f ro m t h e s u r f a c e . 4 ^
The c o m p l e t e e x p r e s s i o n i s :
(2.23)
or
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PB
GA
-
PB
= T ^C
[2CS
PD
- CS PD ] +
As 4
aAs4^ A s 4
s As2 uA s 2
s GA Ga
xAs4 A s 4
47
PB
= Pn +
BA s 4
°
ol
P„
Ga
where
p
-
CsGa
V
°
f 2C3AS2 p
- p ,
Cs Ga ° A s 2
“ Ga’
S
(2-24)
CGa
a =
r A s 4CA s 4
S u b s t i t u t i n g i n t h e e x p r e s s i o n s f o r C^ g i v e s :
^*4
p_. = T
°
MAs 4 1 / 2
1
A s4 4
(m
Ga
T2
)
(if
n
-
sub
fif
/2
As4
'
MGa
12 ( m
As 2
, /2
}
M
T
As4 CGa 1 / 2
AS4
^s^G a
n
1
As2
Ga
(2<25)
(2.26)
MGaTcAs
because the c e l l tem p eratu res ty p i c a l l y vary only over a
sm all ran g e, on ly a sm all e r ro r
i s i n t r o d u c e d by u s i n g an
average tem perature fo r th e c e l l :
T
Ga a v e r a g e
= 115Q°C
2
= 1 2 0 5°C = 1478K
T.
= 300 t - l l O = 315o c = 588K
As4 a v e r a g e
2
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
J
48
M a k in g s u b s t i t u t i o n s f o r t h e c o e f f i c i e n t s :
„
( 0 . 4 + 0 . 6 A x l 4 ' 9 1 / 2 ,300,1/2
po = — O —
<-S 5T T>
4
x [2 ( 69,r.Z-)
x
'2x74.9'
. f M
,300_, V 2
As4
sub
(0.4 + 0 .6
*0.
AS2
_ p
l
DGa
u . 36 p
“ = I*. (0 .4 4 0 .* $ ) )
j
Rs2
(iffi))
,300 , 1 / 2
< f^>
4x74-9
( 69‘7
( 2. 27)
GA
1478
‘
1/ 2
588
3 . 5 0 x 2 , 2 9 = 1 1 • 52
I A s.
*
‘''As.
4
4
( 2. 28)
Thus
P
As4
(3 0 0 _ , 1 / 2 (1
aA s 4
sub
_ p
As2
)+
Ga
US52,
aA s 4 BGa
(2. 29)
A f e a t u r e of th e e x p r e s s i o n which c o n f u s e s th e
a u th o r occurs in th e d e s o rp tio n term .
The p r o b l e m i s
most obvious a t th e c o n g ru en t s u b lim a tio n p o i n t .
p o in t,
2 J fts
= J q , and a c c o r d i n g t o A r t h u r
44
th is
At t h i s
is
e x p r e s s e d i n p r e s s u r e form a s ;
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
49
P,
Since. PAg
2P
As
+ 4P
2
(2.30)
As 4
>> PAs o v e r t h e r a n g e s o f i n t e r e s t ,
2
4
The p r o b l e m w h i c h e x i s t s i s t h a t t h e
then
P„_ = 2 P ,. . .
Gd
2
c o n v e r s i o n o f t h e f l u x e x p r e s s i o n t o p r e s s u r e s y i e l d s PQa
The a u t h o r i s u n c l e a r a s t o t h e r e a s o n f o r
2
t h e d i s c r e p a n c y , a l t h o u g h i t may be r e l a t e d t o t h e
1 . 3 6 PAs
d i f f e r e n t i n t e r n a l d e g r e e s o f f r e e d o m b e t w e e n A s 2 and
As^.
The p r o b l e m a r i s e s
in t h a t th e d e s o r p t i o n term w i l l
n o t be c o n s i s t e n t w i t h t h e e x i s t i n g d a t a .
T h erefo re, the
a u th o r w i l l s e t the c o e f f i c i e n t to 2 in s te a d of 1.36.
Thus:
(2.31)
I t s h o u l d be n o t e d t h a t t h i s i s n o t a t h e r m o d y n a m i c
relatio n ,
b u t r a t h e r a f o r m u la which a llo w s a d e s c r i p t i o n
of th e f lu x r a t i o
desorbing flu x e s .
i n t e r m s o f t h e i n c i d e n t and
(unknown)
The a c t u a l c o n s t a n t s w i l l v a r y
d e p e n d in g on t h e p a r t i c u l a r m ach in e, hence t h e p r e c i s e
value of the c o n sta n ts is not im portant.
Mow c o n s i d e r a
c a s e w h e r e t h e s u b s t r a t e t e m p e r a t u r e i s 700°C .
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
50
The
BEP r a t i o c a n a p p e a r t o b e q u i t e l a r g e , d e p e n d i n g
the
m agnitude of
at
th e d e s o rb in g p r e s s u r e s . For
—7
700°C a n d 1 n m / h r
(~ 5 x 10
torr)
on
exam ple,
w h e r e f ro m
A r t h u r ' s d a t a 44 t h e d e s o r b i n g p r e s s u r e s a r e
PAg
~
3.5 x
1 0 "6 T o r r a n d PQa ~ 3 . 0 x 10- 6 we h a v e
PB
A s4 = 1 . 2 4 6
'
=
( 2 x 3 . 5 x l 0 "6 - 3 . 0 x l 0 " 6 ) + 1 1 %5 2
5 * 10' 7
10.0 + 11.5 = 21.5
I f t h e Ga p r e s s u r e was l o w e r , s a y 2 . 5 x 10
(~
0.5
]jn), t h e n t h e p r e s s u r e r a t i o ,
_7
Torr
bec om es
PB
= (1.246)
*B„
+ n . 5 = 31.5
2.5x10"'
A p o i n t w h ic h r e q u i r e s d i s c u s s i o n i s t h e f u n c t i o n a l
d e p e n d e n c e o f PQ a nd
pressures.
Eq.
(2.31)
a o n t h e i n c i d e n t A s 4 a n d Ga beam
i s c a s e i n a form which s u g g e s t s a
l i n e a r d e p e n d e n c e b e t w e e n PQ
a nd Pn
, y e t t h e r e i s no
As.
Ga
g o o d r e a s o n why o n e s h o u l d e x p e c t PQ a n d a t o be
co n stan t.
The f a c t o r
a c o n ta in s a l l the su rfa c e chem istry
i n v o l v e d i n t h e m e c h a n is m s by w h i c h As i s o b t a i n e d by t h e
s u r f a c e fro m t h e i n c i d e n t A s 4 beam.
T h is i s s u e has been
s t u d i e d by A r t h u r 45 and J o y c e a nd F o x o n 4 ^ .
The p r o p o s e d
r e a c t i o n s e q u e n c e i s t h a t a n As4 o r AS2 m o l e c u l e i s
adsorbed
(precursor s ta te )
on t h e s u r f a c e , and s u r f a c e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
51
d iffu ses u n til a su itab le s ite
it
i s e n c o u n t e r e d , w h e r e upon
i s cheraxsorbed w i t h o u t d i s a s s o c x a t i o n .
A rthur
45
s u g g e s t e d t h a t t h e p r o c e s s was d e p e n d e n t on As c o v e r a g e ,
a n d i n v o l v e d b u l k d i f f u s i o n w h i l e J o y c e a n d Foxon
47
noted
t h a t t h e p r o c e s s was d e p e n d e n t on Ga c o v e r a g e a s w e l l .
T h u s , o n e w o u ld e x p e c t a
to vary a t l e a s t w ith s u rf a c e
s t o i c h i o m e t r y and c o n c e n t r a t i o n .
PQ c o n t a i n s t h e d e s o r b i n g AS2 a nd Ga f l u x e s .
s i m p l e s t models one m ig h t c o n s t r u c t ,
l i f e t i m e c o u l d be i n v o l v e d w h e r e b y
In the
a desorption
46
NX*
j
=
DAS2
j
2
(2.32)
t As 2
= No XGa
°Ga
(2.33)
T Ga
Where NQ i s t h e number o f As s i t e s
a s s u m e d t o be e q u a l .
and Ga s i t e s ,
XAg
a nd XGa a r e t h e AS2 and Ga
2
f r a c t i o n a l o c c u p a n c y , r e s p e c t i v e l y . TAg a nd T Ga a r e t h e
2
d e s o r b t i o n l i f e t i m e s , and a r e s u b s t r a t e t e m p e r a t u r e
d e p e n d e n t and c o v e r a g e d e p e n d e n t .
N o t e t h a t XAg
+ XQa
2
7*1 .
H a v in g e s t a b l i s h e d t h i s a p p r o a c h , o n e c o u l d a t t e m p t
t o make f u r t h e r c o n n e c t i o n s b e t w e e n XAg
and XQa t h r o u g h
2
thermodynamics
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
52
— As*
2
2
As*
+
(2.34)
'
Ga* + As* ♦ Ga As(g)
aAs
a*
As
2 _ *
”
(2.35)
(2.36)
1
1
a* a*
Ga As = „
a GaAs
2
(2.37)
Ga* , Asl!j, and As* d e n o t e s u r f a c e s p e c i e s , w h e r e a g a ,
aAs ' a n d aAs a r e t h e ; '-r r e s p e c t i v e s u r f a c e a c t i v i t i e s .
GaAs^g j d e n o t e s t h e s o l i d ,
a nd a QaAs i s t h e a c t i v i t y o f
t h e s o l i d , w h i c h s e t t i n g a QaAg = 1 g i v e s :
1
a* ^ • a* — K K
A s2
Ga " 1 2
(2.38)
H e r e , t h e s u r f a c e h a s b e e n a s s u m e d t o be i n e q u i l i b r i u m
in th e bulk.
D u r i n g g r o w t h , t h e s u r f a c e m u s t h a v e an
o v e r p o t e n t i a l s o t h a t t h e b u l k re m o v e s a t o m s fro m t h e
s u r f a c e , t h u s t h i s may n o t h o l d e x a c t l y .
The a c t i v i t i e s
c a n be e x p r e s s e d i n t e r m s o f c o m p o s i t i o n u s i n g H e n r y ' s
l a w 48
aj.yjX j
(2.39)
Thus t h e f l u x e s becomes
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
w h e r e t h e a s t e r i s k (*) a g a i n i n d i c a t e s a s u r f a c e q u a n t i t y .
T his g iv e s
( 2. 42)
As a l m o s t e v e r y c o n s t a n t i n t h i s e x p r e s s i o n i s an
unknown v a r i a b l e , t h i s e x p r e s s i o n i s u s e l e s s f ro m a
q u a n t i t a t i v e p o i n t o f view .
However t h e f o rm a l l o w s
comments t o be made c o n c e r n i n g t h e r e l a t i o n s h i p b e t w e e n
beam s t o i c h i o m e t r y and s u r f a c e s t o i c h i o m e t r y .
Several
a u t h o r s h a v e s i m p l y assu m ed t h a t t h e i n c i d e n t beam
p r e s s u r e s c a n be d i r e c t l y r e l a t e d t o t h e b u l k a c t i v i t i e s
th ro u g h e q u i l i b r i u m thermodynamics
+ Ga
(v)
(v)
GaAs
(2. 43)
(s)
or
a GaAs
K
3
(2. 44)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
54
where th e s u b s c r i p t
(v ) i n d i c a t e s a v a p o r s p e c i e s .
The
i m p l i c i t a s s u m p t i o n w h i c h h a s b e e n made i s t h a t
pl/2
B
As 2
V Ga
_
”
(2.43)
As
a s (s)
= a G a(s)
, ,
( 2 - 46)
Where K i s a c o n s t a n t .
It
i s more d e f e n s i b l e t o a s s u m e t h a t t h e s u r f a c e
phase i s
in e q u i l i b r i u m in th e b u l k , where t h e s u r f a c e
a c t i v i t i e s a r e a f f e c t e d by t h e c o m p e t i n g e f f e c t s o f
d e s o r b t i o n and i n c o r p o r a t i o n o f a d a t o m s .
Even t h i s
approach i s r is k y , as th e s u rf a c e chem ical a c t i v i t i e s
m u s t be g r e a t e r t h a n t h e b u l k i n o r d e r f o r g r o w t h t o
occur.
Prom t h i s p o i n t v i e w , t h e b u l k t h e r m o d y n a m i c
p r o p e r t i e s c a n o n l y be a f f e c t e d t o t h e d e g r e e t h a t t h e
s u r f a c e s t o i c h i o m e t r y c a n be a f f e c t e d .
E quation
(2.42)
e x p l i c i t l y in clu d es th e g allium s u rfa c e com position,
which i s a f a c t o r which d e t e r m in e s t h e s u r f a c e
reco n stru ctio n .
At 6 0 0 ° C , t h e w i d t h i n As p r e s s u r e f o rm
th e m etal ric h to th e a r s e n ic r ic h s id e of th e s o lid u s
fie ld
i s v e r y s m a l l , a nd c a n be t r a v e r s e d w i t h a p r e s s u r e
r a t i o o f 10:1 f o r th e m e ta l r i c h edge to 1 5:1 f o r th e
a r s e n i c r ic h edge
49
.
A sch em atic r e p r e s e n t a t i o n of th e
s o l i d u s f i e l d a s i t a p p l i e s t o MBE g r o w t h i s shown i n
Fig.
2.9.
The s u r f a c e r e c o n s t r u c t i o n s a r e i n d i c a t e d on
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
55
SOLIDUS FIELD
Greatly exaggerated
680C
4x2
As rich
Ga rich
600°C
10:1
1.0
0.0
Composition
F igure 2.9.
Schem atic a p p l i c a t i o n o f th e s o lid u s
f i e l d d i a g r a m t o MBE g r o w t h .
BEP r a t i o s
a t edges o f th e s o li d u s f i e l d as determ ined
b y RED p a t t e r n s a r e i n d i c a t e d a t 600°C
a n d 680°C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
56
the fig u re .
A t t e m p t i n g t o make a c o n n e c t i o n w i t h
e q u i l i b r i u m therm odynam ics w ith a p r e s s u r e r a t i o of 30:1
i s c o m p le te ly m e a n in g le s s , as th e a r s e n i c r i c h edge has
a lr e a d y been re a c h e d .
AS2 l i f e t i m e
Once t h i s p o i n t i s o b t a i n e d , t h e
(or As4) s h o u ld d e c r e a s e d r a m a t i c a l l y , a s
s u g g e s t e d by Hancock
en
b o t h b e c a u s e no more a r s e n i c s i t e s
a r e a v a i l a b l e , o r t h e g a l l i u m p o p u l a t i o n i s v e r y low.
Th us t h e d i m e r o r t e t r a m e r p r e c u r s o r s i m p l y d e s o r b s
w ithout chem isorbing.
the s o lid u s f ie ld
At high te m p e r a tu re s , th e w idth of
b r o a d e n s c o n s i d e r a b l y , a nd t h e s u r f a c e
s t o i c h i o m e t r y i s a f f e c t e d o v e r a b r o a d e r r a n g e o f BEP
ratio s.
In t h i s range, the tru e su rface sto ic h io m e try i s
i n d i c a t e d n o t by t h e beam s t o i c h i o m e t r y , b u t r a t h e r t h e
desorbing p re ssu re sto ic h io m e try .
Im p lic it in th is
d i s c u s s i o n i s t h a t t h e b e s t way t o c o n t r o l t h e g r o w t h
sto ic h io m e try i s to c o n tr o l the su rface re c o n s tru c tio n s
over th e w idth of th e s o lid u s f i e l d .
T h is d i s c u s s i o n h a s n o t been a b l e t o answer what th e
d e p e n d e n c e o f PQ an d a a r e on t h e i n c i d e n t beam f l u x e s .
The m e t a l r i c h e d g e i s v e r y i m p o r t a n t i n MBE a s t h e
m a t e r i a l q u a l i t y o f b o t h GaAs a nd AlGaAs a r e b e s t a t t h a t
su rface stoich iom etry.
Th us t h e f o l l o w i n g s e c t i o n w i l l
e v a lu a te th e s to ic h io m e tr y r e l a t i o n s h i p along th e m etal
r ic h edge.
The b e h a v i o r o f a a n d PQ u n d e r t h i s c o n d i t i o n
c a n be d e t e r m i n e d f ro m t h e r e s u l t i n g d a t a .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
57
2.3
Beam P r e s s u r e s A lo n g t h e M e t a l R i c h Ed ge o f GaAs
B e c a u s e t h e f o rm o f E q .
(2.23)
is a lin e a r equation
a m eth o d f o r e v a l u a t i n g t h e c o n s t a n t PQ a n d a i n v o l v e s a
s e t o f Pn
a nd Pn
t o which t h e e q u a t i o n i s f i t t e d .
Ga
As.
The o b v i o u s a p p r o a c h i s t o e v a l u a t e t h e P„
for several
A s.
Pn
, b u t i t i s c l e a r t h a t P„ c a n v a r y somew hat d e p e n d i n g
Ga
°
on t h e s u r f a c e c o n c e n t r a t i o n s , a n d t h e s u r f a c e
com position
(XAs/X Qa) .
The m e a s u r e m e n t s m u s t be t a k e n a t a n i d e n t i f i a b l e
c o n d i t i o n s u c h t h a t t h e v a r i a t i o n s o f PQ a n d a
m inim ized.
are
The m e a s u r e m e n t s h a v e b e e n p e r f o r m e d a l o n g
th e m e ta l r i c h edge o f t h e s u r f a c e r e c o n s t r u c t i o n as
e v i d e n c e d by t h e RED p a t t e r n s .
The m e t a l r i c h e d g e i s
r e a d i l y i d e n t i f i e d and e x h i b i t s a n a r r o w t r a n s i t i o n r a n g e
from t h e 3 x 1
r e c o n s tr u c tio n as a f u n c tio n of flu x e s .
The s e c o n d a d v a n t a g e i s t h a t t h e m e t a l r i c h e d g e
r e p re s e n ts a d e f i n i t e although u n s p e c ifia b le s u rfa c e
c o n c e n t r a t i o n o f Ga a d a t o m s , and t h u s a n e a r l y f i x e d
PD , w i t h i n t h e v a r i a t i o n s ° f T (3a ' Y As / a n d y £ a w i t h
Ga
2
X . 0 . A v a r i a t i o n i n Pn
i s s t i l l e x p e c te d , as i t i s
As 2
p r o b a b l y t h e r a t e l i m i t i n g co m p o n e n t o f t h e r e a c t i o n
G a (s) + A s (s> + G a A s (s)
a s s u c h , t h e s u r f a c e c o n c e n t r a t i o n o f As a d a t o m s m u s t
i n c r e a s e a s Pn
a nd Pn
increase.
T h e r e f o r e , P_ w i l l
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
58
p r o b a b l y v a r y somew hat w i t h t h e i n c i d e n t p r e s s u r e s .
The
d a t a w i l l p r o v id e an e v a l u a t i o n o f t h e e f f e c t .
2 .3 .1
E xperim ental
The MBE s y s t e m
sources,
(G.E.)
i s e q u i p p e d w i t h two a r s e n i c
(As#0, As#7) s o t h i s a l l o w e d a t o t a l o f t h r e e
d i f f e r e n t a r s e n i c p r e s s u r e s , PA s # 0 ,
pAs#7*
pas#7 '
and pAs#0 +
Tlie beam P r e s s u r e s and t h e s u b s t r a t e t e m p e r a t u r e
were measured as s p e c i f i e d
i n S e c t i o n s 2 . 5 and 2 . 6 .
m e t a l r i c h e d g e was d e t e r m i n e d u s i n g RED.
The
The
m e a s u r e m e n t s b e g a n w i t h t h e l o w e s t As p r e s s u r e .
The Ga
c e l l t e m p e r a t u r e was g r a d u a l l y i n c r e a s e d u n t i l a m e t a l
r i c h s u r f a c e r e c o n s t r u c t i o n was o b t a i n e d .
C a r e was t a k e n
t o i n s u r e t h a t a s t e a d y s t a t e c o n d i t i o n was a t t a i n e d
a f t e r t h e v a r i o u s c e l l and c o n t r o l l e r t r a n s i e n t s h ad d i e d
o ut.
The Ga c e l l t e m p e r a t u r e t h a t r e s u l t e d i n a m e t a l
r i c h c o n d i t i o n was n o t e d f o r e a c h As p r e s s u r e .
The
t e m p e r a t u r e s w e r e c o n v e r t e d i n t o p r e s s u r e s by t h e u s e o f
F ig. 2.1.
2 .3 .2
R esults
Two m ain e x p e r i m e n t s w e r e p e r f o r m e d .
The f i r s t
e x p e r i m e n t was o v e r t h e s u b s t r a t e t e m p e r a t u r e r a n g e o f
-7
640°C t o 740°C w i t h Ga p r e s s u r e s ( 4 . 7 x 10
Torr) t h a t
resulted
i n a g r o w t h r a t e o f ~ 1 Um/Hr.
I t was c l e a r
t h a t b e c a u s e PQ was d r o p p i n g r a p i d l y n e a r 640°C ,
s i g n i f i c a n t e r r o r s were r e s u l t i n g b e c a u s e o f t h e r e l a t i v e
l a r g e Pn
Ga
and PQ .
As
To c o m p e n s a t e , a s e c o n d e x p e r i m e n t
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
59
was p e r f o r m e d f r o m 540°C t o 640°C u t i l i z i n g beam
p r e s s u r e s a f a c t o r of te n low er.
The r e s u l t s
o f t h e tw o
experim ents
2.11.
(P„
v s Pn ) i s p l o t t e d i n P i g s . 2 . 1 0 a n d
BAs
Ga
The v a l u e s o f PQ a n d
were d e te rm in e d u s in g a
l e a s t s q u a r e s f i t and p l o t t e d a s a f u n c t i o n o f i n v e r s e
tem perature in P ig . 2.12.
v s PD
c u r v e s show a
As
Ga
s m a l l am ou nt o f c u r v a t u r e , i n d i c a t i n g t h e l i n e a r m o d e l i s
not s tr i c t l y co rrect.
The Pn
I t i s a ls o cu rio u s th a t th e sign
o f t h e c u r v a t u r e c h a n g e s b e t w e e n t h e low t e m p e r a t u r e - l o w
p r e s s u r e e x p e r i m e n t a nd t h e h i g h t e m p e r a t u r e - h i g h
p re ss u re experim ent.
The o t h e r s i g n i f i c a n t d i f f e r e n c e i s
t h a t 01 i s much l a r g e r
(~ 1 0 ) f o r t h e lo w t e m p e r a t u r e
experim ent th an th e high tem p eratu re experim ents
(~ 7 . 5 ) .
S p e c u l a t i n g o n t h e o r i g i n o f t h i s d i f f e r e n c e w o u ld n o t be
v e r y u s e f u l c o n s i d e r i n g t h e l a c k o f knowledge c o n c e r n i n g
t h e s u r f a c e p r o c e s s e s d u r i n g MBE g r o w t h .
The a u t h o r
n o t e s h o w e v e r t h a t t h e s t u d i e s o f F o x o n , H a r v e y , a nd
J o y c e ^ showed t h a t t h e As l i f e t i m e e x h i b i t e d a
d e p e n d e n c e on t h e i n c i d e n t Ga f l u x ,
i n c r e a s i n g Ga f l u x .
in c r e a s in g w ith
T h i s would be c o n s i s t e n t w i t h a
r e d u c e d <* a t h i g h e r Ga p r e s s u r e s .
A lso p l o t t e d in F ig .
2.12 i s th e v a lu e s o b ta in e d f o r
44
by u t i l i z i n g t h e K nuds en c e l l d a t a o f A r t h u r .
P
o J
■
o
g e n e r a l l y fo llo w s th e m easured v a lu e e x c e p t f o r s u b s t r a t e
P
t e m p e r a t u r e s a b o v e 720°C w here t h e c a l c u l a t e d PQ i s
s ig n if ic a n tly higher.
The q u e s t i o n a r i s e s a s t o w h a t t h e
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
60
(x 10 ° Torr)
As a function of Tsub
740
«*
£
flu
7 2 0 ?0 0 < C ^
PBG0 (x IO '7 Torr)
F igure 2.10.
A s4 BEP a s a f u n c t i o n o f t h e Ga BEP
a lo n g th e g a lliu m r i c h edge o f th e
s o lid u s f i e l d param eterized w ith th e
s u b s t r a t e t e m p e r a t u r e , Ts u jj*
Ts u b r a n 9 e s
f r o m 640°C t o 740°C .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
61
II
As a function of TSUb
10
9
8
P Po
7
6
•o
x
5
00
CL
4
3
2
0
0
I
2
3 4
5 6
p
F igure 2.11.
7
B Go
8 9 10 II 12 13 14 15
(X
I O - e T o rr)
A s 4 BEP a s a f u n c t i o n o f Ga BEP a l o n g t h e
g a ll iu m r i c h edge o f t h e s o l i d u s f i e l d
p aram eterized w ith th e s u b s tr a te
t e m p e r a t u r e Tg u b .
Ts u b r a n g e s f ro m
540°C t o 6 4 0 ° C .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(T orr)
Arthur's data
10 8>—■—
0.950
780°C
F ig u re 2.12.
1.000
.150
1.050
.200
1.230
540°C
P l o t o f PQ a n d a a s a f u n c t i o n o f i n v e r s e
s u b s tra te tem perature.
c a l c u l a t e d from F ig .
PQ a n d a a r e
2.10 and F i g .
2.11.
Open c i r c l e s a r e e x p e r i m e n t a l p o i n t s .
S o l i d l i n e i s c a l c u l a t e d from A r t h u r
(44)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
63
s u r f a c e r e c o n s t r u c t i o n i s d u r i n g t h e Knudsen c e l l
experim ents.
From t h e a r g u m e n t s a b o v e , o n e would e x p e c t
t h a t t h e m e a s u r e d PQ w o u ld a l w a y s be e q u a l t o o r l e s s
than th e e q u ilib riu m c a s e ,
m etal r ic h edge.
a s PQ i s m i n i m i z e d a l o n g t h e
The d i s a g r e e m e n t c o u l d a r i s e f ro m a
d iff e r e n c e in s u rfa c e com position.
T h e re i s an i n t e r e s t i n g t r e n d i n
tem perature in c re a s e s .
Above 720OC, a
aas t h e s u b s t r a t e
sta rts
to drop.
The m e a n i n g o f t h i s i s t h a t As i s b e i n g s u p p l i e d more
e f f i c i e n t l y t o th e grow th p r o c e s s , as a
o n ly one
i s co mposed o f
term t h a t i s s u b s t r a t e te m p e ra tu re dependent
,
n A s . MAs .
1_______ 4 . ___ 4
n
'M
w h i l e t h e c e l l t e m p e r a t u r e s a r e c h a n g i n g , t h e maximum
c o n t r i b u t i o n i s o n l y ~ 5%, s o t h e o n l y o t h e r t e r m t h a t
can change i s I ,
, w h i c h i s a m e a s u r e o f t h e number o f
4
As a t o m s c o n t r i b u t e d t o t h e s u r f a c e by an As t e t r a m e r
fro m t h e i n c i d e n t beam f l u x .
arsenic
I t has been s u g g e ste d t h a t
t e tr a m e r s m ight b e g in to c ra c k a t h ig h e r
s u b s t r a t e t e m p e r a t u r e s 51 a n d t h i s c o u l d be e v i d e n c e f o r
th is effect.
An i n c r e a s e i n I
As 4
w o u l d a l s o r e d u c e Po
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64
2.4
C o m p a r i s o n W i t h AlGaAs Growth
E quation
(2.11)
c a n be r e c o n s i d e r e d f o r AlGaAs
g r o w t h by t h e i n c l u s i o n o f t h e s t e a d y s t a t e f l u x
c o n t i n u i t y e q u a t i o n f o r A1
J BA1 = J i n c A1
( 2 *47)
w h e r e A1 d e s o r p t i o n h a s b e e n n e g l e c t e d .
The f l u x
r e l a t i o n s h i p between t h e c o n s t i t u e n t e le m e n ts of th e
AlGaAs c r y s t a l becomes
J.
= J .
l n c As
1
+ J.
Ga
-3-a
A1
~ Jn
BGa
Ga
+ JQ
(0
BA1 ( 2 - 4 8 )
s o t h a t t h e t o t a l f l u x e q u a t i o n becomes
JB
= JD
“ JD
+ JB
+ J3
As
°A s
Ga
Ga
SA1
( 2 *49)
w h i c h becomes
V
As4
= p o + aP Ga + SPAi
( 2 - 50)
where
8 =
1
C a1
XA s 4 CA s 4
I f th e s u b s t r a t e te m p e ra tu re i s not too high
(in th e
a u t h o r ' s w o r l d , < 7 0 0 ° C ) , t h e n t h e Ga d e s o r p t i o n i s n o t
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
65
t o o h i g h , a n d t h e AlGaAs c o m p o s i t i o n c a n be e x p r e s s e d i n
terms of th e i n c i d e n t f lu x e s
53
JB
■=----- ~^=-=----- = x = A1 m o l e f r a c t i o n
Ga
\
(2.51)
l
or
B
Ga = (1/ x - 1 )
'B
Al
(2.52)
C o n v e r t i n g t o beam p r e s s u r e s
p^2 =
BA1
( j - 1) = K ( | - 1 )
<2-53)
Ga
w here
C,
n_
M„ Ta1 n
0 .6 (tt)
As __ Gs # 6 s Al* 1 / 2
14
= n _ . (M . . T *
" n g. ,1 3.
Ga
Al
A l Ga
0.6(y^)
+ 0.4
. n .
+ 0.4
,Q _
«69* 7 ,
27.0
, , C1 ,
1653% 1 / 2
1483
= 3.1
E quation
(2.53)
c a n be s u b s t i t u t e d i n t o t h e a b o v e
expression
P„
= p +a P +
A1
BA s .
°
°
1k s ACk s .
4
4
4
Ga
CA1 .1
s — ( - - 1)
Ga x
(2.54)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
T h i s o n l y h o l d s when t h e Ga d e s o r p t i o n i s n o t s i g n i ­
fic an t.
The m e a n i n g o f t h e e q u a t i o n i s f a i r l y
suggests th a t a / ( l- x )
th e h ig h er group I I I
2 .4 .1
c l e a r , and
c o e f fic ie n t increases to r e f le c t
a rriv a l rate.
Experim ental
E quation
( 2 . 5 5 ) was e v a l u a t e d e x p e r i m e n t a l l y i n t h e
same m an n er a s t h e GaAs e x p e r i m e n t s , b u t was o n l y
evaluated
a t 680°C w i t h GaAs g r o w t h
occurring
a t t h e same t i m e .
measurem ents
The As p r e s s u r e was m e a s u r e d
w i t h PQa o n l y and t h e n w i t h PGa + PAj_ s i m u l t a n e o u s l y .
The P0
vs (P„
+ Pn ) a r e shown i n P i g .
As.
Ga
BA1
The c e l l t e m p e r a t u r e s o f t h e Ga and A l c e l l w ere
2.13.
i n c r e a s e d s u c h t h a t PQa/ P A1r e m a i n e d c o n s t a n t , a s
c a l c u l a t e d from P i g .
2.1.
C a r e was t a k e n t o e n s u r e t h a t
a s t e a d y s t a t e c o n d i t i o n was a t t a i n e d .
2 .4 .2
R e s u l t s a nd D i s c u s s i o n
The r e s u l t s o f t h e m e a s u r e m e n t s and t h e l e a s t
squares f i t t e d
e q u a t i o n s a r e shown i n F i g .
2.13.
The
m ost s t r i k i n g f e a t u r e i s t h e d i f f e r e n c e between th e
e x p e r i m e n t a l a c a l c u l a t e d f o r GaAs (a = 7 . 2 )
AlGaAs
(ot = 6 . 8 ) .
a nd f o r
A c c o r d i n g t o t h e d i s c u s s i o n a b o v e , we
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
67
As pressure required for metal rich growth as
function of Pea for AI020Ga0.80As and GaAs
at Tsub s 680°C
10
9
<0
I
O
PBas=I.2 E-6+7.22 FGa
J
.
8
GaAs S
X
7
6
£
5
* 4
C?
3
2
X
/
' V
-
6 E - 7 + 6 -8 P * °
AlGaAs
I
j
0
I
I
L
t
i
i
i
i
I 2 3 4 5 6 7 8 9
10 II 12
PGa (x I0-7 Torr)
F ig u re 2.13.
As^ BEP a s a f u n c t i o n o f Ga BEP f o r
GaAs a n d A l Q 20Ga0 80As a l o n 9 t h e m e t a l
r i c h edge o f s o li d u s f i e l d .
S ubstrate
t e m p e r a t u r e i s 680 C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
68
<*
1-X
=
7
1 • z2
1-0.2
= 9 0
*’U
I n s t e a d a low er v a lu e i s o b t a i n e d .
C onsider ag ain the
t e r m s o f a Ga:
CGa
f 1'* )
IA s CA s ,
4
4
The o n l y t e r m w h i c h c a n c h a n g e i s I .
.
as4
r a t i o s o f t h e a 1s
C
Gel
ct
Ga
=
“ AlGaAs
^As ^As
4
4
=
CGa________
4
4
*As
4
^ s,
Taking th e
= 7^2
6-8
f1" * )
or
AS
7.2
6.8
1
(1- x )
= 1.32
w h e r e I As
and I '
r e p r e s e n t t h e a v e r a g e number o f As
4
4
monomers o b t a i n e d f r o m As^ f o r Ga a l o n e a n d Ga t o g e t h e r
w i t h alu minum r e s p e c t i v e l y .
A p p a r e n t l y , As i s b e i n g s u p p l i e d m ore e f f i c i e n t l y
the s u rfa c e .
to
I n d u lg i n g i n s p e c u l a t i o n , an e s t i m a t e can
be made o f t h e r e l a t i v e e f f i c i e n c y o f t h i s p r o c e s s f o r A l
a n d Ga.
L e t us assume t h a t t h e v a l u e o f I '
depends
as4
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69
upon t h e i n t e r a c t i o n o f As^ w i t h A l and Ga and t h e i r
c o n trib u tio n s to I ^ s
c a n be d e s c r i b e d by I A1 and I G a ,
4
respectively.
Then, s in c e th e r e l a t i v e c o n c e n t r a t i o n s of
Ga a nd A l a r e g i v e n by t h e c o m p o s i t i o n x , I A^ and I Ga c a n
be r e l a t e d t o l . „
a nd I .
by t h e f o l l o w i n g e x p r e s s i o n :
as4
as4
XGa
^ 4
w h i c h c a n be r e s t a t e d a s :
*A1
XA s4
T “ AS4
( 1-X)
ZGa
w ith the p re s e n t d a ta
I M _ (1.32 Z <Sa '
( 1 - 0 .2 ) ) =2.6
°'2
The r e s u l t s o f t h i s e x p e r i m e n t s u g g e s t t h a t t h e
r e a c t i o n which r e s u l t s
i n c i d e n t beam i s
i n As on t h e s u r f a c e i n t h e
i n f l u e n c e d by t h e t y p e o f a d a t o m s on t h e
s u r f a c e , a n d Al i s more e f f i c i e n t i n t h i s r e a c t i o n .
The s e c o n d i m p o r t a n t a nd p e r h a p s more f a m i l i a r
d i f f e r e n c e o c c u r s between t h e
o
v a l u e s o b t a i n e d f o r GaAs
( 1 . 2 x 10~"® T o r r ) and AlGaAs ( 1 . 2 x 1 0 ” ^ T o r r ) .
E x p e r i m e n t a l l y , t h e l o w e r v a p o r p r e s s u r e o f As
(m olecular
s p e c i e s ) o v e r AlGaAs a s c o m p a r e d w i t h GaAs i s w e l l known.
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
70
The i n t e r e s t i n g f e a t u r e h e r e i s t h a t P Q h a s d r o p p e d much
f a s t e r f o r AlGaAs t h a n o n e w o u ld e x p e c t f ro m a
c o n s i d e r a t i o n o f R a o l t s law
48
f o r th e b in a r y system of
GaAs a n d AlAs a s i n d i c a t e d i n F i g . 2 . 1 4 , w h e r e t h e A s2
p r e s s u r e o v e r AlAs i s t a k e n t o be much l e s s t h a n o v e r
GaAs.
T h i s r e s u l t r e f l e c t s t h e i m p o r t a n c e o f t h e Al
b o n d i n g upon a l l n e a r e s t n e i g h b o r As a t o m s , n o t j u s t a s a
m i x t u r e o f GaA s/A lA s.
From t h e d i s c u s s i o n a b o v e , p a r t i c u l a r l y on t h e
i n t e r a c t i o n b e t w e e n As^ a n d A l on t h e s u r f a c e , i t seem s
p o s s i b l e t h a t an A l a d a t o m on t h e s u r f a c e m i g h t fo rm a
c o m p l e x w i t h s e v e r a l As a to m s f ro m a n As t e t r a m e r .
If
t h i s o c c u r r e d , i t w o u ld p r o b a b l y be r e l a t i v e l y i m m o b i l e ,
a n d w o u ld m o d i f y t h e g r o w t h m e c h a n i s m s .
T h i s w o u ld be a
d i f f e r e n t g r o w t h m e c h a n is m fro m t h e n o r m a l k i n k s i t e
p r o p a g a t i o n w h i c h i s t h o u g h t t o be t h e g r o w t h m ech an is m
f o r GaAs.
T h i s i d e a i s e x p l o r e d b e lo w i n t h e c o n t e x t o f
t h e s u r f a c e m o r p h o l o g y o f AlGaAs.
An i m p o r t a n t a s p e c t o f h e t e r o j u n c t i o n g r o w t h i s
s u r f a c e morphology as t h e m ic ro s c o p ic sm oothness o f t h e
grow ing s u r f a c e i n th e l a r g e m easure d e te rm in e s th e
s t r u c t u r a l p r o p e rtie s of the i n t e r f a c e .
The c u r i o u s
a s p e c t o f t h e work on t h e m o r p h o l o g y o f AlGaAs i s t h e
d i v e r s i t y o f r e s u l t s o b t a i n e d i n v a r i o u s l a b s . Morkoc e t
OC
al.
f o u n d t h a t u s i n g lo w f l u x r a t i o s p o o r m o r p h o l o g y
was o b t a i n e d f o r AlGaAs c o m p o s i t i o n s x >
0.40 u n l e s s th e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
71
Vapor pressure of As2 for pseudobinary
GaAs-AIAs assuming Raolt's law
o-experimental point
t
,2-°?x
Vapor
Pressure
10.0
s\
\
Tsub = 680°C
Raolt's law
\
8.0
(x I0"7 Torr) 6-0 h
\
\
v
\
o
\
4.0 h
X
2.0 J-
\
N
\
j ______ i______ i______ i
0
0.2
GaAs
0.4
0.6
0.8
\
1.0
AlAs
Composition
F ig u re 2.14.
Vapor p r e s s u r e o f As4 a s a f u n c t i o n o f
Al c o m p o s itio n
(x) a s s u m i n g R a o l t ' s l a w .
Open c i r c l e s a t x = 0 a n d x = 0 . 2 0
i n d i c a t e d a t e from F i g u r e 2 . 1 3 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
72
s u b s t r a t e t e m p e r a t u r e was b e l o w 630°C o r a b o v e 70 0 °C .
Some d e p e n d e n c e o n f l u x r a t i o was n o t e d .
al.
36
A lexandre e t
s t u d i e d t h e m orphology as a f u n c t i o n o f growth
t e m p e r a t u r e and g r o w t h c o n d i t i o n s .
M o rk o c's e t a l .
In a d d itio n to
f i n d i n g s , t h e y f o u n d a t 680°C t h a t good
m o r p h o l o g y c o u l d be o b t a i n e d f o r x = 0 . 3 a t v e r y low o r
very high f lu x r a t i o s .
I n p a r t i c u l a r , lo w f l u x r a t i o s
corresponded to being j u s t a t th e m etal ric h edge.
It
s h o u l d be n o t e d t h a t Morkoc e t a l . d i d n o t o b s e r v e
m orphology problem s a t t h i s c o m p o s itio n .
al.
A lexandre e t
a l s o f o u n d some i m p r o v e m e n t w i t h l e s s e r g r o w t h r a t e s .
A v e r y i n t e r e s t i n g f a c t t h a t t h e y f o u n d was t h a t t h e
m o r p h o l o g y i m p r o v e d i n t h e p r e s e n c e o f t h e d o p a n t s S n and
Be.
S i d i d n o t h a v e a ny e f f e c t .
as R .C . M ille r e t a l .
20
This p o in t i s r e le v a n t
h a v e p o s t u l a t e d t h a t an
u n i n t e n t i o n a l i m p u r i t y s u c h a s c a r b o n i n h i b i t s t h e sm o o th
g r o w t h o f AlGaAs .
A l e x a n d r e ' s f i n d i n g s were t h a t t h e
m o r p h o l o g y was s o m e t i m e s i m p r o v e d by i n t e n t i o n a l
im purities.
S tall et a l.
37
observed s u rfa c e roughness a t
c o m p o s i t i o n s a s low a s 2 0 %, w h i c h was a t t r i b u t e d t o a
t h i n l a y e r o f s u r f a c e s e g r e g a t e d Ga.
The p r o b l e m o f t h e s u r f a c e m o r p h o l o g y o f AlGaAs
illu strates
t h e d i f f i c u l t y o f c o m p ariso n s between grow th
c o n d i t i o n s as p e r c e iv e d i n v a r i o u s l a b s .
e v e r y o n e c a n be r i g h t .
C learly not
This au th o r i s o b lig a te d to
r e c o u n t h i s own e x p e r i e n c e s .
G e n e ra lly , t h i s author
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
73
o b t a i n s good m o r p h o l o g y a t low f l u x r a t i o s f o r g r o w t h
r a t e s on t h e o r d e r o f 1 Mm/hr f o r GaAs a t s u b s t r a t e
t e m p e r a t u r e s >, 680°C f o r c o m p o s i t i o n s < 0 . 3 4 .
morphology o c c u r s f o r t h e s e c o m p o s itio n s i t
When p o o r
u su ally
a p p e a r s t o be r e l a t e d t o p o o r i d e n t i f i c a t i o n o r c o n t r o l
of th e s u b s t r a t e te m p e ra tu re such t h a t th e s u b s t r a t e
t e m p e r a t u r e i s b e l o w 68 0 °C .
A p a r t i c u l a r l y im portant
a s p e c t o f AlGaAs g r o w t h i s c o m p l e t e t h e r m a l c o n t a c t
t h r o u g h t h e i n d i u m s o l d e r t o t h e molybdenum s u b s t r a t e
block.
P o o r i n d i u m w e t t i n g c a n o f t e n be c o r r e l a t e d w i t h
p o o r m o r p h o l o g y on t h e s u b s t r a t e , w h e r e t h e t e m p e r a t u r e
is g en erally cooler.
The i d e a o f A s - A l c o m p l e x e s on t h e s u r f a c e may be
r e l e v a n t t o t h e p r o b l e m o f s u r f a c e m o r p h o l o g y o f AlGaAs.
I t see m s p o s s i b l e t h a t A l - A s c o m p l e x e s on t h e s u r f a c e
c o u ld s e rv e as n u c le a tio n s i t e s
sites
f o r Ga a d a t o m s .
i n c o m p e t i t i o n w ith kink
W h e t h e r o r n o t n u c l e a t i o n w o u ld
p r o c e e d o v e r k i n k s i t e p r o p a g a t i o n fro m t h e i n d i v i d u a l
c o m p l e x e s w o u l d d e p e n d on t h e r e l a t i v e s i t e o c c u p a t i o n .
I t see m s t h a t t h e i m p r o v e m e n t i n s u r f a c e m o r p h o l o g y a t
h i g h e r t e m p e r a t u r e s c o u l d be r e l a t e d t o t h e r e d u c e d
p r o b a b i l i t y of n u c le a tio n
a b o u t Al-As co m p lex es.
T his
argum ent im p lie s a thermodynamic l i m i t a t i o n i n th e
s u r f a c e m orphology problem .
The k ey e x p e r i m e n t i s t h e
s u r f a c e m o r p h o l o g y a t low g r o w t h r a t e s a n d low
t e m p e r a t u r e s , w h i c h would s t i l l
r e s u l t i n poor m orphology
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
74
if
t h i s m ode l i s c o r r e c t .
Heiblum e t a l .
h a v e s t u d i e d A1Q 3GaQ ^As g r o w t h a t
s u b s t r a t e t e m p e r a t u r e s o f 600°C u s i n g lo w g r o w t h r a t e s
( 0 . 1 4 M m /h r ).
The y r e p o r t e d e x c e l l e n t o p t i c a l p r o p e r t i e s
and p e r f e c t s u r f a c e m orphology.
T h i s a u t h o r g r e w a 2 urn
t h i c k Alq 25Gao 75As l a ^ e r a t a s u b s t r a t e t e m p e r a t u r e o f
600°C u s i n g a g r o w t h r a t e o f 0 . 1 2 y m / h r w i t h b a r e l y As
sta b le conditions.
The s u r f a c e m o r p h o l o g y was r o u g h a n d
t h e o p t i c a l p r o p e r t i e s were p o o r .
U nfortunately, th is
disagreem ent ren d ers t h i s s e t of experim ents
i n c o n c l u s i v e , a n d more work n e e d s t o be d o n e t o d e t e r m i n e
th e c a u s e o f poor s u r f a c e m orphology.
al.
cc
A n o t h e r p o i n t o f v ie w i s p r o v i d e d by K i r c h n e r e t
eg
and Calawa
where t h e s u r f a c e m orphology i s
l i m i t e d by r e a c t i o n s o f t h e A l w i t h o x y g e n i n t h e s y s t e m .
T h i s w o u ld be d e p e n d e n t on t h e q u a l i t y o f t h e m a c h i n e an d
source m a te ria ls .
The p r o b l e m w o u ld be w o r s e a t low
g r o w t h r a t e s a n d low t e m p e r a t u r e s i f t h e o x y g e n was
p r e s e n t i n th e machine a m b ie n t.
T his could e x p la in the
r e s u l t s o f t h e p r e s e n t work.
A f i n a l comment r e l a t e s t o t h e u s e o f AS2 v s . As4
f o r t h e m o l e c u l a r As s o u r c e .
In th e l i t e r a t u r e ,
surface
m o r p h o l o g y i n AlGaAs h a s b e e n f o u n d t o im p r o v e t h r o u g h
t h e u s e o f ASj a s c o m p a r e d w i t h A s ^ .
54
I f t h e Al adatom s
a r e i n d e e d more e f f i c i e n t t h a n Ga a d a t o m s a t t h e
a q u i s i t i o n o f As ato m s f r o m a n A s4 m o l e c u l e , t h e n i t
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
is
75
l i k e l y t h a t AS2 s h o u l d r e v e a l d i f f e r e n c e s i n t h e g r o w t h
o f AlGaAs.
The s u r f a c e m o r p h o l o g y i s p r o b a b l y a s t r o n g
f u n c t i o n o f t h e number o f n u c l e a t i o n s i t e s ,
and th e use
o f AS2 m i g h t i m p r o v e s u r f a c e m o r p h o l o g y t h r o u g h a
r e d u c t i o n i n t h e number o f A s - A l c o m p l e x e s .
The
e n c o u n t e r b e t w e e n a n A l a d a to m a nd a s As m o l e c u l e may
r e s u l t i n l e s s As c a p t u r e by t h e A l ato m t h a n i n t h e c a s e
o f A s4>
2.5
GaAs and AlGaAs Growth
The MBE g r o w t h o f h e t e r o j u n c t i o n s m u s t f i r s t s t a r t
w i t h t h e g r o w t h o f t h i c k e p i l a y e r s o f GaAs a n d AlGaAs, a s
t h e e l e c t r i c a l and o p t i c a l p r o p e r t i e s a r e e a s i e r t o
ev alu ate.
In t h i s s e c t i o n , th e r e s u l t s of H all
m e a s u r e m e n t s a n d p h o t o l u m i n e s c e n c e m e a s u r e m e n t s on
l i g h t l y d o p e d n - t y p e GaAs a r e r e p o r t e d t o show t h e
g e n e r a l q u a l i t y of the m achine.
H all d a ta fo r lay er
g r o w t h t h r o u g h t h e l a y e r n u m b e r s 570 t o 1538 a r e
in clu d ed .
N e x t H a l l d a t a an d p h o t o l u m i n e s c e n c e d a t a f o r AlGaAs
a r e p r e s e n t t o p r o v i d e a c o m p a r i s o n f o r t h e AlGaAs
s tr u c tu r e s to follow .
The l a y e r s p r e s e n t e d i n t h i s
s e c t i o n w e r e grown a t C o r n e l l ;
2 .5 .1
GaAs
The q u a l i t y o f GaAs c a n be e v a l u a t e d i n s e v e r a l ways
a nd u s u a l l y b e s t c o m p a r e d i n u n i s o n .
The m e a s u r e m e n t s
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76
r e p o r t e d h e r e a r e H a l l m e a s u r e m e n t s a nd p h o t o ­
lum inescence.
The u s e f u l n e s s o f H a l l m e a s u r e m e n t s i s i n
th e d e t e r m in a ti o n of th e background im p u rity
con cen tratio n .
The b a c k g r o u n d i n " g o o d ” MBE m a c h i n e s i s
u s u a l l y low p - t y p e
(~ 10
14
cm
-3
).
The g r o w t h o f undope d
l a y e r s p ro v id e s a m easure o f th e c o n c e n t r a t i o n of
im p u rities,
an(3 t h e q u a l i t y o f t h e m a t e r i a l
t h r o u g h t h e room t e m p e r a t u r e and 77K m o b i l i t i e s .
T ypical
m o b i l i t i e s f o r 1 0 ^ cm"2 m a t e r i a l a r e ~ 450 cm2/ V . s a t
300K a n d - 8500 cm2/ V . s a t 77K.
U nfortunately,
t h e o r e t i c a l work on p - t y p e m a t e r i a l i s l e s s w e l l
d e v e l o p e d t h a n n - t y p e m a t e r i a l , a n d a c a l c u l a t i o n o f NA
and Nd i s n o t p o s s i b l e ,
n - t y p e m a t e r i a l , on t h e o t h e r
h a n d , h a s b e e n e x t e n s i v e l y s t u d i e d e x p e r i m e n t a l l y and
th eo retic ally .
The m e a s u r e m e n t o f t h e H a l l c a r r i e r
c o n c e n t r a t i o n a nd t h e H a l l m o b i l i t i e s
d e t e r m i n a t i o n o f b o t h ND a nd NA.
a t 77K a l l o w a
This te c h n iq u e i s very
im p o rta n t as th e f r e e e l e c t r o n c o n c e n tr a tio n
(ND“NA) i n a
c l o s e l y c o m p e n s a t e d m a t e r i a l may a p p e a r v e r y l o w , e v e n
t h o u g h t h e a c t u a l d o n o r a nd a c c e p t o r c o n c e n t r a t i o n s a r e
very h ig h .
The m e a s u r e m e n t o f t h e c a r r i e r c o n c e n t r a t i o n
a lo n e i s m eaningless w ith o u t a c o n s id e r a t i o n o f the
c a r rie r m obility.
S p e c ific d e t a i l s of the in te r p r e t a tio n
o f n - t y p e H a l l m o b i l i t i e s c a n be f o u n d i n t h e work o f
C h a n d r a 57 and W o lf e e t a l . 5 8 .
The tw o h i g h e s t H a l l
m o b i l i t i e s o b t a i n e d i n th e machine a r e l i s t e d
in T able 2 .1 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
77
T able 2 .1 .
Layer #
1534
1538
Tem perature
H a l l M easurements
2
pm (cm / V . s )
NH ( c n f 3 )
300
3 . 9 E14
8,530
77
4 . 8 E14
92,262
300
4 . 6 E14
7,680
77
4 . 6 E14
101,400
The l a y e r s w e r e grown a t 610°C u n d e r b a r e l y As
s t a b i l i z e d c o n d i t i o n s a t a grow th r a t e of 1 um /hr.
U n less o t h e r w i s e s p e c i f i e d , t h e s e a r e th e g e n e r a l growth
c o n d i t i o n s u t i l i z e d f o r GaAs g r o w t h i n t h i s t h e s i s .
These r e s u l t s a r e p l o t t e d i n P ig . 2 .1 5 as a f u n c t i o n
o f doping c o n c e n t r a t i o n .
A lso p l o t t e d i s th e t h e o r e t i c a l
CO
m o b i l i t y c u r v e f o r n - t y p e GaAs
.
i s d a t a fro m t h e work o f P a l m a t e e r
Included in th e curve
28
.
W h i l e some o f t h e
d a t a compare w e l l , o t h e r ru n s e x h i b i t c o n s i d e r a b l e
com pensation.
This d a t a s e rv e s as a r e f e r e n c e f o r
g e n e r a l m achine q u a l i t y f o r l a y e r s numbers up t o 1538.
P hotolum inescence i s u s e f u l fo r the i d e n t i f i c a t i o n
o f s h a l l o w d o n o r s and a c c e p t o r s a n d t h e g e n e r a l
e v a lu a tio n of the s t r u c t u r a l q u a l i t y of the m a te r ia l.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
78
10,6
'
O •
A
1538 '
a
—
v
T = 77°K
300°K
T H E O R E T IC A L CUR VE
FOR UNCOMPENSATED
^vC aA s
1184
0»
77°K
978
10
10,3
10
lO
1015
101S -
10
17
C AR RIER C O N CEN TRA TION ( cm - *)
F ig u re 2.15.
P l o t o f t h e o r e t i c a l and e x p e r im e n ta l
300K a n d 77K m o b i l i t y v e r s u s c a r r i e r
c o n c e n t r a t i o n f o r n - t y p e S i - d o p e d GaAs.
S o l i d c i r c l e s a n d t r i a n g l e s a r e f ro m
r e f e r e n c e 28.
Open c i r c l e s a n d t r i ­
a n g l e s a r e from t h e p r e s e n t work.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
79
The m o s t up t o d a t e r e f e r e n c e on h i g h p u r i t y MBE grown n t p e GaAs i s Skromme e t a l . ^ 7
The r e s u l t s o f p h o t o ­
l u m i n e s c e n c e a r e shown f o r l a y e r 1534 i n P i g .
2 .1 6 (a).
B o t h s c a n s show a low c a r b o n c o n c e n t r a t i o n a s shown
by t h e r a t i o o f t h e b a n d e d g e e x c i t o n r e c o m b i n a t i o n
( 1 . 5 1 3 eV) t o t h e C ( e , 8 ) t r a n s i t i o n .
2.16(a)
A l s o shown i n P i g .
i s f u r t h e r em issio n s p e c i f i c a t i o n using s p e c t r a
ta k e n a t d i f f e r e n t t e m p e r a tu r e s f o r l a y e r 1534.
The p e a k
a s s i g n m e n t s w e r e made on t h e b a s i s o f p r e v i o u s e n e r g y
assignm ents.
From t h i s d a t a t h e d o m i n a n t a c c e p t o r i s
c a r b o n , w h i l e t h e s i l i c o n a c c e p t o r c o n t e n t i s q u i t e low.
The p e a k a t 1 . 4 9 2 eV i s t h e r e c o m b i n a t i o n b e t w e e n
a d j a c e n t h o l e s bound t o c a r b o n and e l e c t r o n s bound t o
silico n .
The p e a k i n t e n s i t y d i m i n i s h e s w i t h i n c r e a s i n g
t e m p e r a t u r e a s t h e d o n o r s become i o n i z e d , and t h e d e n s i t y
o f bound e l e c t r o n s d e c r e a s e s .
The p e a k a t 1 . 4 9 4 eV c o u l d
e i t h e r be a n unknown a c c e p t o r t r a n s i t i o n , o r a n unknown
d o n o r r e l a t e d bound t o bo und t r a n s i t i o n .
by Skromme, t h e s e c o u l d be s u l f u r r e l a t e d .
From t h e p a p e r
The n e a r ban d
e d g e e x c i t o n s p e c t r a f o r 1534 a r e shown i n F i g . 2 . 1 6 ( b )
respectively.
The t r a n s i t i o n s e x h i b i t good i n t e n s i t y a n d
a re w ell re so lv e d ,
exciton
p a r t i c u l a r l y t h e n e u t r a l d o n o r bound
(D°X) a nd and a c c e p t o r bound e x c i t o n d o u b l e t
i n d i c a t i v e o f good m a t e r i a l q u a l i t y .
I t s h o u l d be n o t e d t h a t h i g h q u a l i t y n - t y p e m a t e r i a l
r e q u i r e s a l o n g w i t h go od q u a l i t y u n d o p e d m a t e r i a l a h i g h
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.5137
£to
1.5131
(dT x)
23 K
C
3
w.
2
J3
oU.
(A
C
I7K
1.5152
12 K
0>
c
to
CL
7K
lO
4K
F ig u re 2.16.
Energy (eV)
P h o to lu m in escen ce s p e c t r a o f l i g h t l y S i doped n l a y e r
(1534)
a) Bandedge and a c c e p t o r r e g i o n a s a f u n c t i o n o f t e m p e r a t u r e .
2
o
Pump p o w e r i s 2W/cm a t 6328A.
b ) B a n d e d g e e x c i t o n r e g i o n a t 4K
Pump p o w e r i s
0.2W/cm^ a t 6 3 2 8 ^ .
81
q u a lity s ilic o n source.
The m o s t r e c e n t h i g h q u a l i t y
l a y e r s were o b t a i n e d u sin g S i s o u rc e #1.
2.5 .2
AlGaAs G row th
T h e r e h a v e b e e n n u m e r o u s s t u d i e s o f AlGaAs g r o w t h .
G e n e r a l l y , t h e m a t e r i a l q u a l i t y h as been found t o improve
b o t h by g r o w i n g a t low f l u x r a t i o s and h i g h s u b s t r a t e
tem peratures
(j> 6 8 0 ° C ) .
Because r e p o r t e d growth
c o n d i t i o n s a r e d i f f i c u l t t o c o r r e l a t e f ro m l a b t o l a b ,
t h e m a in p u r p o s e i n t h e s e c t i o n i s t o d e m o n s t r a t e t h a t
h i g h q u a l i t y AlGaAs c a n be grown u n d e r t h e c o n d i t i o n s
reported h ere.
Because th e e l e c t r o n m o b i l i t i e s a re
l i m i t e d by a l l o y s c a t t e r i n g , a c o m p l e t e e v a l u a t i o n o f
AlG aA s, a l s o r e q u i r e s a n a l y s i s u s i n g p h o t o l u m i n e s c e n c e t o
d e te r m in e a l l o y c o m p o s itio n and i m p u r i t y c o n t e n t .
Good
lu m in e s c e n c e i n t e n s i t y and narrow e x c i t o n l i n e w i d t h s a r e
g e n e r a l c h a r a c t e r i s t i c s o f " s t a t e o f t h e a r t " AlGaAs
a l t h o u g h i t c a n s t i l l l e a v e much t o be d e s i r e d .
A c o m p l i c a t i o n o f H a l l m e a s u r e m e n t s i n AlGaAs a t lo w
t e m p e r a t u r e s i s t h a t t h e m easured H a l l c a r r i e r
c o n c e n t r a t i o n d e p e n d s upon t h e c o n d i t i o n s u n d e r w h i c h t h e
sam ple i s c o o le d .
B o t h t h e s p e e d o f c o o l i n g a nd s a m p l e
i l l u m i n a t i o n e f f e c t th e m easured c a r r i e r c o n c e n t r a t i o n .
Because of th e u n c e r t a i n t y of a t t a i n i n g e q u ilib riu m a t
lo w t e m p e r a t u r e s , o n l y 300K d a t a a r e r e p o r t e d .
Improper
l a y e r d esig n can a l s o r e s u l t in a tw o-dim ensional
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
82
e l e c t r o n g a s a t t h e AlGaAs-GaAs b u f f e r i n t e r f a c e .
CO
The
experim ents in t h i s t h e s i s a re s u it a b l y designed to avoid
t h i s problem .
The r e s u l t s o f H a l l and PL m e a s u r e m e n t s on
A lg 3Gag yAs a r e shown i n T a b l e 2 . 2 .
The l a y e r s w e r e
g row n a t a s u b s t r a t e t e m p e r a t u r e o f 680°C u s i n g m e t a l
s t a b l e growth c o n d i t i o n s n e a r th e m e ta l r i c h edge of th e
so lid u s f ie ld .
T h i s i s d e t e r m i n e d by t h e
(3x1) s u r f a c e
r e c o n s t r u c t i o n a s o b s e r v e d by t h e RED p a t t e r n s .
g r o w t h r a t e was ~ 1 U m /h r .
The GaAs
These a r e th e g e n e r a l growth
c o n d i t i o n s u s e d f o r a l l c o m p o s i t i o n s o f AlGaAs r e p o r t e d
i n t h i s t h e s i s e x c e p t where n o t e d .
Two f e a t u r e s w h ic h
a r e c l e a r f ro m t h e t a b l e a r e t h e v a r i a t i o n o f t h e boun d
e x c i t o n FWHM l i n e w i d t h and H a l l m o b i l i t i e s w i t h c a r r i e r
concentration.
Only a m odest i n c r e a s e o f t h e H a l l
m o b ility w ith decreased c a r r i e r c o n c e n tra tio n i s
observed.
S e c o n d , t h e bound e x c i t o n l i n e w i d t h d i m i n i s h e s
w ith decreasing c a r r i e r c o n c e n tra tio n .
S i n g h e t a l . 60 h a v e p o i n t e d o u t t h a t i o n i z e d
im p u ritie s could c o n tr ib u te to lin e w id th broadening.
The d a t a i n T a b l e 2 . 2 a r e p l o t t e d i n P i g . 2 . 1 7 and
i n d i c a t e a r e l a t i o n s h i p o f Ae FWhM ~
F i g . 2 . 1 7 a n d T a b l e 2 . 2 a r e shown i n F i g .
The PL d a t a o f
2.18.
A
d i f f e r e n c e among t h e s a m p l e s i s t h e p r e s e n c e o f t h e
oq
"d efect exciton"
i n s a m p l e 1249 a l o n e .
The a u t h o r h a s
n o t a t te m p te d t o c o r r e l a t e t h i s peak s t r u c t u r e w ith any
e x p e r im e n ta l f a c t o r s , b u t u s u a ll y o b s e rv e s both th e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
FWHM
Linewidth
(meV)
Full width half m a x im u m linewidth ( A E FWHM' 01
A ln ^ G a n r A s a s a function of Hall c a r rie r con centratio n
1564
layer
-791-1453
Minimum A E FWHM
o b serv ed in this work
1566
1249
T heoretical m in im u m A E FWHM
S in g h 8 B a ja j
Hall carrier concentration at 3 0 0 K
F ig u re 2.17.
4K FWHM l i n e w i d t h o f S i d o p e d A l ^ 3 oG£q 70As a s a f u n c t i ° n o f H a l l
c a r r ie r concen tration.
Open c i r c l e s a r e d a t a f r o m d o p e d l a y e r s .
Dashed l i n e s i n d i c a t e l i n e w i d t h s from undoped l a y e r s and t h e o r e t i c a l
calcu latio n s.
oo
u>
84
1.921 BE
1566
nHoii =2.0 EI6cm ^
6.0 meV
.904
v>
c
3
S
wk
o
.905 BE
o
.891
7.1 meV
V)
c
c
l-l
4)
.862
_l
o.
1.909
1564
nu(,i, =8.3EI6cm"3
.862
Energy
F ig u re 2.18.
8.4 meV
(eV)
PL d a t a f o r d o p e d s a m p l e s shown i n F i g u r e
2.17 and T able 2 . 2 .
BE i n d i c a t e s b o u n d
e x c i t o n a n d DE i n d i c a t e s d e f e c t e x c i t o n .
P e a k a t 1 . 8 6 eV i n a l l s a m p l e s i s t h e S i
acceptor.
30
Pump p o w e r i s 6W/cm
2
at
5145&.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
29
85
d e f e c t a n d bound e x c i t o n s a t h i g h e r A l c o m p o s i t i o n s
(x >
0.30) .
T able 2 .2 .
Layer#
Alg 3GSq
Ae FWHM (meV)
Bound e x c i t o n ( x )
jAs
M easurements
NH a l l (cra” 3)
pH a l l
3 0 OK
(c™2/ 7 *3 )
3 0 OK
1564
8.4
31.6
8 . 3 E16
1584
1249
7.1
31.0
3 . 1 E16
2090
1560
6.0
32.6
1 . 2 E16
2210
79 1
4.5
30.5
Undoped b u f f e r o f m o d u l a t i o n
do p ed s t r u c t u r e
The bound e x c i t o n l i n e w i d t h i n u n d oped m a t e r i a l i s
t h o u g h t t o be t h e r e s u l t o f a v e r a g e a l l o y f l u c t u a t i o n s i n
t h e l o c a l n eig h b o rh o o d where t h e e x c i t o n re c o m b in e s.
S i n g h a nd B a j a j
have t r e a t e d t h i s problem t h e o r e t i c a l l y
a s s u m i n g a random a l l o y d i s t r i b u t i o n t o d e t e r m i n e t h e
ex cito n linew idth .
The r e s u l t o f t h e i r c a l c u l a t i o n w i t h
e x p e r i m e n t a l p o i n t s i n c l u d e d i s shown i n F i g . 2 . 1 9 .
A lso
i n d i c a t e d on t h e f i g u r e a r e d a t a f r o m t h i s a u t h o r ' s own
m aterial.
The m a t e r i a l r e p o r t e d i n t h i s t h e s i s c o m p a r e s
f a v o r a b l y w i t h t h e r e s u l t s r e p o r t e d fro m o t h e r w o r k e r s .
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
86
6.0
791
5.0
4.0
□1453
617
| 3.0
b
1366
2 .0
0.4
0.3
Al CONCENTRATION (X)
0.1
F ig u re 2.19.
0.2
F u l l w i d t h maximum l i n e w i d t h
(a) a s a
f u n c t i o n o f a lu m in u m c o m p o s i t i o n .
S olid
l i n e i s t h e t h e o r e t i c a l c u rv e o f Singh
e t al.
60
Squares r e p r e s e n t th e p r e s e n t
work w i t h l a y e r n u m b e r .
trian g le is re f.
C ircle is r e f.
32, + i s r e f .
29, x i s
r e f e r e n c e 62.
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
30,
CHAPTER 3
N-N HETEROJUNCTION GROWTH
The p r e c e d i n g c h a p t e r h a s p r o v i d e d t h e b a s i s f o r t h e
d i s c u s s i o n o f th e meaning o f s u r f a c e s t o i c h i o m e t r y i n
t e r m s o f t h e i n c i d e n t f l u x e s and i d e n t i f i e d t h e m a i n
components of th e f l u x e q u a t i o n as P , t h e d e s o r b in g
te rm , and a , th e i n c o r p o r a t i o n term .
U sing t h i s
fram ew ork, t h e d i f f e r e n c e s between t h e growth
r e q u i r e m e n t s a t 680°C f o r GaAs and AlGaAs h a v e b e e n
in d icated .
The GaAs and AlGaAs d a t a r e p o r t e d i n d i c a t e
th e h ig h q u a l i t y o f m a t e r i a l t h a t t h e a u th o r has been
a b l e t o o b t a i n by g r o w t h a t t h e c o n d i t i o n s i d e n t i f i e d i n
each s e c tio n .
3.1
In tro d u ctio n
The g r o w t h o f h e t e r o j u n c t i o n s r a i s e s a number o f
q u e s ti o n s co n c e rn in g th e c h o ic e of a p p r o p r i a t e growth
co n d itio n s.
The f i r s t q u e s t i o n r e g a r d s t y p e o f m a t e r i a l
q u a l i t y one i s t r y i n g t o a c h ie v e .
The g r o w t h o f h i g h
q u a l i t y GaAs and AlGaAs i n v o l v e 2 d i f f e r e n t s u b s t r a t e
tem p eratu re ran g es,
610°C v s 680°C r e s p e c t i v e l y ,
if
the
g r o w t h r a t e i s t o be h e l d c o n s t a n t .
As d i s c u s s e d i n C h a p t e r 1 , many e a r l y e f f o r t s a t
AlGaAs-GaAs g r o w t h s i m p l y u s e d t h e low t e m p e r a t u r e g r o w t h
87
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
88
conditions.
S t r u c t u r e s w h i c h a r e m o s t s e n s i t i v e t o GaAs
q u a l i t y g e n e r a l l y h a v e e n j o y e d some m e a s u r e o f s u c c e s s
using t h i s approach.
The s i n g l e i n t e r f a c e A l G a A s r S i
gro wn o n GaAs m o d u l a t i o n d o p e d s t r u c t u r e s h a s b e e n s o
s u c c e s s f u l b e c a u s e i t r e q u i r e s h i g h q u a l i t y GaAs g r o w t h .
The i n s e n s i t i v i t y o f t h e MODFET t o c a p p i n g l a y e r q u a l i t y
i s q u i t e s u r p r i s i n g , a lth o u g h f u r t h e r improvements in
d e v i c e p e r f o r m a n c e may r e v e a l t h e e f f e c t s o f t r a p s i n t h e
microwave p e r f o r m a n c e .
( 6 0 0 ° C ) , lo w g r o w t h r a t e
The u s e o f l o w t e m p e r a t u r e
( 0 . 1 4 u m / h r ) h a s n o t shown any
improvement i n t h e e l e c t r o n m o b i l i t i e s o f t h e s e
s t r u c t u r e s . * ’*
Double h e t e r o s t r u c t u r e l a s e r s ,
in
c o n t r a s t , h a v e shown d r a m a t i c i m p r o v e m e n t s i n t h r e s h o l d
c u r r e n t s through th e grow th
cladding la y e r .
o f h i g h q u a l i t y AlGaAs
The a p p l i c a t i o n s o f t h e
h e t e r o s t r u c t u r e u ltim a te ly d eterm ine the m a te r ia l
requirem ents.
The work p r e s e n t e d i n t h i s
t h e s i s em phasizes th e
g r o w t h o f h i g h q u a l i t y AlGaAs a s t h e p r i m a r y g o a l .
c o n v e n tio n a l growth r a t e
At a
(~ 1 n m / h r ) , t h i s r e q u i r e s
g r o w t h a t 680°C u s i n g l o w f l u x r a t i o s .
Once t h i s s e t o f
c o n d i t i o n s i s s p e c i f i e d , t h e g r o w t h o f GaAs m u s t be
re c o n sid e re d w ith in the c o n s tr a i n t s of the c a p a b i l i t i e s
o f t h e MBE m a c h i n e .
A more s u b t l e p o i n t , h o w e v e r , i s t h e
growth c o n d i t i o n s r e q u i r e d f o r h ig h q u a l i t y m a t e r i a l a t
the h e te r o ju n c tio n .
The n e x t c h a p t e r w i l l show t h a t t h e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
89
g r o w t h o f t h i n l a y e r s o f GaAs on AlGaAs i s c o n s i d e r a b l y
d i f f e r e n t f r o m t h e g r o w t h o f t h i c k GaAs a t a s u b s t r a t e
t e m p e r a t u r e o f 680 °C.
At th e i n t e r f a c e
t h e thermodynamic
p r o p e r t i e s o f t h i n l a y e r s a r e n o t t h e same a s t h i c k
l a y e r s , a n d a r e i n f l u e n c e d by t h e u n d e r l y i n g m a t e r i a l .
T h i s p r o b l e m w i l l be c o n s i d e r e d i n much g r e a t e r d e t a i l i n
C h a p t e r 4.
F o r t h e p r e s e n t , l e t us p r o c e e d b l i n d l y a h e a d
and c o n s i d e r t h e f a r s i m p l e r p r o b l e m o f t h e g r o w t h o f
AlGaAs on GaAs u s i n g t e c h n i q u e s w h ic h r e s u l t i n h i g h
q u a l i t y AlGaAs.
The p u r p o s e o f t h e n e x t s e c t i o n i s t o
d e m o n s t r a t e t h e i m p o r t a n c e o f h i g h q u a l i t y AlGaAs i n t h e
n-n h e te r o ju n c tio n s t r u c t u r e .
A fte r high q u a l i t y
i n t r i n s i c m a t e r i a l has been o b t a i n e d , t h e s o u r c e o f deep
l e v e l s i n t h e j u n c t i o n i s exam ined.
O ut-diffusing
s u b s t r a te im p u ritie s a re i d e n t i f i e d as a fa c to r
i n f l u e n c i n g t h e e l e c t r i c a l an d o p t i c a l p r o p e r t i e s o f t h e
s t r u c t u r e s , b u t t h e DX c e n t e r
33
i s i d e n t i f i e d as the
dom inate f a c t o r in the deep l e v e l b e h a v io r.
3.2
N- n H e t e r o i u n c t i o n E l e c t r i c a l C h a r a c t e r i s t i c s
As t h e r e c t i f y i n g p r o p e r t i e s o f b a r r i e r s a r e
d e t e r m i n e d f o r a l a r g e p a r t by t h e b a r r i e r m a t e r i a l
p r o p e r t i e s as w e ll as t h e i n t e r f a c e , t h e n-n
h e t e r o j u n c t i o n i s a c o n v e n i e n t means o f s t u d y i n g t h e
i n f l u e n c e of b a r r i e r q u a l i t y on th e e l e c t r i c a l p r o p e r t i e s
of the ju n c tio n .
When t h e a u t h o r f i r s t em b a r k e d o n t h i s
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
90
work# h i g h q u a l i t y j u n c t i o n s h ad n o t y e t b e e n r e a l i z e d by
MBE.
As s u c h , t h e f i r s t p r o p e r t y t h e a u t h o r e v a l u a t e d
was t h e " q u a l i t y " o f t h e r e c t i f i c a t i o n .
The e l e c t r o n
tra n sp o rt across th is h etero ju n ctio n is p a rtic u la rly
se n sitiv e to b a rrie r q u a lity .
In t h i s s e c tio n , the
q u a l i t a t i v e p r o p e r t i e s o f t h e I - V c h a r a c t e r i s t i c s w i l l be
used to dem onstrate the b e s t c o n d itio n s fo r
h e t e r o j u n c t i o n grow th.
The d e v e l o p m e n t o f a
mode l f o r
t h e n - n h e t e r o j u n c t i o n i s p r e s e n t e d i n A p p e n d i x B.
To
e v a lu a te th e q u a l i t y of th e r e c t i f i c a t i o n a sim ple
t h e r m i o n i c e m i s s i o n mo del i s f o r m u l a t e d .
As t h e
t r a n s p o r t a c r o s s lo w b a r r i e r h e t e r o j u n c t i o n s
is ty p ically
a m ix tu r e o f b o th th e r m i o n i c e m is s io n and t u n n e l l i n g
c u r r e n t s , th e fo llo w in g d is c u s s io n using th erm io n ic
e m i s s i o n p r o v i d e s an u p p e r e s t i m a t e o f t h e d e g r e e o f
rectificatio n .
The e x p r e s s i o n f o r a S c h o t t k y d i o d e o b e y i n g
t h e r m i o n i c e m i s s i o n i s g i v e n by
2
J = A*T
-q<i>bA T
e
64
qv/kT
[e
..
1
A* = E f f e c t i v e R i c h a r d s o n ' s c o n s t a n t
T h i s e q u a t i o n c o n t a i n s a number o f i m p l i c i t
a s s u m p t i o n s w h i c h a r e a d d r e s s e d i n A p p e n d i x B.
The m a in
d i f f e r e n c e b e t w e e n t h e t y p i c a l S c h o t t k y b a r r i e r and t h e
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
91
h e t e r o j u n c t i o n i s t h a t th e h e t e r o j u n c t i o n has a lower
b a rrie r height.
In a d d itio n ,
i t is
the usual p r a c tic e to
e x p l i c i t l y in c lu d e th e v o lta g e v e rsu s f i e l d dependence of
t h e n a r r o w g a t e m a t e r i a l a s a n o t c h p o t e n t i a l , Vj j (V) .
The b a r r i e r h e i g h t , (Jj ^ , w h i c h i s t h e c u r r e n t c o n t r o l l i n g
m e c h a n is m i n t h e r m i o n i c e m i s s i o n , i s t h e n g i v e n by
♦ b = AVc * VN(V>
where
AV = c o n d u c t i o n ban d d i s c o n t i n u i t y
c
= 0.75 x
x = a l lo y com position.
Vjj (V) = d e p t h o f n o t c h b e l o w F e r m i l e v e l
In g e n e r a l , th e forw ard d i r e c t i o n e x h i b i t s a ra p id
t u r n on w ith a p p l i e d v o l t a g e :
J
-
e 9 V /k T
w h i l e t h e r e v e r s e d i r e c t i o n s h o u l d show a w e a k e r
d e p e n d e n c e on a p p l i e d v o l t a g e
The r e a s o n f o r t h i s b e h a v i o r i s t h a t m o s t o f t h e
a p p l i e d v o l t a g e i s d r o p p e d i n t h e AlGaAs b a r r i e r r e g i o n ,
whereas v ery l i t t l e
T h i s i s a ke y p o i n t ,
i s a c c o u n t e d f o r by t h e GaAs r e g i o n .
as d i o d e s which e x h i b i t slow fo rw a r d
t u r n on v o l t a g e s a r e i n d i c a t i v e o f s i g n i f i c a n t v o lta g e
d r o p s i n t h e GaAs r e g i o n , o r a c u r r e n t c o n t r o l l i n g
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
92
b a r r i e r w h i c h i s n o t l o c a t e d a t t h e AlGaAs-GaAs
in terface.
In the r e v e rs e d i r e c t i o n , th e b a r r i e r low ering only
d e p e n d s o n VN (V).
S i n c e VN (V) i s o n l y w e a k l y d e p e n d e n t o n
V, t h e n :
qVN ( V )/ k T
and t h e c o r r e s p o n d i n g c u r r e n t i n c r e a s e s s l o w l y .
The
behavior a t th e o r ig i n i s p a r t i c u l a r l y i n t e r e s t i n g as the
s l o p e i s d e p e n d e n t on t h e z e r o b i a s b a r r i e r h e i g h t
q V /k T _ 1}
T his e x p re ssio n i l l u s t r a t e s
i s o f te n m isunderstood.
in f in ite resista n ce
a p r o p e r t y o f a d i o d e w h ic h
A good d i o d e d o e s n o t h a v e
(no c u r r e n t f l o w )
near the o r ig in .
I f t h e b a r r i e r i s lo w a f i n i t e r e s i s t a n c e c a n b e o b t a i n e d
a t high te m p e ra tu re s .
Many d i o d e s e x h i b i t a s i g n i f i c a n t s e r i e s r e s i s t a n c e
w h i c h c a n be e s t i m a t e d by t h e h i g h c u r r e n t vs v o l t a g e
behavior.
T hus, i n c l u d i n g t h i s as th e e x t e r n a l l y
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
93
measured v o l t a g e :
Ve x t = V + RJ
3.3
MBE G row th C o n d i t i o n s
As d i s c u s s e d i n C h a p t e r 1 , many f a i l u r e s o f t h e
e a r l y s t u d i e s o f MBE grown n - n h e t e r o j u n c t i o n s c a n be
t r a c e d t o p o o r g r o w t h c o n d i t i o n s f o r t h e AlGaAs b a r r i e r
region.
The f o l l o w i n g s e c t i o n d e m o n s t r a t e s t h a t h i g h
q u a l i t y j u n c t i o n s c a n be grown by o p t i m i z i n g t h e AlGaAs
b arrier quality .
The s t r u c t u r e s
a lso i l l u s t r a t e the
a p p r o a c h o f h e t e r o j u n c t i o n g r o w t h by w h i c h g r o w t h
c o n d i t i o n s a r e c h o s e n by a d j u s t i n g s u b s t r a t e t e m p e r a t u r e s
and f l u x r a t i o s
to produce high q u a l i t y m a te r ia l a t
c o n s t a n t growth r a t e s .
The b a s i c p r o b l e m i s t h a t
d i f f e r e n t grow th te m p e r a tu re s a r e r e q u i r e d f o r p r a c t i c a l
g r o w t h r a t e s f o r AlGaAs and GaAs.
The e x p e r i m e n t a l t e c h n i q u e s o u t l i n e d a b o v e f o r MBE
were fo llo w e d .
The f o l l o w i n g s e t o f r e s u l t s a r e
i l l u s t r a t i v e o f many l a y e r s .
Fig.
3 . 1 shows t h e l a y e r s t r u c t u r e and F i g s .
3*2 and
3 . 3 t h e I - V c h a r a c t e r i s t i c s a t 300K and 77K o f a
s t r u c t u r e w i t h t h e AlGaAs q u a l i t y i n t h e b a r r i e r
optim ized.
T h i s i s a c h i e v e d by r a i s i n g t h e s u b s t r a t e
t e m p e r a t u r e i n t h e GaAs r e g i o n a few m i n u t e s p r i o r t o
AlGaAs g r o w t h .
The GaAs s u r f a c e i s p r e v e n t e d from g o i n g
m e t a l r i c h t h r o u g h t h e a p p l i c a t i o n o f more As by u s i n g a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
94
2 0 0 0 A I018 GaAs: Si
7 0 0 A I018 graded AlGaAs:Si
3 0 0 I I018 AlGaAs: Si
2 0 0 0 A n' 5 xlO,s AlGaAs
5 0 0 0 A I018GaAs: Si
S ubstrate
F ig ure 3.1.
Layer s t r u c t u r e of th e n-n h e te r o ju n c tio n .
The alum inum c o m p o s i t i o n i s
30%, a s d e t e r ­
m i n e d by p h o t o - l u m i n e s c e n c e m e a s u r e m e n t s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
95
D V c(eV )“ . 2 2
100-*
NGaRs“ l . E + 1 8
Lb(CM)“ 2 . E - 5
80N R lG aR s-l.E+18
Nbar**5.E15
80T»300
40"
Rs(OHMS)»10
C urrent
C*R)
20"
b)
V olts
F ig u re 3.2.
300K I - V c h a r a c t e r i s t i c o f n - n h e t e r o j u n c t i o n
a) m e a s u r e d c h a r a c t e r i s t i c s ;
b) c a l c u l a t e d
ch aracteristics.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
a)
.
i
DVc(eV)**.22
5 .0
NGafls**l . E + 1 8
L b ( CM) * 2 . E - 5 4 . 0
N R IG afis-l.E+18
Nbar“ 5.E 15
3 .0
T-77
Rs(OHMS)»10 2 . 0
C urrent
(mR)
1 .0 "
b)
-2
H
-1
+■
1
-1.0
+
2
V olts
-2.0
-3.0
-4.0
-5.0
F i g u r e 3. 3.
77K I - V c h a r a c t e r i s t i c s o f n - n h e t e r o j u n c t i o n
a) m e a s u r e d c h a r a c t e r i s t i c s ;
b) c a l c u l a t e d
c h aracteristics.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
97
s e c o n d As s o u r c e c e l l .
Once AlGaAs g r o w t h i s s t a r t e d ,
o n e o f t h e two As c e l l s i s s h u t t e r e d , s o t h a t op tim um
V :III flu x r a t i o i s used.
A very in t e r e s t i n g e f f e c t th a t
o n e n o t e s i s t h a t o n l y a few m o n o l a y e r s o f AlGaAs g r o w t h
i s r e q u i r e d t o s u s t a i n t h e low f l u x c o n d i t i o n .
By u s i n g
t h i s a p p r o a c h o n e c a n grow h i g h q u a l i t y m a t e r i a l
throughout the s tr u c tu r e .
GaAs i s a much more f o r g i v i n g
compound t h a n AlGaAs i n t h e t e m p e r a t u r e r a n g e o f i n t e r e s t
betw een 600-680°C, so a s m a l l compromise i n i t s
growth
c o n d i t i o n s a l l o w s an i m p r o v e m e n t t h e b a r r i e r q u a l i t y .
The h i g h q u a l i t y o f t h e s t r u c t u r e i s i l l u s t r a t e d by t h e
r a p i d f o r w a r d t u r n o n c h a r a c t e r i s t i c a t b o t h 300K a nd 77K
and P i g s .
3 . 2 a and 3 . 3 a .
A l s o shown i n t h e f i g u r e s a r e
t h e c a l c u l a t e d I-V c h a r a c t e r i s t i c from t h e e q u a t i o n s i n
A p p e n d i x B.
Good q u a l i t a t i v e a g r e e m e n t i s o b t a i n e d .
The
c a l c u l a t e d 300K c h a r a c t e r i s t i c d e m o n s t r a t e s t h a t w h i l e
t h e r e c t i f i c a t i o n i s weak, i t
i s not n e c e s s a r ily
i n d i c a t i v e of poor m a t e r i a l q u a l i t y .
A t 77K, t h e
c a lc u la te d c h a r a c t e r i s t i c in the re v e rse d i r e c t io n i s
p r e d o m i n a t e l y t u n n e l i n g c u r r e n t , and i l l u s t r a t e s
the
im portance of a tte m p tin g a com plete a c c o u n tin g of a l l
c u r r e n t t r a n s p o r t mechanisms.
F o r n - n h e t e r o j u n c t i o n s t h e q u a l i t y o f t h e AlGaAs i s
q u ite im portant.
T his e x p la in s th e poor r e s u l t s
p re v io u sly published of t h i s s tr u c tu r e .
A practical
a p p ro a c h t o t h e growth o f t h e s e s t r u c t u r e s has been
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
98
d i s c u s s e d w ith r e q u i r e s a s p e c i f i c machine c o n f i g u r a t i o n ,
n a m e l y tw o As s o u r c e s .
C o n s i d e r i n g t h a t m o s t MBE
m a c h i n e s h a v e 8 s o u r c e c a p a b i l i t y , a n d t h a t a d d i t i o n a l As
s o u r c e s i n c r e a s e up t i m e f o r t h e m a c h i n e , t h i s a p p r o a c h
is q u ite p ra c tic a l.
3.4
Im purities
in n-n H e te ro ju n c tio n s
As w i t h many p r o b l e m s i n c r y s t a l g r o w t h , t h e
i m p r o v e m e n t i n m a t e r i a l q u a l i t y u s u a l l y r e v e a l s a new s e t
o f p r o b le m s , w h ile p e rh a p s o f l e s s e r m a g n itu d e , which
t a k e o n new i m p o r t a n c e .
The a b o v e t e c h n o l o g y was a p p l i e d
ce
t o t h e V e r t i c a l FET (VFET) d e v i c e
( s e e a l s o A p p e n d i x C)
a n d i n d i c a t e d c o n s i d e r a b l e d e e p l e v e l b e h a v i o r a t low
tem peratures.
To i d e n t i f y t h e s o u r c e o f t h e d e e p l e v e l s ,
a s e t o f e x p e r i m e n t s was p l a n n e d t o t e s t t h e i n f l u e n c e o f
s u b s t r a t e r e l a t e d im p u ritie s on the dev ice perform ance.
As d i s c u s s e d i n C h a p t e r 1 s u b s t r a t e e f f e c t s h a v e r e c e n t l y
become r e c o g n i z e d a s i m p o r t a n t i n d e v i c e g r o w t h , l a r g e l y
b e c a u s e o f t h e c o n t i n u i n g i m p r o v e m e n t i n MBE m a t e r i a l s .
S u b s t r a t e s c a n be a s o u r c e o f b o t h u n i n t e n t i o n a l
i m p u r i t i e s an d i n t r i n s i c d e f e c t s .
In f a c t , s u b s tr a te s
are a very im portant lim itin g f a c to r in th e co n tin u in g
d e v e l o p m e n t o f GaAs t e c h n o l o g y .
A method t o d e p l e t e s u b s t r a t e i m p u r i t i e s i n v o l v e s
t h e a n n e a l i n g and s u b s e q u e n t r e p o l i s h i n g o f c o n v e r t e d
surface m ate ria l.
S u b s t r a t e s a r e h e a t e d a t 750°C f o r 24
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
99
h o u r s i n p a l l a d i u m d i f f u s e d H2 .
The s u r f a c e s a r e
s u b s e q u e n t l y r e p o l i s h e d u s in g B r o m in e - m e th a n o l, removing
ctr
a p p r o x i m a t e l y 20 m i c r o n s o f m a t e r i a l . '
To t r y t o
i m p r o v e t h e d e v i c e b e h a v i o r , t h i s m e th o d was e m p lo y e d f o r
t h e grow th of t e s t h e t e r o j u n c t i o n s t r u c t u r e s .
The
s t r u c t u r e s were a n a ly z e d u s in g p h o to lu m in e s c e n c e
cap acitance-voltage p ro filin g
Spectroscopy
(PL),
( C - V ) , Deep L e v e l T r a n s i e n t
(DLTS), a nd S e c o n d a r y I o n Mass S p e c t r o s c o p y
(SIMS) .
3 .4 .1
Experim ental
S u b s t r a t e s f o r MBE g r o w t h w e r e p r e p a r e d a s f o l l o w s .
An n+ S i d o p e d B r i d g e m a n grown s u b s t r a t e and an un doped
LEC s u b s t r a t e w e r e c l e a v e d i n h a l f .
H alf of each
s u b s t r a t e was t r e a t e d u s i n g t h e a n n e a l e d / r e p o l i s h c y c l e
d e t a i l e d above.
The o t h e r h a l f was u n t r e a t e d .
For each
g r o w t h , t h e t r e a t e d and u n t r e a t e d s u b s t r a t e s o f e a c h t y p e
w e r e m o u n te d s i d e by s i d e , s o t h a t i d e n t i c a l g r o w t h
c o n d i t i o n s were o b t a i n e d .
The two g r o w t h r u n s o n LEC a n d
n + B r i d g e m a n s u b s t r a t e s w e r e grown o n e a f t e r t h e o t h e r t o
m inim ize machine v a r i a t i o n s .
The r u n n u m b e r s a r e l i s t e d
i n t h e t a b l e below.
The s t r u c t u r e s grown a r e shown i n F i g .
3.4.
The
r u n s w e r e i d e n t i c a l e x c e p t t h a t r u n 1128 had a s l i g h t l y
t h i c k e r AlGaAs b a r r i e r .
The s t r u c t u r e s w ere grown u s i n g
the procedure s p e c ifie d in S ectio n 3 .2 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
100
300 A
4 x 1 0 17
GaAs: Si
2000 A
I017
AIGaAs: Si
5000 A
I016
GaAs: Si
o
5000 A
18
10
GaAs: Si
Substrate
F igure 3.4.
Layer s tr u c tu r e fo r n-n h e te ro ju n c tio n
used in s u b s tr a te im p u rity study.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
101
TABLE 3 . 1
S u b s t r a t e Type
Layer #
S ubstrate
1127 A
LEC A n n e a l e d / r e p o l i s h e d
1127 N
LEC u n t r e a t e d
1128 A
n+ B r i d g e m a n S i d o p e d ,
annealed/repolished
1128 N
3 .4 .2
n+ B r i d g e m a n S i d o p e d , u n t r e a t e d
C apacitance-V oltage P r o f ilin g
I s o l a t e d mesa s t r u c t u r e s w i t h Au S c h o t t k y b a r r i e r s
o n t h e t o p o f t h e mesa w e r e f a b r i c a t e d .
The c a r r i e r
p r o f i l e s a s d e t e r m i n e d by t h e c a p a c i t a n c e p r o f i l i n g
t e c h n i q u e a r e shown i n F i g s .
A /N ).
3.5
(1127A/N) a nd 3 . 6
(1128
The p r o m i n e n t p e a k i n t h e c a r r i e r c o n c e n t r a t i o n
l o c a t e d a t a p p r o x i m a t e l y 0 . 2 3 m i c r o n s r e s u l t s fro m t h e
b e h a v i o r o f t h e 2DEG d u r i n g p r o f i l i n g , and a p p r o x i m a t e l y
in d i c a t e s the i n t e r f a c e .
67
The m o s t i m p o r t a n t d i f f e r e n c e
b e t w e e n t h e f o u r s a m p l e s i s t h a t t h e s t r u c t u r e s grown on
n+ s u b s t r a t e s
(Fig. 3.6)
show a s l i g h t bump o f e l e c t r o n s
j u s t t o t h e GaAs s i d e o f t h e 2DEG.
look q u a l i t a t i v e l y c o r r e c t .
In g e n e r a l , th e C -V 's
To t h e r i g h t o f t h e 2DEG,
t h e n-GaAs r e g i o n s a p p e a r a s t h e g e n t l e v a l l e y i n t h e
ele c tro n concentration.
The s l o p e s r e p r e s e n t t h e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
102
10
O
17
•o 10
(b)
Z
o»
o
0.3
0.5
Distance
0.7
12 7 A
,18
10
(O
I
e
a
I017
(a)
O'
O
I0,6L
0.1
0.3
127 N
Figure 3 .5 .
0.7
0.5
D istance i f i m )
l o g Nd v s . d i s t a n c e f o r l a y e r s gr ow n on LEC
s u b s t r a t e s a s d e t e r m i n e d u s i n g C-V p r o f i l i n g ,
a) L a y e r 1127N; b) l a y e r 1127A.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
103
(O
I
e
T>
(b)
O'
o
0.3
0.5
0.7
Distance (/xm)
I2 8 A
18
10'
10
17
0.3
0.7
0.5
Distance (zxm)
II2 8 N
F ig u re
.6.
Log
v s . d i s t a n c e f o r l a y e r grow n on n +
s u b s t r a t e s a s d e t e r m i n e d u s i n g C-V p r o f i l i n g ,
a) L a y e r 1128N; b)
l a y e r 1128A.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
104
e l e c t r o n Debye t a i l s
rig h t)
(on t h e
67
a nd t h e p e c u l i a r p r o p e r t i e s o f 2DEG p r o f i l i n g
(on t h e l e f t ) .
f ro m t h e n+ b u f f e r r e g i o n
To t h e l e f t o f t h e 2DEG, t h e d e p l e t i o n
r e g i o n i n t h e AlGaAs b a r r i e r a p p e a r s a s a s h a r p d i p i n
the fre e e le c tro n co n cen tratio n .
The t e r m i n a t i o n o f t h e
p r o f i l e before th e su rfa c e r e f l e c t s the zero b ias
d e p le tio n depth.
W hile sm a ll d i f f e r e n c e s in th e p r o f i l e s
a r e p r e s e n t among t h e l a y e r s , t h e s e a r e p r o b a b l y n o t
sig n ifican t.
The s m a l l bump i n t h e e l e c t r o n
c o n c e n t r a t i o n t o t h e r i g h t o f t h e 2DEG p r e s e n t i n t h e
s t r u c t u r e s grown on t h e n+ s u b s t r a t e i s s i g n i f i c a n t .
It
c o u l d be t h e r e s u l t o f an i n c r e a s e d d o n o r c o n c e n t r a t i o n
in the reg io n near the i n t e r f a c e in th e se s tr u c t u r e s .
T h i s c o u l d be t h e r e s u l t o f a u n i n t e n t i o n a l d o n o r s u c h a s
s u l f u r o r t h e movement o f s i l i c o n .
3 .4 .3
P hotolum inescence
P h o t o l u m i n e s c e n c e was u s e d t o e v a l u a t e t h e i m p u r i t y
c o n t e n t i n t h e GaAs and AlGaAs b a r r i e r .
L ittle
d i f f e r e n c e c o u l d be f o u n d b e t w e e n t h e s a m p l e s i n t h e GaAs
sp ectra
(not show n).
The s a m p l e s grown o n LEC m a t e r i a l
showed s l i g h t l y m ore p r o m i n a t e boun d t o bound t r a n s i t i o n s
a nd c a r b o n f r e e t o boun d t r a n s i t i o n s .
These c o u ld both
r e s u l t f ro m an i n c r e a s e d c a r b o n c o n t e n t .
The s o u r c e o f
t h e c a r b o n i s u n c l e a r , a l t h o u g h LEC s u b s t r a t e m a t e r i a l
c a n be a s o u r c e o f o u t d i f f u s i n g c a r b o n .
not s p a tia lly s e n sitiv e ,
49
S i n c e PL i s
i t i s d i f f i c u l t to determ ine
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
105
f ro m w h i c h l a y e r t h e s p e c t r a o r i g i n a t e s .
The d e e p
a c c e p t o r r e g i o n o f GaAs was n o t e x a m i n e d .
The m o s t s i g n i f i c a n t d i f f e r e n c e among t h e s t r u c t u r e s
a p p e a r s i n t h e AlGaAs p h o t o l u m i n e s c e n c e .
in P igs.
3.7
(1127A/N) a nd 3 . 8
(1128A/N).
T h i s i s shown
It
is
i m p o r t a n t t o n o t e t h a t t h i s i s e m i s s i o n f ro m o n l y 2000 8
o f p a r t i a l l y o r f u l l y d e p l e t e d m a t e r i a l , t h u s i t would be
s u r p r i s i n g t o s e e any e x c i t o n s t r u c t u r e b e c a u s e of th e
lo cal fie ld s p resen t.
The a p p e a r a n c e o f t h e s c a n s i s
c o n s i d e r a b l y d i f f e r e n t f ro m t h o s e shown i n C h a p t e r 2
(P ig.
2.19).
There a re s e v e ra l p o in ts t o n o te .
there is l i t t l e
F irst,
d i f f e r e n c e betw een th e
a n n e a l e d / r e p o l i s h e d and u n t r e a t e d LEC AlGaAs
(F ig.
3.7)
lum inescence ex cep t i n t e n s i t y .
S e c o n d , b o t h t h e n+
Bridgeman s u b s t r a t e s
are co n sid erab ly
(P ig.
3.8)
d i f f e r e n t , s h o w i n g s e v e r a l p e a k s n o t p r e s e n t i n t h e LEC
m aterial.
T h i s s u g g e s t s a l a r g e r i m p u r i t y c o n t e n t and
the p resen ce of s e v e ra l types of im p u r itie s .
The t h i r d
p o in t i s t h a t th e im p u rity c o n te n t using the an n ealin g
p r o c e d u r e a p p e a r s t o be c o n s i d e r a b l y r e d u c e d , o n t h e
o rd e r of a f a c t o r of 5 (Fig.
3 .4 .4
3 .8 (b )).
S e c o n d a r y I o n Mass S p e c t r o s c o p y
S e c o n d a r y I o n Mass S p e c t r o s c o p y
(SIMS) was u s e d t o
t r y to determ in e th e types of im p u r itie s p r e s e n t.
SIMS
r e q u ire s g re a t c a re in i n t e r p r e t a t i o n as a consequence of
a r t i f a c t s , and i n h e t e r o j u n t i o n s t r u c t u r e s th e s i t u a t i o n
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
106
.76
1127 A
3.3 X
25 K
units)
(b)
3.3 X
(arbitrary
4K
intensity
.76
PL
27 N
1X
25K
1X
4K
Energy (eV)
F igure
.1 .
AlGaAs PL s p e c t r a o f t h e n - n h e t e r o j u n c t i o n
grown o n LEC s u b s t r a t e s ,
b) 1127A.
a)
1127N;
2
Pump p o w e r i s lOW/cm .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(a)
10 7
1.75
28 A
3.3X
.80
25K
W
c
3.3 X
ow
X)
w
O
0>
c
4K
1128 N
c
.80
1.62
_J
CL
25 K
4 K
Energy
Figure 3 .8.
(eV)
AlGaAs PL s p e c t r a o f t h e n - n h e t e r o j u n c t i o n
g r o w n on n + s u b s t r a t e s ,
1128A.
a)
1128N, b)
2
Pump p o w e r i s lOW/cm .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
108
i s c o n s i d e r a b l y worse b ec a u se of m a t r i x a f f e c t s , and th e
p r e s e n c e o f alu minum h y d r i d e s
w h i c h mask t h e S i
sig n als.
SIMS - Donor I m p u r i t i e s
E p i t a x i a l l a y e r s from a n n e a l e d and u n a n n e a l e d LEC
and n+ GaAs s u b s t r a t e s w e r e e x a m i n e d a t b o t h s l o w a n d
f a s t sp u tterin g ra te s.
The s l o w s p u t t e r i n g r a t e a n a l y s i s
was d o n e f o r o n l y t h e t o p 300 8 GaAs l a y e r a nd AlGaAs
lay er.
T h e s e r e s u l t s w e r e r u n u s i n g a 200 nA (25 nA f o r
t h e s l o w s p u t t e r ) , 10 keV Cs + p r i m a r y beam t o e x a m i n e
donor im p u r itie s .
A l l p r o f i l e s were n o rm a liz e d t o a
common m a t r i x i o n i n t e n s i t y i n o r d e r t o a c c o u n t f o r a ny
d i f f e r e n c e s i n t r a n s m i s s i o n e f f i c i e n c y f ro m s a m p l e t o
sam ple.
The d e p t h o f e a c h p r o f i l e was i n d i v i d u a l l y
m easured u s in g a s u r f a c e
pro filo m eter.
One o f t h e d i f f i c u l t i e s
in comparing im p u r ity
p r o f i l e s b e t w e e n t h e AlGaAs a nd GaAs l a y e r s i s t h a t
d i f f e r e n c e s in s p u t t e r i n g r a t e s can a f f e c t th e r e l a t i v e
im purity c o n c e n tra tio n s.
Some i m p u r i t i e s may show a
c h a n g e a t t h e i n t e r f a c e b e t w e e n t h e two compounds w hich
i s o n ly r e l a t e d to a change i n s p u t t e r i n g r a t e s , r a t h e r
t h a n an a c t u a l c o n c e n t r a t i o n c h a n g e .
A d i f f e r e n c e in s p u t t e r i n g r a t e can a ls o acco u n t f o r
some t h e c h a n g e s i n o x y g e n and c a r b o n s i g n a l s .
The
r e a s o n f o r t h i s i s a l o w e r s p u t t e r i n g r a t e w h i c h mea ns a
h i g h e r b a c k g r o u n d f o r any o f t h e r e a c t i v e g a s e s s i n c e t h e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
109
r a t e o f rem oval i s low er and t h e i r s t i c k i n g c o e f f i c i e n t s
are nearly u n ity .
The s l o w s p u t t e r i n g SIMS r e s u l t s
1128A/N a r e shown i n P i g s .
f o r 1127A/N an d
3 . 9 a nd 3 . 1 0 r e s p e c t i v e l y .
In
a l l s c a n s , ch a n g e s i n th e c a rb o n and oxygen io n
i n t e n s i t i e s c a n be o b s e r v e d a t t h e AlGaAs-GaAs i n t e r f a c e
a t a d e p t h o f 200 S .
real.
As n o t e d a b o v e , t h e s e may n o t be
On t h e LEC s u b s t r a t e s , t h e o x y g e n , c a r b o n , a n d
s u l f u r s i g n a l s a r e m o d e s t l y l o w e r on t h e n o n a n n e a l e d
su b strate s
(1127N F i g .
3.9b)
as compared t o t h e a n n e a le d
su b strate s
(1127A F i g .
3.9a)
b u t may n o t be s i g n i f i c a n t .
T he l a y e r s grown on n+ s u b s t r a t e s show l i t t l e
d ifferen ce
b e t w e e n t h e c a r b o n , s u l f u r , a n d o x y g e n a s shown i n F i g .
3 . 1 0 b f o r 1128N a n d F i g .
3 . 1 0 a f o r 1128A.
In a l l four
sam ples, th e s i l i c o n s ig n a ls look very s im i l a r .
t h e s i l i c o n look very s i m i l a r .
Because
Because th e s i l i c o n
s i g n a l t r a c k s t h e al uminum s i g n a l ,
it
a p p e a r s t h a t an
alu m inum h y d r i d e may be c o n t r i b u t i n g t o t h e s i l i c o n
sig n al.
A m o d e s t r i s e i n t h e s i l i c o n s i g n a l c a n be
o b s e r v e d a t t h e AlGaAs-GaAs i n t e r f a c e a t a d e p t h o f 2000
8.
As f a r a s t h e s l o w s p u t t e r i n g r e s u l t s g o , o n e i s
u n a b l e t o s a y w h e t h e r t h e s i l i c o n h a s d i f f u s e d f ro m t h e
AlGaAs l a y e r t o t h e s u r f a c e o r f ro m e i t h e r t h e LEC o r n+
su b strates.
At t h i s e n e rg y
(lOkeV) i t
is u n lik ely th a t
t h e s p u t t e r i n g r a t e has r e a c h e d e q u i l i b r i u m a t 3008.
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
m
N
cn
10
10
10
10
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c
3O
10
■u
c
3
oO
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cn
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iz
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10-4
1
z
IU
Iz
10
... G a
10
z
z
o
o
A1
10
10
10
10
Si
0
800
1000
1800
20 0 0
Si
0
2800
DEPTH (Angstrom s)
1800
20 0 0
(b)
beam o f n - n h e t e r o j u n c t i o n s g r o w n
s
a) 1127A , b) 1127N .
110
S lo w s p u t t e r i n g SIMS s c a n s u s i n g C
on LEC s u b s t r a t e s ,
1000
DEPTH (Angstrom s)
(a )
F igure 3 .9 .
800
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
m
c
3
o
O
>t■p
10
10
10
10
+m
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c
3
o
cj
10
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10
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cn
z
cn
z
Ga
10
UJ
10
10
z
z
o
o
H
10
10
A1
10
A1
10
SI
Si
10
10
0
BOO
1S00
1000
D EPTH
2000
0
2B00
2500
(A n g s tro m s )
a)
s
bea m o f n - n h e t e r o j u n c t i o n s g r o w n on
1128A , b) 1128N .
Ill
su b stra tes.
20 0 0
(b)
S l o w s p u t t e r i n g SIMS u s i n g C
n
1B00
1000
D EPTH
(A n g s tro m s )
(a )
F ig u re 3.10.
BOO
112
T h e r e f o r e , a n y d i f f e r e n c e s w i t h i n t h e f i r s t few h u n d r e d
a n g s t r o m s a r e l i k e l y t o be f o r t u i t o u s .
The f a s t s p u t t e r i n g SIMS s c a n s u s i n g t h e C* beam a r e
w
shown i n P i g . 3 . 1 1 f o r 1127 a nd F i g .
3.12 f o r 1128.
For
t h e s a m p l e s grown o n LEC s u b s t r a t e s , t h e o x y g e n , c a r b o n ,
and s u l f u r s i g n a l s a r e m o d e r a t e l y low er on t h e l a y e r s
grown on t h e a n n e a l e d s u b s t r a t e s .
The a c c u m u l a t i o n s a t
th e s u b s tra te e p ila y e r in te r f a c e are a lso reduced.
t h e l a y e r s grown o n n+ s u b s t r a t e s ,
For
th e oxygen, c arb o n ,
a n d s u l f u r c o n c e n t r a i t o n s i n t h e e p i l a y e r and a t t h e
i n t e r f a c e a r e n e a r l y t h e same f o r t h e a n n e a l e d a n d non
annealed s u b s t r a te s .
I n t h e n “ r e g i o n a t 0 . 5 pm, t h e
s i l i c o n c o n c e n tra tio n appears d i f f e r e n t but i s l i k e l y to
b e an a r t i f a c t .
SIMS - A c c e p t o r I m p u r i t i e s
E p i t a x i a l l a y e r s f r o m a n n e a l e d and u n a n n e a l e d LEC
a n d n+ GaAs s u b s t r a t e s w e r e e x a m i n e d a t b o t h f a s t and
slow s p u t t e r i n g r a t e s .
These r e s u l t s were done u s in g a 1
pA (200 nA f o r t h e s l o w s p u t t e r ) , 15 keV
p r i m a r y beam
a nd p o s i t i v e s e c o n d a r y i o n s p e c t r o m e t r y t o e x a m i n e
acceptor im p u ritie s .
The s lo w s p u t t e r i n g SIMS r e s u l t s u s i n g t h e
a r e shown i n F i g .
1128 A/N.
beam
3 . 1 3 f o r 1127 AN a nd F i g u r e 3 . 1 4 f o r
S e v e r a l d i f f e r e n c e s among t h e s c a n s c a n be
s e e n . B etw e en t h e a n n e a l e d and non a n n e a l e d LEC
s u b s tr a te s , Fig.
3 . 1 3 b a nd F i g .
3.13a r e s p e c t i v e l y , th e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
10
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10
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rz
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10
10
10
10
10
10
0.00
0 .6 0
1.00
l.BO
a.oo
DEPTH (M icrons)
( a)
0.00
o.so
l.BO
1 .00
a . bo
DEPTH (M icro n s)
(b)
F a s t s p u t t e r i n g SIMS s c a n s u s i n g C+ beam o f n - n h e t e r o j u n c t i o n s g r o w n
s
on LEC s u b s t r a t e s .
a) 1127A , b) 1127N .
113
F ig u re 3.11.
S.BO
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
10
10
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3
uO
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H-
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10
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■p
c
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iz
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10
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□
10
10
10
10
10
10
0.00
O.BO
1 .0 0
l.BO
2 .0 0
2. BO
0 .00
DEPTH (M icro n s)
l.BO
2.00
(b )
F a s t s p u tte r in g scans using
on n
1.00
DEPTH (M icro n s)
(a )
F ig u re 3.12.
O.BO
su b stra tes,
a)
1128N , b)
bea m o f n - n h e t e r o j u n c t i o n s g r o w n
1128A.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
10
As
As
\cn
01
-p
c
3
O
u
10
10
cn
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uj
01
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f-l
10
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10
10
z
10
o
Cr
10
10
Fe
A1
Mg
10
A1
10
0
BOO
1200
D EPTH
1800
2400
0
3000
800
(a)
F ig u r e 3.13.
24 0 0
DEPTH
(A n g s tro m s )
3000
(A n g s tro m s )
(b)
S lo w s p u t t e r i n g SIMS s c a n s u s i n g C>2 beam o f n - n h e t e r o j u n c t i o n s
g r o w n o n LEC s u b s t r a t e s .
a)
1127N, b)
1127A.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
10
B
As
AS
n
10
10
4
«
+m
»
c
3
o
C
J
-p
c
3
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u
108
10
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U
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z
U
l
H
Z
M
in
z
111
10
10
Cr
Fe
8
Cr
Mn
10
10
i
B1
Mg
10
10
0
BOO
1200
D EPTH
iboo
24 0 0
A1
o
0
3000
DEPTH
(a )
F ig u re 3.14.
1600
BOO
(A n g s tro m s )
2400
3000
(A n g s tro m s )
(b)
S l o w s p u t t e r i n g SIMS s c a n s u s i n g 0 * bea m o f n - n h e t e r o j u n c t i o n s
grown o n n
su b stra tes,
a)
1128N , b) 1128A .
117
s u r f a c e a c c u m u l a t i o n o f Mg i s a p p a r e n t l y r e d u c e d on t h e
s u r f a c e o f t h e f i l m grown on a n n e a l e d LEC s u b s t r a t e s .
The Mn c o n c e n t r a t i o n i n AlGaAs l a y e r i s a l s o r e d u c e d ,
w h i l e t h e C r , F e , a nd B c o n c e n t r a t i o n s a r e u n a f f e c t e d by
t h e a n n e a l i n g p r o c e d u r e p r i o r t o g r o w t h f o r t h e LEC
su b strates.
On t h e n+ s u b s t r a t e s ,
t o be r e v e r s e d fro m e x p e c t a t i o n s .
t h e r e s u l t s w o u ld seem
The C r , F e , Mn, a nd Mg
c o n c e n t r a t i o n s a r e h i g h e r s i g n i f i c a n t l y i n t h e AlGaAs
l a y e r on t h e a n n e a l e d s u b s t r a t e , F i g .
3 . 1 4 b , a s compared
w i t h t h e non a n n e a l e d s u b s t r a t e , P i g . 3 . 1 4 a .
n + and LEC non a n n e a l e d s u b s t r a t e s ,
t h e s u r f a c e Cr a nd Pe
l e v e l s a p p e a r h i g h e r on t h e n+ , P i g .
F ig.
B e tw e e n t h e
3 . 1 4 a , t h a n t h e LEC,
3 . 1 3 a , w h i l e t h e Mg s u r f a c e a c c u m u l a t i o n i s j u s t t h e
o p p o site.
The f a s t s p u t t t e r i n g SIMS s c a n s u s i n g t h e 0+ beam
a r e shown i n F i g .
3 . 1 5 f o r 1127 a n d P i g .
3 .1 6 f o r 1128.
D i f f e r e n c e s c a n be s e e n b e t w e e n t h e i m p u r i t y
c o n c e n tr a tio n s in the s u b s t r a t e s .
F igs.
The LEC s u b s t r a t e s ,
3 . 1 5 a , b show much h i g h e r B and Cr t h a n t h e n+
s u b s tra te s , P igs.
3 . 1 6 a , b . The h i g h e r b o r o n c o n c e n t r a t i o n
c a n be a t t r i b u t e d
t o t h e b o r o n o x i d e w h i c h i s u s e d a s an
e n c a p s u l a n t i n t h e LEC m e t h o d .
The Cr may h a v e b e e n an
u n in te n tio n a l im p u rity in th e boule p u l l e r .
LEC s u b s t r a t e s ,
a n n ealin g has reduced th e i n t e r f a c e
a c c u m u l a t i o n a s shown i n F i g .
w ith Fig.
Between th e
3 . 1 5 a f o r 1127N.
3 . 1 5 b f o r 1127A a s c o m p a r e d
The C r , F e , Mn, Mg
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
10
r \.
As
As
10
10
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hH-
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0z1
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10
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o
10
10
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8
A# °
10
10
Fe
10
10
Mg
Mn
10
10
o.oo
o . bo
i . oo
i . bo
a.oo
a . bo
Ai___
0.00
O.BO
DEPTH (M icrons)
F ig u re 3 .15.
1.00
l.BO
a.00
DEPTH (M icrons)
(a)
(b )
F a s t s p u t t e r i n g s c a n s u s i n g O , beam o f n - n h e t e r o j u n c t i o n s
grown on LEC s u b s t r a t e s .
a) 1127N , b) 1127A .
a . bo
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
10
As
10
10
m
«
N.
n
-p
c
3
O
CJ
>
I-M
(It
z
Ui
s
c
3
o
10
a
10
at
10
z
UJ
Jz
M
Cr
z
z
o
a
10
10
10
Fe
Mg
10
10 ’
10
10
0.00
O.BO
l.B O
1.00
DEPTH
o .o o
S .00
o . bo
(M ic ro n s )
(a)
F ig u re 3.16.
i
. bo
(M ic ro n s )
(b)
F a s t s p u tte rin g scans u sing
grown on n+ s u b s t r a t e s ,
i.o o
DEPTH
a)
beam o f n - n h e t e r o j u n c t i o n s
1128 N, b)
1128A.
a .o o
a . bo
120
c o n c e n t r a t i o n s seem s i m i l a r i n t h e e p i l a y e r s on t h e LEC.
On t h e n o n a n n e a l e d s u b s t r a t e ,
t h e r e i s a boron
a c c u m u l a t i o n r e g i o n i n t h e n~ GaAs n e a r t h e AlGaAs-GaAs
i n t e r f a c e a t a d e p t h o f 0 . 5 ym ( F i g . 3 . 1 5 a ) w h i c h i s
a b s e n t on t h e a n n e a le d s u b s t r a t e s
(Fig. 3 .1 5 b ).
A lso
n o t e t h e c h a n g e i n t h e As+ s i g n a l on a l l f o u r s c a n s a t
t h e AlGaAs-GaAs i n t e r f a c e , F i g s .
3 . 1 5 a , b and F i g s .
3 .1 6 a,b .
fro m a c h a n g e i n t h e
T h iis probably r e s u l t s
sp u tterin g r a te ,
and c a n e x p l a i n some o f t h e c h a n g e s i n
the o th er im p u ritie s a t th a t in te r f a c e .
On t h e n + s u b s t r a t e s , t h e C r , F e , a n d Mg a r e
m o d estly lower in th e e p i l a y e r s o f th e an n ealed
s u b s tr a te s , Fig.
s u b s tra te s F ig.
3 . 1 6 b , a s c o m a p r e d w i t h t h e non a n n e a l e d
3.16a w hile th e i n t e r f a c e accum ulations
o f Cr a n d Fe a p p e a r t o be s l i g h t l y h i g h e r .
The s u r f a c e
a c c u m u l a t i o n s o f Mg, C r , a n d Mn a p p e a r t o be s l i g h t l y
h ig h e r on t h e no n an n ealed s u b s t r a t e s , F i g .
3.16a.
This
i s t h e o p p o s i t e o f w h at was o b s e r v e d i n t h e s l o w s c a n
case, F ig.
3.14b.
The SIMS d o e s n o t seem t o p r o v i d e a n y c l e a r
i d e n t i f i c a t i o n o f t h e i m p u r i t i e s which a r e o b s e rv e d i n
t h e C-V o r t h e AlGaAs p h o t o l u m i n e s c e n c e .
On t h e n+
s u b s t r a t e s , no i m p u r i t i e s i n t h e n-GaAs r e g i o n n e x t t o
t h e AlGaAs b a r r i e r c a n be f o u n d w h ic h c o u l d be c a u s i n g
t h e bump i n t h e e l e c t r o n d i s t r i b u t i o n , a l t h o u g h m o d e s t S i
tails
fro m t h e AlGaAs r e g i o n c a n be s e e n .
The o n l y
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121
e v i d e n c e f o r i m p u r i t i e s i n t h e b a r r i e r a s o b s e r v e d i n PL
is P ig.
3 . 1 6 a , w h e r e t h e Mg, C r , a n d Mn w e r e s l i g h t l y
h ig h e r, although F ig .
3.14b i s
in c o n f l i c t w ith t h i s
data.
3 .4 .5
Deep L e v e l T r a n s i e n t S p e c t r o s c o p y
S in c e s e v e r a l d i f f e r e n c e s were o b s e rv e d i n
p h o t o l u m i n e s c e n c e a nd C - V ' s , we e x p e c t e d d i f f e r e n c e s
t h e DLTs s p e c t r a .
T h i s was n o t t h e c a s e h o w e v e r .
in
In
f a c t , a l l t h e s t r u c t u r e s showed n e a r l y t h e same s a m p l e
sp ectra.
A r e p r e s e n t a t i v e s c a n i s shown i n F i g .
3.17.
The b e h a v i o r o f a l l t h e s a m p l e s was d o m i n a t e d by t h e d e e p
do nor/D X c e n t e r b e h a v i o r i n t h e AlGaAs b a r r i e r r e g i o n .
T h is i s i n f a c t th e main problem i n th e s t r u c t u r e .
While
th e m easurem ents i n S e c ti o n s 3 . 3 .2 t o 3 . 3 . 4 d e m o n stra te
v a r ia tio n s in im purity c o n te n t are p r e s e n t, th e se are not
determ ining i t s deep le v e l behavior.
3.5
Summary o f n - n H e t e r o i u n c t i o n R e s u l t s
J u n c t i o n s e x h i b i t i n g high q u a l i t y r e c t i f i c a t i o n have
been r e a l i z e d u s in g gro w th t e c h n i q u e s which o p t i m i z e th e
m a t e r i a l q u a l i t y i n t h e AlGaAs b a r r i e r .
The j u n c t i o n s
e x h i b i t c o n s i d e r a b l e d e e p l e v e l b e h a v i o r w h ic h i s r e l a t e d
t o t h e d e e p do nor/D X c e n t e r .
W h i l e my s t u d i e s show t h a t
s u b s t r a t e i m p u r i t i e s e f f e c t t h e e l e c t r i c a l and o p t i c a l
p r o p e r tie s of the b a r r i e r s ,
they are not th e lim itin g
f a c t o r in t h e i r perform ance a t t h i s tim e.
F u r t h e r work
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ll-
cs.
in
E
oCVJ
o
in
E
a>
o
c
a
o
o
Q.
O
U
c
a>
o*
c
o
JZ
O
50
F ig u re 3.17.
150
250
Temperature
450
350
(K)
DLTS s c a n o f AlGaAs b a r r i e r o f t h e n - n h e t e r o j u n c t i o n g r o w n o n a
n o n a n n e a l e d LEC s u b s t r a t e
(1127N).
Trace
(a)
i s e l e c t r i c a l DLTS
a n d i n d i c a t e s e l e c t r o n e m i s s i o n f r o m t h e DX c e n t e r .
o p t i c a l DLTS.
The s t e e p s l o p e o n t r a c e
Trace
(b) i s
(b) f r o m 80K t o 150K i s
122
e v i d e n c e o f e l e c t r o n r e c a p t u r e b y t h e DX c e n t e r .
123
r e l a t e d t o t h i s problem s h o u ld f o c u s on t h e p r o p e r t i e s of
S i d o p e d AlGaAs.
The p r e s e n c e o f t h e h e t e r o j u n c t i o n p r o d u c e s l o c a l
e l e c t r o s t a t i c f i e l d s which a r e p r e s e n t d u r in g growth
after
t h e AlGaAs l a y e r h a s b e e n d e p o s i t e d .
These f i e l d s
and t h e ch an g in g c h e m ic a l a c t i v i t i e s a t t h e i n t e r f a c e
m ight cause im p u rity r e d i s t r i b u t i o n d u rin g grow th,
d e p e n d i n g on t h e m a g n i t u d e o f t h e d i f f u s i o n c o e f f i c i e n t s .
In th e near s u rfa c e re g io n , im p u rity accum ulation la y e rs
/ gg \
h a v e b e e n shown t o r e s u l t f o r m e l e c t r o s t a t i c e f f e c t s '
a s s o c i a t e d w i t h a d i f f e r e n c e i n s u r f a c e a nd b u l k F e r m i
le v e ls.
P alm ateeru sed
an e q u i l i b r u m e l e c t r o s t a t i c
argument to d e s c r ib e th e observed im p u rity accum ulation
i n n e a r s u r f a c e r e g i o n o f GaAs.
I w i l l em p l o y t h i s
a p p ro a c h i n c o n j u n c t i o n w ith an e x p r e s s i o n f o r ch a rg e d
i m p u r i t y movement i n t h e p r e s e n c e o f c o n c e n t r a t i o n
g r a d i e n t s and e l e c t r i c f i e l d s
t o a n a l y z e some o f t h e
b e h a v i o r o f s e g r e g a t i o n on t h e s u r f a c e and n e a r s u r f a c e
region.
A b r i e f d is c u s s io n of the im p u rity d i s t r i b u t i o n
in h e te r o ju n c tio n s fo llow s in o rd er t o see i f Si
r e d is tr ib u tio n is occurring.
3.6
D i s t r i b u t i o n o f I m p u r i t i e s i n an E l e c t r o s t a t i c
P o ten tial
The rm o d y n a m ic e q u i l i b r i u m f o r c h a r g e d i m p u r i t i e s i n
t h e p r e s e n c e o f c o n c e n t r a t i o n g r a d i e n t s a nd e l e c t r i c
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
124
fie ld s
i s a t t a i n e d when t h e g r a d i e n t o f t h e
electrochem ical p o te n tia l is
zero
99
Vn = 7 ( U + q i 4*) = 0
n = electro ch em ical p o te n tia l
]X = c h e m i c a l p o t e n t i a l
= charge of im p arity s p e c ie s .
<{> = e l e c t r o s t a t i c p o t e n t i a l
I f we s u b s t i t u t e H e n r y s
48
la w f o r
U an d s p e c i a l i z e
t o t h e one d im e n s io n a l c a s e
- | j [UQ + kT I n YX + g i <W * 0
where
(3.9)
uQ i s t h e c h e m i c a l p o t e n t i a l o f t h e s p e c i e s i i n
the stan d ard s t a t e ,
and X i s t h e m o le f r a c t i o n o f t h e
im p u rity s p e c ie s , then:
(3.10)
m u l t i p l y i n g by dx and i n t e g r a t i n g •
to y2X2
k T /
+q
*1Tl x2
$2
/
\
d 4> = 0
(3.11)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
125
(3.12)
Y2x 2
—
77—
^ 1 ^ 2
=
" * l ) / kT
(3.13)
e
Y l Xl
The p r e v i o u s d i s c u s s i o n o f t h e e q u i l i b r i u m i m p u r i t y
d i s t r i b u t i o n in th e p resen ce of e l e c t r o s t a t i c f i e l d s
s u g g e s t an e v a l u a t i o n o f t h e a b o v e SIMS m e a s u r e m e n t s f o r
th e e f f e c ts of im purity r e d is t r ib u ti o n .
A s u b t l e and
unknown p o i n t i s t h e a c t i v i t y c o e f f i c i e n t d i f f e r e n c e
b e t w e e n GaAs a nd AlGaAs.
One c a n i m a g i n e t h a t
e q u i l i b r i u m w o u ld be d i f f i c u l t t o d e t e r m i n e i n s t r u c t u r e s
of c o m p lic a te d doping i n th e p re s e n c e of h e t e r o j u n c t i o n s .
S till,
i t i s tem p tin g to se e i f n - ty p e i m p u r i t i e s tend to
a c c u m u l a t e i n t h e d e p l e t i o n r e g i o n i n t h e AlGaAs a d j a c e n t
to the h e te ro ju n c tio n .
I f the d i f f u s i o n p r o f i l e reaches
eq u ilib riu m , then th e in c re a se in im purity co n cen tratio n
w o u ld be d e s c r i b e d by Eq.
(3.1 3 ).
Tak e f o r e x a m p le a
b a r r i e r a t t h e h e t e r o i n t e r f a c e o f 150 meV, a t 680°C w here
kT/q -
82 meV, t h e n
=
e +150/82
=
6 .2
(3.14)
Xb u l k AlGaAs
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126
An e x a m i n a t i o n o f t h e SIMS ( P i g s .
3 . 1 1 a nd 3 . 1 2 )
sh ows o n l y a m o d e s t r i s e i n t h e s i l i c o n p r o f i l e ,
and
s u g g e s t s t h a t r e d i s t r i b u t i o n o f S i d ue t o e l e c t r o s t a t i c
f i e l d s d u r i n g g r o w t h e v e n a t h i g h t e m p e r a t r u e s i s n o t an
im portant e f f e c t .
A more i m p o r t a n t e f f e c t c o u l d r e s u l t f ro m a
d i f f e r e n c e i n a c t i v i t y c o e f f i c i e n t s b e t w e e n AlGaAs an d
GaAs.
In C hapter 4, i t
i s suggested th a t the a c t i v i t y
c o e f f i c i e n t o f s i l i c o n i n AlGaAs i s h i g h e r t h a n t h a t o f
GaAs.
T h i s means t h a t a l o w e r c o n c e n t r a t i o n o f S i i n
AlGaAs would be i n e q u i l i b r i u m w i t h a h i g h e r
c o n c e n t r a t i o n o f S i i n GaAs.
I t is possible th at a
h e t e r o j u n c t i o n S i w o u ld move fro m t h e AlGaAs t o t h e GaAs
to reach e q u ilib riu m .
The a c t i v i t y c o e f f i c i e n t
d i f f e r e n c e i s t h e d i s c o n t i n u i t y t h a t a s i l i c o n atom
e x p e r i e n c e s , r a t h e r t h a n t h e p o t e n t i a l s t e p c a u s e d by t h e
c o n d u c t i o n ban d d i s c o n t i n u i t y , w h i c h i s s t r i c t l y
e le c tro n ic in nature.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 4
GaAs-AlGaAs I n t e r f a c e G r o w th
In th e p re v io u s c h a p t e r , th e im portance of high
q u a l i t y g r o w t h c o n d i t i o n s f o r n - n h e t e r o j u n c t i o n s was
dem onstrated.
The d i s c u s s i o n was m a i n l y c o n c e r n e d w i t h
th e q u a l i t y of the bulk m a t e r i a l .
The p r o b l e m a t t h e
i n t e r f a c e was o n l y b r i e f l y a l l u d e d t o .
This c h a p te r w i l l
f o c u s on t h e g r o w t h o f GaAs on AlGaAs.
The u l t i m a t e
a p p l i c a t i o n of th e growth te c h n o lo g y i s d i r e c t e d tow ards
t h e r e a l i z a t i o n o f s i n g l e quantum w e l l m o d u l a t i o n doped
fie ld effect tra n sisto rs.
I n C h a p te r 1, t h e e le m e n ts o f th e problem o f th e
g r o w t h o f GaAs o n AlGaAs w e r e d e t a i l e d .
called
B r i e f l y , th e so
i n v e r t e d m o d u l a t i o n d o p e d GaAs on A l G a A s : S i
s t r u c t u r e was shown t o e x h i b i t p o o r e l e c t r o n m o b i l i t i e s
compared t o t h e r e g u l a r m o d u la tio n doped s t r u c t u r e .
The
f a i l u r e was a t t r i b u t e d t o two p o s s i b l e m e c h a n is m s
i)
L a ttic e s t r a i n a t the in te r f a c e in tro d u cin g a la rg e
c o n c e n t r a t i o n o f d e f e c t s and d i s l o c a t i o n s .
The low
m o b ility r e f l e c t e d the poor c r y s t a l q u a l i t y .
ii)
Rough AlGaAs g r o w t h .
Under some c o n d i t i o n s , AlGaAs
e x h i b i t s a m a c ro s c o p ic s u r f a c e r o u g h n e s s which i s
d e p e n d e n t on g r o w t h c o n d i t i o n s .
The s u r f a c e
r o u g h n e s s o f t h e MBE g r o w t h i s f r o z e n i n a s GaAs
127
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
128
g r o w t h b e g i n s , and a p p e a r s a s a m e t a l l u r g i c a l
in te r f a c e roughness.
P o o r m o b i l i t y r e s u l t s f ro m
increased in te rf a c e s c a tte r in g .
At t h e tim e t h a t t h e e x p e r i m e n t a l f a c t s o f t h e
i n v e r t e d m o d u l a t i o n d o ped s t r u c t u r e w e r e b e c o m in g known,
p o o r b e h a v i o r o f und o p ed s i n g l e q u a n tu m w e l l s t r u c t u r e s
was o b s e r v e d .
Poor o p t i c a l p r o p e r t i e s were n o t e d ,
s p e c i f i c a l l y l a r g e i m p u r i t y r e l a t e d l u m i n e s c e n c e and
b r o a d e x c i t o n l i n e w i d t h s , t h o u g h t t o be an i n d i c a t i o n o f
poor i n t e r f a c e sm oothness.
The f a i l u r e o f t h e s e
s t r u c t u r e s was a t t r i b u t e d t o c a r b o n r i d i n g t h e g r o w t h
surface,
i n h i b i t i n g sm o o th p l a n a r g r o w t h .
As GaAs g r o w t h
b e g a n , t h e c a r b o n i n c o r p o r a t e d a nd t h e i n t e r f a c e
r o u g h n e s s was f r o z e n i n , a c c o u n t i n g f o r t h e o p t i c a l
behavior.
H i s t o r i c a l l y , t h e f a i l u r e o f t h e s e s t r u c t u r e s was
t h o u g h t t o be t h e c o n s e q u e n c e o f t h e same m e c h a n is m ,
a l t h o u g h v a r i o u s r e s e a r c h e r s d i s a g r e e d as t o what t h a t
m ech an is m w a s .
in h ib ited its
T his li n k a g e o f problem s p ro b a b ly
understanding.
I n t h i s c h a p t e r i t w i l l be
shown t h e r e i s a c t u a l l y two p r o b l e m s .
The f i r s t p r o b l e m
i s o n e o f g r o w i n g a s m o o t h h i g h q u a l i t y AlGaAs i n t e r f a c e .
The i n t e r f a c e s t r u c t u r e i s s t r o n g l y d e p e n d e n t on t h e
s m o o t h n e s s o f t h e AlGaAs d u r i n g g r o w t h .
The s e c o n d
p r o b l e m i s g r o w i n g p u r e GaAs o n S i d o p e d AlGaAs.
The
r e a l f a i l u r e o f t h e i n v e r t e d m o d u l a t i o n do ped
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129
stru ctu re
i s th e r e d i s t r i b u t i o n of the s i l i c o n dopant
d u rin g grow th.
The s i l i c o n bec omes d i s t r i b u t e d
in the
GaAs r e g i o n n e x t t o t h e i n t e r f a c e a nd c a u s e s i n c r e a s e d
im purity s c a tte r in g .
I n te r f a c e q u a l i t y in both c a ses
a p p e a r s t o be u n a f f e c t e d m e t a l l u r g i c a l l y by t h e p r e s e n c e
of im p u rities.
The b e s t q u a l i t y o f t h e u n d o p e d s t r u c t u r e i s
o b t a i n e d by g r o w t h a t low V : I I I f l u x r a t i o s .
To
a p p r e c i a t e th e s i g n i f i c a n c e of t h i s as w e ll as to
u n d e rsta n d th e im p lic a tio n s f o r grow th,
I w ill f i r s t
d i s c u s s th e u n u s u a l e x p e r im e n ta l b e h a v io r of th e growth
s t o i c h i o m e t r y o f t h i n GaAs l a y e r s on AlGaAs.
4.1
G rowth o f I n t e r f a c e s a t Low F l u x R a t i o s
The m a t e r i a l q u a l i t y o f b o t h GaAs a nd AlGaAs i s
g e n e r a l l y found t o improve as f l u x r a t i o s a r e r e d u c e d , a s
long as a m etal r ic h s u r f a c e r e c o n s t r u c t i o n does not
resu lt.
One w o u ld e x p e c t t h a t t h i s w o u ld a l s o be
i m p o r ta n t f o r i n t e r f a c e growth a s w e l l .
q u e s t i o n w h ic h a r i s e s
The i n t e r e s t i n g
i s what happens t o th e growth
sto ich io m etry a t the in te r f a c e .
As n o t e d e a r l i e r i n t h e
c h a p t e r , a t high s u b s t r a t e te m p e ra tu re s
(68Q-700°C) t h e
f l u x r a t i o r e q u i r e m e n t s f o r AlGaAs a nd GaAs g r o w t h a r e
co n sid erab ly d if f e r e n t.
The As p r e s s u r e r e q u i r e d f o r
AlGaAs g r o w t h a l o n g t h e m e t a l r i c h e d g e o f t h e s o l i d u s
fie ld
i s much l o w e r t h a n t h a t r e q u i r e d f o r GaAs growth..
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130
I n terras o f t h e p h e n o m e n o lo g ic a l e x p r e s s i o n d i s c u s s e d
earlier
in Chapter 2
P
Ga
P
(4 -1 )
„
Ga
B o t h t h e PQ and a t e r m s a r e l o w e r .
The b e h a v i o r a t t h e
i n t e r f a c e depends on th e sequence o f grow th.
4 .1 .1
AlGaAs Grown on GaAs
I n t h e c a s e o f AlGaAs g r o w t h on GaAs, g r o w t h i s
p r o g r e s s i n g fro m a h i g h f l u x r a t i o m a t e r i a l
low f l u x r a t i o m a t e r i a l
( A lG a A s) .
(GaAs) t o a
One e x p e c t s t h a t o n c e
AlGaAs g r o w t h b e g i n s a t some p o i n t t h e f l u x r a t i o c a n be
reduced w ith o u t going m etal r i c h .
E x p e rim e n ta lly , only a
c o u p l e m o n o l a y e r s o f AlGaAs g r o w t h a r e r e q u i r e d b e f o r e
low f l u x r a t i o s c a n be u s e d .
seconds d u rin g grow th.
T h i s am o u n ts t o a c o u p l e o f
I n f a c t , o n c e a few m o n o l a y e r s
a r e d e p o s i t e d , t h e m a t e r i a l b e h a v e s v e r y much l i k e t h i c k
AlGaAs.
The f l u x e s c a n be t e r m i n a t e d and t h e s u r f a c e
w i l l s t a y m etal s t a b l e , even a t s u b s t r a t e te m p e r a tu re s
w h e r e GaAs would n o r m a l l y go m e t a l r i c h .
d e s o r p t i o n a l w a y s o c c u r s t o some d e g r e e ,
As g a l l i u m
in these cases
t h e s u r f a c e aluminum c o m p o s i t i o n p r o b a b l y g o e s u p , s o t h e
b e h a v i o r o f t h e s u r f a c e d e p e n d s on many f a c t o r s .
In th e
c o n t i n u o u s g r o w t h c a s e , t h i s i s p r o b a b l y l e s s o f an
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
131
effect.
I n t e r m s o f t h e f l u x m o d e l , t h e b u l k PQ and a
v a l u e s a r e o b t a i n e d a f t e r o n l y a few m o n o l a y e r s .
4 .1 .2
GaAs G row th on AlGaAs
I n t h i s c a s e g r o w t h i s p r o g r e s s i n g f r o m a low f l u x
r a tio m aterial
(AlGaAs) t o a h i g h f l u x r a t i o m a t e r i a l
(GaAs) f r o m t h e c a s e a b o v e , o n e m i g h t e x p e c t t h a t t h e
s u r f a c e would become m e t a l r i c h a f t e r a few m o n o l a y e r s o f
GaAs g r o w t h .
I n s t e a d , a most u n u s u a l e f f e c t o c c u r s as
GaAs g r o w t h p r o c e e d s w i t h o u t g o i n g m e t a l r i c h , e v e n
t h o u g h t h e f l u x r a t i o i s t o o lo w f o r GaAs g r o w t h a t h i g h
tem p eratu res.
T h i s e f f e c t p e r s i s t s t o a d e g r e e w h ic h
v a r i e s w i t h t h e GaAs t h i c k n e s s .
The n o r m a l b u l k
p r o p e r t i e s o f GaAs do n o t a p p e a r f o r s e v e r a l h u n d r e d
angstrom s
(~ 500 8 ) .
The a u t h o r was t h e " d i s c o v e r e r " o f
t h i s e f f e c t and t h e f i r s t t o r e p o r t i t s p r e s e n c e and
sig n ifican ce.
A c r u d e mapping o f t h e f l u x r a t i o a t t h e
m e ta l r i c h edge of th e s o l i d u s f i e l d as a f u n c t i o n of
GaAs g r o w t h t i m e i s shown i n P i g .
po in ts r e f le c ts
4.1.
The d e a r t h o f
th e d i f f i c u l t y of o b ta in in g d a ta w ith a
h i g h d e g r e e o f c o n f i d e n c e , b u t n e v e r t h e l e s s p r o v i d e s an
q u a l i t a t i v e d e s c r ip tio n of the behavior of the t r a n s i e n t
effect.
The s i g n i f i c a n c e o f t h e p l o t i s t h a t t h e l e n g t h
o f t i m e t h a t GaAs g r o w t h c a n o c c u r b e f o r e t h e m e t a l r i c h
c o n d i t i o n i s e n c o u n t e r e d d e p e n d s on t h e As o v e r p r e s s u r e
e m p l o y e d f o r t h e GaAs g r o w t h .
F o r v e r y low f l u x r a t i o s ,
o n l y t h i n GaAs l a y e r s c a n be g r o w n .
For t h i c k l a y e r s ,
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132
T R A N S IT IO N
T IM E
vs P0EAs
log ( P 8 E A . ( T o r r ) )
logfe*
\ rBEn
I I _ EquiMbr[u_m_As_Pjressure_for Metal_Stable Ji£face_fteMn£tmct[on_
10
30
25
-
8-
20
6-
- 15
<
-
4 -
^substrate =
^
growth rate = 0.9^i.m /hr
.-6
10
40
60
Transition
F igure 4 .1 .
- 5
JL
-J_
20
10
80
T im e
100
120
140
(sec)
T r a n s i t i o n tim e from m e t a l s t a b l e t o m e t a l
r i c h c o n d itio n as a fu n c tio n of log
and lo g
(BEP r a t i o )
(BEPAs^)
a t 680°C f o r a GaAs
g r o w t h r a t e o f 0 . 9 p m / h r f o r Alg 20Ga0 80A s "
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133
m or e As m u s t be e m p l o y e d .
To d e s c r i b e t h e e f f e c t , o n e
m i g h t s a y t h a t t h i n GaAs b e h a v e s l i k e t h e u n d e r l y i n g
AlGaAs i n t e r m s o f t h e f l u x r a t i o r e q u i r e m e n t s .
In term s
o f t h e m o d e l , i t seem s p r o b a b l e t h a t t h e l o w e r f l u x r a t i o
r e s u l t s f r o m a d r o p i n t h e P„ t e r m .
o
The
o
term i s
d o m i n a t e d by t h e d e s o r b i n g As f l u x , t h u s t h e e f f e c t w o u ld
seem t o be r e l a t e d t o a d i m i n i s h m e n t o f t h e d e s o r b i n g A s 2
flu x .
I n a n a t o m i s t i c m odel o f t h e s u r f a c e ,
th e desorbing
f l u x w o u ld be d e p e n d e n t o n t h e s u r f a c e c o v e r a g e and t h e
d e s o r b i n g l i f e t i m e s o f t h e As s p e c i e s .
The l i f e t i m e c a n
be d e s c r i b e d by
t
= x0 e -AH/kT
w h e r e AH i s
(4.2)
t h e e n t h a l p y o f d e s o r b t i o n an d
v i b r a t i o n a l tem.
tq
is a
t q i s u s u a l l y c a l c u l a t e d on t h e b a s i s
o f a s im p le harm onic o s c i l l a t o r w hich models t h e bonding
o f t h e atom t o t h e s u r f a c e .
In t h e p r e s e n t c a s e , one can
im a g in e t h e i n c l u s i o n of a d d i t i o n a l v i b r a t i o n te rm s which
w o u ld d e s c r i b e l o c a l i z e d s u r f a c e modes o r t h e c o m p l e t e
v i b r a t i o n a l spectrum of th e s o l i d .
d i s c u s s io n , each of th e s e f a c t o r s
In th e follow ing
i s d i s c u s s e d as
p o s s i b l y a f f e c t i n g t h e d e s o r b i n g f l u x o f As f ro m t h e
surface.
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134
The t h r e e m ain p o s s i b i l i t i e s a r e :
i)
C hange i n t h e s u r f a c e c o n c e n t r a t i o n
(surface
chem istry).
ii)
C hange i n t h e o p t i c a l mode v i b r a t i o n a l f r e q u e n c i e s
r e l a t e d to bonding.
iii)
Changes in th e v i b r a t i o n a l spectrum a t t h e s u r f a c e
which a r e r e l a t e d t o th e l o c a l c r y s t a l e n v iro n m e n t.
The c h a n g e i n s u r f a c e c o n c e n t r a t i o n would h a v e t o
r e s u l t f ro m a c h a n g e i n t h e b a s i c r e a c t i o n
G a ( S ) + As^gj £ GaAs
W h ile t h i s
is possible,
(4.3)
i t see m s r a t h e r u n l i k e l y .
A
s e c o n d e f f e c t i s an i n c r e a s e i n t h e Ga s u r f a c e
p o p u l a t i o n , w h ic h wo uld d i m i n i s h t h e As s u r f a c e
p o p u l a t i o n , m i n i m i z i n g PQ t h r o u g h i n c r e a s e d Ga
desorption.
et al.
70
T h i s c a n be r u l e d o u t , a s r e c e n t l y R a l s t o n
h a v e r e p o r t e d t h a t Ga d e s o r p t i o n i s r e d u c e d f o r
t h i n l a y e r s o f GaAs on AlGaAs.
I n f a c t , o ne w o u ld e x p e c t
t h e two e v e n t s t o be c o n n e c t e d .
C h a n g e s i n t h e l o c a l b o n d i n g c o u l d be c a u s e d by t h e
p r e s e n c e o f Al in th e s u r f a c e l a y e r .
A s i n g l e bonding o f
a n As atom t o an Al atom would be q u i t e e f f e c t i v e i n
reducing d e s o rb tio n .
quantum w e l l .
The Al w o u ld t h e n be p r e s e n t i n t h e
To d a t e , t h e r e h a s b e e n no e v i d e n c e which
s u g g e s t s t h a t Al i s p r e s e n t i n MBE grown q u a n t u m w e l l s .
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135
C h a n g e s i n t h e p h o n o n mode f r e q u e n c i e s wo uld r e f l e c t
d i f f e r e n t modes p r e s e n t f o r t h i n GaAs o n AlGaAs.
seems q u i t e p o s s i b l e ,
This
a s i t i s w e l l known t h a t
v i b r a t i o n a l spectrum near s u r f a c e s i s m o d ifie d .
I t is
l i k e l y t h a t t h e s u r f a c e modes o f GaAs w o u ld be m o d i f i e d
i n t h e d e g r e e o f f r e e d o m p e r p e n d i c u l a r t o t h e s u r f a c e by
t h e p r e s e n c e o f AlGaAs b e n e a t h t h e s u r f a c e .
One c a n a l s o
a r g u e t h a t t h e r e d u c e d Ga d e s o r b t i o n i s a c o n s e q u e n c e o f
t h i s e f f e c t as w e ll.
The s u r f a c e l i f e t i m e i s c o n n e c t e d t o t h e p h o n o n
spectrum through th e appearance of th e v i b r a t i o n a l
p a r titio n fu n ctio n , j
where x
v
, in the l i f e t i m e e x p ressio n
is the h eat of v a p o riz a tio n
( p e r mode) a t T= 0°K,
T i s te m p e ra tu re , h i s Plancks c o n s ta n t,
B oltzm an n 's c o n s ta n t.
71
and k i s
To u s e t h i s e x p r e s s i o n r e q u i r e s an
e n u m e r a t i o n o f t h e v i b r a t i o n a l modes a s s o c i a t e d w i t h a
given l a t t i c e site,.
com plicated.
In th e p r e s e n t c a s e , t h i s i s q u i t e
I t h a s b e e n s u g g e s t e d t h a t s u r f a c e modes
r a t h e r t h a n b u l k modes a r e t h e a p p r o p r i a t e t e r m s t o be
71
included in the p a r t i t i o n fu n c tio n
. On t h e b a s i s o f a
s i n g l e mode p r e s e n t a t t h e s u r f a c e ,
t h e l i f e t i m e wo uld be l o n g e r i f
i t c a n be s e e n t h a t
t h e mode f r e q u e n c y i s
low er th a n th e b u lk c a s e , as
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136
?
=
I ________
S i n c e j V > 1 , t h e n p r e s e n c e o f a d d i t i o n a l modes c o m p a r e d
to th e bulk case a ls o in c r e a s e s th e l i f e t i m e .
S u r f a c e modes a r e p r e s e n t f o r many c r y s t a l s u r f a c e s ,
a nd t h e t y p e o f mode p r e s e n t r e l a t i v e t o t h e b u l k
v i b r a t i o n a l s p e c t r u m d e p e n d s o n t h e t y p e o f ato m ( o r
layer)
a t the su rfa c e .
A g e n e ra l d isc u s s io n of the
f r e q u e n c y p o s i t i o n o f d e f e c t a nd s u r f a c e modes c a n be
f o u n d i n t h e p a p e r s by S i e v e r s
t h e b o o k by M a d e lu n g
74
72
a nd W a l l i s e t a l .
73
, a nd
.
As t h e o p t i c a l mode f r e q u e n c i e s a r e l o w e r i n GaAs
t h a n i n AlGaA s, t h e modes a s s o c i a t e d w i t h t h e t h i n GaAs
s u r f a c e p r o b a b l y a p p e a r a s l o c a l i z e d g a p modes n e a r t h e
b r i l l o u i n zone b o u n d a ry , i n a n a lo g y w ith th e sim p le
d i a t o m i c l i n e a r c h a i n m odel w h e r e t h e d e f e c t ato m mass
lies
i n b e t w e e n t h e tw o m a s s e s o f t h e d i a t o m i c c h a i n
72
.
T h i s c o r r e s p o n d s t o a n o p t i c a l mode f r e q u e n c y b e l o w t h e
b u l k l a t t i c e o p t i c a l mode f r e q u e n c y .
a n d AlGaA s, i t
I n t h e c a s e o f GaAs
i s u n c l e a r w h e t h e r s u c h a mode would be
l o w e r i n f r e q u e n c y t h a n b u l k GaAs.
The l o w e r a c o u s t i c
mode f r e q u e n c i e s may n o t e v e n a p p e a r i f
t h e y a r e b e lo w
t h e v i b r a t i o n a l s p e c t r u m o f t h e a c o u s t i c mode b r a n c h o f
t h e AlGaAs .
As t h e l a y e r t h i c k n e s s i n c r e a s e s , t h e
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137
s u r f a c e bec omes more l i k e t h i c k GaAs.
The i n t e r f a c e r e p r e s e n t s a b o u n d a r y f o r t h e modes
a n d p l a c e s a s p a t i a l c o n s t r a i n t on t h e w a v e l e n g t h .
In
a d d i t i o n , t h e r e s p e c t i v e modes o n e a c h s i d e o f t h e
i n t e r f a c e p r o b a b l y c a n n o t p r o p a g a t e i n t h e medium on t h e
o t h e r s i d e o f t h e i n t e r f a c e , a nd e x h i b i t d e c a y i n g
envelope fu n c tio n s fo r t h e i r d isp lace m e n t.
I t is
p o s s i b l e t h a t a l o c a l i z e d mode a s s o c i a t e d w i t h t h e
i n t e r f a c e may be p r e s e n t .
T h i s would c o n t r i b u t e an
a d d i t i o n a l mode t o t h e p a r t i t i o n f u n c t i o n , i n c r e a s i n g t h e
lifetim e.
At th e s u r f a c e ,
t h e i n f l u e n c e o f t h i s mode
w o u l d d i m i n i s h a s t h e GaAs s u r f a c e l a y e r t h i c k n e s s .
An
e x a c t a n a l y s i s o f t h i s problem i s d i f f i c u l t b ecau se th e
i n t e r f a c e i s a boundary betw een a d ia to m ic l a t t i c e and a
t e r n a r y l a t t i c e w ith d i s o r d e r on t h e g roup I I I
su b lattice.
4.2
P r a c t i c a l C o n s id e ra tio n s of Flux R atio s
I n t h i c k e p i l a y e r g r o w t h o f b o t h GaAs and AlGaAs t h e
m a t e r i a l q u a l i t y i s g e n e r a l l y found t o improve t h ro u g h
lower f lu x r a t i o s .
attrib u ted
The i m p r o v e m e n t i s g e n e r a l l y
t o t h e m i n i m i z a t i o n o f i n t r i n s i c d e f e c t s a nd
k i n e t i c induced d e f e c t s .
The c o n c e p t t h i s a u t h o r p r e f e r s
is th a t m a te ria l q u a lity i s b est a t near sto ic h io m e tric
growth c o n d i t i o n s .
T h i s i d e a seem s i n t u i t i v e l y o b v i o u s
b u t r e a l l y d e f i n i n g w h a t i s m e a n t by i t
i s more
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138
d ifficu lt.
S t o i c h i o m e t r y i m p l i e s XQa = XAg.
What
h a p p e n s o u t s i d e o f t h i s p o i n t i n MBE g r o w t h i s u n c l e a r .
Some b a s i s f o r d i s c u s s i o n c a n be f o u n d i n t h e Ga-As
b in a ry phase diagram ,
75
reproduced in P ig . 4 .2 .
MBE
g r o w t h t e m p e r a t u r e f o r GaAs a nd AlGaAs a r e t y p i c a l l y i n
t h e r a n g e o f 600OC t o 700°C (873K t o 9 7 3 K ) .
On t h e p h a s e
d i a g r a m , p u r e GaAs o n l y e x i s t s o v e r a ( i m p e r c e p t i b l y )
narrow ra n g e .
On t h e Ga r i c h s i d e , l i q u i d
p r e s e n t , w h i l e on t h e As r i c h s i d e , s o l i d
present.
(Ga) i s
(As)
is
I n MBE g r o w t h a V : I I I f l u x r a t i o w h ic h i s m e t a l
ric h r e s u lts
i n Ga d r o p l e t s on t h e s u r f a c e .
As r i c h c o n d i t i o n s r e s u l t s
problem s, b ut t h i s
G row th u n d e r
i n no p e r c e p t i b l e s u r f a c e
i s n o t t h e o n l y mea ns by w h ic h g r o w t h s
s h o u l d be e v a l u a t e d .
I n GaAs, t h e f l u x r a t i o h a s s e v e r a l e f f e c t s on t h e
e l e c t r i c a l p r o p e r t i e s o f u n i n t e n t i o n a l l y doped m a t e r i a l .
F re e e l e c t r o n c o n c e n t r a t i o n s have been o b s e rv e d to
in c r e a s e w ith in c r e a s in g flu x r a t i o s .
The d e e p l e v e l
c o n c e n tr a tio n g e n e r a lly d ecrease s w ith d e c re a sin g flu x
ratio s.
These e f f e c t s have u s u a l l y been i n t e p r e t e d i n
t e r m s o f s i m p l e t h e r m o d y n a m i c a r g u m e n t s b a s e d on t h e
23
T e r a m o t o e q u a t i o n f o r LPE g r o w t h .
The a r g u m e n t u s u a l l y a t t r i b u t e d t h e f r e e e l e c t r o n
c o n c e n t r a t i o n change t o e i t h e r changes in Si s i t e
occupancy
(s i As v s S i g a ) o r c h a n g e s i n t h e c a r b o n
in corporation.
The T e r a m o t o e q u a t i o n d e s c r i b e s a s o l i d
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
139
1600
1400
I5I3K
Liquid
Phase
T(K)
GaAs ♦ Liquid
1200
1083 K
TEMP ERATURE
(000
GaAs + Liquid
T(K) \
1090 \
800
1083 /
0.98
600
400
303 K
GaAs + Solid As
200
GaAs + Solid Ga
0.2
0.4
0.6
0.8
ARSENI C FRACTI ON, XA#
F ig u re 4.2.
C onventional m e t a l l u r g i c a l form at f o r th e
Ga-As b i n a r y p h a s e d i a g r a m , w i t h a r s e n i c
f r a c t i o n X, a s a l i n e a r a b s c i s s a s c a l e .
AS
The i n s e t shows t h e e x t r e m e a r s e n i c - r i c h
end, w ith th e e u t e c t i c f r e e z in g p o in t
d e p r e s s e d below t h a t o f p u r e As.
A fter
r e f e r e n c e 75.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
140
in e q u ilib riu m w ith a m elt of i t s c o n s t i t u e n t elem ents.
The r e l a t i o n r e f l e c t s a c o m p e t i t i o n b e t w e e n t h e g r o u p I I I
and V e le m e n ts w ith i m p u r i t i e s t h a t m ight occupy t h e i r
respective s ite s .
A l t e r n a t i v e l y , one can say the
i m p u r i t i e s c a n o c c u p y v a c a n c i e s on t h e r e s p e c t i v e
s u b l a t t i c e s w h i c h a r e c o n t r o l l e d by t h e r e l a t i v e r a t i o o f
th e chem ical a c t i v i t i e s of the s u b l a t t i c e elem ents.
I n a v e r y i m p o r t a n t p a p e r , Skromme e t a l .
27
have
d e m o n s t r a t e d c o n c l u s i v e l y t h e e l e m e n t a l As s o u r c e
m a t e r i a l i s a n i m p o r t a n t s o u r c e o f b o t h s u l f u r a nd c a r b o n
i n l i g h t l y doped e p i l a y e r s .
As s u l f u r i s a d o n o r i n
GaAs, t h e i n c r e a s e i n f r e e c a r r i e r c o n c e n t r a t i o n w i t h
i n c r e a s i n g As p r e s s u r e r e s u l t s f ro m d o n o r s i n c o r p o r a t e d
from t h e s o u r c e i m p u r i t i e s , n o t c h a n g e s i n t h e S i s i t e
occupancy.
T h i s e f f e c t i s m i n i m i z e d by g r o w t h a t low
flux r a tio s .
A s e c o n d e f f e c t w h i c h h a s b e e n d e m o n s t r a t e d by
Palm ateer e t a l .
28 4Q
'
is th a t s u b s tr a te im purity
r e d i s t r i b u t i o n i s i n f l u e n c e d by t h e V : I I I f l u x r a t i o
d u rin g grow th.
S urface c o n c e n tra tio n s of s u lf u r ,
m a n g a n e s e a nd i r o n a r e r e d u c e d by g r o w i n g u n d e r low f l u x
ratio s.
The r e d u c e d f l u x r a t i o s m i n i m i z e t h e i n t r i n s i c
d e f e c t s w h ic h r e d u c e s t h e d i f f u s i o n c o e f f i c i e n t s o f
i m p u r i t i e s w h ic h move v i a d e f e c t e n h a n c e d r e a c t i o n s ,
p a r t i c u l a r l y t h o s e d e p e n d e n t on g r o u p I I I v a c a n c i e s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
141
The p r e s e n t work e x a m i n e s t h e e f f e c t o f f l u x r a t i o s
o n t h e g r o w t h o f q u an tu m w e l l s t r u c t u r e s .
The o r i g i n a l
i n t e r e s t d ev elo p ed d u r in g n -n h e t e r o j u n c t i o n grow th w ith
the o b se rv a tio n of the e f f e c t d e scrib ed above, the
p e r s i s t e n t m etal s ta b l e s u rfa c e r e c o n s tr u c tio n .
The
m a t e r i a l q u a l i t y o f b o t h AlGaAs a n d GaAs i s g e n e r a l l y
i m p r o v e d by g r o w t h a t t h e l o w e s t f l u x r a t i o s p o s s i b l e .
T h e r e f o r e , an e x a m i n a t i o n o f t h e i n f l u e n c e o f t h e V : I I I
f l u x r a t i o on q u a n t u m w e l l s t r u c t u r e s was u n d e r t a k e n .
T h e s e s t u d i e s h a v e i d e n t i f i e d t h e f a i l u r e m e c h a n is m s o f
b o t h t h e u n d o p e d a n d m o d u l a t i o n d o p e d GaAs grown on
AlGaAs.
4.3
G row th o f Undoped Quantum W e l l S t r u c t u r e s
I n t h i s s e c t i o n t h e m ain f a c t o r s i n f l u e n c i n g t h e
g r o w t h o f h i g h q u a l i t y q u a n tu m w e l l s w i l l be i d e n t i f i e d .
These a r e th e q u a l i t y of th e m achine, t h e V : I I I f lu x
r a t i o d u r i n g w e l l g r o w t h , and t h e i n c o r p o r a t i o n o f
su b strate re la te d im p u rities.
The g r o w t h o f t h e s e
s t r u c t u r e s a t high s u b s t r a te tem p eratu re i s re q u ire d in
o r d e r t o o b t a i n a h i g h q u a l i t y AlGaAs b u f f e r .
The
fo llo w in g s e t of experim ents e v a lu a te s the e f f e c t s of
f l u x r a t i o s and s u b s t r a t e s .
4 .3 .1
E xperim ental
The e p i t a x i a l l a y e r s w e r e gr ow n i n a V a r i a n Gen I I
MBE m a c h i n e .
The s u b s t r a t e s u s e d i n t h i s s t u d y w ere
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
142
u n d o p e d LEC grown GaAs.
The a n n e a l e d and n o n - a n n e a l e d
s u b s t r a t e s came fro m t h e same w a f e r .
A n n e a l e d s u b s t r a t e s w e r e p r e p a r e d by h e a t i n g a t
750°C f o r 24 h o u r s i n p a l l a d i u m d i f f u s e d H2 *
The
s u r f a c e s were s u b s e q u e n t l y r e p o l i s h e d u s in g B rom inem e t h a n o l , r e m o v i n g a p p r o x i m a t e l y 20 m i c r o n s o f m a t e r i a l .
T his procedure i s e f f e c t i v e in reducing s u rfa c e im purity
a c c u m u l a t i o n d u r i n g MBE g r o w t h
G row th r u n s
w i t h an a n n e a l e d
28
.
(#638 a nd #639) w e r e p e r f o r m e d , e a c h
(A) and n o n - a n n e a l e d
m o u n t e d o n t h e same s u b s t r a t e h o l d e r .
(N) s u b s t r a t e
The h o l d e r was n o t
r o t a t e d d u r i n g g r o w th , b u t t h e s u b s t r a t e s were mounted i n
c l o s e p r o x im ity t o m inim ize s p a c i a l v a r i a t i o n s of th e
m o l e c u l a r beam f l u x e s .
For both runs th e s u b s t r a t e
t e m p e r a t u r e a nd GaAs g r o w t h r a t e w e r e h e l d c o n s t a n t a t
680°C and 1 . 4 u m /h r r e s p e c t i v e l y .
The o n l y p a r a m e t e r
v a r i e d b e t w e e n r u n s 638 amd 639 was t h e a r s e n i c t o
g a l l i u m beam p r e s s u r e r a t i o d u r i n g t h e g r o w t h o f t h e
q u a n tu m w e l l .
The e x p e r i m e n t s a r e d e t a i l e d i n T a b l e 4 . 1 .
The s t r u c t u r e grown f o r b o t h 638 a n d 639 shown i n F i g .
4 . 3 , was a 0 . 5 yin GaAs b u f f e r , 1 . 0 ym A1Q 2 0Ga0 80A s ' 160
8 GaAs (QW) and 0 . 5 ym A1Q 2o GaO 80A s *
V : I I I beam e q u i v a l e n t p r e s s u r e
F o r r u n 638 t h e
(P0 g) r a t i o was k e p t a t
1 8 : 1 d u r i n g t h e g r o w t h o f t h e GaAs quantum w e l l .
In run
639 t h e a r s e n i c f l u x was i n c r e a s e d u s i n g a s e c o n d a r s e n i c
s o u r c e t o g i v e a V : I I I PgE r a t i o o f 2 9 : 1 d u r i n g t h e
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143
SINGLE QUANTUM WELL STRUCTURE
0 .5 ^ im
AIq 2q GQq q q As
1 6 0 A Go A s
Quantum Well
AlGaAs Buffer
1. 0
jj.m
0 .5
A Iq 2 q G 0 q s o
fj.rx\
As
GaAs
SUBSTRATE
F ig u re 4.3.
S i n g l e quantum w e l l l a y e r s t r u c t u r e
s t u d i e d i n t h i s work.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
144
g r o w t h o f t h e GaAs q u a n tu m w e l l .
T h i s 2 9 : 1 PBE r a t i o
g a v e t h e minimum As f l u x r e q u i r e d t o grow t h e GaAs b u f f e r
l a y e r s f o r b o t h r u n s a t 680°C .
W ith any l e s s a r s e n i c th e
s u r f a c e would c o n v e r t t o a Ga r i c h l i q u i d p h a s e .
The
A^0 20Ga0 80As on t h e o t h e r ha nd r e m a i n e d w i t h i n t h e
c r y s t a l s t a b i l i t y f i e l d w ith t h e 18:1 f l u x r a t i o as
e v i d e n c e d by a m e t a l s t a b l e
(3x1) e l e c t r o n d i f f r a c t i o n
p a t t e r n m onitored d u rin g grow th.
I t s h o u l d be n o t e d t h a t
t h e s u r f a c e l a y e r i n r u n 638 r e m a i n e d w i t h i n t h e
s t a b i l i t y f i e l d a f t e r t h e alu minum f l u x was t u r n e d o f f
f o r a t i m e l o n g e n o u g h t o grow 160 8 o f GaAs.
T able 4.1
P
Run
BE R a t i o *
Number
AlGaAs
QW
638
14:1
18:1
Growth C o n d i t i o n s
S ubstrate
P reparation
(A) A n n e a l e d / R e p o l i s h e d
(N) N o n - a n n e a l e d
639
14:1
29:1
(A) A n n e a l e d / R e p o l i s h e d
(N) N o n - a n n e a l e d
*Beam e q u i v a l e n t p r e s s u r e r a t i o s (PgE) a r e t a k e n from
t o t a l g a l l i u m and a r s e n i c f l u x e s m e a s u r e d w i t h a n u de i o n
gauge in th e s u b s t r a t e p o s i t i o n .
P „E r a t i o s r e p o r t e d
h e r e a r e p e c u l i a r t o th e machine u s i a i n t h i s s tu d y .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
145
P h o t o l u m i n e s c e n c e s t u d i e s w e r e p e r f o r m e d a t 4k u s i n g
t h e 6328 § l i n e f ro m a He-Ne l a s e r u s i n g e x c i t a t i o n power
d e n s i t i e s as in d ic a te d in th e f i g u r e s .
4 .3 .2
R esults
The l u m i n e s c e n c e s p e c t r a o f P i g s .
4.4(a)
and 4 . 4 ( b )
a r e f r o m s i n g l e q u a n tu m w e l l s t r u c t u r e s on i d e n t i c a l n o n a n n e a l e d GaAs s u b s t r a t e s .
T h e s e s t r u c t u r e s w e r e gro wn a t
t h e same g r o w t h r a t e s b u t d i f f e r e n t a r s e n i c f l u x e s w e r e
u s e d t o grow t h e GaAs q u a n t u m w e l l l a y e r .
F igs.
4 . 5 and
4 . 6 a r e p h o to lu m in e s c e n c e s p e c t r a o f t h e quantum w e l l
peaks of F ig s.
4.4(a)
ex citation lev els.
a nd 4 . 4 ( b )
r e s p e c t i v e l y , a t two
When t h e e x t r a a r s e n i c i s u s e d t o
gr ow t h e GaAs w e l l l a y e r on a n o n - a n n e a l e d s u b s t r a t e t h e
l u m i n e s c e n c e o f t h e QW ( F i g s .
4.4(a)
and 4 . 5 i s weak,
b r o a d and s h i f t e d t o a h i g h e r e n e r g y .
lum inescence observed
(F igs. 4.4(b)
The QW
and 4 .6 )
f ro m a n
i d e n t i c a l s t r u c t u r e u s i n g t h e low a r s e n i c f l u x t o grow
t h e GaAs w e l l h a s by c o n t r a s t an i n t e n s e n a r r o w l i n e
s p e c t r a c e n t e r e d a b o u t t h e e x p e c t e d QW e m i s s i o n e n e r g y .
S im ila r i n t e n s e narrow l i n e s p e c t r a
(F igs.
4 .7 , 4.8)
76
are
o b s e r v e d f ro m SQW s t r u c t u r e s grown on a n n e a l e d
su b strate s.
These s p e c t r a
(using an nealed s u b s t r a t e s )
a r e i n d e p e n d e n t o f t h e a r s e n i c f l u x u s e d t o grow t h e GaAs
w e l l w i t h i n th e ra n g e o f f l u x e s em ployed.
A com parison o f F ig . 4 .4 (b )
638
a nd F i g .
4 .6 o f sample
(N) i n d i c a t e t h a t t h e p e a k a t 1 . 5 3 1 7 eV i s i n t r i n s i c
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
146
1.531 eV
PL
Intensity
(orb. units)
4°K
Quantum
Well —
Pump Power = 0 .2 W /cm '
— 0 .8 meV FWHM
.4 9 2 eV
I 1.515 eV
1.546 eV
Energy
Figure 4.4.
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW a n d o f
GaAs b u f f e r o f s a m p l e s
29:1)
(b)
638 (N)
(a)
6 3 9 (N),
( V :I I I = 18:1)
(V sIII-
on
fig u re.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
147
4 K
PL
Intensity
(arb. units)
—iH
l.5 4 5 9 e V
•3.7 meV FWHM
33 x
1 .5 4 6 2 eV
(a ) 0.10 W/ctn*
F igure 4.5.
E nergy
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW i n s a m p l e
6 3 9 (N)
(V:111=29:1)
0.1W/cm2 ,
(b)
a t pump p o w e r s o f
(a)
0.01W/cm2 .
l.53l7eV
PL
Intensity
(arb. units)
1.5 3 0 6 eV
33 x
— 0.7meV
FWHM
Hh
(b) 0.01 W/cm‘
1.5281 eV
(a) 0.10 W /cm'
E nergy
Figure 4.6.
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW i n s a m p l e
6 3 8 (N)
(V : 1 1 1 = 1 8 : 1 )
O.IW/cm2 ,
a t pump p o w e r s o f
(a)
(b) 0 . 0 lW / c m 2 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PL
Intensity
(arb. units)
1.5331 eV,
33 x
'■5 3 ? 6 e ' f l 5 3 7 ° e v
(b) 0 . 0 1 W /cm'
— I I meV FWHM
1 .5 3 3 2 eVy
3 .3 x
1.5309 e V / ^
E nergy
F igure 4.7.
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW i n s a m p l e
639(A)
(V:111=29:1)
0.1W/cm2 ,
(b)
a t pump p o w e r s o f
(a)
0.01W/cm2 .
PL
In ten sity
(arb. units)
33 x
— 0.7 meV FWHM
Ii
(b) 0.01 W/cm'
I
II
I
I 5 3 5 5 eV
i
(a) 0,10 W/cm2
Energy
F igure 4.8.
P h o t o l u m i n e s c e n c e s p e c t r a o f SQW i n s a m p l e
638(A)
( V : 1 1 1 = 1 8 : 1 ) a t pump p o w e r s o f
O.IW/cm2 ,
(b)
(a)
0 . 0 lW /c m 2 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
149
photolum inescence sin c e i t s
i n t e n s i t y does not s a t u r a t e
w ith i n c r e a s i n g e x c i t a t i o n power.
The l o w e r e n e r g y p e a k s
( 1 . 5 3 6 eV, 1 . 5 2 8 1 eV) do s a t u r a t e a nd a r e a t t r i b u t e d
l u m i n e s c e n c e f ro m i m p u r i t i e s i n t h e w e l l .
to
S im ila rly the
h ig h er energy peaks in P ig .
4.7
( 1 . 5 3 7 0 eV) and F i g .
4.8
( 1 . 5 3 5 5 eV) a r e i n t r i n s i c .
These peaks a r e a t t r i b u t e d t o
th e n= l c o n fin e d e l e c t r o n heavy h o le e x c ito n t r a n s i t i o n
f„
' lh
. 76,77,78
*
T h i s a s s i g n m e n t i s f u r t h e r s u p p o r t e d by t h e b e h a v i o r
o f th e t r a n s i t i o n in th e presen ce of a m agnetic f i e l d
79
The Zeeman e f f e c t p r o v i d e s a p r e d i c t a b l e b e h a v i o r w h i c h
a l l o w s an u n a m b ig u o u s i d e n t i f i c a t i o n o f t h e c o n f i n e d
exciton tr a n s itio n .
This i s
im p o rta n t, as the
i n d e n t i f i c a t i o n on t h e b a s i s o f e n e r g y
alone does n o t
p r o v i d e a d e t e r m i n a t i o n o f t h e t r a n s i t i o n mechanism.
Many q u a n tu m w e l l s t u d i e s do n o t p r o v i d e an a n a l y s i s o f
t h e t r a n s i t i o n m ech anis m a nd t h e a s s u m p t i o n o f a n
e x c i t o n i c m ec han is m when o t h e r m e c h a n i s m s a r e d o m i n a t i n g
resu lts
in c o n c l u s i o n s which a r e f u n d a m e n ta lly f la w e d .
The s p e c t r a shown i n F i g s .
s t r u c t u r e a nd i n t e n s i t y .
4 .6 -4 .8 a re s im ila r in
A l l t h r e e sam ples e x h i b i t
e x tre m e ly narrow lin e w i d t h s fo r th e
t r a n s i t i o n s and
f o r th e lower energy e x t r i n s i c p e a k s .
The i n t r i n s i c p e a k
e n e r g i e s , l i n e w i d t h s and s p a c i n g o f e x t r i n s i c p e a k
energies are lis t e d
in T able 4 .2 .
While th e a s sig n m e n t
o f t h e o r i g i n o f t h e s e e x t r i n s i c p e a k s i s n o t p o s s i b l e on
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
150
T a b le 4 .2
El h Peaic e n e r g i e s , El h FWHM l i n e w i d t h s
a nd s p a c i n g s o f e x t r i n s i c p e a k s r e l a t i v e t o El h
E x t r i n s i c peak s p a c in g s
Elh<eV)
AEi
<3
w
ae3
6 3 9 (N)
1.5459
-----
------
------
3.7
639(A)
1.5371
1.4
4.0
6.5
1.1
6 3 8 (N)
1.5317
1.1
3.6
-----
0.7
638(A)
1.5354
1.1
3.7
———
0.7
CM
Run
(meV)
El h FWHM (meV)
t h e b a s i s o f t h e s e d a t a a l o n e , we n o t e t h e c a l c u l a t i o n s
of M ailhiot
O f]
-
f o r s h a l l o w d o n o r s i n a SQW p r e d i c t a
s e q u e n c e o f e x c i t e d d o n o r l e v e l s l y i n g j u s t b e lo w t h e n = l
c o n d u c tio n subband w i t h i n an en erg y ran g e e q u i v a l e n t t o
t h a t observed h ere.
Thus we t e n t a t i v e l y a t t r i b u t e t h e
e x t r i n s i c peaks to shallow don ors.
The s p e c t r a o f 6 3 9 (N)
(F ig. 4.5)
c o n t r a s t to t h a t o f 638(A),
a re a dram atic
(N) and 6 3 9 ( A ) , e x h i b i t i n g a
r e l a t i v e l y b r o a d QW l i n e w i d t h
( 3 . 7 meV FWHM) w i t h no
r e s o l v e d peak s t r u c t u r e .
The w e l l e m i s s i o n i s a l s o
s h i f t e d upwards i n e n e rg y
(1.5459 e V ) .
The e x c i t a t i o n
i n t e n s i t y d e p e n d e n c e o f t h e PL s p e c t r a o f 639(H) shown i n
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
151
F igs.
4.4(a)
and 4 . 5 s u g g e s t s t h a t t h e 1 . 5 4 5 9 eV
lum inescence in th e s e s p e c t r a i s p red o m in a n tly e x t r i n s i c
in o r ig in since i t s
i n t e n s i t y a ls o s a t u r a t e s w ith
i n c r e a s i n g pump p o w e r .
The two e x c i t o n p e a k s a t 1 . 5 1 2 eV
a nd 1 . 5 1 5 eV p r e s e n t i n b o t h s c a n s o f F i g .
4.4 a re l i k e l y
f r o m t h e 0 . 5 urn GaAs b u f f e r l a y e r b e t w e e n t h e s u b s t r a t e
and t h e 1 . 0 pm AlGaAs l a y e r .
T h e i r p r e s e n c e and
i n t e n s i t y d em o n strate a s im i l a r q u a l i t y of th e b u ffe r
lay er.
The p e a k a t 1 . 4 9 2 eV c o r r e s p o n d s t o a d o n o r
c a r b o n - a c c e p t o r t r a n s i t i o n f o r GaAs an d a l s o o r i g i n a t e s
from t h e b u f f e r .
The p e a k a t 1 . 5 3 7 eV i n F i g .
4.4(b)
is
due t o th e n= l c o n f i n e d e l e c t r o n l i g h t - h o l e e x c i t o n
tra n sitio n
(E ^).
o f t h e AlGaAs,
76
'
78
'
7Q
The p h o t o l u m i n e s c e n c e s p e c t r a
( n o t s h o w n ) , h a v e l i n e w i d t h s r a n g i n g from
2 . 7 meV t o 5 . 5 meV, w h i c h a r e a l s o i n d i c a t i v e o f t h e h i g h
q u a l i t y of t h e s e l a y e r s .
4 .3 .3
D iscussion
The e x p e r i m e n t s d e t a i l e d i n T a b l e I w e r e d e s i g n e d t o
t e s t two s e e m i n g l y s e p a r a t e f a c t o r s ,
the e f f e c t of
s u b s t r a t e a n n e a l i n g a nd o f t h e V : I I I f l u x r a t i o on SQW
lum inescence.
The s i m i l a r i t y o f t h e p r e d o m i n a n t l y
i n t r i n s i c PL s p e c t r a e x h i b i t e d by 639
( a ) , 6 3 8 ( A ) , and
6 3 8 ( N ) , when c o m p a r e d w i t h t h e r e l a t i v e l y p o o r e x t r i n s i c
PL fro m 639 (N ), s u g g e s t s t h a t t h e e f f e c t on t h e SQW PL o f
t h e s e two v a r i a b l e s i s t h e same.
The d i f f e r e n c e s i n PL
b e t w e e n 6 3 9 (N) a nd 639(A) s u g g e s t t h a t t h e p o o r SQW PL o f
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
152
639 (N) i s r e l a t e d t o t h e p r e s e n c e o f o u t d i f f u s i n g
im purities.
The a s s e r t i o n t h a t i m p u r i t i e s a r e r e s p o n s i b l e f o r
t h i s e f f e c t i s b a s e d on p r e v i o u s work
28
w h i c h shows
t h a t a n n e a l i n g a nd s u b s e q u e n t r e m o v a l o f t h e c o n v e r t e d
s u r f a c e o f GaAs s u b s t r a t e s p r i o r t o g r o w t h r e s u l t s
in a
r e d u c t i o n o f u n i n t e n t i o n a l i m p u r i t i e s i n grown e p i l a y e r s .
The s o u r c e o f t h e s e i m p u r i t i e s i s t h e s u b s t r a t e .
This
e f f e c t has been c o n c l u s i v e l y d e m o n stra te d in p r e v io u s
work u s i n g t h e e l e c t r i c a l p r o p e r t i e s o f grown e p i l a y e r s
and SIMS a n a l y s i s ^ '
high V : I I I f lu x r a t i o )
Sa mp l e 6 3 9 (N)
(nonannealed,
which e x h i b i t e d p r e d o m in a te ly
e x t r i n s i c QW PL ( P i g . 4 . 5 )
a n d s a m p l e 639(A)
( a n n e a le d /r e p o lis h e d , high V : I I I f lu x r a t i o )
which
e x h i b i t e d e x c e l l e n t i n t r i n s i c QW PL ( F i g . 4 . 7 ) , w e r e
grown s i m u l t a n e o u s l y u n d e r n e a r l y i d e n t i c a l g r o w t h
conditions.
On t h i s b a s i s we b e l i e v e t h a t t h e m echanis m
w h i c h c a u s e s p o o r SQW PL i s t h e i n c o r p o r a t i o n o f
o u td iffu sin g su b strate re la te d im purities.
The s i m i l a r i t y b e t w e e n t h e QW l u m i n e s c e n c e o f 639(A)
a nd 6 3 8 (N)
( n o n - a n n e a l e d , lo w V : I I I f l u x r a t i o )
when
c o m p a r e d w i t h 6 3 9 (N) s u g g e s t s a d e p e n d e n c e o f i m p u r i t y
i n c o r p o r a t i o n on V : I I I f l u x r a t i o .
A p o s s i b l e m ec han ism
f o r t h i s e f f e c t i s s u g g e s t e d by t h e s h i f t t o h i g h e r
e n e r g y o f t h e QW l u m i n e s c e n c e o f 6 3 9 ( N ) .
I t has been
shown t h a t t h e p r e s e n c e o f i m p u r i t i e s d u r i n g t h e g r o w t h
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
153
o f a GaAs-AlGaAs i n t e r f a c e c a n d e g r a d e t h e a b r u p t n e s s o f
t h a t i n t e r f a c e by e n h a n c i n g i n t e r d i f f u s i o n o f t h e Ga a n d
A l.
82
I f A1 d i f f u s e d i n t o t h e GaAs w e l l , t h e i n c r e a s e d
b a n d g a p and g r a d e d i n t e r f a c e c o u l d r e s u l t i n t h e h i g h e r
e m i s s i o n e n e r g y a nd a b r o a d e r l i n e w i d t h .
The l i n e w i d t h
b r o a d e n in g , i n d i c a t i v e of rough i n t e r f a c e s c o u ld a l s o
r e s u l t fro m i n h o m o g e n e o u s i n t e r d i f f u s i o n a l o n g t h e GaAsAlGaAs i n t e r f a c e .
T his s u g g e s ts t h a t a high a r s e n i c
f l u x , p o s s ib ly g e n e r a tin g group I I I v a c a n c ie s ,
r e q u ir e d along w ith th e i m p u r i t i e s ,
is
for th is
in te rd if f u s io n to occur.
The e f f e c t o f i m p u r i t i e s o n t h e i n t e r f a c e q u a l i t y i s
f u r t h e r c o n s id e r e d below.
6 3 9 (N) i s weak and b r o a d ,
While t h e lu m in e s c e n c e of
i t c o u l d s i m p l y be t h e r e s u l t
of im p u rity lum inescence w ith a l a r g e c o n te n t of nonr a d i a t i v e c e n t e r s com peting w ith th e e x c ito n
lu m n in escen ce.
The r a n d o m i z a t i o n o f h e t e r o j u n c t i o n
i n t e r f a c e s has u s u a l l y been o b s e rv e d to o c c u r th ro u g h
d e fe c t re a c tio n s re q u irin g the presence of im p u ritie s in
v ery high c o n c e n t r a t i o n s
(~ 10
18
—3 82
cm ) .
The i m p u r i t y
c o n c e n t r a t i o n s h e r e a n d t h e c r y s t a l t e m p e r a t u r e m i g h t be
t o o low t o a l l o w t h i s t o o c c u r .
The e f f e c t o f t h e V : I I I f l u x r a t i o o n t h e
l u m i n e s c e n c e o f SQWs i s more s u b t l e .
The
FWHM
l u m i n e s c e n t l i n e w i d t h s f ro m SQWs grown u s i n g lo w V : I I I
f l u x r a t i o s a r e n a r r o w w i t h a v a l u e o f 0 . 7 meV f o r b o t h
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
154
s a m p l e s 638(A)
and 6 3 8 ( N ) .
SQW l u m i n e s c e n c e fro m
s t r u c t u r e 6 3 9 ( A ) , grown w i t h t h e h i g h e r V : I I I f l u x r a t i o
on t h e a n n e a l e d s u b s t r a t e h a d a s l i g h t l y b r o a d e r
l i n e w i d t h o f 1 . 1 meV.
A lthough t h i s d i f f e r e n c e i s s m a ll
t h e t r e n d s u g g e s t s t h a t 6 3 9 (A) h a s l e s s s m o o t h w e l l
i n t e r f a c e s , due t o Al-Ga i n t e r d i f f u s i o n o r t o r e m a in in g
im p u ritie s in c o rp o ra te d w ith the higher V :I I I flu x r a t i o .
A s e c o n d e f f e c t w h i c h m u s t be c o n s i d e r e d i s
b r o a d e n i n g o f t h e e x c i t o n l i n e w i d t h d ue t o i o n i z e d
im p u rities.
The l i n e w i d t h s a r e s u f f i c i e n t l y low t h a t
t y p i c a l GaAs b u l k e f f e c t s m i g h t be i m p o r t a n t .
T his p o in t
i s examined below .
I n t h e l u m i n e s c e n c e o f s a m p l e 6 3 9 (A) a t h i r d
e x t r i n s i c p e a k i s s e e n a t 1 . 6 3 0 5 eV w h i c h i s a b s e n t i n
b o t h 6 3 8 (N) a n d 6 3 8 ( A ) .
T h i s p e a k may be d ue t o
i n c r e a s e d Ga v a c a n c i e s w i t h i n t h e w e l l o r p o s s i b l y t o
a d d i t i o n a l im p u rite s in c o rp o ra te d o nly w ith th e h ig h e r
arsen ic flu x .
The h i g h q u a l i t y o f s i n g l e q u an tu m w e l l s grown u n d e r
low f l u x r a t i o s an d h i g h t e m p e r a t u r e s i s d e p e n d e n t upon
several facto rs.
The f i r s t m a in f a c t o r i s t h e q u a l i t y o f
t h e AlGaAs b u f f e r l a y e r .
The c o n d i t i o n s q u o t e d h e r e
r e s u l t i n AlGaAs s h o w i n g e x c e l l e n t o p t i c a l p r o p e r t i e s and
s m o o th s u r f a c e m o r p h o l o g y .
The s m o o t h n e s s o f t h e g r o w i n g
s u r f a c e p r o b a b l y r e f l e c t s a s m o o t h n e s s o f t h e s u r f a c e on
t h e s c a l e r e l e v a n t t o good e l e c t r o n t r a n s p o r t and n a r r o w
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
155
exciton linew idths.
A second im p o rta n t consequence of
g r o w t h u n d e r low f l u x r a t i o s
is th e m in im ization of
i m p u r i t i e s f ro m t h e As s o u r c e a n d r e d i s t r i b u t i o n fro m t h e
su b strate .
These c o n d i t i o n s m inim ize s u r f a c e s e g r e g a te d
i m p u r i t i e s w h i c h w o u ld o t h e r w i s e be a v a i l a b l e f o r
in c o rp o ra tio n in the w e ll.
Both o f t h e s e c o n c l u s i o n s a r e
s u g g e s t e d by t h e work o f Skromme e t a l . ^
e t a l . 49
a nd P a l m a t e e r
The a u t h o r n o t e s t h a t h e was a c o l l a b o r a t o r
w i t h P a l m a t e e r , and t h e i n t e r e s t i n th e s tu d y o f th e
i n f l u e n c e o f g r o w t h s t o i c h i o m e t r y on s u b s t r a t e i m p u r i t y
r e d i s t r i b u t i o n a r o s e fo rm t h e wo rk a b o v e on q u a n tu m w e l l
stru ctu res.
The work on t h i c k e p i l a y e r s a l l o w e d a
c h e m ic a l e v a l u a t i o n o f t h e p r e s e n c e o f i m p u r i t i e s , and i s
a n i m p o r t a n t s u p p o r t f o r t h e a r g u m e n t t h a t t h e As f l u x a t
t h e s u r f a c e d u r i n g g r o w t h c o n t r o l s t h e i n c o r p o r a t i o n and
r e d i s t r i b u t i o n of i m p u r i t i e s d u rin g w e ll grow th.
A f i n a l p o i n t on g r o w t h c o n d i t i o n s i s a g e n e r a l
s t a t e m e n t on t h e im p o rta n c e o f h ig h p u r i t y m a t e r i a l .
It
i s p r o b a b l e t h a t t h e d i s t r i b u t i o n o f d e f e c t s i n quantum
w e l l s t r u c t u r e s r e a c h e s thermodynamic e q u i l i b r i u m w ith
the claddings la y e rs .
This r e q u i r e s t h a t the g r a d ie n t of
t h e e l e c t r o c h e m i c a l p o t e n t i a l be z e r o .
it
T h i s means t h a t
i s p r o b a b l y i m p o s s i b l e t o gro w p u r e GaAs b e t w e e n
c l a d d i n g l a y e r s o f i m p u r e AlGaAs.
The a c t u a l r e l a t i v e
c o n c e n t r a t i o n s w i l l n a t u r a l l y d e p e n d on t h e d i f f e r e n c e s
b e t w e e n t h e a c t i v i t i e s o f t h e i m p u r i t y i n t h e two
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
156
m aterials.
In th e a u t h o r 's e x p e rie n c e , th e p resen ce of
e x t r i n s i c l u m i n e s c e n c e i n GaAs q u an tu m w e l l l u m i n e s c e n c e
i s a l w a y s a c c o m p a n i e d by i m p u r i t y l u m i n e s c e n c e i n t h e
AlGaAs.
This i s i l l u s t r a t e d
in P igs.
4.9(a)
a nd 4 . 9 ( b )
b o t h o f w h ic h a r e PL s p e c t r a f ro m d i f f e r e n t p o i n t s on an
e p i l a y e r o f a 100 8 SQW.
4.9 (b )
The PL s p e c t r a shown i n F i g .
e x h i b i t s i m p u r i t y l u m i n e s c e n c e i n b o t h t h e AlGaAs
s p e c t r a and t h e q u a n tu m w e l l s p e c t r a .
In the s p e c t r a in
F ig . 4 . 9 ( a ) , th e im p u rity lum inescence i s a b s e n t, or
g r e a t l y reduced in i n t e n s i t y .
The a p p a r e n t v a r i a t i o n o f
i m p u r i t i e s a c r o s s a s u b s t r a t e i s a common e f f e c t i n
su b strate related e ffe c ts.
i m p u r i t i e s which a r e
flu x r a tio s .
T h i s d e m o n s t r a t e s t h a t some
p r e s e n t r e d i s t r i b u t e e v e n u n d e r low
T h i s was d e m o n s t r a t e d q u a n t i t a t i v e l y by
Palm ateer e t a l .
49
T h i s a u t h o r h a s u s e d SIMS m o r d e r t o
i d e n t i f y im p u ritie s but the u su al d i f f i c u l t l y
i s one o f
r e p e a tin g the r e s u l t s .
The r e m a i n i n g two f a c t o r s w h ic h m i g h t be i m p o r t a n t
a r e g r o w t h r a t e an d s u b s t r a t e t e m p e r a t u r e .
The a u t h o r
h a s n o t o b s e r v e d a s t r o n g d e p e n d e n c e on g r o w t h r a t e a t
h i g h t e m p e r a t u r e s i n t h e r a n g e o f GaAs g r o w t h r a t e s o f
1 .0 t o 1 .5 pm /hr.
The l i m i t i n g f a c t o r i s p r o b a b l y t h e
AlGaAs q u a l i t y , and i t s l i m i t a t i o n s w i t h g r o w t h r a t e s .
F o r c o m p o s i t i o n b e l o w 33%, t h e a u t h o r h a s b e e n a b l e t o
grow good q u a l i t y m a t e r i a l a t t h e s e g r o w t h r a t e s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
157
1.55
1.550
1.541
.83
(b)
1.1 meV
1.54
.56
units)
I.85R8I
(arbitrary
1.554
PL intensity
.55
— 1.1 meV
(a)
1.83
Energy
F ig u re 4.9.
(eV)
V a r i a t i o n o f im p u rity lum inescence a c r o s s
w a f e r f o r a 100 $ q u a n tu m w e l l .
S c a n (b)
shows m o re i m p u r i t y l u m i n e s c e n c e a s shown
b y t h e p e a k s a t 1 . 8 1 eV a n d 1 . 5 4 eV w h i c h
a re ab sen t in scan (a).
L e f t s c a n s , pump
2
p o w e r i s lOW/cm . R i g h t s c a n s , pump
2
p o w e r i s 0 . 2 W/cm .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
158
I t w i l l be shown n e a r t h e e n d o f t h i s s e c t i o n ,
th at
t h e r e a p p e a r s t o be a d e p e n d e n c e o f GaAs q u a l i t y on
s u b s t r a t e t e m p e r a t u r e , p a r t i c u l a r l y f o r wide w e l l s
(~ 200
&) w h e r e t h e i n t r i n s i c l i n e w i d t h o f t h e GaAs c a n be a
l i m i t i n g f a c t o r i n l i n e w i d t h s o f q u an tu m w e l l s .
4.4
F a c to r s L im itin g L inew idth
In th e p re c e d in g s e c t i o n , i t has been d em o n strated
t h a t h i g h q u a l i t y s i n g l e quantum w e l l s c a n be grown on
t h i c k AlGaAs.
T h i s i s a c h i e v e d by c a r e f u l l y c o n t r o l l i n g
g r o w t h c o n d i t i o n s an d o p t i m i z i n g t h e m a t e r i a l q u a l i t y i n
the b u ffe r.
T h e s e r e s u l t s c o n t r a d i c t many o f t h e
p r e v i o u s a s s e r t i o n s c o n c e r n i n g t h e q u a l i t y o f GaAs grown
o n u n d o p e d AlGaAs.
T his s e c t io n w i l l r e c o n s id e r the
f a i l u r e m e c h a n i s m s w h i c h w e r e p r e v i o u s l y p r o p o s e d , an d
i d e n t i f y t h e f a c t o r s which l i m i t l i n e w i d t h .
The e x c i t o n l i n e w i d t h c a n be t a k e n t o r e p r e s e n t t h e
c o n v o l u t i o n o v e r a l l t h e m e c h a n is m s w hic h v a r y t h e g r o u n d
s t a t e e n e r g y o f t h e w e l l and t h e i n t r i n s i c e x c i t o n
l i n e s h a p e 18 a s s o c i a t e d w i t h t h e GaAs.
I f we m o d e l t h e
v a r i a t i o n s a s G a u s s i a n , t h e o v e r a l l l i n e w i d t h i s g i v e n by
4E = [4EGaAs +
w h e r e AE_ .
I
iE i ] 1 /2
( 4 ‘ 5>
and AE. a r e t h e f u l l w i d t h h a l f maximum
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
159
lin e w id th s of the v a r i a t io n s .
In la ttic e - m a tc h e d
c o m p o u n d s , o n e wou ld e x p e c t t h a t w e l l t h i c k n e s s
v a r i a t i o n s w o u ld be t h e m o s t i m p o r t a n t f o l l o w e d by a l l o y
flu ctu atio n s
in th e b a r r i e r la y e r .
The a l l o y
f l u c t u a t i o n s w i l l be c o n s id e r e d below .
The w o r s t c a s e
d e p e n d e n c e o f t h e g r o u n d s t a t e e n e r g y v a r i a t i o n on
t h i c k n e s s f l u c t u a t i o n s c a n be e s t i m a t e d f ro m t h e i n f i n i t e
w ell case
83
(4.6)
so
AE_ = — - —r- • AL
L
4mr L3
w h e r e m*
reduced e f f e c t i v e
The l i n e w i d t h would go a p p r o x i m a t e l y a s
(4.8)
4mr L
As L
00
(4.9)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
160
As L g e t s l a r g e ,
t h e l i n e w i d t h a p p r o a c h e s t h e GaAs
c a s e , w h ile as L + 0, th e w e ll th ic k n e s s v a r i a t i o n
dom inates.
We w i s h t o i d e n t i f y t h e s e 2 r e g i o n s
ex p erim en tally .
A more c o m p l e t e c o n s i d e r a t i o n o f t h e
l i n e w i d t h w i l l be c o v e r e d b e l o w .
4 .4 .1
E xperim ental
A s e t o f q u a n tu m w e l l s w e r e grown w i t h l a y e r
stru ctu res
id e n t i c a l to Fig.
4.3.
The g r o w t h c o n d i t i o n s
u t i l i z e d low f l u x r a t i o s w h ic h r e s u l t e d
in a m etal s ta b l e
s u rf a c e r e c o n s t r u c t i o n a t a s u b s t r a t e te m p e ra tu re of
68 0 °C .
The e p i l a y e r s w e r e grown on u n a n n e a l e d LEC
su b strate s.
The g r o w t h r a t e was 1 Um/hr f o r GaAs.
P h o t o l u m i n e s c e n c e s t u d i e s w e r e p e r f o r m e d a t 4K u s i n g t h e
6328 8 l i n e f ro m a He-Ne l a s e r u s i n g e x c i t a t i o n power
d en sities
in d ic a te d in the f ig u r e s .
The h e a v y h o l e
e x c i t o n t r a n s i t i o n f o r t h e q u an tu m w e l l i s shown a t
h i g h e r r e s o l u t i o n w i t h l o w e r pump p o w e r s i n F i g . 4 . 1 0 .
These s c a n s a r e r e p r e s e n t a t i v e of th e r e s u l t s r e p o r t e d in
T able 4 .3 .
Shown i n F i g . 4 . 1 1 and 4 . 1 2 a r e b r o a d s c a n s s how in g
b o t h t h e quantu m w e l l l u m i n e s c e n c e and t h e AlGaAs
c l a d d i n g l a y e r l u m i n e s c e n c e f o r t h e s c a n s shown i n F i g .
4.10.
On t h e b r o a d s c a n s , m o s t show a q u an tu m w e l l t o
c l a d d i n g l a y e r p e a k r a t i o o f > 1 , e x c e p t f o r L = 125 8
(Fig. 4.11
(a)).
Also a b s e n t i s th e l i g h t ho le
t r a n s i t i o n , w h ic h a p p e a r s i n t h e o t h e r s c a n s a s a p e a k on
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
161
.5535
.13 meV
</>
>*
o
-Q
k-
O
1.5462
L=IOOA
0.77 meV
1.5292
(/>
c
*<-0
c
CL
0 .6 2 meV
L= I25A
L =I75 A
.5253
0.98 meV
F ig u re 4.10.
Energy (eV)
4K PL s p e c t r a o f t h e h e a v y h o l e e x c i t o n f o r
SQW t h i c k n e s s
(L ) f r o m 200 2 t o 1 0 0 2 . W e l l
2
t h i c k n e s s , e m i s s i o n e n e r g i e s a n d FWHM l i n e ­
w idths a re in d ic a te d in th e f ig u r e .
E xcita­
t i o n i n t e n s i t y i s 0.2W/cm^ a t 6 3 2 8 2 .
R esolution i s
0 . 6 4 2 (~ 0 . 1 2 m eV ).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
162
1.55
L = 100 A
(b)
+(A
—
C
3
>»
k.
O
k.
1.83
.56
5k.
o
(0
c
<u
1.85
_J
CL
L= 125 A
.55
F ig u re 4.11.
(a)
Energy (eV)
4K PL s p e c t r a o f SQW s a m p l e s s h o w i n g c l a d d ­
i n g l a y e r l u m i n e s c e n c e a n d SQW l u m i n e s c e n c e
Pump p o w e r i s 10W/cm2 a t 5 1 4 5 $ .
Scan ( b ) :
(L =100$) AlGaAs s p e c t r a i s a t 1 . 8 3 eV, a n d
l i g h t h o l e e x c i t o n i s a t 1 . 5 6 eV a n d h e a v y
h o l e e x c i t o n i s a t 1 . 5 5 eV.
S c a n ( a ) : (L =
o
^
125 a ) N o t e t h a t l a s e r p o w e r v a r i e d d u r i n g
scan.
AlGaAs s p e c t r a i s a t 1 . 8 5 e V a n d
h e a v y h o l e e x c i t o n i s a t 1 . 5 5 eV.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
163
1.53
1.84
.83
33 x
.52
.54
1.52
1.82
PL
intensity
(arbitrary
units)
L = I75 A
L=200A
.80
F ig u re 4.12.
33 x
.53
Energy (eV)
4K PL s p e c t r a o f SQW s a m p l e s s h o w i n g c l a d d i n g
l a y e r l u m i n e s c e n c e a n d SQW l u m i n e s c e n c e .
Pump p o w e r i s 10W/cm^ a t 5 1 4 5 $ .
S c a n (b)
(Lz = 1 7 5 $ ) : AlGaAs s p e c t r a i s a t 1 . 8 e V , l i g h t
h o l e e x c i t o n i s a t 1.54eV , heavy h o l e e x c i t o n
i s a t 1.53eV . Peak a t 1.52eV i s i m p u r i t y r e ­
lated .
S c a n (a )
(L = 2 0 0 $ ) : AlGaAs s p e c t r a
z
i s a t 1.8eV , l i g h t h o le e x c i t o n i s a t 1.53eV,
h e a v y h o l e e x c i t o n i s a t 1 . 5 2 eV.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
164
th e high energy s id e of th e heavy h o le e x c i t o n .
These
two d i f f e r e n c e s r e s u l t e d f ro m a u n c o n t r o l l e d d r o p i n t h e
l a s e r power o u t p u t d u r i n g t h e s c a n .
O v e r a l l , th e sam ples a r e of v ery high q u a l i t y ,
e x h i b i t i n g e x c e l l e n t i n t e n s i t y and n a r r o w l i n e w i d t h s f o r
th e heavy h o le e x c it o n t r a n s i t i o n .
An i n t e r e s t i n g
d i f f e r e n c e between th e s p e c t r a i n P i g .
4 .1 0 and F i g s .
4 . 6 , 4 . 7 a n d 4 . 8 i s t h e a b s e n c e o f t h e low e n e r g y p e a k
sequence in F ig . 4 .1 0 .
The h i g h q u a l i t y o f t h e AlGaAs
c l a d d i n g l a y e r s i s d e m o n s t r a t e d by t h e n a r r o w n e s s o f
t h e i r l u m i n e s c e n c e , a s shown i n F i g s .
4 . 1 1 and 4 . 1 2 .
T able 4.3
El h (Lz ) L i n e w i d t h s
Lz
Layer #
FWHM
(meV + . 0 5 meV)
4 .4 .2
200 8
1363
0.80
175 8
1364
0.62
150 8
638N
0.60
125 8
1365
0.74
ioo 8
1366
1.1
75 8
1371
2.2
D iscussions
Some o f t h e q u a n tu m w e l l s c a n s
(F igs.
4.9(a)
and
( b ) ) show v a r y i n g a m o u n ts o f i m p u r i t y c o n t e n t on
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
165
d i f f e r e n t p a r t s o f th e w a f e r , which i s a c h a r a c t e r i s t i c
of s u b s tr a te r e la te d im purity r e d i s t r ib u t i o n .
The
l i n e w i d t h s of th e heavy h o le e x c ito n a r e r e l a t i v e l y
u n a f f e c t e d by t h e p r e s e n c e o f i m p u r i t i e s a nd a l l o w s u s t o
make t h e f i r s t c o n n e c t i o n w i t h p r e v i o u s w o r k .
From t h i s
d a t a th e h y p o th e s is t h a t im p u r i t i e s roughen th e s u r f a c e
d u rin g growth i s p r o b a b ly i n c o r r e c t , a t l e a s t in d i l u t e
c o n c e n t r a t i o n s p r e s e n t i n a go od q u a l i t y MBE m a c h i n e .
The l i n e w i d t h s o f t h e h e a v y h o l e e x c i t o n a r e p l o t t e d
a s a f u n c t i o n o f w e l l t h i c k n e s s on a l o g - l o g p l o t i n F i g .
4.13.
As t h e w e l l t h i c k n e s s i n c r e a s e s bey ond 125 8 t h e
lin e w id th s are co n sid e ra b ly le s s s e n s itiv e to the w ell
t h i c k n e s s , and t h i s p r o b a b l y r e p r e s e n t s t h e i n t r i n s i c
GaAs l i n e w i d t h .
L
—2 2
Below 125 8 , t h e l i n e i n c r e a s e s a s
which i s t h e c o r r e c t o r d e r o f depen d en ce one m ig h t
e x p e c t from t h e above a r g u m e n ts c o n c e r n i n g t h e e f f e c t of
in terface q u ality .
Thus a t 200 8 , t h e l i n e w i d t h s a r e a
s t r o n g f u n c t i o n o f t h e GaAs q u a l i t y i n t h e q u a n t u m w e l l
f o r the range of lin e w id th s r e p o rte d h e re .
T h i s a r g u m e n t i s f u r t h e r s u p p o r t e d by t h e PL s c a n o f
a v e r y w i d e w e l l i n F i g . 4 . 1 4 whose d a t a i s a l s o shown i n
F ig. 4.13.
The l i n e w i d t h i s v e r y n a r r o w , 0 . 3 8 meV FWHM.
The w e l l was grown a t a s u b s t r a t e t e m p e r a t u r e o f 640°C
i n s t e a d o f t h e 680°C t e m p e r a t u r e u s e d f o r t h e w e l l s
re p o rte d in F ig . 4.13.
The n a r r o w l i n e w i d t h r e p r e s e n t s
t h e b e t t e r s t r u c t u r a l q u a l i t y o f t h e m a t e r i a l grown a t
640°C .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
166
lO.OrInterface +
Alloy Fluctuation
S Ci nt ’ I.OA
^AlGaAs= 5 0 meV
AlGaAs ” O .G meV
>
a>
e
2
X
$
1.0
Tsub= 680“C o
Ij l
-C
•Alloy Fluctuation
UJ
aE G(.A»'a6meV
<
TSUb=640°C
aEAIGaAS=50meV
j _______ i
J _____i— i— i . j , i , i
10
i
i
i
i
100
i i
1000
WEL L THICKNESS (A)
F ig u re 4.13.
P l o t o f h e a v y h o l e e x c i t o n FWHM l i n e w i d t h
as a fu n c tio n o f w ell th ic k n e ss.
Open
c i r c l e s a r e f r o m s a m p l e s grown a t 680°C.
S o l i d c i r c l e i s a s a m p l e w i t h GaAs w e l l
gro wn a t 64 0°C .
S olid lin e s w ith
i n d i c a t e d p a r a m e t e r s a r e f orm t h e
c a lc u la tio n s discu ssed in S ection 4 .4 .3 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
167
9K
intensity
(arbitrary
units)
I6 K
PL
2. CD
in m
0 .3 meV
FWHM
4 K
u
L
Energy (eV )
F ig u re 4.14.
PL s p e c t r a o f a w i d e q u a n t u m w e l l a s a
fu n c tio n of tem perature.
C e n tra l peak
a t 1 . 5 1 4 8 eV i s e i t h e r a d o n o r b o u n d
exciton or a fre e exciton.
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
168
T h i s o b s e r v a t i o n a l l o w s us t o make t h e c o n e c t i o n
w i t h t h e p r e v i o u s work o f W e is b u c h e t a l .
17
which im p lie d
t h a t 680°C was a c r i t i c a l t e m p e r a u r e f o r i n t e r f a c e
d iso rd er.
I n f a c t , w h a t 680°C r e p r e s e n t s i s a t r a d e o f f
b e t w e e n t h e AlGaAs q u a l i t y i n t h e c l a d d i n g l a y e r s and t h e
GaAs q u a l i t y i n t h e q u a n tu m w e l l .
o b s e r v e d by W e is b u c h e t a l .
17
The r a p i d d e g r a d a t i o n
a b o v e 690°C was p r o b a b l y
t h e r e s u l t o f d e g r a d i n g GaAs q u a l i t y i n t h e 200 8 quantum
w e lls , not increased in te rf a c e d iso rd e r.
Th us t h e m a i n m ech an is m a f f e c t i n g i n t e r f a c e q u a l i t y
i s l i k e l y t o be t h e g r o w t h t e m p e r a t u r e a n d f l u x r a t i o o f
t h e AlGaAs c l a d d i n g l a y e r s .
The d e g r a d a t i o n o f t h e
l i n e w i d t h a b o v e 680°C r e f l e c t GaAs q u a l i t y , n o t i n t e r f a c e
q u ality .
The i m p u r i t y c o n t e n t o f t h e w e l l s i s a s t r o n g
f u n c t i o n o f b o t h t h e p u r i t y o f t h e GaAs a n d t h e AlGaAs
cladding la y e rs .
The p u r i t y o f t h e m a t e r i a l i s o p t i m i z e d
by a low m a c h i n e b a c k g r o u n d , h i g h p u r i t y s o u r c e
m a t e r i a l s , and h ig h q u a l i t y s u b s t r a t e s .
The i m p u r i t y
c o n t e n t i s m i n i m i z e d by g r o w t h a t low f l u x r a t i o s .
4 .4 .3
F i n i t e W e l l Model f o r I n t e r f a c e and B a r r i e r
A lloy F lu c tu a tio n s
The i n f i n i t e w e l l m odel f o r w e l l t h i c k n e s s
f l u c t u a t i o n s o v e r s t a t e s th e d e r i v a t i v e of th e ground
s t a t e w ith r e s p e c t to th e w e ll t h ic k n e s s .
The i n f l u e n c e
o f b a r r i e r h e i g h t v a r i a t i o n s r e s u l t i n g fro m AlGaAs a l l o y
f l u c t u a t i o n s a l s o c a n n o t be q u a n t i f i e d .
In order to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
169
i n c l u d e t h e s e e f f e c t s t h e a n a l y s i s must b e g in w ith th e
f i n i t e w e l l m odel.
The e n e r g y o f t h e e m i t t e d p h o t o n up o n
e x c i t o n r e c o m b i n a t i o n i s g i v e n by
Eo = Eg + Ee + Eh “ Eex
where
(4.10)
= GaAs b a n d g a p
h^]t 2
E + e
e
h^k^
= 5 -^ - + SJL. = 2JLh
2m*
e
2m£
h
2in
m*
= r e d u c e d e f f e c t i v e m as s
E
= e x c ito n binding energy
(4.11)
r
The a d m i t t e d l y o v e r s i m p l i f i e d a p p r o a c h t o be u s e d
h e r e i s t o compute t h e d e r i v a t i v e o f t h e ground s t a t e
w i t h r e s p e c t t o t h e v a r i a b l e o f i n t e r e s t and m u l t i p l y i t
by t h e f l u c t u a t i o n i n t h e v a r i a b l e t o e s t i m a t e th e
i n f l u e n c e on t h e g r o u n d s t a t e e n e r g y .
■
i r
•
[§ 5 T 1 + fc *“ } 4“
< 4 ' 12>
The n e x t s i m p l i f i c a t i o n i s t h a t t h e e x c i t o n b i n d i n g
e n e r g y d e p e n d e n c e on 8 i s n e g l e c t e d s o t h a t a l l t h a t i s
le f t is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
170
_ 3_ h V
a * o " 3a 2m*
(4.13)
_ h2 k . 3k
“ 2m*
3a
w h e r e t h e r e d u c e d e f f e c t i v e m ass d e p e n d e n c e on a i s a l s o
neglected.
I n o r d e r t o p r o c e e d f u r t h e r a model m u s t be
a s s u m e d f o r t h e d e p e n d e n c e o f k on a .
The u s e o f t h e
f i n i t e p o t e n t i a l w e l l model c a n be j u s t i f i e d
follow ing reaso n s.
fo r the
S e v e r a l a u t h o r s have c a l c u l a t e d th e
g r o u n d s t a t e e n e r g y o f an e x c i t o n i n a quantu m w e l l u s i n g
a v a r i e t y o f a p p r o a c h e s a nd m o d e l s .
J ia n g has p rovided a
s h o r t r e v i e w o f t h e s e w o r k s a s w e l l a s c o n t r i b u t i n g an
a d d i t i o n a l m eth o d i n w h ic h t h e m o t i o n s p e r p e n d i c u l a r t o
t h e w e l l i n t e r f a c e s a r e a s s u m e d t o b e s e p a r a b l e f ro m t h e
i n p l a n e m o t i o n s a nd t h e c o u p l i n g t h r o u g h t h e e x c i t o n i s
tre a te d a t a p ertu rb atio n .
84
T his a llo w s th e use of th e
f i n i t e w e l l model a s i n t h e o n e d i m e n s i o n a l c a s e .
The
c o r r e c t n e s s of t h i s a s su m p tio n has r e c e n t l y been
c o n f i r m e d e x p e r i m e n t a l l y i n Zeeman e f f e c t s t u d i e s o f
e x c i t o n s i n quantu m w e l l s .
79
The v a l u e o f k a s a f u n c t i o n o f t h e w e l l t h i c k n e s s ,
L, a nd b a r r i e r h e i g h t , VQ, c a n be f o u n d from t h e s o l u t i o n
of the equation
83
(4.14)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
171
w h e r e £ = ka and
/2m * V
8 = ----- c - 2- L
(4.15)
T aking th e d e r i v a t i v e o f both s i d e s g iv e s
1_ . aB2- u d . I S
2 _ 3£ _ 1 £2
3ot
£3
0Ot
SeC ? 9 a " 2
T2
172
(Sy “ 1)
(4.16)
r
or
IS
1
.2
2
3a
2 [ £2 (
for
ill
3a
(4.17)
ITT
-l)
'
sec2£ + -U
r
a = VQ,
2m* V L2
2m*zL2
(4.18)
so
2m* L2
3£
and f o r
(4.19)
(“ V
h ”)
0 r2
1/2
q2
2 [ £ 2 (A t - 1 )
sec £ +
]
S2
C2
a = 1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
172
2m*L
H
_ _________
7T 2
175
[C2 ( ^ - l )
52
Z
(4.20)
7T~
s e c 2C + ^ ]
i 1
R ecall th a t
^ 5
= * + L ^
<4.2!,
These a r e com bined, a g a in assum ing G au ssia n
d istrib u tio n s
Ae aE -
to give:
TAe2
+
'• GaAs + (m*
r
ffv“o • AV
AV ) 2 ++ (m
*
m*
^
3L
• A L ) 2 11 / 2
w h e r e k i s d e t e r m i n e d f ro m t h e f i n i t e w e l l m o d e l .
fu n ctio n is p lo tte d
param eters.
in F ig .
AL) ]
T his
4.13 along w ith th e in d i c a te d
The b a r r i e r a l l o y f l u c t u a t i o n h a s b e e n
c a l c u l a t e d f r o m t h e FWHM l i n e w i d t h o f b u l k AlGaAs and
r e p r e s e n t s t h e sum o f t h e u n c o r r e l a t e d b a r r i e r
v ariatio n s.
The o n l y f i t t e d p a r a m e t e r i s t h e w e l l
thickness v a ria tio n .
Two comments c a n be made.
F irst,
the b a r rie r a llo y
f l u c t u a t i o n c o n t r i b u t i o n i s s i g n i f i c a n t l y lower th a n th e
w e l l t h i c k n e s s v a r i a t i o n c o n t r i b u t i o n a s d e m o n s t r a t e d by
t h e l o w e r s o l i d c u r v e i n F i g . 4 . 1 3 , w h e r e AL = 0 .
Secondly, the c a l c u l a t i o n i s extrem ely s e n s i t i v e to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
173
u n r e a l i s t i c a l l y s m a l l w e l l t h i c k n e s s v a r i a t i o n s , a s shown
by t h e u p p e r s o l i d c u r v e w h ic h f i t s
1.0 8 .
t h e d a t a w i t h 5L =
This v a r i a t i o n i s l e s s than h a l f a m onolayer,
which re d u c e s t h e c o n f i d e n c e o f c a l c u l a t i n g t h e i n t e r f a c e
sm oothness f o r w e l l s t h a t a r e o f such h igh q u a l i t y .
a p p ro a c h o f S in g h and B a j a j
18
The
w h i c h a s s u m e s a random
d i s t r i b u t i o n of i s l a n d - l i k e s tr u c tu r e s a t the in te r f a c e
s e e m s more r e a s o n a b l e .
When l i n e w i d t h s become t h i s
n a r r o w h o w e v e r , o n e b e g i n s t o wonder w h e t h e r o t h e r
b r o a d e n i n g m e c h a n i s m s s h o u l d be i n c l u d e d a s w e l l .
A
f i n a l comment i s t h a t t h e e x c i t o n s i z e h a s b e e n
c o m p l e t e l y n e g l e c t e d , a nd w o u ld f u r t h e r i n f l u e n c e t h e
r e l a t i v e c o n tr ib u tio n of the b a r r ie r a llo y f lu c tu a tio n s
a n d t h e GaAs c o n t r i b u t i o n s .
I t i s p o s s ib le th a t the
s l i g h t r i s e o f t h e l i n e w i d t h b e y o n d 175 8
( 0 . 6 meV -*• 0 . 8
meV) c o u l d r e f l e c t t h e t r a n s i t i o n f ro m a two d i m e n s i o n a l
e x c i t o n t o t h e t h r e e d im e n s io n a l e x c i t o n , where th e
o v e r a l l e x c ito n shape changes.
4.5
S i R e d i s t r i b u t i o n i n M o d u l a t i o n Doped S t r u c t u e s
In preceding s e c tio n s of th e c h a p te r ,
I have
d e m o n s t r a t e d t h a t t h e r e a r e no i n t r i n s i c p r o b l e m s i n t h e
g r o w t h o f GaAs o n u n d o p e d AlGaAs.
T h is r e s u l t h as been
o b t a i n e d by c o n t r o l l i n g g r o w t h c o n d i t i o n s a n d t a k i n g c a r e
o f m a c h i n e , s o u r c e , and s u b s t r a t e i m p u r i t y e f f e c t s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
174
The p r a c t i c a l a p p l i c a t i o n o f q u an tu m w e l l s t r u c t u e s
t o d e v i c e s r e q u i r e s t h a t d o p i n g be i n t r o d u c e d i n t o t h e
stru ctu re.
T h i s a g a i n b r i n g s us b a c k t o t h e p r o b l e m o f
t h e i n v e r t e d m o d u l a t i o n d o p e d s t r u c t u r e a nd t h e i s s u e o f
i n t e r f a c e q u a l i t y i n GaAs gro wn on S i d o p e d AlGaAs.
The
p r e c e d in g s e c t i o n has d e m o n stra te d th e growth of h ig h
q u a l i t y u n d o p e d s t r u c t u r e s a n d now i t
i s n a tu r a l to
r e c o n s i d e r t h e i n t e r f a c e problem i n th e c o n t e x t o f t h e
p re se n c e of S i doping.
4 .5 .1
M a t e r i a l s G row th a nd H a l l E f f e c t M e a s u r e m e n t s
T y p i c a l s t r u c t u r e s i n v e s t i g a t e d a r e shown i n F i g .
4.15.
They h a v e b e e n grown a t a s u b s t r a t e t e m p e r a t u r e o f
680°C w i t h GaAs g r o w t h r a t e s o f l u m / h r a n d a m e t a l s t a b l e
(3x1) s u r f a c e r e c o n s t r u c t i o n .
The m e a s u r e d H a l l
m o b i l i t i e s a nd s h e e t c o n c e n t r a t i o n s f o r t h e s e s t r u c t u r e s
a r e shown i n F i g . 4 . 1 5 .
The h i g h m o b i l i t y m o d u l a t i o n
d o p e d q u an tu m w e l l w h e r e d o p i n g i s i n t r o d u c e d a f t e r t h e
l a s t gro wn i n t e r f a c e i s t y p i c a l o f w hat i s o b t a i n e d f o r
t h e c o n v e n t i o n a l m o d u l a t i o n do p ed s t r u c t u r e s .
The
m o b i l i t y f o r t h e i n v e r t e d c o n f i g u r a t i o n i s p o o r by
com parison.
4 .5 .2
SIMS S e c o n d a r y I o n Mass S p e c t r o s c o p y
S e c o n d a r y I o n Mass S p e c t r o s c o p y
(SIMS) m e a s u r e m e n t s
were p e rfo rm e d t o m easure t h e S i p r o f i l e s
i n t h e two
q e
stru ctu res.
The m e a s u r e m e n t o f S i p r o f i l e s
m AlGaAs
i s c o m p l i c a t e d by t h e p r e s e n c e o f A1H” w h ic h h a s a
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
175
GaAs
80A
< AI0.3Ga0e7As >
2400 A
AIq 3 Ga0 7 As: Si
AI0
3
Go0 .7 As
140A
115^
GaAs
quantum
noA
<J
/j .77 = 9 0 ,0 0 0 cm2/V-sec <
Ns
= 5 x I0"cm ‘ z
77
(a)
F ig u re 4.15.
I I 5A
Alo.3 Gao.7 As
I40A
Alo.3 Gao.7 As: Si
AI0 3 Ga07As
o
>
7200A
GaAs
buffer
a
. ®
.
substrate
strate
>
2.
/m 77 = 12,000 cmVV-sec
M
Ns
1 3 „ ,n 12^-2
= 1.3
x I0,2 cm'
77
(*>)
M o d u la tio n doped l a y e r s t r u c t u r e s used
s i l i c o n r e d i s t r i b u t i o n experim ents,
a)
S i l i c o n i n t r o d u c e d a f t e r quantum w e l l g ro w th ,
b)
S i l i c o n i n t r o d u c e d b e f o r e q u a n tu m
w e ll grow th.
The r e s u l t s o f H a l l m e a s u r e ­
m ents on e a c h s t r u c t u r e i s i n d i c a t e d a t
th e bottom .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
176
m o l e c u l a r w e i g h t 28 AMU and l i m i t s t h e s e n s i t i v i t y o f S i
d etection.
To a v o i d t h i s p r o b l e m , t h e s i g n a l f o r 30S i +
was o b s e r v e d f o r c o m p a r i s o n t o 2 8 g i .
where agreem ent
b e t w e e n t h e tw o p r o f i l e s i s g o o d , i t c a n be a s s u m e d t h a t
the 28si p r o f i l e i s r e p r e s e n ta tiv e of the a c tu a l Si
p ro file.
F igs.
The r e s u l t s o f t h e m e a s u r e m e n t a r e s e e n i n
4 .1 6 and 4 . 1 7 .
Si appears to e x i s t predom inantly
in t h e l o c a t i o n s where i t has been i n t e n t i o n a l l y i n t r o ­
duced.
Some a c c u m u l a t i o n o f S i a t t h e s u r f a c e o f e a c h
s tr u c tu r e is a lso observed.
a d d e d a f t e r t h e GaAs w e l l
In th e s t r u c t u r e w ith Si
(F ig . 4.16)
a Si accum ulation
i s o b s e r v e d i n t h e n e a r s u r f a c e r e g i o n , 2000 8 b e y o n d t h e
r e g i o n w h e r e i t was i n t e n t i o n a l l y i n t r o d u c e d , and i s
evidence of s u rfa c e s e g re g a tio n of the dopant.
4 .5 .3
P hotolum inescence
While p h o to lu m in e s c e n c e
(PL) f r o m e x c i t o n s i n
q u an tu m w e l l s i s a u s e f u l t e c h n i q u e f o r e v a l u a t i n g
i n t e r f a c e q u a l i t y and t h e p r e s e n c e o f i m p u r i t i e s i n
u n d o p e d q u a n tu m w e l l s t r u c t u r e s , t h e l u m i n e s c e n c e fro m
m o d u l a t i o n d o p e d w e l l s i s more d i f f i c u l t t o c o r r e l a t e
w ith s t r u c t u r a l p r o p e r t i e s .
A n e t c a r r i e r d e n s i t y and
b a n d b e n d i n g i n t h e q u a n tu m w e l l r e g i o n h a s b e e n shown t o
cause broadened lum inescence lin e w id th w h ile th e
s tru c tu re s t i l l ex h ib its ex cellen t e le c tr ic a l
properties.
Qg
The l u m i n e s c e n c e i s l i k e l y d u e t o f r e e
c a r r i e r r e c o m b i n a t i o n r a t h e r t h a n t h e e x c i t o n i c m ech an is m
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
177
,24
,20
Concentration
(a to m s /c c )
,22
28
0
Figure
.16.
0.10
0.20
0 .3 0
Depth (m icrons)
0 .4 0
0.50
SIMS s c a n o f s a m p l e w i t h s i l i c o n i n t r o d u c e d
a f t e r quantum w e l l g ro w th .
S im ultaneous
p e a k s i n t h e m a s s 30 a n d m a s s 28 s i g n a l s
indicate tru e s ilic o n p ro file s.
Peak a t
0.25 m icrons i n d i c a t e s i n t e n t i o n a l
s i l i c o n doping.
Peak a t 0.04 m icro n s
in d ic a te s su rfa c e segregated s ilic o n .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0
F ig u re 4.17.
0.10
0 .2 0
0 .3 0
Depth m icrons
0.40
0 .5 0
SIMS s c a n o f s a m p l e w i t h s i l i c o n i n t r o d u c e d
p r i o r t o quantum w e l l g ro w th .
Sim ultaneous
p e a k s i n m a s s 30 a n d m ass 28 i n d i c a t e
tru e silic o n p ro file .
Peak a t 0 .30 m icro n s
i s i n t e n t i o n a l s i l i c o n doping.
Note red u ced
s i l i c o n s u r f a c e accu m u latio n a t 0.04 m icrons.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
179
o f undoped w e l l s .
The u s e f u l n e s s o f PL h e r e i s i n a
co m p ariso n between t h e sam ples r a t h e r th a n t h e e v a l u a t i o n
of in terface q u ality .
The g e n e r a l l i n e s h a p e f u n c t i o n f o r
a q u a n t u m w e l l m o d u l a t i o n d o p e d s t r u c t u r e w i l l be
discu ssed in th is ch ap ter.
In P ig .
4 . 1 8 , t h e l u m i n e s c e n c e o f a SQW ( P i g .
w i t h S i d o p i n g a f t e r t h e w e l l a nd SQW ( F i g . 4 . 1 8 b )
S i d o p i n g b e f o r e t h e w e l l a r e shown.
r e g i o n o f 1 . 9 eV i n P i g s .
w ith
The p e a k s i n t h e
4 . 1 8 a , b a r e fro m t h e
and i n d i c a t e t h e s i m i l a r h ig h q u a l i t y o f t h e
c la d d in g l a y e r s in both sam ples.
4.18a)
(AlGa)As
(AlGa)As
A d is tin c t d ifferen ce
b e t w e e n t h e two s a m p l e s i s o b s e r v e d i n t h e m ai n
lu m in e sc e n c e peak a t lower e n e r g i e s
( 1 . 5 - 1 . 6eV).
P ig.
4 . 1 8 a shows an a s y m m e t r i c e m i s s i o n w h ic h i s
c h a r a c t e r i s t i c o f a m o d u l a t i o n d o p e d SQW w h i l e P i g . 4 . 1 8 b
i s b r o a d e r a nd n e a r l y s y m m e t r i c . I t w i l l be shown b e l o w
t h a t t h e b r o a d low e n e r g y t a i l o b s e r v e d f ro m t h e q u an tu m
w e l l i n P i g . 4 .1 8 b i s c h a r a c t e r i s t i c o f a h e a v i l y doped
d e g e n e r a t e i m p u r i t y ban d c o n t r i b u t i n g t o t h e d e n s i t y o f
s t a t e s below t h e normal f i r s t c o n d u c tio n subband and i s
in d ic a tiv e of the presence of im p u ritie s .
87
According
t o t h e m o d e l d i s c u s s e d b e l o w , t h e s y m m e t r i c FWHM p e a k
w i d t h o f 31 meV c o r r e s p o n d s t o a s i l i c o n c o n c e n t r a t i o n o f
4 x 1 0 11 cm” 2 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
180
.90
1.54
.87
.49
31 meV FWHM
Intensity
(orb. units)
.89
PL
1.90
1.55
1.86
I
!3meV FWHM
1.49
Energy (eV)
F ig u re 4.18.
4K PL s c a n o f m o d u l a t i o n d o p e d SOW shown i n
Fig.
4.15.
P e a k s a t 1 . 9 eV a r e f r o m t h e
AlGaAs c l a d d i n g l a y e r s ,
a f t e r SQW.
a) S i i n t r o d u c e d
P e a k a t 1 . 5 5 i s f r o m t h e SQW.
b) S i i n t r o d u c e d b e f o r e SQW.
i s f r o m SQW.
Peak a t 1.54eV
Note b ro a d sym m etric l i n e s h a p e .
A l s o n o t e l a r g e p e a k a t 1 . 8 6 eV i n
( a ) , which
may i n d i c a t e a l a r g e s i l i c o n c o n t e n t i n t h e
AlGaA s.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
181
4 .5 .4
TEM M e a s u r e m e n t s
The s i n g l e q u a n tu m w e l l s t r u c t u r e s w e r e s t u d i e d by
means o f
(002) d a r k - f i e l d m i c r o s c o p y a nd l a t t i c e
The c r o s s - s e c t i o n a l TEM r e s u l t s g i v e i n f o r m a t i o n
im aging.
88
regarding:
a)
t h e e x a c t quantum w e l l t h i c k n e s s and t h i c k n e s s
hom ogeneity
b)
th e chem ical a b ru p tn e ss of the i n t e r f a c e a t the
GaAs q u a n t u m w e l l
c)
the l a t t i c e
s t r u c t u r e o f t h e q u a n tu m w e l l and
in terfaces
d)
th e e f f e c t of p o s s i b l e sm earing o f th e Si
doping p r o f i l e a s a r e s u l t o f Si s e g r e g a t i o n t o
t h e s u r f a c e d u r i n g MBE g r o w t h .
In Pig.
4 .1 9 (a), a
(002) d a r k - f i e l d
G a A s /n :A lG aA s h e t e r o j u n c t i o n
(Pig.
q u a n t u m w e l l t h i c k n e s s i s 93 8 .
im ag e o f n o r m a l
4.15a)
i s sh own.
The q u a n t u m w e l l
i s o f ho mo gen eo us t h i c k n e s s and c o m p o s i t i o n .
4 . 1 9 ( b ) , a high r e s o l u t i o n l a t t i c e
h e t e r o i n t e r f a c e i s s how n.
The
layer
In F ig.
im ag e o f t h e
T h e r e i s no e v i d e n c e o f
d e f e c ts or d is lo c a tio n s a t the in te r f a c e .
In F ig.
4.20(a)
a
(002) d a r k f i e l d
i n v e r t e d G a A s / n :A lG a A s h e t e r o j u n c t i o n
sh ow n.
image o f an
(Pig.
The q u a n tu m w e l l t h i c k n e s s i s 110 8 .
4.15b)
is
The qu antum
w e l l i s o f h o m ogeneous t h i c k n e s s a nd c o m p o s i t i o n a nd i s
s i m i l a r i n s t r u c t u r e t o t h e image i n F i g .
4.19(a)
o f the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
F ig u re 4.19.
(a)
Dark f i e l d image o f t h e sam ple
shown i n F i g .
4 .1 5 (a).
(b) L a t t i c e
image o f h e t e r o i n t e r f a c e .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
183
F ig u re 4.20.
(a)
D a r k f i e l d im age o f t h e s a m p l e
shown i n F i g .
4 .15(b).
(b) L a t t i c e
im a g e o f h e t e r o i n t e r f a c e .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
184
r e g u l a r m o d u la tio n doped s t r u c t u r e .
high r e s o l u t i o n l a t t i c e
sh ow n.
In F igure 4 .2 0 (b ), a
im age o f t h e h e t e r o i n t e r f a c e i s
T h e r e i s no e v i d e n c e o f d e f e c t s o r d i s l o c a t i o n s
and i s s i m i l a r i n s t r u c t u r e t o F i g .
In F ig. 4 .2 1 (a ),
in v erted
4.1 9 (b ).
(002) d a r k - f i e l d
i m a g e s o f an
h e t e r o j u n c t i o n i s shown ( n o t t h e same a s 4 . 1 4 b ) .
The 3 nm GaAs q u a n tu m w e l l was f o u n d t o be homogeneous
t h i c k n e s s and c o m p o s i t i o n .
in
The t o p 7 nm GaAs l a y e r
h o w e v e r shows c l e a r b l a c k t r i a n g u l a r f e a t u r e s bound by
the
(001) s u r f a c e a n d t h e two
ill
type p la n e s .
These
l a t t i c e f e a t u r e s w i l l be r e f e r r e d t o a s " p y r a m i d s . "
The
d e n s i t y o f p y r a m i d s v a r i e s a n d i s f o u n d t o be h i g h e s t a t
th e edge of th e w a f e r.
s t u d i e d by l a t t i c e
d irectio n .
T h e s e p y r a m i d a l d e f e c t s wer e
imaging i n th e
[110] c r o s s - s e c t i o n a l
The r e s u l t s a r e s u m m a r i z e d i n F i g . 4 . 2 1 b .
M ost o f t h e s u r f a c e i s r o u g h w i t h a r o u g h n e s s o f
2 - 3 nm ( F i g . 4 . 2 1 a ) .
Pyramids w ith c u t - o f f to p s ly in g
e n t i r e l y w i t h i n t h e t o p GaAs l a y e r
(F ig.
4 . 2 1 b ) and f u l l y
d e v e l o p e d p y r a m i d s w h i c h e x t e n d down i n t o t h e
(AlGa)As
c a n be o b s e r v e d i n i n v e r t e d h e t e r o j u n c t i o n s t r u c t u r e .
From t h e TEM c o n t r a s t , t h e p y r a m i d s a p p e a r t o c o n s i s t o f
am o r p h o u s m a t e r i a l o f a t o m i c number l o w e r t h a n GaAs.
c o n c l u s i o n s o f t h e TEM work a r e t h e f o l l o w i n g :
a.
The S i - d o p i n g d o e s n o t seem t o
in flu e n c e the
q u a l i t y o f GaAs quan tu m w e l l s .
T h e r e i s no
e v i d e n c e f o r t h e p r e s e n c e o f A1 i n t h e GaAs
w ells.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The
F ig u re 4.21.
(a) D a r k f i e l d im a g e o f a 30& m o d u l a t i o n
d o p e d w e l l q u a n t u m s t r u c t u r e s h o w in g
p y r a m i d s on t h e s u r f a c e .
(b)
L attice
image o f t h e s u r f a c e py ram id s t r u c t u r e .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
186
b.
The TEM o b s e r v a t i o n s r e v e a l t h a t t h e s u r f a c e
s e g re g a tio n o f S i a p p a re n tly can g iv e r i s e to
t h e f o r m a t i o n o f s u r f a c e r o u g h n e s s and p y r a m i d s
o f amorphous m a t e r i a l .
The same r e s u l t s w e r e
a ls o observed fo r in v e rte d h e te ro ju n c tio n
s t r u c t u r e s grown on a s u p e r l a t t i c e b u f f e r o r
w i t h a s u p e r l a t t i c e s u b s t i t u t e d f o r t h e und o p ed
spacer layer
89
The o b s e r v a t i o n o f S i p y r a m i d s i s r e m i n i s c e n t o f t h e
work by D u t t e t a l . ^ ® o n g r a d e d b a n d g a p
gro wn by LPE.
( A l G a ) A s i S i L E D 's
A lthough t h e c h e m is tr y o f th e pyram ids
c o u l d n o t be d e t e r m i n e d by means o f SIMS, AES o r AEM,
qn
p r e v i o u s work
a nd t h e p r e s e n t TEM r e s u l t s s u g g e s t t h a t
t h e o b s e r v e d s u r f a c e f e a t u r e s a r e c a u s e d by t h e s u r f a c e
s e g re g a tio n of th e Si dopant.
4 .5 .5
D iscussion
The r e s u l t s o f t h e PL, SIMS, H a l l m e a s u r e m e n t s , a n d
TEM s t r o n g l y s u g g e s t t h a t t h e f a i l u r e o f t h e i n v e r t e d
m o d u l a t i o n d o p e d s t r u c t u r e i s d ue t o s i l i c o n s u r f a c e
s e g r e g a tio n r a t h e r than i n t e r f a c e roughness or l a t t i c e
stra in .
91
The l i k e l y m e c h a n is m d u r i n g g r o w t h i s t h a t a s
s i l i c o n d o p i n g p r o c e e d s i n AlGaAs, a s u r f a c e p o p u l a t i o n
o f S i adatoms b u i l d up.
As GaAs w e l l g r o w t h b e g i n s , t h e
S i s u r f a c e a to m s i n c o r p o r a t e i n t h e GaAs r e g i o n n e x t t o
the in te rf a c e .
The p r e s e n c e o f s i l i c o n i n t h e w e l l c a n
be o b s e r v e d i n t h e PL and t h e s e g r e g a t i o n c a n be o b s e r v e d
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
187
i n t h e SIMS.
The s t r u c t u r a l q u a l i t y b e t w e e n t h e
i n t e r f a c e s o f t h e two s t r u c t u e s i s e q u i v a l e n t w i t h i n t h e
r e s o l u t i o n o f TEM.
The low H a l l m o b i l i t y o f t h e i n v e r t e d
s t r u c t u r e r e s u l t s f ro m s c a t t e r i n g f ro m a s c r e e n e d
d e g e n e ra te im p u rity band.
4.6
Si R e d is tr ib u tio n
The p r o b l e m o f i m p u r i t y s u r f a c e s e g r e g a t i o n h a s b e e n
c o n s i d e r e d f r o m a v a r i e t y o f view p o i n t s .
In g en eral
t e r m s , t h e two m ai n p r o c e s s e s w h i c h m i g h t i n f l u e n c e b u l k
i m p u r i t y d i s t r i b u t i o n s a r e s u r f a c e e f f e c t s , where t h e
i m p u r i t y s i t s on t h e s u r f a c e , o r n e a r s u r f a c e e f f e c t s ,
w h e r e t h e i m p u r i t y moves i n t h e b u l k b e c a u s e o f d r i v i n g
f o r c e s unique to th e s u r f a c e .
S u r fa c e e f f e c t s have been
u s e d t o e x p l a i n t h e o b s e r v e d s u r f a c e s e g r e g a t i o n o f Sn
d u r i n g GaAs g r o w t h by MBE.
92
Nea r s u r f a c e b u l k e f f e c t s
s u c h a s s u r f a c e e l e c t r i c f i e l d s and d i s t a n c e d e p e n d e n t
d i f f u s i o n c o e f f i c i e n t s h a v e b e e n s u g g e s t e d a s t h e m ain
r e d i s t r i b u t i o n m echanis m f o r i m p u r i t i e s w i t h r e l a t i v e l y
f a s t d i f f u s i o n c o e f f i c i e n t s s u c h a s F e , C, Mn, and C r .
69
The m ain c o n s t r a i n t b e t w e e n t h e two t y p e s o f
mechanisms i s t h e m ag n itu d e o f th e d i f f u s i o n c o e f f i c i e n t
for surface f ie ld s ,
t h e i o n d r i f t v e l o c i t y m u s t be a t
l e a s t th e o r d e r o f th e grow th r a t e ,
J = vc = U cE
thus:
(4.23)
R e p r o d u c e d with permission o f the copyright owner. Further reproduction prohibited without permission.
188
or:
v = \iE
(4.24)
where E i s th e e l e c t r i c f i e l d , c i s th e io n
c o n c e n t r a t i o n , and u i s t h e m o b i l i t y .
I f we u s e t h e
E i n s t e i n r e l a t i o n f o r u and D:
v = —2 - _ E
kf7q
<4 *25)
a t 1 u m / h r , T = 68 0 °C , a n d E = 100 kV/cm, t h e minimum
d iffu sio n c o e ffic ie n t is:
D = 2 . 3 E - 1 4 cm2/ s
The d i f f u s i o n c o e f f i c i e n t o f S i i n GaAs a t 680°C i s on
t h e o r d e r o f 2 x 10” ^
th is
cm2/ s , ® 2 t h u s i t
is unlikely th at
i s a s i g n i f i c a n t r e d i s t r i b u t i o n mechanism, even w ith
i n c r e a s e s i n D t h a t m ight o c c u r w ith i n c r e a s e d v a c a n c ie s
near the s u rf a c e .
T herefore, i t
i s pro b ab le t h a t the
s e g r e g a t i o n i s o c c u r r i n g on t h e s u r f a c e .
sectio n ,
th is
In the next
i s d i s c u s s e d i n a c o m p a r i s o n b e t w e e n GaAs,
w h e r e t h e s e g r e g a t i o n i s l o w , and AlGaAs, w h e r e i t
high,
The hope i s t o i d e n t i f y t h e d i f f e r e n c e s
is
in
c h e m i c a l a c t i v i t y b e t w e e n GaAs a nd AlGaAs a s s o l v e n t s f o r
d ilu te so lu te concentrations.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
189
4 .6 .1
S i l i c o n S u r f a c e S e g r e g a t i o n o n AlGaAs
The i n c r e a s e d i n c o r p o r a t i o n o f S i a s GaAs g r o w t h
b e g i n s p r o b a b l y r e s u l t s f ro m a c h a n g e i n t h e G i b b s f r e e
e n e r g y o f s e g r e g a t i o n a t t h e s u r f a c e b e t w e e n t h e two
m ateria ls.
The e f f e c t see m s s i m i l a r t o t h e c a s e o f t i n
Q9
i n GaAs a s f i r s t r e p o r t e d by Cho
and s u b s e q u e n t l y
9 4 —9 7
95
s t u d i e d by many a u t h o r s
. Wood e t a l .
and
95
A lexandre e t a l .
a ttr ib u te d the s e g re ta tio n to k in e tic
07
lim itatio n s.
The m o d el p r o p o s e d by R o c k e t t e t a l .
a t t r i b u t e d the s e g re g a tio n of t i n to i t s
r e l a t i v e t o t h e GaAs l a t t i c e .
l a r g e atom s i z e
They c o n n e c t e d t h e atom
s i z e t o a s e g r e g a t i o n p o r e n t i a l w h i c h w o u ld r e s u l t i n a
su rfa c e p opulation of t i n .
A lexandre e t a l .
96
has
p o i n t e d o u t t h a t t h e t r e n d see m s c o n s i s t e n t w i t h t h e r e s t
o f t h e column IV e l e m e n t s , w h e r e s e g r e g a t i o n i n c r e a s e s
w i t h i n c r e a s i n g a to m ic number.
I n t h e c a s e o f S i i n GaAs a n d AlGaAs, S i l i e s
b e t w e e n Ga and Al i n a t o m i c n u m b e r .
in
On t h e b a s i s o f t h e
s i z e a r g u m e n t , i t m i g h t seem t h a t S i wou ld s e g r e g a t e more
on t h e AlGaAs s u r f a c e .
U n f o r tu n a te ly t h i s argument i s
w e a k e n e d by t h e d i f f e r e n c e i n l a t t i c e c o n s t a n t b e t w e e n
AlAs a n d GaAs, a s d i s c u s s e d b e l o w .
I t s h o u l d a l s o be
n o t e d h e r e t h a t S i s e g r e g a t e s on t h e s u r f a c e o f GaAs,
which i s o f t e n n o t r e c o g n i z e d .
An a d d i t i o n a l
c o m p l i c a t i o n i n t h e c a s e o f AlGaAs i s t h e d i s o r d e r e d
g r o u p I I I s u b l a t t i c e a nd t h e s m a l l l a t t i c e c o n s t a n t
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
190
d i f f e r e n c e b e t w e e n GaAs a nd A lA s .
The p r e s e n c e o f a
s m a l l am ount o f l a t t i c e p o s i t i o n d i s o r d e r
( n o t j u s t G roup
III
c o n f i g u r a t i o n a l d i s o r d e r ) w o u ld c a u s e d i f f e r e n c e s i n
AQ
CO
t h e b o t h t h e e n t h a l p y ° and e n t r o p y
(v ib ratio n al
entropy)
of a given s i t e .
com paring th e d i f f e r e n c e s
T h i s c a n be u n d e r s t o o d by
in the lo c a l c r y s ta l
e n v i r o n m e n t b e t w e e n GaAs a n d AlGaAs a s s e e n by a s i l i c o n
a to m on a g r o u p I I I s i t e .
The p r e s e n c e o f alum inum on
th e n e a r e s t n eig h b o r group I I I s i t e
resu lts
in a
com pression o f th e s i l i c o n s i t e over t h a t f e l t in a pure
GaAs c r y s t a l .
T his i s because th e l a t t i c e c o n s ta n t of
AlAs i s s l i g h t l y l a r g e r t h a n GaAs.
AlGaAs x <
0.50,
I n low c o m p o s i t i o n
the average lo c a l l a t t i c e co n sta n t is
m ore l i k e GaAs, a nd AlAs w o u l d t e n d t o i n t r o d u c e
com pressive s t r a i n a t th e s i l i c o n s i t e .
One m i g h t e x p e c t
d i f f e r e n t G ibbs f r e e e n e r g i e s o f s i t e o c c u p a t i o n on t h e
b a s i s o f b o t h t h e number o f A l a nd Ga a t o m s on t h e n e x t
n e a r e s t n e i g h b o r s i t e s and on t h e i r c o n f i g u r a t i o n .
This
v a r i a t i o n o f e n e r g i e s m i g h t e f f e c t i v e l y remove some s i t e s
fo r occupation.
Thus, i t
i s p o s s ib le th a t the in creased
S i s u r f a c e s e g r e g a t i o n on AlGaAs r e l a t i v e t o GaAs a r i s e s
f ro m an e f f e c t i v e r e d u c t i o n i n t h e number o f s i t e s
a v a i l a b l e f o r o c c u p a t i o n a s c o m p a r e d w i t h GaAs.
be i n t e r e s t i n g
I t would
to ev a lu a te ex p erim en tally the s e g re g a tio n
o f S i on AlAs i n l i g h t o f t h e s e a r g u m e n t s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
191
T h i s a r g u m e n t i m p l i e s t h a t t h e s e g r e g a t i o n wo uld be
present if
t h e s u r f a c e were a t e q u i l i b r i u m in t h e b u lk .
T h i s m i g h t be a n i m p o r t a n t p o i n t , a s a way t o r e d u c e t h e
s e g r e g a t i o n m i g h t be t o u s e g r o w t h s t o p s , a s i s d i s c u s s e d
b e l o w i n t h e c a s e o f s u r f a c e s e g r e g a t i o n i n GaAs.
I n t h i s t h e s i s some p r e l i m i n a r y work u t i l i z i n g
grow th i n t e r r u p t i o n s has been s t a r t e d .
SIMS h a s b e e n
u t i l i z e d to e v a lu a te the d iff e r e n c e in Si s u rfa c e
s e g r e g a t i o n on GaAs b e t w e e n l a y e r s grown c o n t i n u o u s l y and
l a y e r s u t i l i z i n g growth i n t e r r u p t i o n s .
s t r u c t u r e i s shown i n F i g .
4.22.
The l a y e r
The l a y e r s t r u c t u r e s
a r e grown w i t h i d e n t i c a l g r o w t h c o n d i t i o n s a t 600°C
e x c e p t t h a t d u r i n g g r o w t h , l a y e r 333 was i n t e r r u p t e d and
a l l o w e d t o s i t u n d e r a n As f l u x p r i o r t o g r o w i n g t h e
undoped c a p p in g l a y e r .
As shown by SIMS i n F i g . 4 . 2 3 ,
t h e S i a c c u m u l a t i o n l a y e r a t t h e s u r f a c e o f 333 i s
re d u c e d u s in g t h e grow th i n t e r r u p t i o n a s compared w ith
t h e SIMS s c a n i n F i g .
4.24 of 334.
S im ila r approaches
may h e l p t o i m p r o v e t h e s e g r e g a t i o n b e h a v i o r o f S i d o p i n g
i n AlGaAs.
T h i s would be i n e f f e c t i v e a c c o r d i n g t o t h e
above a r g u m e n ts , which im ply a thermodynamic l i m i t a t i o n ,
w h ile growth s t o p s a r e e f f e c t i v e f o r k i n e t i c l i m i t a t i o n s .
The i n i t i a l s t u d i e s h e r e i n AlGaAs h a v e n o t f o u n d g r o w t h
s t o p s t o be e f f e c t i v e .
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
192
334
-----------------------
Continuous
growth
333
GaAs undoped
20001
10 min.
growth
interruption
GaAs: Si
IOl7cm'3
4000 A
GaAs undoped
Substrate
F ig u re 4.22.
L a y e r s t r u c t u r e f o r r u n s 333 a n d 334
f o r c o n tin u o u s and grow th i n t e r r u p t i o n
experim ents.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
193
o
o
</>
CONCENTRATION
E
o
o
0 .3 0
F ig u re 4.23.
0.60
DEPTH
0.90
(microns)
SIMS s c a n s o f l a y e r
in terru p tio n .
Note
tio n p r o f i l e in the
.20
.50
333 w i t h g r o w t h
th e s ilic o n co ncentra­
to p 0.2 m icrons.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
194
o 10
2 10
< 10
0
F ig u re 4.24.
0 .3 0
0 .6 0
0 .9 0
DEPTH (m icrons)
1.20
1.50
SIMS s c a n o f l a y e r 334 w i t h o u t g r o w t h
in terru p tio n .
Note t h e h i g h e r s i l i c o n
c o n c e n tr a tio n p r o f i l e in th e to p 0.2
m ic ro n s a s compared w i t h F i g .
4.23.
A lso n o te t h e s i m i l a r i t y o f th e c arb o n
and oxygen p r o f i l e s t o F ig .
4.23.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
195
4 .6 .2
S ilic o n D iffusion
I n s t r u c t u r e s s u c h a s n a r r o w s p a c e r HEMTs, e v e n
modest r e d i s t r i b u t i o n can cause s i g n i f i c a n t r e d u c t i o n s in
electro n tran sp o rt p ro p erties.
A m e c h a n is m w h ic h i s
o f t e n t h o u g h t t o be i m p o r t a n t i s t h e h i g h e l e c t r i c f i e l d
in the spacer region.
In the next s e c tio n t h i s e f f e c t is
in c lu d e d in a sim ple e v a lu a tio n of th e d r i f t / d i f f u s i o n of
a c h a r g e i m p u l s e i n t h e p r e s e n c e o f an e l e c t r i c f i e l d .
S t a r t i n g w ith th e flu x e q u a tio n
j
.
CD
3 jj
kT 'li*
B± .
q 3x 1
99
(4.26)
and t h e c o n t i n u i t y e q u a t i o n :
3c _ 3J
3 t " 3X
u t i l i z i n g H e n r y s law
u = u
o
CD
kT
(4.27)
48
+ kT Jin C
“
(4.28)
(4.29)
(4.30)
(4.31)
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
I f c o u p le d w ith P o i s s o n s e q u a t i o n , t h i s problem
q u i c k l y bec omes u n s o l u a b l e .
Instead,
I sh all ju st tre a t
t h e e l e c t r i c f i e l d a s c o n s t a n t s o t h a t an a n a l y t i c
e x p r e s s i o n c a n be o b t a i n e d .
ac _ _ 32C , qD _ 3C
I t " D ^ 2 + k T E 7x
(4.33)
R ew riting t h i s eq u atio n
3C
2 32C .
^
3 ?
.
3C
(4.34)
M
where
= D
b = f§E
The b o u n d a r y c o n d i t i o n s a r e
C ( x , o) = g ( x )
where g(x)
(4.35)
i s the i n i t i a l im purity p r o f i l e .
a s s u m e C + o a t x + + M.
F u rth er,
F o u rie r tra n sfo rm in g the x
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
197
v a r i a b l e , where th e tr a n s f o r m p a i r
fa, = 4 .
/2 ir
r
-00
is defined as:
£(x) e- w x dx
<4-36>
+ 00
f (x) =
/2
tt
/
-®
F (X) e +lXx dX
r e s u lts in:
f f = - a 2X2C - ibXC
= -
<4 *37)
( a 2X2 + ibX) C
C (X, 0) = G (x )
who se s o l u t i o n s i s :
C (X, ( t )
= G(X) e
- ( a 2X2 + i b L ) t
(4.38)
w h i c h up on i n v e r s e t r a n s f o r m i n g g i v e s
C (* , t ,
.-L .
/2 n
-.»>
r
a - ( ^ 2- l ( b t - K ) X ) / d X ( 4 . 3 9 )
00
To p r o c e e d f u r t h e r g ( x ) m u s t be s p e c i f i e d .
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
198
R e c a l l t h e a s s u m p t i o n s t h a t l e a d t o E q.
m a in a s s u m p t i o n was t h a t t h e e l e c t r i c f i e l d
(4 .3 9 ).
The
is co n stan t,
or t h a t th e im p u rity d i s t r i b u t i o n i s not coupled to
P o is s o n 's eq u atio n .
When t r y i n g t o c a l c u l a t e t h e t i m e
re q u ire d fo r s i l i c o n t o d i f f u s e through th e sp a c e r to the
GaAs r e g i o n , o n e w o u ld l i k e a n i m p u r i t y d i s t r i b u t i o n t h a t
a l l o w s o n e t o p r e d i c t t h e s p a t i a l movement o f t h e
im purity f ro n t.
in itial
H e r e we w i l l
im pulse of d o p a n t.
c o n s i d e r th e d r i f t o f an
W hile th e e l e c t r i c f i e l d w i l l
vary as th e t o t a l charge r e d i s t r i b u t e s ,
i f t h e am ount o f
c h a r g e i n th e im pulse i s low, t h e e l e c t r i c f i e l d w i l l n o t
c h a n g e much a c r o s s i t .
So l e t
g(x)
= Nq 5 (x)
(4.40)
w h e r e 5 (x) i s t h e d i r a c d e l t a f u n c t i o n , and
2
Nq h a v i n g u n i t s o f #/cra .
Then
G (X) = - ==2
__
/ 2 ir
J
5 ( x ) e " iXxdx
(4.41)
N_
G (X) = - z z 2
/ 2 tt
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
199
so t h a t
C ( ,
t )
=
I
s Z
e - a 2 x 2 t - l < b t - x )X
dx
<4 ‘ 4 2 >
w h i c h c a n be e x p r e s s e d i n t h e f o l l o w i n g s u g g e s t i v e f o r m :
C (x ,t)
= ~
J
e " a tX
cos { (bt-x)X }dx
( 4 *4 3 >
o
T h i s i n t e g r a l c a n be d o n e r a t h e r c l e v e r l y o r by t a b l e s ,
th e r e s u l t i n g e x p re ss io n being:
C (x ,t)
No
r
= ------------- e x p {-------------------------}
2 /ir D t
(4.44)
4DT
w h i c h i s s i m p l y a n e x p a n d i n g G a u s s i a n p r o f i l e whose
c e n t e r d r i f t s due t o t h e e l e c t r i c f i e l d w ith a v e l o c i t y
_ gO E
v " kT
(4.45)
The i m p o r t a n c e t h a t r e s u l t s fro m t h i s e x p r e s s i o n i s
th a t for s h o rt tim es,
it
is the d if f u s iv e term s, not the
d r i f t t e r m s , which r e s u l t in t h e g r e a t e s t r e d i s t r i b u t i o n .
The i o n d r i f t i n a f i e l d o f 100 kV/cm a t 680°C w h e r e
D . = 4 x 1 0 ~ 15 cm2/ s
bX
i n AlGaAS
go
i s g iv e n by:
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
200
_ (4X1Q-16 a ! ) (1 0 0 ,0 0 0
(4>4S)
82 meV
= 4 . 8 8 x 1 0 "10 | S
f o r t = 240 s e c
(4 m i n ) , t h e d i s t a n c e t h e c e n t e r o f t h e
G a u s s i a n moves i s o n l y
x = ( 4 . 8 8 x 1 0 "10 c m / s )
(240 s e c )
= 11.7 8
W hile th e d i f f u s i o n l e n g t h g iv e s
L = /D t
2
= ( ( 4 x l 0 “ 16 J 2- ) (240 s e c ) ) 1 / 2
= 31 8
The d o m i n a n t c o n t r i b u t i o n i s t h e d i f f u s i o n l e n g t h .
From t h i s c a l c u l a t i o n ,
it
i s c l e a r t h a t th e growth of
h i g h q u a l i t y s h o r t s p a c e r HEMTs (~ 40 8 ) c a n o n l y be
a c h ie v e d a t th e low er te m p e r a tu r e s .
4.7
E l e c t r i c a l P r o p e r t i e s o f M o d u l a t i o n Doped S i n g l e
Quantum W e l l s
I n t h e p r e c e d i n g s e c t i o n i t was shown t h a t d u r i n g
h ig h te m p e r a tu re growth
(TSUk = 6 8 0 ° ) o n l y a m o d u l a t i o n
d o p e d q u an tu m w e l l w i t h S i i n t r o d u c e d a f t e r w e l l g r o w t h
e x h i b i t s high m o b ility .
The c o m b i n a t i o n o f h i g h e l e c t r o n
m o b i l i t y w i t h a n AlGaAs b u f f e r makes t h e s t r u c t u r e i d e a l
f o r a s h o r t g a t e m i c r o w a v e FET a s t h e p a r a s i t i c b u f f e r
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
201
c u r re n t i s reduced.
I n t h i s s e c t i o n some o f t h e
t r a n s p o r t p r o p e r t i e s a r e e x a m i n e d and a mod el f o r t h e
photolu m in escen ce lin e s h a p e i s developed in th e fo llo w in g
sectio n .
One o f t h e i m p o r t a n t i s s u e s t o a d d r e s s i s w h a t
t h e c o n t r o l l i n g f a c t o r s a r e i n e l e c t r o n m o b i l i t y , a nd t o
l o o k f o r t h e p r e s e n c e o f i m p u r i t i e s i n PL.
4 .7 .1
Experim ental
A t y p i c a l l a y e r d i a g r a m f o r t h e m o d u l a t i o n dope d
s t r u c t u r e i s shown i n P i g .
4.25.
The i m p o r t a n t f e a t u r e
i s t h e t h i c k A1Q 3 QGao 7QAs b u f f e r , a p p r o p r i a t e f o r
device a p p lic a tio n s .
To p r e v e n t t h e f o r m a t i o n o f a
p a r a s i t i c 2DEG, t h e i n t e r f a c e b e t w e e n t h e GaAs b u f f e r and
t h e AlGaAs i s c o m p o s i t i o n a l l y g r a d e d .
The c a p p i n g l a y e r
a b o v e t h e GaAs q u a n tu m w e l l e x h i b i t s a t y p i c a l m o d u l a t i o n
d o p e d g e o m e t r y o f an u n d o p e d s p a c e r b e t w e e n t h e
h e t e r o i n t e r f a c e a nd t h e s i l i c o n d o p i n g r e g i o n .
c o n d u c t i o n ban d d i a g r a m i s shown i n F i g .
The
4.26 w ith th e
q u an tu m w e l l r e g i o n e x p a n d e d i n P i g . 4 . 2 7 .
The s t r u c t u r e s w e r e gr ow n f o l l o w i n g t h e p r o c e d u r e s
discussed in S ection 4 .3 .
This i s q u i t e im p o rta n t, as a
h i g h q u a l i t y b u f f e r a nd q u a n tu m w e l l r e s u l t .
The
l i m i t a t i o n o f t h e m eth o d i s t h a t a s t h e c a p p i n g l a y e r i s
grown a t 6 8 0 ° C , s i l i c o n d i f f u s i o n i s i m p o r t a n t and
u l t i m a t e l y l i m i t s t h e n a rro w n e s s o f t h e undoped s p a c e r .
H a l l s a m p l e s w e r e p r e p a r e d u s i n g a v an d e r Pauw
p a t t e r n w ith a llo y e d t i n dopts fo r c o n ta c ts .
The 77K
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
202
SQW HEMT EPILAYER STRUCTURE
SURFACE
AIGaAs CAPPING LAYER
IONIZED DONORS
SPACER
2 DEG
AIGaAs
BUFFER
GRADED AIGaAs
GaAs
BUFFER
1800 A AI0 30Ga070As undoped
90 X AIq jq GOqtqAs n ~ 5 x lO l7/cm
120 A Al0 3 0 Ga070As undoped
120 A GaAs SQW
5000 A AI030Ga070As undoped
700 A GRADED undoped
AIGaAs
5000 A GaAs undoped
SEMI-INSULATiNG
GaAs
SUBSTRATE
F ig u re 4.25.
Schem atic c r o s s s e c t i o n o f a s in g l e
q u a n t u m w e l l HEMT.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
203
CONDUCTION BAND DIAGRAM FOR EPILAYER
AE.
GaAs
"Buffer
5000 A
SUBSTRATE
F ig u re 4.26.
AIGaAs
’ Buffer
5000 A
GRADED
AIGaAs
-7 0 0 A
'v-'
n+
AIGaAs
2000 A
GaAs
SQW
120 A
C onduction band diagram f o r a s in g l e
q u a n t u m w e l l HEMT.
CONDUCTION BAND AT SINGLE QUANTUM WELL
F ig u re 4.27.
D e t a i l e d c o n d u c t i o n b a n d d i a g r a m a t q u a n tu m
w e l l w i t h g r o u n d s t a t e e n e r g y EQ, wave
fu n ctio n
, a n d F e r m i l e v e l Et
F.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
204
H a l l m e a s u r e m e n t s w e r e made a f t e r i l l u m i n a t i o n and
e x h ib ite d strong p e r s is te n t p h o to co n d u ctiv ity .
T his i s
d i s c u s s e d m o re e x t e n s i v e l y b e l o w .
4 .7 .2
H a l l Measurement R e s u l t s
The 300K a nd 77K H a l l m o b i l i t i e s a r e p l o t t e d
in Fig.
4 . 2 8 a s a f u n c t i o n o f s p a c e r t h i c k n e s s f o r a 12 0 8 w e l l
th ick n ess.
The 300K m o b i l i t i e s t y p i c a l l y l i e
between
6000 a n d 7000 cm / V - s a n d a r e i n d e p e n d e n t o f s p a c e r
thickness.
T h e s e m o b i l i t i e s a r e somewhat l o w e r t h a n
th o se o b tain ed fo r the b e s t s in g le in te r f a c e re g u la r
m o d u la tio n doped s t r u c t u r e .
co n sid erab le s c a tte r
The 77K m o b i l i t i e s e x h i b i t
in d a t a , b ut th e peak m o b i l i t i e s
r a n g e f r o m 1 2 0 , 0 0 0 t o 1 4 0 , 0 0 0 cm2/ V - s a nd a r e r e l a t i v e l y
in s e n s itiv e to spacer thickness.
The m o b i l i t y o f 1 4 0 , 0 0 0 cm / V - s i s t h e h i g h e s t
m o b i l i t y r e p o r t e d t o d a t e f o r s i n g l e quantum w e l l
m o d u la tio n doped s t r u c t u r e s .
T h i s m o b i l i t y i s somew hat
low er th a n th e h i g h e s t m o b i l i t i e s r e p o r te d f o r r e g u l a r
2
s i n g l e i n t e r f a c e d e v i c e s (~ 1 6 0 , 0 0 0 cm / V - s f o r t h i s
spacer).
The 77K H a l l m o b i l i t i e s a r e r e p l o t t e d i n F i g .
4.29
as a f u n c t io n of s h e e t c h a rg e w ith s p a c e r t h i c k n e s s as a
param eter.
This reduces th e s c a t t e r
i n t h e d a t a and
dem on strates t h a t w hile n e a rly e q u iv a le n t m o b i l i t ie s are
o b t a i n e d f r o m s p a c e r t h i c k n e s s e s o f 90 8 t o 150 8 , t h e s e
occur a t d if f e r e n t sh eet charges.
N ar ro w s p a c e r s r e q u i r e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
205
fL vs SPACER THICKNESS
200
1236
>1026
>1192
100
80
>1250
oil 57
60
J I 86
o
40
r*79l
1237
*790
V)
£
e
20
,792
o
PO
3
=1 .
|
10
1236 _79l
I250_ o
o
||86%Oll5 7 °7 9 0
8
H 'T tk
j ^300K
q1026
'1237
1192
792
0
±
50
±
100
_L
150
_L
200
L SP (A)
F igure 4.28.
H all e le c tr o n m o b ility as a fu n c tio n of
spacer th ick n ess.
C i r c l e s a r e 77K d a t a ,
w h i l e s q u a r e s a r e 300K d a t a .
C ornell
l a y e r numbers a r e i n d i c a t e d .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
206
vs Ns
/J .J Y
1236
1026
100
80
60
o
V
40
'
/ .V’
\ I 2 5 o V 91
0
V L sp=l20A
1*57
/W237
f
848
/T,86 \
''*79°
V)
,=150A
E
o
20
to
•7 9 2
956
O
N
4
6
8
10
12
Ns ( # /c m 2 x I011)
F ig u re 4.29.
77K H a l l e l e c t r o n m o b i l i t y a s a f u n c t i o n o f
s h e e t c h a r g e , N , p a r a m e t e r i z e d by s p a c e r
s
th ick n ess.
C o r n e l l l a y e r numbers a r e
in d icated .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
207
h ig h c h a r g e d e n s i t i e s th a n wide s p a c e r s t o o b t a i n high
m o b ilities.
The p e a k m o b i l i t i e s f o r t h e q u a n tu m w e l l
s t r u c t u r e s a ls o occur a t hig h er s h e e t ch arg es than the
conventional reg u lar s in g le in te rf a c e .
T h i s co m b in e d
w i t h t h e low er peak m o b i l i t i e s l e a d one t o s u s p e c t t h a t
th e im p o rta n t f a c t o r in th e high m o b ility of t h i s
s t r u c t u r e i s a high e l e c t r o n k i n e t i c energy o b ta in e d a t
t h e Fermi l e v e l f o r h ig h s h e e t c h a r g e s , which re d u c e s
im purity s c a tte rin g e f f e c ts .
4 .7 .3
L ight S e n s i t i v i t y
A s a m p l e was e t c h e d w i t h l i g h t and d a r k H a l l
m e a s u r e m e n t s made b e t w e e n e t c h i n g s .
C o n t a c t s w e r e masked
o f f w i t h p h o t o r e s i s t a n d t h e s a m p l e was e t c h e d u s i n g
50:3:1
:H20 2 .
The m o b i l i t y a s a f u n c t i o n o f
s h e e t charge is p lo tte d in F ig.
dark b ehavior.
4 . 3 0 f o r t h e l i g h t an d
The i n t e r e s t i n g p o i n t i s t h a t w h i l e t h e
d a r k s h e e t c h a r g e and m o b i l i t y were d i m i n i s h i n g , t h e
l i g h t s h e e t c h a r g e and m o b i l i t y s t a y e d r e l a t i v e l y
co n stan t.
The d a r k m o b i l i t y a l s o d r o p s v e r y r a p i d l y w i t h
dark sh e e t charge.
The l i g h t s e n s i t i v i t y i s v e r y s t r o n g ,
b u t d e p e n d s o n t h e am ount o f c h a r g e i n t h e c a p p i n g l a y e r .
I f t h e d a r k s h e e t c o n c e n t r a t i o n was h i g h e r , l e s s l i g h t
s e n s i t i v i t y w ou ld o c c u r .
4 .7 .4
S e n s itiv ity to Im p u rities
S a m p l e s w er e grow n w i t h a p l a n e o f s i l i c o n d o p i n g
i n t e n t i o n a l l y i n t r o d u c e d i n t h e q u a n tu m w e l l .
The
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
208
Dark points
Light points
10
9
8
7
W
I
>
6
5
NN*
=L
4
3
2
0
0
1
2
3
4
5
6
7
8
Nss d 0 " # /c m 2 )
F ig u re 4.30.
77K H a l l e l e c t r o n m o b i l i t y a t a s t e p e t c h e d
sample as a f u n c t i o n o f H a ll s h e e t c h a r g e .
S o l i d l i n e s c o n n e c t i l l u m i n a t e d and d a rk
m easurem ents o f t h e sam ple a t a g iv e n
etch step
( i n d i c a t e d by n u m b e r s n e x t t o
dark p o i n t s ) .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
209
stru ctu res,
i m p u r i t y s h e e t c o n c e n t r a t i o n a nd 77 m o b i l i t y
r e s u l t s are in d ic a te d in F ig.
4.31.
The m o b i l i t i e s a r e
2
s u r p r i s i n g l y h i g h a t 6 5 , 0 0 0 cm V . s , e v e n f o r a s h e e t
i m p u r i t y c o n c e n t r a t i o n t h a t i s 2 x 10
10
2
/cm .
The
e x tra p o la te d im purity sh e e t charge a t th e h ig h e st
m o b i l i t i e s r e p o r t e d i n S e c t i o n 4 . 8 . 2 i s on t h e o r d e r o f a
t y p i c a l b a c k g r o u n d d o p i n g i n t h e MBE m a c h i n e a t C o r n e l l .
4 .7 .5
Summary o f H a l l M e a s u r e m e n t s
W hile th e m o b i l i t i e s r e p o r t e d i n S e c t i o n 4 . 7 . 2 a r e
among t h e h i g h e s t r e p o r t e d f o r a SQW s t r u c t u r e ,
it
is
u n c l e a r w h e t h e r h i g h e r m o b i l i t i e s m i g h t be o b t a i n e d f o r
w e l l t h i c k n e s s e s o f 120 8 .
The a u t h o r s u s p e c t s t h a t t h e
s t r u c t u r e s a r e l i m i t e d by t h e b a c k g r o u n d i m p u r i t i e s
p r e s e n t i n t h e MBE m a c h i n e , p a r t i c u l a r l y f ro m t h e r e s u l t s
in S ection 4 .7 .4 .
The s t r u c t u r e e x h i b i t s a s u r p r i s i n g l y l a r g e d r o p i n
m o b i l i t y w i t h low s h e e t c h a r g e , w h i c h c o u l d be d e t r i ­
m e n t a l t o t h e b e h a v i o r o f FETs w h e r e o p e r a t i o n a t low
s h e e t c h a r g e i s i m p o r t a n t f o r low n o i s e o p e r a t i o n .
narrow er spacer d e v ic e s ,
For
t h e c a p p i n g l a y e r a nd GaAs w e l l
s h o u l d be grown a t l o w e r t e m p e r a t u r e s
f
640°C)
in order
t o r e d u c e s i l i c o n d i f f u s i o n th r o u g h t h e undoped s p a c e r
layer.
In a d d itio n ,
t h e GaAs m a t e r i a l i n t h e quantum
w e l l w o u ld be o f h i g h e r s t r u c t u r a l q u a l i t y ,
in S ectio n 4 .4 .2 .
as d isc u s s e d
G r o w th s w i t h n a r r o w s p a c e r s <
680°C t y p i c a l l y e x h i b i t e d low m o b i l i t i e s , <
80S a t
40,000
cm2/ V . s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
210
WELL IMPURITIES ( # /c r n )
Ns = 8 x IO „cm “ 2
L sp = IO O A o
L weu = 12 0 A
IK
I0 K
100 K
1000 K
^t77(cm2/V -s)
F ig u re 4.31.
77K H a l l m o b i l i t i e s a s a f u n c t i o n o f
in te n tio n a l w ell im p u ritie s.
Inset
in d icates s ilic o n lo catio n .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
211
4.8
P h o t o l u m i n e s c e n c e L i n e s h a p e s o f M o d u l a t i o n Doped
S tru ctu res
As h a s b e e n n o t e d by s e v e r a l a u t h o r s , 8 6 , 1 0 0 , 1 0 1 t ^ e
lu m in e s c e n c e l i n e s h a p e o f a m o d u la tio n doped s t r u c t u r e i s
so m ew hat d i f f e r e n t fo rm t h e u n d o p e d s t r u c t u r e .
The
presen ce of the dense e le c tr o n gas in tro d u c e s screen in g
e f f e c t s which p r e c l u d e e x c i t o n form ation.^® ®
The main
r e c o m b i n a t i o n m ech anis m i s f r e e c a r r i e r r e c o m b i n a t i o n
betw een t h e e l e c t r o n g as and p h o t o e x c i t e d h o l e s .
The
g e n e r a l f e a t u r e s o f t h e l i n e s h a p e a r e d e t e r m i n e d by t h e
e l e c t r o n a nd h o l e d e n s i t y o f s t a t e s and t h e i r r e s p e c t i v e
d is tr ib u tio n fu n ctio n s:
00
L(hu)) = A J p ( E ) f
O
w h e r e P(E)
(E )f.(E )
5 (E-hm)dE
is the d e n sity of s t a t e s ,
f
(4.47)
a nd f ^ a r e t h e
e l e c t r o n and h o l e d i s t r i b u t i o n s .
S i n c e p(E)
i s a c o n s ta n t above th e ground s t a t e
e n e r g y , and a t low t e m p e r a t u r e s f e (E) i s n e a r l y a s t e p
f u n c t i o n f o r d e g e n e r a t e s y s t e m s a t low t e m p e r a t u r e s , t h e
l i n e s h a p e above t h e f i r s t c o n d u c tio n subband i s
c o n t r o l l e d by t h e h o l e d i s t r i b u t i o n .
The p h o t o e x c i t e d
h o l e d i s t r i b u t i o n i s B o ltz m a n n - lik e w ith an e f f e c t i v e
h o l e t e m p e r a tu r e s l i g h t l y above t h e c r y s t a l t e m p e r a t u r e .
T his i s
illu strated
i n P i g . 4 .3 2 where t h e lo g o f th e
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
212
-3x10'
1.530
log
PL intensity
(PL
intensity)
(arbitrary units)
h i/=1.676 eV
3 W /cm 2
3x10'
1.540
F ig u re 4.32.
1.560
1.550
Energy (eV)
1.570
1.580
2K PL l i n e s h a p e o n a 2D e l e c t r o n g a s a s
a f u n c t i o n o f e n e r g y on b o t h l i n e a r a n d
log i n t e n s i t y s c a le s .
I n s e t equation
i n d i c a t e s g e n e ra l high energy s id e
lineshape fu n ctio n s.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
213
lu m in e sc e n c e i n t e n s i t y i s p l o t t e d vs E along w ith th e
lin ear
in ten sity .
The c r y s t a l t e m p e r a t u r e i s 2K and t h e
e f f e c t i v e h o l e t e m p e r a t u r e i s 6K.
T his accounts fo r th e
h ig h energy s i d e lin e s h a p e of th e peak q u i t e w e ll.
The e f f e c t s o f r e d u c e d e l e c t r o n d e n s i t y a r e
illu strated
in P ig.
4 . 3 4 , w h e r e t h e s a m p l e shown i n F i g .
4.33 has been e tc h e d t o reduce th e e l e c t r o n d e n s it y .
The
h i g h e n e r g y s i d e l i n e w i d t h h a s b e e n r e d u c e d by t h e
reduced e le c tr o n d e n s ity .
T y p i c a l b e h a v io r f o r Fermi
e l e c t r o n d i s t r i b u t i o n a nd a h o l e B o l t z m a n n d i s t r i b u t i o n
c a n be s e e n i n t h e l i n e s h a p e a s a f u n c t i o n o f t e m p e r a t u r e
a s shown i n F i g . 4 . 3 5 .
The e v o l u t i o n o f t h e e l e c t r o n
F e r m i f u n c t i o n a b o u t t h e e l e c t r o n F e r m i l e v e l c a n be s e e n
a t th e high energy t a i l of th e l in e s h a p e .
The
in t e r e s t i n g p a r t of the lin esh ap e is the fa c to rs
d e t e r m i n i n g t h e low e n e r g y l i n e s h a p e .
4 .8 .1
L o c a liz e d S t a t e C o n tr ib u tio n to Lineshape
W hile t h e d e n s i t y of s t a t e s
(valence)
fo r th e f i r s t conduction
subband i d e a l l y e x h i b i t s a s t e p l i k e b eh av io r
a t t h e ban d e d g e , t h e l u m i n e s c e n c e s p e c t r a e x h i b i t s a low
energy t a i l .
P r e v io u s s t u d i e s have a t t r i b u t e d t h i s t a i l
t o b a n d g a p r e n o r m a l i z a t i o n d u e t o many body e f f e c t s f o r
t h e 2DEG. 100
An a l t e r n a t i v e e x p l a n a t i o n i s o f f e r e d h e r e
b a s e d o n t h e p r e s e n c e o f l o c a l i z e d s t a t e s b e lo w t h e f i r s t
c o n d u c tio n subband.
T h e s e s t a t e s m i g h t r e s u l t from
i m p u r i t i e s i n t h e q u an tu m w e l l ,
in te rf a c e roughness, or
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
214
HALL SAMPLE
4°K
AIGaAs
SUFFER
*■ 0 .3 0
35
1.888
z
1.874
O
m
tr
QUANTUM
WELL
<
AIGaAs
SURFACE
x » 0 .3 4
1.339
1.931
L945
to
Z
LJ
Z
a
GaAs
BUFFER
1497
1.859
ENERGY
P ig.
4.33.
(eV)
P h o to lu m m e sc e n c e s p e c t r a o f a H a ll sample.
2
E x c i t i o n p o w e r i s lOW/cm .
ETCHED SAMPLE
(SURFACE AIGaAs PARTIALLY REMOVED)
AIGaAs
BUFFER
x*0.30
1.878
4°K
QUANTUM
WELL
1.865
1.358
AIGaAs
SURFACE
*■0.34
1.925
<0
I-
CD
X
<
>
to
z
LJ
K
Z
5 meV FWHM
GaAs
BUFFER
_J
1.850
a. 1.490
11.493
1908
1.515
ENERGY (eV)
F ig u re 4.34.
P h o t o l u m i n e s c e n c e s p e c t r a o f an e t c h e d s a m p l e .
2
E x c i t a t i o n p o w e r i s lOW/cm .
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
215
Eo
1.539
1.570
30 K
V)
c3
>»
O
22 K
JQ
w.
O
V)
c
4)
C
CL
4K
.514
Energy (meV)
F ig u re 4.35.
PL l i n e s h a p e o f a 2D e l e c t r o n g a s a s a
fu n c tio n of energy fo r s e v e ra l c r y s t a l
t e m p e r a t u r e s f r o m 4K t o 44K.
EQ i n d i c a t e s
ap p ro x im a te ground s t a t e e n erg y .
Ef
i n d i c a t e s approxim ate p o s i t i o n o f e l e c t r o n
Fermi en erg y .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
216
t h e e f f e c t s o f t h e co u lo m b p o t e n t i a l s f ro m t h e r e m o t e
i o n i z e d i m p u r i t i e s i n t h e b a r r i e r AIGaAs l a y e r .
This
e x p l a n a t i o n i s s u p p o r t e d by e x p e r i m e n t s w h ic h i n d i c a t e
t h a t t h e s e a r e s t a t e s u n r e l a t e d t o th e s t a t e s above th e
band edge e x c e p t f o r t h e s c r e e n i n g e f f e c t s .
In th e
p re se n c e of a m agnetic f i e l d , th e s c re e n in g c a p a b i l i t i e s
o f a 2DEG c a n be r e d u c e d
(in p r in c ip l e )
t o z e r o by
p o s t i o n i n g t h e Fermi l e v e l betw een a d j a c e n t Landau
l e v e l s , so t h a t f u l l bands r e s u l t .
i s achieved,
When t h i s c o n d i t i o n
in th e sam ples r e p o r t e d h e r e , e x c ito n s can
be o b s e r v e d t o f o r m . 1 ^*
This su g g e sts t h a t th e r e a re
s t a t e s p r e s e n t which a r e n o t r e l a t e d t o t h e f r e e e l e c t r o n
s t a t e s a b o v e t h e ban d e d g e .
A s i m p l e p h e n o m e n o l o g i c a l t h e o r y c a n be f o r m u l a t e d
t o d e s c r i b e t h e low e n e r g y s i d e l i n e s h a p e .
The e l e m e n t s
t o be i n c l u d e d a r e l o c a l i z a t i o n o f t h e p a r t i c l e
w a v e f u n c t i o n , t h e c o u lo m b i n t e r a c t i o n b e t w e e n l o c a l i z e d
h o l e s a nd e l e c t r o n s , a n d t h e s c r e e n i n g e f f e c t s o f t h e
2DEG.
The m ain e n e r g y d e p e n d e n c e o f t h e l i n e s h a p e i s
a s s u m e d t o r e s u l t from t h e co u lo m b i n t e r a c t i o n , n o t t h e
e n e r g y o f t h e s t a t e s b e lo w t h e b a n d e d g e .
The a n a l y s i s i s s i m i l a r t o t h e t h r e e d i m e n s i o n a l
c a s e of re c o m b in a tio n between rem ote i o n i z e d donor
acceptor p a ir s .
102
The mai n a s s u m p t i o n i s t h a t t h e
w a v e f u n c t i o n s a r e n o t p e r t u r b e d by t h e o t h e r c a r r i e r ,
th e m a trix elem en t f o r r e c o m b in a tio n i s sim ply th e
J
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
and
217
o v e rla p of the c a r r i e r w avefunctions.
re q u ire s th a t the f ie ld
This c o n d itio n
f ro m t h e r e c o m b i n i n g c a r r i e r n o t
p e r tu r b the p o t e n t i a l about th e o th e r c a r r i e r too
g reatly .
W ith s c r e e n i n g e f f e c t s ,
th e long range p o r t i o n
o f t h e c o u lo m b p o t e n t i a l i s d e c a y i n g a p p r o x i m a t e l y a s
e " r / r s , thus th e approxim ation i s b e t t e r fo r c a r r i e r
s p a c i n g s beyond t h e s c r e e n i n g c o n s t a n t r .
The s e c o n d
i s s u e i s w hether or n ot th e h o l e - e l e c t r o n p a i r can b in d ,
b u t f ro m t h e a b o v e a r g u m e n t , i t
i s assumed t h a t t h i s
cannot occur.
The s i m p l e s t way t o i n c l u d e s c r e e n i n g e f f e c t s
is to
u s e t h e s c r e e n e d co u lo m b p o t e n t i a l o f t h e t h r e e
dim ensional case
/ \
but to u tiliz e
«~r / r s
(4.48)
the value of the screen in g co n sta n t
c a l c u l a t e d i n t h e Thomas F e r m i a p p r o x i m a t i o n f o r a t w o d im en sio n al system .
The s c r e e n e d c o u lo m b p o t e n t i a l
d e p e n d e n c e on r a d i u s i s somew hat d i f f e r e n t i n two
d im e n s io n s a s compared w ith t h r e e d i m e n s i o n s .
c a s e i s used h e r e b ecause of i t s
a nd t h e d e s i r e t o i l l u s t r a t e
103
a n a ly tic a l sim p licity
the q u a lita tiv e c o n trib u tio n
of lo c a liz e d s t a t e s to the lin e sh a p e .
For a two-
d i m e n s i o n a l s y s t e m a t t h e q u a n tu m e l e c t r i c l i m i t ,
screening co n stan t r
d
The 3D
i s g i v e n by :
the
103
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
w h e r e a Q i s t h e e f f e c t i v e Bohr r a d i u s , a n d i s o n l y w e a k l y
d e p e n d e n t on c a r r i e r c o n c e n t r a t i o n .
F o r GaAs, r s » 50 S.
To m o d el a l o c a l i z e d wave f u n c t i o n , as su m e a
G aussian dependence in th e m otions p a r a l l e l to th e w e ll
interfaces
( x , y)
'•'e = g e (z) e
*h = 9e U )
3
e
" r e/B e
,
9
(4.50)
e
and 3 U a r e l o c a l i z a t i o n r a d i i .
h
The m a t r i x e l e m e n t f o r
t h e o v e r l a p i s g i v e n by
|He h l 2 -
Ae- * 2/ * 2
<4 - 5 1 ’
w h e r e A i s a c o n s t a n t , 3^ = 3^ + 3^> a n d R i s t h e
i n t e r p a r t i c l e spacing.
The d e n s i t y o f s t a t e s a s a f u n c t i o n o f e n e r g y i s :
N(E)dE = N (R) d r
(4.52)
N(R) = 2irRn
(4.53)
w h ere
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219
f o r a two d i m e n s i o n a l s y s t e m , a n d n Q i s t h e number o f
c a r r i e r s p e r cm .
I t i s l i k e l y t h a t t h e number o f
l o c a l i z e d e l e c t r o n a nd h o l e s t a t e s a r e c l o s e t o t h e same,
T h e s e a r e c o m b in e d t o g i v e :
H(E> - 2" Rn°
N(E' ' ~3e73r
<4 ‘ 54)
F o r t h e u n s c r e e n e d c o u lo m b p o t e n t i a l , t h i s g i v e s
■ ,«, . J L J ^
c
16 tteE
<4-” >
W hile th e s c re e n e d p o t e n t i a l g i v e s
2ir«n *R
Es c r < E> ■
B (i + pi)
‘
’
and
E = -4 ttc
a -R e"RAs
e
<4-57>
a
Co mbined w i t h t h e m a t r i x e l e m e n t , t h i s g i v e s t h e
lineshape:
L (E) = N(S) e ^
2
(4- 58>
The u n s c r e e n e d co u lo m b p o t e n t i a l l i n e s h a p e i s p l o t t e d i n
F ig.
4.36 f o r v a r io u s l o c a l i z a t i o n r a d i i .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
m o y
U N SC R E EN ED
P O T E N T IA L
1.00
lin e s h a p e
COULOMB
CQ. 02. QQ.
0 .9 0
0 .8 0
0 .7 0
Normalized
0 .6 0
0 .5 0
0 .4 0
0 .3 0
0.20
0.10
0.00
-I
-15
I
I " —9
L.
-10
-5
0
Energy below groundstate of quantum well
F ig u re 4,36.
(m eV)
N orm alized l i n e s h a p e a s a f u n c t io n
o f e n e rg y below t h e ground s t a t e o f
a q u a n t u m w e l l f o r a n u n s c r e e n e d Coulomb
p o ten tial.
6 in d ic a te s th e v alue of
t h e l o c a l i z a t i o n r a d i u s i n Eq.
and Eq.
(4.58)
(4 .5 5 ).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The s c r e e n e d co u lo m b p o t e n t i a l i s p l o t t e d
4 . 3 7 f o r fj = 100 8 a nd r g - 59
in Fig.
i t can be seen t h a t
t h e e f f e c t o f s c r e e n i n g i s t o g r e a t l y s h a r p e n t h e low
e n e r g y l i n e s h a p e , a s t h e s c r e e n e d l i n e s h a p e f o r 3 = 100
i s s i m i l i a r t o t h e u n s c r e e n e d l i n e s h a p e f o r 3 = 2000 8 .
The a b o v e c a l c u l a t i o n s d e m o n s t r a t e t h a t
r e c o m b i n a t i o n f ro m l o c a l i z e d s t a t e s c a n a c c o u n t f o r t h e
low e n e r g y s i d e l i n e s h a p e fro m a 2DEG.
As t h e
c a l c u l a t i o n i n v o l v e d two a d j u s t a b l e p a r a m e t e r s , 3 a n d nQ
the c a lc u la tio n i s of lim ite d u s e fu ln e s s.
F urther
e v a l u a t i o n s s h o u l d a t t e m p t t o a s s u m e a m ech an is m f o r
l o c a l i z a t i o n , and c a l c u l a t e a c t u a l w a v e f u n c t i o n s and
p o ten tials.
In the next s e c tio n ,
th e lim itin g ease of a
l a r g e d e n s i t y o f i m p u r i t i e s w h i c h i n t e r a c t t o f o r m an
i m p u r i t y b and i s c o n s i d e r e d .
4.8 .2
C o n t r i b u t i o n o f a D e g e n e r a t e I m p u r i t y Band t o
th e D ensity of S ta te s
In S ectio n 4 .5 .3 ,
th e broad symm etric li n e s h a p e of
t h e quantum w e l l w ith s i l i c o n p l a c e d b e f o r e t h e quantum
w e l l was a t t r i b u t e d t o a d e g e n e r a t e i m p u r i t y b a n d .
In
t h i s s e c t io n , the c o n tr ib u tio n to th e 2 dim ensional
d e n s i t y o f s t a t e s f u n c t i o n below t h e f i r s t c o n d u c tio n
subband i s c o n s id e r e d .
th r e e dim ensional case
The a n a l y s i s c l o s e l y f o l l o w s t h e
104
b a s e d o n t h e T h o m a s - F e rm i
approxim ation.
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
222
£=I00A
1.0 0 -
SCREENED COULOMB
POTENTIAL
rs =50A
/!
lin e sh a p e
0 .9 0 0 .8 0 -
I
0 .7 0 0 .6 0 -
Normalized
0 .5 0 0 .4 0 -
o
o
0 .3 0 -
0.2 0 0. 10-
•o-
-o-
0.00 *— L
-15
-10
-5
0
Energy below groundstate of quantum well (meV)
F ig u re 4.37.
N orm alized l i n e s h a p e a s a f u n c t i o n of
e n e r g y below t h e g round s t a t e o f a quantum
w e l l f o r t h e s c r e e n e d coloum b p o t e n t i a l .
8 and r
s
are the lo c a liz a tio n rad iu s
and s c r e e n i n g p a r a m e t e r , r e s p e c t i v e l y ,
f o r Eqs.
(4 .5 8 ),
(4.56)
and ( 4 .5 7 ) .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
223
The s p a t i a l l y v a r y i n g d e n s i t y o f s t a t e s o f a two
d i m e n s i o n a l s y s t e m i s g i v e n by
g (e r r )
=
U[e + q * ( r ) ]
( 4 *59)
hz
where u[e]
i s the heavyside s te p fu n c tio n .
To c o m p u te
t h e t o t a l d e n s i t y of s t a t e s one must i n t e g r a t e t h i s over
a l l c l a s s i c a l l y a l l o w e d r e g i o n s a nd d i v i d e by t h e t o t a l
a r e a A_
g(e)=jp
o
J
e> -q<t>
u [ e + q<j>(r)]dr
(4,60)
h
o r , l e t t i n g V = -q<J>
4irm
h
a
r
(4 .6 1 )
f
u [ e ’ v]
dr
The c o o r d i n a t e i n t e g r a t i o n c a n be r e p l a c e d by
averaging over a l l values of th e p o t e n t i a l energy
*
G(e) =
e
J
F (v) dV
h
w h e r e P(V)
i s the p o t e n t i a l energy d i s t r i b u t i o n fu n c tio n ,
w h i c h r e m a i n s t o be d e t e r m i n e d .
F o r a random
d i s t r i b u t i o n o f s c r e e n e d c o u lo m b p o t e n t i a l s ,
i t c a n be
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
224
shown t h a t m o s t o f t h e c o n t r i b u t i o n s t o t h e p o t e n t i a l
f l u c t u a t i o n s a r e from a r e g i o n w i t h i n a s c r e e n i n g
c o n s ta n t of the im p u rity .
T h i s c o m b in e d w i t h t h e
f l u c t u a t i o n in th e d e n s ity of im p u r itie s w ith in t h i s
r e g i o n r e s u l t s i n t h e f o l l o w i n g form:
F (V) =
1
y /i r
*-V2/
y2
e
(4.63)
where
y 2 = — ~ . 2t V2 ~2
2 • Ns • r s2 = ( 7 . 6 8 x 1 0 " 16V2 -cm 2 )Ne
(4ir) e r s
_2
a n d Ns i s t h e d e n s i t y o f i m p u r i t i e s i n cm
a nd r s i s t h e
s c r e e n i n g c o n s t a n t f o r a 2DEG.
The d e n s i t y o f s t a t e s
f u n c tio n s gives
1£
- 2 - • e‘v2/lf2 dv
-«
(y/rTj
g(e, . I ® !
h
<4-64>
w hich i s sim p ly th e com plem entary e r r o r f u n c t i o n .
The l u m i n e s c e n c e l i n e s h a p e w i l l be g i v e n by
L(hoj) = J g ( E )
f h (Eh ) f e (E)
6(E-haj) dE
( 4 *65)
w h e r e E^ i s d e t e r m i n e d by a c o n s e r v a t i o n o f k f o r b o t h
e l e c t r o n s and h o l e s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
225
T h i s f u n c t i o n was progr am med o n t o a c o m p u t e r , a nd
t h e r e s u l t s a r e shown i n F i g . 4 . 3 8 .
d istrib u tio n ,
The e l e c t r o n
f g (E), i s th e F erm i-D irac f u n c tio n w ith the
p o s i t i o n o f t h e c h e m i c a l p o t e n t i a l d e t e r m i n e d by t h e
s h e e t e l e c t r o n c h a r g e , N , and t h e te m p e r a tu r e i s ta k e n
ss
t o b e t h e c r y s t a l t e m p e r a t u r e , Tc r y St i ’
The h o *e
d i s t r i b u t i o n i s a s s u m e d t o be B o l t z m a n n - l i k e , w i t h a non ­
e q u i l i b r i u m h o l e t e m p e r a t u r e d e t e r m i n e d by T ^.
a t o m d e n s i t y i s g i v e n by Nd s .
The d o n o r
The c a l c u l a t i o n o n l y
in c lu d e s th e f i r s t subband in th e w e l l .
b o t h g( E ) a n d L(E) a r e p l o t t e d .
In th e f i g u r e ,
The l i n e s h a p e i s b r o a d
and s y m m e tr ic , r e f l e c t i n g t h e t a i l o f t h e d e n s i t y o f
s t a t e s d i s t r i b u t i o n , g ( E ) , below t h e z e ro p o i n t e n e rg y o f
the w ell.
Compared t o t h e PL r e s u l t s p r e s e n t e d e a r l i e r
in F ig . 4 .1 7 , the s i l i c o n c o n c e n tra tio n appears in the
w e l l a p p e a r s t o be o n t h e o r d e r o f 4 x 10
11
cm
-2
The e f f e c t o f t h e d o p i n g o n t h e l u m i n e s c e n c e l i n e s h a p e i s
t o b o t h b r o a d e n t h e l i n e a nd t o s h i f t t h e p e a k t o l o w e r
energies.
The e s t i m a t e o f 4 x 1 0 ^ cm” 2 comes fro m
m a t c h i n g t h e e q u i v a l e n t f u l l w i d t h h a l f maximum
linew id th.
T h i s m o d e l c a n be u s e d a t v e r y low s h e e t
c o n c e n t r a t i o n s , a s shown i n F i g .
c h a r g e i s 7 x 10
8
cm
—2
4 .39 where th e s h e e t
.
T h i s i s on t h e o r d e r o f t h e
1/1
"3
b a c k g r o u n d d o p i n g i n t h e m a c h i n e , 7 x 10
cm" . The
i n t e r e s t i n g p o i n t i s t h a t t h e l i n e s h a p e c a n be d e s c r i b e d
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
226
N s s = l . 0E+12#/cm2
T c r s t 1** 4 . 0K
Th= 6. 0K
Nds=s4 . 0 E + l l # / c
g(E)
- 8 0-7 0-G 0-50-4 0-3 0-20-10
ENERGY CmeV)
WRT QW GROUNDSTRTE
F ig u re 4.38.
10 20 30
C a lc u la te d n o rm alized lin e s h a p e L (E ), as a
f u n c t i o n o f e n e rg y below t h e f i r s t c o n d u c tio n
subband f o r a quantum w e l l w i t h a d e g e n e r a t e
donor im p u rity band.
The e l e c t r o n s h e e t
d e n s i t y , N g , c r y s t a l t e m p e r a t u r e , Tc r s t } /
e f f e c t i v e h o l e t e m p e r a t u r e , T ^, a n d d o n o r
d e n s i t y , N^g , a r e i n d i c a t e d .
The n o r m a l i z e d
two d i m e n s i o n a l d e n s i t y o f s t a t e s ,
g(E),
indicated.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
is
227
Nss=5.0E+l l#/cm !h
T c r s t l = 4.0 K
T h - 6.0 K
N d s = 7 . 0E+08#/cm
5
10
15
ENERGY CmeV)
4-HRT QW GROUNDSTRTE
F ig u re 4.39.
C a lc u la te d norm alized lin e s h a p e , L(E ),
as a
f u n c t i o n o f en erg y below t h e f i r s t con­
d u c t i o n subband f o r a quantum w e l l w i t h a
l i g h t donor d e n s ity .
of s ta te s ,
The n o r m a l i z e d d e n s i t y
g(E), is in d icated .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
228
as e q u a lly w e ll as in th e p r e v io u s s e c t i o n , even though
t h e mechanisms a r e q u i t e d i f f e r e n t .
T h i s m odel i s b a s e d
on a s p a t i a l l y v a r y in g d e n s i t y o f u n l o c a l i z e d s t a t e s ,
w hile th e previous s e c tio n d iscu ssed lo c a liz e d s t a t e s .
T h i s m odel i s b a s e d on a Thomas F e r m i t y p e mo del o f a
d e g e n e r a t e i m p u r i t y b a n d , w h ic h w o u ld r e q u i r e t h a t t h e
i n t e r - i m p u r i t y atom s p a c i n g a p p r o a c h t h e Bohr r a d i u s
100 8 ) .
case.
C learly th is
is not s a tis f ie d
(~
in the d il u t e
In th e c a l c u l a t i o n , th e s p a t i a l l y varying d e n s ity
s t a t e s o n l y d e p e n d s on t h e s c r e e n e d i m p u r i t y p o t e n t i a l
c o n tr ib u tio n s to the p o te n tia l f lu c tu a tio n s .
is a s lig h t co n trad ictio n ,
Here t h e r e
in t h a t th e s h o r t range
s t r e n g t h o f t h e s c r e e n e d p o t e n t i a l v i o l a t e s t h e Th om as F e r m i a p p r o x i m a t i o n , and f r e e e l e c t r o n l i k e s t a t e s would
not see the p o t e n t i a l .
Two f i n a l p o i n t s c a n be made f ro m t h e a b o v e
d iscu ssio n .
The i m p u r i t y ban d was shown t o b o t h b r o a d e n
t h e l i n e w i d t h and s h i f t t h e p e a k o f t h e e m i s s i o n t o l o w e r
energies.
P i n c z u k e t a l . 100 o b s e r v e d r e c o m b i n a t i o n a t
e n e r g i e s b e l o w t h e ban d e d g e o f GaAs, i n m u l t i p l e QW
m o d u l a t i o n d o p e d s t r u c t u r e s w h ic h was a t t r i b u t e d t o
bandgap r e n o r m a liz a tio n .
I t i s p o s s i b l e , d e p e n d i n g on
t h e s a m p l e g e o m e t r y , t h a t S i s u r f a c e s e g r e g a t i o n may h a v e
o c c u r r e d , w h i c h m i g h t r e s u l t i n t h e i m p u r i t y band e f f e c t s
d isc u sse d above.
b a n d g a p o f GaAs i f
T h i s c o u l d r e s u l t i n e m i s s i o n b e lo w t h e
t h e d o n o r d e n s i t y i s h i g h e n o u g h , and
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
229
th e w e lls a r e broad enough.
A second p o in t r e l a t e s to
t h e b ro ad l i n e w i d t h s sometim es o b s e rv e d i n AlInAs-GalnAs
quantum w e l l s .
As t h e i m p u r i t y ban d d e n s i t y i n c r e a s e s ,
t h e l u m i n e s c e n c e l i n e s h a p e bec om es more s y m m e t r i c .
An
a s y m t o t i c e x p a n s i o n o f t h e l i n e s h a p e r e s u l t s i n a form
t h a t goes as:
- (E + - 1 — )
+ 2 kT
n
^
L(E)’ ^ A e
w h i c h i s j u s t t h e p r o d u c t o f t h e e x p a n s i o n o f E q.
a nd t h e h o l e d i s t r i b u t i o n .
(4.64)
The c o n s t a n t , A, i s a
f u n c t i o n o f pump power a nd d o n o r d e n s i t y .
AlInAs-GalnAs
quantum w e l l s som etim es e x h i b i t a bro ad sym m etric
l i n e s h a p e w i t h a FWHM l i n e w i d t h w h i c h i s n e a r l y
in d ep en d en t of w e ll t h ic k n e s s .
inc
I t is po ssib le th a t
t h i s r e s u l t s f ro m a s c r e e n e d d e g e n e r a t e i m p u r i t y b a n d .
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
CHAPTER 5
CONCLUSIONS AND FUTURE WORK
T h is t h e s i s has o u t l i n e d a g e n e r a l approach to th e
g r o w t h o f h e t e r o s t r u c t u r e s by m o l e c u l a r beam e p i t a x y .
T his in v o lv e s th r e e p a r t s .
F irst,
t h e growth o f each
s e p a r a t e m a t e r i a l i n t h e h e t e r o j u n c t i o n s h o u l d be
o p t i m i z e d and u n d e r s t o o d , p a r t i c u l a r l y i n t h e r a n g e o f
g r o w t h c o n d i t i o n s o v e r w h i c h h i g h q u a l i t y m a t e r i a l c a n be
obtained.
S e c o n d l y , t h e s e g r o w t h r a n g e s m u s t be
o v e r l a p p e d w i t h i n t h e p r a c t i c a l c o n s t r a i n t s o f t h e MBE
tec h n o lo g y so t h a t high q u a l i t y m a t e r i a l i s o b ta in e d in
the ju n c tio n region.
These c o n s t r a i n t s
in v o lv e th e speed
w i t h w h i c h t h e t h r e e m a i n g r o w t h p a r a m e t e r s c a n be
adjusted.
F l u x r a t i o s a nd g r o w t h r a t e s c a n be c h a n g e d
w ith in th e tim e i t ta k e s to open or c l o s e a s h u t t e r ,
w h ile changes in s u b s t r a t e t e m p e r a tu re in v o lv e s tim e
s c a l e s on t h e o r d e r o f t e n s o f s e c o n d s .
T h ird ly , the
o v e r a l l m a t e r i a l c o n s i d e r a t i o n s m u s t be d i s c u s s e d i n t h e
c o n te x t of the device req u irem en ts.
T h is i s one o f th e
m o s t d i f f i c u l t p o i n t s , a s t h e d e v i c e p e r f o r m a n c e and
m a t e r i a l q u a l i t y a r e i n t i m a t e l y c o n n e c t e d i n ways w h ic h
a re o fte n not c le a r ly understood.
Our k n o w l e d g e o f t h e
t h e o r e t i c a l p erfo rm an ce l i m i t s o f d e v ic e s i s always based
on t h e p e r f o r m a n c e o f a c t u a l d e v i c e s .
R elaxing ones
230
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
231
s ta n d a rd s in m a te r ia l q u a l i t y w i l l produce i n f e r i o r
d e v i c e s i n wa ys w h i c h c a n n o t b e p r e d i c t e d .
High q u a l i t y r e c t i f y i n g n - n h e t e r o j u n c t i o n s have
been d e m o n s tra te d .
The g r o w t h t e c h n i q u e u t i l i z e d a low
s u b s tr a te tem perature
(600°C)
f o r t h e t h i c k GaAs r e g i o n
a nd a h i g h s u b s t r a t e t e m p e r a t u r e
AlGaAs r e g i o n .
(680OC) f o r t h e t h i c k
The s u b s t r a t e t e m p e r a t u r e c h a n g e i n
b e t w e e n was acco m m o d a te d by t h e u s e o f a s e c o n d As c e l l
to s ta b iliz e
t h e GaAs g r o w t h a t h i g h t e m p e r a t u r e s .
l i m i t a ti o n s of the device p r e s e n tly l i e
The
in th e poor
e l e c t r i c a l p r o p e r t i e s o f S i d o p e d AlGaAs.
W hile
s u b s t r a t e i m p u r i t y e f f e c t s c a n be o b s e r v e d , t h e y a r e n o t
sig n ifican tly
in flu e n c in g the d e v ic e perform ance.
T h i s g r o w t h t e c h n o l o g y was a p p l i e d i n a
co llab o rativ e e f fo r t,
the v e r t i c a l f i e l d e f f e c t
C C
t r a n s i s t o r w ith a h e te r o ju n c tio n cath o d e.
o f t h i s e f f o r t i s d e s c r i b e d i n A p p e n d i x B.
The r e s u l t s
O n g o in g
c o l l a b o r a t i o n s i n c l u d e b a l l i s t i c Gunn d i o d e s ,
s tu d ie s of l i g h t s c a tte rin g of hot e le c tro n s .
In g e n e r a l,
10 6
and
107
f u t u r e work i n t h i s a r e a s h o u l d f o c u s on
an u n d e r s t a n d i n g o f t h e p r o p e r t i e s o f t h i c k A lGaAsrSi,
and i t s
behavior in the b a r r i e r re g io n of a
h e te ro ju n c tio n diode.
A com plete u n d e rsta n d in g o f the
d i o d e b e h a v i o r w i l l o n l y o c c u r when a n u n d e r s t a n d i n g o f
the n a tu re of the e le c tr o n ic d e fe c ts
is obtained.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
232
The g r o w t h o f q u an tu m w e l l s t r u c t u r e s
s u b s ta n tia l p o rtio n of th is th e s is .
is a
As t h e s t o i c h i o m e t r y
o f GaAs on AlGaAs i s an i m p o r t a n t i s s u e ,
a
p h e n o m e n o l o g i c a l m o d e l o f BEP r a t i o s was f o r m u l a t e d a nd
e v a l u a t e d f ro m e x p e r i m e n t a l d a t a on t h i c k m a t e r i a l .
The
form o f t h i s e x p r e s s i o n i s
As.
Pc
Ga
BGa
Where PQ c o n t a i n s t h e d e s o r b i n g f l u x e s o f A s 2 and
Ga, a n d a c o n t a i n s t h e e f f i c i e n c y w i t h w h i c h As i s
s u p p l i e d t o t h e s u r f a c e f ro m t h e i n c i d e n t A s4 f l u x .
The
e x p e r i m e n t a l d a t a showed t h a t f o r GaAs, a i s r e l a t i v e l y
c o n s t a n t up t o Ts u b = 720°C and t h a t t h e m a i n c o m p o n e n t
of flu x r a t i o in c r e a s e w ith in c r e a s in g tem perature
r e s u l t s f ro m t h e i n c r e a s e i n t h e d e s o r b i n g f l u x e s
c o n t a i n e d i n PQ.
Above 7 2 0 ° C , a b e g i n s t o d e c r e a s e ,
which c o u ld r e f l e c t th e i n c r e a s e d c r a c k i n g p r o b a b i l i t y o f
A s4 t o A s 2 .
In terestin g
i n f o r m a t i o n a b o u t AlGaAs was a l s o
o b t a i n e d i n s p i t e o f t h e s e e m in g s i m p l i c i t y o f t h e m o d e l .
By c o m p a r i n g t h e e x p e r i m e n t a l d a t a f o r AlGaAs t o GaAs t h e
f o ll o w i n g r e s u l t s were o b t a i n e d .
F irst,
PQ i s l o w e r t h a n
GaAs a t t h e same t e m p e r a t u r e , a s o n e m i g h t e x p e c t s i n c e
AlAs i s more s t a b l e t h a n GaAs.
Secondly, a r s e n ic appears
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
233
t o b e s u p p l i e d more e f f i c i e n t l y
p r e s e n t a s c o m p a r e d w i t h Ga.
t o t h e s u r f a c e when A1 i s
A rough e s tim a te s u g g e s ts
tw ice as e f f i c i e n t l y .
The a n a l y s i s o f t h i s m o d el was u s e f u l i n t h e
d i s c u s s i o n o f th e t r a n s i e n t growth s t o i c h i o m e t r y o b se rv e d
f ro m t h i n GaAs g r o w t h on AlGaAs.
E xperim entally,
i t is
o b s e r v e d t h a t t h e f l u x r a t i o f o r t h i n GaAs on AlGaAs i s
much l o w e r t h a n t h e r a t i o r e q u i r e d f o r t h i c k GaAs a t t h e
same s u b s t r a t e t e m p e r a t u r e .
The s i g n i f i c a n c e o f t h e
tr a n s ie n t sto ich io m etry a t h e te ro ju n c tio n in te rfa c e l i e s
i n t h e f a c t t h a t GaAs and AlGaAs m a t e r i a l q u a l i t y i s
o p t i m i z e d a t low f l u x r a t i o s .
Thus t h e g r o w t h o f t h i n
GaAs on AlGaAs c a n be a c h i e v e d a t v e r y low r a t i o s i f o n e
i s aware o f t h i s e f f e c t .
The low f l u x r a t i o s
reduce
i m p u r i t y i n c o r p o r a t i o n f r o m As s o u r c e s and m i n i m i z e s t h e
r e d i s t r i b u t i o n o f many s u b s t r a t e r e l a t e d i m p u r i t i e s .
U n i n t e n t i o n a l i m p u r i t i e s a r e t h e main l i m i t i n g f a c t o r i n
high p u r i t y m a t e r i a l .
A ll th e se e f f e c t s a re im portant in
t h e g r o w t h o f h i g h p u r i t y and h i g h q u a l i t y quan tum w e l l
stru ctu res.
The s m o o t h n e s s o f t h e i n t e r f a c e ,
th e second
im portant c h a r a c t e r i s t i c of h e te ro ju n c tio n s tr u c tu r e s i s
i m p r o v e d by g r o w t h o f AlGaAs a t h i g h t e m p e r a t u r e a l t h o u g h
in c o n c e rt w ith t h i s
q u ality .
i s a d e g r a d a t i o n o f t h e GaAs
The c h o i c e o f g r o w t h c o n d i t i o n s d e p e n d s on t h e
u ltim a te a p p lic a tio n of the s tr u c t u r e .
I f one i s t r y i n g
t o m inim ize e x c ito n l i n e w i d t h s , th e q u a l i t y of both the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
234
i n t e r f a c e a nd t h e GaAs i s
w e l l s ~ 200 S.
im portant, e s p e c ia lly fo r th ic k
The b e s t l i n e w i d t h w i l l be o b t a i n e d by
g r o w i n g t h e AlGaAs c l a d d i n g l a y e r s a t 680°C a nd t h e n
r a p i d l y d r o p p i n g t h e s u b s t r a t e t e m p e r a t u r e down t o l o w e r
t e m p e r a t u r e s f o r GaAs g r o w t h a nd t h e n r a i s i n g t h e
te m p e ra tu re fo r th e second c la d d in g l a y e r .
The h i g h q u a l i t y u n d o p e d q u a n t u m w e l l s r e p o r t e d on
i n C h a p te r f o u r have been u t i l i z e d
in a stu d y o f the
Zeeman e f f e c t o f e x c i t o n s i n q u a n t u m w e l l s
tran sp o rt.
108
79
a nd e x c i t o n
T h i s work h a s p r o v i d e d a new u n d e r s t a n d i n g
o f t h e e l e c t r o n i c s t a t e s a nd ban d s t r u c t u r e o f e x c i t o n s
i n q u a n tu m w e l l s .
F u r t h e r work i n t h e a r e a o f q u a n t u m w e l l and GaAs on
AlGaAs i n t e r f a c e g r o w t h s h o u l d f o c u s o n h i g h c o m p o s i t i o n
AlGaAs (x > 0 . 3 5 ) .
T h i s would be v e r y c h a l l e n g i n g a s
h i g h c o m p o s i t i o n AlGaAs g r o w t h i s r e l a t i v e l y u n e x p l o r e d .
The new p r o b l e m s w o u ld be t h e s m o o t h n e s s o f t h e s u r f a c e
d u r i n g g r o w t h , and w h e t h e r l a t t i c e s t r a i n m i g h t f i n a l l y
become a p r o b l e m .
T h is problem has im p o rta n t
a p p l i c a t i o n s i n b o t h q u a n tu m w e l l l a s e r s a nd AlGaAs
b u f f e r e d MODFET.
I n t h e c a s e of th e m o d u la tio n doped s t r u c t u r e ,
the
f a i l u r e of the in v e rte d s tr u c tu r e is lin k ed to th e
s u rfa c e s e g re g a tio n of s i l i c o n r a th e r than a re d u c tio n in
i n t e r f a c e q u a l i t y d ue t o l a t t i c e
s u g g e s t e d by p r e v i o u s w o r k e r s .
stra in ,
a s h ad b e e n
The s u r f a c e s e g r e g a t i o n
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
235
is a ttrib u te d
to a d i f f e r e n c e in the chem ical a c t i v i t y
c o e f f i c i e n t s o f S i d i s s o l v e d i n GaAs a n d AlGaAs .
d ifference
This
is a t t r i b u t e d to the e f f e c t s of n e a re s t
n e i g h b o r al uminum a t o m s m o d i f y i n g t h e e n t h a l p y and
e n tro p y of group I I I
s i t e occupation.
The e l e c t r i c a l
p r o p e r t i e s o f t h e r e g u l a r m o d u l a t i o n d o p e d s i n g l e q u an tu m
w e l l s t r u c t u r e were s t u d i e d .
D a t a h a s b e e n p r e s e n t e d on
t h e e l e c t r o n m o b i l i t y a s a f u n c t i o n o f s h e e t c h a r g e and
s p a c e r t h i c k n e s s , and t h e l i m i t a t i o n s o f th e h ig h
t e m p e r a t u r e g r o w t h o f t h e AlGaAs c a p p i n g f o r n a r r o w
s p a c e r s has been i d e n t i f i e d .
As s i g n i f i c a n t s i l i c o n
d i f f u s io n can o c c u r, th e capping la y e r s
of
device
s t r u c t u r e s which r e q u i r e narrow s p a c e r s
s h o u l d be grown
a t low t e m p e r a t u r e s t o r e d u c e t h e b u l k d i f f u s i o n e f f e c t s .
The c o n t r i b u t i o n o f a d e g e n e r a t e i m p u r i t y band t o
the d e n s ity of s ta t e s
f u n c t i o n has been c a l c u l a t e d i n a
q u a n tu m w e l l and s u g g e s t s t h e p r e s e n c e o f a l a r g e s i l i c o n
c o n c e n t r a t i o n i n t h e q u a n tu m w e l l o f t h e i n v e r t e d
m o d u la tio n doped s t r u c t u r e i n p h o to lu m in e s c e n c e .
T h i s g r o w t h t e c h n o l o g y was a p p l i e d
in
a
c o lla b o r a tiv e e f f o r t to the f a b r i c a t i o n
of
the
f i r s t low
n o i s e s u b m i c r o n s i n g l e q u an tu m MODFET w i t h a n AlGaAs
buffer.
109
The r e s u l t s a r e r e p o r t e d m A p p e n d i x C.
The
s tr u c t u r e dem onstrates d e f i n i t e re d u c tio n s in b u ffe r
c u r r e n t s a s compared w ith th e r e g u l a r s i n g l e i n t e r f a c e
MODFET.
At th e tim e of t h i s w r i ti n g t h i s d e v ic e e x h ib ts
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
236
a s t a t e of the a r t cryogenic
K a t 3 . 3 GHz.
( 1 2 . 5K) n o i s e f i g u r e o f 1 0 . 5
O t h e r o n g o i n g c o l l a b o r a t i v e e f f o r t s on t h e
m o d u l a t i o n d o p e d s i n g l e q u an tu m w e l l s i n c l u d e s t u d i e s o f
t h e o p t i c a l p r o p e r t i e s o f 2DEG i n t h e p r e s e n c e o f
m agnetic f i e l d s , c y c l o t r o n
r e s o n a n c e o f 2DEG i n
SQW,^® an d e l e c t r o n s p i n r e s o n a n c e o f n e u t r a l d o n o r s i n
2 d im en sio n al system s.
F u r t h e r work i n t h i s a r e a s h o u l d f o c u s on t h e
t r a n s p o r t p r o p e r t i e s o f t h e q u an tu m w e l l s t r u c t u r e s o v e r
th e e n t i r e range of s h e e t c h a rg e , p a r t i c u l a r l y in the
r a n g e a p p r o p r i a t e t o MODFETs, n o t j u s t t h e h i g h e s t H a l l
m o b ilitie s obtain ab le.
D ata on l i g h t s e n s i t i v i t y
i s r e p o r t e d , a nd shows t h e
unusual behavior of th e s t r u c t u r e .
T h i s s h o u l d be
f u r t h e r ex p lo red , although in p r a c t i c e the author n o tes
a n im p r o v e m e n t i n b e h a v i o r when l o w e r g r o w t h t e m p e r a t u r e s
a r e used f o r the capping l a y e r .
N aturally, a g reat deal
o f work r e m a i n s i n MODFET w o r k , r a n g i n g f ro m optimum w e l l
t h i c k n e s s e s and b u f f e r c o m p o s i t i o n t o a n u n d e r s t a n d i n g o f
e l e c t r o n t r a n s p o r t confinem ent in s h o r t g a te s t r u c t u r e s .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX A
ELIMINATION OF FLUX TRANSIENTS IN MOLECULAR BEAM EPITAXY
P . A . M a k i, S . C . P a l m a t e e r , A.R . C a law a
and B .R . Lee
G e n e r a l E l e c t r i c Company,
E le c tro n ic s L aboratory
S y r a c u s e , New Yor k 13221
P r e s e n t e d a t t h e M3E Workshop
M i n n e a p o l i s , MN, S e p t e m b e r 1985
237
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238
E lim in a tio n o f Flux T ra n sie n ts in Molecular Beam Epitaxy
P.A. Maki*, S.C. Palmateer
A.R. Calava* and B.R. Lee
General E l e c t r i c Company
E le c tro n ic s Laboratory
Syracuse, Nev York 13221
ABSTRACT
The a p p li c a t i o n of molecular beam ep itax y (MBE) to the growth o f compound
semiconductors f o r devices and m onolithic c i r c u i t s re q u ire s e x c e l le n t film
uniform ity and r e p r o d u c i b i l i t y of growth c o n d itio n s .
Conventional c o n ic a l
c r u c ib le c e l l s f o r group I I I elements ty p ic a ll y provide e x c e lle n t film
u n ifo rm ity but e x h ib i t a s i g n i f i c a n t f lu x t r a n s ie n t upon s h u t t e r opening due
to the cooling o f the melt su rfa c e.
Flux t r a n s ie n t s r e s u l t in poor c o n tr o l
over i n i t i a l growth stoichiom etry and u n c e rta in ty over the i n i t i a l film
param eters which l i m i t film r e p r o d u c ib i li t y .
This paper r e p o r ts on a unique
p y r o ly t ic boron n i t r i d e (PBN) c ru c ib le arrangement for group I I I elements
which sim ultaneously provides e x c e lle n t film uniform ity and a low flu x
tra n sie n t.
The c e l l u t i l i z e s a deep PBN c ru c ib le fo r a la r g e recessed melt
volume and a PBN i n s e r t to maintain beam uniform ity.
The c e l l provides film
thickness and doping v a r ia tio n s of l e s s than 2% across a 2 inch diam eter
v a fe r and a flu x t r a n s ie n t o f l e s s than 3 % when the s h u tte r i s opened.
The
usable melt volume i s 20 cc and the o p e ra tin g temperature i s approxim ately
100°C lo v e r than th a t of the conical c ru c ib le c e l l .
The la rg e c e l l volume
in c re a s e s run time between c e l l recharging over conventional c o n ic a l c ru c ib le
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239
c e lls.
The melt lo c a tio n deep in the furnace provides long t e r n temperature
s t a b i l i t y and reduces long term g ro v th r a t e v a r i a t i o n s .
The high m a te ria l
q u a l i t y and lov oval d e fe c t d e n s ity <<500cm') a re comparable to conventional
c o n ic a l c e l l s .
The r e s u l t s of the measurement of film parameters a re
p resented and the o p e ra tio n of the c e l l i s described.
I.
INTRODUCTION
Group I I I f lu x t r a n s i e n t s in MBE grovth l im it film r e p r o d u c ib i li t y as
v e i l as being an inconvenience to the MBE u ser.
The flu x t r a n s i e n t r e s u l t s
from a cooling of the melt s u rfa c e vhich occurs vhen the beam s h u t t e r i s
opened.
Short term flu x t r a n s i e n t s cause poor c o n tr o l over grovth
sto ic h io m e try and u n c e rta in ty over i n i t i a l grovth r a t e s .
Control o f grovth
r a t e s during the f i r s t fev minutes a f t e r s h u tt e r opening i s important for the
reproducible grovth of such submicron s tr u c tu r e s as the high e le c tr o n
m o b ility t r a n s i s t o r .
I n i t i a l flu x t r a n s ie n ts a ls o i n t e r f e r e v i t h b a s ic
r ese a rc h in MBE pro ce sse s, such as r e f l e c t i o n e le c tr o n d i f f r a c t i o n (RED)
i n t e n s i t y o s c i l l a t i o n s t u d i e s , and a r e la rg e ly a nuisance to the res e a rc h e r.
Flux t r a n s ie n t s can be reduced by the use of p a r t i a l l y f i l l e d c r u c ib le s so
th a t the in flu e n ce of changes in the r a d ia tiv e s h ie ld in g provided by the
s h u t t e r are reduced.
This approach i s im practical as the reduced charge
volume l i m it s machine o p eration betveen c e l l recharging.
Recently, ve have
rep o rted a nev p y r o ly tic boron n i t r i d e (PBN) c ru c ib le arrangement fo r Group
I I I elements v i th a p r a c t i c a l c e l l capacity vhich provides e x c e lle n t film
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240
u n ifo rm ity v i t h l i t t l e o r no flu x t r a n s i e n t * 11.
involves a 40 cc c r u c ib le v i t h a c o n ic a l i n s e r t .
The nev c ru c ib le arrangement
The large re c e ssed melt
volume i s l e s s s e n s i t i v e to r a d i a t i v e s h ie ld in g a t the c e l l o r i f i c e and the
c o n ic a l i n s e r t c o n tr o ls the beam un ifo rm ity .
In t h i s paper, ve expand on the
c e l l p r o p e r t i e s , r e p o rt a d d it io n a l device uniform ity data and dem onstrate the
p u r it y o f GaAs film s grovn v i t h t h i s c e l l .
II.
EXPERIMENTAL
The machine used in t h i s vork i s a conventional Varian GEN I I MBE
machine.
The only m odification i s in the PBN c ru c ib le c o n fig u ra tio n v i t h i n a
standard source furnace.
The c ru c ib le arrangement i s shovn in Figure 1.
A
16 cc c o n ic a l c r u c ib le v i t h i t s bottom removed i s centered v i t h i n a standard
40 cc c r u c i b l e .
Since the m odification i s i n t e r n a l to the 40 cc c r u c i b le , a
conventional furnace can be used to heat the group I I I melt.
In our i n i t i a l
s t u d i e s , the c e l l has been implemented as a gallium source.
Elemental Ga i s
placed v i t h i n the 40 cc c ru c ib le and the 16 cc conical c ru c ib le i s then
in s e r t e d .
th re a d in g .
Both c ru c ib le s a re secured v i t h conventional tantalum v i r e
The machine i s a ls o equipped v i t h a standard Ga c e l l v i t h a 16 cc
c o n ic a l c ru c ib le so th a t c o n tro lle d comparisons betveen the tvo c e l l s can be
made v i th o u t opening the MBE machine.
Beam e q u iv a len t pressure (BEP) measurements vere made v i t h a nude ion
gauge lo c a te d in the beam path a t the grovth p o s itio n .
The time dependent
beam i n t e n s i t i e s vere recorded by p l o t t i n g the analog output of the ion gauge
c o n t r o l l e r on a s t r i p c h art rec o rd e r.
BEP measurements vere made of fluxes
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241
GALLIUM
F ig u re 1.
D ia g r a m o f t h e new c r u c i b l e a s s e m b l y .
The
e x t e r i o r c r u c i b l e i s a s t a n d a r d 40 c c c r u c i b l e ,
w h i l e t h e i n s e r t i s a m o d i f i e d 16 c c c o n i c a l
cru cib le.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
242
from the nev c ru c ib le reported here and from the conventional 16 cc co n ic a l
c r u c i b le .
The p ressure readings vere made v i t h a background p ressu re of l e s s
than 10"10 Torr in the grovth chamber.
GaAs film s ve re grovn using elem ental a rs e n ic as the group V source and
e lem ental s i l i c o n as the n-type dopant.
All e p i t a x i a l la y e rs reported on
he re ve re grovn on 2 inch diameter undoped s u b s t r a t e s grovn by the liq u id
encapsulated Czochralski technique.
The films ve re grovn a t a grovth r a t e of
lpm/hr using a s u b s tr a te temperature o f 600°C v i t h a s u b s tr a te r o ta t io n speed
of 10 rptn under b arely a rs e n ic s t a b l e co n d itio n s as determined using RED
p attern s.
Film thickness and doping d e n s ity uniform ity across a 2 inch diameter
v a f e r vere measured using an e le c trochem ical p r o f i l e r .
M aterial p u r it y vas
e v alu ated by Hall measurements on l i g h t l y doped n-type GaAs l a y e r s .
Hall
samples vere made v i t h a Van der Pauv c lo v e r le a f p a tte rn and alloyed t i n
c o n ta c ts .
Hall measurements vere made v i t h a magnetic f ie l d of 2K Gauss
v ith o u t sample illu m in a tio n .
F ie ld e f f e c t t r a n s i s t o r s vere f a b r ic a te d on 2 " MBE v a f e r s , grovn v i t h the
conventional c onical c e l l , and v i t h the nev c e l l design.
The s tr u c tu r e
c o n s is te d of a 1pm undoped GaAs b u f fe r , a 0.4pm lX1017at/cm3 a c ti v e region
and a 0.2pm 2X101 , at/cm3 cap la y e r .
p h o to lith o g r a p h ic a lly and etched.
Mesa i s o l a t i o n p a tte rn s vere defined
Source and d r a in p a tte rn s vere developed
and AuGeNi ohmic contacts evaporated and alloyed in a rapid thermal an n e ale r.
1.0 X 400 micron gate p a tte rn s vere defined by deep UV lithography and
recessed using a vet chemical e tc h .
TiPtAu vas used as the g a te metal.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
243
III.
RESULTS
Figure 2 i s a comparison of the beam equivalent p ressu res as a function
of time f o r the nev c ru c ib le and a conventional 16 cc c onical c r u c i b l e , both
c o n ta in in g a 25 gram gallium charge.
The ty p ic a l i n i t i a l p re s s u re t r a n s ie n t
f o r a conventional 16 cc c o n ic a l c ru c ib le c e l l i s 30? to 50? l a r g e r than the
ste a d y s t a t e value.
The t r a n s ie n t ty p ic a ll y l a s t s 1 - 3 minutes.
The nev
c e l l re p o rte d here e x h ib its a p ressu re t r a n s ie n t of l e s s than 3? over the
same time pe rio d .
The film th ic k n e ss, doping d e n s ity p r o f i l e , and la y e r
s t r u c t u r e o f a uniformly doped GaAs film grovn using the nev c e l l a re shovn
in Figure 3.
The e le c tro n c oncentration p r o f i l e as a function o f depth a t
s e v e r a l p o in ts across the v a fe r ( i n s e t ) i s in d ic a te d .
The th ic k n e ss
v a r i a t i o n i s <+2? over the f u l l 2 inch v a fe r diameter.
FETs f a b r ic a te d on v a fe r s grovn v i th the nev c ru c ib le design e x h ib i t
comparable DC performance to va fe rs grovn v i t h the conventional c o n ic a l c e l l .
Figure 4 shovs a mapping o f the f u l l channel c u rre n t uniform ity a c ro s s a 2
inch v a f e r grovn v i th the nev c e l l design.
The f u l l channel c u rre n t
v a r i a t i o n a c ro ss the wafer i s l e s s than 3? v i th a standard d e v ia tio n of
3 .0 mA (v e t chemical etch used for channel r e c e s s ) .
This i s comparable to
v a fe r s grovn v i t h the conventional co n ic a l c e l l .
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244
GROWTH R A T E ~}xm /H R
7.4x10
(a) CONVENTIONAL
CONICAL CELL
5.4x10
(b) CELL ASSEMBLY
SHOWN IN FIG 1
3.4x10
6
5
4
3
2
TIME (MINUTES)
F ig u re 2.
1
SHUTTER
OPENED
C o m p a r i s o n o f t h e g a l l i u m c e l l BEP t r a n s i e n t .
Trace
(a)
i s th e response of a standard c e l l
w i t h a 16 c c c o n i c a l c r u c i b l e a n d t r a c e
(b) i s
a c e l l w i t h t h e new c r u c i b l e a s s e m b l y .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1018
t<2%
d < 2%
z
o
10
17
<
cc
H
z
UJ
u
16 .
8 10
OS
cc
UI
03
cc
cc
STRUCTURE
1cm
<
m x 8. 5 x
10
”
at/cm ^
1/Xm U N D O P E D B U F F E R
SUBSTRATE
101 5 ;
0.2
0.M
0.6
0.8
1.0
T H IC K N ES S (/im)
F ig u r e 3.
Doping c o n c e n t r a t i o n a s a f u n c t i o n o f d e p t h f o r a u n i f o r m l y doped
GaAs f i l m .
The f i l m s t r u c t u r e i s
245
i n d i c a t e d on t h e w a f e r i n s e t .
show n a n d t h e p r o f i l e d p o i n t s a r e
246
92.5
92.5
93.5
96.5
89.5
93.5
95.5
94.5
96.5
96.5
96.5
91.5
95.5
F ig u re 4.
92.5
95.5
U n i f o r m i t y map o f t h e f u l l c h a n n e l
(mA) o f 1
x 400 ym g a t e FETs f a b r i c a t e d o n a
GaAs f i l m
grown u s i n g t h e two c r u c i b l e c e l l .
The mean
f u l l channel c u rre n t i s
9 4 . 5 mA a n d t h e
s t a n d a r d d e v i a t i o n i s 3 . 0 mA.
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
The r e s u l t s of B a ll measurements a t 300R and 77K a re rep o rted in Table I
f o r a l i g h t l y doped n-type GaAs film of 15pm th ic k n e s s.
Table 1
Temperature
W
Hall
M obility (cmJ /V-s)
B all E lectron
Concentration (#/cm3)
300
7,870
1 X 1014
77
129,900
1 X 1014
These r e s u l t s a re eq u iv a len t to those normally obtained in our machine
using the conventional 16 cc c onical c ru c ib le c e l l .
IV.
DISCUSSION
The lo v e r t r a n s i e n t flu x from the nev c r u c ib le c e l l i s l a r g e ly the r e s u l t
o f the recessed p o s itio n of the group I I I melt deep v i th i n the furnace.
The
temperature o f the melt deep in the furnace i s l e s s s e n s i t i v e to changes in
the r a d i a t i v e s h ie ld in g provided by the c e l l s h u t t e r .
This lov flu x
tr a n s ie n t behavior i s obtained vhen the c e l l i s f u l l y charged, compared to
the conventional c o n ic a l c ru c ib le c e l l vhich only e x h ib its lov flu x
tr a n s ie n t s vhen n e arly empty.
The nev tvo c r u c ib le c e l l provides both e x c e lle n t thickness uniform ity,
as shovn by Figure 3, and an e x c e lle n t doping-thickness product uniform ity as
demonstrated by the FET r e s u l t s in Figure 4.
Since the doping uniform ity
a ls o depends on the s i l i c o n source c o n fig u ra tio n , the r e s u l t s here in d ic a te
th at the s i l i c o n source, vhich i s comprised of a nearby empty c onical
c r u c ib le , provides e x c e lle n t doping uniform ity as v e i l .
The e x c e lle n t
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248
u n ifo rm ity of the nev gallium source i s a consequence o f the re c e ssed
a p e r t u r e of the conical c r u c ib le i n s e r t , vhich c o n tro ls the beam u n ifo rm ity .
Deep 40 cc c ru c ib le s normally e x h ib it poor beam un ifo rm ity 41’ 31 r e s u l t i n g
from a change in the p r o je c te d melt a re a a t larg e angles from the c e l l a x is .
V ith the nev source, the a p e rtu re of the c ru c ib le i n s e r t p r e s e n ts a c o n stant
melt a re a to a l l points a c ro s s a 2 inch diameter s u b s tr a te .
Because the
a p e r t u r e s iz e and p o s itio n i s defined by the cone i n s e r t , the unobstructed
m elt area v i t h i n the cone angle i s constant as long a s the p r o je c ti o n o f the
a p e r t u r e l i e s on the melt s u rf a c e .
g r a p h i c a l l y in Figure 5.
This c o n s tru c tio n i s in d ic a te d
One should note th at the r o le of the c r u c ib le
i n s e r t i s to lim it the m olecular (atomic) t r a j e c t o r i e s from the melt su rfa c e
vhich reach the s u b s tr a te .
Because the a p e rtu re i s la r g e the i n s e r t i s not
l i k e l y to cause an equilibrium group I I I vapor to be maintained over the melt
as in a Knudsen c e l l 14’ .
An important c o n sid e ratio n in using a beam l im it in g
a p e r tu r e i s the trade o f f betveen the beam i n te n s it y and the c e l l o p e ra tin g
tem perature.
As ve noted p r e v io u s ly ’11, the apertured c ru c ib le provides the
same flu x v i th an apparent o p e ra tin g temperature about 100°C lo v e r than the
c onventional 16 cc c onical c ru c ib le .
T herefore, the flu x and o p e ra tin g
temperature are not lim ite d by the prese n t a p erture s i z e . .
Ve have a ls o observed th a t the film grovth r a t e using t h i s c e l l a t a
fix e d temperature s e t t i n g i s s t a b l e fo r a s much as 300 microns o f t o t a l film
g ro v th .
This long term grovth r a t e s t a b i l i t y r e f l e c t s both the temperature
s t a b i l i t y and uniformity deep v i t h i n the furnace and the constant flu x
provided
by the a p e rtu re .
The c ru c ib le geometry compensates f o r the
receding melt surface by in c re a s in g the projected melt a re a , keeping the flu x
c o n s ta n t.
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249
SUBSTRATE
CRUCIBLE
APERTURE
MELT
SURFACE
Ca
F ig u re 5.
G eom etrical c o n s tr u c tio n f o r c o n s ta n t m elt are a
as seen a t th e s u b s tr a te p o s itio n .
The d i a g r a m
i s n o t drawn t o s c a l e and i s i n a v e r t i c a l
o rie n ta tio n for c la rity .
W ithin t h e cone a n g le ,
a n early c o n sta n t m elt area i s o btained.
O ut­
s i d e t h e c o n e a n g l e , p o o r beam u n i f o r m i t y
r e s u l t s f r o m a s h a d o w i n g by t h e c r u c i b l e l i p .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
250
The a d d itio n of the c ru c ib le i n s e r t has not reduced the p u r it y of the
GaAs grovn using the nev gallium source, a s shovn by the high 77K e le c tr o n
m o b ility re p o rte d in Table 1.
This r e s u l t i s t y p ic a l of s t a t e of the a r t MBE
m a te ria l and i s e quivalent to the r e s u l t s obtained in our machine using the
conventional 16 cc co n ic a l c ru c ib le c e l l .
Oval d e fe c t d e n s it ie s a r e an im portant f a c t o r vhich can l im it the y i e l d
o f m onolithic c i r c u i t s fa b r ic a te d on MBE grovn GaAs.
Vhile ve have not y e t
completed sy ste m atic s tu d i e s , ve have observed th a t the oval d e fe c t d e n s ity
i s c o n s is t e n t ly lover using the c o n ic a l i n s e r t c ru c ib le c e l l .
Defect
d e n s i t i e s o f <500cm"J fo r th in la y e rs a re obtained v i t h good machine
co n d itio n s and good s u b s tr a te p re p a ra tio n f o r both the nev c ru c ib le and a
conventional 16 cc co n ic a l c r u c ib le .
be s u b s tr a te lim ite d .
These d e fe c t d e n s it ie s a re b e lieved to
For lay e rs th ic k e r than 5 microns, the d e fe c t d e n s ity
i s a f a c t o r o f 2 to 3 lo v e r than th a t o b tained using the conventional c e l l .
P a r t of t h is decrease may be due to the lo v e r o perating temperature.
V.
CONCLUSION
Ve have demonstrated a larg e c a p ac ity gallium c e l l vhich sim ultaneously
provides e x c e lle n t film uniform ity and l i t t l e or no flux t r a n s ie n t behavior.
The c e l l i s e a s i l y implemented and provides s i g n i f i c a n t improvements in both
a la r g e r melt capacity and long term g rovth r a t e s t a b i l i t y over the
conventional co n ic a l c ru c ib le c e l l .
v i t h lov oval d e fe c t d e n s i t i e s .
The c e l l produces high p u r ity m a te ria l
Vhile demonstrated for gallium, t h i s
approach should vork equally v e i l fo r o t h e r group I I I melts.
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251
ACKNOWLEDGEMENTS
The authors vould l i k e to thank J.C.M Hvang, R.L. F i n i d e and J.D . Berry
£or u s e f u l tec h n ic a l d is c u s s io n s .
*
School of E l e c t r i c a l Engineering, Cornell U n iv e rsity , I th a c a , Nev York
14853.
+ C ornell L ectu rer, Permanent Address:
Lincoln Laboratory, Massachusetts
I n s t i t u t e of Technology, Lexington, Massachusetts
02173.
REFERENCES
1.
P.A. Maki, S.C. Palmateer, A.R. Calava, and B.R. Lee, to be published,
Jo u rn a l o f the Electrochemical Society, 1985.
2.
P.E. Luscher and D.H. C o llin s, Progress in C ry s ta l Grovth and
C h a r a c te riz a tio n , 1979, Vol. 2, pp. 15-32, Pergamon Press LTD.
3.
V. Schaff, Ph.D.
4.
For a d isc u ssio n o f effu sio n c e l l s , see fo r example refe re n c e 3 or
K. Ploog, "Molecular Beam Epitaxy of III-V Compounds", C ry s ta l Grovth
P r o p e rtie s and A pplications, 1980, Vol. 3, Springer-V erlag.
D is s e r ta tio n , Cornell U n iv e rs ity , 1985.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX B
BARRIER HEIGHT DEPENDENCE OP AN N-N HETEROJUNCTION
ON APPLIED VOLTAGE
The p u r p o s e o f t h i s a p p e n d i x i s t o f o r m u l a t e t h e
dependence o f t h e b a r r i e r h e i g h t o f an n -n h e t e r o j u n c t i o n
on a p p l i e d v o l t a g e .
p a p e r by K r o e m e r .
■jin
The d e r i v a t i o n s c a n be f o u n d i n t h e
Kr oem er f o r m u l a t e d t h e e q u a t i o n s on
t h e b a s i s o f t h e Jo y ce-D ix o n a p p ro x im a tio n f o r weakly
d e g e n e r a t e Fermi s y s te m s .
T h is f o r m u l a t i o n i s t h e most
accu rate of the a n a ly tic a lly a c c e s sib le c la s s ic a l
tre atm e n ts.
As Kroem er h a s n o t e d , h o w e v e r , t h e c l a s s i c a l
t r e a t m e n t c a n o n l y be r e g a r d e d a s a n a p p r o x i m a t i o n .
B ecause o f th e s t r e n g t h o f th e f i e l d w i t h i n th e narrow
g a p m a t e r i a l and t h e c o n f i n i n g i n t e r f a c e , bound s t a t e s
c a n be f o r m e d , w h i c h v i o l a t e t h e b a s i c a s s u m p t i o n s o f t h e
c la s s ic a l treatm en t.
The m o s t c o m p l e t e t r e a t m e n t o f t h e
p r o b l e m o f a c c u m u l a t i o n l a y e r s i n n - t y p e I I I - V compounds
i s p r o b a b l y by B a r a f f and A p p elb au m .
113
They i n c l u d e d
m u l t i p l e boun d s t a t e s and s c r e e n i n g e f f e c t s from
co n duction e l e c tr o n s in a s e l f c o n s i s t e n t num erical
so lu tio n .
K roem er's approach i s used h e re because of i t s
a n a l y t i c a l s i m p l i c i t y an d f o r t h e n e e d f o r q u a l i t a t i v e
r a th e r than q u a n t i t a t i v e r e s u l t s .
K roem er's f o rm u la tio n
o f t h e h e t e r o j u n c t i o n problem c o n s i s t s of a s e t o f
p a r a m e tric e q u a tio n s w ith th e e l e c t r o n c o n c e n tr a tio n in
252
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253
th e narrow gap m a t e r i a l as th e in d e p e n d e n t p a r a m e te r .
The a p p l i e d v o l t a g e i s g i v e n b y:
v = |T
q
+ [e
{^dGaAs—
dAlGaAs
N
+ _L _
*£
d
nc
- Ae + ? + k T ] }
c
^32
2 -
In
nc
(£-) ]
nd
GaAs
(volts)
where
= k T ( H n 2 - + i = 2 _ )GaAS
The b a r r i e r h e i g h t ,
eB = Aec -
(Joules)
e0 , i s g i v e n by
eN
(Joules)
H ere, k i s B oltzm ann's c o n s t a n t , T i s th e te m p e ra tu re in
K elvin, q i s th e e l e c t r o n i c ch arg e, n ,
and n .
GaAs
AlGaAs
a r e t h e n - t y p e d o p i n g c o n c e n t r a t i o n i n t h e GaAs and
AlGaAs, r e s p e c t i v e l y .
is
w
The c o n d u c t i o n b a n d d i s c o n t i n u i t y
w h ile 5 i s th e e q u i l i b r i u m p o s i t i o n o f th e Fermi
l e v e l i n t h e AlGaAs r e l a t i v e t o t h e c o n d u c t i o n b and e d g e .
Some o f t h e b r a c k e t e d q u a n t i t i e s a r e t o be e v a l u a t e d i n
t h e GaAs a s i n d i c a t e d .
They a r e :
the e f f e c t i v e
c o n d u c t i o n b and d e n s i t y o f s t a t e s , n , t h e f r e e e l e c t r o n
V
c o n c e n tra tio n a t the in te r f a c e ,
n^.
n, th e doping d e n s i t y ,
T h e s e e q u a t i o n s w e r e pr ogra m m ed o n t o a c o m p u t e r , and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
254
t h e p a r a m e t e r s were u sed i n th e t h e r m i o n i c e m is s io n
e q u a tio n o f C hapter 3.
A t low t e m p e r a t u r e s , t h e r m i o n i c c u r r e n t d e n s i t i e s
a r e v e r y l o w , a n d t u n n e l i n g c u r r e n t s c a n be a s i g n i f i c a n t
component of th e t o t a l c u r r e n t .
The f o l l o w i n g
c a l c u l a t i o n p r o v id e s an e s ti m a t e o f th e m agnitude o f th e
tunneling c u rre n t.
a t the beginning.
p artic le s
The c a l c u l a t i o n a s s u m e s a bound s t a t e
The t u n n e l i n g c u r r e n t o f l o c a l i z e d
t h r o u g h a b a r r i e r i s g i v e n b y:
J tu n = ^ [Tunneling p r o b a b i l i t y ]
x [# o f P a r t i c l e s ]
x [frequency]
I n t h e WKB a p p r o x i m a t i o n , t h e t u n n e l i n g p r o b a b i l i t y i s
, 64
g i v e n by
r
4 /
T = exp [- ^ /
m
2m*
, 25
h
<eB>V 2
E
w h e r e m* i s t h e e f f e c t i v e m a s s , h i s P l a n c k ' s c o n s t a n t ,
eB i s t h e b a r r i e r h e i g h t , a n d E i s t h e e l e c t r i c f i e l d a t
the i n t e r f a c e .
The number o f p a r t i c l e s
i s a s s u m e d t o be
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
255
w h e r e e i s t h e d i e l e c t r i c c o n s t a n t o f t h e AlGaAs.
T his
j u s t i m p l i e s t h a t a l l t h e e l e c t r o n s w h ic h t e r m i n a t e t h e
e l e c t r i c f i e l d a r e l o c a t e d i n a bound s t a t e a t t h e
in terface.
The f r e q u e n c y w i l l be e s t i m a t e d u s i n g t h e t r i a n g u l a r
w e l l a p p r o x im a tio n f o r t h e ground s t a t e o f an e l e c t r o n
c o n f i n e d by t h e f i e l d t o t h e i n t e r f a c e .■L14
h« = ----- ^ 73 ■ * [ | tt * q hE ] 2 / 3
(2m*)1 / 3
8
T h is c u r r e n t component i s added t o th e t h e r m io n ic
em ission c u r r e n t to o b ta in th e t o t a l c u r r e n t.
These
e q u a t i o n s a r e a l s o i n c lu d e d in th e com puter program .
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APPENDIX C
VERTICAL ELECTRON TRANSISTOR (VET) IN GaAs
WITH A HETEROJUNCTION (AlGaAs-GaAs) CATHODE
U. M i s h r a , P . A . M a k i , J . R . W e n d t , W. S c h a f f ,
E. Kohn a nd L . F . E a s t m a n
School of E l e c t r i c a l E ngineering
C o r n e l l U n i v e r s i t y , I t h a c a , NY 14853
P u b lish e d in E l e c t r o n i c s L e t t e r s , V ol.
20,
No. 3 , p . 1 4 5 , 1985
256
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257
VERTICAL ELECTRON TRANSISTOR (VET)
IN GaAs WITH A HETEROJUNCTION
(AlGaAs-GaAs) CATHODE
Indexing terms: Semiconductor devices and materials. Electron
devices
We wish to report on the successful fabrication of submicrometre channel length (0-75 jim) and gate length
(0-15 pm) vertical electron transistors with AlGaAs cathodes.
Lack of electron velocity enhancement has been proposed to
be due to high operating channel temperatures, and low tem­
perature measurements were hindered by carrier freeze-out.
Introduction: Vertical electron transistors have attracted
attention because of their capability of delivering a high power
density at a high frequency. Since the early work in Si such as
the analogue transistor o f Zuleeg1 and N ishizawa’s SIT 2 the
primary effort to obtain microwave performance has been in
GaAs led by the PBT, developed by Lincoln Laboratories.2
W estinghouse's X-band FET4 and the vertical electron tran­
sistor fabricated at Cornell.5 In contrast to the published work
we wish to report on preliminary results obtained on vertical
transistors fabricated with AlGaAs cathodes to tap the ballis­
tic electron launching property o f the AlGaAs-GaAs junction.
Purpose o f heterojunction: The AlGaAs-GaAs abrupt n-n het­
erojunction shown schematically in Fig. 1 exhibits a conduc­
tion band discontinuity, which is, by Dingle’s rule, --85% of
the bandgap difference. On forward-biasing the junction elec­
trons are launched into the GaAs with a kinetic energy equal
to the conduction band discontinuity. The electrons are then
accelerated by the drift field in the GaAs and at the same time
decelerated by dissipative forces such as polar optical phonon
AI03®a o-7As
■*------
GaAs
near
flat b and
zero b i a s
Fig. 1 AlGaAs-GaAs n-n heterojunction under zero bias and forward
bias
E L E C T R O N IC S L E TTE R S
2 n d February 1 9 8 4
Vol. 2 0
N o. 3
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scattering (small-angle forward-scattering) and intervalley
transfer (randomising). M onte Carlo calculations o f electron
velocity against distance under these conditions have been
performed by Tang and Hess.6 Thev predict a ‘collision free
window’ o f approximately 1000 A for a drift field of
10 k V c m " 1 in GaAs. Furthermore the only self consistent
M onte Carlo calculation at 77 K. o f a 0-25 /im S-D spacing
FET with an AlGaAs cathode performed by Tomizawa et a l.'
predicted an average electron velocity through the channel
(iVp = 7 x 1016 c m -3 ) o f ~ 7 x 107 c m s ' 1 and therefore a gm
o f 1250 mS mm * 1 a n d /™ , o f 250 G H z.
W e report on the first successful fabrication o f a unipolar
electron transistor in GaAs with an AlGaAs cathode.
Device structure and fabrication: The epitaxial layers were
grown by MBE on an n~ substrate in a Varian Gen II
source
topered
2000 A
500 A
G a As
A lG a A s
A lG aA s
channel
GaAs
d ra in
GaAs
tOOOA
•7500 A
s u b s t r a te
7
ohm ic m etal g
'UKifitumumtmtMi
y/t>> >>>>>>>}!} >>rrn>yrr
AlGaAs
GaAs
n*
0
-
so u rc e
Ti (gate
(drain)
is12/ri
Fig. 2
a Cross-section of starting material for fabricating heterojunction
VETs
b Cross-section of a single post of the multifinger VET
of the copyright owner. Further reproduction prohibited without permission.
machine. A schematic diagram o f a cross-section o f the start­
ing material is shown in Fig. 2a. The AlGaAs was chosen to
be Al0.:5G a 0.75As to obtain a conduction band discontinuity
o f 0-25 eV. T he Al m ole fraction was decided as a compromise
between the need for a high initial kinetic energy of launching
and the need to avoid intervalley transfer, i.e. A £c>>ii =
A £ r _ L = 0 3 1 eV. The n~ contact layers were doped at
3 x 1018 cm - 3 , the AlGaAs at 8 x 1016 c m -3 and the GaAs
a t 2 x 1016c m - 3 .
The fabrication steps were very similar to the scheme
employed for fabricating the vertical electron transistor with a
traditional n* source, as detailed in a previous publication.5
The processing steps will therefore be just summarised below.
The wafers were first metallised with an AuGeNi/Ag/Au
system with 1000 A Ti overlay. The source fingers (1 nm wide
with 2 fim periodicity) and pad were then defined optically
and the pattern transferred to the Ti using an H F r H -O ,:
H 20 etch. T he wafer was then milled in Ar in 15% partial
pressure o f oxygen to a depth o f ~ 1000 A into the GaAs.
N ext the wafer was etched in an H 3P 0 4 : H , 0 2 : H20 etch
which served the triple purpose of:
(a) removing the milling damage
(b) providing an overhanging source metallisation
(c) producing 0-6 pm-wide fingers for each pinch-off.
The overhang was then used to deposit a 1500 A gate of Ti by
shadow evaporation. Definition o f the gate pad and a back
ohm ic contact evaporation followed by an alloy completed the
device fabrication. A cross-section of a single post of the VET
is shown in Fig. 2b.
Results and discussion: The devices were measured at room
temperature and yielded a gm o f - 6 0 r a S m m ''. Fig. 3a shows
an SEM micrograph of the device, and Fig. 3h the I V charac­
teristics. The pinch-off voltage was - 4 V, which is larger than
the geometrical pinch-off, which can be explained by shortchannel effects such as electron spill-over from the contacts.8-*
N o obvious electron velocity enhancement could be deduced
from either current density or gm considerations. This could be
either due to the high operating channel temperature (which
reduces the electron mean free path) or large channel length.
Attempts to cool the device led to carrier freeze-out which
made interpretation of I V characteristics difficult. Effort is
under way to fabricate devices o f shorter channel length and
improve material quality to render low-temperature measure­
ments more meaningful.
of the copyright owner. Further reproduction prohibited without permission.
260
b
1512/31
Fig. 3
a SEM micrograph of VET. T op fingers form the source. Recessed
metallisation forms the gate. Back contact is the drain
b D C characteristics of VET
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
261
Conclusion: Successful fabrication o f submicrometre channel
length (0-75 pm) and gate length (0-15 pm) vertical electron
transistors is reported. The devices had a gm o f - 60 mSm m - 1 . Lack of apparent velocity enhancement has been pro­
posed to be due to high operating channel temperatures and
low-temperature measurements have been hindered by carrier
freeze-out.
Acknowledgm ents: We would like to thank P. M. Smith and J.
Berry for their invaluable technical assistance. We thank E. D.
W olf for permission to use the facilities o f the US National
Research & Resource Facility for Submicron Structures. This
work was supported by the Joint Services Electronics
Program (JSEP) under the US Air Force Office o f Scientific
Research.
U. M1SHRA
12th December 1983
P. A. MAK1
J. R. W EN D T
W. SC H A FF
E. K O H N
L. F. EASTMAN
School o f Electrical Engineering
Cornell Unicersitv
Ithaca, S Y 14853. USA
References
1
2
3
z u l e e g . r .: ‘A silicon space-charge-limited triode and analog tran ­
sistor’, Solid State Electron., 1967, 10. pp. 449-460
N iS H iZ A W A , j .- i. , t e r a s a k i . t . , and s h i b a t a , j . : IE E E Trans., 1975,
ED-22
B O ZLER , C . 0 . . ALLEY, G . D .. M U R PH Y , R. A., FLANDERS, C. D .. and
l i n d l e y , w . t . : ‘Permeable base transistor'. Proc. Seventh Biennial
Conference on active microwave devices and circuits. Cornell.
USA, 1979
4
C L A R K E . R. C ., N A TH ANSON, H . C „ OAKES, 3. G ., a n d HA RD ISO N. G . T..‘
5
m is h r a ,
DRC, 1982
U ., K O H N . E..
and
Ea s t m a n , l . F .:
I EDM Tech. Dig., 1982,
p. 594
6
7
8
IEEE Trans., 1982, ED-29
, y ., h a s h i z u m e . n „ and k a w a s h i m a . m .:
‘Sim ulation of GaAs submicron FE T with hot-electron injection
structure’. Electron. Lett.. 1983.19, pp. 697-698
K O H N . E., m i s h r a . u„ and E a s t m a n , l . F .: I EEE Electron Device
Lett., 1983. EDL-4
t a n g , j..
and
t o m iz a w a
h e ss, k .:
. k„
aw ano
* l y d e n , c . , a n d C a m p b e l l , j . s .: t o b e p u b l i s h e d
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX D
SUB-MICROMETER QUANTUM WELL HEMT WITH
AN A1q 3GaQ 7AS BUFFER LAYER
L.H . C a m n itz , P.A . M aki, P . J . T a s k e r
a nd L . F . E a s t m a n
School of E l e c t r i c a l Engineering
C ornell U n iv ersity ,
I t h a c a , NY 14853
P u b lish ed in th e p ro ceed in g s of the I n t e r n a t i o n a l
Symposium on GaAs an d R e l a t e d Compounds, 1984
262
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263
Inst. Phys. Conf. Ser. .Vo. 74: Chapter 5
Paper presented at Int. Symp. GaAs and Related Compounds, Biarritz, 1984
S u b - m i c r o m e t e r q u a n t u m w e l l H E M T w i t h A I 0 3G a 0<7A s b u f f e r l a y e r
L.H. Camnitz, P.A. Maki, P . <3. Tasker, and L.F. Eastman
School of E l e c t r i c a l E n gin eerin g, P h i l l i p s Hall
Cornell U n i v e r s it y , Ith a c a , NT 14853
USA
Abstract
The f i r s t modulation doped AlGaAs/GaAs h e te r o s tr u c tu r e f i e l d e f f e c t
t r a n s i s t o r s have been f a b r ic a t e d u t i l i z i n g an undoped A l o . 3Gao. 7 As
b u ffe r la y e r . Transconductances (gm) o f 125 mS/mm and 210 mS/mm have
been measured at 300K and 77K, r e s p e c t i v e l y f o r 0 .3 5 urn length
enhancement mode d e v ic e s . The d e v ic e s e x h i b i t t h i s high g^ at low
drain current with e x c e l l e n t p in c h o f f c h a r a c t e r i s t i c s , a ttr ib u t e d to
a high q u a l i t y GaAs quantum w ell channel and AlGaAs b u ffe r la y e r . A
s t a t e of the art 8 . 3 GHz n o is e temperature o f 1 0 . 5K was measured at a
p hysical temperature o f 1 2 . 5K, demonstrating great p o te n t ia l of t h e s e
d e v ic e s f o r low n o is e a m p li f ic a t io n .
1)
Introduction
The use of an AlGaAs b u ffe r layer t o reduce th e p a r a s i t i c b u ffer current
or s u b s t r a t e conduction in GaAs MESFET's has been proposed by Eastman e t
al (1979) and i n v e s t i g a t e d by many la b o r a t o r i e s (Hal 1 ias e t al 1979).
Improvements in output r e s i s t a n c e have been noted (Ghosh e t al 1984,
Wang e t al 1981) and some improvement in microwave power e f f i c i e n c y has
been found (S c h a ff e t al 1983). However, very l i t t l e d i r e c t evidence of
s i g n i f i c a n t l y improved high freq u en cy behavior has y e t been observed.
An important reason f o r t h i s lack of e v id e n c e may be t h a t most of the
d e v ic e s i n v e s t ig a t e d e i t h e r had ga te le n g th s longer than one micrometer
or were not t e s t e d at microwave f r e q u e n c ie s . One would exp ect the
p a r a s i t i c s u b s t r a t e conduction to become dominant f o r very short ga te
le n g t h s , f o r i t s c a l e s in v e r s e l y as th e g ate le n g th squared (Eastman e t
al 1979).
A problem which e x i s t s in MESFET's with AlGaAs b u ffer la y e rs i s the
d i f f i c u l t y of growing high q u a l i t y GaAs on AlGaAs. Such d ev ices may
show degradation o f gm at low Id or poor p in c h o f f c h a r a c t e r i s t i c s ,
in d i c a t i n g a poor h eter o ju n c tio n i n t e r f a c e or b u ffe r l a y e r . The use of
smoothing la y e r s (Kopp e t al 1982, Arnold e t al 1984) or m u lt i- la y e r
b u ffe r s (S c h aff e t al 1983) r e s u l t s in improved p in ch off
ch a r a c te r i s t i e s .
Recent modeling work (Widiger e t al 1984) has shown th at in High
E lectron M o b ility T ra n sistor (HEMT) s t r u c t u r e s th e e x c it e d elec tr o n
energy s t a t e s o f the two dimensional e l e c t r o n gas (2DEG) are not
con fined by th e p o te n t ia l w e l l , ra th er th ey are bulk -1 ik e s t a t e s s im ila r
t o th o se in the MESFET. Since hot e l e c t r o n s are p revalent in the
channel of an op eratin g sh ort channel HEMT, one would exp ect a large
© 1985 Adam Hilger Ltd
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
264
Gallium Arsenide and Related Compounds 1984
s u b s t r a t e conduction component. Indeed, s t a t e o f the a rt 0 .3 5
micrometer HEMT's e x h i b i t values of output r e s i s t a n c e as low as 43 Q<mm
(Berenz e t al 1984). An in v e r te d HEMT or double h e t e r o s tr u c tu r e HEMT
which in co r p o r a te s a t h ic k AlGaAs b u ffer laye r might then outperform a
s h o r t channel con ven tiona l HEMT.
In t h i s paper, we report on th e su c c e s s f u l f a b r ic a t io n of a submicron
low n o ise quantum w e ll HEMT. A conventional HEMT s t r u c t u r e with the
a d d itio n of an undoped A1Q ,Gag 7 AS b u ffer la y e r was chosen over the
in v e r te d HEMT or double h e t e r o s tr u c tu r e HEMT in order to avoid the
problems of S i-d o p in g in the AlGaAs below the GaAs channel (Drummond e t
al 1982). This work r e f l e c t s t h e a p p lic a t io n of p r e v io u s ly reported M3E
growth techn iqu es f o r undoped s i n g l e quantum w e l l s (Maki e t al 1983a)
and modulation doped s i n g l e quantum w e ll s (Maki e t al 1983b) on th ic k (1
yn) AlGaAs b u f f e r s . The important a sp ec ts are high temperature growth
(680°C) at con ven tional growth r a t e s ( 1 - 1 . 4 yn/hr) with carefu l control
o f s u r fa c e s t o ic h io m e t r y through th e V: III f l u x r a t i o . These c o n d itio n s
r e s u l t in a quantum w ell s t r u c t u r e e x h ib it in g narrow pnotoluminescence
lin e w id th and high e l e c t r o n m o b i l i t y , in d i c a t in g smooth w ell in t e r f a c e s
and low d e f e c t and im purity c o n c e n t r a tio n . In a d d it io n , a high q u a l i t y
AlGaAs b u ffe r i s ob tain ed which i s important t o th e d ev ic e r e s u l t s
p resen te d h ere.
E p ilay er Growth
The s t r u c t u r e was grown by MBE in a Varian
• 3 n ero n i
Gen II machine on an undoped SI LEC sub­
.35 iic ro m
s t r a t e . The AlGaAs la y e r s and the GaAs
quantum w ell were grown at a su b s t r a t e
C a te
temperature of 630°C at a GaAs growth r a t e
|
/
\ .91 •itran
Drain
d o u rcs
“
ir’
o f 1 yn/hr under metal s t a b l e growth con­
d i t i o n s . The d ev ic e s t r u c t u r e grown i s
n* G afc .0 3 5 11 c ro n
-.97 aicrpn
shown in F ig . 1. F i r s t , a 2100 8 GaAs
b u f fe r la y e r was grown, fo llo w e d by a
n+ RIGaRs ..035 ■ ic ro n
graded com position 1000 A t h ic k AlGaAs
m icron
Undoped RIGafls sp a c e r
l a y e r to preclude th e formation of a second
Unriooed
GaRs
c
h
a
n
n
e
l
.0
1
3
a
ic
ro n
2DEG. The 1 yn t h ic k n e s s of the uninten­
t i o n a l l y doped AlGaAs b u ffe r was chosen
as t h i s i s a depth over which the fringing
Undoped fll Ga fis 1 l i c r o n
f i e l d s are high due t o th e 0 .3 5 yn ga te
______ U t7________
geometry (Eastman, 1979). The undoped
Grided fll C« f e .1 r e r a n
130 A th ic k GaAs quantum w ell channel
Undoped Gift: .27 r e r a n
i s follo w ed by an 35 8 undoped AlGaAs
LEC undoped SI s u b s tra te
sp acer la y e r and 350 A Si-doped
AlGaAs at a Si atom c o n ce n tr a tio n o f 1.5
x IQ18 cm*3 . F i n a l l y , a 350 A
F ig . 1. Oevice s t r u c t u r e .
n+ c on tact la y e r was grown. Van der
Pauw Hall measurements y ie ld e d a m o b ility
o f 5 ,4 0 0 c n r /V - s e c and 70,000 cm2 /V -se c with sh ee t c a r r i e r d e n s i t i e s of
7 . 6 x 1 0 ^ cm- 2 and 6.1 x 1 0 ^ cm- 2 at 300K and 77K, r e s p e c t i v e l y . The
77K sh ee t c a r r i e r d e n s i t y was - 4 x 1 0 ^ cm "2 when measured in the
dark, in d i c a t i n g a s i g n i f i c a n t p e r s i s t e n t p h oto co n d u ctiv ity (PPC) e f f e c t
in the Si-doped AlGaAs.
D evice F a b r ic ation
Lew n o is e sub-micrometer gate length HEMT's were f a b r ic a t e d using mesa
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Gallium Arsenide and Related Compounds 1984
i s o l a t i o n , Ni/AuGe/Ag/Au a llo y e d ohmic c o n t a c t s , and re c e sse d ga tes
formed by d i r e c t w r it e EBL. The ohmic c o n t a c t i s a 100 ft Ni, 900 ft
AuGe, 1000 ft Ag, and 1000 ft Au m e t a l l i z a t i o n d eep ly a llo y e d a t a peak
temperature o f 570OC in a 3 minute t r a n s i e n t c y c l e (Zwicknagl e t al
1984, J o n e s ) . The ohmic co n ta c t r e s i s t i v i t y was measured to be 0 .5 fl-mm
a t 77K and at 300 K. Wet chemical e tc h in g was used to etc h o f f - 100 ft
o f the n+ GaAs la yer b efo re the Ti/Pt/Au g a te metal d e p o s it io n . The
edges o f the r e ce sse d g a te trench coincided with the f o o t p r i n t o f the
0 .3 5 pm long g a t e . The 300 pm width gate was centered in the 3 pm
channel formed by the source and drain ohmic c o n t a c t s , and fed by 2 g a te
pads in the standard ir c o n f ig u r a ti o n .
DC C h a r a c t e r is t ic s
The drain c h a r a c t e r i s t i c s at 300K (Figure 2) showed an enhancement-mode
l i k e b eh avior, with a th r esh o ld o f v o lt a g e o f - 0 . 2 V and a maximum
transconductance o f 38 mS, or 125 mS/mm. The r e ce sse d Schottky gate
metal t o GaAs quantum w e ll sp a cin g , a, was 700 ft, as confirmed by the
gate c a p a c ita n c e . This e x p la in s the r e l a t i v e l y low transconductance
compared t o HEMT's with sp acin gs o f 350-400 ft. Considering t h a t the
channel a sp ec t r a t i o , L /a, i s o n ly about 5 : 1 , the drain output
r e s i s t a n c e value o f 500 SI or 150 Sl-mm i s rath er high.
10
GM
lnul
(mS)
SO.001
S0.00L
6* 00!
0000
0000
OOOOl________ _________ _____ _____ _____ ____ ____
. 0000
.2 5 0 0 /d lv
I V)
F ig. 2: 300K enhancement mode
drain c h a r a c t e r i s t i c ( Vds
O .lV /ste p )
10
5 000/4IV lrfu>
F ig . 3: 300K gm vs Id
A very important c h a r a c t e r i s t i c f o r low n o is e FET's is to have high
transconductance at low current in order t o reduce the n o is e generated
by v e l o c i t y saturated e l e c t r o n s (P u ce ll e t a l , 1975 ) . A p lo t o f g vs
I . (F ig . 3) r e v e a ls t h a t the AlGaAs buffered HEMT ach ieves i t s maximum
transconductance at o n ly 11 mA. In a d d it io n , i t does not e x h i b i t gm
degradation with in c r e a sin g drain b i a s . This behavior i s much d i f f e r e n t
than t h a t seen f o r an equal ga te le ngth HEMT with a GaAs b u ffe r lay e r
and a channel asp ect r a t i o , L/a, o f 10:1 (F ig . 4) (Camnitz e t al 1984).
With the GaAs b u f f e r , in c r e a sin g the drain v o lt a g e degrades the gm at
low c u r r e n t , in d ic a t in g an in cr ea se in s u b s t r a t e current which i s not
w ell c o n t r o ll e d by the g a te v o lt a g e .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
266
Gallium Arsenide and Related Compounds 1984
(mS)
2 0 0. 0
50
00
£*00
20.00
5
OOOj
/d jv
1 X
.ogoo;
10
1 0 .0 0 /d i v
0000
f« A /
2 500
F ig . 5: 77K enhancement mode drain
F ig. 4: 300K g v s .
f o r 0 .3 3
c h a r a c t e r i s t i c ( Vds O .lV /ste p )
urn GaAs b uffered HEMT
At 77K in l i g h t , th e th r esh o ld v o lta g e and transconductance in creased to
OV and 210 mS/mm, r e s p e c t i v e l y ( F ig . 5 ) .
In th e dark, th e drain curves
c o l la p s e d when V. exceeded IV, due to the deep tra p s in the doped
AlGaAs (R o c h e tte , 1982). The p lo t of gm vs I j at 77K measured in l i g h t
(F i g . 6 ) shows th at the maximum transconductance i s reached at o n ly 8
mA, as compared to 30-50 mA f o r con ven tional MESFET's. The v a r ia tio n in
gm at h igher current i s caused by th e onset of drain current c o l l a p s e .
1000 0.
0
£*
'
X
■V\
•vN.V -V
'vN
X
X X
WSLATCff*. GAIN
X.
10
F ig. 6 : 77K gm vs
SC
S
0 0 0 /o jv
(mA)
00
IC * 0 t
F r a q u s n a y (G H t)
fT
IC*82
F ig . 7: Gain vs f r e q . a t I^lSmA
Microwave C h a racterization
S-parameters were measured at 300K from 2-18 GHz at varying bias
c o n d it i o n s . The maximum microwave gain was observed at 15 mA drain
current and 2V drain b i a s . The maximum s t a b l e gain was 1 5 .4 d 8 at 8
GHz. F ig . 7 shows a p lo t of cu rrent gain and u n il a t e r a l gain vs
freq u en cy . E xtrap olation at 6 dB/octave y i e l d s 32 GHz f o r f y and a very
good 50 GHz f^ax fo r t h i s op era tin g cu rr en t.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
267
Gallium Arsenide and Related Compounds 1984
Noise Measurements
Some c h ip s were packaged and evalu a ted f o r n o is e temperature in a slu g
tuned 8 GHz t e s t s e t by M. P o s p ie s z a is k i o f the National Radio Astronomy
Observatory. The minimum 300K n o is e temperature was 104K (1 .3 3 dB) with
an a s s o c ia t e d gain o f 8 . 4 dB at a drain b ias o f 5 mA and 2V ( F ig . 8 ) .
The n o is e f i g u r e i s f a i r l y low between 2 mA and 10 mA w hile the gain
degrades on ly below 5 mA. This optimum current i s s i g n i f i c a n t l y lower
than in GaAs MESFET’s , whose optimum current is u s u a ll y 8 to 15 mA a t a
b ia s Of 3—3 . 5V. This n o is e f i g u r e i s U n ite d by th e p a r a s i t i c g a te and
source r e s i s t a n c e s .
Using the Fukui equation and the measured values
Rg = 7 fl. Rs = 9 fi, gm0 = 47 mS and Cgs = *2 pF one ob tain s a f i t t i n g
f a c t o r Kf = 1 .8 2 . This value compares c l o s e l y t o th a t obtained fo r
co n v en tion al HEMT s t r u c t u r e s by Laviron e t al (1984) and i s lower than
normally seen f o r GaAs FET's.
8 . 5 0 *t NOISE "IGLBE RNQ GRIN ST 300K
8.35** NCISS PIGURG *NC
12.5*
»S3CCISTEa GRIN
8 s s o c :°t c c SRIN
noise
riffjpc
:** i*SJ 8 vat-2.0v
F ig. 8: 300K n o i s e parameters
vs l d
lot * 9 «3t•^.2v
F ig . 9: 1 2 . 5K n o is e parameters
vs I .
The t e s t s e t and d ev ic e were cooled to 1 2 . 5K p h y sic a l temperature and
the minimum n o is e temperature o f 10.5 K with an a ss o c ia t e d gain o f 10.5
d8 was measured f o r the a m p lifie r at a drain b ias o f 1.5 mA and 1.2 V.
This i s a s t a t e o f the art value o f n o ise temperature w hile the optimum
b ia s cu rrent i s f a r lower than normally measured on s i m i l a r l y siz e d GaAs
MESFET's ( - 8 mA). See F ig . 9.
D isc u ssion
The"abiT'iTy for t h i s HEMT to keep high gain at very low current i s aided
by the high e l e c t r o n m o b il it y and confinement o f the e le c tr o n s to the
quantum w e ll ch an n el. This help s to lower the n o is e f ig u r e and a lso
lower the optimum b ia s cu rren t f o r minimum n o is e f i g u r e .
The advantage o f an AlGaAs b u ffe r la ye r becomes more apparent when
GaAs b u ffe r la ye r conduction becomes s i g n i f i c a n t at very sh ort gate
le n g t h s . A major improvement i s seen in the sharper p in c h off
c h a r a c t e r i s t i c , and the increased output r e s i s t a n c e . A high q u a li t y
AlGaAs b u ffe r la y e r and quantum w ell channel are e s s e n t i a l to high
performance.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
268
Gallium Arsenide and Related Compounds 1984
Conclusion
We have f a b r ic a t e d th e f i r s t submicrometer g a te HEMT with an undoped
A l o . 3 Gao. 7 As b u ffe r l a y e r . We have demonstrated i t s su p e rior p in c h o ff
c h a r a c t e r i s t i c s and high gain at low cu rr en t. I t s n o is e performance i s
comparable t o co n ven tional HEMT's at 300K and s t a t e of th e art at 1 2 . 5K
p h y sic a l tem perature. The HEMT's f a b r ic a t e d in t h i s work were lim it e d
by high gate r e s i s t a n c e , low s h e e t c a r r ie r d e n s i t y , and lower than
d e s ir e d doping in th e n* AlGaAs. High source r e s i s t a n c e and lower than
optimum transconductance r e s u l t e d . By in c r e a sin g th e doping d e n s i t y and
a d ju stin g la y e r t h i c k n e s s e s , th e transconductance and p a r a s i t i c
r e s i s t a n c e s can be made e q u iv a le n t t o GaAs b u ffered HEMT s t r u c t u r e s .
Without t h e se l i m i t a t i o n s , th e AlGaAs b uffered HEMT may surpass the
con ven tional HEMT in n o is e performance.
Acknowledgements
The authors g r a t e f u l l y acknowledge help fu l d is c u s s io n s with H. Lee, W.
S c h a ff , and D.W. Van der Merwe, and te c h n ic a l a s s i s t a n c e from J. Berry.
We thank Dr. E. Wolf f o r the use o f the NRRFSS e le c tr o n beam
m ic r o fa b r ic a t o r , P ro f. J .P . Krusius f o r a s s i s t a n c e in DC measurements,
and M. P o sp ie sz a lsk i of NRAO fo r n o is e f i g u r e measurements. This work
was supported by NRAO, IBM C orporation, and t h e Air Force O f f i c e o f
S c i e n t i f i c Research under co n tra c t no. F49620-81-C-0082.
R eferences
«
Arnold D, Kopp W, F isc h e r R, Henderson T, and Morkoc H 1984 IEEE E l e c t .
Dev. L e t t . EDL-5 pp 8 2 -8 4 .
Berenz J J, Nakano K, and W eller K P 1984 IEEE MTT-S D ig est pp 98-101.
Camnitz L H, Lee H, Tasker P J, Van der Merwe D, and Eastman L F to be
p resented at 1984 I n t . E l e c t . Dev. Mtg.
Chevrier J, and Linh N T 1984 Dev. Res. C o n f., Santa Barbara, C a lif o r n ia
Drummond T J, F is h e r R, M i l l e r P, Morkoc,H and Cho A V 1982 J. Vac. S c i .
Tech. 21 pp 684-688.
Eastman L F and Shur M S 1979 IEEE Trans. E lectron Dev. ED-26 1359-1361.
Ghosh C L and Laynan R L 1984 IEEE E le c . Dev. L e t t . EDL-5 pp 3 - 5 .
H a l l a i s J, Andre J P, Baudet P, and Boccon-Gibod D 1979 I n s t . Phys.
Conf. Ser. 45 p. 361.
Jones W L p r iv a t e communication.
Kopp W, Su S L, F isc h e r R, Lyons W G, Thorne R E, Drummond T J, Morkoc
H, and Cho A Y 1982 Appl. Phys. L e tt . 41 pp 563-565.
Laviron M, D elagebeaudeuf, D, Rochette J F, Tung P N, D e le s c lu e s e P,
Chevrier J and Linh N T 1984 Dev. Res Conf. Santa Barbara, CA USA
Maki P A, Palmateer S C, Wicks G W, Eastman L F and Calawa A R 1983a
Jour. E le c tr o n . M t r ls . 12 pp 1051-1063.
Maki P A, Wicks G W, and Eastman L F 1983b Proc. o f IEEE/Cornell Conf.
on High Speed Semi. Dev. and C i r c u i t s , 209-217.
Pucel R A, Haus H A, and S t a t z H 1975 Adv. in E l e c t r . and E l e c t . Phys.
38 pp 195-265.
Rochette J F, D e le s c lu s e P, Laviron M, Delagebeaudeuf D, Cheurier J, and
Linh N T 1982 I n s t Phys Conf Ser 65 385-392.
S ch a ff W J and Eastman L F 1983 Proc o f IEEE/Cornell Conf on High Speed
Semi. Dev. and C ir c u i t s pp 226-233.
Wang, W I , Judaprawira S.Wood C E C, and Eastman L F 1981 Appl. Phys.
L e t t . 38 pp 708-710.
Widiger D, Hess K, and Coleman J J 1984 IEEE E lect Dev L e tt 5 266-269.
Zwicknagl P, Mukherjee S D, Jones W L, Lee H, Caoan’ P M, Griem, H T,
Berry J D, Rathbun L, and Eastman L F 1984 I n s t . Phys. Conf .S e r . 74 575
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX E
LIST OF PRESENTATIONS AND PUBLICATIONS
269
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
270
L is t of P u b lic a tio n s
].
"A S t u d y o f S u b s t r a t e E f f e c t s on P l a n a r Doped S t r u c t u r e s
i n G a l l i u m A r s e n i d e Grown by M o l e c u l a r Beam E p i t a x y " ,
S . C . P a l m a t e e r , P . A . M a k i , M.A. H o l l i s , L . F . E a s t m a n ,
C. H i t z m a n a n d I . Ward, I n s t . P h y s .
C o n f . S e r . No.
65, C h .3, 149-156, (1983).
2.
" E n h a n c e d M o b i l i t i e s i n S i n g l e Quantum W e l l S t r u c t u r e s
on A l . 30G a . 70As B u f f e r s Grown by MBE", P . A . M a k i,
G . W ., * W i c k s a n d L . F .
Eastm an, P r o c e e d in g s IE E E /C o rn ell
C o n f e r e n c e on H i g h - S p e e d S e m i c o n d u c t o r D e v i c e s and
C i r c u i t s , C o r n e l l U n i v e r s i t y , I t h a c a , NY 2 0 9 - 2 1 7 .
3.
" E f f e c t o f S u b s t r a t e A n n e a l i n g and V : I I I F l u x R a t i o
on t h e M o l e c u l a r Beam E p i t a x i a l Growth o f AlGaAs GaAs S i n g l e Quantum W e l l s " , P .A . M a k i, S . C .
Palm ateer,
G.W. W i c k s , L . F .
E a s t m a n a n d A.R . C a l a w a , J . E l e c .
M a t . , 12 ( 6 ) 1 0 5 1 - 1 0 6 3 (Nov.
1983).
4.
" V e r t i c a l E l e c t r o n T r a n s i s t o r (VET) i n GaAs w i t h a
H e t e r o j u n c t i o n (AlGaAs/GaAs) C a t h o d e " , U. M i s h r a ,
P . A . M a k i , J . R . W e n d t, W. S c h a f f , E. Kohn a nd L . F .
E a s t m a n , E l e c t r o n i c L e t t . , 20 (3) 1 4 5 - 1 4 6 ( F e b . 1 9 8 4 ) .
5.
" E x c i t o n T r a n s p o r t i n A l xG a i _ xAs o f O p t i c a l l y E x c i t e d
S i n g l e Quantum W e l l s " , H. L e , B. L a x , P . A . M a k i,
S.C . P a lm a t e e r , and L .F .
Eastm an, J . A ppl. Phys.
55, 4367-4372 (1984).
6.
"The I n f l u e n c e o f V : I I I F l u x R a t i o on U n i n t e n t i o n a l
I m p u r i t y I n c o r p o r a t i o n D u r i n g M o l e c u l a r Beam E p i t a x i a l
Grow th", S.C .
P a l m a t e e r , P .A . M a k i , A .R . C a l a w a ,
J . C . M . Hwang a n d L . F . E a s t m a n , P r o c . 1 1 t h I n t . Symp.
on GaAs & R e l a t e d C p d s . ( 1 9 8 5 ) .
7.
" S u b - M i c r o m e t e r Quantum W e l l HEMT w i t h A l o . 3G a o . 7As
B u f f e r L a y e r " , L . H . C a m n i t z , P .A . M a k i, P . J . T a s k e r
an d L . F . E a s t m a n , P r o c .
1 1 t h I n t . Symp. on GaAs &
R e la te d C pds. (1985).
8.
"The E f f e c t o f D opin g on t h e I n t e r f a c e B e t w e e n GaAs
an d A lG a A s " , W . J . S c h a f f , P .A . M a k i, L . F . E a s t m a n ,
L . R a t h b u n , B . C . De Cooman, a nd C .B . C a r t e r .
To be
p u b lish ed in Proc. of th e M a te ria ls Research S o c ie ty ,
November 1984 m e e t i n g .
9.
" E l i m i n a t i o n o f F l u x T r a n s i e n t s i n M o l e c u l a r Beam
E p i t a x y " , P .A . M a k i, S . C . P a l m a t e e r , A .R . C a la w a and
B . R . L e e , t o be p u b l i s h e d Nov. 1 9 8 5 , i n J o u r .
E lectrochem ical S o ciety .
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271
L is t of P resen tatio n s
1.
" F a b r i c a t i o n and P e r f o r m a n c e o f
B a r r i e r Diodes and T r a n s i s t o r s " ,
P a lm a te e r , L .F . Eastm an, C.E.C.
A . S . Brow n, WOCSEMMAD, P h o e n i x ,
1982) .
GaAs P l a n a r Doped
M. H o l l i s , S .
Wood, P . A . Maki and
A r i z o n a , (March
2.
"G row th o f P l a n a r Doped B a r r i e r S t r u c t u r e s i n
G a l l i u m A r s e n i d e by M o l e c u l a r Beam E p i t a x y " ,
C o llo q u e I n t e r n a t i o n a l Sur l e s R e l a t i o n s E n t r e Les
C o n d i t i o n s de C r o i s s a n c e E p i t a x i a l e e t Les
P r o p r i e t e s Des C o u c h e s E p i t a x i a l e s S e m i c o n d u c t r i c e s ,
P e rp ig n a n , F ra n c e , A ugust 1982, P o s te r p r e s e n t a t i o n .
3.
"A S t u d y o f S u b s t r a t e E f f e c t s on P l a n a r Doped
S t r u c t u r e s i n G a l l i u m A r s e n i d e Grown by M o l e c u l a r
Beam E p i t a x y " , S . C . P a l m a t e e r , P . M a k i , M. H o l l i s ,
L.F.
E a s t m a n a nd I . W ar d, 1982 I n t e r n a t i o n a l
Symposium on G a l l i u m A r s e n i d e a nd R e l a t e d C om p o u n d s ,
A l b u q u e r q u e , New M e x i c o , ( S e p t e m b e r 1 9 - 2 1 , 1 9 8 2 ) .
4.
" S u b m i c r o n GaAs V e r t i c a l E l e c t r o n T r a n s i s t o r " , U.
M i s h r a , E. Kohn, J . R . S h e a l y , P . A . M a k i , S . C .
P a l m a t e e r and L . F . E a s t m a n , WOCSEMMAD, San A n t o n i o ,
TX ( F e b .
20-22, 1983).
5.
" E n h a n c e d M o b i l i t i e s i n S i n g l e Quantum W e ll
S t r u c t u r e s o n A l . 30 G a . 70As B u f f e r s Grown by MBE",
P . A . M a k i , G.W. W i c k s ’ a n d L . F .
Eastm an, N inth
B i e n n i a l H ig h S p e e d S e m i c o n d u c t o r D e v i c e s and
C i r c u i t s C o n f . , C o r n e l l U n i v e r s i t y , I t h a c a , NY (Aug.
1 5 - 1 7 ,, 1983).
6.
"MBE Growth C o n d i t i o n s f o r H ig h Q u a l i t y S i n g l e
Quantum W e l l s o n T h i c k A lG a A s " , P .A . M a k i , S . C .
P a l m a t e e r , G.W. W i c k s , L . F . E a s t m a n a n d A .R . C a l a w a ,
5 t h MBE W o r k s h o p , A t l a n t a , G e o r g i a , ( O c t o b e r 6 - 7 ,
1983) .
7.
"The P h y s i c a l E l e c t r o n i c s o f L a u n c h i n g B a l l i s t i c
E l e c t r o n s i n t o N - t y p e GaA s", L . F . E a s t m a n , P . M a k i ,
U. M i s h r a , WOCSEMMAD, S an F r a n c i s c o , CA ( F e b . 1 9 - 2 1 ,
1984).
8.
" V e r t i c a l E l e c t r o n T r a n s i s t o r i n GaAs w i t h a H e t e r o ­
j u n c t i o n (AlGaAs-GaAs) C a t h o d e " , U.K. M i s h r a , P .A .
M aki, J . R .
W en d t, W. S c h a f f , L . F . E a s t m a n , P.M.
S m ith , J.C .M .
Hwang, WOCSEMMAD, San F r a n c i s c o , CA,
(Feb. 1 9 -2 1 , 1 9 8 4 ).
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272
9.
"Zeeman S t r u c t u r e o f E x c i t o n s i n GaAs-AlGaAs S i n g l e
Quantum W e l l s " / B. L a x , H. L e , P . A . M a k i , S . C .
P a l m a t e e r , and L . F . E a s t m a n , A m e r i c a n P h y s i c a l
S o c i e t y M e e t . , C h i c a g o , I L , (March 2 6 - 3 0 , 1 9 8 4 ) .
10.
" E x c i t o n T r a n s p o r t i n O p t i c a l l y E x c i t e d A l xG a i _ xA s GaAs H e t e r o s t r u c t u r e s " , H. L e , B. L a x , P . A . M a k i,
S .C . P a lm a te e r and L .F . Eastm an, American P h y s i c a l
S o c i e t y M e e t . , C h i c a g o , I I , (March 2 3 6 - 3 0 , 1 9 8 4 ) .
11.
"The I n f l u e n c e o f V : I I I F l u x R a t i o on U n i n t e n t i o n a l
I m p u r i t y I n c o r p o r a t i o n D u r i n g M o l e c u l a r Beam
E p i t a x i a l G rowth", S.C .
P a l m a t e e r , P .A . M a k i , W.
K a t z , A.R. C a l a w a , J . C . M . Hwang a nd L . F .
Eastm an,
11th I n t .
Symp. on GaAs & R e l a t e d C p d s , B i a r r i t z ,
F rance (S ept.
26-28, 1984).
12.
" S u b - M i c r o m e t e r Quantum W e l l HEMT w i t h A l g . 3G a o . 7As
B u f f e r L a y e r " , L .H . C a m n i t z , P . A . M a k i, P . J . T a s k e r
and L .F . Eastm an, 1 1 th I n t .
Symp. on GaAs & R e l a t e d
Cpds, B i a r r i t z , F rance (S ept. 26-28, 1984).
13.
" P r o g r e s s T ow ard s P e r f o r m a n c e L i m i t s f o r MODFET and
HBJT D e v i c e s " , P . J . T a s k e r , L . H . C a m n i t z , H. L e e ,
P . A . M a k i , L. L u n a r d i , P.
E n q u i s t a nd L . F . E a s t m a n ,
9 t h E u r o p e a n S p e c i a l i s t Workshop on A c t i v e M ic r o w a v e
S e m i c o n d u c t o r D e v i c e s , E i n d h o v e n , The N e t h e r l a n d s
(Oct.
10-12, 1984).
14.
"The E f f e c t o f Dopin g on t h e I n t e r f a c e B e t w e e n GaAs
a n d AlGaAs, W . J . S c h a f f , P .A . M a k i , L . F . E a s t m a n , L.
R a t h b u n , B. De Cooman a nd C.B. C a r t e r , M a t e r i a l s
R e s e a r c h S o c i e t y M e e t . , B o s t o n , MA (Nov. 2 6 - 3 0 ,
1984).
15.
" S u b - M i c r o m e t e r MODFET's W ith an A l o . 3G a o . 7As B u f f e r
L a y e r " , L .H . C a m n i t z , P .A . M a k i , P . J . T a s k e r , H.
Le e a nd L . F . E a s t m a n , Workshop on S e l e c t i v e l y Doped
H e t e r o s t r u c t u r e M a t e r i a l s , D e v ic e s and C i r c u i t s ,
S a n t a B a r b a r a , CA (Dec. 3 - 7 , 1 9 8 4 ) .
16.
"The R o l e o f C h a r g e C o n t r o l on D r i f t M o b i l i t y i n
AlGaAs/GaAs MODFET's", L .H . C a m n i t z , P . J . T A s k e r ,
P . A . M a k i, H. L e e , J . Huang and L . F . E a s t m a n , 1 0 t h
I E E E / C o r n e l l B i e n n i a l C o n f e r e n c e on A dvan ce d
C o n c e p t s i n Hi gh S p e e d S e m i c o n d u c t o r D e v i c e s and
C i r c u t s , C o r n e l l U n i v e r s i t y , I t h a c a , NY, ( J u l y 2 9 31, 1985).
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