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Electrical properties and physical characteristics of polycrystalline diamond films deposited in a microwave plasma disk reactor

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Order Number 9223201
E lectrical properties and physical characteristics o f polycrystalline
diam ond films deposited in a microwave plasm a disk reactor
H uang, B ohr-ran, P h.D .
Michigan State University, 1992
Copyright ©1992 by Huang, Bohr-ran. All rights reserved.
UMI
300 N. ZeebRd.
Ann Arbor, MI 48106
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ELECTRICAL PROPERTIES A N D PHYSICAL
CHARACTERISTICS OF POLYCRYSTALLINE
DIAM OND FILMS DEPOSITED IN A MICROWAVE
PLASM A DISK REACTOR
By
Bohr-ran Huang
A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
D epartm ent of Electrical Engineering
1992
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Copyright by
Bohr-ran Huang
1992
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ABSTRACT
ELECTRICAL PROPERTIES A N D PHYSICAL
CHARACTERISTICS OF POLYCRYSTALLINE
DIAM OND FILMS DEPOSITED IN A MICROWAVE
PLASM A DISK REACTOR
By
Bohr-ran Huang
D iam ond possesses m any excellent properties which m otivate investigations of
th e potential of diamond for a wide range of electronic, m echanical, and optical
applications. This work experim entally investigates techniques for high quality dia­
m ond synthesis and develops m eans for electrical and physical characterization of the
films. T he films are deposited by plasm a assisted chemical vapor deposition using a
m ethane/hydrogen plasm a in a microwave plasm a disk reactor system .
B oth a diamond paste nucleation m ethod and a diam ond powder nucleation
m ethod are studied in this research. Although as indicated by R am an spectroscopy
both m ethods produced sim ilar quality diamond films, th e powder nucleation m ethod
produced fine grain, sub-m icron sized crystallite, films whereas th e paste nucleation
m ethod produced large grain, several-m icrometer size crystallite, films. This differ­
ence is due to a higher density of surface nucleation sites in the powder polished
films. G rain size could also be varied by using microwave power, pressure, and the
m ethane/hydrogen ratio as processing variables, however, th e effect is sm aller th an
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th e differences caused by th e two nucleation m ethods.
For electrical characterization, a new sam ple preparation m ethod was developed
in cooperation w ith th e U niversity of W uppertal which allows m etallic access to
both sides of th e diam ond film. Using this technique, th e properties of a variety
of m etal/diam ond contacts were investigated. A lthough films were not intention­
ally doped, therm o-pow er m easurem ents show all films to be p-type w ith activation
energies between 0.1 and 0.5 eV.
For powder polished films, all m etallic contacts were ohmic. These samples were
used to explore the high electric field properties of diam ond. It was discovered th a t for
fields larger th an approxim ately 1 xlO 5 V /cm the electrical properties are dom inated
by defects, where defect is used generically for either an im purity or a structural
defect. For low electric fields, th e electrical conductivity was constant which resulted
in ohmic behavior. B ut for high fields, th e conductivity was field activated according
to Poole’s law. This behavior was m odeled as being due to ionizable defects and
indicates th a t there is approxim ately one ionizable defect per 10,000 host atom s. As
a result of such defects, th e breakdown field for these films was somewhat less th an
l x l O 6 V /cm . A large concentration of defects is com patible w ith th e observation
of ohmic contact behavior regardless of m etallic work function since contact space
charge layers would be sufficiently thin to allow tunneling.
Non-ohmic, Schottky barrier contacts were achievable on th e paste polished films.
For A l/diam ond/silicon structures diode characteristics were observed. These I-V
characteristics were m odeled as an ideal Schottky barrier diode in series w ith bulk
diamond, for which th e property of the bulk diam ond follows an IcxV m relation­
ship, indicative of space charge lim ited current in an insulating m aterial. T he rec­
tifying behavior was determ ined to be at th e A l/diam ond surface rath er th an the
diam ond/silicon surface. T he best rectification ratios were 2 x 1 0 s.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
To my paxents
Chien-Ching, Tsui-Kuan C. Huang
and my wife
Pey-Nan C. Huang
V
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ACKNOW LEDGEMENTS
The au th o r would like to thank Dr. Donnie K. R einhard for his g u id an ce, support,
and encouragem ent. Special thanks is extended to Dr. Jes Asmussen Jr. and Dr.
T im othy G rotjohn for their valuable discussions, and Dr. P e te r A. Schroeder for his
comments. A dditional thanks is given to Dr. Engem ann and Mr. Hans Keller in
W uppertal University (G erm any) for th eir help on fabricating back-etched diam ond
samples, Dr. Kevin Gray of N orton Company and Dr. M ark Holtz in the Physics
departm ent of Michigan S tate U niversity for providing th e R am an spectra, and Dr.
Kevin Hook in th e Composite and Structural M aterials C enter of Michigan S tate
University for perform ing th e XPS analysis.
This work was supported in p a rt by grants from W avem at, Corp. and the Michigan
Research Excellence Fund.
vi
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TABLE OF CONTENTS
LIST OF TABLES
x
LIST OF FIG URES
xi
1 Introduction
1
1.1
M otivation for D iam ond R e s e a r c h ...............................................................
1
1.2
Research O b jectiv es..........................................................................................
3
1.3
Dissertation O utline
4
......................................................................................
2 Synthetic Diam ond : Background R eview
5
2.1
In tro d u c tio n .......................................................................................................
5
2.2
Brief History of Diam ond S y n th e s is ............................................................
6
2.3
2.4
2.5
2.2.1
Historical B a c k g r o u n d ........................................................................
6
2.2.2
Growth M echanism of Diamond F i l m s .........................................
13
Review of Diamond Film T e c h n o lo g ie s .....................................................
17
2.3.1
Hot Filam ent Chemical Vapor Deposition T e c h n iq u e s ..............
18
2.3.2
Plasm a Assisted Chemical Vapor Deposition Techniques
20
2.3.3
O ther Deposition M e th o d s .................................................................
24
2.3.4
Nucleation M ethods...............................................................................
24
Physical A ttributes of D ia m o n d ..................................................................
27
2.4.1
Comparison of Bulk and Film M aterial P r o p e r tie s .....................
27
2.4.2
Comparison of Semiconductor P r o p e r tie s ......................................
30
Review of Diamond Diodes and T ra n sisto rs..............................................
33
3 Film Deposition and
...
Sam ple Preparation
38
3.1
In tro d u c tio n .......................................................................................................
38
3.2
The M PD R S y s te m ..........................................................................................
38
3.2.1
The Microwave C a v i t y .......................................................................
38
3.2.2
T he Deposition S y s te m .......................................................................
41
The Nucleation M e th o d s ................................................................................
45
3.3
vii
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3.4 Diamond Film D e p o s iti o n ..............................................................................
48
3.4.1
O peration of T he S y ste m ....................................................................
48
3.4.2
Experim ental Param eters
.................................................................
53
3.4.3
Substrate T e m p e ra tu re ........................................................................
54
Fabrication of E lectrical S a m p l e s .................................................................
61
3.5.1
Four-Point P robe S a m p le s .................................................................
61
3.5.2
The M etal/D iam ond/Silicon S a m p le s .............................................
65
3.5.3
Back-Etched S a m p le s ............................................
65
3.5
4
5
Physical Characterization
72
4.1
In tro d u c tio n .........................................................................................................
72
4.2
R am an S p e c tro s c o p y ........................................................................................
73
4.3
X-ray Photoelectron S p ectro sco p y .................................................................
79
4.4
Dek-Tak A n a ly s is ...............................................................................................
89
4.5
Scanning Electron Microscope A n a ly s is .......................................................
94
4.6
Film Uniformity A n a ly sis..................................................................................
107
Electrical Characterization
113
5.1
In tro d u c tio n .........................................................................................................
113
5.2
Four Point Probe C haracterization
..............................................................
113
5.3
5.4
6
5.2.1
Experim ental M ethod
.......................................................................
113
5.2.2
Theory of C onductivity vs. T e m p e ra tu re ......................................
116
5.2.3
Comparison w ith Physical C haracterization
...............................
118
5.2.4
Experim ental Results and M atch w ith T h e o r y ............................
122
Back-Etched Sam ples..........................................................................................
130
5.3.1
The Back-Etched Samples and I-V M easurem ent Set-Up
. . .
130
5.3.2
C ontact Effects on Back-Etched S a m p le s ......................................
132
5.3.3
High Field E f f e c t .................................................................................
132
5.3.4
Photo E f fe c t............................................................................................
145
T he M etal/D iam ond/Silicon S a m p le s ..........................................................
147
5.4.1
The I-V M easurem ent S e t - u p ..........................................................
147
5.4.2
C ontact Effects on M etal/D iam ond/Silicon S a m p le s ..................
151
5.4.3
Photo E f fe c t............................................................................................
158
5.4.4
Diamond Schottky Barrier D i o d e ....................................................
165
Sum m ary and Future Research
183
6.1
Sum m ary of Im p o rtan t R e s u l t s ....................................................................
183
6.1.1
184
Nucleation M e t h o d ..............................................................................
viii
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6.1.2 Q uality of D iam ond F i l m s .................................................................
185
6.1.3 Back-Etching T e c h n iq u e .....................................................................
185
6.1.4 Diam ond/Silicon I n te r f a c e .................................................................
186
6.1.5 A ctivation Energy of Diam ond F i l m s .............................................
186
6.1.6
6.2
Electric Field D ependent C onductivity of D iam ond Film s . . .
187
6.1.7 Diamond Schottky B arrier D i o d e ....................................................
188
Future R e se a rc h ..................................................................................................
188
6.2.1
Improvement for D iam ond D ep o sitio n .............................................
189
6.2.2
T he Techniques for Physical C h a r a c te r iz a tio n ............................
189
6.2.3
T he Role of S iC a t th e D iam ond/Silicon In te rfa c e .....................
190
6.2.4
Defects States in D iam ond F i l m s .....................................................
191
6.2.5
Diamond D e v ic e s ..................................................................................
191
A D eposited Diamond Film s
193
B Electrical Samples
198
BIB LIO G R APH Y
202
ix
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LIST OF TABLES
2.1 Physical properties of bulk n atu ral diam ond.[6 8 ] ......................................
28
2.2 Properties of bulk natu ral diam ond and synthetic diam ond film s.[13] .
29
2.3 Properties of diam ond, i-C, and graphite. [21] . . ' ......................................
31
2.4 Working definition for different carbon coatings. [ 6 9 ] ...............................
32
2.5 Comparison of sem iconductor properties for diam ond, Si, GaAs, /9Sic.[70] 34
4.1
Relation of average grain size vs. m ethane concentration, microwave
power and plasm a pressure by th e paste-polished m eth o d ........................... 105
4.2 R elation of average grain size vs. microwave power and plasm a pressure
by th e powder-polished m ethod.......................................................................
4.3
Relation of deposition rate vs.
106
m ethane concentration, microwave
power and plasm a pressure by the paste-polished m eth o d........................... 108
4.4
Relation of deposition rate vs.
m ethane concentration, microwave
power and plasm a pressure by th e powder-polished m eth o d ....................... 109
x
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LIST OF FIGURES
2.1
The atom ic stru ctu re of (a) diam ond (b) graphite.....................................
7
2.2
Phase diagram of carbon.[12]..........................................................................
8
2.3
Illustration of free energy difference between diam ond and graphite. .
10
2.4
Low pressure CVD diam ond synthesis in th e phase diagram of carbon.[13] 12
2.5
Relationship between diam ond properties and engineering applica­
tions. [70] ...............................................................................................................
3.1
35
The cross section of th e M PD R and processing cham ber, and a
schem atic display of th e electric (E) and m agnetic (H) field p attern s
of the TM ow m ode..............................................................................................
39
3.2
The microwave power source, circuit, and cavity applicator...................
42
3.3
The diagram of th e M PD R CVD system .....................................................
44
3.4
SEM pictures of th e silicon substrate prepared by diam ond paste method. 47
3.5
SEM pictures of the silicon surface prepared by diamond powder method. 49
3.6
The M PD R CVD system during the diam ond film deposition...............
52
3.7
Top view of th e M PD R processing cham ber...............................................
56
3.8
(a) Typical tem p eratu re profile and (b) b e tte r tem p eratu re uniform ity
of th e silicon su b strate.......................................................................................
3.9
Si substrate tem perature vs. microwave input power for
1
% and 0.5
% m ethane concentration at 50 T orr..............................................................
3.10 Si substrate tem perature vs. microwave input power for
1
58
59
% and 0.5
% m ethane concentration at 60 T orr..............................................................
60
3.11 Si substrate tem perature vs. microwave input power for 50 and 60 Torr
at 0.5 % m ethane concentration......................................................................
62
3.12 Si substrate tem perature vs. microwave input power for 50 and 60 Torr
at 1 % m ethane concentration.........................................................................
63
3.13 T em perature difference between silicon and silicon nitride for 0.5 %
m ethane concentration at 50 T orr...................................................................
xi
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64
3.14 Fabrication of th e four-point probe samples. T he actual surface was
m ore uneven th a n is indicated here, (a) T he startin g point is a silicon
nitride substrate which has received a nucleation procedure, (b) Since
th e substrate is insulating, th e sam ple is ready for th e 4 point probe
m easurem ent.........................................................................................................
66
3.15 Fabrication of th e m etal/diam ond/silicon samples, (a) T he surface is
nucleated by diam ond powder or diam ond paste, (b) the diam ond film
is deposited, and (c) top contacts are evaporated through a shadow
m ask. A lum inum is shown here as an exam ple...........................................
67
3.16 Fabrication of back-etched diam ond samples for surface analysis, (a)
Silicon is coated w ith diam ond using conventional m ethod, (b) The
sam ple is secured to an epoxy substrate, (c) T he silicon is removed by
chemical etching...................................................................................................
69
3.17 Fabrication of dual-sided m etal contacts on isolated diam ond films.
(a) T he silicon is coated w ith diam ond and (b) a m etallic contact is
evaporated on th e first diam ond surface, (c) A wire lead is attached
to th e evaporated m etal and (d) th e su b strate is placed face down in
epoxy, (e) T he silicon is removed by etching and (f) m etal contacts
are evaporated on th e second diam ond surface...........................................
4.1
R am an spectroscopy is based on a process of inelastic scattering of
photons by phonons............................................................................................
4.2
.............................................................
1%
m ethane, 1000 °C and (d)
1%
m ethane, 1040 °C . (From Dr. G r a y ) ..........................................................
R am an spectra of samples of 0.5% m ethane at 1000 °C for (a)
deposition and (b)
4.7
80
R am an spectra of samples w ith conditions : (a) 0.5% m ethane, 1000
°C (b) 0.5% m ethane, 1040 °C (c)
4.6
78
Ram an spectra of 0.5% m ethane samples at (a) 936 °C (b) 1040 °C
and (c) 1090 °C . (From Dr. Gray)
4.5
76
R am an Spectroscopy set-up for th e R am an spectrum analysis.(From
Dr. G r a y ) ............................................................................................................
4.4
74
Ram an spectra for diam ond and silicon on (a) absolute wave num ber
scale and (b) relative wave num ber scale......................................................
4.3
70
6
10
81
hours’
hours’ deposition. (From Dr. G r a y ) ....................
82
R am an spectra of samples w ith 0.5% m ethane at 1040 °C by (a)
powder-polished m ethod and (b) paste-polished m ethod. (From Dr.
Gray)
...................................................................................................................
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
83
4.8
R am an spectra of (a) th e as-deposited film and (b) th e free standing
film of the sample w ith 1.5% m ethane at about 1040 °C. (From Dr.
Gray)
4.9
..................................................................................................................
84
Principle of X-ray Photoelectron Spectroscopy. W hen a core level elec­
tron absorbs a photon of energy (hi/) greater th an its binding energy
(E b ), the electron is ejected from th e atom w ith kinetic energy (K .E.).
86
4.10 ESC A spectra of sam ples exposed to air for (a) a long tim e and (b) no
tim e before th e analysis. (From Dr. H o o k ) ...............................................
88
4.11 (a) Elem ental survey scan of the sam ple before th e A r sputtering (b)
higher resolution narrow scan of (a) in the carbon region. (From Dr.
Hook)
..................................................................................................................
90
4.12 (a) Elem ental survey scan of the sample after the A r sputtering (b)
higher resolution narrow scan of (a) in the carbon region. (From Dr.
Hook)
..................................................................................................................
91
4.13 SEM view of the paste-polished produced diam ond film on a cleaved
sam ple showing th e silicon substrate, the interface, and the side view
and top surface of th e diam ond film..............................................................
93
4.14 T he surface profile on (a) the top surface of an as-deposited pastepolished produced diam ond film and (b) the back surface of the film
after transfer to the epoxy su b strate..............................................................
95
4.15 SEM view of the powder-polished produced diam ond film on a cleaved
sam ple showing the silicon substrate, the interface, and th e side view
and top surface of th e diam ond film..............................................................
96
4.16 T he surface profile on (a) the top surface of an as-deposited powderpolished produced diam ond film and (b) the back surface of the film
after transfer to the epoxy su b strate..............................................................
97
4.17 T he extrem e top surface profiles of (a) large-grain-size and (b) smallgrain-size diamond film s....................................................................................
98
4.18 Typical SEM photo of the top view of the diam ond film prepared by
the paste-polished m ethod. The triangular shapes indicate [111] crys­
tallite faces. (MW power: 600 W, plasm a pressure: 60 Torr, m ethane
concentration: 0 .5 % .) .......................................................................................
101
4.19 Typical SEM photo of the top view of the diam ond film prepared by
the powder-polished m ethod. (MW power: 600 W , plasm a pressure:
60 Torr, m ethane concentration: 0 .5 % .)......................................................
xiii
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102
4.20 G row th mechanism of diam ond film prepared by (a) paste-polished
and (b) powder-polished m ethod. T he latter has a higher nucleation
density and therefore a finer grain film .........................................................
103
4.21 This SEM photo shows large and small grain size diam ond growth on
th e sam e substrate...............................................................................................
104
4.22 (a) Cross section view of laser scan from plane A to D. (b) Thickness
(//m ) uniform ity analysis on an area of 1 cm X 2 cm of a sample.
. .
112
5.1
T h e set-up of the four point probe m ethod..................................................
115
5.2
R am an spectra of (a) NDF-S3, (b) NDF-S4, and (c) NDF-S 6 . (From
D r. H o l t z ) ............................................................................................................
120
5.3
R am an spectrum of NDF-S1. (From Dr. Gray)
.....................................
121
5.4
C onductivity vs. 1000/T of N D F-Sl
..........................................................
125
5.5
C onductivity vs.
of NDF-S3
..........................................................
126
5.6
C onductivity vs. 1000/T of NDF-S4
..........................................................
127
5.7
C onductivity vs. 1000/T of NDF-S 6
..........................................................
128
5.8
R am an spectrum of th e diam ond film as deposited on th e silicon sub­
5.9
1 0 0 0 /T
s tra te w ith substrate tem p eratu re at 1000 °C. (From Dr. Gray) . . .
131
T h e I-V m easurem ent set-up of the back-etched sam ples........................
133
5.10 I-V characteristics of a small grain size sample w ith gold contacts top
and bottom . It shows linear and sym m etric behavior a t low voltages.
(Electrical sample : P 4 7 - T 1 ) ..........................................................................
134
5.11 I-V characteristics of a small grain size sample w ith gold contacts top
and bottom . It shows nonlinear behavior at high voltages. (Electrical
sam ple : P 4 7 - T 1 ) ..............................................................................................
136
5.12 T he I — V 3/ 2 characteristic and (b) the I — V 2 characteristic of Figure
5.11. (Electrical sam ple : P 4 7 -T 1 )................................................................
137
5.13 I-V characteristic of diam ond film with gold contacts top and bot­
tom .
Experim ental d a ta is shown by d ata points.
T he solid line
represents the model results of Eq. (5.12) w ith G oo= 2.6xlO -10 ft- 1 ,
Go = 2.1 xlO -11 ft-1 , and a — 0.042 V-1 . (Electrical sam ple : P47-T1) 139
5.14 I-V characteristic of diam ond film with top indium contacts and
bottom gold contact.
Experim ental d ata is shown by d a ta points.
T he solid line represents the model results of Eq.
(5.12) with
G00= 2 .1 x lO -10 f t - 1, Go = 2 .0 x l0 - n f t - 1, and a = 0.045 V ~ \ (Elec­
trical sample : P 4 7 -T 3 )....................................................................................
xiv
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
140
5.15 I-V characteristic of diam ond film w ith silver contacts top and bot­
tom .
Experim ental d a ta is shown by d a ta points.
T he solid line
represents the model results of Eq. (5.12) w ith G oo=2.9xlO ~10 ft- 1 ,
Go = 4 .0 xlO -12 O-1 , and a — 0.08 V ~ x. (Electrical sam ple : P49-T2)
141
5.16 T he photoconductance is nearly independent of th e applied voltage.
Shown here are d a ta for different photon fluxes corresponding to dif­
ferent optical density (O.D) filter values. (Electrical sam ple : P47-T1)
143
5.17 (a) T he calibration factors corresponding to different optical wave­
length for Si and Ge photodetector, (b) T he relative power intensity
factors for Si and Ge photodetector a t different wavelength of th e op­
tical monopass filters..........................................................................................
148
5.18 R elation of photoconductance vs. photon energy for Au-Au contact
sam ple (a) before norm alization and (b) after norm alization. (E lectri­
cal sam ple : P 4 7 - T 1 ) ..................................................................................
149
5.19 Relation of photoconductance vs. photon energy for In-Au contact
sam ple (a) before norm alization and (b) after norm alization. (E lectri­
cal sam ple : P 4 7 - T 3 ) ..................................................................................
5.20 The I-V m easurem ent set-up of the m etal/diam ond/silicon samples.
150
.
152
5.21 I-V characteristic of th e small grain size A u/diam ond/silicon samples.
(Electrical sam ple : P 3 9 ) ...........................................................................
154
5.22 I-V characteristic of th e small grain size In/diam ond/silicon samples.
(Electrical sample : P 3 9 ) ...........................................................................
155
5.23 I-V characteristic of th e large grain size A l/diam ond/silicon samples.
The average grain size is 2.1 fim. (Electrical sam ple : P 6 ) ...............
156
5.24 I-V characteristic of th e large grain size A l/diam ond/silicon samples.
The average grain size is 1.4 /zm. (Electrical sam ple : P 7 ) ...............
157
5.25 Photoconductance versus photon energy (a) before norm alization and
(b) after norm alization for th e small grain size A u/diam ond/silicon
sample. (Electrical sam ple : P 3 9 ) ..........................................................
159
5.26 Photoconductance versus photon energy (a) before norm alization and
(b) after norm alization for the small grain size In/diam ond/silicon sam ­
ple. (Electrical sam ple : P 3 9 ) .................................................................
160
5.27 Photoconductance versus photon energy (a) before norm alization and
(b) after norm alization for the large grain size A l/diam ond/silicon sam ­
ple. T he average grain size is 2.1 fim. (Electrical sam ple : P 6 ). . . .
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161
5.28 Photoconductance versus photon energy (a) before norm alization and
(b) after norm alization for th e large grain size A l/diam ond/silicon sam ­
ple. The average grain size is 1.4 fim . (Electrical sam ple : P7) . . . .
162
5.29 Photoconductance versus photon energy (a) before norm alization and
(b) after norm alization for th e large grain size A l/diam ond/silicon sam ­
ple. The average grain size is 1.1 /xm. Electrical sam ple :P20) . . . .
163
5.30 The energy band diagram at the A l/p -ty p e diam ond interface................
164
5.31 The I-V characteristic of the point contact/diam ond/silicon stru ctu re
showed ohmic behavior. The top contact is a tungsten point probe and
th e bottom contact is th e silicon wafer. (Electrical sam ple : P20)
. .
166
5.32 The best Schottky barrier diode (SBD) characteristic of the large grain
size A l/diam ond/silicon samples. T he top contact is alum inum and th e
bottom contact is th e silicon wafer. (Electrical sam ple : P 2 0 ) ..............
167
5.33 The R am an spectrum of NDF-P20. The peak between 1550cm-1 and
1600cm-1 indicates th a t the existence of graphitic com ponent in the
film...........................................................................................................................
168
5.34 The SBD I-V characteristic of a film w ith coplanar surface contacts.
One contact is alum inum and the other contact is a tungsten point
probe. (Electrical sam ple : P 2 0 ) ....................................................................
5.35 The log(I)-V characteristic of the SBD. (E lectrical sample :P20) . . .
169
171
5.36 The forward bias I-V characteristic of th e SBD. T he o represents the
experim ental d ata, th e solid line shows the ideal diode w ith
77
7 o = 5 .2 x l0 - 1 6 A and th e dashed line shows th e ideal diode w ith
77
=
1,
= 2,
7 o = 2 .9 x l0 - 1 4 A. (Electrical sam ple : P 2 0 ) ...............................................
173
5.37 Conductance-bulk voltage characteristic. It follows the field activated
model above 10 V. (Electrical sam ple : P 2 0 ) ............................................
5.38 T he I-H 3/ 2 characteristic of the SBD. (Electrical sam ple : P20)
5.39 T he I-Vj,2 characteristic of the SBD. (Electrical sam ple : P20)
. . .
. . . .
174
176
177
5.40 (a) The energy band diagram of th e SBD. The barrier height (f>p equals
to the sum of built-in potential Vi and activation energy E a. (b) The
equivalent circuit of th e SBD...........................................................................
178
5.41 Capacitance versus frequency characteristic at different reverse bias.
(Electrical sam ple : P 2 0 ) .................................................................................
180
5.42 C apacitance versus reverse bias characteristic at 50 kHz. (Electrical
sample : P 2 0 ) .....................................................................................................
xvi
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182
CHAPTER 1
In trod u ction
1.1
M otivation for Diamond Research
Diam ond is well known as one of the m ost valuable gems and possesses m any excellent
properties, such as an extrem ely high degree of hardness, high therm al conductivity,
high electrical resistivity, chemical inertness, and good optical transparency. W hen
properly doped w ith im purities, diamond also has potential as a useful semiconducting
m aterial.
C urrently in industry, diam onds are m ostly used in hard-surfacing and abrasive
applications. T he general m ethods to get synthetic diam ond are variations on the
high pressure and high tem p eratu re technique (H PH T ), reported by B undy et al. [1]
a t General Electric several decades ago. T h a t process is a m ature and economically
feasible technology, but it has some lim itations. F irst, commercial H PH T diamond
synthesis needs a large, expensive system. Secondly, low im purity diam ond is rather
difficult and costly by this m ethod. Finally, and m ost im p o rtan t it is only suitable
for production of diam ond in the form of small pieces, grit, and powder. It is not
practical for m any technological applications where a continuous film is required.
Now there appears to be a high likelihood th a t these lim itations can be overcome
by new diamond thin film deposition techniques which allow large area coating with
1
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lower cost. These new m ethods are based on m etastable synthesis of diam ond at
pressures of an atm osphere or less. So far this new m ethod of synthesis has m oti­
vated a m ajor international research effort aim ed a t developing techniques for high
quality, low cost, large area, and high deposition ra te diam ond thin film production.
This research raises the possibility of m any new potential applications in mechanical,
electronics, and optical industries.
Following are some of th e potential diam ond film applications:
* Tribology and abrasive coating applications.
* Thermal heat sinks.
* Electronic packaging and passivation.
* Electrical isolation.
* High temperature, high speed electronic devices;
* Optical windows.
* Microwave and millimeter wave power devices.
* Corrosion resistant applications.
Significant issues still rem ain th a t hinder widespread commercial applications of
diam ond films. These include reproducibility, film quality, film uniformity, and high
deposition rate of the diam ond films.
T he initial steps of this research were to develop techniques on new apparatus for
producing high quality diam ond films. Subsequent steps focused on the correlation
between processing techniques and diam ond film physical properties and specifically
on studying electrical properties of the diam ond films.
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1.2
Research Objectives
One objective of this research was to develop techniques for high quality diam ond film
synthesis using a microwave plasm a disk reactor (M PD R ) chemical vapor deposition
system developed by Asmussen e t al. [2, 3] which had previously been proven success­
ful in plasm a assisted etching of semiconductors [4, 5,
6,
7] and in plasm a oxidation
techniques [8 , 9]. This objective, which was p art of a group effort, was m et, and in
fact during th e past year a com m ercial version of the M PD R was placed in use on an
industrial processing line.
A second objective was to correlate diamond film properties w ith substrate prepa­
ration m ethods and synthesis conditions, providing insight into the factors affecting
diam ond quality. Two different nucleation m ethods were shown in this research to
produce films of sim ilar diam ond quality as m easured by R am an spectroscopy, b u t
considerably different morphology and different electrical characteristics. A variety
of physical characterization m ethods were used for film analysis.
T he th ird and final objective was to specifically focus on the electrical properties of
th e films in order to provide insight into the relationship between synthesis variables
and electrical properties, and to investigate electron device related film properties.
Toward this objective, new sam ple preparation m ethods were used to study the role
of m etal contacts. New inform ation about the role of defects in lim iting the use of
diam ond films at very high electrical fields, above 105 V /cm , was provided by this
research. Also, the im portance of nucleation procedures on the properties of m etaldiam ond junctions, specifically on the properties of diam ond Schottky barrier diodes,
was dem onstrated.
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1.3
Dissertation Outline
This dissertation is divided in to four subjects. These are (1) background literatu re
review, (2) diamond film deposition and sam ple fabrication, (3) physical characteri­
zation, and (4) electrical properties analysis.
C hapter 2 of this dissertation reviews th e diam ond synthesis theory, diam ond film
production m ethods, and previous diam ond electronic device results from the lit­
erature. C hapter 3 describes diam ond film deposition by using the M PD R chemical
vapor deposition system and also describes the sample preparation techniques used in
this research. C hapter 4 reviews characterization results of th e diam ond films by R a­
m an spectroscopy, scanning electron microscope, X -ray photoelectron spectroscopy,
and surface profiling, The electrical conductivity versus tem p eratu re of the diam ond
films as m easured by the four point probe m ethod is presented in chapter 5 (section
2). C hapter 5 (section 3 and 4) also analyzes the electrical contact effects on back
etched diam ond samples and on diam ond/silicon structures. C hapter
6
concludes this
work w ith a sum m ary of im p o rtan t results and discussions for future research.
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CHAPTER 2
S y n th etic D iam on d : B ackground
R ev iew
2.1
Introduction
This chapter begins by describing th e historical background of diam ond synthesis from
high pressure high tem perature m ethods to low pressure low tem p eratu re m etastable
production m ethods.
Then th e growth mechanisms of th e m etastable production
m ethods are covered. In chapter 2 (section 3), several categories of the diam ond film
production technologies are discussed, such as chemical vapor deposition techniques,
plasm a assisted chemical vapor deposition techniques, and ion beam deposition tech­
niques. T hen several nucleation m ethods are also described. T he physical attrib u tes
of diam ond, such as m aterial properties and sem iconductor properties, and some of
the potential applications are explained in chapter 2 (section 4). Finally, some of the
diam ond device work is reviewed in chapter 2 (section 5).
5
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2.2
2.2.1
Brief History of Diamond Synthesis
Historical Background
Diamond was first known to be a form of carbon in 1797. L ater it w as discovered th a t
diam ond is formed by carbon atom s which are arranged in a p articu lar crystalline
structure. It h a s a cubic lattice which is b u ilt up from sp 3 -hybridized, tetrahedrally
arranged carbon atom s. A m uch more common form of solid state carbon is graphite,
in which carbon atom s are arranged in a hexagonal crystalline stru ctu re. The graphite
lattice consists of layers of condensed sp2-hybridized rings. T he atom ic structure of
diam ond and graphite are shown in Figure 2.1. Most recently, however, scientists
have just found th e th ird form of carbon, th e fullerenes [11]. W ith th e knowledge of
the fact th a t diam ond is form ed by th e sp 3 bonding arrangem ent of carbon atom s, a
scientific approach w as open for research on synthetic diam ond technologies.
Therm odynam ic calculations based on th e G ibb’s free energy show the conditions
under which carbon atom s prefer to form diam ond or graphite. A t ordinary tem ­
peratures and pressures, diam ond is a m etastable form of carbon. This means th at
diam ond is not th e m ost stable form of carbon compared to graphite in nature, be­
cause the hexagonal stru ctu re of graphite has a lower therm odynam ic potential than
the diamond crystal structure. Therefore, graphite is favored in term s of therm ody­
nam ic stability. However, at very high pressures, greater th a n 100,000 atmospheres,
the phenomenon is reversed and diam ond becomes the preferred state. The phase
diagram of carbon, shown in Figure 2.2, illustrates the regions of tem p eratu re and
pressure corresponding to diam ond, graphite, liquid carbon, and vapor carbon [1 2 ].
Although diam ond is m etastable, it does not degrade to graphite spontaneously
under ordinary conditions. A significant potential energy barrier prevents such degra­
dation except at very high tem peratures as shown in Figure 2.3. However, the phase
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(a)
Diamond (sp3bonding)
(b ) Graphite (s^bonding)
Figure 2.1. T he atom ic stru ctu re of (a) diam ond (b) graphite.
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0
2,000
4,000
6,000
8,000
10,000
Temperature (°F)
□
Vapor
Liquid
Graphite
Diamond
Figure 2.2. Phase diagram of carbon.[12]
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diagram does show th a t at ordinary conditions graphite can ’t transform to diam ond
because graphite has the lower therm odynam ic p otential under such conditions.
M any experim enters in th e 19th century and early 20th century tried unsuccess­
fully to m ake diam ond by subjecting graphite and m any o th er carbon compounds to
the conditions where diam ond is th e favored state of carbon, nam ely under high heat
and great pressure. Eventually, a group of researchers a t G E successfully developed
the first technique to m ake ”synthetic diamond! ', or ” man-m ade diamond P ’ in 1955
[I]-
This technique involves processing at high tem peratures and high pressures, which
are in th e range of 30,000-100,000 k g /c m 2 and 1000-3000 ° K respectively. It also in­
volves a m etal, such as iron, nickel, cobalt, manganese, chrom ium , or tan talu m to
serve as a solvent and catalyst. T he G E work not only lead directly to present indus­
trial production of synthetic diam ond for abrasive applications, b u t also to advanced
carbon research in general w ith more understanding of th e carbon phase diagram .
T here is another im portant high pressure m ethod to synthesize diamond which is
the so-called shock wave technique. Diamond containing m aterial was achieved first
by m eans of shock waves by Paul S. DeCarli and John C. Jam ieson in 1961. This
technique, industrialized by Du P o n t, utilizes the high pressure in the shock wave of
explosions to directly convert crystalline graphite into diam ond. The particles are
then separated from the starting m aterial by sedim entation [13].
W ith th e success of these two techniques, over 30 tons of industrial diam ond ab ra­
sive grain are m ade each year around the world. However, it has long been recognized
th a t there is a possibility th a t diam ond may be formed in th e m etastable portion of
the phase diagram , i.e. in th e graphite stable region, under the conditions where
’’nascent” (un-combined) carbon atom s are liberated [12]. Such synthetic conditions
would be easier to achieve and also b e tte r suited for diam ond film coating at relatively
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Energy
o
0.02 e v
A to m ic
S p a cin g
Diam ond
A
E ( fre e carbon, g ra p h ite ) =
Graphite
Vapor
170 K c a l/g -a to m
A E ( diamond, graphite ) = 0 .5 K c a l/g -a to m
Figure 2.3. Illustration of free energy difference between diam ond and graphite.
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low cost over larger areas.
Following this spirit of thought, attem p ts were m ade in th e 1950’s, 1960’s, and
1970’s to form diam ond films on non-diam ond substrates by depositing carbon atom s
at relatively low pressures and low tem peratures under a variety of conditions.
T he first results of the diam ond film work were th e developm ent of a few exper­
im ental m ethods to form diamond-like-carbon (DLC) films by vapor deposition or
ion beam deposition. [14, 15, 16, 17] However, th e atom ic stru ctu re of DLC films is
different from th a t of diam ond films. The difference will be explained in chapter 2
(section 4).
A significant breakthrough occurred when researchers a t th e Moscow In stitu te of
Physical C hem istry developed m ethods for depositing diam ond films on both dia­
m ond and, for the first tim e, non-diam ond substrates. They suggested three different
approaches to achieve a ”super-equilibrium” w ith atom ic hydrogen : catalysis, heated
filament, and electric-discharge plasma [18]. However, th eir work did not get much
attention in the west until Ja p a n ’s N ational In stitu te for Research in Inorganic Ma­
terials (NIRIM ) repeated th e work and also reported tru e diam ond films. The key
feature of this work is the addition of hydrogen gas associated w ith m ethane gas into
the deposition cham ber [19]. This result confirmed th e hypothesis th a t diam ond syn­
thesis techniques can take place under the conditions in which diam ond is m etastable
with respect to graphite.
T he region for low pressure chemical vapor deposition
(CVD) technique of diam ond synthesis is illustrated in Figure 2.4. Not only did this
developm ent trigger m any research efforts on diam ond film technology but it also
pushed the synthetic diam ond films towards industrial applications.
Since 1983, m any groups successfully deposited diam ond films or DLC films on
all kinds of non-diam ond substrates by all kinds of techniques. Some representative
techniques and their experim ent param eters will be described in details in chapter
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2
400
Liquid carbon
Pressure
( K b a r)
Shock wave
synthesis
300
Diamond
Diamond &
m etastable
graphite
200
Catalytic
high-pressure,
high-temperature
synthesis
High-pressure,
high-temperature
synthesis
100
Graphite &
m etastable
diamond
Chemical vapor
deposition
0
1000
2000
3000
T e m p e ra tu re ( £
4000
5000
)
Figure 2.4. Low pressure CVD diam ond synthesis in the phase diagram of carbon.[13]
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(section 3).
2.2.2
Growth Mechanism of Diamond Films
In general, there are two ways to form m etastable crystal:
1. I f a stable crystal is brought into a new temperature or pressure range in which it
doesn’t transform into the more stable form ;
2. A precipitate or transformation m ay fo r m a new metastable phase. [20]
M etastable crystal will rem ain present because th e high activation energy required
for the conversion into more stable phase causes a low rate of transition. This explains
why diam ond doesn’t spontaneously degrade to graphite at ordinary tem p eratu re and
pressure.
The possibility of synthesis of diam ond in the graphite region of th e phase-diagram
is based on th e fact th a t the free energy difference between diam ond and graphite (0.5
K cal/g-atom ) is so small under am bient conditions. See Figure 2.3. There is some
probability th a t diam ond and graphite can both nucleate and grow simultaneously
from nascent carbon atom s, especially under conditions in which kinetic factors can
dom inate, e.g. high energy. If graphite can be prevented from form ing, or if it can
be removed preferentially, diam ond can be recovered. [21] A sam pling of theories and
models for diam ond growth m echanism from m ethane/hydrogen m ixture are briefly
described below.
In 1986, M. Tsuda, et al proposed the ” Epitaxial Growth Mechanism o f Diamond
Crystal in CH4-H2 Plasma.” They believe th a t no phase transition of graphite or
amorphous carbon takes places under the conditions of low tem p eratu re plasm a reac­
tion. R ather, th e diam ond form ation in vacuum is considered to proceed by chemical
reactions which are different from th e phase transition of graphite under ultrahigh
pressure at very high tem perature. By a quantum chemical calculation a mechanism
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was found to explain th e process in two steps. F irst, th e (111) plane of diamond
surface is covered by m ethyl groups (C H 3) via m ethylene insertion or hydrogen ab­
straction followed by m ethyl radical addition. Second, epitaxial grow th occurs when
three neighboring m ethyl groups standing on th e ( 1 1 1 ) plane of the diam ond surface
bind spontaneously to form th e diamond stru ctu re via m ethyl cation interm ediates
[22]. However, S. J. Harris also proposed a m echanism for hom o-epitaxial grow th on
an finite (100) diamond surface from the m ethyl radicals (C H 3) [23].
In 1987, M. Frenklach and K. E. Spear proposed a different growth m echanism of
vapor-deposited diamond. This central postulate is th a t th e m onom er grow th species
is ”acetylene” {C2 H 2 ). This m echanism basically consists of two alternating steps.
T he first is surface activation by the H abstraction of a bonded hydrogen atom from
a surface carbon. Then the activated carbon radical acts as a site for the addition of
one or two acetylene molecules. During the addition reaction cycle a num ber of solid
C-C bonds are formed and hydrogen atom s m igrate from a lower to a upper surface
layer. This paper also discusses the two possibilities for th e production of graphite
on a growing diamond layer. F irst, graphite precursors m ay form in th e gas phase
followed by surface deposition and further growth. Second, the initiation and growth
of graphite m ay happen directly on the surface [24].
A dditional insight into grow th mechanism is provided by the fact th a t m ethane
and hydrogen is not a unique recipe for diam ond film growth. For exam ple, Hirose
and Terasaw a reported th a t diam ond thin films w ith good crystallinity and quality
were quickly synthesized by therm al CVD using several organic compounds, some of
which include oxygen [25]. However, the specific roles of oxygen were not discussed
in th a t work.
T. K awato and K-I Kondo were th e first to specifically study the effects of oxygen.
W ith th e addition of oxygen, th e deposition of graphitic or am orphous carbon could be
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suppressed, the growth ra te of diam ond was increased and the pressure range for the
synthesis of diamond was extended. It was suggested th a t these effects resulted from
th e fact th a t the acetylene concentration is significantly reduced upon th e addition
of oxygen [26].
S. J. Harris and A .M . W einer also reported th a t O 2 can reduce th e effective initial
hydrocarbon mole fraction, which is im p o rtan t because higher quality diam ond is
grown at a lower initial hydrocarbon mole fraction. Perhaps more im portantly, it was
also noted th a t there is a possibility the form ation of sufficient gas phase O H will
remove non-diamond (pyrolytic) carbon from th e film [27]. Y. Liou, et al. also found
th a t the addition of oxygen was critical for diam ond grow th at tem peratures below
500 °C [28].
It is likely th a t details of th e growth mechanism depend on the synthesis m ethod.
For example, with the acetylene flame technique, Y. M atsui, et al. [29] proposed
th a t stable species such as C H 4 and C 2 H 4 are rapidly produced, followed by the
m ethyl radical form ation. T hen the C-radicals adsorbed on the diam ond surface are
etched by H-atoms to form C H 4 . The growth rate depends both on th e substrate
tem perature and on the C 2 H 2 /O 2 ratio. This result is also confirmed by Y. Tzeng,
et al. [30].
Although there are m any postulates attem p tin g to explain diam ond synthesis,
th e experim ental evidence in support of any p articu lar growth m echanism is lim ited.
However, there are several hypotheses most researchers believe th a t can explain parts
of th e growth mechanism for th e diamond synthesis. T here are listed as follows.
F irst, the best quality diam ond films have been generally obtained in a predom ­
inantly hydrogen atm osphere, with only small am ounts of hydrocarbon (typically
m ethane, 1 % or less) present in the reaction m ixture. T h a t is, th e hydrogen atom s
should be in a super-equilibrium state for a successful diamond growth. Moreover, H
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atom s play a very crucial role in enhancing the film grow th. T he effects of hydrogen
are described in the following [31].
1
. H y d ro g e n a c ts a s t h e e tc h in g a g e n t fo r n o n - d ia m o n d c a r b o n :
T he form ation of diam ond kinetically competes w ith th e form ation of graphite. W hen
graphite and diam ond are form ed simultaneously, the atom ic hydrogen created from
th e hydrogen gas either by therm al energy or by electric energy is considered a pref­
erential etching agent of graphite over diamond.
2
. H y d ro g e n p ro v id e s s ta b iliz a tio n o f sp 3 b o n d in g :
A tom ic hydrogen serves to satisfy th e dangling bonds of surface carbon atom s, keep­
ing them in the sp 3 configuration and thus preventing the diam ond surface from
reconstruction into graphitic (sp2) or carbonic (sp) structures.
3. H y d ro g e n p r o m o te s t h e m a in m o n o m e r o f t h e g a s -p h a s e p r o d u c tio n fo r
d ia m o n d g ro w th :
E ither acetylene ((7 2 # 2 ) [24, 32] or m ethyl radical (C H 3 ) [22,23] is the m ain m onomer
for the diamond growth and hydrogen is believed to support th e form ation of theses
species.
Secondly, the tem p eratu re dependence of the rate of diamond growth exhibits
a m aximum. T h at is, it initially increases with tem p eratu re and then decreases.
G raphite form ation rates also exhibit a tem p eratu re m axim um , but at higher tem ­
peratures than th a t of diam ond.
Finally, the addition of oxygen not only increases the deposition rate but also
improves aspects of the film quality. The roles of oxygen are suggested as follows [32].
1. O x y g e n m a y in c re a s e t h e H a to m c o n c e n tr a tio n w h ic h s e le c tiv e ly e tc h e s
g r a p h itic a n d a m o r p h o u s c a r b o n s .
2
. O x y g e n m a y c a u s e t h e r e d u c tio n o f p y ro c a r b o n - f o r m in g s p e c ie s .
Consequently, it will increase th e growth rate of diamond.
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3. O x y g e n m a y also a c t a s a g o o d s e le c tiv e e t c h a n t o f n o n - d ia m o n d c a r ­
bons.
Oxygen is b e tte r th an hydrogen in the efficiency of etching graphite, however, th e
addition of an excessive am ount of oxygen m ay result in a decreased growth rate,
because th e diam ond crystals are also etched.
Besides th e growth m echanism of th e diamond synthesis, th e nucleation m ech­
anism still needs to be briefly considered. The initial nucleation m echanism of a
diam ond crystallite is distinct from the m echanism for th e extension of a preexisting
diam ond lattice. W ithout special su b strate preparation, th e initial nucleus density
of diam ond is extrem ely low, and a continuous film does n o t result. To improve th e
am ount of nucleation sites becomes necessary. Generally speaking, it is well recog­
nized th a t the nucleation rate can be enhanced prom inently bo th by scratching th e
substrate surface or by attaching small seed crystallites to th e su b strate before th e
deposition. Nucleation m ethods will be described further in chapter 2 (section 3).
U ntil now, the understanding of the growth m echanism is only a t th e ice-breaking
stage.
M any more experim ents with b e tte r control of experim ent param eters are
needed to verify the physics and chem istry of the diam ond film synthesis.
2.3
Review of Diamond Film Technologies
In the past few years the potential applications of diam ond films have stim ulated a
great deal of research interest on various synthesis technologies, each of which has
advantages and disadvantages.
A broad sam pling of these technologies is briefly
presented in this section in which th e synthesis m ethods are grouped into th ree cate­
gories. Theses are ( l) h o t fila m e n t a s s is te d c h e m ic a l-v a p o r-d e p o s itio n (C V D );
(2 )p la s m a a s s is te d C V D ; and (3 )o th e r d e p o s itio n m e th o d s . Additionally, in
the last p art of this section, several nucleation m ethods will be described.
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2.3.1
Hot Filament Chemical Vapor D eposition Techniques
Generally, CVD is a reaction in which two types of gas, C(g) and D(g), react at a
high tem p eratu re to form a solid phase A(s) and a gas phase B(g). Sometimes, more
than two kinds of gases are introduced for certain applications. One way of enhancing
th e reaction ra te is to add therm al energy. In hot filam ent CVD th e therm al energy
comes from a hot filament located in th e reaction cham ber in to which the gases are
introduced.
In 1982, M atsum oto, et al. reported synthesis of m icrocrystal diam ond using a hot
filament CVD process from a m ixture of m ethane and hydrogen gas. This technique
involves th e use of a hot tungsten filam ent in the vicinity of th e substrate.
The
hot filam ent causes the dissociation of the gas as well as heating of the substrate,
providing appropriate conditions for th e growth of diam ond [19].
A modified version of th e therm al CVD m ethod has been used by Sawabe, et al.
to improve th e growth rate of diam ond films. It involves th e electron bom bardm ent
of the su b strate surface by biasing th e substrate positively w ith respect to the hot
filament. It is suggested th a t the decomposition of C H 4 and H i by electrons occurs
m ainly at th e substrate surface, because the scattering cross section for electrons
is fairly sm all in space under these conditions. It is also found th a t the num ber of
nucleation sites on the substrate surface is increased by electron bom bardm ent. Thus,
growth rates as high as 3 - 5 /im /h o u r were achieved [33].
Hirose and Terasawa experim ented w ith many organic gases other than m ethane
using th e hot filament CVD m ethod. These experim ents were based on their hypothe­
sis th a t th e m ethyl radical (G Hz) w ith sp 3 hybridized orbits and atom ic hydrogen (H )
play im p o rtan t roles in the synthesis of diamond. Consequently, they choose m ethyl
alcohol (C H zO H ), ethyl alcohol (C 2 H 5 O H ), acetone (C H zC O C H z), dim ethyl ether
(C 2 H 5 OC 2 H 5 ) , and some other organic gases which can generate m ethyl radicals
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
more easily in order to increase the grow th rate. They found th a t good quality dia­
mond films can be grown on silicon substrates w ith high grow th rates
(8 - 1 0
/zm /hour)
under the pressure range 1 - 800 Torr. T he growth rate is about 10 tim es faster than
therm al CVD m ethod using C H 4 or C 2 H 2 gas [34].
H.
Aikyo and K.-I. Kondo reported diam ond synthesis by a new hot filament
CVD m ethod in which m ethane is decomposed m ainly by an abstraction reaction of
C H 4 w ith atom ic hydrogen. T he film obtained by this m ethod was com pared with
th a t obtained by th e m ethod which involves the therm al decom position of C H 4 near
a filament. They found th a t th e film quality of th e form er m ethod was b e tte r in
term s of the ratio of crystalline to am orphous carbon, b u t slightly lower in deposition
rate com pared to th a t by th e la tte r m ethod. They hypothesize th a t production of
acetylene (C 2 H 2 ) is suppressed by th e separate introduction of the gases and th a t
there is consequently an im provem ent in the film quality [35].
Y. H. Lee, et al. used th e bias-controlled therm al CVD m ethod to do diam ond
synthesis. They found th a t high quality films w ith highly facetted cubo-octahedral di­
am ond grow th was observed under low current bias conditions, in which the substrate
is negative w ith respect to th e filament. In contrast, m ultiply tw inned m icrocrystal
growth incorporating sp2-bonded carbon was obtained under high current bias condi­
tions. They suggested th a t this is due to a decrease in electron and ion bom bardm ent
of the sam ple during the low bias conditions which minimizes surface damage. Clearly
this observation is quite different from Sawabe’s point of view about the role of the
electrons [36].
Lately, M. Ihara, et al. reported th a t they successfully deposited good quality
diam ond on silicon substrates at a tem perature as low as 135 °C by controlling the
tem perature of the substrate holder. A tan talu m filam ent was typically used and
m aintained a t 2300 °C in this experim ent [37].
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There are some advantages and lim itations associated w ith the hot filam ent CVD
process. The fact th a t th e apparatus is inexpensive and easy to operate is th e most
attractiv e advantage.
However, film non-uniform ity due to the high tem p eratu re
gradient of th e filament and the difficulty to scale up th e size lim its the possibilities
for a production process. Also, th e hot filament can be a source of im purities in the
films.
2.3.2
Plasma A ssisted Chemical Vapor D eposition Tech­
niques
Plasm a assisted chemical vapor deposition (PACVD) processes used for th e synthesis
of diam ond and diam ond-like films involve th e decom position and dissociation of
hydrocarbon gas in a plasm a by direct-current (dc), radio-frequency (rf), or microwave
excitation. Several variant techniques of the PACVD process as applied to diam ond
are described in the following.
DC Plasm a CVD Techniques
In 1987, K. Suzuki, et al. first successfully deposited good quality diam ond films by
the DC plasm a parallel p late CVD m ethod. Film s w ith growth rates of about 20
/im /h o u r were grown on silicon and a-alum ina substrates from a m ethane/hydrogen
m ixture gas at a pressure of 200 Torr w ithout surface scratching by diam ond or cBN powder. By experim ent they also observed th a t electrons could enhance the film
growth.
[38] Later epitaxial growth of diam ond thin films on cubic boron nitride
(111) surfaces [39] and deposition of diam ond onto an alum inum substrates at low
tem perature (below 480 ° C ) [40] by dc plasm a CVD technique were also reported by
other groups.
K. K urihara, et al. reported diam ond synthesis with th e use of a DC plasm a jet
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(cham ber pressure: 100 - 400 Torr). A plasm a je t, form ed by the dc arc discharge of
C H \ diluted with H%, was sprayed onto a water-cooled substrate. Because of th e high
density of radicals, a high grow th rate of 80 /im /h o u r was achieved by this technique
[41].
Diamond films synthesized by dc arc discharge plasm a CVD in a hydro­
gen/argon/ethanol m ixture gas was reported by F. A katsuda, et al. (cham ber pres­
sure: 200 - 400 Torr). T he grow th rate of diam ond films was reported to be about
200 - 250 //m /hour which is higher th an th a t of other CVD techniques [42].
S. M atsum oto, et al. also studied the substrate bias effect on diamond deposition
by th e dc plasm a je t m ethod. W hen they applied a positive bias voltage to substrates
in a dc plasm a jet of th e A r-H i-C H ^ system , th e deposition rate increased more
th a n twice, and a m axim um ra te of 15 /rni/m in was obtained. The deposition area
also increased b u t the uniform ity of film thickness did not improve. They deduced
th a t positive bias on th e su b strate enlarges the plasm a region near th e substrate
by inducing a secondary discharge between the arc plasm a and bias electrode. As
th e concentration of activated species increased near the substrate, the deposition
of diam ond also increased. However, too much bias current changes the secondary
discharge from glow-like to an arc discharge, causing th e over heating of the su b strate
and deposition of graphite instead of diamond [43].
RF Plasm a CVD Techniques
In 1987, S. M atsum oto, et al. developed a technique using an rf (4 MHz) induc­
tive heating of an argon-hydrogen-m ethane m ixture plasm a under
1
atm pressure.
M icro-crystals of diam ond and continuous polycrystalline films were deposited on
m olybdenum substrates w ith a
1
/im /m in deposition rate, however the films were not
uniform in thickness and had poor adhesion [4 4 ].
D. E. Meyer, et al. studied the 13.56 MHz inductive coupled plasm a CVD tech­
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nique. They found th a t th e diamond-like carbon scratching only enhanced th e form a­
tion of diam ond particles on silicon and niobium, b u t not on stainless-steel substrates
[45].
Microwave Plasm a CVD Techniques
Based on th e num ber of journal articles, the literatu re indicates th a t the microwave
plasm a CVD process is th e m ost often used technique by diam ond film researchers.
Various techniques for this approach are described in th e following.
For microwave (usually 2.45 GHz) plasm a excitation, typically a quartz tu b e is
positioned inside a microwave waveguide, a gas flow is introduced through the tube,
and a plasm a is produced and confined in th e center of th e tube. Substrates are
m ounted in the tu b e and exposed directly to the plasm a which is formed at a gas
pressure in th e range of 10 to several hundred Torr.
In 1983, M. Kamo, et al. reported diamond synthesis from th e gas phase in a
microwave plasm a [46]. W ith a m ethane/hydrogen plasm a th e deposition rates were
approxim ately 0.5 - 1 //m /h r. A num ber of subsequent investigations used similar
apparatus for diamond synthesis or used a variation where a bell ja r is placed in a stubtuned microwave reactor. For exam ple, single-crystal grow th rates over 20 //m /h o u r
were obtained from a m ethane/hydrogen/oxygen gas m ixture by C.-P. Chang, et al.
[47]. I. W atanabe and K. Sugata used a microwave plasm a to synthesize diam ond with
different organic gases. Film s w ith deposition rates higher th a n m ethane/hydrogen
plasm as and with a uniform film thickness over a 5 cm by 3.3 cm area were achieved
[48]. J. S. Ma, et al. studied the selective nucleation and growth of diam ond on a
S iO i dot-patterned Si su b strate exposed to a microwave C O / C H 4 / H i plasm a [49].
Epitaxial growth of high quality diamond film on diam ond substrate [50] and local
epitaxial diamond growth on Si (100) substrate [51] have also been studied by different
microwave plasm a CVD (M PCV D ) techniques.
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In an alternative microwave approach, Y. M itsuda, et al. developed a microwave
plasm a torch (M W plasm a je t) system , which can generate a pure H 2 plasm a jet
at atm ospheric pressure from th e end of a center electrode. T he electric field has
m axim um strength at th e end of the electrodes, so the plasm a is initially ignited by
the electric breakdown. Then the plasm a je t was powered and sustained by th e elec­
trom agnetic field generated betw een th e electrodes a n d /o r in th e cham ber. Diamond
films were successfully synthesized a t a high rate (30 /im /h o u r) on an area of 2.5 x 2.5
c m 2 from a A r - H ^ - C m ixture gas [52].
Recently, low tem perature (< 500 °C ) CVD of diam ond films has been addressed
as an im portant task for diam ond synthesis because m any m aterials will melt or be­
come unstable a t high tem perature deposition conditions. Microwave plasm a systems
have played an im portant role in this effort. Usually, low tem p eratu re deposition is
achieved by lowering the gas pressure and the microwave pow er from norm al depo­
sition conditions. For example, a magneto-microwave plasm a for deposition of wide
area diam ond films was reported by Hiraki and his coworkers. In this case, the pres­
sure was reduced to 0.1 Torr and a m agnetic field higher th a n th a t required for ECR
resonance was added to the plasm a. T he plasm a, which is 16 cm diam eter wide, filled
the reactor. High quality diam ond films were formed on a positively biased silicon
substrate from a m ixture gas of C H 4 / H 2 or C O /H 2 [53]. L ater, they also successfully
deposited low tem perature (500 ° C ) diam ond films on Al su b strates at 0.1 Torr from
the C H 4 I C O 2 I H 2 m ixture gas [54].
Growth of diam ond films on silicon a t 500 °C has been accom plished in a M PCVD
system by W . L. Hsu, et al. [55]. Y. Liou, et al. also found th a t they could deposit
diamond films on silicon, MgO, and fused silica at about 400 °C in a bell ja r MPCVD
system [56]. D. J. Pickrell, et al. successfully deposited diam ond b o th in the plasm a
and dow nstream at about 400 °C by using the same system [57]. However, all low
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tem perature techniques have th e disadvantage of extrem ely low grow th rates because
of the reduction in th e plasm a density.
Recently, N orton C om pany has begun a unique microwave reactor approach, called
the microwave plasm a disk reactor (M PD R ), which was developed a t Michigan State
University. This is th e reactor type used for th e research in this dissertation. It is
described in detail in chapter 3 (section 2).
2.3.3
Other Deposition M ethods
In addition to hot filament and plasm a approaches to diam ond th in film deposition,
a num ber of oth er interesting m ethods have been reported. A t Rice university, re­
searchers showed th a t diam ond can also be deposited by using th e m ixture gas of
halogen, hydrogen and carbon atom s on a variety of substrates a t tem peratures as
low as 300 °C [58]. In perhaps the sim plest procedure in term s of experim ental ap­
paratus, acetylene torches (C 2 H 2 and O 2 ) have been used for diam ond synthesis. Ion
beam deposition, in which C + ions are directed a t a substrate, have been reported
by several groups [14, 15, 59, 60, 61, 62, 63, 64] for ”diam ond-like” carbon films.
These films do not have the diam ond crystal structure and the 1332 cm - 1 Ram an
signal, b u t they do have some diam ond-like properties in term s of transparency and
hardness, although generally to a lesser extent th an diam ond. T he m ethod appears
less successful a t this tim e for true diam ond synthesis. In all diam ond film deposition
approaches, th e challenge is to produce high quality diam ond films over large areas
with useful deposition rates.
2.3.4
Nucleation M ethods.
In chapter 2 (section 2) it was noted th a t enhancing the num ber of nucleation sites
on the surface of th e substrates is a precursor to m ost diam ond deposition processes.
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Typically th e substrates are either abraded w ith diam ond powder or diam ond paste,
in th e size of approxim ately 0.1 /zm to 40 /zm, or seeded w ith sm all diam ond crys­
tallites in order to increase th e num ber of nucleation sites. A fter a surface abrasion
treatm ent, a cleaning procedure is always followed to remove residue and minimize
the contam ination. Substrates are cleaned by one or m ore organic solvents, such as
acetone, m ethanol, ethyl alcohol, ... etc. Sometimes, ultrasonic tre a tm e n t or acid
solutions are also used in order to clean m ore effectively. T here are m any kinds of
nucleation m ethods [11-70]. Only a sampling of nucleation m ethods will be described
in th e following.
1. C. P. Chang, et al. described th a t they abraded polished silicon substrates w ith
diam ond powder (0.5 /zm), rinsed the substrate in de-ionized w ater and blew it dry
[47].
2. I. W atanabe and K. Sugata reported th a t the substrates were roughened for 40
m in by diam ond powder w ith a particle size of 40 /zm suspended in ethyl alcohol
where ultrasonic waves were passing [48].
3. H. Shiomi, et al. reported th a t the substrates were polished w ith a diam ond paste
of 1/4 /zm size for 5 m in and were de-greased in acetone. Then th e substrates were
treated in a
1 :1
solution of H F , H N O 3 and in a 1:3 solution of H N O 3 , H C l. Sub­
strates were finally treated w ith de-ionized w ater and acetone [50].
4. J.-I. Suzuki, et al. reported th a t the substrates received ultrasonic treatm en t for
30 m in in a suspension of diam ond powder (20 - 40 /zm) and ethyl alcohol. After
th a t, they were cleaned in organic solvents [53].
5. Dr. Richard G uarnier, senior engineer in IBM W atson research center, described
an electrophoresis seeding m ethod [65]. Diamond powder (0.1 /zm) is first put in an
ethyl alcohol solution and m ixed homogeneously. The su b strate is positively biased
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at 40 V and placed face to face w ith a 2 cm gap away from a reference electrode for
about 30 sec. The substrate is gently taken out of the solution and dried on a spinner.
As a result, nuclei are homogeneously seeded on th e substrate. In this case there is
no subsequent cleaning procedure since th a t would remove th e seeds. In a sim ilar
technique, th e diamond pow der is dispensed on the su b strate by an aerosol m ist.
6.
Professor Aslam and colleagues have described a seeding m ethod whereby diam ond
powder is in solution w ith photo-resist, which allows p a tte rn ed seeding and p attern ed
growth of diamond films [6 6 ].
For th e nucleation techniques involving direct seeding by diam ond powder, th e nu­
cleation action is clear. The diam ond particles serve as sites for homo-epitaxial grow th
and th e crystallites size increases. Eventually a continuous polycrystalline film results.
For nucleation m ethods which involve abrasion, the nucleation action is less obvious
since microscopy generally indicates a clear surface. However, recently S. Iijim a, et
al. [67] reported th a t extrem ely high power transm ission electron microscopy (TEM )
indicates th a t abrasion does, even after cleaning, leave very sm all plaques of diam ond,
< 50 A in size, th a t serve as homo-epitaxial growth sites for individual crystallites
in th e polycrystalline film. Therefore a hypothesis is th a t nucleation m ethods work
by providing sites for hom o-epitaxial growth. However, it should be noted th a t the
literature does contain some cases where nucleation of substrates is claimed w ithout
the use of diamond powder a t all. For example, D. E. M eyer, et al. reported th a t
all the substrates were polished or roughened by sandpaper, sand-blasting, or silicon
to silicon abrasion. Substrates were cleaned by sonicating in trichloroethylene and
acetone [45].
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2.4
Physical A ttributes of Diamond
2.4.1
Comparison o f Bulk and Film M aterial Properties
The great interest in diam ond is due to its several extrem e properties. It has the
highest atom num ber density of any m aterial.
Because of its high atom num ber
density and th e strong covalent bonding, diamond has th e highest hardness and elastic
modulus of any m aterial and is th e least compressible substance known [64]. These
properties contribute to its wide use as an abrasive and cu ttin g m aterial.
B ut, if you look further in to diam ond’s physical properties as listed in Table 2.1
[6 8 ], you will find th a t the therm al conductivity of diam ond is higher than th a t of
any other m aterial, even 5 tim es th a t of copper a t room tem perature. Its therm al
expansion coefficient is very low. Electrical resistivity is extrem ely high and chemical
reactivity is extrem ely low.
T he com bination of these excellent properties makes
diam ond unique, special, and a ttra ctiv e for m any applications.
However, these are the properties of bulk natural diam ond. There are some dif­
ferences betw een natural diam ond and synthetic diam ond films which are synthesized
by th e different kinds of techniques mentioned in chapter 2 (section 3). Those m ajor
difference are shown in Table 2.2 [13]. The most prom inent difference is th a t the
properties reported for n atural diam ond are for single crystalline m aterial, b u t syn­
thetic diam ond films on non-diam ond substrates are polycrystalline. ”Diamond-like”
carbon is often amorphous. Consequently, the properties of synthetic diamond films
are often not as great as n atural diam ond. Therm al conductivity, electrical resistivity,
and hardness are smaller depending on the techniques used for th e diamond synthesis
and their experim ental param eters.
It should be noted th a t th e ” diam ond films” produced by those techniques may
exist as composites of crystalline diamond a n d /o r am orphous diamond-like carbon
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Property
Diamond’s Value
Chemical Reactivity
Extremely Low
Hardness
9000 k g /m m -2
Heat Conductivity
20 W / cm / degree K @ 3CP C
Tensile Strength
Compressive Strength
0 .5 x 106 psi (natural)
6
1 4 x 1 0 psi (natural)
Themal Expansion Coeff.
.8 x 10*6 / degree K
Refractive Index
2.41 @ 590 nm
Transmissivity
225 nm - far IR
Friction Coefficient
0.05 (dry)
Band Gap
5.4 eV
Electrical Insulation
10
Young’s Modulus
10.35x10*2 dy n es/cm 2
A
16
ohm-cm (natural)
Table 2.1. Physical properties of bulk natural diam ond. [6 8 ]
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Property
Crystal structure
Diamond and
Dlamond-llke Films
Face-centered
cubic
Amorphous,
micrograined
polycrystalline
1
0 .0 5 -0 .1 5
CO
CO
3.51
10
1—
Density ( am / cm 3)
Hardness ( M ohs)
Coefficient of friction
( against s te e l)
Bulk Diamond
7 -9
0.002 - 0.2
2.42
1 .5 -3 .0
Yes
Yes, but shows
interference color
Optical transparency
( infrared region)
Yes
Yes
Electrical resistivity
( O hm .cm )
Type la, b, and
lla: 1016
Type lib: 10 3
Thermal conductivity
(W/cm^K)
1 0 -2 0
5-18
Most inorganic
acids and solvents
Many inorganic
acids and solvents
Refractive Index
Optical transparency
( visible region)
Chemical inertness
1 0 2- 1 0 14
Table 2.2. Properties of bulk natural diam ond and synthetic diam ond films.[13]
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(i-C) an d /o r graphite.
The relative concentration of these phases depend on the
synthetic processing conditions. Likewise, th e properties of th e film strongly depend
on the relative concentration of each respective phases. T he characteristic properties
of these three phases are given in Table 2.3 [21].
So far, we have m entioned diam ond films and diam ond-like films very often. Some­
tim es in the literature, particularly for early papers, the term s ’’diam ond films” and
” diamond-like films” are used w ithout a clear distinction. B ut m ost researchers are
trying to clearly specify these two as different categories. Table 2.4 carefully sets the
definitions for diam ond films, diamond-like films, carbon and graphite coatings [69].
It should be noted th a t film properties can vary considerably for different deposi­
tion techniques. T he chemical structure, crystallinity, and thickness have a significant
influence on the final film properties. Consequently, the physical, m echanical, elec­
trical, optical and therm al properties of th e films are not solely determ ined by the
intrinsic properties of th e bulk diamond. In some cases this m ay be advantageous
since film properties can be tailored to th e specific requirem ents of certain applica­
tions.
2.4.2
Comparison of Semiconductor Properties
Diamond is a very wide bandgap sem iconductor ( £ a=5.45 eV). Table 2.5 compares
diamond sem iconductor properties to Si, GaAs, and SiC. D iam ond’s therm al conduc­
tivity is nearly 20 tim es th a t of silicon. Also, diam ond’s satu rated carrier velocity,
the velocity at which electrons move in a high electric field, is nearly 3 tim es th a t of
Si and GaAs. T he satu rated carrier velocity of diam ond is also greater th an the peak
velocity of GaAs. Unlike GaAs, the satu rated carrier velocity of diam ond m aintains
its high rate even in a large electric field. T he Johnson’s figure of m erit, of which
saturated carrier velocity and peak electric field at breakdown are factors, predicts
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Property
Diamond
Crystal structure
Cubic
( spa )
Density (gm /cm 3)
Chemical stability
Hardness
( Vickers, kg / mm )
Thermal conductivity
( W /cm 2.K)
Optical properties:
Refractive Index
Transparency
Electrical properties:
Resistivity ( Ohm.cm)
Dielectric constant
3.51
Inert, inorganic acids
and solvents
7000 - 1GC00
20
i-C
Graphite
Amorphous with small
crystal regions mixed
Hexagonal
( sp2 )
with sp2and sp3 bonds
1.8-2.0
Inert, inorganic acids
and solvents
2.26
Inert, inorganic acids
900-3000
...
2.42
UV-VIS-IR
1.8-2.2
VIS - IR
1016
5.7
10’°-1013
4 -9
...
K//30 - 40, K-l 1-2
2.15 (// c ), 1.8 ( j_C)
Opaque
0.04 ( //c), 0.2 (-i-C)
2.6 ( / / c ), 3.28 ( jl C )
Table 2.3. Properties of diamond, i-C, and g rap h ite.[21]
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1. Diamond Film
* sem i-transparent to transparent.
* hard.
* resistivity : 103 to 1016ohm-cm.
* chemically unreactive.
* crystalline to X-ray diffraction.
* show s Ram an p eak a t 1332 cm'1 .
2. Diamond-Like Film
* o p ag u e to sem i-transparent.
* hard.
* insulating.
* chemically unreactive.
* non-crystalline to X-ray diffraction.
* show s only a single broad Raman peak around 1550 cm '1 .
3. Carbon
* black.
* hard.
* electrically conducting.
* non-crystalline to X-ray diffraction.
* Ram an show s broad p e ak s centered at 1350 - 1360 cm '1
and 1580 - 1610 c m -1.
4. Graphite
* black.
* soft.
* electrically conducting.
* X-ray diffraction show ing d value of about 3.3 to 3.4 Angstroms.
* Ram an show s well defined peak at 1590 cm*1, may or may not
show well developed peak at 1355 cm'1 depending on crystal
size and ordering.
Table 2.4. Working definition for different carbon coatings. [69]
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the suitability of a sem iconductor for high power applications [70, 71]. The Keyes’
figure of m erit, of which satu rated carrier velocity, dielectric constant and therm al
conductivity are factors, indicates th e therm al lim itation of a m aterial on its high
frequency electrical perform ance [70, 71]. From Table 2.5 th e com bination of high
Johnson’s figure of m erit and high Keyes’ figure of m erit m ake it an excellent poten­
tial m aterial for high power microwave and m illim eter wave devices. It is expected to
outperform GaAs and /?SiC in microwave device application based on the available
d ata [70, 71]. Moreover, due to the large bandgap of diam ond, it can also be used for
UV detectors. T he com bination of high bandgap and high resistivity would reduce
the contribution to the photocurrent of therm al and background radiation, thereby
make it possible to fabricate low noise UV detectors.
Diamond is also of interest in electronic systems for uses other th an as an active
electronic m aterial.
For example, diam ond heat sinks can be fabricated for high
power, high tem perature, electronic and photonic devices. D iam ond windows m ay be
used to perm it viewing of an object over a wide optical range from infrared to UV
light.
In Figure 2.5 diam ond’s physical properties are related to various potential en­
gineering applications. B ut can diam ond films really be used to fabricate diamond
devices? Yes, however, the fabrication techniques for diam ond devices are not quite
m ature yet. Some of the prelim inary diam ond devices results will be reviewed in the
next section.
2.5
Review of Diamond Diodes and Transistors
In this section selective examples of diam ond device work are briefly reviewed.
In 1982, J. F. Prins dem onstrated a bipolar transistor action in ion im planted
diamond. P-type sem iconducting natural diam ond was used as th e substrate. Then
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PROPERTY
B andgap ( e V )
DIAMOND
e-s/c
SI
GaAs
5.45
2.3
1.11
1.43
2.7
2.5
1.0
S aturated Electron Velocity
(1 0 ^ cm / s )
2.0
peak
velocity
Carrier Mobility (cm 2/ V- s )
electron
2200
1000
1350
8500
hole
1600
50
480
400
Dielectric Constant
5.7
9.7
11.9
12.5
Dielectric Strength ( MV / cm )
10
4.0
0.5
0.6
Thermal Conductivity ( W/ cm . C )
20
5.0
1.45
0.5
8206.0
1137.8
1.0
6.9
32.2
5.8
1.0
0.5
Jo h n so n ’ s Fig. Merit
( ratio to silicon)
K eyes’ Fig. Merit
( ratio to silicon)
Table 2.5. Comparison of sem iconductor properties for diam ond, Si, GaAs, /3Sic.[70]
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Properties
Applications
Hard
Abrasive coatings for cutting tools
Low friction
Tribological applications
Electric insulator
Heat sinks for electronic devices
High therm al conductivity
Large band gap
Microwave power devices
Low dielectric constant
RF electronic devices
High hole mobility
High sp e e d electronic devices
Acid/base resistive
Radiation resistive
Electronic devices for
severe environm ents such a s
in sp a c e or in nuclear reactor
T ransparent
Large refractive index
Figure 2.5.
tions.[70]
Electro-optical devices
Relationship between diamond properties and engineering applica­
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n-type layers were induced by im plantation of carbon ions. However, very low current
gain was obtained by this configuration [72].
In 1987, M. W . Geis and coworkers fabricated high-tem perature point-contact
transistors and Schottky diodes form ed on synthetic boron-doped diamond.
The
boron-doped single crystal diam ond was produced by the high-tem perature highpressure process. The transistors exhibited power gain a t 510 °C and the Schottky
diodes were operational even a t 700 °C [73].
Since 1986 several groups developed m ethods to synthesize boron-doped diam ond
films by th e microwave plasm a CVD technique or the hot-filam ent CVD technique.
A m ixture gas of B 2 H q /C H 4 [74, 75, 76, 77] or a saturation solution of B 2Oz powder
in C H 3 O H m ixed w ith acetone [78] are the m ost often used m ethods. Also, placing
boron pow der [79] or boric acid [51] near the substrate during th e diam ond synthesis
is also used to get boron-doped films.
In 1989, H. Shiomi et al. fabricated m etal-sem iconductor field-effect transistors
(M ESFET) using boron-doped diam ond epitaxial films. This is th e first report of a
planar type transistor using a diam ond film. T he single crystal diam ond substrate
was prepared by the high-tem perature and high-pressure process.
A high-quality
boron-doped epitaxial diam ond film was then obtained on th e single crystal diamond
by a plasm a assisted chemical vapor deposition m ethod. Ti was evaporated for the
source and drain ohmic contacts by the electron beam m ethod. Al was therm ally
evaporated for the gate Schottky contact. Basic transistor operation was observed
[81].
In 1991, K. Okano et al. m ade a diamond p-n junction diode by the hot-filament
CVD m ethod. Since silicon was used as the substrate, th e synthetic diam ond films
were polycrystalline. P2Os and B 2Oz were used for th e doping source. Rectifying
characteristics were observed [82].
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G. Sh. Gildenblat and coworkers fabricated a high-tem perature thin-film diam ond
field-effect transistor by using a selective growth m ethod. A natural single crystal
diam ond was used as th e substrate. Selective grow th of boron-doped hom o-epitaxial
diam ond films was achieved using sputtered S i0 2 as a m asking layer. T he device was
operational a t 300 °C. [83]
W . Tsai, et al. developed a diam ond M ESFET using ultra-shallow rapid therm al
processing (R TP) boron doping. A novel doping technique was used to drive in and
activate boron in n atural ty p e Ila diamond. An ultra-shallow p-doped channel of less
than 500
A was
created by R T P solid-state diffusion using cubic boron n itrid e as the
dopant source. A device pinch-off phenomenon was observed for the first tim e [84].
M. W . Geis and coworkers also found th a t th e metal-.S'zCVdiamond stru ctu re
on (lOO)-oriented substrate is more acceptable for a depletion-m ode m etal-oxide­
sem iconductor field-effect transistor (M O SFET). T ype lib n atu ral diam ond was used
as th e substrate in this work [85].
It is noted th a t commercial fabrication of diam ond electronic devices is not a
reality, however, m any researchers are devoted to optim ize th e deposition processes
and develop techniques for th e fabrication of diam ond devices. For exam ple, the
technique of selective grow th of diamond has been much improved in recent years
[86, 87]. N -type doping has been perform ed by incorporation w ith lithium [88] and
by using F 2 G 5 as a doping source in the process [82]. In term s of achieving heteroepitaxial films, large-area mosaic diamond films approaching single crystal quality
has been obtained on th e silicon substrate [89]. Consequently, significant progress is
being m ade on overcoming technological problem s associated with the fabrication of
diam ond devices.
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CHAPTER 3
F ilm D ep o sitio n and Sam ple
P rep aration
3.1
Introduction
In this chapter, the detailed configuration of the M PD R deposition system will be
explained in chapter 3 (section 2). Then the description of the nucleation m ethods
and th e film deposition process will be presented in ch ap ter 3, section 3 and section 4
respectively. Finally, the fabrication processes of the sam ples for physical characteri­
zation and electrical studies, such as four-point probe sam ples, metal-diamond-silicon
samples, and back-etched diam ond samples, are described in chapter 3 (section 5).
3.2
The M PD R System
3.2.1
The Microwave Cavity
A tunable microwave plasm a disk reactor (M PDR) system [2, 3] is used for microwave
plasm a assisted CVD (M PACVD) diam ond synthesis. T he cross section of th e M PD R
and quartz processing cham ber are shown in Figure 3.1.
38
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Optical Pyrometer
Cavity
Sliding Short
Microwave
Input Probe
Quartz
Chamber
O-ring
Plasma
Silicon
Baseplate
Water
Cooling
Gas Inlet
araphite
Quartz
Substrate
Tube
Perforated /
Stainless Steel Sheet
Stainless Steel
Vacuum
Chamber
Pump
Figure 3.1. The cross section of the M PD R and processing cham ber, and a schem atic
display of the electric (E) and m agnetic (H) field patterns of th e T M qh mode.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The body of th e cavity is a 17.8 cm (7 inch) inner diam eter double-wall brass
cylinder, w ith w ater cooling betw een the walls. T he u pper end of the cylinder is
term inated by a movable sliding short. The baseplate and quartz chamber are in the
lower end of this structure. Microwaves are introduced in to th e system via a short
antenna, which is a coaxial microwave input probe. T he length of the probe inside
th e cavity, L p, is adjustable and serves as one of th e tw o tuning param eters. The
other tuning param eter, th e length of the cavity (Ls), m ay be varied by adjusting
th e sliding short. Using these two degrees of tuning , it is possible to m atch the
im pedance of the cavity/plasm a system to th e microwave pow er source. A perforated
stainless steel sheet is affixed to th e baseplate in the b o tto m p art of the processing
cham ber and serves to term in ate the cavity and still allow gas to flow through.
The baseplate consists of an opening for the discharge cham ber, gas inlets, and
w ater cooling. T he plasm a is centrally contained under a qu artz disk which is vacuum
sealed to th e baseplate by a viton O-ring.
The height of th e discharge cham ber
m easured from the bottom of the baseplate to th e top surface of the quartz disk is
9.5 cm. It is 3.5 cm from th e perforated stainless steel sheet to the stage where the
quartz disk sits on, and 2 cm from th e stage to the top surface of the cavity bottom
respectively. The height of th e quartz disk is 6 cm and its inner diam eter is 9.25 cm.
Thus, th e top of the quartz disk is 4 cm above the cavity b o tto m surface.
T here are 16 small holes which serve as the gas inlets located in the inner surface
of th e baseplate. Pre-mixed gas from m ultiple gas tanks is uniform ly distributed to
each of these gas inlets by a common gas channel, which is located at the inner region
of the baseplate.
W ater cooling of the system is accomplished through th e double wall cylinder
and through a circum ferential channel inside the baseplate. T he la tte r cooling chan­
nel protects the viton O-ring. Also, the baseplate is designed to allow low pressure
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electron cyclotron resonance (ECR) operating conditions. For such operation, eight
rare-earth m agnets are placed inside th e baseplate. [6] Since th e m agnets are sensi­
tive to high tem perature, cooling is achieved through the w ater channel surrounding
the m agnets. However in this work, plasm a pressures were too high to allow ECR
operation, so th e m agnets were removed.
3.2.2
The D eposition System
The input microwave system consists of a 2.45 GHz Cheung pow er source (model
MPS2450-1200), a three port circulator, a m atched dum m y load, a cross waveguide
directional coupler, power m eters and waveguides as shown in Figure 3.2 [90]. The
Cheung microwave power source can produce stable power from 200 to 1200 w atts.
The ou tp u t of th e microwave source is attached to a three p o rt circulator, which
protects th e m agnetron from reflected power by directing radiation tow ard th e m i­
crowave source into a 50 fi m atched dum m y load. Incident and reflected power are
sampled by a cross waveguide direction coupler and are fu rth er dim inished by 30 db
attenuators before being m easured by th e power meters. Microwave power is then
coupled from a waveguide to th e cavity input probe.
The H P 435A microwave power m eters are used to m easure incident power P,and reflected power Pr at th e cavity. T he microwave power coupled into the plasm a
loaded applicator, which is th e microwave absorbed power, is given by Pa = P,- - Pr.
By tuning th e wall probe and sliding short, Pr is generally less th a n or equal to 10
% of P{. Due to losses in the input system , waveguide and cavity, about 15 % of the
power is dissipated as heat. Consequently, the microwave absorbed power is about
75 % of th e th e power supplied from th e microwave source.
Figure 3.3 gives a schem atic diagram of the M PD R CVD system . Up to four
different gases m ay be mixed by a 4-channel (model MKS 1159A) m ass flow controller.
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0
2 .4 5 G H z
M icrow ave
P o w er S o u rc e
30 dB
C irculator
Incident
P o w er
C avity
A pplicator
D irectional
C o u p ler
Pi
Pr
W av eg u id e
3 0 dB
D um m y L oad
R eflected
P o w er
Figure 3.2. The microwave power source, circuit, and cavity applicator.
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The flow controllers have m axim um operating flows of 10,000, 500, 100, and 10 seem
respectively. T he gas output of each m ass flow controller is m ixed simultaneously into
a single gas channel which feeds th e quartz processing cham ber through the baseplate.
A large stainless steel vacuum cham ber is below th e baseplate and quartz chamber.
In some cases, this could be used as a dow nstream processing cham ber, although it was
not used as such in this research. [6] Pressure m easurem ent w ithin the vacuum system
is perform ed through four vacuum gauges. There are th ree gauges connected to the
stainless steel vacuum chamber; a baratron pressure gauge, a th erm al conductivity
vacuum gauge, and an ion pressure gauge. T he baratron pressure gauge (model MKS
122A) can determ ine pressure from 0.1 Torr to 1000 Torr. The therm al conductivity
vacuum gauge (TC2) (model MKS 286) can accurately m easure pressure from 1 m Torr
to 1 Torr, and th e ion gauge (model MKS 290) is able to m easure in th e high vacuum
region (10-3 to 10-9 Torr). These th ree different pressure gauges can therefore cover
the range from very good vacuum to above an atm osphere depending on the operating
conditions. In addition, a second therm al conductivity vacuum gauge (T C I) with the
same operating pressure range is located between the roughing valve and roughing
pum p (mechanical pum p).
T he system is pum ped down by a Alcatel 2033 roughing pum p. The mechanical
pum p is filled w ith Alcatel 200 oil which is safe for pum ping hydrogen and m ethane
gases. In order to minimize th e backstream oil from the pum p, a baffle trap is placed
between the pum p and the cham ber. Because the hydrogen and m ethane are explosive
gases, nitrogen gas is also used to purge the gas exhaust for safety reasons.
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44
Mcrowave
MPDR 300
Mcrowave Cavity
Flow Controller
Source
M u tt : MKS USB A )
Sliding Short
Baseplate
W ater Cooling
Probe
G as
Inlet
Diamond Rim Processing
Quartz Chamber
G as
Roughing
Valve
Vacuum Cham ber
Baratron Pressure Gauge
Flow Controller
( U oM : U K S 122A )
Ion P ressure Gauge
( Mb<M : M KS 290 )
System Vent
G as
TC1
TC2
( F oreline) ( C ham ber)
Thermal Conductivity
Vacuum
G auge
Thermal Conductivity
Vacuum
G auge
I
( MbcM : MKS 2SS )
Baffle Trap
uom
:
u k s
a s )
Exhaust
G as Balast
To Air
Purge
Roughing Pump
Uodtl : AkMU 203J )
Figure 3.3. T he diagram of th e M PD R CVD system.
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3.3
The Nucleation M ethods
A wide variety of nucleation m ethods are described in chapter 2. In this research, two
approaches were used to do th e sample polishing and cleaning before th e diamond
synthesis. They are 1. t h e d ia m o n d p a s te n u c le a tio n m e th o d , and 2. t h e d ia ­
m o n d p o w d e r n u c le a tio n m e th o d . The two nucleation m ethods will be described
in detail as follows.
T h e d ia m o n d p a s te n u c le a tio n m e th o d :
1.
0.25 n m Buehler M etadi synthetic diam ond paste was placed on a Buehler
TE X M E T polishing cloth sheet, which is w ater and oil resistant.
Then Buehler
M etadi lubricant oil was sprayed onto the paste. T he su b strate was placed onto the
diam ond paste position and was gently polished by hand for about 20 m in by using
rubbing back and forth m otions.
2. Step 1 produces a su b strate surface which has a d irty appearance. A fter the pol­
ishing, a kimwipe w ith some acetone was used to get rid of th e dirty stain on the
substrate surface as much as possible.
3. N ext the substrate was placed in a boiling T C E solution for 3 m in. T hen it was
rinsed im m ediately in running acetone, running m ethanol, and running de-ionized(DI)
w ater for about 60 sec each. Finally it was dried w ith nitrogen gas (N 2 ).
4. The substrate was then im m ersed in a freshly prepared degrease etch ( 5 H 2 O :
I H 2 O 2 : 1N H 4 OH ) for 10 m in at about 70 °C to remove residual organic contam i­
nation left over from the solvent cleaning. Then it was rinsed in DI w ater and dried
w ith N 2.
5. The substrate was next im m ersed in a freshly prepared de-m etal etch ( 8 H 2 O :
2
H 2 O 2 : 1H C l) for 10 m in a t about 70 °C to remove ionic and m etallic contam i­
nants. Again, it was rinsed in DI w ater and dried w ith N 2 .
Step 4 and 5 from the RCA cleaning procedure are widely used in the silicon industry.
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[91]
6. A fter th e RCA clean, the substrate was placed in a 9:1 H^O'.HF diluted solution
in order to get rid of th e th in oxide layer. This was again followed by a rinse in DI
w ater and a dry w ith A^.
The substrate a t this point was ready for th e diam ond film deposition. If the
substrate was a silicon wafer polished to a m irror finish, then after step 6 one cannot
see scratches easily on th e shining surface by eye unless it is observed under a strong
light. No diam ond particle residue was found either by th e optical microscope or by
the scanning electron microscope as shown in Figure 3.4.
The diamond powder nucleation m ethod :
1. The substrate was cleaned by following step 3 to step 6 of th e diam ond paste
m ethod.
2. Dry 1 /zm n atu ral diam ond powder was spread on a 3-inch diam eter sacrificial
silicon wafer which was used as a lapping surface. The su b strate surface was then
prepared by gentle polishing by hand on th e sacrificial wafer for about 2 min.
3.
Following th e polishing process, th e substrate receives an ultrasonic ace­
tone/m ethanol cleaning for 5 min respectively.
4. T he substrate also received a diluted H F solution treatm en t, DI w ater rinse and
blow dry w ith nitrogen.
A t this point the substrate was ready for th e diam ond film deposition. If the
substrate was originally m irror-polished silicon, after step 4 one can easily see a
m arked difference of the surface. The surface is filled w ith a lot of scratches and is
now hazy com pared to th e previous mirror-like surface. U nder the optical microscope
and the scanning electron microscope, th e surface is seen to be rough and full of
scratches as com pared to the diam ond paste nucleation m ethod. In Figure 3.5 one
can easily see th e difference of the surface prepared by the powder nucleation m ethod.
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15KU X10000
0001
'
1 . 0U CE091
Figure 3.4. SEM pictures of th e silicon substrate prepared by diamond, paste method.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
W hat are the effects of th e nucleation m echanism on th e diam ond film growth?
In other words, is there any difference on th e film growth after different surface
treatm ents? The answer is yes! It will be described in the next section.
3.4
Diamond Film Deposition
3.4.1
Operation of The System
In th e M PD R , a microwave (M W ) plasm a is generated w ithin a resonant cavity. There
are several advantages over radio-frequency (rf) and direct current (dc) plasm a. For
a given power input and plasm a size, th e electron density is higher in a MW plasm a
by virtue of th e higher frequency, so its reactivity is expected to be very high. Since
MW plasm a can be electrodeless, electrode contam ination can be elim inated.
Although the electrom agnetic modes of a perfect cylindrical cavity are well known
[92], the introduction of a plasm a into the cavity can significantly alter th e field
geometry. However, the M PD R can create a microwave discharge when excited in a
single cavity electrom agnetic mode. For example, in the p ast experim ental discharges
have been sustained in the T E m [93], T £ 2 1 1 [3], T M 012 and T M qw modes [94]. Each
m ode was experim entally evaluated for its potential to deposit films at discharge
pressures of 30 - 90 Torr. T he cavity applicator was first length (L s) and probe (Lv)
tuned to a specific mode. T hen a discharge was started a t a gas pressure of 5 - 10
Torr, by applying microwave power which then ignited a discharge th a t entirely filled
th e quartz processing cham ber. The discharge pressure was then increased to an
operating condition of 30 - 90 Torr by manually adjusting the roughing valve while
length and probe tuning the cavity to a m atched condition. It was found th a t the
T M qu m ode is superior to th e others in this pressure range and set-up. [10]
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Figure 3.5. SEM pictures of th e silicon surface prepared by diamond powder method.
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The experim ental p aram eter space is large, since the microwave power, C H ^ f H z
ratio, substrate m aterial, shape of the substrate, position of the substrate, discharge
pressure (plasm a pressure) all m ay contribute to th e quality of th e diam ond depo­
sition.
A system atical operation procedures should be conducted in order not to
inadvertently introduce new param eters. Consequently, a well defined procedure of
operation for ’’turning on” and ’’shutting off’ th e M PD R CVD diamond system was
developed as follows.
The procedures of turning on the system :
1. T he substrate received th e surface treatm en t either by diam ond powder nucleation
m ethod or diamond paste nucleation m ethod before p u ttin g it into th e quartz pro­
cessing chamber. The size of the substrate was always 2 cm x 2cm for silicon and
1.2 cm x 1.2 cm for silicon nitride.
2. T he system stand-by condition is under vacuum . So th e first step was to close the
baratron valve, open th e system vent valve, and allow th e nitrogen gas to flow into
th e quartz chamber until th e cham ber pressure reaches atm osphere. Then th e quartz
disk and the sliding short are removed from th e system.
3. Silicon substrates were typically m ounted on a 0.1 inch thick graphite holder with
a 2 cm x 2 cm recessed area. The graphite holder was placed on a quartz tube, ty p i­
cally 3.25 cm high, which stands on the perforated stainless steel sheet, contained in
th e quartz processing cham ber. T he silicon nitride substrates were m ounted directly
on the quartz tube w ithout a graphite susceptor. In both cases, the substrates were
in a position along the cavity axis.
4. T he quartz disk was positioned such th a t it sm oothly fit the viton O-ring. The
system vent valve was closed and th e processing cham ber was evacuated by the rough­
ing pum p with an u ltim ate pressure of 10-4 Torr. T he baratron readout was then
adjusted to zero.
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5. The sliding short was placed in th e cavity, then the cavity length L a was set to
approxim ately 7 cm which is th e position for th e T M o u m ode. T he probe position
L p was set a t about 1.8 cm which corresponds to m axim um absorbed power and m in­
im um reflected power.
6. A fter th e reading in both of th e therm al conductivity vacuum gauges (T C I, TC2)
was alm ost zero, gas was introduced into the discharge cham ber. 99.999 % H 2 and
99.99 % C H \ Airco gases are used in this research. The flow rate and ratio of the
gases were m onitored through th e 4-channel gas flow controller. T he nitrogen purge
and w ater cooling were on from this point throughout th e whole process .
7. T he roughing valve was ’’th ro ttle d ” to increase th e cham ber pressure up to 5 to 10
Torr as m easured by the baratron gauge, then MW power was applied. A t ignition
th e discharge completely filled th e quartz cham ber, but as the pressure increased to
30 - 90 Torr (It takes about 5 - 1 5 m in), the discharge contracted and separated from
th e surrounding quartz walls and form ed a roughly semi-spherical shape when the
T M o u m ode was used. D uring th e plasm a pressure increase, cavity length and probe
tuning are still necessary to minimize th e reflected power. Visual inspection showed
th a t the discharge uniformly covered th e whole substrate under this particular mode.
Typically, th e microwave inp u t power and plasm a pressure are in th e range of 400 900 w atts and 50 - 80 Torr respectively.
8. Typically the system ran for 5 - 10 hours in order to get th e desired thickness of
th e diam ond films. For the duration of th e run, the roughing valve was m onitored to
provide the desired plasm a pressure.
O peration of the M PD R CVD system during the film growth is shown in Fig­
ure 3.6. One can see the discharge glowing inside th e cavity through a transparent
metal-screened side window.
The procedures of shutting off the system :
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i)2
tV .rv rm ir
Figure 3.6. T he M PD R CVD system during the diamond film deposition.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1. The M W power was reduced from several hundred w atts down to 0 im m ediately and
the gas flow of both hydrogen and m ethane gases were tu rn ed olf. Also the roughing
valve was fully open to rapidly elim inate th e gases from the discharge cham ber. At
this point, th e T C I, TC2, and baratron pressure gauges all read alm ost zero.
2. A 20 m in period was allowed to let th e system cool down. Then th e w ater cooling
was turned off and th e system was vented to nitrogen and th e sam ple was removed.
Unless otherwise noted, these were th e procedures for startin g and ending a de­
position process.
The procedure to term inate the deposition process is crucial to th e question of
form ation of a surface conducting layer on top of the diam ond film. For example,
if reactive gases continue to flow while th e substrate is cooling, a graphite layer
m ay be formed. Early in th e research, another approach was used to tu rn off the
system. F irst, th e m ethane flow was turned off to prevent graphite form ation on the
surface of the diam ond film bu t the hydrogen was left on. Second, th e hydrogen flow
was gradually reduced,and then the microwave power was turned down immediately.
However, R am an characterization showed a graphite peak instead of the diamond
peak on one of th e samples under this procedure. The energy of th e hydrogen plasm a
is high enough to convert diam ond into carbonaceous conducting layer. [32, 79]
3.4.2
Experimental Parameters
The experim ental deposition param eters varied in this research are sum m arized as
follows.
M e th a n e C o n c e n tr a tio n : The percentage of m ethane in the m ethane/hydrogen
m ixture was varied from 0.5% to 1.5%.
M ic ro w a v e P o w e r : T he microwave input power was varied from 400 W to 900 W.
P la s m a P r e s s u r e : T he plasm a pressure range investigated was from 50 to 80 Torr.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
G a s F lo w : Total gas flow was varied from 100 seem to 250 seem.
S u b s tr a te P o s itio n : T he quartz pedestal length was varied from 2.5 cm to 3.5 cm.
A dditional deposition param eters, as described elsewhere in this chapter, are the
choice of substrates (silicon or silicon nitride) and th e nucleation procedures (dia­
m ond paste m ethod or diam ond powder m ethod). It should be noted th a t there is a
big difference between the deposited diam ond films which are prepared by diam ond
powder m ethod and diam ond paste m ethod. Large grain (greater than 1 /m i) size
diam ond films were obtained by th e diam ond paste prepared m ethod, and small grain
(sub-micron) size films were achieved by the diam ond powder prepared m ethod. A
more detailed study of th e grain size difference will be discussed in chapter 4.
3.4.3
Substrate Temperature
In the last subsection, it was noted th a t during the film grow th th e position of the
substrate, flow rate, C H 4 / H 2 concentration, microwave power, and plasm a pressure
are all experim ental param eters. Each of these m ay contribute to the film proper­
ties either directly or indirectly in th a t they affect the su b strate tem perature. In
this research, the substrate was self-heated by the plasm a and microwave power.
Consequently, changing the deposition conditions generally changed the substrate
tem perature.
T em perature is a very im portan t param eter in any CVD process, including dia­
m ond film deposition. Most often, good quality diam ond films are deposited with
a substrate tem perature between 800 °C and 1100 °C [10-60]. Usually, there are
two ways to determ ine the sub strate tem perature. One way is to determ ine th e sub­
strate tem perature by direct contact as for exam ple by a therm ocouple through the
substrate holder, which is on the down side of the substrate. However, microwave
coupling to th e therm ocouple wire is a potential problem in this approach. A nother
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way is to determ ine the tem p eratu re w ithout direct contact as for exam ple w ith an
optical or infrared pyrom eter. For a microwave applicator one m eans of optical access
is through th e system window, which is on the top side of th e substrate. Generally
th e windows are shielded by a m etal screen which provides microwave power leakage
protection. In this case the m easurem ent goes through th e m etal screen, transparent
window quartz wall, and discharge to the substrate. Pyrom eter calibration is always a
questionable m a tte r in such a situation. The tem peratures reported for good quality
diam ond films by direct contact w ith a therm ocouple (800 °C to 950 °C) are often
lower th an those reported by th e pyrom eter m ethods (900 °C to 1050 °C).
In our design, th e original sliding short on the M PD R can be replaced w ith a short
constructed w ith an axial opening to allow vertical optical access to th e substrate
as shown in Figure 3.7. A brass collar is used for shielding the MW leaks. The
advantage of this set-up is th a t one can m easure the tem p eratu re through only quartz
and plasm a to the substrate. W ith o u t a m etal screen, calibration for the measuring
device is much easier and tem p eratu re m easurem ent accuracy improved. Generally
these vertical pyrom eter m easurem ents produced higher (100 °C - 150 ° C ) readings
than pyrom eter readings through th e m etal screened window.
An Ircon Ultim ax optical pyrom eter (model UX-20), w ith working tem peratures
between 600 to 3000 °(7, was used for m easuring th e su b strate tem perature. This
is a small spot size instrum ent which has a spot size approxim ately equal to D /100
m m where D is the focusing distance. This device has a focal range from 500 m m to
infinity, b u t close focus lenses can be used to reduce the focal range. There are four
different close focus lens w ith focal ranges of 100 - 130 m m , 130 - 180 mm, 180 - 290
m m , and 250 - 540 mm respectively. The unit was generally used with a 370 m m
focus distance, which corresponds to a spot size of 3.7 m m . This makes it possible to
m easure the tem perature distributions across the substrates.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Optical
Pyrometer
P
i i
I
,
f
s
r
-
i -i.
■ -*-!t »
..v .r* .
1 \
I »
T --4
I I
i i,
; -1i. i. •
I
i
i
Circular
Opening
t '
I
I
B rass
Collar
j
Sliding Short
Cavity
Quartz
Disk
Plasm a
G as Inlet
( 16)
Quartz Tube
Substrate and
Graphite Holder
Perforated
Stainless Steel
Sheet
to Vacuum Chamber
Figure 3.7. Top view of the M PD R processing cham ber.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Each m aterial has different emissivity, which is th e m ain calibration factor for
this device. W hen the silicon emissivity of 0.89 and th e silicon nitride em issivity of
0.92 is m ultiplied by the quartz correction factor of 0.95, the corrected emissivity
comes to 0.85 and 0.88 for m easuring th e tem peratures of silicon and silicon nitride
respectively. Then th e calibration is also confirmed using silicon wafers a t com parable
tem peratures in therm al diffusion furnaces.
For several runs a t various deposition conditions, 9 spot m easurem ents were m ade
on each substrate. The corner and side m easurem ent points were approxim ately 2
m m from th e edge of the substrate. Figure 3.8 (a) shows a typical tem p eratu re profile.
However th e uniform ity is quite dependent on substrate position and cavity tuning.
In some cases much b e tte r profiles were observed. A b e tte r uniform ity case is shown
in Figure 3.8 (b). Generally, all points showed tem peratures w ithin plus/m inus 5 %
of th e center tem perature. However, b e tte r uniform ity was sometime achieved within
less than 1 %.
Figure 3.9 and Figure 3.10 show th e substrate tem p eratu re dependence on power
for 1 % and 0.5 % C H ^ / H ^ concentrations for two different plasm a pressures, 50 Torr
and 60 Torr. A t both cases, a 2 cm x 2 cm silicon wafer was placed on a 0.1 inch thick
graphite holder which is supported by a 3.25 cm high quartz pedestal. T here is no
obvious tem perature difference for the 1 % and 0.5 % concentration depositions. The
tem perature increases w ith increasing microwave power in a nearly linear fashion.
Figure 3.11 and Figure 3.12 show the the sam e d a ta p lo tted so as to dem onstrate
the pressure dependence of substrate tem perature. W ith higher plasm a pressure,
higher substrate tem peratures, were observed. A t higher pressures, th e plasm a ball
shrinks and th e energy is concentrated nearer th e substrate. In all the above cases, the
microwave input powers listed are shown as m easured by th e Cheung microwave power
supply. The actual power delivered into the cavity is about 75 % of the microwave
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
( a)
905° C
930° C
96CTC
9 1 0 °C
950° C
980° C
9 1 5 °C
950° C
985° C
M icrowave
P robe
S c re e n e d Window
1070°C
1070°C
1060°C
1075°C
1076°C
1080° C
1080°C
1085°C
1090°C
M icrowave
P robe
S c re e n e d Window
Figure 3.8. (a) Typical tem p eratu re profile and (b) b etter tem perature uniform ity of
th e silicon substrate.
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Substrate Temperature (°C)
1080
1060
Substrate m a te ria l: S i
Plasm a pressure : 50 Torr
1040
1020
1000
980
960
940
450
500
550
600
650
700
750
800
Microwave Input Power (W)
Figure 3.9. Si substrate tem peratu re vs. microwave input power for 1 % and 0.5 %
m ethane concentration at 50 Torr.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1120
Substrate Temperature (°C)
11 0 0
Substrate m a te ria l: Si
Plasm a pressure : 60 Torr
1080
1060
1040
1020
1000
980
960
940
350
400
450
500
550
600
650
700
750
800
850
Microwave Input Power (W)
Figure 3.10. Si substrate tem peratu re vs. microwave in p u t power for 1 % and 0.5 %
m ethane concentration at 60 Torr.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
powers from th e Cheung microwave source.
W hen silicon nitride substrates were used, a 1.2 cm x 1.2 cm x 0.5 cm substrate
was supported on a 2.75 cm high quartz pedestal w ithout a graphite holder. Fig­
ure 3.13 shows the tem perature difference between silicon and silicon nitride for 0.5
% m ethane concentration a t 50 Torr. The substrate tem p eratu re of silicon nitride
increases alm ost linearly w ith microwave power and is higher th an th a t of silicon at
the sam e deposition conditions. T h e reason for the higher tem p eratu re w ith silicon
nitride is th a t the surface area is sm aller than th e silicon. T he plasm a will then con­
trib u te m ore energy per unit area on th e silicon nitride sam ples as compared to the
silicon samples.
3.5
Fabrication of Electrical Samples
3.5.1
Four-Point Probe Samples
Silicon nitride, which is an insulating composite ceram ic m aterial, was used as th e
substrate for fabricating four-point probe samples. As described in chapter 3 (section
4), a 1.2 cm x 1.2 cm x 0.5 cm silicon nitride sample was placed directly on a 2.75
cm high quartz pedestal for deposition. Both diam ond pow der and diam ond paste
nucleation m ethods were used to prepare the substrate. T he as-received silicon n itride
substrates showed an uneven surface upon microscope exam ination, w ith m ultiple
trenches and ridges which were on the order of a few m icrom eters wide and deep.
However, th e deposited diam ond film followed the surface topography and successfully
covered this uneven surface.
As shown in Figure 3.14, as-deposited diamond films were ready for the four-point
probe m easurem ents which determ ines the sheet resistance of th e diam ond films.
Results of th e 4 point probe resistivity m easurem ents will be discussed in chapter 5
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1120
Substrate Temperature (°C)
1100
60 Torr
. 50 Torr
Substrate m a te ria l: Si
C H 4 / H 2 concentration : 0.5%
1080
1060
1040
1020
1000
980
960
940
350
400
450
500
550
600
650
700
750
800
850
Microwave Input Power (W)
Figure 3.11. Si substrate tem p eratu re vs. microwave input power for 50 and 60 Torr
at 0.5 % m ethane concentration.
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1100
Substrate Temperature (°C)
Torr 4 Torr
1080
Substrate m a te ria l: Si
C H ^ / H i concentration : 1%
1060
1040
1020
1000
980
960
940
350
400
450
500
550
600
650
700
750
800
Microwave Input Power (W)
Figure 3.12. Si substrate tem p eratu re vs. microwave in p u t power for 50 and 60 Torr
at 1 % m ethane concentration.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Substrate Temperature (°C)
1100
C H 4 / H 2 concentration : 0.5%
Plasm a pressure : 50 Torr
1050
1000
950
900
250
300
350
400
450
500
550
600
650
700
750
Microwave Input Power (W)
Figure 3.13. T em perature difference between silicon and silicon nitride for 0.5 %
m ethane concentration at 50 Torr.
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(section 2).
3.5.2
The M etal/D iam ond/Silicon Samples
For th e m etal/diam ond/silicon samples, both n and p-type silicon substrates with
(100) orientation and resistivity of 1-2 fl*m were used, w ith a area of 4 c m 2. Some
samples were prepared w ith th e diam ond paste nucleation m ethod and some by the
diam ond powder nucleation m ethod. A fter deposition, a shadow m ask w ith 900 fim
diam eter openings was used to form m etal contacts on th e top of th e diam ond films.
Typically, A1 contacts were m ost often used in th e diam ond paste prepared samples
as shown in Figure 3.15. B ut on the diamond powder prepared samples, a shadow
mask w ith smaller openings (400 fim) was used. Au, Ag, and In were used as the top
m etal contacts in this case. T he electrical properties will be described in chapter 5
(section 4).
3.5.3
Back-Etched Samples
For film characterization, it is useful to have access to both sides of the film. Towards
this purpose, a procedure suggested by Dr.
J. Engem ann from the University of
W uppertal was used. Some of th e preparation details were developed by H. Keller at
the U niversity of W uppertal. Specifically, after th e diam ond deposition on th e silicon
substrate, a back-etched process was performed to transfer the diam ond film from
th e silicon substrate to an epoxy substrate.
For samples intended for back surface analysis, the coated silicon was cleaved into
approxim ately 5 m m x 5 m m samples which were placed diam ond film down into
a T o r r — S e a l ™ [96] m ixture formed by 1 p art of hardener and 2 p arts of resin
on an alum inum oxide substrate. After a 70 °C anneal for 30 m in, th e silicon was
removed by etching. Both NaOH (25 % by weight to w ater) and KOH (44 % by
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(a)
N u cle a te d
S u rfa c e
Silicon Nitride
( Si3 N 4 )
D iam ond Film
Silicon Nitride
(S i3N 4 )
Figure 3.14. Fabrication of th e four-point probe samples. T h e actual surface was more
uneven than is indicated here, (a) T he starting point is a silicon nitride substrate
which has received a nucleation procedure, (b) Since the su b strate is insulating, the
sample is ready for the 4 point probe m easurem ent.
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Nucleated
Surface
Silicon
(b)
Diamond Film
•Silicon
(c)
m m rn .
\m
m
i
Al Dot
•Diamond Film
Silicon
Figure 3.15. Fabrication of the m etal/diam ond/silicon samples, (a) The surface is
nucleated by diam ond powder or diam ond paste, (b) the diam ond film is deposited,
and (c) top contacts are evaporated through a shadow m ask. Aluminum is shown
here as an exam ple.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
weight to w ater) were successfully used w ith a b ath tem p eratu re of 58 °C. For a
substrate thickness of 15 m ils, it takes about 25 hours to etch th e silicon. W ith
lower tem perature, longer tim e is needed to finish th e etching process.
There is
sufficient adhesion between th e top polycrystalline surface of the diam ond film and
the epoxy th a t th e intact film rem ains, attached to th e hardened epoxy substrate,
thereby providing th e back surface for analysis as shown in Figure 3.16. In some
cases, however, film peeling and wrinkling were observed from th e epoxy, indicating
either films w ith higher internal strain or poor film-epoxy adhesion [95]. Some of the
diam ond/silicon interface (back-etched diam ond surface) characterization results will
be discussed in chapter 4.
For electrical access to b o th sides of th e film, th e sam ple preparation procedure
was modified as follows. A fter diam ond deposition on th e silicon substrate, the first
(bottom ) m etal contact was evaporated onto the diam ond surface through a shadow
mask. Then a wire lead is attached to th e evaporated m etal w ith Epo — T e k ™
polym ide silver conducting epoxy [97] which is cured first for 30 m in a t 50 °C and
then 60 m in at 150 °C. The sam ple is placed top down in th e T or r — S e a l ™ epoxy
and the silicon is removed by back etching. A t this point, the evaporated m etal
contact is visible through th e tran sp aren t diam ond film, and the second diam ond
surface is available for electrical contacts as shown in Figure 3.17. The second (top)
m etal contacts then can be evaporated onto the back-etched surface through another
shadow m ask w ith much sm aller openings. A careful study of diamond film samples
prepared by this m ethod will be discussed in chapter 5 (section 3).
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U J7
(a)
Diamond Rim
— Silicon
( P or N ty p e )
Silicon
Torr Seal
Diamond Rim
W A W .V .V
■ w
(c)
Torr Seal
Diamond Rim
/
s S > SS
A ■*'*•>.'■ i* .■«■*>
^ .W W W W X fJO ^ -M
^ # !® ^ ^ ^ ^ ^ ^ ^ ^ ^ W ^ ^ ^ ^ ^ ^ S !8 !8 ^ S S 8 ® S S S ® S ® 3 S S * S S !3 S s S S S 8 S S S S S S § S
'Al20 3
Figure 3.16. Fabrication of back-etched diam ond samples for surface analysis, (a)
Silicon is coated with diam ond using conventional m ethod, (b) The sam ple is secured
to an epoxy substrate, (c) T h e silicon is removed by chemical etching.
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— Diamond Film
. Silicon
( P or N type)
- First Metal Contact
- Diamond Film
- Silicon
(b)
(c )
Ag Epoxy.
- • First Metal Contact
- Diamond Film
—Silicon
/
Contact Wire
Contact Wire
mrnm
Ag Epoxy
Torr Seal
Diamond Film
First Metal Contact
( Bottom Metal Contact)
Silicon
Contact Wire
-+ - Diamond Film
-
-
ai2o 3
Figure 3.17. Fabrication of dual-sided m etal contacts on isolated diam ond films, (a)
T h e silicon is coated w ith diam ond and (b) a m etallic contact is evaporated on the
first diamond surface, (c) A wire lead is attached to th e evaporated m etal and (d)
th e substrate is placed face down in epoxy, (see also Engem ann, et’ al. [95])
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■(e)
Torr Seal
\
C o n tact Wire
D iam ond Film
_____________
C o n tact W ire
\
^ rwTiTil rFrFR ^>7^ n 5TWnr7r7? «
\ ^
Ag Epoxy
^
S e c o n d Metal C o n tact
( T op Metal C o n ta c t)
^
_
r~ '
D iam ond R im
First Metal C o n ta ct
( Bottom Metal C o n ta c t)
Torr S eal
C o n tact W ire
- D iam ond Film
Figure 3.17 (cont’d): Fabrication of dual-sided m etal contacts on isolated diam ond
films, (e) T he silicon is removed by etching and (f) m etal contacts are evaporated on
the second diam ond surface.
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CHAPTER 4
P h ysical C h aracterization
4.1
Introduction
Physical characterization of th e deposited film is very im p o rtan t to determ ine w hether
the film is in fact diam ond and if it is indeed diam ond, the film quality.
There
are several m ethods which m ay be used for film characterization, such as Ram an
spectroscopy, X-ray diffraction, reflection high-energy electron diffraction (RH EED ),
scanning electron m icroscope (SEM ), X-ray photoelectron spectroscopy (X PS), Auger
analysis, and others. In this chapter, we will use R am an spectroscopy to study th e film
quality in term s of sp 3 bonding, XPS to perform surface analysis, SEM to exam ine
th e surface morphology, a D ektak profiler to evaluate th e surface profile, and laser
scanning microscopy to observe the film uniformity. D etailed descriptions of these
m ethods are in the following sections.
72
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4.2
Raman Spectroscopy
R am an spectroscopy is based on an inelastic light scattering process. T he incident
photons, which are m onochrom atic, interact w ith th e m aterial either by creating or
annihilating one or several phonons and then emerge w ith energies different from
th e incident photons.
As in Figure 4.1, the strong line centered a t h v is due to
elastic scattering of photons and is known as Rayleigh scattering. The weak lines
a t h v ± h 8 originate from inelastic scattering of photons by phonons and constitutes
th e R am an spectrum. T he R am an bands at frequencies u — 8 are called Stokes lines,
corresponding to phonon generation, and those a t frequencies v +
anti-Stokes lines, corresponding to phonon absorption.
8
are known as
T he intensity of th e anti-
Stokes lines are usually considerably weaker th a n those of the Stokes lines [98] and
th e R am an spectra reported in this chapter are based on th e Stokes-line signals.
Figure 4.2 shows th e R am an spectra for diam ond and silicon on the absolute wave
num ber scale and on the relative wave num ber scale. Most traditionally R am an spec­
tra on the relative wave num ber scale were presented. For example, in this research,
a wavelength of 488 nm laser was used by the R am an operators. It corresponds to
th e photon energy of 2.541 eV and wave num ber at 20492 cm -1 . Since only the
first-order Ram an scattering, which involves th e optical phonons w ith wave vector k
~ 0, is considered, for diam ond the energy of optical phonon involved is 0.165 eV.
Considering only the S to k es signal, the photon energy becomes 2.376 eV after the
phonon generation. Photon energy of 2.376 eV corresponds to wavelength of 522 nm
and wave num ber of 19160 cm -1 . A strong peak of diam ond is then observed at 19160
cm -1 if an absolute wave num ber scale is used. It is 1332 cm -1 , which is called the
R am an shift for diam ond, between 19160 cm -1 and 20492 cm -1 . T he reported natural
diam ond peak at 1332 cm -1 is on the relative wave num ber scale [99]. Similarly, for
silicon the optical phonon energy is 0.063 eV. A fter calculation, the silicon peak will
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Raman Spectroscopy
(a)
Argon
Laser
E la stic S ca tte rin g
hi/
hi/
hi/ + h <5
hi/ -
Sample
h6
j Inelastic S ca tte rin g
h6 s
: O p tic a l phonon energy
0>)
R a y le ig h S pectrum
Raman Spectrum
Raman Spectrum
( Stokes line )
( A nti-S tokes line )
JL
hv
-
h <5
hi/
hi/ + h 6
Figure 4.1. R am an spectroscopy is based on a process of inelastic scattering of photons
by phonons.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
be a t 19984 cm -1 on the absolute wave num ber scale and 508 cm "1 on th e relative
wave num ber scale.
It is also known th a t graphite shows broad peaks a t about 1360 cm -1 , 1590 cm -1
[100] and amorphous carbon a t about 1600 cm -1 [41]. Diamond-like carbon, which is
am orphous b u t has properties close to those of diam ond, is reported to have a broad
peak at about 1500 - 1550 c m '1. [101, 102, 103] T he larger the R am an signal at
1332 cm -1 and the sm aller th e other peaks, th e higher quality of the diam ond films
in th a t th e results indicate a preponderance of sp 3 bonding (diamond) ra th e r than
sp2 bonding (graphite). A t th e present tim e, R am an spectroscopy is considered to
be th e best technique to distinguish between th e diam ond, diamond-like carbon and
graphite.
Dr. Kevin Gray in N orton Company (N orthboro, M assachusetts) perform ed most
of th e m easurem ents for th e R am an spectrum analysis. T he principle set-up of the
instrum entation is illustrated in Figure 4.3 [104]. T he m onochrom atic and polarized
light of the laser passes through an interference filter th a t rejects spurious lines and
background from the laser source. The light beam th en enters the beam -splitter and
is focused by the lens to th e samples. If m icro-Ram an analysis is perform ed, the
microscope shown in Figure 4.3 is added to reduce th e beam size. Light scattered
from th e sample through a variable beam -splitter th en is focused by th e lens onto the
entrance slit of the 1/8 m eter double-grating m onochrom ator. Before entering the
m onochrom ator, a notch filter was used to reduce th e laser light. The double-grating
m onochrom ator acts as a tunable filter of extrem ely high contrast; its purpose is to
prevent the internally scattered intense Rayleigh light from overpowering th e weak
R am an lines. Light leaving th e final slit of th e double-grating m onochrom ator is
collected by a 1024 elem ent silicon-array detector, whose o u tp u t is processed w ith a
” photon counting” electronics. It includes a program m able detector controller and
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Laser
D iam ond
>N
-t—•
to
c
Silicon
Q)
C
19160
19984
20492
A b so lu te W a v e N um ber ( cm '1 )
(b)
D iam ond
to
&
c
Silicon
508
1332
R elative W ave N um ber ( cm '1 )
Figure 4.2. Ram an spectra for diam ond and silicon on (a) absolute wave num ber
scale and (b) relative wave num ber scale.
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an oscilloscope, which is used for m onitoring th e real tim e signal synchronously with
the spectrom eter. Finally, th e d a ta processing and d a ta acquisition were collected
and processed w ith th e personal com puter.
In this work, typically a wavelength of 488 nm from an Argon laser and a laser
power of 400 m W was used to excite th e Ram an signals.
In Figure 4.4 R am an
spectra of 0.5 % C H ^ / H 2 powder-polished samples are com pared a t different substrate
deposition tem peratures. All spectra show the characterization peak at 1332 cm -1 .
At 936 °C th e 1332 cm -1 peak is quite small and a broad shoulder around 1500
cm -1 indicates am orphous diam ond-like component in th e film. Around 1040 °C a
b e tte r defined diam ond peak shows up. Then at 1090 °C a graphite peak a t 1600
cm -1 becomes obvious. In all cases th e peak a t 508 cm ~l is th e silicon Ram an signal.
For depositing good quality diam ond films, the depositing tem p eratu re m ust be in a
certain range. If the tem perature is too low, an am orphous diamond-like com ponent
is dom inant in the film. If th e tem p eratu re is too high, graphitic com ponent will play
a m ajor role in the film. It is observed th a t the preferential tem perature range for
good quality diam ond films is between 1030 °C and 1060 °C.
The R am an spectrum is also dependent on gas flow composition. In Figure 4.5
the R am an spectra for 0.5 % and 1 % gas m ixture powder-polished samples are
compared a t a fixed su b strate tem perature of 1000 °C and 1040 °C. It shows th a t
the 1 % samples have a larger graphitic com ponent th an th e 0.5 % samples for both
substrate tem peratures. It indicates th a t for the same tem p eratu re, a smaller m ethane
concentration of th e gas flow produces b etter quality diam ond films than a higher
m ethane concentration does.
Figure 4.6 shows th a t at th e same growth condition a 10 hours deposition sam ple
has a sim ilar R am an spectrum to a 6 hours deposition sam ple, b u t th e sam ple w ith
longer deposition tim e has a stronger diam ond signal and weaker silicon signal because
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Optical
Notch Filter
( A=488 nm)
Aperture
Microscope
eyepiece
L _
Aperture
1/8 Meter
M onochromator
Variable
Beamsplitter
Ar Ion L aser
Silicon
Array
Detector
( S p e c tra P h y sic s 2 0 2 0 )
Detector
Controller
Beamsplitter
A=488 nm
Tunable L aser
Line Filter
O scilloscope
32 X M icroscope
Objective L ens '
X.
A /\ /j
>A A |
Sam ple
S tag e
I
A
C om puter
Com
r1— ^
Figure 4.3. Ram an Spectroscopy set-up for the Ram an spectrum analysis.(From Dr.
Gray)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
of the thicker diam ond film.
In Figure 4.4 to Figure 4.6, only powder-polished sam ples were analyzed.
In
Figure 4.7 R am an spectra of sam ples w ith the same deposition conditions but w ith
different nucleation m ethods were exam ined. It showed th a t th e R am an spectra were
sim ilar for samples w ith 0.5% m ethane at 1040 °C by b o th paste-polished and powderpolished m ethods.
Consequently, th e quality of diam ond films shows no obvious
difference between different kinds of preparation m ethods under th e sam e deposition
conditions. R am an spectra are based on bulk properties of th e sam ple rath er than
on surface or interface characteristics. However, the m ethod was used to indirectly
identify the location of th e graphite signal in one case.
Figure 4.8 shows a good R am an spectrum w ith a sm all graphitic peak for a sample
prepared w ith 1.5% m ethane a t about 1040 °C. The tem p eratu re was not m easured
from the pyrom eter directly b u t by estim ation from th e tem p eratu re m easurem ent
on the sim ilar deposition conditions. A p art of the film peeled off when th e sam ple
cleavage was perform ed. Absence of th e graphitic peak was observed on the R am an
spectrum of the free standing film. This suggests th a t some graphitic com ponent
exist in th e diam ond/silicon interface during th e film deposition. W hen the diam ond
film peeled off, most of th a t graphitic com ponent stayed on th e silicon surface.
Generally, Ram an analysis is essential for characterizing th e deposited diam ond
films. However, it is not appropriate to do the surface or interface analysis. Another
characterization technique, X -ray photoelectron spectroscopy, is then introduced for
the chemical analysis in the surface and interface layers.
4.3
X-ray Photoelectron Spectroscopy
X-ray Photoelectron Spectroscopy (X PS) is also known as Electron Spectroscopy for
Chemical Analysis (ESCA). This technique is able to analyze a wide of variety of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
8U
(a)
RAHAN SPECTRUM
SAM PLE:
N 0 F -P 4 9
1 1 1 0 .0
9 9 0 .0 0
8 7 0 .0 0
7 9 0 .0 0
6 3 0 .0 0
4 3 0 .0 0
7 0 0 .0 0
9 8 0 .0 0
1 2 6 0 .0
1 5 4 0 .0
1 8 1 0 .0
R E C IPR O C A L CM
(b)
RAMAN SPECTRU M
SAM PLE:
N O F -P 3 B
9 0 8 .0 0
8 2 9 .0 0
7 4 4 .0 0
M
6 8 3 .0 0
M
9 0 1 .6 0
4 3 0 .0 0
7 0 0 .0 0
9 8 0 .0 0
1 8 1 0 .0
1 2 6 0 .0
REC IPR O C A L CM
(c)
RAMAN SPECTRUM
SAM PLE:
M 0 F -P 5 0
6 8 6 .0 0
6 3 8 .0 0
586.00
5 3 5 .0 0
4 3 8 .4 8
7 0 0 .0 0
9 8 0 .0 0
1 5 4 0 .0
1 8 1 0 .0
R E C IPR O C A L CM
Figure 4.4. R am an spectra of 0.5% m ethane sam ples at (a) 936 °C (b) 1040 °C and
(c) 1090 °C. (From Dr. G ray)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
U X
(a)
(b)
RAMAN SPECTRUM
HAMAH SPECTRUM
SAMPLE: HOF-P39
U iO lt: HOE-WO
801.00
9 0 0 .0 0
731.00
0 ^ 9 .0 0
7 4 4 .0 0
t
99 1 .0 0
9 2 1 .0 0
9 8 2 .0 0
20
I2S0.0
7 0 0 .0 0
1 9 4 0 .0
is io .o
4 3 0 .0 0
7 0 0 .0 0
RECIPROCAL CM
1 9 4 0 .0
1 0 1 0 .0
RECIPROCAL CM
(c)
(d)
RAMAN SPECTRUM
SAMPLE: H0P-P40
AAMAM SPECTRUM
SAMPLE: W F -P 3 7
6 4 7 .0 0
0 4 0 .0 0
99 9 .0 0
9 9 3 .0 0
9 4 3 .0 0
9 4 0 .0 0
00
4 S 7 .0 0
•3
7 0 0 .0 0
9 6 0 .0 0
1 2 6 0 .0
RECIPROCAL CM
1 9 4 0 .0
1 0 1 0 .0
4 3 0 .0 0
7 0 0 .0 0
9 6 0 .0 0
12 9 0 .0
1010
RECIPROCAL CM
Figure 4.5. R am an spectra of samples w ith conditions : (a) 0.5% m ethane, 1000 °C
(b) 0.5% m ethane, 1040 °C (c) 1% m ethane, 1000 °C and (d) 1% m ethane, 1040 °C.
(From Dr. G ray)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
RAMAN SPECTRUM
SAMPLE:
N 0 F -P 4 7
86Q.00
793.00
702.00
609.00
316.00
423.11
430.00
700.00
980.00
1260.0
1540.0
1810.0
RECIPROCAL CM
(b)
RAMAN SPECTRUM
SAMPLE: N0F-P39
801.00
731.00
661.00
u
991.00
921.00
491.20
430.00
700.00
9 0 0 .0 0
1 2 00.0
1 9 4 0 .0
1010.0
RECIPROCAL CM
Figure 4.6. R am an spectra of samples of 0.5% m ethane a t 1000 °C for (a) 10 hours’
deposition and (b) 6 hours’ deposition. (From Dr. Gray)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(a)
RAMAN SPECTRUM
SAMPLE: N0F-P3B
SOB.00
B29.00
744.00
tM
5
BBS.00
H
X
M
982.00
901.80
430.00
700.00
SB0.00
12B0.0
1940.0
1810.0
RECIPROCAL CM
(b)
RAMAN SPECTRUM
SAMPLE: N0F-N1B
2150.0
1840.0
1530.0
ui 1220.0
910.00
600.20
430.00
700.00
980.00
1540.0
1810.0
RECIPROCAL CM
Figure 4.7. R am an spectra of samples w ith 0.5% m ethane at 1040 °C by (a) powderpolished m ethod and (b) paste-polished m ethod. (From Dr. G ray)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
o*±
(a)
RAMAN SPECTRUM
SAMPLE: N0F-N6 GROWTH SIDE
2 0 8 0.0
>
hM
cn
5 1470.0
h*
Z
1000.0
5 3 3 .0 0
430.00
7 0 0.00
9 8 0 .0 0
1260.0
1540.0
1810.0
RECIPROCAL CM
(b)
RAMAN SPECTRUM
SAMPLE: N0F-N6 FREE STANDING FILM
2 1 9 0.0
1850.0
1510.0
>•
tM
w
£
tz
1170.0
»-«
8 3 0 .0 0
4 9 3.60
430.00
7 0 0 .0 0
9 8 0 .0 0
1260.0
1810.0
RECIPROCAL CM
Figure 4.8. R am an spectra of (a) the as-deposited film and (b) the free standing film
of the sample w ith 1.5% m ethane at about 1040 °C. (From Dr. Gray)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
samples in term s of elem ental and chemical composition on the surface layer of a
sample, typically 5 to 50 A in depth.
XPS involves the energy analysis of electrons ejected from a surface under energetic
bom bardm ent by X-rays as shown in Figure 4.9. T he photoelectric process occurs
when a core level electron absorbs a photon of energy greater th an its binding energy.
W hen this occurs, the electron is ejected from the atom w ith kinetic energy (K.E.)
expressed by th e following equation:
K .E . — hv — E g
(4*1)
where hi/ is th e X-ray photon energy and E b is the photoelectron binding energy. E b
provides both elem ental inform ation as well as chemical bonding inform ation. For
example it is possible to distinguish carbon bonded as diam ond from carbon bonded
to silicon as S iC . Typically, K a X-ray emission from the light m etals is th e photon
source used by m ost XPS [105].
XPS characterization of th e samples was perform ed in a Perkin-Elm er PH I 5400
X-ray Photoelectron Spectrom eter in this research. Dr. Kevin Hook of M SU’s Com­
posite and S tructural M aterials C enter perform ed the XPS analysis. Samples were
attached directly to the instrum ent analysis stubs and placed in the system pre­
pum ping cham ber. System pressure during XPS analysis was approxim ately 10-9
mbar. The spectrom eter is equipped w ith both a M g K a standard source and a A l
K a m onochrom atic source. T he A l K a m onochrom atic source was used m ostly in this
study and was operated at 600 W (15 KV, 40 m A ). A continuously variable stage was
used and set to 65° w ith respect to the sam ple surface. T h e portion of th e sample
was analyzed through an initial lens system in this instrum ent. For all analyses, the
lens system was set in the large area, small solid angle mode. T he size of the electron
beam was 3.3 m m diam eter circle. D ata points were collected in the fixed analyzer
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
A + h?
►A+* + e KE= h p - E B
(ESCA)
Figure 4.9. Principle of X -ray Photoelectron Spectroscopy. W hen a core level electron
absorbs a photon of energy (h v ) greater than its binding energy ( E b ), the electron is
ejected from the atom with kinetic energy (K .E.).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
transmission m ode utilizing a position sensitive detector and a 180° hemi-spherical
analyzer. There are two types of samples analyzed by this technique : as-deposited
samples as shown in Figure 3.16 (a) and back-etched samples as shown in Figure 3.16
(c). All the samples analyzed by XPS were treated by th e paste-polished m ethod.
For the as-deposited samples, th e top surface of the diam ond film was analyzed.
Figure 4.10 shows th e spectra of two different samples; one was left in the air for
several weeks before th e analysis and for th e other the analysis was perform ed within
15 m inutes after th e deposition.
Oxygen peaks were observed in addition to the
carbon peak on th e sample exposed to th e air for a long tim e. Since the sample
has some pinholes, silicon dioxide m ight in principle be a contributor to the oxygen
peaks. However, as th e analyzing angle (the angle of th e beam relative to the sur­
face) decreases th e O peak increases. This suggested th a t th e contam ination came
from the diam ond film surface rath er th an pinholes. This does not necessarily mean
th a t the diam ond surface has formed carbon-oxygen bonds. T he oxygen signal could
be simply a result of non-outgassing H 2 O on th e surface, since hydrogen is not de­
tectable w ith XPS. A lternatively, oxidation of im purities in grain boundaries m ight
be a contributor, although no other elem ental signal were observed.
Back-etched sam ples were used to analyze th e diam ond/silicon interface. In Fig­
ure 4.11 (a) C , O, K and S i were observed on th e surface of the back-etched sample
under the elem ental survey scans. The K peak came from th e K O H which was in the
silicon etching solution. S i peaks m ay have come from th e silicon residue or S iC (as
discussed below) which was not completely etched, or silicon from th e etching bath.
An additional source of S i, as well as O, m ay come from contam ination from the
quartz disk. Such contam ination has been previously reported for microwave CVD
diamond deposition. [106] However, it is noted th a t the S i signal was not observed
for the top surface X PS analysis. The big O peak, m ay also be due to contam ination
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
o o
10
3
8
7
6
5
3
2
••
0
1
0
1100.0
990.0
880.0
2 /0 .0
860.0
550.0
440.0
BINDING ENERGY. eV
330.0
220.0
110.0
0 .0
990.0
880.0
7 /0 .0
660.0
550.0
440.0
BINDING ENERGY. eV
330.0
220.0
110.0
0.0
(b)
10
9
8
7
G
5
3
2
1
0
1100.0
Figure 4.10. ESCA spectra of samples exposed to air for (a) a long tim e and (b) no
tim e before the analysis. (From Dr. Hook)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
of the torr-seal during the back-etch process. However, another contributor might
be the oxidation of th e silicon residue after th e back-etch process. Figure 4.11 (b)
shows more detailed inform ation about th e C peak. The m ost intense contribution
to the overall peak shape is from the peak centered at 284.25 eV due to the deposited
diam ond surface layer. T he sm aller peak present at the lower binding energy side of
th e diamond peak is a contribution from th e S iC interface layer (282.75 eV). It has
been suggested th a t th e growth of a thin layer of S iC (20 - 100
A)
in th e early stage
of deposition is crucial to th e form ation of diam ond films. [107, 108, 109, 110] In any
case, the observation of a S iC layer is consistent w ith several other groups’ studies.
[ I l l , 112, 113]
Typically Argon sputtering is the m ost used m ethod to clean th e surface im purities
from an XPS sam ple. In this work, 3 KeV Argon sputtering at 15 m P a gun pressure
was perform ed for 30 seconds. After th e surface cleaning, it was observed th a t the
O peak is obvious reduced from Figure 4.12 (a) which indicates th a t p art of the
contam ination was removed. Also from Figure 4.12 (b) it is shown th a t th e S iC
shoulder has com pleted disappeared.
Based on the sputtering ra te of S iC , it is
estim ated th a t the carbide layer was approxim ately 30
A
thick. However, the S iC
signal was not observed in some of the back etched samples. For these samples, it is
believed th at th e S iC layer was etched by the back-etch solution because the samples
were not removed from the etching solution right after th e silicon was completed
etched.
4.4
Dek-Tak Analysis
In chapter 3 (section 3), it was m entioned th a t the diamond pow der preparation
approach produced much finer grained films th an did the diam ond paste preparation
m ethod. Microscope exam ination shows th e powder-polished film to be m ade of fine,
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yu
(a)
10
9
8
7
s -■
5
-•
4 -•
3 '.'.-M
2
■■
<1
1 -•
0 ----1100.0
990.0
880.0
770.0
660.0
550.0
449.0
BINDING ENERGY. eV
330.0
220.0
110.0
0.0
(b)
10
284.25 eV
9
8
7
6
5
4
282.75 eV
3
SiC
2
0
300.0
298.0
296.0
292.0
290.0
288.0
BINDING ENERGY. eV
286.0
284.0
282.0
280.0
Figure 4.11. (a) Elemental survey scan of the sample before the Ar sputtering (b)
higher resolution narrow scan of (a) in th e carbon region. (From Dr. Hook)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
9
8
7
6
5
3
S i ■ Si
2
1
0
1100.0
990.0
880.0
770 .0
660.0
550.0
-140.8
330.0
220.0
110.0
0 .0
BINDING ENERGY. eV
10
284.25 eV
9
8
7
282.75 eV t
6
5
SiC
4
3
2
1
0
300.0
298.0
296.0
292.0
290.0
283.0
BINDING ENERGY. eV
286.0
284.0
282.0
280.0
Figure 4.12. (a) Elem ental survey scan of the sam ple after the Ar sputtering (b)
higher resolution narrow scan of (a) in the carbon region. (From Dr. Hook)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
sub-m icron crystallites w hereas the paste-polished surface produced crystallites which
were larger than one micron for similar deposition conditions. The difference is also
obvious to th e eyes. Prior to deposition, a silicon su b strate has a m irror finish and
reflected images are easily observed.
After deposition on a paste-polished silicon
substrate, the deposited diam ond films show a highly specular surface w ith a gray
appearance and no observable reflected images. However, w ith th e powder prepared
surfaces, th e diam ond film is flat and somewhat shiny w ith some ability to produce
reflected images.
T here are also some o th er interesting visual observations. F irst, both types of
films appear glittery under th e sun or strong light, b u t the film produced by the
paste-polished m ethod did show a darker background. Secondly, because th e film
prepared by the powder m ethod is less specular, some of th e scratches on th e silicon
substrate surface could be seen through the film. T hird, if the grain sizes are small
enough, film produced by th e powder-polished m ethod showed colorful interference
rings due to thickness variation across the sample.
For the samples produced by the paste-polished m ethod, optical and electron
microscopy of the transferred film shows a relatively flat back surface com pared to
the original top surface of th e film. This observation is consistent w ith the crosssectional view of th e film and underlying substrate shown in Figure 4.13. It shows
the relative smoothness of th e back surface relative to th e top surface.
In order to quantify film sm oothness, a Sloan DekTak II surface profiler was used.
T he relative smoothness of th e two surfaces of th e paste-polished diam ond film is
shown in Figure 4.14. Shown here are surface profiles m easured over a 50 fim scan
across the samples. A stan d ard deviation of 940 A is observed about the m ean for
the top surface of the diam ond film as deposited on the silicon substrate. W hen the
silicon is removed as shown in Figure 3.16 (c) and th e back surface is scanned, a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
*
Top Surface of
Diamond
Side View of
Diamond
Interface
Si Substrate
15KU X7200
0003
1 . 0U CEO9.0
Figure 4.13. SEM view of th e paste-polished produced diam ond film on a cleaved
sam ple showing the silicon substrate, the interface, and th e side view and top surface
of th e diam ond film.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
standard deviation of 130
A
is observed.
For th e samples produced by th e powder-polished m ethod, SEM cross-sectional
view shows relative sm oothness of th e top and bottom surfaces in Figure 4.15. It is
clearly seen th a t the difference between these two surfaces is not as big as th a t of the
paste-polished produced samples. T he quantified sm oothness of th e two surfaces is
shown in Figure 4.16. Typically, a standard deviation of 630
top surface and 570
A
A
is observed for the
is for th e bottom surface of th e diam ond film. As shown in
Figure 3.4 and Figure 3.5, a rougher surface on th e back side of th e diamond film
produced by the powder-polished m ethod is expected.
Figure 4.17 shows the top surface profiles of two extrem e samples produced by the
paste-polished and the powder-polished nucleation m ethods respectively. Standard
deviations of 1600
A
and 420
A
are observed for the large-grain-size and small-grain-
size diam ond film respectively.
4.5
Scanning Electron M icroscope Analysis
The scanning electron microscope (SEM) has unique capabilities for analyzing sur­
face. It is analogous to the conventional optical microscope, b u t a different radiation
source serves to produce th e require illum ination. W hereas th e optical microscope
forms an image from light reflected from a sample surface, the SEM uses electrons
for image form ation. T he different wavelengths of these radiation source result in
dram atically different resolution levels. Electrons have a much shorter wavelength
th an light photons, and shorter wavelengths are capable of generating high-resolution
inform ation. Enhanced resolution th en perm its higher m agnification w ithout loss of
details.
The m axim um magnification of the optical microscope is about 2000X and the
theoretical resolution lim it is about 0.17 /im. In practice it is difficult to clearly dis-
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
VO
(a)
-(J
o
4)
ca
V
3 ,6 0 0
<u
Ul
a
03
10
scan length (/im)
(b)
866
•<
'a
o
o
V
03
4)
•a
43
Ju
-608
3
03
10
15
20
scan length (/im)
Figure 4.14. The surface profile on (a) the top surface of an as-deposited pastepolished produced diamond film and (b) the back surface of th e film after transfer to
the epoxy substrate.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Top Surface of
Diamond
<
Side View of
Diamond
— Interface
— Si Substrate
15KU.X7200
4606
1 .611 CEC90
Figure 4.15. SEM view of the powder-polished produced diam ond film on a cleaved
sample showing the silicon substrate, the interface, and th e side view and top surface
of the diam ond film.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(a)
L
M
---d
o
»
-4 J
u
sa
m
,1
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j
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t
ti
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t
Y
i/i ^
i
.
ft- >
r*
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500
U J ;
.
ili
'I
w
5,000
r-
k-
scan length (/im)
(b)
2 , 500
d
o
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-43)
O
V
txS
V
33
0)
u
<4—4
1-4
3
CA
2 ,0 0 0
J\ i
,
Ad
i a j r/W
I V\
A
\
1 ,5 0 0
1 , 000
*JT ""”"
!f kj
'A i
'lA 'J, vty
I
\
500
[
0
-500
IV
-
0
5
10
15
20
25
30
35
40
45
1,000
0
scan length (/im)
Figure 4.16. T he surface profile on (a) the top surface of an as-deposited powderpolished produced diam ond film and (b) the back surface of the film after transfer to
the epoxy substrate.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(a)
1 , 000
*<
'f l
O
000
s
Cfl
50
scan length (/on)
(b)
ii
kr<111 0
11
11 5 0 0
1
u 1 .. 000
11
l—1 . 5 0 0
111
- 2,000
1i
>. —2 . 5 0 0
a
o
i\
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<U
cd
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u
t-ij
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\
k
A
7 (|
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F-V'l./T
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I*
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(Mr
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f I
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,1
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1\ 1
13 .0 0 0
1
I
1
3 .5 0 0
1
1
>
1
1
4 .0 0 0
1
0
5
10
15
20
25
30
35
40
45
50
scan length (/ira)
Figure 4.17. The extrem e top surface profiles of (a) large-grain-size and (b) smallgrain-size diam ond films.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
tinguish features smaller th an 1 /zm. In comparison, th e wavelength of the electrons
is less th an 0.5
A,
and theoretically the m axim um m agnification of electron beam
instrum ents is beyond 800,000X. However, because of th e instrum ental param eters,
practical magnification and resolution lim its are about 75,000X and 40
A in
a conven­
tional SEM [114]. Since poly crystalline diam ond films often have sub-micron feature
sizes, th e high resolution capabilities of SEM are crucial for determ ining crystal m or­
phology.
In chapter 4 (section 4), it was noted th a t different grain-size polycrystalline di­
amond films result from different nucleation techniques. Relatively large-grain-size
diam ond films, as in Figure 4.18, are formed by the paste-polished m ethod and rel­
atively small-grain-size films, as in Figure 4.19, are form ed by th e powder-polished
m ethod. T he m ajor reason for the difference is th a t th e powder-polished m ethod pro­
vides a much higher density of nucleation sites than did th e paste-polished m ethod.
Diamond particle residue and th e scratches associated w ith polishing by diam ond par­
ticles provide nucleation sites which enhance the local configuration for the diam ond
growth. As shown in Figure 3.4 and Figure 3.5, more scratches and particle residue
were observed by the powder-polished m ethod than the paste-polished m ethod. As
the nucleation sites density increases, the space of lateral grow th of diam ond for each
site is lim ited. Correspondly, the grain size becomes much sm aller as shown in Fig­
ure 4.20. This phenomenon also can be proved by a careful experim ent described as
follows. T he surface was intentionally polished with less nucleation sites in one place
than in th e other places by th e powder-polished m ethod. A fter the deposition, the
difference of th e crystal growth is obvious under the SEM in Figure 4.21. Large grain
size diam ond particles show up in the area with less nucleation sites com pared to
the small grain size diamond particles in the area with a higher density of nucleation
sites. This shows th a t the density of nucleation sites is a m ajo r factor in determ ining
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
grain size. This observation is also confirmed by J. F. DeNatale and coworkers [115].
T he m ain objectives for using SEM analysis in this research are to examine the
surface morphology and grain size and to determ ine the film thickness. From the
surface micrograph th e average grain size of a film can be estim ated by th e m ethod
of linear intercepts, which is explained by the following equation.
total path length
1
< g ra m s ize > = ------- —------------- x ----------— ----- :—
no. o f in tercep ts
m a g n ific a tio n
,.
(4.2)
The results in Table 4.1 and Table 4.2 show th a t for a given preparation m ethod
the average grain size has a tendency to increase as the microwave power or plasm a
pressure increases. The higher the microwave power, the higher th e plasm a energy,
then the higher the reaction rate, so th e bigger the grain size. On the other hand,
plasm a density increases as the plasm a pressure increases, so does th e reaction rate
and grain size. In Table 4.1 th e d ata also indicates th a t in the paste-polished sam ­
ples the
concentration plays an im p o rtan t role since th e average grain size
increases significantly as m ethane concentration increases. It is understandable since
more m ethyl radicals can form the same crystal at the sam e tim e.
It was noted th a t the samples prepared by the powder-polished m ethod were
deposited at lower microwave power and plasm a pressure conditions th an those pre­
pared by the paste-polished m ethod since the experim ent was perform ed sequentially
for these two groups. The samples were prepared by the paste-polished m ethod in
the beginning. However, it was found from the Ram an spectrum th a t a sample at the
condition of 700 W , 70 Torr, and 1 % m ethane concentration showed only graphite
peaks instead of a diamond peak since the deposition tem p eratu re is too high. Con­
sequently the deposition conditions for later samples prepared by powder-polished
m ethod were lower.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.18. Typical SEM photo of the top view of the diam ond film prepared by the
paste-polished m ethod. The triangular shapes indicate [111] crystallite faces. (MW
power: 600 W , plasm a pressure: 60 Torr, m ethane concentration: 0.5%.)
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Figure 4.19. Typical SEM photo of the top view of the diam ond film prepared by the
powder-polished m ethod. (M W power: 600 W , plasm a pressure: 60 Torr, m ethane
concentration: 0.5%.)
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(a)
V oid s
D iam on d
C o lu m n s
N u cleation
S it e s '
S iC Layer
*3*
- Silicon
(b)
V oids
D iam ond
C olu m n s
S iC Layer
- Silicon
Figure 4.20. G rowth m echanism of diam ond film prepared by (a) paste-polished and
(b) powder-polished m ethod. The latter has a higher nucleation density and therefore
a finer grain film.
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Figure 4.21. This SEM photo shows large and small grain size diam ond growth on
the same substrate.
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(a)
CH4/H
2
C on cen tration : 0.5 %
A v e r a g e g r a in s i z e s ( f i m ) a r e lis te d b e lo w .
Microwave
Input Power
(b)
Plasm a Pressure
60 Torr
600 W
0 .8 3
700 W
0 .8 9
800 W
1 .0 1
80 Torr
70 Torr
—
—
1 .1 8
1 .2 8
—
—
CH 4 /H 2 Concentration : 1.25 %
600 W, 60 Torr : 1.41
A*m
CH 4 /H 2 Concentration : 1.5 %
600 W, 70 Torr : 2.14
fim
Table 4.1. Relation of average grain size vs. m ethane concentration, microwave
power and plasm a pressure by the paste-polished m ethod.
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CH 4 /H 2 Concentration : 0.5 %
Average grain sizes (fim) are listed below.
M icrow ave
Input P ow er
P la sm a P r e ssu r e
5 0 Torr
6 0 Torr
8 0 Torr
400 W
—
0.47
—
500 W
—
0.57
—
600 W
0.55
0.63
0.76
Table 4.2. Relation of average grain size vs. microwave power and plasm a pressure
by the powder-polished m ethod.
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T he relationship of th e growth rate or th e deposition rate (//m /h o u r) versus
m ethane concentration, microwave power and plasm a pressure by th e tw o different
preparation m ethods are listed in Table 4.3 and Table 4.4. M ethane concentration
is a m ajor factor for the grow th rate since th e grow th ra te increases as th e m ethane
concentration increases in both preparation m ethods. The higher th e m ethane concen­
tratio n , the more m ethyl radicals (C H 3 ) are available for deposition, so th e growth
ra te increases. Likewise, th e growth rates also had a tendency to increase as the
microwave power and plasm a pressure increase since the reaction ra te increased as
plasm a energy or plasm a density increased , however, the effect is sm aller th an the
m ethane concentration effect.
4.6
Film Uniformity Analysis
So far, in this chapter R am an Spectroscopy was used for film quality analysis, XPS
was perform ed for diam ond/silicon interface studies, DekTak profiles were used for
th e film surface sm oothness test, and SEM were used for the grain size and growth
ra te studies. B ut there is another im portant p aram eter, which is th e film uniformity.
W hen depositing th e diam ond film by using th e plasm a CVD technology described
in chapter 3 (section 4), usually an approxim ately hem ispherical shape discharge is
formed right above th e substrates. This phenom enon becomes particularly evident
when the discharge pressure is above 50 Torr. T he discharge size increases as the
pressure decreases because a t about 10 Torr or less the discharge tends to fill the
quartz confinement cham ber. The deposition rates of the film are dependent on the
positions correspond to th e density of the discharge. If at low pressures, th e discharge
volume is large enough to uniformly cover the sample, th e deposition ra te will be
fairly uniform over the sam ple. At high pressures as the discharge size decreases, the
deposition rates over the sam ple area will show m ore variation. T he trade-off is th a t
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(a )
CH4/H
D e p o s itio n
2
r a te s
M icrowave
Input P o w er
(b)
C on cen tration : 0.5 %
(p m /h o u r )
a re
P la sm a P re s s u re
60 Torr
70 Torr
—
600 W
0 .3 4 -0 .3 8
7 00 W
0 .3 5 - 0 .4
0 .3 6 - 0 .4 1
800 W
0 .3 9 -0 .4 4
—
8 0 Torr
—
.
0 .3 7 -0 .4 2
—
CH 4 /H 2 Concentration : 1.25 %
600 W, 6 0 Torr : 0.54 - 0.6
(c )
lis te d b e lo w .
p m /h o u r
CH 4 /H 2 Concentration : 1.5 %
600 W, 7 0 Torr : 0.92 - 1.06
p m /h o u r
Table 4.3. Relation of deposition rate vs. m ethane concentration, microwave power
and plasm a pressure by th e paste-polished m ethod.
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(a)
CH4 /H
2
C on cen tration : 0.5 %
D e p o s itio n r a te s ( p m / h o u r ) a r e lis te d b e lo w .
Plasm a Pressure
Microwave
Input Power
80 Torr
60 Torr
400 W
—
0 .2 8 -0 .3 2
—
500 W
—
0 .3 3 -0 .3 8
—
0 .4 2 -0 .4 5
0 .5 8 -0 .6 3
600 W
(b)
50 Torr
0 .4 -0 .4 2
CH4/H2 Concentration : 1
%
D e p o s itio n r a te s ( p m / h o u r ) a r e lis te d b e lo w .
Plasm a Pressure
Microwave
Input Power
50 Torr
60 Torr
600 W
0 .4 8 -0 .5 2
0 .5 6 -0 .6 2
Table 4.4. Relation of deposition rate vs. m ethane concentration, microwave power
and plasm a pressure by the powder-polished m ethod.
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deposition rate goes down as th e pressure and plasm a density decreases.
To determ ine the thickness on different p arts of th e film, SEM is still th e m ost
accurate measurem ent m ethod. However, a gold coating of the sample is necessary to
obtain good image resolution and sample cleavage is required. Consequently, th e SEM
technique is destructive. In order to determ ine a technique for m easuring thickness
uniform ity non-destructively, a laser scanning confocal microscope (LSM) was used
in this research.
The LSM is a light microscope, but one w ith a difference! In th e microscope’s
nam e th e words ’’laser” and ’’scanning” refer to th e m ethod of illum ination, while
’’confocal” refers to th e m ethod of image form ation. The microscope has four light
sources: a fiber optic tungsten lam p, a m ercury lam p, an argon-ion laser w ith 488
and 514 nm lines, and a helium -neon laser w ith a 543 nm line. So th e LSM can be
operated both as a conventional microscope (using th e tungsten or the m ercury lam p)
and a laser scanning instrum ent.
The term ’’confocal” indicates th a t the microscope is aligned so th a t th e illum i­
nated spot and imaged spot coincide precisely, which is not the case in conventional
light microscopes. In a conventional microscope, light reaching the observer comes
from all parts of the specim en w ithin the field of view - b o th from th e narrow hori­
zontal plan which is in focus and from out-of-focus regions above and below it. The
light from out-of-focus areas, which severely degrades th e focal-plane image, generally
lim its to just a few microns th e depth to which a specim en can be examined.
In a confocal instrum ent th e specimen is scanned point-to-point w ith a finely
focused beam , most effectively by a laser, and a pinhole ap ertu re is placed directly in
front of the detector at th e focal point of light coming from the in-focus p art of the
specim en. The effect of these modifications is to block light from out-of-focus regions
because th e focal point of such light falls either in front of or behind the pinhole
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aperture. The result of this instrum ent configuration on th e final image is a dram atic
increase in resolution and contrast.
The theoretical lim it of resolution for a conventional light microscope is about
0.17 ilm. B ut much sm aller objects can be easily seen in th e LSM. T here are m ultiple
reasons for this, chief among them being the elim ination of out-of-focus light by the
confocal pinhole. In addition, m onochrom atic light (the laser) elim inates chrom atic
aberration; scanning a specim en w ith a small spot, no m a tte r w hat th e light source,
always improve resolution; and the photom ultiplier detector is more sensitive than
th e hum an eye. M agnification in th e LSM ranges from 200x to 16,000x [116]. This
upper lim it is significantly higher th an a conventional optical microscope.
In our case, however, th e samples prepared by paste-polished m ethod have a
b e tte r resolution in th e diam ond/silicon interface th an those prepared by powderpolished m ethod since the interface was flatter by the form er m ethod. A sam ple with
th e biggest average grain size and thickest diam ond film was used to perform the
uniform ity study. U nder the SEM analysis, th e thickness of th e diam ond film on an
area of 0.2 cm X 0.4 cm is between 6.5 and 7.5 fim. By th e LSM technique, a scan
in the Z (or vertical) axis of th e sam ple can be carefully observed since diam ond is
transparent to visible light. A 488 nm laser scan was used for the best resolution in
our studies. The thickness of the film was then determ ined by the distance between
A plane and D plane as shown in Figure 4.22(a). Eight thickness m easurem ents were
perform ed on an area of 1 cm x 2 cm of the same sam ple in different locations as in
Figure 4.22(b). A distribution of thickness between 5.8 and 8.0 fim were obtained.
This study did show th a t th e LSM technique indeed can perform the uniform ity
studies efficiently w ithout significant loss of accuracy and also w ithout any sam ple
destruction and contam ination.
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(a)
Voids
Diamond
~ ' Columns
Nucleation \
Sites 'v-v-j
/ SiC Layer
-Silicon
(b)
6.9
8 .0
6 .5
7
7.8
6.1
7 .4
5 .8
Figure 4.22. (a) Cross section view of laser scan from plane A to D. (b) Thickness
(^m ) uniform ity analysis on an area of 1 cm X 2 cm of a sample.
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CHAPTER 5
E lectrical C h aracterization
5.1
Introduction
In this chapter, conductivity vs. tem perature results for th e as-deposited diam ond
films will be described in chapter 5 (section 2). Then current-voltage characteristic of
samples w ith dual-side m etal contacts on the isolated diam ond films will be discussed
in chapter 5 (section 3). Finally, current-voltage characterization of the samples w ith
m etal-diamond-silicon stru ctu re will be discussed in chapter 5 (section 4).
5.2
5.2.1
Four Point Probe Characterization
Experimental M ethod
The m easurem ent of bulk resistivity, particularly vs. tem p eratu re, provides a fun­
dam ental m eans of electrically characterizing m aterials and is essential to investigate
current transport in diam ond films. In order to avoid contact effects, a m ethod of
choice for m easurem ent of electrical resistivity is the four p oint probe m ethod. How­
ever, on high resistivity thin films (such as diamond films), th e m easurem ent technique
is nontrivial because of the requirem ent of accurately m easuring small voltages and
113
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currents. A standard instrum entation approach such as is used on silicon wafers is not
adequate. Consequently, a sensitive four point probe apparatus, w ith th e capability
of changing th e m easurem ent tem perature, was constructed as in Figure 5.1.
The sheet resistance of diam ond films on insulating silicon nitride substrates
(S is N 4 ) was m easured w ith a Signatone four point probe station as follows. Four
equally spaced point probes are brought into contact w ith the sam ple surface. A
Hewlett Packard 4145 B Sem iconductor P aram eter Analyzer is used as th e am m eter
to m easure the current (I), which is supplied by a dc power supply, through the two
outer probes. The supply voltage is set at 40 V in this study. A high im pedance
digital voltm eter (Fluke Model 8840 A) connected to th e two inner probes is used for
the voltage (V) m easurem ent. This arrangem ent largely elim inates contact resistance
effects, since the voltage m easurem ent probes draw negligible current. T he probe sta­
tion is then placed in a T herm otron Environm ental Control C ham ber (Model
S i.2)
which allows th e tem p eratu re to be varied from —75°C' to 175°C.
The sheet resistance Ra has a constant of proportionality between V /I, and is
expressed as follows
Ra = « y )
(5.1)
where £ is a correction factor, the value of which is determ ined by the shape of the
test sam ple and th e ratio betw een the size of the sample and th e probe spacing. In
this experim ent, the size of th e S ^ A ^ substrate is 0.5” x 0.5” x 0.2” and the probe
spacing is 0.0625 inches. For a square su b strate and a substrate size to probe spacing
ratio of 8, th e correction factor £ is determ ined to be approxim ately 4 [117].
T he resistivity (p) of the diam ond film is then obtained by
p = Ra • t
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(5.2)
Ammeter
(M odel : HP 4145 B )
Voltmeter
T em perature
Controlled C ham ber
( Model : Fluke 8840 A
( Model : Thermotron S 1 .2 )
Four - point Probe
Diamond Film
Silicon Nitride
( S i 3N 4 )
Figure 5.1. T he set-up of th e four point probe m ethod.
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where t is the film thickness, and the conductivity (cr) of the film is
a = P
5.2.2
(5.3)
Theory of Conductivity vs. Temperature
The conductivity of th e diam ond film on insulating m aterial (S i 3 N 4) was studied at
different tem perature levels in order to understand th e conduction mechanism.
As is generally th e case for semiconductors, th e electrical properties of diamond
are dom inated by the effect of dopants and other im purities or defects. Experim ental
m easurem ents of th e electrical conductivity provide inform ation about th e concentra­
tion of electrically active im purities and defects as well as their energy levels.
Thermopower m easurem ents on the diam ond films used in this research show th a t
the films are p-type, indicating the presence of acceptor-type im purities or defects even
though the films were not intentionally doped w ith acceptors. For a semiconductor
w ith a single acceptor level a t energy E a in th e energy gap, the hole concentration,
p, is given by [118, 119]
p = JV„exp - [(E f — E v)/kT ]
(5.4)
where N v is th e effective density of states in the valence band, E j is Fermi level, E v
is top level of the valence band and k and T denote th e B oltzm ann constant and
absolute tem perature, respectively. From charge neutrality,
p = n + N a~
(5.5)
where n is the electron concentration, N a is the concentration of acceptors, and N a~
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is the concentration of ionized acceptor. Consequently,
P
n + l + 4 e x p [{Ea - E f )/k T }
The term on the left side of th e equation corresponds to positive charges,
on the right
(5>6)
and those
side correspond to negative charges. However, a t the tem p eratu re of
interest here th e electron density in p-type m aterial is very small, so th e first term
on th e right hand side can be neglected and Eq. (5.6) becomes [118]
Na
P = l+ 4 e x p [ { E a - E f )/k T ]
(5' ?)
From Eq.(5.4) and (5.7), one obtains
V=
X [-JV./4 + ^ /V „7 l6 + N M e x p i ^ - ^ - ) ]
(5.8)
The hole concentration is directly related to the m easured conductivity by th e ex­
pression
(T = qnPp
(5.9)
where q and p p are electron charge and carrier m obility respectively. T he reported
value of hole mobility fip is 1200 cm 2 /V -s. [120]
In this research, N v was calculate from
N v = 2{2irmpmk T / h 2 f
2
(5.10)
where m p* is effective hole m ass and h is Planck constant. [121] The reported value
of m p*(= 0.75 mo) of natural diam ond was used [120], which yields
7V„ = 3 .1 3 7 -1015- T 3/2cm "3
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(5.11)
From Eq.(5.8), (5.9) and (5.11), and th e m easured conductivities a t different tem per­
atures, (E a-E v) and N a may be used as param eters to fit th e experim ental data. The
comparison of the experim ental results and theoretical calculation will be discussed
in chapter 5 (section 2.4).
5.2.3
Comparison w ith Physical Characterization
T here are four samples in this four point probe study, th ree of which are prepared by
th e diam ond powder nucleation m ethod and one which is prepared by the diam ond
paste nucleation m ethod. T he three powder-polished samples (NDF-S3,S4,S6) have
sim ilar sub-micron crystallite morphology and the paste-polished sam ple (NDF-S1)
has a larger (greater than 1 fim ) grain size diam ond film based on optical microscope
analysis.
T he deposition conditions for th e sam ples are described as follows. The deposition
tim es are all 6 hours.
N D F -S l, CH4 IH2 : 0.5 %(1.25 sccm /250 seem), M .W in p u t power : 600 W ,
plasm a pressure : 60 Torr, tem p eratu re : 1060 °C.
N D F-S3, C H ^ I H 2 : 0.5 %(0.75 s e e m /150 seem), M.W in p u t power : 500 W ,
plasm a pressure : 50 Torr, tem p eratu re : 1035 °C.
N D F-S4, C / / 4 / / / 2 : 0.5 %(0.75 sccm /150 seem), M.W in p u t power : 400 W,
plasm a pressure : 50 Torr, tem p eratu re : 1000 °C.
N D F-S6, C H 4 / H 2 : 0.5 %(0.75 sccm /150 seem), M.W input power : 320 W,
plasm a pressure : 50 Torr, tem p eratu re : 950 °C.
Samples NDF-S3, S4 and S6 are as-deposited diamond films, for which the deposition
was term inated by turning off the microwave power while b o th m ethane and hydrogen
gas flows were on.
However, the deposition for N D F-Sl was term inated by first
turning off th e m ethane gas flow, exposing the sample to a hydrogen plasm a and
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then turning off the microwave power.
This sam ple was subsequently treated in
a solution (2 H 2 O A 6 H 3 P O 4 A H N O 3 A acetic acid) a t 80 °C for 1 hour in order to
remove th e conducting layer caused by the hydrogen plasm a during th e process.
The corresponding R am an spectrum for each sam ple is shown in Figure 5.2 and
Figure 5.3. In Figure 5.2, th e R am an spectra, which were perform ed by Dr. M ark
Holtz in Physics departm ent of Michigan S tate University, of NDF-S3, S4, S6 are
compared. N ote th a t the horizontal wavenumber scale is in a b s o lu te w a v e n u m b e r s w ith photon energy increasing from left to right and phonon energy increasing
fro m right to left. More typically, diam ond R am an spectra are plotted vs. shifted
wavenumber. In term s of shifted wavenumber, the peak th a t is apparent in each spec­
tru m is a t 1331 cm -1 plus or minus 1 cm -1 and corresponds to the diam ond peak.
A t all tem peratures shown in this figure a strong diam ond peak is noted. T here are
two indicators th a t the diam ond films approach th e R am an properties of n atu ral di­
am ond as tem perature increases from 950 °C to 1035 °C. F irst, the height of the
diam ond peak relative to the background level increases as the tem p eratu re increases.
Secondly, the w idth of th e peak increases as the tem p eratu re decreases. For n a tu ­
ral diam ond, th e peak is very sharp, with full-w idth-half-m axim um (FW H M ) of 2.1
cm -1 . For diam ond films, FW HM values are substantially larger, in the range of 7 17 c m "1 [102].
In Figure 5.3, the R am an spectrum , which was perform ed by Dr. Kevin G ray in
N orton Company, of N D F-Sl is shown. T he spectrum shows a very sharp diam ond
peak along w ith a small graphitic peak on the right shoulder. This spectrum is similar
to the spectrum of NDF-S3, w ith a FW HM equal to 9 cm -1 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
II
I
I
.
t
•
I
;
[_ .._.70pJ______
I
17700
17840
17080
18120
18260
[
'soocJ
(b)
17700
17840
18280
r
i
i
i
i
h-
30001
17700
17640
17080
18260
18400
MAVENUHBERS
Figure 5.2. R am an spectra of (a) NDF-S3, (b) NDF-S4, and (c) NDF-S6. (From Dr.
Holtz)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
RAMAN SPECTRUM
SAMPLE:
NDF-Sl
1010.0
890.00
INTENSITY
770.00
650.00
530.00
415.20
430.00
700.00
980.00
1260.0
1540.0
RECIPROCAL CM
Figure 5.3. R am an spectrum of N D F-Sl. (From Dr. Gray)
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18 1 0
5.2.4
Experimental Results and Match w ith Theory
The films in this study were not intentionally doped w ith im purities. However, im­
purities m ay come from a variety of sources including im purities in th e feed gases.
Although th e m ethane used was the highest purity readily available (99.99 %), this is
not high purity by sem iconductor standards. A m ethane pu rity of 99.99 % essentially
corresponds to one im purity in 10,000 m ethane molecules. If, as a case in point, there
is one im purity per 10,000 carbon atom s in th e diam ond film, this would correspond
to an im purity density of 1 x lO 19 cm -3 which is more th an sufficient to dom inate the
electrical properties. In conventional sem iconductors like silicon , im purity densities
in active device regions are typically on th e order of lx lO 15 cm -3 . O ther possible
sources of im purities include residual gases in the vacuum system , air leaks, and sam­
ple handling. Only a mechanical roughing pum p was used to evacuate th e system.
Furtherm ore, structural defects in the crystallites and grain boundaries can introduce
states in th e gap which m ay act as traps, acceptors, or donors. Consequently, even
though the films are not intentionally doped, it is anticipated th a t electrical properties
of the film will be controlled by defects and im purities.
Simple hot probe m easurem ents based on the therm opow er phenom enon showed
th a t all of the samples were p-type, indicating th a t defects a n d /o r im purities play
the role of acceptors. By combining the theory of chapter 5 (section 2.2) w ith exper­
im ental results, inform ation is obtained about the concentration of these states, and
their energies.
Perform ing the four point probe m easurem ent, one can get the (V /I) ratio. Then
following Eq. (5.1), it was found th a t the sheet resistances (R a) for N D F-Sl, S3,
S4 and S6 at room tem perature are 1.2 x 10s , 1.3 x 106, 5.6 x 105, and 2.1 x 10s
f! /□ respectively. It was also noted th a t the sheet resistance of N D F-Sl before the
cleaning procedure was 1 x 104 D /n . All of these resistivities are low compared to
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natural, un-doped, diam ond. However th e variation of resistivities am ong th e samples
is consistent w ith th e R am an indication th a t higher deposition tem p eratu re produced
properties closer to natu ral diam ond since th e diamond peak is m ore sim ilar to th a t
of natural diamond.
Since the thickness of th e samples were estim ated to be about 3 fim thick for
6 hours’ deposition, from Eq. (5.2) th e resistivities (p) a t room tem perature were
calculated to be 3.6 x 104, 3.9 x 102, 1.68 x 102 and 6.3 x 101 D-cra for N D F-S l, S3, S4
and S6 respectively. Taking the inverse of th e resistivity (p), th en th e conductivity
(<t) was determ ined and plotted.
Figure 5.4, Figure 5.5, Figure 5.6 and Figure 5.7 show th e tem p eratu re dependence
of the conductivity for N D F-Sl, S3, S4 and S6 respectively. In all cases, points show
experim ental results and lines show the results of theoretical calculation from Eq.
(5.8), (5.9) and (5.11) assuming a constant mobility (pp = 1200 c m 2 /V -s).
The
values of E a-E v and N a are presented in th e following.
N D F-Sl, E a-E v= 0.51 eV and iVa= 2 .5 x l0 12 cm -3 .
NDF-S3, E a-E v= 0.31 eV and N a= 2 .0 x l 0
' 3
cm "3.
NDF-S4, E a-E v—0.24 eV and A^a= 4 .0 x l0 13 cm -3 .
NDF-S6, E a-E v= 0.22 eV and Na= 1 .0 x l0 14 cm~3.
However, in reality the carrier m obility for polycrystalline diam ond films is much
smaller than th a t in single crystal diam ond. If it is assumed th a t p p = 12 cm 2 / V - s ,
the values of E a-E v and N a after the sam e curve fitting procedure are presented in
the following.
N D F-Sl, E a-E V= 0A 2 eV and A^a= 5 .5 x l0 14 cm~3.
NDF-S3, E a-E v= 0.23 eV and N a= 3 .5 x l0 ls cm -3 .
NDF-S4, E a-E v= 0.15 eV and Afa= 4 .0 x l0 15 cm -3 .
NDF-S6, E a-E v= 0.13 eV and iVa= 1 .2 x l0 16 cm -3 .
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Generally, th e model calculated d a ta for both fxp = 1200 cm 2/ V 'S and fip = 12
cm ?/V -s fit quite well to the experim ental results. However, there are some deviations
from the experim ental plots. This m ay indicate the existence of other levels in the
forbidden gap or non-monoenergetic states, also th e m obility is general not constant
w ith tem perature, b u t m ay either decrease or increase w ith tem p eratu re depending
on w hether im purity scattering or lattice scattering dom inates.
The paste-polished sam ple N D F-Sl has a much higher activation energy th an the
powder-polished samples NDF-S3, S4, S6. Also, the concentration of acceptors is an
order of m agnitude less than for the other samples. From chapter 4 (section 4), it is
known th a t paste-polished samples usually have bigger grain size diam ond crystals
than th e powder-polished samples. Comparatively, the concentration of grain bound­
aries for paste-polished samples are sm aller th an those of powder-polished samples.
This is consistent w ith a hypothesis th a t acceptor type states are associated with
grain boundaries. However, th e situation is evidently m ore complex since the chang­
ing value of E a-E v indicates th a t the n atu re of acceptor level changes as well as the
concentration. For comparison purposes, it is interesting to note th a t type lib di­
amonds which are naturally doped with boron are reported to have an activation
energy of 0.37 eV [120].
An annealing process was also perform ed on all the sam ples to study if the tem per­
ature dependence of the conductivity would change. The samples first were cleaned
by acetone, m ethanol, DI w ater and dried with N 2. Then they were annealed at 500
°C for 1 hour in a furnace atm osphere of nitrogen with a 500 seem flow rate. After
th e annealing, the sheet resistance of the NDF-S6 is 1.6 xlO 9 !) /□ a t room tem per­
ature. It increased by nearly 4 orders of m agnitude. T he sheet resistance of the
other samples could not be measured because the current was too small. For these
samples, the sheet resistance is beyond the lim it of the m easuring equipm ent used in
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.001
Theory —
Experiment O
Conductivity (Q
0.0001
le -0 5
le -0 6
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
1000/T (°K_1)
Figure 5.4. C onductivity vs. 1000/T of N D F-Sl
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4.8
o .oi
Conductivity (Q
Theory —
Experiment O
0.001
0.0001
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
1000/T (°K_l)
Figure 5.5. Conductivity vs. 1000/T of NDF-S3
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4.8
o.i
Conductivity (Q
Theory —
Experiment O
;
0.01
0.001
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
1000/T (°K_1)
Figure 5.6. C onductivity vs. 1000/T of NDF-S4
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4.8
Theory(before
Theory(after
Experiment(before
Experiment(after
Conductivity (Q
g
anneal)
anneal)
anneal)
anneal)
•••
—
□
O
0.01
0.001
0.0001
le -0 5
le -0 6
le -0 7
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
1000/T (°K_1)
Figure 5.7. C onductivity vs. 1000/T of NDF-S6
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4.8
this investigation, th a t is greater th a n 1010 f l / □.
T he calculated E a-E v and N a for NDF-S6-anneal are 0.44 eV, 1.9 xlO 10 c m ~ 3 when
Up = 1200 cm2/ V - s and 0.36 eV, 2 .2 xlO 12 cm -3 when fip = 12 cm 2 /V - s respectively.
No m a tte r w hat the value of th e m obility is, the significant decrease of the acceptor
concentration indicates th a t p a rt of the defects or im purities are expelled from the
samples during th e annealing process. The increase of th e activation energy suggests
th a t there are m ultiple im purity energy levels existing in th e forbidden gap. W hen
one level is annealed out, th e previous m inor level becomes dom inant.
L andstrass and Ravi presented a specific explanation for such an annealing effect;
a m odel of hydrogen passivation of deep donor-like trap s in th e inter-band states. Hy­
drogenation electrically neutralizes th e traps and the resistivity is governed by shallow
acceptor levels, th e effective acceptor concentration increases and the conductivity in­
creases. T he annealing process on th e other hand causes de-hydrogenation resulting in
electrical activation of deep traps. These donor-like traps cause the effective acceptor
concentration to decrease because of compensation and th e conductivity decreases.
W hen th e hydrogenated diam ond films are heat treated in a neutral am bient, th e hy­
drogen can be expelled from th e crystals, restoring the high resistivity. For example,
with a 780 °(7, 2 hour tre a tm e n t in flowing nitrogen am bient, th e resistivity increases
from - 1 0 s to ~ 1 0 14 fl-cm. [122, 123] Albin et al. [124] also had a similar observation
th a t th e conductivity of the diam ond films will increase several orders of m agnitude
after hydrogenation by the hydrogen plasm a. However these m easurem ents were per­
formed by th e two probe m ethod instead of the four point probe m ethod.
Defects states due to dangling bonds and grain boundaries and inter-band levels
due to im purities play m ajor roles in the conduction m echanism of the diamond films.
Consequently, from the four point probe experim ent, there are indeed m ultiple in ter­
band energy levels existing in th e forbidden gap and some of th e im purities, such as
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hydrogen, can be expelled from th e samples by th e annealing process. T he effect of
defects states will be discussed in chapter 5 (section 3).
5.3
Back-Etched Samples.
5.3.1
The Back-Etched Samples and I-V Measurement SetUp
Typical deposition param eters and some physical characterization of diam ond films
deposited for back-etched sam ples are described as follows. A m ixture of m ethane
and hydrogen, with flow rates of 0.75 seem and 150 seem respectively, was allowed
to flow into th e cham ber and a gas discharge was m aintained w ith 600 W microwave
input power. T he plasm a pressure was 50 Torr and th e su b strate tem p eratu re was
approxim ately 1000°C'. Film s were deposited on a sacrificial silicon substrate th a t
were diam ond powder polished.
Scanning electron microscopy of the top surface of th e resulting films showed well
defined crystallites w ith an average grain size of 0.55 fim as determ ined by the linear
intercepts m ethod. A surface profile as m easured by a stylus profileometer showed a
standard deviation of 630
A.
Ram an characterization of th e films, as illustrated in
Figure 5.8, showed the characteristic diamond peak at 1332 cm -1 and indicates an
absence of appreciable non-diam ond carbonaceous m aterial.
A fter deposition, dual side contacts were m ade to the top and bottom of the film
as described in chapter 3 (section 5.3). The top m etal contacts are circular w ith a
diam eter of 400 fim and approxim ately 20 contacts are on each transferred film. This
effectively provides 20 tw o-term inal devices for electrical characterization, w ith the
bottom contact being common to all devices.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
RAMAN SPECTRUM
SAMPLE:
N 0F - P 4 7
888.00
795.00
702.00
>H
w
CO
g
Iz
M
609.00
516.00
423.11
430.00
700.00
980.00
1260.0
1540.0
18 1 0 . 0
RECIPROCAL CM
Figure 5.8. Ram an spectrum of the diam ond film as deposited on the silicon substrate
with substrate tem perature at 1000 °C. (From Dr. G ray)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The m easurem ent set-up is illustrated in Figure 5.9. T he sam ple was placed on the
Signatone m icroprobe station. C urrent-voltage (I-V) characteristics were m easured
at room tem perature, w ith and w ithout photo-excitation. For voltages up to 100 V,
an HP 4145B semiconductor param eter analyzer was used to collect the data, w ith
a current resolution of approxim ately 100 pA. For higher voltages, a Tektronix 577
curve tracer was used.
5.3.2
Contact Effects on Back-Etched Samples
In this study, Au, Ag, and In contacts were used in order to investigate the contact
effect of different m etal work functions. T he work functions for Au, Ag, and In are
5.2 eV, 4.42 eV, 3.97 eV respectively [125].
For low voltages, less th an 35 V, th e I-V characteristic of powder-polished samples
were nearly linear and sym m etric for gold, silver, and indium contacts, as shown
in Figure 5.10.
For a film thickness of 3.5 fim , 35 V corresponds to an electric
field of 105 V /cm . This indicates th a t for electric fields below 105 V /cm , the films
exhibited predom inantly ohmic behavior with a conductivity th a t was independent
of the applied voltage for th e high work function m etal contacts (Au) and low work
function m etal contact (In). However, for large grain films described later in chapter
5 (section 4), results show th a t under certain conditions, m etal-diam ond contacts are
non-ohmic and dom inate device I-V characteristics.
5.3.3
High Field Effect
The com bination of dual-side contacts and relatively th in films facilitates m easure­
m ent of electrical properties a t higher electric fields th an have been generally reported
for previous studies of diam ond films w ith m etallic contacts [126]. In the last section,
it is shown th a t ohmic behavior was predom inant at low voltages fpr different m etal
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
SMU4 SMU3
0 ”6
SMU2 SMU1
O
Q
HP 4145 B
Semiconductor Param eter Analyzer
Tungsten
' Top Metal Contact
Contact
Wire
- Torr Seal
. Diamond Film
Bottom Metal Contact
—AI2 Q 3
Al Coating
Si Substrate
/
Ag Paste
/
Signatone Microprobe Station
Figure 5.9. The I-V m easurem ent set-up of the back-etched samples.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6e — 09
4e — 09
2e — 09
£
s£
O
4e — 09
—6e - 09
—8e - 09
-2 0
-1 5
-1 0
-5
0
5
10
15
20
Voltage (V)
Figure 5.10. I-V characteristics of a small grain size sam ple w ith gold contacts top
and bottom . It shows linear and sym m etric behavior at low voltages. (Electrical
sample : P47-T1)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
contacts. However, a t high voltages, th e current increases m ore rapidly than linearly,
as shown in Figure 5.11.
As shown in Figure 5.12 (a) and (b), current-voltage relationship did not follow
the space charge lim ited current-voltage power law where in (a) IocV 3 ^ 2 and in (b)
/ocV2.
However, th e nonlinear behavior at high voltages is well m odeled by a voltage
activated conductivity. The to ta l dark conductance of the device m ay be expressed
as
G = Goo “h Goexp(aV)
(5.12)
where the first term represents ohmic behavior corresponding to low field conduc­
tion in the diam ond film and second term represents the nonlinear behavior a t high
voltages, with V being the applied voltage and a representing th e slope of the curve.
Figure 5.13 and Figure 5.14 show th a t this model fits th e d a ta over the entire
m easurem ent range for both Au-Au contacts on the top and b o tto m , and In contacts
on the top and Au contact on the bottom respectively. For both cases the diamond
film thickness was 3.5 fim. A t voltages higher than 250 V , a breakdown regime is
entered in which a negative resistance state is followed by an irreversible breakdown.
For voltages less th an 250 V, however, no hysteresis in th e I-V characteristic is ob­
served. T he experim ental d a ta is repeatable and the same for increasing voltage as for
decreasing voltage. Since both contact com binations gave th e sam e low and high field
characteristics, th e work function of the contact does not appear to contribute to the
I-V characteristics of the small grain size samples. This result is consistent w ith some
other reported work on m etal-diam ond contacts [127]. T he slopes a of Figure 5.13
and Figure 5.14 are 0.042 V-1 and 0.045 V-1 respectively. B oth Au-Au and In-Au
contacts gave rise to nearly sym m etric I-V characteristics and exhibited a low-field,
constant conductance (ohmic) region w ith a conductivity cr0o. For Figure 5.13 and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2e — 07
1.5e - 07
le - 0 7
5e — 08
4-S
G
a;
(-i
t-4
G
o
5e — 08
—le — 07
—2e - 07
-1 0 0
50
0
50
100
Voltage (V)
Figure 5.11. I-V characteristics of a small grain size sam ple w ith gold contacts top and
bottom . It shows nonlinear behavior at high voltages. (Electrical sam ple : P47-T1)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
XU I
1.6e - 07
1.4e —07
<
a(U
1-4
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200
400
600
800
1000
8000
10000
Voltage3/2 (V3/2)
(b)
1.8e - 07
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2U.
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8e —08
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0
2000
4000
6000
Voltage2 (V2)
Figure 5.12. The / — V 3/ 2 characteristic and (b) the I — V 2 characteristic of Figure
5.11. (Electrical sam ple : P47-T1)
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Figure 5.14, <Joo is equal to 7.3x10 11 (fl-cm) 1 and 5.9x10 11 (fi-cm) 1 respectively.
For samples deposited at sim ilar conditions w ith a thickness of 2 //m. Figure 5.15
shows th a t this m odel also fits th e d a ta over the entire m easurem ent range for AgAg contacts on th e top and bottom . T he breakdown regim e happens a t voltages
higher than 150 V. T he slope a of Figure 5.15 is 0.08 V -1 . T he Ag-Ag contacts also
gave rise to nearly sym m etric I-V characteristics and exhibited a low-field, constant
conductance (ohmic) region w ith a conductivity (Too- For Figure 5.15, (Too is equal to
4 .6 x l0 - n (fl-cm )-1 .
The slope a is approxim ately 0.045 V -1 for a sam ple w ith a thickness of 3.5 /im.
However, the slope a is 0.08 V -1 for a sam ple w ith a thickness of 2 fim. For samples
of varying thickness, th e slope of the conductivity vs. voltage is found to be inversely
proportional to th e sam ple thickness, indicating th a t th is increase is related to the
applied electric field. Consequently, th e conductivity m ay be expressed as
cr = cr00 + <r0e x p (a F ) = cr0o + <r0e x p (F /F 0)
(5.13)
where F is th e electric field and a represents the slope of the log of th e conductivity
vs. electric field and Fo is th e inverse of the a. The value of a and F q for Figure 5.13,
Figure 5.14 and Figure 5.15 are 1 .4 7 x l0 -5 (V /cm )-1 , 6 .8 x l0 4 V /cm ; 1 .5 8 x l0 -s
(V /cm )-1 , 6 .3 x l0 4 V / c m ; and 1 .6 x l0 -5 (V /cm )-1 , 6 .2 5 x l0 4 V /cm respectively.
In order to investigate w hether the high voltage increase in conductance is due
to an increase in dark carrier concentration or an increase in mobility, the voltage
dependence of th e photoconductivity was also investigated. A tungsten light source
was used to illum inate the devices so th a t th e current increased by an am ount Alph-
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
le — 05
Experiment O
Model —
O
s
le — 07
<v
w
§
le — 09
le — 10
0
50
100
150
200
250
Voltage (V)
Figure 5.13. I-V characteristic of diamond film w ith gold contacts top and bottom .
Experim ental d a ta is shown by d a ta points. The solid line represents the model
results of Eq. (5.12) w ith G oo=2.6xlO -10 fl-1 , Go = 2.1 x lO -11 fl-1 , and a = 0.042
V ~ l . (Electrical sam ple : P47-T1)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
l e — 05
Experiment <0
Model —
le — 06
O
^
6
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u
le - 07
§
•3
c
le - °8
o
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le - 10
0
50
100
150
200
250
Voltage (V)
Figure 5.14. I-V characteristic of diamond film w ith top indium contacts and bottom
gold contact. Experim ental d a ta is shown by d a ta points. The solid line represents
the model results of Eq. (5.12) with G oo=2.1xlO -10 ft-1 , Go = 2 .0 x l0 -11 fl-1 , and
a = 0.045 V -1 . (Electrical sam ple : P47-T3)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0 .0 0 0 1
Experiment O
Model —
;
'
le — 05
O
-2
s
le - 06
CD
0
1
lc -0 7
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g
o
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le - 08
le — 09
le - 10
0
20
40
60
80
100
120
140
160
180
Voltage (V)
Figure 5.15. I-V characteristic of diamond film with silver contacts top and bottom .
Experim ental d a ta is shown by d ata points. The solid line represents the model
results of Eq. (5.12) with Goo=2.9xlO -10 fi-1 , Go = 4 .0 x l0 ~ 12 fI- 1 , and a = 0.08
V -1 . (Electrical sam ple : P49-T2)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
T he photoconductance is defined here as
( 5. 14)
Gph = AIph/V.
T he results of th e photoconductance are shown in Figure 5.16 for several photon flux
intensities. In all cases, the photocurrent was less th an 10 % of th e dark current, and
so m ay be considered as a sm all signal contribution to th e to tal current. For a given
light intensity, the photoconductance is
Gph = qAtiphfiA/t
(5.15)
where A is th e area of the top contact, t is the film thickness, fi is th e carrier mobility,
and Ariph is the increase in carrier concentration due to photo-excitation. Eq. (5.15)
is w ritten generally for any carrier type, th a t is A n ph m ay refer to photo-generated
electrons, or holes, or both.
If the increase in dark conductivity at high voltages
is prim arily due to a field activated mobility, then th e photoconductance would be
expected to show a sim ilar increase at high voltages. In fact, however, the d a ta in
Figure 5.16 shows th a t Gph is essentially constant w ith respect to voltage. Over
th e sam e voltage range, the dark conductance increased by approxim ately an order of
m agnitude. Consequently, th e field activated dark conductivity seems to be prim arily
due to an increase in carrier concentration.
An electric field activated conductivity has been previously reported in a wide
variety of non-single crystal insulating and sem iconductor films.
A linear plot of
the logarithm of th e conductivity versus the applied field is evidence of P o o le ’s
L aw and is often interpreted as being a result of Poole-Frenkel reduction of the
ionization energy associated with Coulombic potentials surrounding ionizable sites
resulting from im purities, local non-stoichiometry, or defects.
[128, 129, 130] The
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
l e — 08
O.D.: 1.0 A O.D.: 0.6 "S—
O.D.: 0.2 -O -
a
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4-=
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-a
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le - 11
10
20
30
40
50
60
70
80
90
100
Voltage (V)
Figure 5.16. T he photoconductance is nearly independent of the applied voltage.
Shown here are d a ta for different photon fluxes corresponding to different optical
density (O.D) filter values. (Electrical sam ple : P47-T1)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Poole-Frenkel mechanism causes an increase in conductivity due to an increase in
carrier concentration, resulting from ionization of Coulombic centers by th e applied
electric field. This is consistent w ith our experim ental evidence th a t it is a carrier
concentration increase, not a m obility increase, th a t gives rise to the field activated
conductivity.
W hen Coulombic potentials m ay be considered as non-overlapping, their contri­
bution to conductivity is proportional to exp(ftF 1/ 2/ ^ ) where /? is the Poole-Frenkel
constant, equal to e3/ 2(7re)-1/ 2, and F is the electric field [130]. However, when the
Coulombic center density is sufficiently high th a t there is appreciable overlap of the
Coulombic potentials, then th e contribution of th e Coulombic centers to the conduc­
tivity is proportional to exp( a F ) where a is a function of tem p eratu re and distance
between centers [130]. The conductivity exhibited by th e films in this study are ex­
amples of th e la tte r case, as indicated by Eq. (5.13) and Figure 5.13 to Figure 5.15.
A direct indication of th e density of the Coulombic centers m ay be found from
the slope of th e log of conductivity vs. voltage in Figure 5.13 to Figure 5.15. From
Hill, the expected slope would be e s /2 k T t where s is th e separation of Coulombic
centers (or defect centers) and t is the sample thickness. [130] For th e experim ental
slope of 0.045 V -1 for t = 3.5 /zm, the corresponding value of s is 4 x l 0 -7 cm. Taking
s to be equal to ./V-1/3 where N is the Coulombic center density, the value of N
is 1.6xlO 19 cm -3 . The overlap of Coulombic potentials is a m a tte r of degree, and
setting a m axim um separation for overlap is necessarily arbitrary. Following Hill, the
m axim um separation which still produces appreciable overlap m ay be taken, as an
approxim ation, as twice the distance from a site to th e m axim um in the barrier. The
lower lim itation of the density for overlapping is (e/?-1 ^ 1/ 2)3 [130]. Taking
(I)3
= N
s
= (e /T 1^
2)3
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(5.16)
this approxim ation leads to a relationship between s and th e electric field according to
s = f l / e F 1!2. As the electric field increases, this m axim um separation decreases. For
s = 4 x l 0 -7 cm, th e corresponding electric field is calculated to be 6 x 1 0 s V /c m . This
value is w ithin the range of fields a t which we observed field activated conductivity
and, given th e rough approxim ation used as an overlap criteria, is an indication th a t
overlap of Coulombic potentials is a reasonable hypothesis a t these densities.
Since th e lattice constant of diam ond is 3.567A and th e lattice configuration is
face center-cubic structure, th e diam ond crystal has 1.76 xlO 23 atom s/cm ?.
The
calculated Coulombic center density corresponds to approxim ately one center per
10,000 host atom s. W hile this value is high by single crystal sem iconductor standards,
it would not necessarily be unexpected for polycrystalline films deposited by chemical
vapor deposition. States at grain boundaries m ay be a contributor to these centers.
Additionally, thin film deposition system purity considerations are also a factor in
term s of im purity or defects states w ithin individual crystallites.
Because of th e field activated conductivity, the current is substantially larger at
high electric field than would otherw ise be the case. T he negative resistance observed
prior to breakdown is evidence of therm al effects which apparently cause therm al
runaway. For 3.5 fim and 2 fim thick films, this occurs at a voltage of 250 V and 150
V respectively. It corresponds to a breakdown field of 7 .2 x l0 5 and 7.5x10s V /cm .
Consequently, the dielectric strength of th e polycrystalline diam ond samples in this
study are substantially less than those reported for single crystal diam ond (0.6 lx lO 7 V /cm ) [131].
5.3.4
Photo Effect
If an incident photon has sufficient energy to excite a valence electron into th e con­
duction band, then, with a certain probability, th a t photon will be absorbed in the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
m aterial creating a hole-electron pair. Alternatively, a valence electron m ay be excited
to a higher lying defect state, creating a free hole, or a trap p ed electron m ay be excited
to the conduction band, creating a free electron. In any case, these photo-generated
carriers, being in therm al equilibrium v/ith their surroundings, recombine after some
tim e, generally via trapping dynam ics. During th e lifetim e of the photo-generated
carriers, an increase in electrical conductivity will be observed.
P hoton absorption in high purity, single crystal sem iconductors and insulators is
generally characterized by an absorption edge which occurs at the m inim um energy
required to free a valence electron and cause band-to-band excitation. Below this
energy there is little photon absorption, above it th e absorption increases sharply.
T he presence of defects and im purities allow different absorption phenom ena which
m ay be exhibited by a tail on th e absorption edge which extends into th e energy gap,
or by stru ctu re in the absorption spectra w ithin th e energy gap. This investigation
studied photo-conductivity due to photon energies sm aller th an the energy gap, and
therefore corresponding to excitation involving defects or im purities.
In this research, two sets of optical filters, which are visible light filters and infrared
filters, were used to study th e photo-effect. The monopass wavelengths of th e visible
filters are between 412 nm and 714 nm and those of th e infrared filters are between 775
nm and 1480 nm. Consequently, the covered range corresponds to photon energies
between 3 eV and 0.84 eV. Before the m easurem ent were conducted on the backetched samples, the relative power density for each filter was measured. A Newport
power m eter (model 815) w ith Si photodetector (model 818-SL) and Ge photodetector
(model 818-IR) were used to m easure the power for photon energies above 1.2 eV and
below 1.6 eV respectively.
As shown in Figure 5.17 (a), a calibration factor was
applied to the power m eter for different wavelengths of light for each photodetector
in order to get the accurate readings. Since the photon flux passing through each
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
filter is different, th e relative power intensities corresponding to each filter are shown
in Figure 5.17 (b).
T he photoconductance versus photon energy at -20 V and 20 V for the Au-Au
contact sample is shown in Figure 5.18(a) before th e norm alization of the relative
power intensity factors and in Figure 5.18(b) the norm alized photoconductance versus
photon energy is shown. Similarly, for th e In-Au (In contact on th e top) contact
sam ple, th e relationship of th e photoconductance versus energy before and after the
norm alization are shown in Figure 5.19(a) and (b) respectively.
For both the Au-Au contact sam ple and th e In-Au contact sample, th e relationship
of photoconductance versus photon energy is nearly the sam e for positive and neg­
ative bias. The existence of appreciable photoconductance well below th e band gap
indicates m any defect states existing in the polycrystalline diam ond films. T he phe­
nomenon of increasing sub-bandgap photo-conduction w ith increasing photon energy
is also observed by a group w ith an international cooperation effort using a differ­
ent approach [132]. They found th a t all CVD diam ond films exhibited an alm ost
m onotonically increasing absorption with increasing photon energy. As the absorp­
tion increases, the generation of electron-hole pairs increases and th e photo current
increases.
5.4
5.4.1
The M etal/D iam ond/Silicon Samples
The I-V Measurement Set-up
T he m etal/diam ond/silicon samples as described in chapter 3 (section 5.2) were placed
on the Signatone m icroprobe station as shown in Figure 5.20. Current-voltage (IV) characteristics were m easured at room tem perature, w ith and w ithout photo­
excitation. As for the back-etched samples, an HP 4145B sem iconductor param eter
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(a)
800
Si D etector
G e D etector
700
=3
J3
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600
<
500
u*
400
a
vs
a)
300
jo
13
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200
100
400
600
800
1000
1200
1400
1600
W avelength of Optical M onopass F ilter (nm)
0
>)
1.2
Si D etector
G e D etector -®—
ha
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0
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0.6
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13
s
ka
o
0.2
Z
400
600
800
1000
1200
1400
1600
W avelength of O ptical Monopass Filter (nm)
Figure 5.17. (a) The calibration factors corresponding to different optical wavelength
for Si and Ge photodetector, (b) T he relative power intensity factors for Si and Ge
photodetector at different wavelength of the optical monopass filters.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
J.'Zt?
(a)
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Photon Energy (eV)
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bias: 20V(Ge
biai:-20V(Si
biaa:-20V(Ge
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9
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3
3.5
Photon Energy (eV)
Figure 5.18. Relation of photoconductance vs. photon energy for Au-Au contact
sam ple (a) before norm alization and (b) after norm alization. (Electrical sam ple :
P47-T1)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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bias: 20V
bias:-20V
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Photon Energy (eV)
(b)
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bias: 20V(G e
bias:-20V(Si
bias:-20V(Ge
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0.5
1.5
2.5
3.5
Photon Energy (eV)
Figure 5.19. Relation of photoconductance vs. photon energy for In-Au contact
sam ple (a) before norm alization and (b) after norm alization. (Electrical sam ple :
P47-T3)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
analyzer was used to collect th e d a ta w ith a current resolution of approxim ately 100
pA . In all cases, p-type silicon was used as the substrate.
5.4.2
Contact Effects on M etal/D iam ond/Silicon Samples
T here are two ways of fabricating th e m etal/diam ond/silicon samples as previously
m entioned in chapter 3 (section 5.2). The I-V characteristic of th e large grain size
diam ond samples (paste-polished samples) w ith A1 contacts and th e small grain size
diam ond samples (powder-polished samples) w ith Au, Ag, and In contacts will be
described and compared to th e results reported in th e last section for back-etched
samples. Silver paste was used to form contact on th e back side of the silicon wafer.
For th e p-type silicon used here, th e paste contact was ohmic.
For low voltages, less th a n 25 V, the current-voltage (I-V) characteristics were
nearly linear and sym m etric for all the Au, Ag, and In contacts as shown in Figure 5.21
and Figure 5.22 on fine grain, diamond-powder prepared samples.
It is strongly
believed th a t in these small grain size diamond films, th e defect state density is so
high th a t th e tunneling process dom inates the m etal/d iam o n d contact I-V properties.
Consequently, ohmic behavior is observed for both high work function m etal contacts
(Au) and low work function m etal contacts (In) of the m etal/d iam o n d contact. This
is consistent w ith the m etal/diam o n d contacts on th e back-etched samples.
The d a ta in Figure 5.21 and Figure 5.22 also indicate th a t there is no rectifying
behavior at th e diam ond/silicon interface. On th e diam ond side of this interface,
it is reasonable to assume th a t, if a barrier did exist, a high defect density again
produces such a thin barrier th a t tunneling allows ohmic behavior. For the silicon,
the startin g wafer which has been severely abraded as shown in Figure 3.5. Con­
sequently, the defect density in the powder abraded silicon is also expected to be
high, which would result in a silicon barrier th a t was also sufficiently thin to tun-
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
SMU4 SMU3
SMU2
SMU1
HP 4145 B
Semiconductor Param eter Analyzer
Tungsten
Probe
r
Al Dot
Diamond Film
P-type Si
Ag Paste
Al Coating
Si Substrate
/
/'
Signatone Microprobe Station
Figure 5.20. The I-V m easurem ent set-up of th e m etal/diam ond/silicon samples.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
nel through. In fact, several decades ago, ohmic contacts to sem iconductors were
sometimes m ade by abrading th e sem iconductor w ith sandpaper in order to create a
defect-laden surface. A lternately, th e lack of a rectifying barrier may also be due to
a lack of appreciable band bending in the silicon. As will be elaborated on later, the
results of photo-excitation experim ents described in th e next section indicate th a t
there is no significant band bending in the silicon, either for powder-polished samples
or for paste-polished samples.
In contrast to th e pow der polished, small grain samples, for the large grain size
paste-polished diam ond sam ples, the I-V characteristics of the A l/diam ond/silicon
stru ctu re showed rectifying behavior as shown in Figure 5.23 and Figure 5.24. This
indicates th a t the defect density is not as high as in th e sm all grain size diam ond films.
It is reported th a t polycrystalline diamond films have a m uch higher defect density
th an the single crystal, and th e concentration of defect increases as th e concentration
of m ethane increases [132]. This research indicates th a t different nucleation tech­
niques a n d /o r different grain sizes are also factors contributing to defect states in the
films.
Rectifying phenom ena of m etal/diam ond/silicon structures have been previously
reported by several groups [133,134] in recent years, and sim ilar behavior was also ob­
served for bulk boron-doped diam ond synthesized under ultrahigh pressure conditions
[135]. T he rectifying property has been variously attrib u te d to th e band bending at
the top-contact/diam ond interface [133] and back-contact/diam ond/silicon interface
[134]. It is im portant to know which interface in fact contributes to th e rectifying
characteristics in our case. F urther investigation using additional inform ation from
photo-excitation will be discussed in the next subsection.
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
0.00015
Current (A)
0.0001
5e — 05
—5e - 05
-
0.0001
-0.00015
-2 5
-2 0
-1 5
-1 0
-5
0
5
10
15
20
25
Voltage (V)
Figure 5.21. I-V characteristic of the small grain size A u/diam ond/silicon samples.
(Electrical sam ple : P39)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
le — 05
8e — 06
Current (A)
6e — 06
4e — 06 2e — 06
—2e - 06
—4e — 06
—6e - 06
—8e - 06
25
-2 0
-1 5
-1 0
-5
0
5
10
15
20
25
Voltage (V)
Figure 5.22. I-V characteristic of the small grain size In/diam ond/silicon samples.
(Electrical sample : P39)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2e — 09
le -0 9
-le -0 9
ta
cu
b
^
2e - 09
—3e - 09
—4e - 09
—5e - 09
—6e - 09
25
-2 0
-1 5
-1 0
-5
0
5
10
15
20
25
Voltage (V)
Figure 5.23. I-V characteristic of the large grain size A l/diam ond/silicon sampl
The average grain size is 2.1 /zm. (Electrical sam ple : P6)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4e — 06
2e — 06
Current (A)
2e — 06
—4e — 06
6e — 06 —8e - 06
—le — 05
—1.2e — 05
—1.4e — 05
—1.6e — 05
—1.8e — 05
25
-2 0
-1 5
-1 0
-5
0
5
10
15
20
25
Voltage (V)
Figure 5.24. I-V characteristic of the large grain size A l/diam ond/silicon samples.
T he average grain size is 1.4 fim. (Electrical sam ple : P7)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5.4.3
Photo Effect
T he experim ents of th e photo effects on the m etal/diam ond/silicon samples used the
sam e m ethod and technique as th a t described in chapter 5 (section 3.4).
Figure 5.25 and Figure 5.26 show the relationship of photoconductance versus
photon energy for th e sm all grain size A u/diam ond/silicon sam ple and the small
grain size In/diam ond/silicon sample respectively. Similarly, th e photoconductance
versus photon energy characteristics for different large grain size A l/diam ond/silicon
samples are shown in Figure 5.27, Figure 5.28 and Figure 5.29. If th e rectification
happens at the diam ond/silicon interface and is a result of silicon band bending,
then silicon should play a role in contributing to th e photo current. Consequently,
th e photo current should abruptly increase as th e photon energy increases above 1.1
eV, which is the energy gap of the silicon. However, in bo th large and small grain
size m etal/diam ond/silicon samples there are no indications th a t photoconductance
abruptly increases a t photon energies above 1.1 eV. This showed th a t th e rectifying
properties are not contributed by the diam ond/silicon interface b u t rath er by the
A l/diam ond interface and th a t band bending happens at th e A l/diam ond interface.
It is also observed th a t a negative bias did generate m ore photo current than a
positive bias in Figure 5.27, Figure 5.28, and Figure 5.29. Figure 5.30 shows a possible
energy band diagram of th e A l/p-ty p e diam ond interface. D uring the photo-excitation
th e valence electrons are excited into the higher lying acceptor level creating more
holes in the valence band. Since the band bending happens a t the A l/diam ond inter­
face, th a t a negative bias drawing more photo current from th e diam ond is reasonable.
Generally, this research indicates th a t large grain size diam ond films prepared
by the paste-polished m ethod provided b e tte r m etal/diam ond rectifying properties
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
le-06
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u
§
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3
-3
3
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1.5
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(b)
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biaa: 15V(Si
biai: 15V(Ge
biu:-15V (S i
b iu:-15V (G e
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0.5
1
1.5
2
2.5
3
3.5
Photon Energy (eV)
Figure 5.25. Photoconductance versus photon energy (a) before norm alization and (b)
after norm alization for the sm all grain size A u/diam ond/silicon sample. (Electrical
sam ple : P39)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
le-07
b i u : 15V 9 biu:-15V
C
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2
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2.5
Photon Energy (eV)
(b)
le -0 6
b i u : 15V(Si
bias: 15V(Ge
biu:-15V (S i
biu:-15V (G e
'2
D
D etector) • 9 D etector) -B—
D etector)
D etector)
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a>
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s
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3
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8
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0.5
1
1.5
2
2.5
3
3.5
Photon Energy (eV)
Figure 5.26. Photoconductance versus photon energy (a) before norm alization and
(b) after norm alization for th e small grain size In/diam ond/silicon sam ple. (Electrical
sam ple : P39)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(a)
le-06
b iu : 15V ■ $ — '
biu:-15V • * - ]
S3
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0.5
1
1.5
2
2.5
3
3.5
Photon Energy (eV)
(b)
le -0 6
C
b i u : 15V(Si
b i u : 15V(Ge
biu:-15V (S i
biu :-1 5 V (G e
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D elector) •O - :
D etector) -B— |
D etector)
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1
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O
o
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o
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0.5
1
1.5
2
2.5
3
3.5
Photon Energy (eV)
Figure 5.27. Photoconductance versus photon energy (a) before norm alization and (b)
after norm alization for the large grain size A l/diam ond/silicon sample. The average
grain size is 2.1 /zm. (Electrical sample : P6)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10Z
(a)
le-06
u.
le -0 7
§
*a
u
3
T3
S
O
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Photon Energy (eV)
(b)
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bias: 15V(Ge
bias:-15V(Si
bias:-15V(Ge
D etector) •O—
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w
3
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ocj
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-3
a.
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0.5
1
1.5
2
2.5
3
3.5
Photon Energy (eV)
Figure 5.28. Photoconductance versus photon energy (a) before norm alization and (b)
after norm alization for the large grain size A l/diam ond/silicon sample. The average
grain size is 1.4 /im. (Electrical sam ple : P7)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(a)
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0.5
1
1.5
2
2.5
3
3.5
Photon Energy (eV)
(b)
0.001
bias: 15V(Si
bias: 15V(Ge
bias:-l5V (S i
bias:-15V (Ge
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D
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0.5
1.5
2.5
3.5
Photon Energy (eV)
Figure 5.29. Photoconductance versus photon energy (a) before norm alization and (b)
after norm alization for the large grain size A l/diam ond/silicon sample. The average
grain size is 1.1 fim. Electrical sam ple : P20)
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Al
999999^
(-)
D iam ond
------- 1— f— J— §• —f -------------- F
TO
O
O
O
O"
Figure 5.30. The energy band diagram in the A l/p -ty p e diam ond interface.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
than the small grain size diam ond films prepared by th e powder-polished m ethod.
However, it doesn’t m ean th a t the larger th e grain size the b e tte r th e rectifying
properties, independent of other variable. In this work, the sam ple which has the
best rectifying properties is not the sam ple w ith biggest grain size diam ond crystal.
The degree of rectifying is also dependent on other properties of th e m etal/diam ond
interface. Details of studies on the device w ith th e best rectifying properties in this
study will be discussed in th e next subsection.
5.4.4
Diamond Schottky Barrier Diode
The best m etal/diam ond/silicon rectifying structures were prepared by the pastepolished m ethod. The best of these rectifying characteristics were formed on film
NDF-P20, deposited at conditions of 700 W microwave input power, 60 Torr pressure,
1 % m ethane/hydrogen concentration (1.5 seem : 150 seem). Typical I-V characteris­
tics of the tungsten point contact/diam ond/silicon and A l/diam ond/silicon structures
on th e film are shown in Figure 5.31 and Figure 5.32 respectively. T he R am an spec­
tru m of NDF-P20 is shown in Figure 5.33. The point contact stru ctu re shows the
ohmic behavior which again supports the m echanism th a t the rectifying property is
a result of th e A l/diam ond interface. W hen two coplanar contacts are m ade, with
one contact being the evaporated alum inum circle and th e other being th e tungsten
point probe, SBD characteristics are again observed as shown in Figure 5.34. In this
case the current passes via th e rectifying diamond/A1 contact through th e diamond
film to the silicon, through th e silicon, and back up through the diam ond film to the
ohmic tungsten point contact.
Figure 5.35 shows th a t th e forward bias current (Ip) to the reverse bias current
(I r ) ratio a t 10 V is alm ost 2x 1 0 s. However, the ratio drops to 2 x l0 2 when the
voltage increases up to 25 V. Sim ilar phenom ena have been reported by other groups
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
8e —
6e —
4e —
2e —
—4e —
—6e —
—8e —
-2 5
-2 0
-1 5
-1 0
-5
0
5
10
15
20
25
Voltage (V)
Figure 5.31. The I-V characteristic of the point co n tact/diam ond/silicon structure
showed ohmic behavior. T he top contact is a tungsten point probe and the bottom
contact is the silicon wafer. (Electrical sample : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5e — 05
—be - 05
■g
£
t-i
3
O
-0.0001
-0.00015
-
0.0002
-0.00025
-2 5
-2 0
-1 5
-1 0
-5
0
5
10
15
20
25
Voltage (V)
Figure 5.32. The best Schottky barrier diode (SBD) characteristic of the large grain
size A l/diam ond/silicon samples. The top contact is alum inum and the bottom con­
ta c t is the silicon wafer. (Electrical sam ple : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
intensity
100000
80000
60000
400
600
800
1000
1200
1400
1600
wavenumber (cm"1)
Figure 5.33. The R am an spectrum of NDF-P20. The peak between 1550cm-1
1600cm-1 indicates th a t the existence of graphitic com ponent in th e film.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5e - 0 6
—5e — 06
—le — 05
i
-t-s
c
O
—1.5e — 05
—2e - 05
—2.5e - 05
—3e - 05
—3.5e - 05
4e - 05
—4.5e - 05
-2 5
-2 0
-1 5
-1 0
-5
0
10
15
20
25
Voltage (V)
Figure 5.34. T he SBD I-V characteristic of a film w ith coplanar surface contacts.
One contact is alum inum and the other contact is a tungsten point probe. (Electrical
sample : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
[76, 80, 137]. It is noted th a t the samples of [76, 80] are intentionally boron-doped
and th e samples of [133, 136, 137] and of this research are self-p-typed-doped during
the deposition.
The current-voltage results of the diode are best analyzed by considering the
current as a function of both the voltage across th e junction and th e voltage across
the bulk region. For th e form er
l K l 0 exp(qVj/rjkT)
where Vj is th e junction voltage,
77
is th e diode ideality factor (1 <
(5-17)
77
< 2), T is
tem perature, k is B oltzm ann’s constant and Io is the saturation current. Then for
any current, the junction voltage is found to be
V = ^ l n ( / / / 0)
(5.18)
and the bulk voltage is next found from
Vb = VApplied - Vj
(5.19)
Consequently, th e forward I-V characteristic of the total stru ctu re is modeled by
V = T^ - \ n ( I / I 0) + f ( I )
7
(5.20)
where Vb — f ( I ) represents th e I-V characteristics of the bulk m aterial. In its sim plest
form, the relationship between the bulk voltage and the current would be ohmic,
Vb = I R
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
(5.21)
0.001
0.0001
le — 05
<*-—- s
<
le — 06
C
le -0 7
2u
A
le - 08
le -0 9
le - 10
le - 11
25
-2 0
-1 5
-1 0
-5
0
5
10
15
20
25
Bias Voltage (V)
Figure 5.35. The log(I)-V characteristic of the SBD. (E lectrical sample : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
where R = p L / A is the resistance of the diamond film. However non-ohmic behavior
is often observed in diamond films, an example being th e field activated conductance
reported in chapter 5 (section 3.3) of this research.
T he d a ta to be modeled by Eq. (5.20) is the forward I-V characteristic of th e diode
shown in th e semilogarithmic plot of Figure 5.36. The param eters used in fitting th e
equation to th e d ata are /o, V, and the functional relationship f(I).
T he d a ta shown in Figure 5.36 doesn’t show any straig h t line region in which
the I-V characteristics are dom inated by the diode portion of Eq. (5.20). For all
currents in th e range of m easurem ents, the bulk voltage plays a m ajor role in th e to tal
voltage. However, two straight lines representing the diode portion of the voltage are
superim posed on the plot of F igure 5.36 which are consistent w ith th e m easured data.
These straight lines correspond to quite different Iq and rj values. The solid line shows
the diode voltage versus current for 7 o = 5 .2 x l0 -16 A and for
77
=
1.
shows th e diode voltage versus current for 7 o = 2 .9 x l0 -14 A and for
The dashed line
77
= 2. As will be
shown below, both lead to sim ilar conclusions about th e bulk I-V relationship, f(I).
One m ight expect th a t th e f(I) relationship would follow th e field activated con­
ductivity model of Eq. (5.12). However Figure 5.37 shows th a t this is not the case.
Above 10 V th e d ata is consistent with a field activated conductivity w ith 7?b = 6 x l0 4
V /cm , consistent with the field activated model. However, below 10 V th e m odel
does not fit, since the conductivity decreases rapidly instead of approaching a con­
stan t value.
T he d a ta is b etter fit by assum ing a power law for f(I) where
I<xVbm
(5.22)
which m is the degree of power. As shown in Figure 5.38 and Figure 5.39 both m = 3 /2
and m = 2 give a reasonable fit to the data, with the m = 3 /2 fit being m arginally
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0 .0 1
0.001
Experiment -0 — :
Ideal diode: J/=1 ----Ideal diode: T]=2
0.0001
^
le - 05
<
le — 06
S
fc
^
l e - 07
le -0 8
le -0 9
le — 10 r
i
ie - 11 ---------------- 1----------------- 1---------------- 1----------------- 1----------------:
0
5
10
15
20
25
Bias Voltage (V)
Figure 5.36. The forward bias I-V characteristic of the SBD. T he o represents the
experim ental data, the solid line shows the ideal diode w ith tj = 1, 7o= 5 .2 x l0 -16 A
and the dashed line shows th e ideal diode with 77 = 2, /o = 2 .9 x l0 ~ 14 A. (Electrical
sam ple : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.0001
Conductance (mho)
Bulk Voltage(77=l) —
Bulk Voltaga(?7=2) • • •
. le - 05
le — 06
le-07
le-08
le-09
0
5
10
15
20
25
Bulk Voltage (V)
Figure 5.37. Conductance-bulk voltage characteristic. It follows the field activated
model above 10 V. (Electrical sam ple : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
b e tte r. T he energy band diagram and the equivalent circuit of th e SBD are shown in
Figure 5.40(a) and (b) respectively. The exact choice of
Iq
and
77
is not critical to the
fits since th e diode portion of th e voltage drop is relatively negligible, especially at
the higher range of voltages. T he range of
Iq
values corresponds to a barrier height
range of between 1.1 eV and 1.2 eV.
T he significance of th e pow er law dependence m ay b e considered further as fol­
lows. L am pert and M ark loosely defined m aterials w ith band gap E g < 2 eV as
sem iconductors and those w ith E g > 2 eV as insulators [138]. By this definition,
un-doped diam ond w ith E g = 5.5 eV can be considered .as an insulator. In a perfect
tra p free solid state insulator, th e space-charge lim ited current-voltage relation can
be expressed as
IocV 2
(5.23)
ra th e r th an the V 3/ 2 . In considering the difference it is noted th a t the derivation of
Eq. (5.23) is based on the assum ption th a t carriers drift a t a velocity, u, with
v = fiE
(5.24)
where \i is carrier m obility and E is applied electric field. However, if the field strength
is too high, th e drift velocity of th e carriers may vary as th e square root of the applied
electric field when above a certain critical field strength E r [138]. This is known as a
characteristic of ’’w arm ” carriers when acoustic phonon scattering is dom inant. Then
for
E > E r,
V
= p(E E r ) l / 2
(5.25)
T he current-voltage relation becomes
IocErV 2 V 3 / 2
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(5.26)
0.0003
Bulk Voltage(»7=l) ----Bulk Voltage(»7=2)
Current (A)
0.00025
0.0002
0.00015
0.0001
5e — 05
0
0
20
40
60
80
100
120
Bulk Voltage3/2 (V 3/2)
Figure 5.38. The I-V;,3^2 characteristic of the SBD. (Electrical sam ple : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.0003
Bulk Voltage(r/=l)
Bulk Voltage(?/=2)
Current (A)
0.00025
0.0002
0.00015
0.0001
5e — 05
0
0
100
200
300
400
500
600
Bulk Voltage2 ( V 2)
Figure 5.39. The I-V&2 characteristic of the SBD. (E lectrical sample : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Al
D iam ond
(b)
f ( I)
'CVy'I&Ioexp(qVj/r)kT)
I<xVt,m
Figure 5.40. (a) The energy band diagram of the SBD. The barrier height </>p equals to
the sum of built-in potential V{ and activation energy E a. (b) T he equivalent circuit
of the SBD.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
which has th e same functional form as th e observed data.
For single crystal diam ond, th e high field phenom enon happens above 1x10s
V /cm . In this case th e film thickness is about 4 fim , so a voltage of 10 V corresponds
to an electric field of 2.5 xl O4 V /cm . The high field effect m ight not contribute to
the phenomenon observed here.
An obvious point is th a t th e powder-polished samples of chapter 5 (section 3.3) fol­
low a field activated conductivity and exhibit ohmic, non-Schottky behavior whereas
the paste-polished samples of this section follow a power law I-V and Schottky behav­
ior. Both of these observations are consistent w ith the hypothesis th a t th e powderpolished films have a larger defect concentration. Consequently, m etal/d iam o n d space
charge layers are sufficiently thin to allow tunneling and ohmic behavior and their
high field properties are dom inated by ionizable defects.
It m ust also be noted, here th a t some of th e SBD exhibited a p articu lar form of
instability. T he current becomes much sm aller after several tim es of m easurem ents.
However, th e rectifying behavior still exist.
Capacitance versus frequency m easurem ent were also perform ed on the
A l/diam ond/silicon diodes w ith a HP4192A im pedance analyzer and th e d a ta is pre­
sented in Figure 5.41 for different values of reverse bias.
T he barrier height <f>v can also be determ ined by the capacitance versus voltage
m easurem ent. By plotting 1 / C 2 against reverse bias, one can get built-in potential Vi
from the extrapolated intercept on the horizontal axis. From the slope of the straight
line one can also calculate th e doping concentration. As shown in Figure 5.40(a) the
barrier height (f>v can be found from
fa ^V i + E a
(5.27)
where E a is activation energy. However, from Figure 5.42 th e d a ta shown did not lie
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
le — 07 ----- 1----1—' i i i i 11------- 1---r—i—i i-m |-------1--- 1—i i i i i i"|
<h
Capacitance (F)
V
_l ^
A
le - 11
O
+
□
X
A
★
°
+ O
o
° a \
le-lO f
i i r-n
bias: -IV
bias: 0V
bias: IV
bias: 2V
bias: 3V
bias: 4V
bias: 5V
i „ na
l e “ 08 t
le — 09 Tr
1
R3-,
b b i& m
&XSHE aas^a» 4 g s
le - 12
1
10
100
1000
10000
Frequency (KHz)
Figure 5.41. C apacitance versus frequency characteristic at different reverse bias.
(Electrical sam ple : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
in a straight line. This may indicate th a t th e self-doping of th e film is non-uniform.
Consequently, th e barrier height can not be determ ined from Figure 5.42.
In Penn S tate University’s work [133,136, 137], they determ ine th e ideality factor
as 1.8, and th e barrier height as 1.15 eV for A1 and Au contacts to p-type CVD
diamond films. They were not able to determ ine barrier height from the I-V curve,
instead they use the technique of internal photoem ission to determ ine the barrier
height. In H. Shiomi, et al.’s work [76], an ideality factor of 2.1 and barrier height
of 1.2 eV were all determ ined from th e I-V m easurem ent of th e W contact to borondoped diam ond films. They noted th a t barrier height could not be obtained from
C-V curve due to the high series resistance in th e bulk. In D. G. Jeng, et al.’s work
[80], they noted th a t an ideality factor of 1.85 was determ ined from the low voltage
region (0.1-0.5 eV) slope for th e A l/diam ond Schottky diode. It is obvious th a t the
understanding of th e synthetic diam ond film diodes is still prelim inary. M any factors
contribute to th e film properties and since the deposition conditions are so different
from group to group, more experim ents are needed to fu rth er investigation.
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Capacitance
2 (1021 F 2)
5
4
3
2
1
0
-3
-2
-1
0
1
2
3
4
5
Bias Voltage (V)
Figure 5.42. C apacitance versus reverse bias characteristic at 50 kHz. (Electrical
sam ple : P20)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 6
Sum m ary and Future R esearch
6.1
Summary of Important Results
The application of the microwave plasm a disk reactor (M PD R ) chemical vapor depo­
sition system for diamond synthesis has been successfully developed. Good quality
polycrystalline diam ond films have been successfully deposited and characterized on
silicon and silicon nitride substrates. Two sample preparation m ethods, which are
paste-polished and powder-polished nucleation techniques have been used for deposit­
ing large grain size diamond films and small grain size diam ond films respectively. A
back-etching processing technique was also developed to isolate the deposited dia­
mond films from the silicon su b strate in order to study th e diam ond/silicon interface
and allow access to evaporate m etal contacts on both sides of the diamond films in
order to study the electrical properties of the diamond films.
A goal of this research has been to provide an increased understanding about the
relationship between properties of thin film diamond and th e preparation conditions.
Toward this purpose several im po rtan t physical characterizations of the diamond
films, such as R am an spectroscopy, X-ray photoelectron spectroscopy, DekTak pro­
file m easurem ent, scanning electron microscopy, and laser scanning microscopy were
183
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carefully studied (see chapter 4). A particular focus of this research has been on the
electrical properties of the films, and consequently several im p o rtan t electrical prop­
erties of th e diam ond films, such as activation energy, I-V characteristics, high field
eifect, and contact phenom enon were also studied (see chapter 5). T he im portance
of preparation conditions on th e n atu re of m etal contacts is described in chapter 5
and new inform ation about th e relationship between defects and high electric field
properties is provided by this research. T he following subsections provide specific
sum m ary highlights.
6.1.1
Nucleation M ethod
The nucleation m ethod is im p o rtan t for the diamond synthesis since as was shown
in this research, th e density of nucleation sites is a m ajor factor in determ ining the
grain size of th e diamond films.
The paste-polished nucleation m ethod produced com paratively larger grain size di­
amond films than the powder-polished nucleation m ethods did since th e latter m ethod
provides m any more nucleation sites than the form er m ethod. It was also observed
experim entally th a t th e average grain size obviously increased as the m ethane con­
centration increased for the paste-polished nucleation m ethod b u t not the powderpolished nucleation m ethod. For both nucleation m ethods, th e average grain size of
diamond increased as the microwave power or plasm a density increased, but th e effect
was smaller than the effect of increasing m ethane concentration in the case for the
paste-polished nucleation m ethod.
For both m ethods, the grow th rate of diam ond increased as m ethane concentration
increases. T he growth rate also increased as the microwave power or plasm a density
increased, however, the effect was sm aller than the m ethane concentration. At the
same deposition conditions, the growth rate of diamond by th e powder-polished nu-
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cleation m ethod is slightly higher th an th a t by th e paste-polished nucleation m ethod.
6.1.2
Quality of Diamond Films
Ram an spectroscopy is used as th e m ajor technique to distinguish th e quality of the
diam ond films. T he reported natural diam ond peak is at 1332 cm -1 . T he larger the
R am an signal a t 1332 cm -1 and the sm aller th e other peaks, th e higher quality of
th e diam ond films. It is known th a t th e tem peratures for depositing good quality
diamond films is a critical variable for diam ond synthesis.
If th e tem perature is
too low, an am orphous diamond-like com ponent can become dom inant and graphite
becomes dom inant instead of diam ond if th e tem p eratu re is too high.
Generally, from this research it was observed, based on R am an results, th a t at
lower m ethane concentration one can get good quality diam ond films in a wider
tem perature range than at higher m ethane concentration. It was found th a t for the
range of power and pressure investigated, th e preferential tem p eratu re range for good
quality diam ond films is betw een 1030 °C and 1060 °C. It was also noted th a t the
two nucleation m ethods, under the sam e deposition conditions, appears to make no
obvious difference in th e quality of the diam ond film.
6.1.3
Back-Etching Technique
For film characterization, it is advantageous to have access to both sides of the film. In
this research, a back-etching process was successfully developed in cooperation w ith
Dr. Engem ann at the University of W uppertal (G erm any) to transfer the diamond
film from the silicon sub strate to an epoxy substrate.
There are two kinds of back-etched samples fabricated by this technique. One
kind of samples were used for physical characterizations (specifically XPS) of the
back surface (diam ond/silicon interface). T he other kind of samples were processed
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to have m etal contacts on both sides of th e film for electrical p roperty studies of the
diamond film.
6.1.4
D iam ond/Silicon Interface
From th e back-etched samples prepared by the paste-polished nucleation m ethod, it
is found th a t a thin layer of S i C existed a t the diam ond/silicon interface by X-ray
photoelectron spectroscopy analysis. This supports the hypothesis th a t th e formation
of a thin layer of S i C is an inherent p a rt of the growth of diam ond on the silicon
substrate.
6.1.5
A ctivation Energy of Diamond Films
A four-point probe technique was used for sheet resistance m easurem ents of the di­
amond films. Diam ond films were deposited on an insulating com posite m aterial,
silicon nitride ( S i s N ^ , using both paste-polished and powder-polished nucleation
m ethods. All th e diam ond films were determ ined as p-type by th e hot probe tech­
nique.
The sheet resistance for th e as-deposited samples prepared by th e powder-polished
nucleation m ethod was on th e order of 10s to 106 D /D . By varying the tem perature
for the sheet resistance m easurem ent, activation energies of 0.22 to 0.31 eV and 0.13 to
0.23 eV were obtained for carrier m obility equals to 1200 and 12 c m 2/ V ' S respectively
(see chapter 5, section 2.4). T he samples were then annealed in a nitrogen am bient
furnace a t 500 °C for one hour. This caused the sheet resistance to increase by at
least four orders of m agnitude and th e activation energy to increase as well.
However, th e sheet resistance of th e as-deposited samples prepared by the pastepolished nucleation m ethod was on the order of 104 D /D . This occurred because a
different system shut off procedure caused form ation of a conducting layer by the
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hydrogen plasm a. W hen the sample was cleaned by a solution (see chapter 5, section
2.4) in order to remove th e conducting layer, th e sheet resistance was determ ined
on the order of 108 fl/D w ith an activation energy of 0.51 eV and 0.42 eV for hole
mobility equals to 1200 and 12 cm 2/ V - s respectively (see chapter 5, section 2.4).
6.1.6
Electric Field Dependent Conductivity of Diamond
Films
The dc electrical conductivity of polycrystalline diam ond films w ith submicron grain
sizes and ohmic contacts was studied as a function of th e applied electric field up
to th e point of electrical breakdown.
For electric fields below approxim ately 105
V /cm , the films exhibited predom inantly ohmic behavior w ith a conductivity th a t
was independent of the applied voltage for both high work function m etal contacts
(Au) and low work function m etal contacts (In). For higher electric fields, however,
the conductivity was field activated according to Poole’s Law. T he d a ta is consistent
w ith a Poole-Frenkel reduction of the ionization energy associated w ith Coulombic
potentials surrounding ionizable centers, where th e Coulombic potentials overlap. The
ionizable centers m ay result from im purities or defects. In this study, experim ental
d a ta showed th a t it is a carrier concentration increase, not a m obility increase, th a t
gave rise to th e field activated conductivity which is consistent w ith th e Poole-Frenkel
mechanism.
Because of the field activated conductivity, the current at high electric fields is
substantially larger than would otherwise be the case. For 3.5 /*m and 2 fim thick
films, the breakdown happens at voltages higher than 250 V and 150 V respectively,
corresponding to a breakdown field of 7.1x10s and 7.5 x10s V /cm respectively. Con­
sequently, th e dielectric strength of th e sub-micron grain size polycrystalline diamond
films in this study are substantially less th an those reported for single crystal dia­
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m ond (0.6—l x l O 7 V /cm ). T he slope of the conductivity versus electric field indicates
approxim ate one ionizable im purity or defect per 10,000 host atom s in such films.
6.1.7
Diamond Schottky Barrier Diode
Polycrystalline diam ond Schottky barrier diodes were fabricated by using the large
grain size diamond films w ith the A l/diam ond/p-silicon structure. Rectifying behav­
ior was determ ined in th e A l/diam ond interface as opposed to th e diam ond/silicon
interface. The
If/Ir
ratio is alm ost 2 x l 0 5 a t 10 V in th e best device, b u t only 2 x l 0 2
at 25 V. The I-V characteristic can be modeled as an ideal Schottky diode in series
w ith an insulator, for which th e property (IcxV m relationship), is indicative of a space
charge lim ited current effect in the bulk diam ond. However, th e m etal/d iam o n d /p silicon samples for th e small grain size diam ond films exhibited ohm ic property since
the defect states in these films are so large th a t carriers can easily tunnel through the
barrier.
6.2
Future Research
This research has studied th e synthesis of diam ond films in th e M PD R deposition
system; physical characterizations, and electrical characterization of the diamond
films and diam ond/silicon samples. However, th e understanding of th e relationship
between the physical characterization and electrical properties is still in the early
stage.
More detailed investigation into the topics covered in this dissertation as
well as th e effects of other param eters (such as the vacuum purity, the addition of
oxygen, and other gases, and lower plasm a pressure) are still needed to com plete our
understanding of this research. Recom m endations for future research follow.
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6.2.1
Improvement for Diamond Deposition
T he deposition system described in this research can be considerably improved with
regard to system p u rity and with regard to deposition over larger area substrate.
W ith regard to th e la tte r, considerable progress has been m ade by a colleague, J.
Zhang [139]. W ith regard to system purity, th e vacuum system used in this research
was mechanically pum ped w ith a ultim ate pressure of l x l O -4 Torr. Residual gas
composition and leak /o u t gas rates were not specially quantified and m onitored on
a run-to-run basis. In order to b etter control im purities it would be useful to up­
grade the system to a l x l O -7 Torr u ltim ate pressure and to perform residual gas
analysis prior to runs. By providing a lower pressure capability, this also offers the
opportunity to investigate E C R plasm a deposition of diam ond at lower pressures and
tem peratures. If th e EC R plasm a technique can be successfully used in an M PD R
system , th e deposition area could be quite large, 6 inches in diam eter or larger.
O ther things being equal, there is always a trade-off between a large deposition
area and a high deposition rate since the am ount of reaction species is fixed for a
given reaction m ixture, gas flow and microwave power. Consequently, th e larger the
deposition area, th e lower th e deposition rate. W ith the addition of oxygen, however,
it is known th a t the quality and deposition rate can be improved. The extent to
which oxygen affects th e quality and deposition rate needs to be carefully studied on
different configurations of this system.
6.2.2
The Techniques for Physical Characterization
It is known th a t R am an spectroscopy is the best technique to distinguish diamond,
graphite, and am orphous carbon. However, R am an spectroscopy only provides lim­
ited inform ation about the sample, specifically it determ ines if th e film is similar
to natural diam ond having a sharp peak at 1332 cm -1 . For exam ple, R am an spec­
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troscopy can’t show the difference between a sample w ith and w ithout a very thin
layer of conducting graphite or chemical contam ination on top of the films. Also as
shown in Figure 4.7, R am an spectroscopy can’t show th e difference between a large
grain size diam ond film and a small grain size diam ond film deposited at the same
conditions.
O ther characterization techniques are therefore also needed in future work to
provide th e basic properties of the film to assist the R am an spectroscopy in order to
have a b e tte r understanding of the over-all physical properties of th e film. Examples
include secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy
(AES) to determ ine the elem ental composition of the film; X -ray diffraction (XRD) to
determ ine the crystal stru ctu re and lattice spacing; electron energy loss spectroscopy
(EELS) to determ ine the localized bonding in the bulk film; X -ray photoelectron
spectroscopy (XPS) to exam ine the chemical composition at th e surface of the film;
and scanning electron microscopy (SEM) to exam ine the surface morphology.
6.2.3
The Role of S iC at the Diam ond/Silicon Interface
It is known th a t a thin layer of S iC exists at the diam ond/silicon interface from
this and other research. It m ay be hypothesized th a t S i C contributed to the ohmic
behavior of the diam ond/p-silicon interface for both small and large grain size dia­
mond films in this research. However, it is not clear a t this point how S i C affects the
electrical properties of the diam ond/silicon heterojunction. It would be of interest in
future research to carefully prepare back-etched samples w ith and w ithout the S iC
layer in order to study this further. It would also be interesting to investigate how
the interfacial electrical properties vary if the nucleation m ethods is changed from
abrasion to seeding since the abrasion nucleation m ethod scratches the silicon surface
and the seeding nucleation m ethods does not.
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6.2.4
Defects States in Diamond Films
T here are clearly defect states in polycrystalline diam ond films because of th e exis­
tence of grain boundaries. However, defect states in each single crystal is also possible
since th e m ethane and hydrogen gases used in this research are 99.99 % and 99.999 %
pure respectively. In this research, it was shown th a t defect states play an im portant
role contributing to th e electrical properties. For small grain size m etal/d iam o n d /p silicon samples, th e num ber of defect states is so large th a t it contributes to th e ohmic
contact behavior. Large grain size A l/diam ond/p-silicon samples, however, did show
some rectifying properties since there are less defect states. F u rth er knowledge of
the defect states in th e diam ond films under different plasm a pressure and different
deposition system becomes an im portant future task for understanding th e electrical
properties of synthetic diam ond films.
6.2.5
Diamond Devices
A m ajor future goal of diam ond synthesis research is to fabricate useful dia­
m ond devices, such as diam ond diodes, diam ond transistors, diam ond metal-oxidesem iconductor structures, and diam ond integrated circuits (IC) w ith perform ance
advantages based on th e excellent properties of diam ond (see chapter 2, section 4).
T here is a need for considerable future work tow ard the developm ent of electronic
grade diamond.
A difficult, but im p o rtan t, future task for diam ond device research is the devel­
opm ent of the capability to synthesize epitaxial diam ond films on non-diam ond sub­
strates. The existing techniques only can deposit epitaxial diam ond films on diamond
substrates md poly crystalline diamond films on non-diam ond substrate.
Further­
more, it is im portant to evaluate and improve th e perform ance of th e poly-diam ond
devices. Future device related tasks also include th e development of improved tech­
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niques for diam ond doping and etching.
T he investigation of th e electrical properties of synthetic diam ond is still in the
prelim inary stage and th e fabrication of diam ond devices is still far from m ature.
Consequently, m uch m ore future research effort is needed in order to explore the
potential developm ent of diam ond devices.
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A P P E N D IC E S
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
A P P E N D IX A
D e p o site d D iam on d F ilm s
(1 )
Samples by Diamond-Paste Nucleation Method
M icrowave
P la sm a
Input
P
r e s s u r e Time
Pow er
(W )
( T o rr) ( ho u r)
S a m p le
N um ber
( NDF- # )
4/ h 2
and
Flow R a te
( seem )
N6
1 .5 %
(3 : 200 )
600
60
7
6 /1 7 7 8 9
P6
1 .5 %
( 3 : 200 )
600
60
7
6 /1 8 /’8 9
N7
1 .2 5 %
(2 .5 : 200 )
600
50
7
6 /1 9 /’89
P7
1 .2 5 %
( 2.5 : 2 0 0 )
600
50
7
6 /2 0 7 8 9
P 8 , N8
0.5 %
(1 .2 5 :2 5 0 )
700
80
6
8 /3 1 7 8 9
P 9 , N9
0.5 %
(1 .2 5 :2 5 0 )
700
80
6
9 /2 7 8 9
P 1 0 , N10
0.5 %
(1 .2 5 :2 5 0 )
700
80
6
9 /3 /’89
P 1 1 ,N 1 1
0.5 %
(1 .2 5 :2 5 0 )
825
80
6
9 /4 789
ch
D a te
193
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
S a m p le
Num ber
( NDF- # )
P 1 2 , N12
C H 4/ Ho
and 2
Flow R a te
( seem )
0.5 %
(1 .2 5 :2 5 0 )
M icrowave
P la sm a
Input
P
r e s s u r e Tim e
Pow er
(W )
( T o rr) ( h o u r)
D a te
700
70
6
9 /5 7 8 9
P 1 3 , N13
0.5 %
( 1 .2 5 :2 5 0 )
700
70
6
9 /67 89
P 1 4 , N14
0 .5 %
(1 .2 5 :2 5 0 )
700
60
8
9/7 7 8 9
P 1 5 , N15
0.5 %
(1 .2 5 :2 5 0 )
800
60
8
9 /8 7 8 9
P 1 6 , N16
0.5 %
(1 .2 5 :2 5 0 )
600
60
8
9 /9 7 8 9
S1
0.5 %
(1 .2 5 :2 5 0 )
600
60
6
9 /1 0 7 8 9
S2
0.5 %
(1 .2 5 :2 5 0 )
600
60
6
9 / 1 1/’8 9
P 1 7 , N17
0.5 %
(1 .2 5 :2 5 0 )
700
60
10
9 /1 2 7 8 9
P 1 8 , N18
2%
(2 :1 0 0 )
70 0
60
5
10/18 /’89
P 1 9 , N19
2%
(2 :1 0 0 )
700
50
5
10 /1 9 7 8 9
P 2 0 , N20
1%
(1 .5 :1 5 0 )
700
60
8
11/578 9
P 2 1 .N 2 1
1%
(1 .5 :1 5 0 )
700
70
8
1 1 /678 9
P 2 2 , N22
1%
(1 .5 :1 5 0 )
800
60
10
11/77 89
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
M icrow ave
P la sm a
Input
P r e s s u r e T im e
Pow er
(W )
( T o rr) ( h o u r)
S a m p le
Num ber
( NDF- # )
4/ h 2
and 2
Flow R a te
( seem )
P 2 3 , N23
1%
(1 .5 :1 5 0 )
800
50
10
11/8789
P 2 4 , N 24
1%
(1 .5 :1 5 0 )
700
70
10
11/9789
P25, P26
1%
(1 .5 :1 5 0 )
700
70
14
11/107 89
P 2 7 , N 27
1%
(1 .5 :1 5 0 )
700
70
10
1/11790
P 2 8 , N28
1 .5 %
(2 .2 5 :1 5 0 )
700
70
6
1/22790
P 2 9 , N29
1 .3 3 %
(2 :1 5 0 )
700
70
6
1/23790
P 3 0 , N30
1.1 %
( 1 .7 5 :1 5 0 )
70 0
70
6
1/24790
P 3 1 , N31
1%
(1 .5 :1 5 0 )
700
70
5
2/6 7 9 0
P 3 2 , N 32
1 .2 %
(1 .8 :1 5 0 )
700
70
6
2/8 7 9 0
N 25, N 26
0 .7 %
(1 .0 5 :1 5 0 )
700
70
6
2 /1 3 7 9 0
ch
D ate
»
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(2)
Samples by Diamond-Powder Nucleation Method
Microwave P la sm a
Input
P re ss u re Time
Power
(W )
( T orr) ( h o u r)
Sam ple
Number
( NDF- # )
4/ h 2
and
Flow R ate
( seem )
P33
0.5 %
(0 .7 5 :1 5 0 )
600
80
6
3/9790
P34
0.5 %
(0 .7 5 : 150)
600
70
6
3/13790
P35
1%
(1 .5 : 150)
600
70
5
3/15790
P36
1 .5 %
(2 .2 5 :1 5 0 )
600
70
5
3/16790
P37
1%
(1 .5 :1 5 0 )
600
60
5
3/19790
P38
0.5 %
(0 .7 5 :1 5 0 )
600
60
6
3/22790
P39
0.5 %
(0 .7 5 : 150)
600
50
6
3/23790
600
50
5
3/27790
(1 .5 : 150)
750
50
5
3/29790
P42
0.5 %
(0 .7 5 : 150)
500
60
6
4/6790
P43
0.5 %
(0 .7 5 : 150)
500
50
6
4/9790
500
50
6
4/11790
400
50
6
4/12790
P40
P41
S3
S4
ch
1%
(1 .5 : 150)
1%
0.5 %
(0 .7 5 :1 5 0 )
0.5 %
(0 .7 5 : 150)
Date
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
M icrowave P l a s m a
Input
P r e s s u r e T im e
Pow er
(W )
(T o rr) ( h o u r)
S a m p le
N um ber
( NDF- # )
4/ h 2
and 2
Flow R a te
( seem )
S5
0 .5 %
(0 .7 5 :1 5 0 )
600
50
6
4 /1 3 7 9 0
P44
0 .5 %
(0 .7 5 :1 5 0 )
500
60
10
4 /1 4 7 9 0
P45
0 .5 %
( 0 .7 5 : 1 5 0 )
400
60
10
4 /1 5 7 9 0
P46
0 .5 %
(0 .7 5 : 1 5 0 )
60 0
60
10
4 /1 6 7 9 0
P47
0 .5 %
( 0 .7 5 : 1 5 0 )
60 0
50
10
4 /1 7 7 9 0
P48
0 .5 %
(0 .7 5 :1 5 0 )
700
60
10
4 /1 8 7 9 0
S6
0 .5 %
( 0 .7 5 : 150 )
320
50
6
4 /1 9 7 9 0
P49
0 .5 %
( 0 .7 5 : 1 5 0 )
400
50
6
4 /2 0 7 9 0
P50
0 .5 %
(0 .7 5 :1 5 0 )
800
60
6
4 /2 3 7 9 0
P51
1%
(1 .5 :1 5 0 )
400
60
5
4 /2 4 7 9 0
P52
1%
(1 .5 :1 5 0 )
500
60
5
4 /2 5 7 9 0
P53
1 %
(1 .5 :1 5 0 )
700
60
5
4 /2 7 7 9 0
S7
1%
(1 .5 :1 5 0 )
400
50
5
4 /2 9 7 9 0
ch
D a te
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
A P P E N D IX B
E lectrical Sam ples
(1 )
Metal/Diamond/Silicon Samples
Film #
Metal Contact
P6
Al
N6
Al
P7
Al
N7
Al
P8
Al
N8
Al
P9
Al
N9
Al
198
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Film #
Metal Contact
P10
Al
N10
Al
P20
Al
N20
Al
P21
Al
N21
Al
P39
Au
P39
Ag
P39
In
P49
Au
P49
Ag
P49
In
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
( 2 ) Metal/Diamond/Metal Back-Etched Samples
T op
Film #
Metal C o n tac t
Bottom
Metal C o n ta c t
N14-T1
—
Cr/Au
N 14-T2
—
Au
P33-T1
Ag
Ag
P 3 3 -T 2
Ag
Pt/Ag
P 33 -T 3
In
Pt/Ag
P 4 4 -T 1
Au
Ag
P 44 -T 2
In
In
P4 4-T 3
In
Pt/Ag
P46-T1
Au
Ag
P 4 6 -T 2
Au
Ag
P4 6-T 3
In
In
P 4 6 -T 4
Au
In
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Film #
Top
Metal Contact
Bottom
Metal Contact
P45-T1
—
Ag
P45-T1
—
Ag
P45-T1
—
Ag
P47-T1
Au
Au
P47-T2
Au
Au
P47-T3
In
Au
P48-T1
—
Au
P48-T2
—
Au
P48-T3
—
Au
P49-T1
Ag
Ag
P49-T2
Ag
Ag
P49-T3
Au
Ag
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
B IB L IO G R A P H Y
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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