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LOW THRESHOLD 0.98pm
ALUMINIUM-FREE
STRAIN ED-QUANTU M-WELL
InGaAs/lnGaAsP/lnGaP LASERS
C. J. C h a n g - H a s n a i n , R. Bhat, H . Leblanc a n d
M. A. Koza
Indexing terms: Semiconductor lasers, Laserc
Aluminium-free strained-quantum-well In, ,Ga, ,As lasers
employing novel two-stepped InGaAsP confinement layers
and InGaP cladding layers on a GaAs substrate are demonstrated for the first time. Threshold current density as low as
58A/cm2 is obtained with broad stripe lasers. This threshold
current density is, to the authors' knowledge, the lowest
reported for 0.98pm lasers grown by organlc chemical
vapour deposition.
80A strained In,,Ga,,As
quantum well was used as the
active layer. The undoped two-step InGaAsP confinement
layers consist of two 600A layers each on both sides of the
quantum well with bandgap wavelengths of 0.85 and 0.8pm.
The top and bottom optical claddings are p- and n-doped
I-2pm-thick InGaP, respectively. A 0 . l p m p-GaAs and a
0.2pm p+-GaAs cap layer are grown on top of the p-cladding
layer. The entire heterostructure was grown on a GaAs substrate in a horizontal low pressure (76 torr) MOCVD reactor
at 650°C. Broad-stripe 308 pm-wide lasers were fabricated by
etching off the highly conductive GaAs layers and -0.3pm
I n G a P in between the laser stripes. The facets were uncoated
and lasers were tested under pulsed operation (20011s at
10kHz) without heatsinks.
Fig. 2a shows the light against current (L-I) characteristics
for 16 neighbouring lasers on one bar. The lasers have a cavity
length of 690pm and a centre-to-centre spacing of 508pm.
:ori!pi
600
Semiconductor lasers emitting in the 0.98 pm wavelength
regime are important for numerous applications including
pumping optical fibre amplifiers and solid state lasers. In addition, because the optical fibre loss at 0.98 pm is only 1 dB/km
(twice that at 1.3pm), these lasers are also promising for short
distance optical interconnects. Typical heterostructure design
of 0.98pm lasers includes an InGaAs strained quantum well
active region sandwiched between GaAs confinement layers
and AlGaAs optical cladding layers [l-41. The high oxidation
rate of AI in AlGaAs causes rapid facet degradation [5] and
difficulty for high quality regrowth. The former leads to reliability problems. High quality regrowth, on the other hand, is
essential for fabricating buried-heterostructure lasers for stable
singlemode operation, distributed feedback lasers, and lasers
integrated with other electronic or photonic components. One
way to circumvent these problems is t o reduce the AI concentration in the cladding layers [4]. However, the resulting
lower bandgap energy difference reduces both carrier and
optical confinements, which tends to result in inferior laser
performance.
A more promising alternative is to replace AlGaAs by
I n G a P lattice-matched to GaAs such that the heterostructure
is aluminium-free. In addition to the lower oxidation rate,
I n G a P has a substantially lower bandgap energy for the direct
transition from the I' valley than the indirect transition from
the X valley, which is very important for electron capture/
confinement for high temperature operation. Recently, there
have been many publications on InGaAs lasers with I n G a P
claddings [6-81 and InGaAsP claddings [9]. All of them used
a separate confinement heterostructure (SCH) with GaAs for
the confinement layers. In this Letter, we report AI-free lasers
with a novel two-stepped InGaAsP layer, lattice-matched to
GaAs, as the confinement layer to provide a better tailored
electron and optical confinement. We obtained a record low
threshold current density of 58A/cmZ for 0.98pm lasers
grown with metal organic chemical vapour deposition
(MOCVD).
The device schematic diagram is shown in Fig. 1. A single
-:300
;Loo
0
Y
-,zoo
0
E 200
100
100
8CL
C8
12
16
pulsed drlve current A
a
0
0 4 08
12
16
pulsed drive c u r r e n t , A
h
Fig. 2 Light against current characteristics for 16 neighhouring lasers
on same bar with cavity lengths oJ690pm, andfive neighbouring lasers
on one bar with 2-53 mm long caoities
The uniformity of the lasers is excellent with all the lasers having
the same threshold current and nearly the same differential
quantum eficiencies
U 16 neighbouring lasers: W = 308 pm, L = 690pm
b Five neighbouring lasers' W = 308pm, L = 2.53pm
The uniformity of the lasers is excellent with all the lasers
having the same threshold current of 194mA and a differential
quantum efficiency of 40 1%. Fig. 2b shows the L-I curves
for five neighbouring lasers on a 2.53mm long bar. Again,
linear and highly uniform L-I characteristics were obtained.
The laser emission wavelength for the 690pm-long lasers is
0.99 pm. For lasers which are 25 pm wide and 600 pm long,
the emission wavelength is near 0.98 pm.
Fig. 3 shows threshold current. density as a function of
inverse cavity length for 11 bars with different cavity lengths.
Each bar is represented by the lowest, the highest and the
mean threshold densities. The mean values are typically averaged over approximately 10 lasers on the same bar. For the
four bars with long lengths (2.53, 2.60, 2.96 and 3.05mm) the
-
TiiAu
~~
0 2 pm P'GaAs
0 1 p m P -GaA5
Q(A,=
-
0 85
1 2 lim P - Ino L9Gao 5,P
strained
quantum
.-.
well
1 2pm
P-lno*9Gu0 5 p
~n'-GaAssubstrate
18731'
_
i
Fig. 1 Schematic diagram and heterostructure oJ InGaAs strained-quantum-well laser with nooel two-stepped InGuAsP confinement layers
and InGaP claddings
ELECTRONICS LETTERS
7th January 1993
Vol. 29 No. 1
1
threshold densities are nearly the same with the lowest and
the average values being 58 a n d 60A/cmZ, respectively. Both
peratures. the To values are typically reduced to between 105
and 120K. Although similar behaviour was observed in previous reports [6], the reason for this change in To is not clear
and is currently under investigation.
do-
inverse covlty length t I L , c m
a-?
10
3~
~~~
zc
these values are lower than the record reported t o date for
InCaAs/GaAs/AlGaAs lasers [I] grown by MOCVD.
Fig. 4 shows the near field intensity patterns for a typical
broad stripe laser at current levels below and above lasing
threshold. The fact that the entire aperture of 308 p m starts t o
lase without dead spots at the threshold current indicates very
good material uniformity. The same low threshold current
density is thus also expected for narrower stripes.
:c
iz
fir
tcrnperct,re
Fig. 3 Threshold current density as function of cac-rry length
11 laser bars with different cavity lengths were mearured and each
average value was calculated with approximately I O lasers on the
same bar
x maximum
0 average
A minimum
3c
-
~-
~~~
~
*7c
BD
1
90
100
C
Fig. 5 Threshold current usfunc~ronof sirhsrrate temperature measured
,hr 2 5 p m wide gain-guided laser7
A characteristic temperature (To)value as high as 194K IS obtained
for the temperature range 10-50 C
In summary. we have demonstrated the first strainedquantum-well InGaAs lasers with two-step InGaAsP layers as
confinement layers a n d I n C a P optical cladding layers. The
lasers exhibit linear a n d highly uniform light against current
characteristics. A record low threshold current density of
58 A/cm2 was obtained for 0.98 pm lasers grown by MOCVD.
2nd November 1992
C . J . Chang-Harnain (Cinrron Lab. and Department of Electrical
Engineering, Stanford University. Stanford, C A 943054085. U S A )
R. Bhat, H. Lehlanc and M. A. Koza (Bellcore, 331 Newman Springs
Road. Red Bank h’J 07701, U S A )
References
CHOI, H. K . and WANG, c. A . : ‘1nGaAs:AlGaAs strained single
quantum well diode lasers with extremely low threshold current
density and high eficiency’, Appl. Phys. Lert., 1990, 57, pp.
321-323
2 CHANU, NARESH, BECKER, E. E., VAN UER ZIEL, 1. P., CHU,s. N. G., and
UUTTA,N K . . ‘Excellent uniformity and very low (<SOA/an’)
threshold current density strained InGaAs quantum well diode
lasers on GaAs substrate’, Appl. Phys. Lett., 1991, 58, (16), pp.
1704-1706
3 WILLIAMS, R. L., UlON, M., CHATENOUD, F., and DZURKO, K.:
‘Extremely low threshold current strained InGaAs/AIGaAs lasers
by molecular beam epitaxy’, Appl. Phys. Lett., 1991, 58, (17). pp.
1
I8 16-1818
4
5
i
0 991,
6
+ 308pm l a t e r a l d i s t a n c e 4
Fig. 4 Near field intensity patterns for typical broad stripe laser at
current levels below and above lasing threshold
The entire aperture of 308 pm starts to lase without dead spots at
the threshold current
W = 308 pm, L = 3.05 pm
The threshold current as a function of substrate temperature is measured for 25 pm wide gain-guided lasers. Fig. 5
shows such measurement for one typical laser. A very high
characteristic temperature (To)value of 194K is obtained for
the temperature range 10-50°C. At higher substrate tem”
L
7
8
P. K., BEERNINK, K. I., FERNANDEZ, G . E., and COLEMAN, J . J.:
‘InCaAs-GaAs strained-layer quantum well buried heteroz I pm) by metalorganic chemical vapor deposistructure lasers (i
tion‘, Appl. Phys. Lett., 1989. 54,pp. 499-501
GARBUZOV, D. 7., KATSAVETS, U. I . . KOCHERGIN, A. v., and KHALFIN,
v. B: ‘An experimental and theoretical study of the local temperature nse of mirror facets in InGaAsiCaAs and AIGaAs/GaAs
SCH SQW laser diodes’ Joint Soviet-American Workshop on the
Physics of Semiconductor Lasers, 20th May-3rd June 1991, Leningrad, USSR
IJICHI. I., OHKUBO,
M., MATSUMOTO, N., and OKAMOTO, H.: ‘High
power CW operation of aluminium-free InGaAs/GaAs/lnGaP
strained layer single quantum well ridge waveguide lasers’. 12th
IEEE Int. Semiconductor Laser C o d , 9th-14th September 1990,
pp. 4 - 4 5
CHEN, Y. K., wu, M. c., KUO, J. M.,CHIN,M. A., and SERGENT,
A. M.:
‘Self-aligned InGaAs/GaAs/InGaP quantum well lasers prepared
by gas-source molecular beam epitaxy with two growth steps’,
Appl Phys. Lerr., 1991,59, (23). pp. 2929-2931
LIAIJ, Z. L., PALMATEER, s. c., GROVES,
s. H., W A L m L E , 1. N., and
MISACCIA,
L. 1.: ‘Low-threshold InGaAs strained-layer quantumwell lasers .;( = 0-98pm) with GalnP cladding layers and masstransported buried heterostructure’, Appl. Phys. Lett., 1992, 60,pp.
YORK,
6-8
9
YOUNG, M G., KOREN, U,, MILLER, B. I., RAYBON, G., and BURRUS,
C.
A.: ‘InGaAs,’GaAs0.98 mm semi-insulating blocked planar buried
heterostructure lasers employing InGaAsP cladding layers’, I E E E
Photonics Techno/. Lert.. February 1992.4, (2)
ELECTRONICS LETTERS 7th January 1993 Vol. 29
No. 7
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