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trigger current level are shown in Fig. 2, and simulated ones
are shown in Fig. 3. By the optimisation of conductivity
modulation resistors in addition to diffusion capacitors, the
GaAIAs BURIED-HETEROSTRUCTURE
LASERS GROWN BY A TWO-STEP MOCVD
PROCESS
Indexing terms: Lasers and applications. Semiconductor lasers
9
OmA/div)
The fabrication and characterisation of GaAIAs buriedheterostructure laser diodes having low threshold currents
(10 mA), high uniformity and planar surface structure, and
grown exclusively by metalorganic chemical vapour deposition (MOCVD) are described. Single-longitudinal and
transverse-mode operation of these devices has been
observed. In addition, considerable suppression of relaxation
resonance effects has been observed in the high-frequency
modulation of these devices.
(20mA/div)
•g
(1mA /div)
'trg
(20mA«iv)
g
(1mA /div)
gth
'trg
(20mA/di\J
[oi677]
Fig. 2 Experimental results of transient current waveforms of latch-up
turn-on process at x = 1 us
2-5
30
I5T6751
Fig. 3 Simulation results of transient current waveforms of latch-up
turn-on process at x = / [is
calculated transient current waveforms are in good agreement
with the experimental ones.
In conclusion, a measurement technique that evaluates the
latch-up threshold current level for trigger pulse current from
the P-well node is established. Through the comparison
between simulation results (considering the capacitor model
and the conductivity modulated resistor model) and experimental results, the main factors which determine the latch-up
transient characteristics are the base-emitter diffusion capacitors.
T. AOKI
R. KASAI
S. HORIGUCHI
26th July 1983
Atsugi Electrical Communication Laboratory
Nippon Telegraph & Telephone Public Corporation
1839, Ono, Atsugi-shi, Kanagawa 243-01, Japan
The buried-heterostructure (BH) laser1 facilitates lowthreshold currents and wide modulation bandwidths by virtue
of its unique built-in dielectric waveguide and current confinement structure. It is a very desirable light source for fibre-optic
communications systems. However, the growth of these
devices by two-step liquid-phase epitaxy (LPE) has proven to
be a difficult and involved process. Problems such as melt
dissolution or nonwetting occur in the second step, burying
layer growth. In addition, growth of composite burying layers
to form a current blocking structure often results in notable
current leakage due to the junction misalignment. Although
an improved structure with a single resistive burying layer has
been developed,2 the growth of such a high-purity material by
LPE requires a long-time bake-out of the melt. On the other
hand, growth of semi-insulating GaAIAs by MOCVD has
been readily obtained without any prebaking procedure. With
this capability, the advantages of using MOCVD in the
growth of BH structures become important. In addition to the
advantages of improved uniformity,3 the use of MOCVD in
the second-step growth eliminates the need for the control of
etch-back and regrowth. In this letter we report the first
demonstration of GaAIAs BH lasers which are grown exclusively by a two-step MOCVD process. The first growth
creates the double heterostructure and the second provides the
semi-insulating GaAIAs burying layer. As a result of this
improved fabrication technique, the devices show good uniformity in terms of thresholds and single-mode characteristics.
In this work, both growth steps for the BH structure were
carried out in a vertical MOCVD reactor at a temperature of
around 750°C. The device fabrication procedure is as follows.
First, a standard laser structure consisting of a 0-5 /im
n+-GaAs buffer layer, a 1-5 j/m n-Gao.7Alo.3As confinement
layer, a 01 nm undoped GaAs active layer, a 1-5 /an
p-Gao.7Alo.3As top confinement layer and a 01 nm p+-GaAs
cap layer was grown on an n+ (100) GaAs substrate. After the
growth, a photoresist mask consisting of 6 ^/m-wide stripes on
500 urn centres was delineated on the wafer for the mesa
etching. The stripes were aligned along the (011) direction to
achieve a desirable waveguide structure after etching. The
masked wafer was etched 2 to 3 /im deep in a 20:1:1 solution
of H 2 O:NH 4 OH:H 2 O 2 . Then a 3 pm-thick semi-insulating
Gao.5Alo.5As burying layer was grown over the exposed
mesas. A free surface etch (with 0-1 % Br-CH3OH) was used to
re-expose the mesa top for electrical contact. A representative
SEM photograph of the cross-section at the stripe region is
shown in Fig. 1. As can be seen, this process results in a nearly
planar surface with a height deviation of ~ 1 ^m. The mea-
References
1
SCHROEDER, J. E., (XHOA, A., a n d DRESSENDORFER, P. V.: ' L a t c h Up
elimination in bulk CMOS LSI circuits', IEEE Trans., 1980,
NS-27, pp. 1735-1738
2 ESTREICH, D. B. : 'The physics and modeling of latch-up and CMOS
integrated circuits'. Stanford Univ. Tech. Rep., G-201-9, 1980
3 HUFFMAN, D. D.: 'Prevention of radiation induced latch up in commercially available CMOS devices', IEEE Trans., 1980, NS-27, pp.
1436-1441
4 TROUTMAN, R. R., and ZAPPE, H. p.: 'A transient analysis of latch-up
in bulk CMOS', ibid., 1983, ED-30, pp. 170-179
5 RUNG, R. D., and MOMOSE, H.: improved modeling of CMOS latchup and VLSI implications'. Proc. Symp. VLSI Tech. 4.1, 1982
6
S l G o a 5 A l 0 5 As
burying layer
P5C/1|
WIEDER, A. w., WERNER, c , and HARTER, J.: 'Design model for bulk
CMOS scaling enabling accurate latch up prediction',
Trans., 1983, ED-30, pp. 240-245
IEEE
Fig. 1 SEM photograph of a stained facet of the BH laser grown by a
two-step MOCVD process
ELECTRONICS LETTERS 15th September 1983 Vol. 19 No. 19
759
sured stripe width at the active region of the structure is
~ 2 nm. The wafer was subsequently processed into individual
laser diodes. Ohmic contacts of alloyed Au-Ge and Cr-Au
were formed on the back and front sides of the device, respectively. For simplicity, broad contact stripes of 150 fim centred
on the stripes were used as the frontside metallisation.
Fig. 2 shows the CW power/current (L/I) characteristic
the strip-buried-heterostructure (SBH) lasers fabricated by a
hybrid growth technique4 and 25 ± 5 mA (20%) for the
planar buried-heterostructure5 lasers fabricated by an
improved two-step LPE process. Although the process in Reference 4 results in similar uniformity, the SBH devices typically have comparatively high threshold.
The lasers have been packaged in a high-speed fixture to
analyse the frequency dependence of the modulation characteristics. A typical frequency response curve (solid circles) is
shown in Fig. 4. The data have not been corrected for the
20
• 2pmBHstripe laser. 1 b = 1-1 I t h
° 4 p m oxide stripe laser.lb=12 I t h
10
CD
•o
£ o
5
o o
*
o
o 850
-10
-20
0
0
0
10
I,mA
02
20° 40'
0-5
20
2 0
10
frequency. GHz
1050/A|
Fig. 4 Frequency response of lasers
Fig. 2 Power-current and far-field characteristics of BH laser
curve and corresponding far-field intensity distribution parallel to the junction plane of a 250 //m-long device. CW threshold currents as low as 9 mA and differential quantum
efficiencies as high as 40% per facet are observed. The corresponding threshold current density is 1-8 kA/cm2 as compared
with ~ 1 kA/cm2 for the broad-area lasers fabricated from the
material.3 The external leakage current of the device is estimated to be 0-4 mA from measurements made by contacting
only the blocking layer. These devices operate in a single
transverse mode with linear L/I characteristics up to 2-5 times
the threshold current or 8 mW output power. The measured
divergent angle at FWHM of the far-field patterns (Fig. 2) is
~ 20°. The corresponding optical aperture calculated from the
diffraction theory agrees well with the stripe width of the laser
noted above. Single-longitudinal-mode (A ~ 8810 A) operation
with the ratio of the main to side mode power of ~15 dB is
also observed. The distribution of threshold currents in the
form of cumulative probability plot for ten devices (~250 nmlong) randomly selected from a processed wafer is shown in
Fig. 3. The average threshold current is 10-5 mA. The varia-
detector frequency response; also the bandwidth of the RF
signal generator is limited to 1-8 GHz. For comparison, the
corresponding response curve (open circles in Fig. 4) for a 4 ^m
oxide stripe laser (fabricated from the same DH wafer) is also
shown. The oxide stripe laser exhibits enhanced relaxation
resonance near the high-frequency cutoff. This resonance
limits the available device bandwidth. The built-in confinement structure in the BH laser provides more efficient interaction of electrons and photons; thus, the relaxation frequency
resonance is depressed.6
In conclusion, we have developed a two-step MOCVD
process for fabricating a planar BH laser structure. The resultant devices exhibit low thresholds and single-mode operation.
These devices also show desirable high-frequency modulation
characteristics. The device operating parameters and uniformity obtained using this process compare favourably with
the best obtained by a two-step LPE or hybrid (MBE-LPE)
growth technique.
C. S. HONG
D. KASEMSET
M. E. KIM
R. A. MILANO
2nd August 1983
Rockwell International
Microelectronics Research & Development Center
Thousand Oaks, CA 91360, USA
References
1
2
TSUKADA, T.: 'GaAs-GaAlAs buried-heterostructure
lasers', J. Appl. Phys., 1974, 45, pp. 4899^906
TSANG,
w.
T,
and
LOGAN,
R. A.: 'GaAs-AlGaAs
injection
buried-
heterostructure lasers grown by molecular beam epitaxy with
Al o . 6 5 Ga O 3 5 As (Ge-doped) liquid phase epitaxy overgrown layer
for current injection confinement', Appl. Phys. Lett., 1980, 36, pp.
730-733
5 10 20 304050 60 70 80 90 95 98 99
% of points with value less than ordinate
Fig. 3 Distribution plot of threshold current for BH lasers selected from
a processed wafer
tion in threshold is mostly due to the variation in the waveguide stripe width of each individual laser. 80% of these
devices have a threshold variation of only ±1-5 mA or 15%.
The best previously published results are 42 ± 5 mA (12%) for
760
3
HONG, C. S., KASEMSET, D., PATEL, N. B., KIM, M. E., a n d DAPKUS, P. D . :
4
'Low-threshold oxide stripe GaAs/GaAlAs lasers grown by
MOCVD', Electron. Lett., 1982, 18, pp. 497-499
TSANG, w. T., and LOGAN, R. A.: '(AlGa)As strip buriedheterostructure lasers prepared by hybrid crystal growth', ibid.,
1982, 18, pp. 397-398
5
MITO, I., KITAMURA, M., KAEDE, K., ODAGIR1, Y., SEKI, M., SUGIMOTO,
M., and KOBAYASHI, K.: 'InGaAsP planar buried heterostructure
laser diode (PBH-LD) with very low threshold current', ibid., 1982,
18, pp. 2-3
6 LAU, K. Y., and YARIV, A.: 'Nonlinear distortions in the current
modulation of non-self-pulsing and weakly self-pulsing GaAs/
GaAlAs injection lasers', Opt. Commun., 1980, 34, pp. 424-428
ELECTRONICS LETTERS 15th September 1983 Vol. 19 No. 19
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