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AIGaAs/GaAs MICROCLEAVED FACET (MCF)
LASER MONOLITHICALLY INTEGRATED
WITH PHOTODIODE
Indexing terms: Integrated optics, Gallium compounds,
Semiconductor injection lasers, Junction
photodetectors,
Monolithic integration
A new AIGaAs/GaAs double-heterostructure laser with
microcleaved facets, restricted just at stripe contact edges,
has been developed. This laser and a junction photodiode
have been monolithically integrated in a single GaAs
substrate. Low-threshold current lasing and high-fidelity
monitoring characteristics have been demonstrated.
Monolithic integration of injection lasers with junction
photodiode and other electronic components such as FETs on
a single semiconductor chip is considered to be an attractive
target in the future development of optical semiconductor
devices and their applied systems.1"4 One of the technological
keys towards such monolithic integration is to establish a
method of fabricating lasers at arbitrary location within the
substrate. The cleavage of substrate of III-V alloys along their
(110) planes, which has been commonly employed to prepare
partially transparent facets for a laser cavity, cannot be used
owing to its geometrical restriction. Wet2"4 and dry5 etching
techniques have recently been used by several investigators
and shown to possess a potential to realise high-quality facets.
In this letter we describe a new method of preparing vertical
facets exhibiting no geometrical restriction, in which a
microcleavage is carried out just at the edges of a stripe
contact. This method could be applied widely in laser
fabrication because there is no need to cleave the substrate. A.
Yariv and his co-workers have recently fabricated a laser
using a similar idea.6 We report here our independent result
on laser fabrication together with a demonstration of its
monolithic integration with a monitoring detector. It is
significant to note that the present laser structure also has an
advantage in realising short cavity lasers,7 which can not only
reduce the threshold current but also stabilise the longitudinal
oscillation mode.
The double-heterostructure (DH) as shown in Fig. la was
grown on the (100) surface of an n+-GaAs substrate by
molecular beam epitaxy. The growth conditions including the
substrate temperature were chosen to maximise the
photoluminescence intensity of the GaAs active layer. Dopant
impurities used were Be and Si for p- and n-type layers,
respectively. The carrier concentrations of the active and the
n- and p-cladding layers were, respectively, 3 x 1017, 2 x 1017
and 3 x 1017 cm" 3 . A microcleaved facet (MCF) laser
integrated with a junction photodiode, the structure of which
is schematically shown in Fig. \b, was fabricated using this
wafer. To obtain an MCF-laser, a 8 /im-wide stripe ohmic
contact of Au-Zn alloy, with its length along the <011>
direction, was formed after Zn diffusion into the top GaAs
contact layer.
An Au/Cr overlay was deposited to facilitate the bonding
area. The procedure for facet formation is as follows. A stripe
pattern with its both ends protruding over the contact length,
together with the metallised area, was covered by a
photoresist layer. Then, by using undercut etching, a couple of
DH beams were left at both ends. A solution of 8H2O2
+ 1H2SO4 + 1H2O was used to remove unprotected part of
DH, and a 30H2O2 + 1NH4 solution, for which AlGaAs is
etch-resistive, was adopted for GaAs undercut etching. These
Au/Zn
p*-GaAs (0-5|jm)
p-AI Q 3 Ga 0 As (l
n-GaAs (015pm)
n-AI
Ga
As (16pm)
rT-GaAs (37pm)
rv-GaAs (-100pm)
substrate
Au-Ge/Ni
0
80
etching processes, successively carried out at room
temperature, produced DH beams without side-etch of GaAs
active layer. These beams were then microcleaved at the
contact edges by the application of ultrasonic agitation. The
geometry of a facet ultimately formed is illustrated by a heavy
line in Fig. la. The cavity length obtained in this experiment
was as small as 40 fim. A photodetector consisted of the same
DH with a Au-Zn contact on the top layer. The detector facet
was made simultaneously during the DH etching, in which an
oblique facet plane was generated by the orientation
dependent etch so that the possibility for the light being
reflected back into the laser is eliminated. The distance
between the laser and the detector was 140 /mi. Finally, an
Fig. 1
a Cross-section of an MCF-laser
Heavy line indicates the contour of MCF
b Configuration of an integrated laser-photodiode
4th March 1982
120
|i66/2l
Fig. 2 Typical light/current characteristics of an MCF-laser under
pulse bias at room temperature
Contact width is 8 fxm and cavity length is 40 (tm
light
output
ELECTRONICS LETTERS
40
current.mA
MCF laser
Vol. 18
No. 5
189
Au-Ge-Ni alloy contact was formed on the back wafer surface
as a common n-side electrode.
Fig. 2 shows a typical trace of current/light (//L)
characteristics of the MCF-laser under pulse bias at room
temperature. The threshold current Ith as small as 45 mA was
obtained, primarily due to the short cavity achieved in this
structure. A good reproducibility of microcleavage has been
confirmed by the variation of Ith within + 10%. From the
comparison of such I/L characteristics with those of ordinary
substrate-cleaved lasers taken from the same wafer, no
degradation of the facet reflectivity has been found to be
induced by the MCF structure. The monitoring characteristic
measured by the same chip is shown in Fig. 3. The
photocurrent into a 50 Q load resistor responding to the laser
light from the inner facet is plotted against the laser power
measured at the outer facet. It is found that the photocurrent
is perfectly in proportion to the power after lasing. Incubation
of the photocurrent below threshold is explained by a large
divergence of the spontaneous emission.
In conclusion, a new AlGaAs/GaAs MCF-laser structure
has been developed and demonstrated to be applied to
laser-detector integration. This structure and its fabrication
method are highly useful for laser monolithic integration,
because the substrate is not required to cleave and there is the
possibility of realising short cavities.
Acknowledgments: The authors wish to thank H. Hashimoto,
Y. Toyama, K. Dazai and T. Misugi for their encouragement
throughout this work. The work is supported by the Agency
of Industrial Science & Technology, MITI, of Japan, in the
frame of the National Research & Development Project
'Optical measurement and control systems'.
O. WADA
S. YAMAKOSHI
T. FUJII
S. HIYAMIZU
T. SAKURAI
Fujitsu Laboratories Ltd.
Optical Semiconductor Devices Laboratory
1015 Kamikodanaka
Nakahara, Kawasaki 211, Japan
25th January 1982
60 r
O
References
1
YUST, M., BAR-CHAIM, N., IZADPANAH, S. H., MARGALIT, S., URY, I.,
WILT, D., and YARIV, A.: 'A monolithically integrated optical
repeater', Appl. Phys. Lett., 1979, 35, pp. 795-797
<
ZL
2
\^
:A0
O
<
o
Q.20
MERZ, J. L., and
LOGAN, R. A.: 'Integrated
GaAs-Al^Ga^^s
injection lasers and detectors with etched reflectors', ibid., 1977,
30, pp. 530-533
3
KISHINO,
K., SUEMATSU,
Y., UTAKA,
K., a n d KAWANISHI, H . :
'Monolithic integration of laser and amplifier/detector by
twin-guide structure', Jpn. J. Appl. Phys., 1978, 17, pp. 589-590
4 IGA, K., and MILLER, B. L: 'GalnAsP/InP laser with monolithically
integrated monitoring detector', Electron. Lett., 1980, 16, pp.
342-343
o
5
COLDREN, L. A., IGA, K., MILLER, B. I., a n d RENTSCHLER, J. A.:
'GalnAsP/InP stripe-geometry laser with a reactive-ion-etched
facet', Appl. Phys. Lett., 1980, 37, pp. 681-683
6
1
2
3
A
laser output,mW
h66/3l
Fig. 3 Typical
monitoring
characteristic
of an
laser-photodiode under pulse bias at room temperature
integrated
HIGH-STRENGTH SMALL REINFORCEMENT
FOR SPLICED OPTICAL FIBRE
Indexing terms: Optical fibres, Fibre reinforcement
A high-strength small reinforcement using a nylon and a
steel wire has been developed. The size of the reinforcement
is 1 mm in diameter and 35 mm in length. An average tensile
strength of 5-4 kg has been achieved with negligible loss
increase.
An arc-fusion splicing is the most practical method for optical
fibre splicing because of its low loss a n d easy handling.
However, some reinforcement is required for the spliced fibres,
since the mechanical strength around the splicing point is
much reduced, owing to removal of a primary coat. Among
various reinforcement methods proposed,1 nylon moulding 2 is
one of the most favourable methods from a viewpoint of small
size, although the reported value of tensile strength is much
lower compared with that of an original fibre. In this letter, we
molten nylon
metal mould
/-
spliced fibre
tension
steel wire
Fig. 1 Method of reinforcement
190
IT797T1
BLAUVELT, H., BAR-CHAIM, N., FEKETE, D., MARGALIT, S., a n d YARIV,
A.: 'AlGaAs lasers with micro-cleaved mirrors suitable for
monolithic integration'. Presented at toptical meeting on
integrated and guided-wave optics, California, 1982
7 MATSUMOTO, N., and ANDO, s.: 'Short cavity semiconductor laser',
Jpn. J. Appl. Phys., 1977, 16, pp. 1697-1698
0013-5194/82/050189-02$1.50/0
report a high-strength small reinforcement using a nylon and
a steel wire.
Fig. 1 shows the reinforcement method used here. A spliced
fibre and a steel wire are mounted in a metal cavity, and then
the cavity is heated up to the melting point of nylon (180°C).
After molten nylon is slowly poured into the interior of the
cavity, the cavity is cooled to room temperature in about 5
min. Throughout this process, the fibre is pulled under a
tension of 500 g.
The theoretical tensile strength F for this reinforcement
method has been estimated by eqn. 1 under the assumption
that the fibre, the nylon and the steel wire shrink as one body
on cooling.
=
EHSH Al-*fAT
F
F'
(1)
where E is Young's modulus, S is the cross-sectional area, A/ is
the shrinkage coefficient of the reinforced part, a is the
coefficient of linear thermal expansion, AT is the temperature
difference between the melting point of nylon and room
temperature, and F' is a tensile strength of the spliced bare
fibre. Subscripts n, s and / represent nylon, a steel wire and a
fibre, respectively. Using the physical constants in the
literature and the measured value of F' (900 g), it has been
estimated from eqn. 1 that the 1 mm-diameter reinforcement
with the steel wire whose diameter is above 0-2 mm should
have a tensile strength higher than that of the original fibre (7
kg).
ELECTRONICS LETTERS
4th March 1982 Vol. 18 No. 5
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