U.S. patent application number 10/803956 was filed with the patent office on 2004-10-21 for semiconductor optical device and method for manufacturing the same.
This patent application is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Kawasaki, Kazushige, Shigihara, Kimio.
Application Number | 20040208214 10/803956 |
Document ID | / |
Family ID | 33156941 |
Filed Date | 2004-10-21 |
United States Patent
Application |
20040208214 |
Kind Code |
A1 |
Kawasaki, Kazushige ; et
al. |
October 21, 2004 |
Semiconductor optical device and method for manufacturing the
same
Abstract
A method for manufacturing a semiconductor optical device
includes forming an epitaxial structure containing at least an
active layer which can emit light, of a III-V group semiconductor
material; forming an insulating layer over the epitaxial structure,
which prevents the V group element from escaping from the epitaxial
structure during heat treatment; heat treating the epitaxial
structure at at least 800 degrees C.; and removing the insulating
layer, thereby enhancing the reliability of the device.
Inventors: |
Kawasaki, Kazushige; (Tokyo,
JP) ; Shigihara, Kimio; (Tokyo, JP) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
700 THIRTEENTH ST. NW
SUITE 300
WASHINGTON
DC
20005-3960
US
|
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha
Tokyo
JP
100-8310
|
Family ID: |
33156941 |
Appl. No.: |
10/803956 |
Filed: |
March 19, 2004 |
Current U.S.
Class: |
372/43.01 ;
257/E21.326; 438/22 |
Current CPC
Class: |
H01S 5/34366 20130101;
H01S 2301/173 20130101; H01L 33/0062 20130101; H01L 33/0095
20130101; H01S 5/34313 20130101; H01L 21/3245 20130101; H01S
2304/00 20130101; H01S 5/2214 20130101; B82Y 20/00 20130101; H01S
5/2231 20130101 |
Class at
Publication: |
372/043 ;
438/022 |
International
Class: |
H01L 021/00; H01S
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 15, 2003 |
JP |
2003-109960 |
Claims
1. A method of manufacturing a semiconductor optical device
comprising: growing an epitaxial structure containing at least an
active layer which can emit light, of a III-V semiconductor
material including a group V element; forming an insulating layer
over the epitaxial structure, which can prevent the V group element
from escaping during heat treatment; heat treating the epitaxial
structure at a temperature of at least 800 degrees C.; removing the
insulating layer.
2. The method of manufacturing a semiconductor optical device
according to claim 1 comprising performing a photoluminescence
measurement after the heat treating.
3. A semiconductor optical device comprising an epitaxial structure
of a III-V group semiconductor material, containing at least an
active layer which can emit light, wherein composition of the
epitaxial structure continuously changes near an interface.
4. The semiconductor optical device according to claim 3, wherein a
photoluminescence wavelength of the optical device is blue-shifted,
as compared to a semiconductor optical device which has an active
layer with the same composition as said active layer and an
epitaxial structure with a composition that changes stepwise near
the interface.
5. The semiconductor optical device according to claim 4, wherein
the photoluminescence wavelength is blue-shifted by at least 20
meV.
6. The semiconductor optical device according to claim 3, wherein
distortion of the epitaxial structure is reduced compared to a
semiconductor optical device which has an active layer with the
same composition as said active layer and an epitaxial structure
with a composition changing stepwise near the interface.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a semiconductor optical
device and method for manufacturing the same, which can be used for
optical information processing and optical communication and the
like, suitably for a pumping light source for a fiber
amplifier.
[0003] 2. Description of the Related Art
[0004] A light source for optical information processing and
optical communication is required to have high power output and
high reliability, particularly in case of a pumping light source
for a fiber amplifier which can be used for an optical repeater in
a submarine optical cable, a semiconductor optical device is
required to have a long lifetime and high reliability.
[0005] As the pumping light source for a fiber amplifier, the
semiconductor optical device having an emission wavelength of 0.98
.mu.m or 1.02 .mu.m is generally selected and a strained quantum
well structure is adopted for an active layer thereof. For example,
there is a distortion of about 1% between an InGaAs quantum well
layer and a GaAs guide layer.
[0006] For main degradation causes of the semiconductor optical
device, end face degradation due to optical absorption on an
optical exit face and internal degradation due to dislocation in a
crystal or distortion between epitaxial growth layers are
known.
[0007] To counter the end face degradation, a window structure in
which the band gap energy of the optical exit face is larger than
that of the active layer can be adopted to prevent the optical
absorption or an appropriate coating on the end face can be
designed.
[0008] To counter the internal degradation, a substrate having a
low dislocation density can be used or an active layer with a
strain-compensated structure can be adopted.
[0009] The related prior arts are listed as follows:
[0010] [Document 1]
[0011] G. Beister et al., "Monomode emission at 350 mW and high
reliability with InGaAs/AlGaAs (.lambda.=1020 nm) ridge waveguide
laser diodes", ELECTRONICS LETTERS, 16 April 1998, Vol. 34,No. 8,
pp. 778-779
[0012] [Document 2]
[0013] Toshiaki Fukunaga et al., "Reliable operation of
strain-compensated 1.06 .mu.m InGaAs/InGaAsP/GaAs single quantum
well lasers", Appl. Phys. Lett., Vol. 69(2), 8 Jul. 1996, pp.
248-250
[0014] In FIG. 2 of the above-mentioned document 1, the result of a
reliability examination of the semiconductor optical device of 1.02
.mu.m wavelength band is illustrated using graphs. The examination
conditions are atmosphere at 40 degree-C., a constant output of 300
mW and a measurement size of ten. The horizontal axis shows an
aging time by 1,000 hours, and the vertical axis shows a driving
current (mA).
[0015] These graphs show initial degradation for three of ten
samples in which the driving current rapidly increases in an
initial stage. For the remaining seven samples, the driving current
gradually increases with time progress and a degradation rate is
calculated by linear approximation with 1.5.times.10.sup.-5 to
8.6.times.10.sup.-5 (/h), resulting in 1.5% to 8.6% of increase in
current at 1,000 hours.
[0016] In FIG. 6 of the above-mentioned document 2, the result of a
reliability examination of the semiconductor optical device of 1.06
.mu.m wavelength band is illustrated using graphs. The examination
conditions are atmosphere at 25 degree-C., a constant output of 250
mW and a measurement size of both ten of SC-SQW (strain-compensated
single quantum well) lasers and ten of SL-SQW (superlattice single
quantum well) lasers. The horizontal axis shows an aging time by
1,000 hours, and the vertical axis shows a driving current
(mA).
[0017] These graphs show that all of the SL-SQW lasers are degraded
before 1,000 hours, on the other hand, all of the SC-SQW lasers are
not remarkably degraded because the driving current does not
increase even at 1,000 hours.
SUMMARY OF THE INVENTION
[0018] The purpose of the present invention is to provide a
semiconductor optical device and method for manufacturing the same
which can remarkably enhance the reliability of devices.
[0019] A method for manufacturing a semiconductor optical device
according to the present invention includes:
[0020] step for forming an epitaxial growth layer containing at
least an active layer which can emit light, using a III-V group
semiconductor material;
[0021] step for forming an insulation layer over the epitaxial
growth layer, which can prevent the V group element from escaping
during heat treatment;
[0022] step for applying heat treatment to the epitaxial growth
layer at a temperature of 800 degree-C. or more;
[0023] step for removing the insulation layer.
[0024] Moreover, a semiconductor optical device according to the
present invention includes:
[0025] an epitaxial growth layer formed of a III-V group
semiconductor material, containing at least an active layer which
can emit light;
[0026] wherein the composition of the epitaxial growth layer is
changing continuously near the interface.
[0027] Such a semiconductor optical device can attain a
significantly longer continuous operation time and extremely higher
reliability, as compared to the conventional device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIGS. 1A to 1C and 2A to 2C are illustrative diagrams
showing an example of a manufacturing process of a semiconductor
optical device according to the present invention.
[0029] FIG. 3 is a graph showing an example of the reliability
examination result of the semiconductor optical device to which
heat treatment has been applied.
[0030] FIG. 4 is a band diagram of the epitaxial growth layer
before heat treatment.
[0031] FIG. 5 is a band diagram of the epitaxial growth layer after
heat treatment.
[0032] FIG. 6 is a graph showing an example of the reliability
examination result of the semiconductor optical device subject to
another condition of heat treatment.
[0033] FIG. 7 is a graph showing an example of the reliability
examination result of the semiconductor optical device subject to
yet another condition of heat treatment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] This application is based on an application No. 2003-109,960
filed Apr. 15, 2003 in Japan, the disclosure of which is
incorporated herein by reference.
[0035] Hereinafter, preferred embodiments will be described with
reference to drawings.
Embodiment 1
[0036] FIGS. 1A to 1C and 2A to 2C are illustrative diagrams
showing an example of a manufacturing process of a semiconductor
optical device according to the present invention. Here, although
an example in which GaAs, AlGaAs and InGaAs are used as III-V group
semiconductor materials is explained, binary, ternary or quaternary
or more compound semiconductor in combination with III group
element(s) such as B, Al, Ga, In and Tl, and V group element(s)
such as N, P, As, Sb and Bi may be used in the present
invention.
[0037] As shown in FIG. 1A, on a substrate 1 with low dislocation
which is formed of n-type GaAs or the like, an n-type cladding
layer 2 formed of Al.sub.0.3Ga.sub.0.7As or the like, a light guide
layer 3 formed of GaAs or the like, a quantum well layer 4 formed
of In.sub.0.14Ga.sub.0.86As or the like, a barrier layer 5 formed
of GaAs or the like, a quantum well layer 6 formed of
In.sub.0.14Ga.sub.0.86As or the like, a light guide layer 7 formed
of GaAs or the like and a p-type cladding layer 8a formed of
Al.sub.0.3Ga.sub.0.7As or the like are epitaxially grown in this
sequence using deposition process such as MOCVD (Metal Organic
Chemical Vapor Deposition).
[0038] For an active layer which can generate light, double quantum
well (DQW) structure in which two quantum well layers 4 and 6 are
disposed on both sides of the barrier layer 5 is adopted.
[0039] The p-type cladding layer 8a is part of a final p-type
cladding layer, which has such a thickness as photoluminescence
(PL) measurement can be performed at a back step.
[0040] Next, as shown FIG. 1B, on the p-type cladding layer 8a an
insulation layer 10 is formed of SiO, SiN, SiON or the like using
deposition process such as CVD (Chemical Vapor Deposition) to
prevent the V group element (herein As) from escaping during heat
treatment at a back step.
[0041] Next, heat treatment is applied to the substrate 1 with the
layers 2-10 for nearly 30 minutes, for example, in an annealing
furnace of quartz tube type with nitrogen (N.sub.2) atmosphere at a
temperature of 800 degree-C. or more.
[0042] After heat treatment, photoluminescence (PL) measurement is
performed by irradiating light of an energy higher than the band
gap energy of the active layer and then analyzing the emission
spectrum from the active layer.
[0043] In case the similar PL measurement has been performed before
heat treatment, the emission spectrum after heat treatment can be
compared with the emission spectrum before heat treatment. If the
result of the comparison shows that the PL wavelength (i.e. peak
wavelength of the emission spectrum) after heat treatment is
blue-shifted and moved toward the short wavelength side, compared
to the PL wavelength before heat treatment, the effect of annealing
by heat treatment can be checked.
[0044] Next, after removing the insulation layer 10 using wet or
dry etching, etc., as shown in FIG. 1C, a residual p-type cladding
layer 8b formed of Al.sub.0.3Ga.sub.0.7As or the like and a contact
layer 11 formed of GaAs or the like are epitaxially grown using
MOCVD.
[0045] Next, after forming a mask pattern for ridge on the contact
layer 11 using a photoresist and an insulating film, as shown in
FIG. 2A, a ridge 11a is formed by removing portions of the contact
layer 11 and the p-type cladding layer 8 using wet or dry etching,
etc. Then, the mask pattern for ridge is removed.
[0046] Next, as shown in FIG. 2B, an insulation layer 12 is formed
of SiO, SiN, SiON or the like using CVD except the top of the ridge
11a.
[0047] Next, as shown in FIG. 2C, a p-side electrode 13 is formed
over the insulation layer 12 using spattering, etc. Next, after
scraping the undersurface of the substrate 1 so thinly as to
perform chip cleavage easily, an n-side electrode 14 is formed over
the undersurface of the substrate 1 using spattering, etc. Then,
the substrate 1 is divided into chips by cleavage.
[0048] The semiconductor optical device obtained in this way has
the composition of the epitaxial growth layer which is changing
continuously near interfaces, since heat treatment has been
performed at a temperature of 800 degree-C. or more. Consequently,
distortion between the epitaxial growth layers is eased, thereby
remarkably enhancing the reliability of the device.
[0049] FIG. 3 is a graph showing an example of the reliability
examination result of the semiconductor optical device to which
heat treatment has been applied. The semiconductor optical devices
which have been annealed for 30 minutes in nitrogen atmosphere at a
temperature of 820 degree-C. in the heat treatment of FIG. 1B are
evaluated by measuring change of the driving current with time
progress using an APC (Automatic Power Control) circuit which can
maintain a constant light output. The examination conditions are
atmosphere at 50 degree-C., a constant output of 300 mW and a
measurement size of ten.
[0050] This graph shows that the driving currents of all ten of
samples do not increase even after 13,000 hours and conspicuous
degradation is not seen. As a result, the semiconductor optical
devices can be continuously operated for 10,000 hours or more and
can be realized with extremely high reliability.
[0051] In addition, the result of PL measurements before and after
the annealing process for 30 minutes at a temperature of 820
degree-C. shows that the PL wavelength before the annealing process
is 1,010 nm and PL wavelength after the annealing process is 974 nm
with blueshift of 45 meV in terms of photon energy. This fact
proves that distortion between the epitaxial growth layers is eased
due to heat treatment and the band gap energy of the active layer
is increased.
[0052] Next, structure analysis of the epitaxial growth layer using
SIMS (Secondary Ion Mass Spectroscopy) will be described.
[0053] FIG. 4 is a band diagram of the epitaxial growth layer
before heat treatment. FIG. 5 is a band diagram of the epitaxial
growth layer after heat treatment. The vertical axis shows the
position of thickness direction and the horizontal axis shows In
composition y leftward and Al composition x rightward with the
center of GaAs.
[0054] The n-type cladding layer 2, the light guide layer 3, the
quantum well layer 4, the barrier layer 5, the quantum well layer
6, the light guide layer 7 and the p-type cladding layer 8a are
deposited in sequence from the bottom, and it can be seen that the
composition of each layer is changed stepwise near the interface
before heat treatment as shown FIG. 4.
[0055] After heat treatment as shown FIG. 5, on the other hand, it
can be seen that the composition of each layer is continuously
changed near the interface and the steepness of the epitaxial
growth interface is reduced. This fact proves that distortion which
may cause degradation of the device is eased.
[0056] FIG. 6 is a graph showing an example of the reliability
examination result of the semiconductor optical device subject to
another condition of heat treatment. Here, the other semiconductor
optical devices which have been annealed for 30 minutes in nitrogen
atmosphere at a temperature of 810 degree-C. in the heat treatment
of FIG. 1B are evaluated under an APC operation as in FIG. 3. The
examination conditions are atmosphere at 50 degree-C., a constant
output of 300 mW and a measurement size of ten.
[0057] This graph shows that one sample is degraded about 1,200
hours, another sample is degraded about 7,000 hours and the other
is degraded about 12,000 hours while the remaining samples are not
remarkably degraded since the driving current does not increase at
13,000 hours.
[0058] Furthermore, the PL wavelength before and after heat
treatment is changed from 1,010 nm to 984 nm with blueshift of 32
meV in terms of photon energy.
[0059] FIG. 7 is a graph showing an example of the reliability
examination result of the semiconductor optical device subject to
yet another condition of heat treatment. Here, the still other
semiconductor optical devices which have been annealed for 30
minutes in nitrogen atmosphere at a temperature of 800 degree-C. in
the heat treatment of FIG. 1B are evaluated under an APC operation
as in FIG. 3. The examination conditions are atmosphere at 50
degree-C., a constant output of 300 mW and a measurement size of
twenty two.
[0060] This graph shows that twelve sample are degraded by 13,000
hours while the remaining ten of samples are not remarkably
degraded since the driving current does not increase at 13,000
hours.
[0061] Furthermore, the PL wavelength before and after heat
treatment is changed from 1,010 nm to 993 nm with blueshift of 2.1
meV in terms of photon energy.
[0062] In this way, applying heat treatment at a temperature of 800
degree-C. or more to the epitaxial growth layers enables the
semiconductor optical device to attain a longer continuous
operation time and extremely higher reliability than that of the
conventional device.
[0063] Moreover, the insulation layer formed over the epitaxial
growth layers can prevent the V group element from escaping during
heat treatment, thereby suppressing variations of the layer
composition.
[0064] Furthermore, performing heat treatment so that the PL
wavelength is blue-shifted preferably by 20 meV or more can weaken
the steepness of the epitaxial growth interface, resulting in the
distortion relief effect which can attain high reliability.
[0065] In addition, the PL measurement after heat treatment,
preferably both before and after heat treatment, enables in-line
evaluation during manufacturing process, thereby improving the
manufacture yield of the semiconductor optical devices.
[0066] The above description exemplifies that the p-type cladding
layer 8 is deposited by two separate steps and heat treatment is
performed between the steps. However, heat treatment may be
performed during forming another layer, or heat treatment may be
performed after depositing all the layers by onetime epitaxial
growth, resulting in the similar distortion relief effect
[0067] Although the present invention has been fully described in
connection with the preferred embodiments thereof and the
accompanying drawings, it is to be noted that various changes and
modifications are apparent to those skilled in the art. Such
changes and modifications are to be understood as included within
the scope of the present invention as defined by the appended
claims unless they depart therefrom.
* * * * *