U.S. patent application number 11/088994 was filed with the patent office on 2005-08-04 for optical semiconductor device having an active layer containing n.
Invention is credited to Sato, Shunichi.
Application Number | 20050169334 11/088994 |
Document ID | / |
Family ID | 27530103 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050169334 |
Kind Code |
A1 |
Sato, Shunichi |
August 4, 2005 |
Optical semiconductor device having an active layer containing
N
Abstract
A laser diode includes a substrate, a lower cladding layer or a
lower optical waveguide layer substantially free from Al and
provided on the substrate, an active layer of a mixed crystal
containing Ga and In as a group III element and N, As and/or P as a
group V element, provided on the lower cladding layer; and an upper
cladding layer or an upper optical waveguide layer substantially
free from Al and provided on the active layer.
Inventors: |
Sato, Shunichi; (Miyagi,
JP) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L Street, NW
Washington
DC
20037
US
|
Family ID: |
27530103 |
Appl. No.: |
11/088994 |
Filed: |
March 24, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11088994 |
Mar 24, 2005 |
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10213072 |
Aug 7, 2002 |
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6879614 |
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10213072 |
Aug 7, 2002 |
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09688875 |
Oct 17, 2000 |
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6449299 |
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09688875 |
Oct 17, 2000 |
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08917141 |
Aug 25, 1997 |
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6233264 |
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Current U.S.
Class: |
372/45.01 |
Current CPC
Class: |
B82Y 20/00 20130101;
H01S 5/343 20130101; H01S 2302/00 20130101; H01S 5/32325 20130101;
H01S 5/3235 20130101; H01S 5/32366 20130101; H01L 33/32 20130101;
H01L 33/30 20130101; H01S 5/32375 20130101; H01S 5/34326 20130101;
H01S 5/3434 20130101; H01S 5/34313 20130101; H01S 5/3219 20130101;
H01L 33/305 20130101 |
Class at
Publication: |
372/045 |
International
Class: |
H01S 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 27, 1996 |
JP |
NO. 8-244244 |
Aug 28, 1996 |
JP |
NO. 8-245597 |
Sep 5, 1996 |
JP |
NO. 8-255413 |
Oct 4, 1996 |
JP |
NO. 8-283422 |
Oct 4, 1996 |
JP |
NO. 8-283424 |
Claims
1-13. (canceled)
14. A semiconductor device, comprising: an InP substrate; an active
layer provided over said substrate, said active layer containing
Ga, In, N, As and P; and another layer free from Al formed adjacent
to said active layer at either one side or both sides thereof, said
another layer having a composition represented as
Ga.sub.dIn.sub.1-dN.sub.cAs.sub.fP.sub.1-c-f (0.ltoreq.d.ltoreq.1,
0.ltoreq.e<1, 0.ltoreq.f.ltoreq.1).
15. A semiconductor device as claimed in claim 14, said active
layer and said another layer accumulating therein a strain of
respective different types.
16. A semiconductor device as claimed in claim 15, wherein said
active layer accumulates therein a compressive strain, and said
another layer accumulates therein a tensile strain.
17. A semiconductor device, comprising: an InP substrate; an active
layer provided over said substrate; and another layer free from Al
formed adjacent to said active layer at either one side or both
sides thereof, said another layer containing N and a group V
element other than N.
18. A semiconductor device as claimed in claim 17, wherein said
semiconductor device comprises a further layer free from Al
provided at a side of said another layer away from said active
layer.
19. A semiconductor device as claimed in claim 18, wherein said
further layer has a composition represented as
Ga.sub.dIn.sub.1-dN.sub.cAs.sub.fP- .sub.1-e-f
(0.ltoreq.d.ltoreq.1, 0.ltoreq.e<1, 0.ltoreq.f.ltoreq.1).
20. A semiconductor device, comprising: an InP substrate formed of
GaAs; an active layer provided over said substrate, said active
layer containing N and a group V element other than N; and another
layer free from Al provided adjacent to said active layer at one
side or both sides thereof, said another layer containing N and a
group V element other than N, said active layer and said another
layer accumulating therein a strain of respective, different
types.
21. A semiconductor device as claimed in claim 20, wherein said
active layer accumulates therein a compressive strain and said
another layer accumulates therein a tensile strain.
22. A semiconductor device as claimed in claim 20, wherein said
semiconductor device comprises a further layer free from Al
provided at a side of said another layer away from said active
layer.
23. An optical semiconductor device, comprising: an InP substrate;
and an active region provided over said InP substrate, said active
region at least comprising: a quantum well layer containing N and a
group V element other than N, said quantum well layer acting as a
light-emitting layer; and another layer free from Al contacting
said quantum well layer at one side or both sides of said quantum
well layer, said another layer containing N and a group V element
other than N.
24. An optical semiconductor device as claimed in claim 23, wherein
said quantum well layer and said another layer accumulate therein a
strain of respective, different types.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention generally relates to optical
semiconductor devices and more particularly to an optical
semiconductor device for use in a 1.3 .mu.m or 1.5 .mu.m wavelength
band.
[0002] Optical wavelength band of 1.3 .mu.m or 1.5 .mu.m is used
commonly in optical telecommunication systems that use optical
fibers. It should be noted that a quartz glass optical fiber has an
optical transmission band in the wavelength of 1.3 .mu.m or 1.5
.mu.m.
[0003] In correspondence to the foregoing specific optical
transmission band of the optical fibers, conventional optical
telecommunication systems generally use a laser diode constructed
on an InP substrate. Such a laser diode typically uses an active
layer of InGaPAs having a lattice constant matching the lattice
constant of the InP substrate and a bandgap corresponding to the
optical wavelength of 1.3 .mu.m or 1.5 .mu.m.
[0004] While the foregoing laser diode that uses InGaAsP active
layer performs well in conventional optical telecommunication
systems, particularly optical telecommunication trunks, the laser
diode, requiring an expensive temperature regulation system such as
a Peltier cooling device for a proper operation thereof, is deemed
to be inappropriate for optical subscriber systems such as optical
home terminals because of the increased cost of the temperature
regulation system. In the foregoing laser diode that uses the
InGaPAs active layer in combination with the InP substrate, the
discontinuity of conduction band at the interface between the
active layer and the surrounding cladding layer or optical
waveguide layer is not sufficient for effective confinement of the
carriers in the active layer, and there is a tendency that the
carriers escape or overflow from the active layer when the device
is not properly cooled. Because of such a poor confinement of the
carriers, the laser diode generally shows a poor efficiency of
laser oscillation. This problem becomes particularly serious in a
high temperature operation of the laser diode where the carriers
experience extensive thermal excitation.
[0005] On the other hand, recent investigations on a GaAs-GaN
system have discovered that the bandgap of a GaAs mixed crystal
containing therein a small amount of N decreases with increasing N
content in the GaAs mixed crystal. GaN itself has been known to
have a very large bandgap and is used for an active layer of an LED
or laser diode that emits a blue or violet optical radiation.
[0006] FIG. 1 shows the bandgap of such a GaAs-GaN mixed crystal
system together with other group III-v compound semiconductor
materials (Kondow, M., et al., Extended Abstracts of the 1995
International Conference on Solid State Devices and Materials,
Osaka, 1995, pp. 1016-1018).
[0007] Referring to FIG. 1, it should be noted that, while GaN or a
mixed crystal thereof containing a small amount of As has a very
large bandgap suitable for emission of blue or violet optical
radiation, the mixed crystal of GaAs containing a small amount of N
has a small bandgap suitable for emission of the 1.3 .mu.m or 1.5
.mu.m optical wavelength band used for optical telecommunications
systems. It should be noted that the bandgap of the GaAs mixed
crystal decreases rapidly with increasing N content therein.
Further, FIG. 1 indicates that the lattice constant of the GaAs
mixed crystal decreases substantially with increasing N content
therein.
[0008] Thus, the Japanese Laid-Open Patent
[0009] Publication 6-334168 describes a technology of growing a
III-V mixed crystal film containing N on a Si substrate
epitaxially. For example, the foregoing reference describes a laser
diode and a photodiode that use a GaNP cladding layer having a
composition of GaN.sub.0.03P.sub.0.97 in combination with an active
layer having a strained superlattice structure in which a GaNP
layer and a GaNAs layer are stacked alternately and repeatedly. The
foregoing cladding layer successfully establishes a lattice
matching with the Si substrate. According to the teaching of the
foregoing reference, it becomes possible to form a III-V device on
a Si substrate without inducing misfit dislocations in the
epitaxial layers. Further, the disclosed technology enables
formation of an integrated circuit in which the III-V optical
semiconductor devices are integrated monolithically with other Si
devices.
[0010] Further, various mixed crystal compositions that establish a
lattice matching with a substrate of GaAs, InP or GaP are reported
for various N-containing III-V systems such as GaInNAs, AlGaNAs and
GaNAs in the Japanese Laid-Open Patent Publications 6-037355.
[0011] Conventionally, no III-V composition has been known that has
a bandgap smaller than the bandgap of GaAs and simultaneously a
lattice constant that matches the lattice constant of GaAs, until a
mixed crystal of GaInNAs is discovered. Provided that the N content
is held small, the GaInNAs mixed crystal successfully establishes a
lattice matching with a GaAs substrate and simultaneously has a
bandgap smaller than the bandgap of GaAs. See the band diagram of
FIG. 1. Thus, the GaInNAs mixed crystal is thought to be a
promising material for an active layer of an optical semiconductor
device that operates in the 1.3 .mu.m or 1.5 .mu.m wavelength band.
However, little is known about the properties of the mixed crystal
of GaInNAs.
[0012] Thus, Kondow, M., et al., op. cit., proposes a laser diode
structure that uses a GaInNAs mixed crystal for the active layer of
a laser diode. The reference further discloses the use of a
cladding layer of AlGaAs in contact with the active layer of
GaInNAs for securing a large discontinuity in the conduction band
at the heterojunction interface across the cladding layer and the
active layer. Because of the very large band discontinuity at the
heterojunction interface, the laser diode is expected to show a
high efficiency of laser oscillation and improved temperature
characteristic associated with an efficient confinement of the
carriers in the active layer.
[0013] On the other hand, it is known that the epitaxial growth of
a GaInNAs mixed crystal is substantially difficult at high
temperatures because of the tendency of the N atoms escaping from
the deposited epitaxial layer of GaInNAs In order to obtain a film
containing a substantial amount of N atoms, it is necessary to
carry out the deposition process at a temperature of about
680.degree. C. or less. However, the epitaxial growth at such a low
temperature is not preferable for the growth of a layer containing
Al, such as a layer of AlGaAs used for the cladding layer, because
of the tendency of the highly reactive Al atoms in the cladding
layer reacting with a small amount of O atoms remaining in the
deposition chamber or in the source gases as impurity. The O atoms
thus incorporated form a non-optical recombination center in the
epitaxial layer, while the non-optical recombination centers thus
formed tend to annihilate the carriers without emitting photons. It
should be noted that the problem of oxidation of Al cannot be
avoided-even when the deposition is carried out under an
environment where the air is purged by a high-performance vacuum
system.
[0014] In order to avoid the foregoing problem of incorporation of
the O atoms into the cladding layer of AlGaAs, it is necessary to
carry out the deposition of the cladding layer at a high
temperature of at least 750.degree. C. However, the use of such a
high temperature growth is contradictory to the requirement of low
temperature growth of the GaInNAs active layer as noted previously.
Even in the case in which the substrate temperature is lowered
after the deposition of the cladding layer of AlGaAs for allowing
the deposition of the GaInNAs layer thereon, such a lowering of the
substrate temperature also allows unwanted incorporation of the O
atoms in the reaction chamber to the exposed surface of the lower
cladding layer, and the formation of non-optical recombination
centers at the heteroepitaxial interface between the lower cladding
layer and the active layer is inevitable. It should be noted that
the non-optical recombination centers reduce the lifetime of the
optical semiconductor device such as a laser diode.
[0015] In addition to the foregoing problem, the inventor of the
present invention has discovered that a direct epitaxial growth of
a GaInNAs layer on an AlGaAs layer is difficult, as in the case of
forming a laser diode that uses a cladding layer of AlGaAs in
combination with the GaInNAs active layer.
[0016] FIG. 2 shows the surface morphology of a GaInNAs layer grown
directly on an epitaxial layer of AlGaAs, which in turn is grown on
a GaAs substrate. The GaInNAs layer is grown with a thickness of
0.1 .mu.m and has a composition of
Ga.sub.0.9In.sub.0.1N.sub.0.03As.sub.0.97 for a successful lattice
matching to the GaAs substrate. The AlGaAs layer underlying the
GaInNAs layer has a composition of Al.sub.0.4Ga.sub.0.6As and is
grown with a thickness of about 0.2 .mu.m.
[0017] Referring to FIG. 2, it will be noted that the surface of
the GaInNAs layer is not smooth but includes minute projections and
depressions, indicating a non-uniform or island-like growth of the
GaInNAs layer occurring on the surface of the AlGaAs layer. The
GaInNAs layer having such an irregular surface morphology performs
poorly when used for the active layer of a laser diode due to
various reasons such as scattering of light at the irregular
surface or non-optical recombination of the carriers caused by the
defects that accompany with such irregular heteroepitaxial
interface.
[0018] Thus, it has been difficult to fabricate a double hetero
laser diode that uses a III-V layer containing N for the active
layer.
SUMMARY OF THE INVENTION
[0019] Accordingly, it is a general object of the present invention
to provide a novel and useful optical semiconductor device wherein
the foregoing problems are eliminated.
[0020] Another and more specific object of the present invention is
to provide an optical semiconductor device operable in a 1.3 .mu.m
or 1.5 .mu.m optical wavelength band without a temperature
regulation.
[0021] Another object of the present invention is to provide an
optical semiconductor device that includes an active layer
containing N atoms therein, wherein a large band discontinuity is
secured between the active layer and a cladding layer while
reducing non-optical recombination centers simultaneously.
[0022] Another object of the present invention is to provide an
optical semiconductor device, comprising:
[0023] a substrate;
[0024] a lower cladding layer substantially free from Al and
provided on said substrate;
[0025] an active layer of GaInNPAs provided on said lower cladding
layer; and
[0026] an upper cladding layer substantially free from Al and
provided on said active layer.
[0027] According to the present invention, the upper and lower
cladding layers are free from Al. Thus, the oxidation of Al in the
upper or lower cladding layer at the time of low-temperature
deposition of the active layer is successfully avoided. by using a
mixed crystal of GaInNPAs for the active layer, it becomes possible
to achieve a photo-electronic interaction in the active layer for
an optical radiation having a 1.3 .mu.m or 1.5 .mu.m band.
[0028] Another object of the present invention is to provide a
laser diode, comprising:
[0029] a substrate of GaAs having a first conductivity type;
[0030] a lower cladding layer of a semiconductor material having
said first conductivity type and provided on said substrate, said
lower cladding layer having a composition substantially free from
Al;
[0031] an active layer of a group III-V compound semiconductor
material provided on said lower cladding layer, said active layer
containing Ga and In as a group III element and N and As as a group
V element;
[0032] an upper cladding layer of a semiconductor material having a
second, opposite conductivity type and provided on said active
layer, said upper cladding layer having a composition substantially
free from Al;
[0033] a contact layer of a group III-V compound semiconductor
material having said second conductivity type and provided on said
upper cladding layer;
[0034] a first ohmic electrode provided in ohmic contact with said
contact layer; and
[0035] a second ohmic electrode provided in ohmic contact with said
substrate.
[0036] According to the present invention, the upper and lower
cladding layers are free from Al. Thus, the oxidation of Al in the
upper or lower cladding layer at the time of low-temperature
deposition of the active layer is successfully avoided. By using
the group III-V semiconductor material containing Ga, In, N and As
in combination with the GaAs substrate, the laser diode oscillates
successfully in the 1.3 .mu.m wavelength band with high efficiency.
As the carriers are confined effectively in the active layer in the
laser diode of the present invention, the laser diode is operable
without external cooling. By setting the composition of the active
layer in lattice matching to the GaAs substrate, the active layer
can be formed with a desired thickness, without inducing a lattice
misfit strain therein. Thereby, the laser diode can have a
double-hetero structure in which the active layer having a
thickness suitable for acting as an optical waveguide is sandwiched
directly by the cladding layers.
[0037] Another object of the present invention is to provide laser
diode, comprising:
[0038] a substrate of GaAs having a first conductivity type;
[0039] a lower cladding layer of AlGaAs having said first
conductivity type and provided on said substrate without
accumulating a substantial lattice misfit strain;
[0040] a lower optical waveguide layer of GaInPAs provided on said
lower cladding layer;
[0041] an active layer of a GaInNAs provided on said lower optical
waveguide layer, said active layer being substantially free from a
lattice misfit strain;
[0042] an upper optical waveguide layer of GaInPAs provided on said
active layer;
[0043] an upper cladding layer of AlGaAs doped to a second,
opposite conductivity type and provided on said upper optical
waveguide layer without accumulating a substantial lattice misfit
strain;
[0044] a contact layer of a group III-V compound semiconductor
material having said second conductivity type and provided on said
upper cladding layer;
[0045] a first ohmic electrode provided in ohmic contact with said
contact layer; and
[0046] a second ohmic electrode provided in ohmic contact with said
substrate.
[0047] According to the present invention, the upper and lower
optical waveguide layers directly in contact with the active layer
are free from Al. Thus, the oxidation of Al in the upper or lower
optical waveguide layers at the time of low-temperature deposition
of the active layer is successfully avoided. By using GaInPAS for
the optical waveguide layers sandwiching the active layer, the
carriers injected to the laser diode are confined effectively in
the active layer as compared with the case of using GaAs for the
optical waveguide layers due to the increased bandgap energy of
GaInPAs that achieves a lattice matching to the GaAs substrate.
[0048] Another object of the present invention is to provide a
laser diode, comprising:
[0049] a substrate of GaAs having a first conductivity type;
[0050] a lower cladding layer of AlGaAs having said first
conductivity type and provided on said substrate;
[0051] a lower optical waveguide layer of GaInPAs provided on said
lower cladding layer;
[0052] an active layer of a GaInNAs provided on said lower optical
waveguide layer, said active layer having a bandgap energy
corresponding to a 1.3 .mu.m optical wavelength;
[0053] an upper optical waveguide layer of GaInPAs provided on said
active layer;
[0054] an upper cladding layer of AlGaAs having a second, opposite
conductivity type and provided on said upper optical waveguide
layer;
[0055] a contact layer of a group III-V compound semiconductor
material having said second conductivity type and provided on said
upper cladding layer;
[0056] a first ohmic electrode provided in ohmic contact with said
contact layer; and
[0057] a second ohmic electrode provided in ohmic contact with said
substrate.
[0058] According to the present invention, the upper and lower
optical waveguide layers directly in contact with the active layer
are free from Al. Thus, the oxidation of Al in the upper or lower
optical waveguide layers at the time of the low-temperature
deposition of the active layer is successfully avoided. By using
AlGaAs for the upper and lower optical waveguide layers, the
thermal resistance of the cladding layer is reduced substantially,
and the high-temperature stability of the laser-diode operation is
improved substantially. By using a mixed crystal of GaInNAs that
contains N for the active layer of the laser diode, a laser
oscillation in the 1.3 .mu.m band is successfully achieved while
simultaneously achieving a lattice matching between the active
layer and the GaAs-substrate. By increasing the In content in the
active layer, the bandgap of the mixed crystal forming the active
layer is reduced and the laser oscillation wavelength is increased
for the mixed crystal composition of the active layer in which the
N content is reduced. By reducing the N content of the active layer
as such, the quality of the crystal forming the active layer is
improved, and the efficiency of laser oscillation and the laser
oscillation spectrum are improved substantially.
[0059] Another object of the present invention is to provide a
laser diode, comprising:
[0060] a substrate of GaAs having a first conductivity type;
[0061] a lower cladding layer of AlGaInP having said first
conductivity type and provided on said substrate;
[0062] a lower optical waveguide layer of GaInPAs provided on said
lower cladding layer;
[0063] an active layer of a GaInNAs provided on said lower optical
waveguide layer, said active layer having a bandgap energy
corresponding to a 1.3 .mu.m optical wavelength;
[0064] an upper optical waveguide layer of GaInPAs provided on said
active layer;
[0065] an upper cladding layer of AlGaInP doped to a second,
opposite conductivity type and provided on said upper optical
waveguide layer;
[0066] a contact layer of a group III-V compound semiconductor
material having said second conductivity type and provided on said
upper cladding layer;
[0067] a first ohmic electrode provided in ohmic contact with said
contact layer; and
[0068] a second ohmic electrode provided in ohmic contact with said
substrate.
[0069] According to the present invention, the upper and lower
optical waveguide layers directly in contact with the active layer
are free from Al. Thus, the oxidation of Al in the upper or lower
optical waveguide layers at the time of low-temperature deposition
of the active layer is successfully avoided. By using a mixed
crystal of GaInNAs that contains N for the active layer of the
laser diode, a laser oscillation in the 1.3 .mu.m band is
successfully achieved while simultaneously achieving a lattice
matching between the active layer and the GaAs substrate. By using
GaInPAs for the upper and lower optical waveguide layers, a large
band discontinuity is secured at the interface between the optical
waveguide layer and the active layer, and the carriers injected to
the laser diode are confined into the active layer with high
efficiency. Thereby, the laser diode shows an excellent high
temperature performance.
[0070] Another object of the present invention is to provide a
laser diode, comprising:
[0071] a substrate of GaAs having a first conductivity type;
[0072] a lower cladding layer having said first conductivity type
and provided on said substrate;
[0073] a lower optical waveguide layer of GaInPAs provided on said
lower cladding layer;
[0074] an active layer provided on said lower optical waveguide
layer, said active layer comprising an alternate repetition of a
quantum well layer of GaInNAs and a barrier layer of GaInPAs;
[0075] an upper optical waveguide layer of GaInPAs provided on said
active layer;
[0076] an upper cladding layer doped to a second, opposite
conductivity type and provided on said upper optical waveguide
layer;
[0077] a contact layer of a group III-V compound semiconductor
material having said second conductivity type and provided on said
upper cladding layer;
[0078] a first ohmic electrode provided in ohmic contact with said
contact layer; and
[0079] a second ohmic electrode provided in ohmic contact with said
substrate;
[0080] each of said quantum well layers accumulating therein a
compressional lattice misfit strain and each of said barrier layers
accumulating therein a tensile lattice misfit strain.
[0081] According to the present invention, the upper and lower
optical waveguide layers directly in contact with the active layer
are free from Al. Thus, the oxidation of Al in the upper or lower
optical waveguide layers at the time of low-temperature deposition
of the active layer is successfully avoided. By forming a strained
superlattice structure in the active layer by the quantum well
layers of GaInNAs and the barrier layers of GaInPAs, it is possible
to reduce the bandgap energy and increase the oscillation
wavelength of the laser diode while simultaneously reducing the N
content in the quantum well layers. Thereby, the quality of the
crystal forming the quantum well layers is improved substantially
and an efficient laser oscillation is achieved with a sharply
defined oscillation spectrum.
[0082] Another object of the present invention is to provide a
laser diode, comprising:
[0083] a substrate of GaAs having a first conductivity type;
[0084] a lower cladding layer having said first conductivity type
and provided on said substrate;
[0085] a lower optical waveguide layer of GaInPAs provided on said
lower cladding layer;
[0086] an active layer provided on said lower optical waveguide
layer, said active layer comprising an alternate repetition of a
quantum well layer of GaInNAs and a barrier layer of GaInNPAs;
[0087] an upper optical waveguide layer of GaInPAs provided on said
active layer;
[0088] an upper cladding layer doped to a second, opposite
conductivity type and provided on said upper optical waveguide
layer;
[0089] a contact layer of a group III-V compound semiconductor
material having said second conductivity type and provided on said
upper cladding layer;
[0090] a first ohmic electrode provided in ohmic contact with said
contact layer; and
[0091] a second ohmic electrode provided in ohmic contact with said
substrate.
[0092] said quantum well layers accumulating therein a
compressional lattice misfit strain and said barrier layers
accumulating therein a tensile lattice misfit strain.
[0093] According to the present invention, the upper and lower
optical waveguide layers directly in contact with the active layer
are free from Al. Thus, the oxidation of Al in the upper or lower
optical waveguide layers at the time of low-temperature deposition
of the active layer is successfully avoided. By forming a strained
superlattice structure in the active layer by the quantum well
layers and the barrier layers, it is possible to reduce the bandgap
energy and increase the oscillation wavelength of the laser diode
while simultaneously reducing the N content in the quantum well
layers. Thereby, the quality of the crystal forming the quantum
well layers is improved substantially and an efficient laser
oscillation is achieved with a sharply defined oscillation
spectrum. Further, the laser diode of the present invention is easy
to fabricate, as the quantum well layers and the barrier layers are
formed in the same deposition apparatus by merely switching the
supply of a source of P on and off repeatedly.
[0094] Another object of the present invention is to provide a
laser diode, comprising:
[0095] a substrate of GaAs having a first conductivity type;
[0096] a lower cladding layer of GaInP having said first
conductivity type and provided on said substrate;
[0097] a lower optical waveguide layer of GaInPAs provided on said
lower cladding layer;
[0098] an active layer of GaInNPAs having a bandgap energy in a 0.8
.mu.m band and provided on said lower optical waveguide layer;
[0099] an upper optical waveguide layer of GaInPAs provided on said
active layer;
[0100] an upper cladding layer of GaInP having a second, opposite
conductivity type and provided on said upper optical waveguide
layer;
[0101] a contact layer provided on said upper cladding layer;
[0102] a first ohmic electrode provided in ohmic contact with said
contact layer; and
[0103] a second ohmic electrode provided in ohmic contact with said
substrate.
[0104] According to the present invention, the upper and lower
optical waveguide layers directly in contact with the active layer
are free from Al. Thus, the oxidation of Al in the upper or lower
optical waveguide layers at the time of low-temperature deposition
of the active layer is successfully avoided. By using a mixed
crystal of GaInNPAs for the active layer, the laser diode
oscillates successfully at an optical wavelength of about 0.81
.mu.m. As a very large band-discontinuity is secured in the laser
diode of the present invention that uses GaInP for the cladding
layer, the laser diode has an excellent high temperature
performance and is suitable for high power applications.
[0105] Another object of the present invention is to provide a
laser diode, comprising:
[0106] a substrate of InP having a first conductivity type;
[0107] a lower cladding layer of InP having said first conductivity
type and provided on said substrate;
[0108] an active layer of a GaInNPAs provided on said lower
cladding layer;
[0109] an upper cladding layer of InP having a second, opposite
conductivity type and provided on said active layer;
[0110] a contact layer of a group III-V compound semiconductor
material having said second conductivity type and provided on said
upper cladding layer;
[0111] a first ohmic electrode provided in ohmic contact with said
contact layer; and
[0112] a second ohmic electrode provided in ohmic contact with said
substrate.
[0113] According to the present invention, the upper and lower
cladding layers directly in contact with the active layer are free
from Al. Thus, the oxidation of Al in the upper or lower optical
waveguide layers at the time of low-temperature deposition of the
active layer is successfully avoided. By using a mixed crystal of
GaInNPAs for the active layer, the laser diode oscillates
successfully at an optical wavelength band of about 1.5 .mu.m. As
the carriers are confined effectively in the active layer in the
laser diode of the present invention, the laser diode is operable
without external cooling. By setting the composition of the active
layer in lattice matching to the InP substrate, the active layer
can be formed with a desired thickness, without inducing a lattice
misfit strain therein. Thereby, the laser diode can have a
double-hetero structure in which the active layer having a
thickness suitable for acting as an optical waveguide is sandwiched
directly by the cladding layers.
[0114] Another object of the present invention is to provide a
laser diode, comprising:
[0115] a substrate of InP having a first conductivity type;
[0116] a lower cladding layer of InP having said first conductivity
type and provided on said substrate;
[0117] a lower optical waveguide layer of GaInPAs provided on said
lower cladding layer;
[0118] an active layer provided on said lower optical waveguide
layer, said active layer comprising an alternate repetition of a
quantum well layer of GaInNPAs and a barrier layer of GaInPAs;
[0119] an upper optical waveguide layer of GaInPAs provided on said
active layer;
[0120] an upper cladding layer of InP having a second, opposite
conductivity type and provided on said upper optical waveguide
layer;
[0121] a cap layer provided on said upper cladding layer;
[0122] a first ohmic electrode provided in ohmic contact to said
cap layer; and
[0123] a second ohmic electrode provided in ohmic contact to said
substrate.
[0124] According to the present invention, the upper and lower
optical waveguide layers directly in contact with the active layer
are free from Al. Thus, the oxidation of Al in the upper or lower
optical waveguide layers at the time of low-temperature deposition
of the active layer is successfully avoided. By using a mixed
crystal of GaInNPAs for the active layer, the laser diode
oscillates successfully at an optical wavelength band of about 1.5
.mu.m. As the carriers are confined effectively in the active layer
in the laser diode of the present invention as a result of the use
of InP cladding in combination with the GaInNPAs active layer, the
laser diode is operable at high temperatures without external
cooling.
[0125] Another object of the present invention is to provide a
laser diode, comprising:
[0126] a substrate having a first conductivity type;
[0127] a lower cladding layer having said first conductivity type
and provided on said substrate;
[0128] an active layer of a group III-V compound semiconductor
material containing Ga and In as a group III element and N and As
as a group V element;
[0129] an upper cladding layer having a second, opposite
conductivity type and provided on said active layer;
[0130] a current confinement structure provided on said upper
cladding layer, said current confinement structures including first
and second patterns disposed on said upper cladding layer at both
lateral sides of an optical axis of said laser diode when viewed
perpendicularly to said substrate so as to expose a part of said
upper cladding layer along said optical axis, each of said first
and second patterns being formed of a group III-V compound
semiconductor material containing Ga and In as a group III element
and N and As as a group V element, said first and second patterns
having said first conductivity type and a bandgap energy not
exceeding a bandgap energy of said active layer;
[0131] a second upper cladding layer of said second conductivity
type provided on said current confinement structure so as to cover
said first and second patterns forming said current confinement
structure and in contact with said exposed part of said upper
cladding layer;
[0132] a contact layer of said second conductivity type provided on
said second upper cladding layer;
[0133] a first ohmic electrode provided in ohmic contact with said
contact layer; and
[0134] a second ohmic electrode provided in ohmic contact with said
substrate.
[0135] According to the present invention, the first and second
patterns, formed of a mixed crystal containing Ga, In, N and As,
effectively absorb the optical radiation produced in the active
layer. Thereby, the refractive index of the first and second
patterns is changed and an optical waveguide structure confining an
optical radiation laterally along the optical axis of the laser
diode is formed along the axial direction of the laser diode. In
other words, the optical radiation produced in the active layer is
confined effectively in the lateral direction, and the stimulated
emission of photons is substantially facilitated as the optical
beam is guided by the optical waveguide structure thus formed back
and forth in the axial direction of the laser diode. As the first
and second patterns are formed to have the first conductivity type,
the confinement of the carriers occurs also to the exposed part of
the upper cladding layer that contacts with the second upper
cladding layer. It is preferred to form the first and second
patterns of the same material forming the active layer.
[0136] Other objects and further features of the present invention
will become apparent from the following detailed description when
read in conjunction with the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0137] FIG. 1 is a diagram showing the bandgap energy of various
group III-V compound semiconductor systems;
[0138] FIG. 2 is a diagram showing a rough surface morphology
obtained for a GaInNAs layer grown directly on an AlGaAs layer;
[0139] FIG. 3 is a diagram showing a horizontal MOVPE apparatus
used in the present invention for growing various group III-V
semiconductor epitaxial layers;
[0140] FIGS. 4A and 4B are diagrams showing the construction of a
ridge-waveguide laser diode according to a first embodiment of the
present invention respectively in a longitudinal cross-sectional
view and an end view;
[0141] FIG. 5 is a laser oscillation spectrum obtained for the
laser diode of the first embodiment;
[0142] FIG. 6 is a diagram showing an optical emission spectrum of
an active layer composition used in the laser diode of the first
embodiment;
[0143] FIG. 7 is a diagram showing the construction of a laser
diode according to a second embodiment of the present invention in
a longitudinal cross-sectional view;
[0144] FIG. 8 is a diagram showing the construction of a laser
diode according to a third embodiment of the present invention in a
longitudinal cross-sectional view;
[0145] FIG. 9 is a diagram showing the construction of a laser
diode according to a fourth embodiment of the present invention in
a longitudinal cross-sectional view;
[0146] FIG. 10 is a diagram showing the construction of a laser
diode according to a fifth embodiment of the present invention in a
longitudinal cross-sectional view;
[0147] FIG. 11 is a diagram showing the construction of a laser
diode according to a sixth embodiment of the present invention in a
longitudinal cross-sectional view;
[0148] FIG. 12 is a diagram showing the construction of a laser
diode according to a seventh embodiment of the present invention in
a longitudinal cross-sectional view;
[0149] FIG. 13 is a diagram showing the construction of a laser
diode according to an eighth embodiment of the present invention in
a longitudinal cross-sectional view;
[0150] FIG. 14 is a diagram showing the construction of a laser
diode according to a ninth embodiment of the present invention in a
longitudinal cross-sectional view;
[0151] FIG. 15 is a diagram showing the construction of a laser
diode according to a tenth embodiment of the present invention in a
longitudinal cross-sectional view;
[0152] FIGS. 16A and 16B are diagrams respectively showing a band
diagram of a conventional laser diode and a band diagram of the
laser diode of FIG. 15;
[0153] FIG. 17 is a diagram showing the construction of a laser
diode according to an eleventh embodiment of the present invention
in a longitudinal cross-sectional view;
[0154] FIG. 18 is a diagram showing the construction of a laser
diode according to a twelfth embodiment of the present invention in
a longitudinal cross-sectional view;
[0155] FIG. 19 is a diagram showing the construction of a laser
diode according to a thirteenth embodiment of the present invention
in a longitudinal cross-sectional view;
[0156] FIG. 20 is a diagram showing the construction of a laser
diode according to a fourteenth embodiment of the present invention
in an end view; and
[0157] FIG. 21 is a diagram showing a modification of the laser
diode of FIG. 20.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0158] FIG. 3 shows the construction of a deposition apparatus 10
used in the present invention for fabricating epitaxial layers
forming the optical semiconductor device of the present
invention.
[0159] Referring to FIG. 3, the deposition apparatus 10 is a
horizontal type apparatus and includes a quartz glass reactor 12
defining therein a reaction chamber 11, wherein a cooling jacket is
provided so as to surround the reactor 12 for water cooling.
Further, the reactor 12 includes an inlet 14 for introducing
various gaseous sources into the reaction chamber 11 together with
a carrier gas. Further, the reaction chamber 11 is evacuated at an
exhaust unit 15, which is connected to a scrubber not
illustrated.
[0160] Inside the reaction chamber 11, a carbon susceptor 16 is
disposed for supporting a substrate 2 on which a deposition is
carried out, and a radio-frequency coil 17 is provided so as to
surround the reactor 12 for energizing the susceptor 16. The
temperature of the substrate 2 is measured by a thermocouple 18
provided in the susceptor 16 in the vicinity of the substrate
2.
[0161] FIGS. 4A and 4B show the construction of a double-hetero
laser diode 1 according to a first embodiment of the present
invention fabricated by the deposition apparatus 10 respectively in
a longitudinal cross sectional view and an end view.
[0162] Referring to FIGS. 4A and 4B, the laser diode 1 includes a
substrate 2 of n-type GaAs on which a lower cladding layer 3 of
n-type GaInP is provided epitaxially with a thickness of typically
about 1500 nm. It should be noted that the lower cladding layer 3
is substantially free from Al, and the deposition of the lower
cladding layer 3 is carried out in the deposition chamber 11 of
FIG. 3 at a substrate temperature of 450-700.degree. C. while
supplying TMG (trimethylgallium) or TEG (triethylgallium) as the
source of Ga, TMI (trimethylindium) or TEI (triethylindium) as the
source of In, AsH.sub.3 as the source of As and PH.sub.3 as the
source of P, together with a carrier gas of H.sub.2. During the
deposition of the lower cladding layer, an n-type dopant such as Si
or Se is added to the foregoing source gases in the form of
SiH.sub.4 or H.sub.2Se. In a typical example, the lower cladding
layer 3 has a composition of Ga.sub.0.5In.sub.0.5P and a bandgap of
1.91 eV, wherein the composition of the lower cladding layer 3 is
set so as to establish a lattice matching to the GaAs substrate 1.
Thereby, the cladding layer 3 can be grown to the foregoing
thickness without creating dislocations caused as a result of
lattice misfit between the cladding layer 3 and the substrate
2.
[0163] After the lower cladding layer 3 is thus formed, an active
layer 4 of undoped GaInNAs having a composition of approximately
Ga.sub.0.9In.sub.0.1N.sub.0.03As.sub.0.97 is grown epitaxially on
the lower cladding layer 3 at a temperature of about
450-700.degree. C., while supplying TMG, TMI, AsH.sub.3 and
dimethylhydradine (DMHy), an organic nitride compound, respectively
as the sources of Ga, In, As and N. The deposition may be conducted
by setting the flow rates of TMG, TMI, AsH.sub.3 and DMHy to
4.0.times.10.sup.-6-4.0.times.10.sup.-5 mol/min,
4.4.times.10.sup.-4.4.times.10.sup.-6 mol/min,
6.0.times.10.sup.-5-2.2.ti- mes.10.sup.-3 mol/min (0.4-46.4 SCCM)
and 5.0.times.10.sup.-4 mol/min, respectively. The total flowrate
including the carrier gas may be set to 6 l/min, and the AsH.sub.3
partial pressure in the reaction chamber 11 is set to 0.9-102 Pa.
The total pressure in the reaction chamber is set to about
1.3.times.10.sup.4 Pa. It should be noted that DMHy is used
commonly as a CVD source of N when growing a SiN film. Further, it
is also possible to use materials such as monomethylhydradine
(MMHy) or TBA (tertiarybutylamine) for the source of N.
[0164] In a preferable example, the flow rate of TMG is set to
about 2.0.times.10.sup.-5 mol/min, the flow rate of TMI is set to
about 2.2.times.10.sup.-6 mol/min, the flow rate of AsH.sub.3 is
set to about 3.3.times.10.sup.-4 mol/min or 7 sccm, and the
flowrate of DMHy is set to about 6.4.times.10.sup.-3 mol/min, and
the deposition is carried out at a substrate temperature of about
630.degree. C. under a pressure of about 15.4 Pa, which is almost
identical to the partial pressure of AsH.sub.3 in the reaction
chamber 11. Under the foregoing deposition condition, the active
layer 4 having a composition of Ga.sub.0.9In.sub.0.1N.sub.0.03As.s-
ub.0.97 is grown with a growth rate of about 1.7 .mu.m/H.
[0165] It should be noted that the foregoing composition of
Ga.sub.0.9In.sub.0.1N.sub.0.03As.sub.0.97 of the active layer 4
successfully achieves a lattice matching to the GaAs substrate 2.
Thus, it is possible in the present embodiment to form the active
layer 4 with an increased thickness of about 100 nm, without
inducing dislocations as a result of lattice misfit between the
active layer 4 and the substrate 2.
[0166] Once the active layer 4 is thus formed, an upper cladding
layer 5 of InGaAsP is grown thereon similarly to the case of
forming the lower cladding layer 3, except that a p-type dopant gas
such as dimethylzinc ((CH).sub.32Zn) is added in place of the
n-type dopant gas noted previously. Further, a cap layer 6 of
p.sup.+-type GaAs is grown on the upper cladding layer 5, and the
cap layer 6 and the underlying cladding layer 5 are subjected to an
etching process to form a ridge structure SR extending in the axial
direction of the laser diode. In order to stop the etching of the
upper cladding layer 5 before the active layer 4 is exposed, it may
be preferable to interpose a thin etching stopper layer of p-type
GaInP (not shown) inside the cladding layer 5.
[0167] After the ridge structure SR is thus formed, a p-type ohmic
electrode 7 having an AuZn/Zn stacked structure is provided on the
cap layer 6, and an n-type ohmic electrode 8 having an AuGe/Ni/Au
stacked structure is provided on the lower major surface of the
substrate 2.
[0168] In the longitudinal cross sectional view of the laser diode
1 of FIG. 4A, it should be noted that an optical cavity is formed
between a front cleaved surface M1 acting as a first mirror and a
rear cleaved surface M2 acting as a second, opposing mirror as
usual in an edge-emission type laser diode. Further, the laser
diode 1 has a so-called double-hetero structure in which the active
layer 4, having a substantial thickness, acts also as an optical
waveguide layer.
[0169] FIG. 5 shows a room-temperature oscillation spectrum of the
laser diode 1 of FIGS. 4A and 4B for a case in which the stripe
structure 5R has a width of about 2 .mu.m. Referring to FIG. 5, it
will be noted that the laser diode oscillates in the optical
wavelength of 1293 nm, which is well inside the desired 1.3 .mu.m
wavelength band (1.25-1.35 .mu.m). This result is in good agreement
with the result of photoluminescent (PL) spectroscopy conducted on
the GaInNAs mixed crystal by illuminating the GaInNAs mixed crystal
by a laser beam of an Ar laser having a wavelength of 448 nm.
[0170] The result of FIG. 5 indicates that the problem of
non-optical recombination of carriers explained previously is
successfully avoided as a result of use of the Al-free composition
for the cladding layers 3 and 5. As the cladding layers 3 and 5 are
substantially free from Al, no problem of oxidation occurs even
when the deposition of the cladding layer 3 or 5 is conducted at a
relatively low temperature as noted above or when the substrate
temperature is lowered in preparation for the deposition of the
GaInNAs active layer 4 on the pre-existing lower cladding layer 3.
Thereby, the laser diode 10 thus obtained is characterized by a
high efficiency of laser oscillation and has a long lifetime.
[0171] It should be noted that the composition of the GaInNAs
active layer 4 that establishes a lattice matching to the GaAs
substrate 2 is by no means limited to the foregoing specific
composition but other compositions are also possible. For example,
a composition of Ga.sub.0.94In.sub.0.06N.sub.0.2As.sub.0.98provides
also a successful lattice matching to the GaAs substrate and a
bandgap smaller than the bandgap of GaAs. Generally, the lattice
constant of the GaInNAs mixed crystal increases with increasing In
content and decreases with increasing N content. Further, the
bandgap decreases with increasing N content and/or increasing In
content. See the relationship of FIG. 1.
[0172] Thus, the foregoing second composition,
Ga.sub.0.94In.sub.0.06N.sub- .0.02As.sub.0.98, provides a photon
emission at the wavelength of 1176 nm as indicated in FIG. 6,
wherein FIG. 6 shows an optical output spectrum of the laser diode
of FIGS. 4A and 4B in which the foregoing second composition is
used for the GaInNAs active layer 4. It should be noted that the
result of FIG. 6 is for the case in which the laser diode is
operated as an LED by supplying a drive current of 50 mA.
[0173] It should be noted that the foregoing second composition,
providing an increased bandgap, can be used also for the cladding
layers 3 and 5. In this case, the composition of the GaInNPAs
cladding layer, generally represented as
GadIn.sub.1-dN.sub.eAs.sub.fP.sub.1-e-f (0.ltoreq.d.ltoreq.1,
0.ltoreq.e.ltoreq.1, 0.ltoreq.f.ltoreq.1), is adjusted such that
the lattice matching is achieved to the GaAs substrate 2 and such
that the bandgap is maximized. As long as Al is not contained in
the cladding layer 3 or 5, no substantial problem occurs when
growing the active layer 4 on the substrate 2.
[0174] Further, it should be noted that the active layer 4 of
GaInNAs may further contain P in addition to Ga, In, N and As. In
this case, the composition of the active layer 4 is generally
represented as Ga.sub.aIn.sub.1-aN.sub.bAs.sub.cP.sub.1-b-c
(0.ltoreq.a.ltoreq.1, 0<b<1, 0.ltoreq.c<1), wherein it
should be noted that the bandgap of the active layer 4 tends to
increase with increasing P content in the mixed crystal.
Second Embodiment
[0175] FIG. 7 shows the construction of a laser diode 20 according
to a second embodiment of the present invention in a longitudinal
cross sectional view.
[0176] Referring to FIG. 7, the laser diode 20 has a so-called
SCH-SQW (separate confinement hetero-structure single quantum well)
structure and constructed on a substrate 21 of n-type GaAs on which
a lower cladding layer 22 of n-type GaInPAs is provided epitaxially
with a thickness of typically about 1500 nm. The lower cladding
layer establishes a lattice matching with the GaAs substrate 21 and
may have a composition of
Ga.sub.0.57In.sub.0.43P.sub.0.85As.sub.0.15 and a bandgap energy of
1.85 eV.
[0177] On the lower cladding layer 22, a lower optical waveguide
layer 23 of undoped GaInPAs having a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.- 25As.sub.0.75 and a bandgap energy
of 1.53 eV is provided epitaxially with a thickness of 100 nm in
lattice matching to the substrate 21, and an active layer 24 of
undoped GaInNAs having a composition of
Ga.sub.0.87In.sub.0.13N.sub.0.04As.sub.0.96 is provided further on
the optical waveguide layer 23 with a thickness of about 20 nm or
less, also in lattice matching to the GaAs substrate 21. Further,
an upper optical waveguide layer 25 of GaInAsP is provided on the
active layer 24 similarly to the lower optical waveguide layer 23
with a composition of Ga.sub.0.39In.sub.0.13P.sub.0.25As.sub.0.75
and a thickness of 100 nm in lattice matching to the GaAs substrate
21, and an upper cladding layer 26 of p-type GaInAsP is provided on
the upper optical waveguide layer 25 with a thickness of typically
about 1.5 .mu.m in lattice matching to the GaAs substrate 21.
Thereby, it should be noted that the active layer 24 forms a
quantum well.
[0178] Further, a cap layer 27 of p.sup.+-type GaAs is provided on
the upper cladding layer 26, and an upper ohmic electrode 28 having
the AuZn/Zn stacked structure is provided on the cap layer 27.
Further, a lower ohmic electrode 29 having the AuGe/Ni/Au stacked
structure is provided on the lower major surface of the substrate
21.
[0179] Similarly to the laser diode 1 of FIG. 4A, the laser diode
20 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 22-27 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0180] In the present embodiment, the active layer 24 emits, when
in a bulk crystal state, an optical radiation having a wavelength
longer than the desired 1.3 .mu.m band because of the increased In
content in the active layer 24. However, the oscillation wavelength
of the laser diode is successfully controlled to the desired
wavelength of about 1.3 .mu.m by reducing the thickness of the
active layer 24 to about 20 nm or less, such that quantum levels
are formed in the active layer 24 with appropriate energy
separation between the quantum levels. Of course, the composition
of the active layer 24 is not limited to the foregoing composition
but other compositions, including those that do not achieve lattice
matching, may be used. As long as the thickness of the active layer
24 is within a critical thickness, the development of dislocations
in the active layer 24 is successfully avoided. Further, the
optical waveguide layers 23 and 25 may have a composition that does
not provide a lattice matching to the GaAs substrate 21, as long as
the thickness is within the critical thickness of such a strained
system formed of the GaAs substrate 21 and the layer 23 or 25.
[0181] In the present embodiment, too, the cladding layers 22 and
26 are substantially free from Al. Further, the upper and lower
optical waveguide layers 23 and 25 are also free from Al. Thereby,
the growth of the active layer 24 of GaInNAs can be conducted
continuously to the process of growing the cladding layer 22 or the
optical waveguide layer 23, which process is conducted at a low
temperature suitable for the growth of the GaInNAs active layer 24.
The laser diode 20 thus obtained is characterized by a high
efficiency of laser oscillation and has a long lifetime.
[0182] Alternatively, the layers 22 and 23 may be formed at a high
temperature. In this case, the substrate temperature is lowered
before the deposition of the active layer 24 is commenced. Even in
such a case, the problem of oxidation does not occur, as any of the
cladding layer 22 and the guide layer 23 is substantially free from
Al.
[0183] Further, it should be noted that, in view of the Al-free
composition of the lower and upper cladding layers 22 and 26, it is
possible to form the optical waveguide layers 23 and 25 by a mixed
crystal of GaInNPAs. In this case, the composition
Ga.sub.dIn.sub.1-dN.sub.eAs.sub.fP.sub.1-e-f (0.ltoreq.d.ltoreq.1,
0.ltoreq.e<1, 0.ltoreq.f.ltoreq.1) of the mixed crystal is
appropriately adjusted such that the bandgap of the optical
waveguide layers 23 and 25 is smaller than the bandgap of the
cladding layers 22 and 26. For example, the composition of
Ga.sub.0.97In.sub.0.03N.sub.0.01A- s.sub.0.99 may be used for the
optical waveguide layers 23 and 25 in combination with the
composition of Ga.sub.0.5In.sub.0.5P for the cladding layers 22 and
26.
[0184] In the present embodiment, it should be noted that the
active layer 24 may be provided with plural numbers, with
intervening barrier layers of GaInNPAs or GaInPAs. In this case,
the composition of the barrier layers is set such that the bandgap
is larger than the bandgap of the active layers 24 but smaller than
the cladding layers 22 and 26.
Third Embodiment
[0185] FIG. 8 shows the construction of a laser diode 30 according
to a third embodiment of the present invention in a longitudinal
cross sectional view.
[0186] Referring to FIG. 8, the laser diode 30 is a SCH-SQW type
laser diode similar to the laser diode 20 of FIG. 7 and is
constructed on a substrate 31 of n-type GaAs on which a lower
cladding layer 32 of n-type AlGaAs is provided epitaxially with a
thickness of typically about 1500 nm. The lower cladding layer 32
establishes a satisfactory lattice matching to the GaAs substrate
31 and may have a composition of Al.sub.0.8Ga.sub.0.2As.
[0187] On the lower cladding layer 32, a lower optical waveguide
layer 33 of undoped GaInPAs having a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.- 25As.sub.0.75 and a bandgap energy
of 1.53 eV is provided epitaxially with a thickness of 100 nm in
lattice matching to the substrate 31, and an active layer 34 of
undoped GaInNAs having a composition of
Ga.sub.0.87In.sub.0.13N.sub.0.04As.sub.0.96 is provided further on
the optical waveguide layer 33 with a thickness of about 20 nm or
less, also in lattice matching to the GaAs substrate 31. Further,
an upper optical waveguide layer 35 of GaInAsP is provided on the
active layer 34 similarly to the lower optical waveguide layer 33
with a composition of Ga.sub.0.87In.sub.0.13P.sub.0.25As.sub.0.75
and a thickness of 100 nm in lattice matching to the GaAs substrate
31, and an upper cladding layer 36 of p-type AlGaAs is provided on
the upper optical waveguide layer 35 with a thickness of typically
about 1500 nm in lattice matching to the GaAs substrate 31.
Thereby, it should be noted that the active layer 34 forms a
quantum well. The optical waveguide layers 33 and 35 may have a
bandgap energy of 1.53 eV as noted already.
[0188] Further, a cap layer 37 of p.sup.+-type GaAs is provided on
the upper cladding layer 36, and an upper ohmic electrode 38 having
the AuZn/Zn stacked structure is provided on the cap layer 37.
Further, a lower ohmic electrode 39 having the AuGe/Ni/Au stacked
structure is provided on the lower major surface of the substrate
31.
[0189] Similarly to the laser diode 1 of FIG. 4A, the laser diode
30 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 32-37 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0190] In the present embodiment, too, it should be noted that the
layers 33 and 35 that contact directly to the active layer 34 is
free from Al. Thus, the problem of formation of non-optical
recombination centers on the surface of the active layer 34 as a
result of oxidation of Al is effectively avoided. Thereby, an
efficient laser diode having a long lifetime is obtained.
[0191] Further, it should be noted that the cladding layers 32 and
36, now formed of AlGaAs, has a low thermal resistance, which is
about one-half the thermal resistance of InGaP. In view of the
large thickness of the upper and lower cladding layers 32 and 36,
the use of AlGaAs for the cladding layers 32 and 36 is advantageous
for suppressing the temperature rise in the active layer 34 during
the operation of the laser diode. Further, the use of AlGaAs,
having a low refractive index and a very large bandgap, for the
cladding layers 32 and 36, is advantageous for enhancing the
optical confinement of the optical radiation produced by the active
layer 34 and the confinement of carriers injected to the active
layer 34.
[0192] Similarly to the laser diode 20 of FIG. 7, the active layer
34 may have a composition different from the composition noted
above, including the composition that does not achieve a lattice
matching to the GaAs substrate 31, provided that the thickness of
the active layer 34 does not exceed a critical thickness of the
strained system formed of the active layer 34 and the substrate 31.
Similarly, the active layer 34 may be a mixed crystal of GaInNPAs
that contains P, wherein the optical waveguide layers 33 and 35 of
GaInPAs may have a composition In.sub.xGa.sub.1-xAs.sub.yP.sub.1-y
(0.ltoreq.x<1, 0.ltoreq.y.ltoreq.1) that does not achieve a
lattice matching to the GaAs substrate 31, provided that the
thickness of the layers 33 and 35 is smaller than a critical
thickness of such a strained system formed by the substrate 31 and
the layer 33 or 35. The cladding layers 32 and 36 of AlGaAs may
have a composition different from the foregoing composition of
Al.sub.0.8Ga.sub.0.2As as long as the conduction band energy is
larger than the conduction band energy of the guide layers 33 and
35.
Fourth Embodiment
[0193] FIG. 9 shows the construction of a laser diode 40 according
to a fourth embodiment of the present invention in a longitudinal
cross sectional view.
[0194] Referring to FIG. 9, the laser diode 40 is a SCH-SQW type
laser diode similar to the laser diode 20 of FIG. 7 and is
constructed on a substrate 41 of n-type GaAs on which a lower
cladding layer 42 of n-type AlGaInP is provided epitaxially with a
thickness of typically about 1500 nm. The lower cladding layer 42
establishes a satisfactory lattice matching with the GaAs substrate
41 and may have a composition of
Al.sub.0.35Ga.sub.0.15In.sub.0.5P.
[0195] On the lower cladding layer 42, a lower optical waveguide
layer 43 of undoped GaInPAs having a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.- 25As.sub.0.75 is provided
epitaxially with a thickness of 7 nm in lattice matching to the
substrate 41, and an active layer 44 of undoped GaInNAs having a
composition of Ga.sub.0.87In.sub.0.13 NO.sub.0.04As.sub.0.96 is
provided further on the optical waveguide layer 43 with a thickness
of about 20 nm or less, also in lattice matching to the GaAs
substrate 41. Further, an upper optical waveguide layer 45 of
GaInAsP is provided on the active layer 44 similarly to the lower
optical waveguide layer 43 with a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.25As.sub.0.75 and a thickness of 100
nm in lattice matching to the GaAs substrate 41, and an upper
cladding layer 46 of p-type AlGaInP is provided on the upper
optical waveguide layer 45 with a thickness of typically about 1500
nm in lattice matching to the GaAs substrate 41. Thereby, it should
be noted that the active layer 44 forms a quantum well.
[0196] Further, a cap layer 47 of p.sup.+-type GaAs is provided on
the upper cladding layer 46, and an upper ohmic electrode 48 having
the AuZn/Zn stacked structure is provided on the cap layer 47.
Further, a lower ohmic electrode 49 having the AuGe/Ni/Au stacked
structure is provided on the lower major surface of the substrate
41.
[0197] Similarly to the laser diode 1 of FIG. 4A, the laser diode
40 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 42-47 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0198] In the present embodiment, too, it should be noted that the
layers 43 and 45 that contact directly to the active layer 44 is
free from Al. Thus, the problem of formation of non-optical
recombination centers on the surface of the active layer 44 as a
result of oxidation of Al is effectively avoided. Thereby, an
efficient laser diode having a long lifetime is obtained.
[0199] Further, the use of AlGaInP, having a very large bandgap
even larger than the bandgap of AlGaAs, for the cladding layers 42
and 46, is advantageous for maximizing the optical confinement of
the optical radiation produced by the active layer 44 and for
maximizing simultaneously the confinement of carriers in the active
layer 44. In addition, it should be noted that, by using AlGaInP
for the cladding layers 42 and 46, it becomes possible to achieve a
perfect lattice matching to the GaAs substrate 41, contrary to the
case of the laser diode 30 of FIG. 8 that uses AlGaAs for the
cladding layers 32 and 36. When AlGaAs is used, the lattice misfit
strain is not completely eliminated.
Fifth Embodiment
[0200] FIG. 10 shows the construction of a laser diode 50 according
to a fifth embodiment of the present invention in a longitudinal
cross sectional view.
[0201] Referring to FIG. 10, the laser diode 50 is a SCH-SQW type
laser diode similar to the laser diode 30 of FIG. 8 and is
constructed on a substrate 51 of n-type GaAs on which a lower
cladding layer 52 of n-type AlGaAs is provided epitaxially with a
thickness of typically about 1500 nm. The lower cladding layer
establishes a satisfactory lattice matching with the GaAs substrate
51 and may have a composition of Al.sub.0.8Ga.sub.0.2As.
[0202] On the lower cladding layer 52, a lower optical waveguide
layer 53 of undoped GaInPAs having a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.- 25As.sub.0.75 is provided
epitaxially with a thickness of 100 nm in lattice matching to the
substrate 51, and an active layer 54 of undoped GaInNAs having a
composition of Ga.sub.0.8In.sub.0.2N.sub.0.2As.sub.0.98 is provided
further on the optical waveguide layer 53 with a thickness of about
20 nm or less, also in lattice matching to the GaAs substrate 51.
Further, an upper optical waveguide layer 55 of GaInAsP is provided
on the active layer 54 similarly to the lower optical waveguide
layer 53 with a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.25As.sub.0.98 and a thickness of 100
nm in lattice matching to the GaAs substrate 51, and an upper
cladding layer 56 of p-type AlGaAs is provided on the upper optical
waveguide layer 55 with a thickness of typically about 1500 nm in
lattice matching to the GaAs substrate 51. Thereby, it should be
noted that the active layer 54 forms a quantum well similar to the
quantum well 34. The optical waveguide layers 53 and 55 may have a
bandgap of 1.53 eV.
[0203] Further, a cap layer 57 of p.sup.+-type GaAs is provided on
the upper cladding layer 56, and an upper ohmic electrode 38 having
the AuZn/Zn stacked structure is provided on the cap layer 57.
Further, a lower ohmic electrode 59 having the AuGe/Ni/Au stacked
structure is provided on the lower major surface of the substrate
51.
[0204] Similarly to the laser diode 1 of FIG. 4A, the laser diode
50 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 52-57 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0205] In the foregoing structure, it should be noted that the
cladding layer 52 or 56 containing therein Al does not contact to
the active layer 54 that contains N therein. Thereby, the problem
of oxidation of Al in the cladding layer 52 or 56 at the time of
the formation of the active layer 54 is successfully avoided.
[0206] In the present embodiment, the active layer 54 has a
composition of Ga.sub.0.8In.sub.0.2N.sub.0.02As.sub.0.98, in which
it will be noted that the In content of the active layer 54 exceeds
the foregoing lattice matched composition of
Ga.sub.0.87In.sub.0.13N.sub.0.04As.sub.0.96 of the active layer 34
of the laser diode 30 substantially, while the N content is reduced
substantially as compared with the foregoing, lattice matched
composition. As a result of the increased In content, the GaInNAs
mixed crystal forming the active layer 54 has a lattice constant
substantially larger than the lattice constant of GaAs. Thereby,
the active layer 54 accumulates therein a compressive lattice
misfit strain, while such a compressive lattice misfit strain acts
to decrease the bandgap of the GaInNAs mixed crystal forming the
active layer 54, and the oscillation wavelength of the laser diode
50 is successfully tuned to the 1.3 .mu.m wavelength band by
decreasing the N content correspondingly.
[0207] Thus, by using the GaInNAs mixed crystal thus accumulating a
compressive lattice misfit strain for the active layer 54, it is
possible to reduce the amount of N to be incorporated into the
active layer 54 for use in the laser diode operable in the 1.3
.mu.m wavelength band. It should be noted that the quality of the
GaInNAs mixed crystal deteriorates rapidly with increasing N
content, while the active layer 54, containing therein only a small
amount of N, provides a sharp oscillation spectrum, reflecting the
excellent quality of the crystal forming the active layer 54.
Further, the use of a strained quantum well for the active layer 54
reduces the threshold current of laser oscillation.
Sixth Embodiment
[0208] FIG. 11 shows the construction of an SCH-MQW (separate
confinement hetero-structure multiple quantum well) laser diode 60
according to a sixth embodiment of the present invention in a
longitudinal cross sectional view.
[0209] Referring to FIG. 11, the laser diode 60 is a is constructed
on a substrate 61 of n-type GaAs on which a lower cladding layer 62
of n-type AlGaInP is provided epitaxially with a thickness of
typically about 1500 nm. The lower cladding layer 62 establishes a
lattice matching with the GaAs substrate 61 and may have a
composition of Al.sub.0.35Ga.sub.0.15In.- sub.0.5P.
[0210] On the lower cladding layer 62, a lower optical waveguide
layer 63 of undoped GaInPAs having a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.- 25As.sub.0.75 is provided
epitaxially with a thickness of 100 nm in lattice matching to the
substrate 61, and an active layer 64 having a multiple quantum well
(MQW) structure in which a quantum well layer of GaInNAs having a
composition Ga.sub.0.8In.sub.0.2N.sub.0.02As.sub.0.98 and a barrier
layer of GaInAsP having a composition GaAs.sub.0.92P.sub.0.08 are
stacked alternately and repeatedly, is provided further on the
optical waveguide layer 63 with a thickness of about 12 nm for the
quantum well layer and a thickness of about 10 nm for the barrier
layer. Thereby, the composition of the quantum well layer is set so
as to accumulate a compressive lattice misfit strain therein, while
the composition of the barrier layer is set so as to accumulate a
tensile strain therein. The quantum well layer and the barrier
layer may be repeated for 3 times. Thereby, the oscillation
wavelength of the laser diode 60 is successfully tuned to the
desired 1.3 .mu.m band while reducing the N content in the quantum
well layer similarly to the laser diode 50.
[0211] Further, an upper optical waveguide layer 65 of GaInAsP is
provided on the active layer 64 similarly to the lower optical
waveguide layer 63 with a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.25As.sub.0.75 and a thickness of 100
nm in lattice matching to the GaAs substrate 61, and an upper
cladding layer 66 of p-type AlGaInP is provided on the upper
optical waveguide layer 65 with a thickness of typically about 1500
nm in lattice matching to the GaAs substrate 61.
[0212] Further, a cap layer 67 of p.sup.+-type GaAs is provided on
the upper cladding layer 66, and an upper ohmic electrode 68 having
the AuZn/Zn stacked structure is provided on the cap layer 67.
Further, a lower ohmic electrode 69 having the AuGe/Ni/Au stacked
structure is provided on the lower major surface of the substrate
61.
[0213] Similarly to the laser diode 1 of FIG. 4A, the laser diode
60 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 62-67 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0214] In the foregoing structure, it should be noted that the
cladding layer 62 or 66 containing therein Al does not contact to
the active layer 64 that contains N therein. Thereby, the problem
of oxidation of Al in the cladding layer at the time of the
formation of the active layer 64 is successfully avoided.
[0215] In the present embodiment, it is possible to change the
composition and hence the lattice constant of the quantum well
layer and the barrier layer forming the MQW active layer as desired
while maintaining the same bandgap energy. This is not possible in
the system of AlGaAs, in which the lattice constant changes little
even when the composition, and hence the bandgap, is changed
variously. By compensating the compressive lattice misfit strain of
the quantum well layers by the tensile lattice misfit strain of the
barrier layers, it becomes possible to form the MQW structure by
stacking the quantum well layer and the barrier layer a plurality
of times.
[0216] Further, it should be noted that the compositions of the
GaInNAs quantum well layer and the GaInPAs barrier layer are not
limited to the foregoing specific compositions but other
compositions are also possible. In relation to this, it should be
noted that the compositions of the GaInNAs quantum well layer and
the GaInAsP barrier layer may be set such that the quantum well
layer accumulates a tensile lattice misfit strain and the barrier
layer accumulates a compressive lattice misfit strain.
Seventh Embodiment
[0217] FIG. 12 shows the construction of a laser diode 70 according
to a seventh embodiment of the present invention in a longitudinal
cross sectional view.
[0218] Referring to FIG. 12, the laser diode 70 is a is constructed
on a substrate 71 of n-type GaAs on which a lower cladding layer 72
of n-type AlGaInP is provided epitaxially with a thickness of
typically about 1500 nm. The lower cladding layer 72 establishes a
lattice matching with the GaAs substrate 71 and may have a
composition of Al.sub.0.35Ga.sub.0.15In.- sub.0.5P.
[0219] On the lower cladding layer 72, a lower optical waveguide
layer 73 of undoped GaInPAs having a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.- 25As.sub.0.75 is provided
epitaxially with a thickness of 100 nm in lattice matching to the
substrate 71, and an active layer 74 having a multiple quantum well
(MQW) structure in which a quantum well having a composition
Ga.sub.0.8In.sub.0.2N.sub.0.02As.sub.0.98 and a barrier layer
having a composition
Ga.sub.0.8In.sub.0.2N.sub.0.02P.sub.0.23As.sub.0.75 are stacked
alternately and repeatedly, is provided further on the optical
waveguide layer 73 with a thickness of about 8 nm for the quantum
well layer and a thickness of about 10 nm for the barrier layer.
Thereby, the composition of the quantum well layer is set so as to
accumulate a compressive lattice misfit strain therein, while the
composition of the barrier layer is set so as to accumulate a
tensile lattice misfit strain therein. The quantum well layer and
the barrier layer may be repeated for 8 times. As a result of the
compressive lattice misfit strain of the GaInNAs quantum well
layers, the bandgap of the quantum well layers is reduced as
compared with the nominal bandgap of the GaInNAs mixed crystal free
from a lattice misfit strain. Thereby, the oscillation wavelength
of the laser diode is successfully tuned to the desired 1.3 .mu.m
band.
[0220] Further, an upper optical waveguide layer 75 of GaInAsP is
provided on the active-layer 74 similarly to the lower optical
waveguide layer 73 with a composition of
Ga.sub.0.87In.sub.0.13P.sub.0.25As.sub.0.75 and a thickness of 100
nm in lattice matching to the GaAs substrate 71, and an upper
cladding layer 76 of p-type AlGaInP is provided on the upper
optical waveguide layer 75 with a thickness of typically about 1500
nm in lattice matching to the GaAs substrate 71.
[0221] Further, a cap layer 77 of p.sup.+-type GaAs is provided on
the upper cladding layer 76, and an upper ohmic electrode 78 having
the AuZn/Zn stacked structure is provided on the cap layer 77.
Further, a lower ohmic electrode 79 having the AuGe/Ni/Au stacked
structure is provided on the lower major surface of the substrate
71.
[0222] Similarly to the laser diode 1 of FIG. 4A, the laser diode
70 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 72-77 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0223] In the foregoing structure, too, it should be noted that the
cladding layer 72 or 76 containing therein Al does not contact the
active layer 74 that contains N therein. Thereby, the problem of
oxidation of Al in the cladding layer at the time of the formation
of the active layer 74 is successfully avoided.
[0224] In the present embodiment, it is possible to change the
composition and hence the lattice constant of the quantum well
layer and the barrier layer forming the MQW active layer 74 as
desired while maintaining the same bandgap energy. This is not
possible in the system of AlGaAs, in which the lattice constant
changes little when the composition, and hence the bandgap, is
changed variously. By compensating for the compressive lattice
misfit strain of the quantum well layers by the tensile lattice
misfit strain of the barrier layers, it becomes possible to form
the MQW structure by stacking the quantum well layer and the
barrier layer a plurality of times.
[0225] Further, it should be noted that the compositions of the
GaInNAs quantum well layer and the GaInNPAs barrier layer are not
limited to the foregoing specific compositions but other
compositions are also possible. In relation to this, it should be
noted that the compositions of the GaInNAs quantum well layer and
the GaInNAsP barrier layer may be set such that the quantum well
layer accumulates a tensile lattice misfit strain and the barrier
layer accumulates a compressive lattice misfit strain.
[0226] In the present embodiment, it should be noted that the
growth of the GaInNAsP barrier layer is carried out in the
deposition apparatus of FIG. 3 similarly to the growth of GaInNAs
quantum well layer, by merely adding a source of P in the form of
PH.sub.3, without changing the supply rate of the gaseous sources
of In, Ga N and As.
[0227] Further, it should be noted that the quantum well layer in
the active layer 74 may be formed of GaInNPAs, provided that the P
content of the barrier layer is increased correspondingly so that
an effective potential well is formed in correspondence to the
quantum well layer.
Eighth Embodiment
[0228] FIG. 13 shows the construction of a high-power double-hetero
laser diode 80 according to an eighth embodiment of the present
invention in a longitudinal cross sectional view. The laser diode
80 of the present embodiment is suitable for energizing a solid
laser operable in a 0.8 .mu.m wavelength band.
[0229] Referring to FIG. 13, the laser diode 80 is a is constructed
on a substrate 81 of n-type GaAs on which a lower cladding layer 82
of n-type GaInP is provided epitaxially with a thickness of
typically about 1500 nm. The lower cladding layer 82 establishes a
lattice matching to the GaAs substrate 81 and may have a
composition of Ga.sub.0.51In.sub.0.49P and a bandgap of about 1.91
eV.
[0230] On the lower cladding layer 82, a lower optical waveguide
layer 83 of undoped GaInPAs having a composition of
Ga.sub.0.57In.sub.0.43P.sub.0.- 85As.sub.0.15 is provided
epitaxially with a thickness of 100 nm in lattice matching to the
substrate 81, and an active layer 84 having a composition of
Ga.sub.0.4In.sub.0.6N.sub.0.01P.sub.0.94As.sub.0.05 and a bandgap
energy of 1.54 eV is provided further on the optical waveguide
layer 83 with a thickness of about 20 nm.
[0231] Further, an upper optical waveguide layer 85 of GaInAsP is
provided on the active layer 84 similarly to the lower optical
waveguide layer 83 with the same composition of
Ga.sub.0.57In.sub.0.43P.sub.0.85As.sub.0.15 with a thickness of 100
nm, in lattice matching to the GaAs substrate 81, and an upper
cladding layer 86 of p-type GaInP is provided on the upper optical
waveguide layer 85 with a thickness of typically about 1500 nm in
lattice matching to the GaAs substrate 81, with the same
composition, and hence the bandgap, to the lower cladding layer 82.
The optical waveguide layers 83 and 85 may have a bandgap of 1.83
eV.
[0232] Further, a cap layer 87 of p.sup.+-type-GaAs is provided on
the upper cladding layer 86, and an upper ohmic electrode 88 having
the AuZn/Zn stacked structure is provided on the cap layer 87.
Further, a lower ohmic electrode 89 having the AuGe/Ni/Au stacked
structure is provided on the lower major surface of the substrate
81.
[0233] Similarly to the laser diode 1 of FIG. 4A, the laser diode
80 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 82-87 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0234] In the foregoing structure, too, it should be noted that the
cladding layer 82 or 86 is free from Al. Thereby, the problem of
oxidation of Al in the cladding layer at the time of the formation
of the active layer 84 is successfully avoided.
[0235] The laser diode 80 of the present embodiment has a SCH-SQW
structure and oscillates at the wavelength of about 0.81 .mu.m. As
the laser diode 80 does not use AlGaAs, which is susceptible to
optical damages, the laser diode 80 operates with reliability in
the high-power applications such as energizing a solid laser. It
should be noted that the laser diode 80, which uses GaInNAsP for
the active layer 84 in combination with the cladding layers 82 and
86 of GaInP, realizes a very large band discontinuity at the
conduction band as well as at the valence band, and the efficiency
of laser oscillation or high-temperature stability of the laser
oscillation is significantly improved.
[0236] It should be noted that the composition of the active layer
84 is not necessarily be chosen so as to establish a lattice
matching to the GaAs substrate 81. The active layer 84 may have a
composition that does not provide the lattice matching to the GaAs
substrate 81, as long as the thickness of the active layer 84 is
within a critical thickness, above which dislocations would develop
in the active layer 84. Similarly, the guide layers 83 and 85 may
have a composition offset from the lattice matching composition,
provided that the guide layers 83 and 85 have a thickness smaller
than the critical thickness of the guide layers 83 and 85.
[0237] As the cladding layers 82 and 86 are free from Al in the
laser diode 80 of the present embodiment, the guide layers 83 and
85 may also contain N. Further, the active layer 84 may be provided
in plural numbers to form a MQW structure.
Ninth Embodiment
[0238] FIG. 14 shows the construction of a double-hetero laser
diode 90 operable in the 1.4 .mu.m wavelength band according to a
ninth embodiment of the present invention in a longitudinal cross
sectional view.
[0239] Referring to FIG. 14, the laser diode 90 is constructed on a
substrate 91 of n-type InP on which a lower cladding layer 92 of
n-type InP is provided with a thickness of typically about 1500
nm.
[0240] On the lower cladding layer 92, an active layer 93 of
undoped GaInNAsP is provided with a thickness of typically about
100 nm, wherein the active layer 93 has a bandgap wavelength of 1.3
.mu.m and establishes a lattice matching to the InP substrate 93,
wherein the foregoing composition is set also such that the active
layer 93 has a bandgap energy of 0.88 eV corresponding to the
optical wavelength of about 1.4 .mu.m. As the active layer 93 has a
lattice matching composition as noted above, the active layer 93
can be formed with a desired thickness.
[0241] Further, the active layer 93 is covered by an upper cladding
layer 94 of p-type InP, and a cap layer 95 of p.sup.+-type InGaAs
is provided further thereon with a thickness of 300 nm. The InGaAs
cap layer 95 has a lattice matching composition of
Ga.sub.0.47In.sub.0.53As.
[0242] The cap layer 95 is covered by a p-type ohmic electrode 96
having the AuZn/Zn stacked structure. Further, the lower major
surface of the InP substrate 91 is covered by an n-type ohmic
electrode 97 having the AuGe/Ni/Au stacked structure.
[0243] Similarly to the laser diode 1 of FIG. 4A, the laser diode
90 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 92-95 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0244] As the active layer 97 has the bandgap corresponding to the
optical wavelength of about 1.4 .mu.m as noted before, the laser
diode 90 successfully oscillates in the 1.5 .mu.m optical
wavelength band. Thereby, a large band discontinuity is secured in
the conduction band at the heterojunction interface between the
active layer 93 and the cladding layer 92 or 94, and the threshold
current of laser oscillation is reduced substantially. In other
words, the laser diode 90 shows an excellent efficiency of laser
oscillation. Further, the overflowing of the carriers from the
active layer 93 is successfully suppressed by the foregoing
increased band discontinuity at the heterojunction interface, and
the laser diode oscillates efficiently at high temperatures,
without external cooling. In other words, the laser diode 90 shows
an excellent temperature characteristic.
[0245] In the present embodiment, it is also possible to use a
composition that is offset from a lattice matching composition,
provided that the active layer 93 is formed with a thickness not
exceeding a critical thickness above which dislocations develop in
the active layer 93. Further, the upper and lower cladding layers
92 and 94 may be formed of GaInPAs with a general composition of
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y (0.ltoreq.x<1,
0.ltoreq.y.ltoreq.1), as long as a lattice matching is achieved
with respect to the InP substrate 91 and as long as the cladding
layers 92 and 94 have a bandgap energy larger than that of the
active layer 93.
[0246] Further, the laser diode 90 can oscillate at a wavelength
exceeding 1.7 .mu.m when the composition
Ga.sub.aIn.sub.1-aN.sub.bAs.sub.cP.sub.1-b- -c of the active layer
93 is set such that the compositional parameter a is smaller than
0.47 and that the compositional parameters b and c satisfy the
relationship of b+c=1, in other words, the active layer 93 is free
from P. This oscillation wavelength has hitherto been possible only
when a GaSb substrate is used, while the laser diode 90 of the
present embodiment realizes the laser oscillation of this
wavelength while using a low cost InP substrate.
Tenth Embodiment
[0247] FIG. 15 shows the construction of an SCH-MQW laser diode 100
according to a tenth embodiment of the present invention in a
longitudinal cross sectional view.
[0248] Referring to FIG. 15, the laser diode 100 is a is
constructed on a substrate 101 of n-type InP on which a lower
cladding layer 102 of n-type InP is provided epitaxially with a
thickness of typically about 1500 nm.
[0249] On the lower cladding layer 102, a lower optical waveguide
layer 103 of undoped GaInPAs having a bandgap wavelength of 1.3
.mu.m is provided epitaxially with a thickness of 100 nm in lattice
matching to the InP substrate 101, and an active layer 104 having a
multiple quantum well (MQW) structure in which a quantum well layer
of GaInNAsP having a bandgap wavelength of 1.55 .mu.m and a barrier
layer of GaInAsP having a bandgap wavelength of 1.3
.tangle-solidup.m are stacked alternately and repeatedly, is
provided further on the optical waveguide layer 103 with a
thickness of about 6 nm for the quantum well layer and a thickness
of about 10 nm for the barrier layer. Thereby, the composition of
the quantum well layer and the composition of the barrier layer are
set so as to achieve a lattice matching to the InP substrate 101.
The quantum well layer and the barrier layer may be repeated for 4
times. The GaInNAsP quantum well layer of the foregoing composition
has a bandgap wavelength of about 1.55 .mu.m while the GaInAsP
barrier layer of the foregoing composition has a bandgap wavelength
of about 1.3 .mu.m.
[0250] Further, an upper optical waveguide layer 105 of GaInAsP is
provided on the active layer 104 similarly to the lower optical
waveguide layer 103 with a bandgap wavelength of 1.3 .mu.m and a
thickness of 100 nm in lattice matching to the InP substrate 101,
and an upper cladding layer 106 of p-type InP is provided on the
upper optical waveguide layer 105 with a thickness of typically
about 3 nm.
[0251] Further, a cap layer 107 of p.sup.+-type InGaAs is provided
on the upper cladding layer 106, and an upper ohmic electrode 108
having the AuZn/Zn stacked structure is provided on the cap layer
107. Further, a lower ohmic electrode 109 having the AuGe/Ni/Au
stacked structure is provided on the lower major surface of the
substrate 101.
[0252] Similarly to the laser diode 1 of FIG. 4A, the laser diode
100 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 102-107 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0253] In the foregoing structure, it should be noted that the
cladding layer 102 or 106 is free from Al, and the problem of
oxidation of Al in the cladding layer at the time of the formation
of the active layer 104 is successfully avoided.
[0254] In the laser diode 100, the oscillation wavelength of 1.55
.mu.m is successfully achieved by incorporating a small amount of N
into a conventional GaInPAs composition, while simultaneously
maintaining a lattice matching to the InP substrate 101. The
incorporation of N into GaInPAs mixed crystal composition decreases
the bandgap and hence the laser oscillation wavelength. See the
relationship of FIG. 1.
[0255] FIGS. 16A and 16B show the band structure of the conduction
band Ec of the laser diode 100 taken along a vertical cross
section, wherein FIG. 16A shows a conventional case of using
GaInPAs for the quantum well while FIG. 16B shows the case of the
present invention that uses GaInNPAs for the quantum well in the
active layer 104.
[0256] Referring to FIGS. 16A and 16B, it will be noted that the
band discontinuity .DELTA.Ec corresponding to the depth of the
quantum well increases substantially when GaInNPAs is used
(.DELTA.Ec.sub.2) for the quantum well layer as compared with the
case of conventional GaInPAs is used (.DELTA.Ec.sub.1). Thereby,
the overflowing of the carriers away from the active layer 104 is
substantially suppressed in the laser diode 100 of the present
embodiment, and the laser diode 100 shows an excellent high
temperature performance.
[0257] In the present embodiment, the cladding layers 102 and 106
are not limited to InP but other composition such as
Ga.sub.xIn.sub.1-xAs.sub.yP.- sub.1-y (0.ltoreq.x<1,
0.ltoreq.y.ltoreq.1) may also be used, provided that the cladding
layers 102 and 106 have a bandgap energy larger than the bandgap
energy of the barrier layer of the active layer 104 or the optical
waveguide layers 103 and 105.
[0258] Further, it should be noted that a similar band structure is
obtained also in the SCH-MQW laser diodes 60 and 70 explained
already.
Eleventh Embodiment
[0259] FIG. 17 shows the construction of an SCH-MQW laser diode 110
according to an eleventh embodiment of the present invention in a
longitudinal cross sectional view.
[0260] Referring to FIG. 17, the laser diode 110 is constructed on
a substrate 111 of n-type InP on which a lower cladding layer 112
of n-type InP is provided epitaxially with a thickness of typically
about 1500 nm.
[0261] On the lower cladding layer 112, a lower optical waveguide
layer 113 of undoped GaInPAs having a bandgap wavelength of 1.3
.mu.m is provided epitaxially with a thickness of 100 nm in lattice
matching to the InP substrate 111, and an active layer 114 having a
multiple quantum well (MQW) structure in which a quantum well layer
of InNPAs having a bandgap wavelength of 1.55 .mu.m and a barrier
layer of GaInAsP having a bandgap wavelength of 1.3 .mu.m are
stacked alternately and repeatedly, is provided further on the
optical waveguide layer 113 with a thickness of about 5.5 nm for
the quantum well layer and a thickness of about 10 nm for the
barrier layer. Thereby, the composition of the barrier layer is set
so as to achieve a lattice matching to the InP substrate 111, while
the composition of the quantum well layer is set such that the
quantum well has a lattice constant exceeding the lattice constant
of the InP substrate 111. The quantum well layer having such a
composition accumulates therein a compressive lattice misfit
strain. The quantum well layer and the barrier layer may be
repeated for 4 times. The GaInNAsP quantum well layer of the
foregoing composition has a bandgap wavelength of about 1.55 .mu.m
or larger while the GaInAsP barrier layer of the foregoing
composition has a bandgap wavelength of about 1.3 .mu.m.
[0262] As the InNPAs quantum well layer of the present embodiment
has a reduced bandgap, only a small amount of N is sufficient for
achieving the laser oscillation in the 1.55 .mu.m band. As the
amount of N introduced into the active layer is very small, the
GaInNAsP mixed crystal forming the quantum well layer maintains an
excellent quality and the laser diode 110 provides a sharp
oscillation spectrum.
[0263] Further, an upper optical waveguide layer 115 of GaInPAs is
provided on the active layer 114 similarly to the lower optical
waveguide layer 113 with a bandgap wavelength of 1.3 .mu.m and a
thickness of 100 nm in lattice matching to the InP substrate 111,
and an upper cladding layer 116 of p-type InP is provided on the
upper optical waveguide layer 115 with a thickness of typically
about 1500 nm.
[0264] Further, a cap layer 117 of p.sup.+-type InGaAs is provided
on the upper cladding layer 116, and an upper ohmic electrode 118
having the AuZn/Zn stacked structure is provided on the cap layer
117 . . . Further, a lower ohmic electrode 119 having the
AuGe/Ni/Au stacked structure is provided on the lower major surface
of the substrate 111.
[0265] Similarly to the laser diode 1 of FIG. 4A, the laser diode
110 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 112-117 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0266] In the foregoing structure, it should be noted that the
cladding layer 112 or 116 is free from Al, and the problem of
oxidation of Al in the cladding layer at the time of the
low-temperature formation of the MQW active layer 114 is
successfully avoided.
[0267] Further, the laser diode structure of FIG. 17 is effective
for confining the carriers in the active layer 114 due to the large
band discontinuity .DELTA.Ec similarly to the case of FIG. 16B.
Thereby, the laser diode shows an excellent performance at high
temperatures without a temperature regulation.
[0268] In the present embodiment, it should be noted that the
GaInNAsP quantum well layer in the active layer 114 may accumulate
a tensile lattice misfit strain in place of a compressive lattice
misfit strain.
Twelfth Embodiment
[0269] FIG. 18 shows the construction of an SCH-MQW laser diode 120
according to a twelfth embodiment of the present invention in a
longitudinal cross sectional view.
[0270] Referring to FIG. 18, the laser diode 120 is a is
constructed on a substrate 121 of n-type InP on which a lower
cladding layer 122 of n-type InP is provided epitaxially with a
thickness of typically about 1500 nm.
[0271] On the lower cladding layer 122, a lower optical waveguide
layer 123 of undoped GaInPAs having a bandgap wavelength of 1.3
.mu.m is provided epitaxially with a thickness of 100 nm in lattice
matching to the InP substrate 121, and an active layer 124 having a
multiple quantum well (MQW) structure in which a quantum well layer
of GaInNAsP having a bandgap wavelength of 1.55 .mu.m and a barrier
layer of GaInAsP having a bandgap wavelength of 1.3 .mu.m are
stacked alternately and repeatedly, is provided further on the
optical waveguide layer 123 with a thickness of about 4 nm for the
quantum well layer and a thickness of about 10 nm for the barrier
layer. Thereby, the compositions of the quantum well layer and the
composition of the barrier layer are set such that the quantum well
has a lattice constant exceeding the lattice constant of the InP
substrate 121 while the barrier layer has a lattice-constant
smaller than the lattice constant of the InP substrate. Thereby,
the quantum well layer accumulates therein a compressive lattice
misfit strain, while the barrier layer accumulates therein a
compensating tensile lattice misfit strain. The quantum well layer
and the barrier layer may be repeated for 6 times. As the
compressive lattice misfit strain of the quantum well layer is
compensated for by the tensile lattice misfit strain of the barrier
layer, the stacking of the quantum well layer and the barrier layer
may be repeated as desired. The GaInNAsP quantum well layer of the
foregoing composition has a bandgap wavelength of about 1.55 .mu.m
or larger while the GaInAsP barrier layer of the foregoing
composition has a bandgap wavelength of about 1.3 .mu.m.
[0272] As the GaInNAsP quantum well layer of the present embodiment
is strained, the quantum well layer inherently has a reduced
bandgap. Thus, only a small amount of N is sufficient for achieving
the laser oscillation in the 1.55 .mu.m band. As the amount of N
introduced into the active layer is very small, the GaInNAsP mixed
crystal forming the quantum well layer maintains an excellent
quality and the laser diode 120 provides a sharp oscillation
spectrum.
[0273] Further, an upper optical waveguide layer 125 of GaInAsP is
provided on the active-layer 124 similarly to the lower optical
waveguide layer 123 with a bandgap wavelength of 1.3 .mu.m and a
thickness of 1500 nm in lattice matching to the InP substrate 121,
and an upper cladding layer 126 of p-type InP is provided on the
upper optical waveguide layer 125 with a thickness of typically
about 1500 nm.
[0274] Further, a cap layer 127 of p.sup.+-type InGaAs is provided
on the upper cladding layer 126, and an upper ohmic electrode 128
having the AuZn/Zn stacked structure is provided on the cap layer
127. Further, a lower ohmic electrode 129 having the AuGe/Ni/Au
stacked structure is provided on the lower major surface of the
substrate 121.
[0275] Similarly to the laser diode 1 of FIG. 4A, the laser diode
120 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 122-127 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0276] In the foregoing structure, it should be noted that the
cladding layer 122 or 126 is free from Al, and the problem of
oxidation of Al in the cladding layer at the time of the
low-temperature formation of the MQW active layer 124 is
successfully avoided.
[0277] Further, the laser diode structure of FIG. 18 is effective
for confining the carriers in the active layer 124 due to the large
band discontinuity .DELTA.Ec similarly to the case of FIG. 16B.
Thereby, the laser diode shows an excellent performance at high
temperatures without a temperature regulation.
[0278] In the present embodiment, it should be noted that the
GaInNAsP quantum well layer in the active layer 124 may accumulate
a tensile lattice misfit strain in place of a compressive lattice
misfit strain. In this case, the GaInNAsP barrier layer accumulates
a compressive lattice misfit strain.
Thirteenth Embodiment
[0279] FIG. 19 shows the construction of an SCH-MQW laser diode 130
according to a thirteenth embodiment of the present invention in a
longitudinal cross sectional view.
[0280] Referring to FIG. 19, the laser diode 130 is a is
constructed on a substrate 131 of n-type InP on which a lower
cladding layer 132 of n-type InP is provided epitaxially with a
thickness of typically about 1500 nm.
[0281] On the lower cladding layer 132, a lower optical waveguide
layer 133 of undoped GaInPAs having a bandgap wavelength of 1.3
.mu.m is provided epitaxially with a thickness of 100 nm in lattice
matching to the InP substrate 131, and an active layer 134 having a
multiple quantum well (MQW) structure in which a quantum well layer
of InNAsP having a bandgap wavelength of 1.55 .mu.m and a barrier
layer of GaInNAsP having a bandgap wavelength of 1.3 .mu.m are
stacked alternately and repeatedly, is provided further on the
optical waveguide layer 133 with a thickness of about 5.5 nm for
the quantum well layer and a thickness of about 10 nm for the
barrier layer. Thereby, the compositions of the quantum well layer
and the composition of the barrier layer are set such that the
quantum well has a lattice constant exceeding the lattice constant
of the InP substrate 131 while the barrier layer has a lattice
constant smaller than the lattice constant of the InP substrate
131. Thereby, the quantum well layer of InNAsP accumulates therein
a compressive lattice misfit strain, while the barrier layer of
GaInNAsP accumulates therein a compensating tensile lattice misfit
strain. The quantum well layer and the barrier layer may be
repeated for 4 times. As the compressive lattice misfit strain of
the quantum well layer is compensated for by the tensile lattice
misfit strain of the barrier layer, the stacking of the quantum
well layer and the barrier layer may be repeated as desired. The
InNAsP quantum well layer of the foregoing composition has a
bandgap wavelength of about 1.55 .mu.m or larger while the GaInAsP
barrier layer of the foregoing composition has a bandgap wavelength
of about 1.3 .mu.m.
[0282] As the InNPAs quantum well layer of the present embodiment
has a reduced bandgap, only a small amount of N is sufficient for
achieving the laser oscillation in the 1.55 .mu.m band. As the
amount of N introduced into the active layer is very small, the
InNAsP mixed crystal forming the quantum well layer maintains an
excellent quality and the laser diode 130 provides a sharp
oscillation spectrum.
[0283] Further, an upper optical waveguide layer 135 of GaInAsP is
provided on the active layer 134 similarly to the lower optical
waveguide layer 133 with a bandgap wavelength of 1.3 .mu.m and a
thickness of about 100 nm in lattice matching to the InP substrate
131, and an upper cladding layer 136 of p-type InP is provided on
the upper optical waveguide layer 135 with a thickness of typically
about 1500 nm.
[0284] Further, a cap layer 137 of p.sup.+-type InGaAs is provided
on the upper cladding layer 136, and an upper ohmic electrode 138
having the AuZn/Zn stacked structure is provided on the cap layer
137. Further, a lower ohmic electrode 129 having the AuGe/Ni/Au
stacked structure is provided on the lower major surface of the
substrate 131.
[0285] Similarly to the laser diode 1 of FIG. 4A, the laser diode
130 is defined in the longitudinal or axial direction thereof by
cleaved surfaces M1 and M2 that form an optical cavity. Further,
the deposition of the layers 132-137 may be made in the deposition
apparatus of FIG. 3 similarly to the process of forming the layers
2-6 of FIGS. 4A and 4B.
[0286] In the foregoing structure, it should be noted that the
cladding layer 132 or 136 is free from Al, and the problem of
oxidation of Al in the cladding layer at the time of the
low-temperature formation of the MQW active layer 134 is
successfully avoided.
[0287] Further, the laser diode structure of FIG. 19 is effective
for confining the carriers in the active layer 134 due to the large
band discontinuity .DELTA.Ec similarly to the case of FIG. 16B.
Thereby, the laser diode shows an excellent performance at high
temperatures without a temperature regulation.
[0288] In the present embodiment, it should be noted that the
InNAsP quantum well layer in the active layer 134 may accumulate a
tensile lattice misfit strain in place of a compressive lattice
misfit strain. In this case, the GaInNAsP barrier layer accumulates
a compressive lattice misfit strain. As the barrier layer and the
quantum well layer in the active layer 134 both contain N, the
alternate deposition of the barrier layer and the quantum well
layer can be achieved in the deposition apparatus of FIG. 3 by
continuously supplying the gaseous sources of TMI or TEI, DMHy,
AsH.sub.3 and PH.sub.3, while switching the supply of TMGa on and
off repeatedly. Thereby, the MQW structure of the active layer 134
is formed easily and efficiently. As the supply of DMHy, which has
to be supplied with a substantial flow rate, is made continuous,
the gas flow in the reaction chamber 11 is stabilized, and the
epitaxial growth of the quantum well layers and the barrier layers
is achieved with excellent reproducibility.
Fourteenth Embodiment
[0289] Heretofore, it was assumed that the various laser diodes has
a ridge guide structure as indicated in the end view of FIG. 4B. On
the other hand, a laser diode generally uses a current confinement
structure for confining the injected carrier to a small region in
the active layer such that the laser oscillation occurs in such a
small region with high efficiency.
[0290] Thus, FIG. 20 shows a stripe laser diode 140 according to a
fourteenth embodiment of the present invention in an end view,
wherein the laser diode 140 is a modification of the
ridge-waveguide laser diode 30 of FIG. 8. However, it should be
noted that the current confinement structure of the present
embodiment is applicable not only to the laser diode 30 but to any
of the laser diode structures described heretofore in the preceding
embodiments. In FIG. 20, those parts corresponding to the parts
described previously are designated by the same reference numerals
and the description thereof will be omitted.
[0291] Referring to FIG. 20, the laser diode 140 now includes a
buffer layer 31A as of n-type GaAs on the GaAs substrate 31, and
the upper cladding layer 36 carries thereon a pair of current block
regions 141A and 141B disposed symmetrically at both lateral sides
of a stripe region 141 of the laser diode, in which the optical
radiation is to be confined. In the illustrated example, the
current blocking regions 141A and 141B are both formed of n-type
GaInNAs grown epitaxially on the upper cladding layer 36, and a
second upper cladding layer 142 of p-type AlGaAs is provided
further thereon in contact with the upper cladding layer 36 at the
foregoing stripe region and so as to bury the current blocking
regions 141A and 141B underneath. Thereby, the contact layer 37 is
provided so as to cover the second upper cladding layer 142, and
the upper ohmic electrode 38 covers the contact layer 37.
[0292] By providing the n-type regions 141A and 141B, the drive
current (holes) injected at the ohmic electrode 38 to the contact
layer 37 is successfully confined to the foregoing stripe region
141 due to the p-n junction and associated depletion region formed
at the interface of the current blocking structure 141A or 141B to
the cladding layer 142.
[0293] It should be noted that the current blocking regions 141A
and 141B has a composition identical to the composition of the
GaInNAs forming the active layer 34 or a composition that provides
a bandgap energy smaller than the bandgap energy of the active
layer 34. Thereby, the current blocking regions 141A and 141B
absorb the optical radiation emitted in the active layer 34, while
such an optical absorption induces a refractive index change in the
current blocking regions 141A and 141B. Thus, the optical radiation
is guided, due to the refraction index difference thus induced,
along the strip region in the axial direction of the laser diode,
and the stimulated emission occurs efficiently in such a strip
region where the optical radiation is thus concentrated.
[0294] It should be noted that the structure of FIG. 20 is formed
easily, by forming a GaInNAs layer on the upper cladding layer,
followed by a photolithographic patterning process of the same into
the foregoing current blocking regions 141A and 141B.
[0295] Further, in view of the fact that the current blocking
regions 141A and 141B contain N therein, it is also possible or
even more preferable to use GaInP or GaInPAs mixed crystal for the
cladding layers 36 and 142 as in the case of the laser diode 20 of
FIG. 7 or 80 of FIG. 13. Further, any n-type semiconductor
material, such as GaInAs, GaAlAs, GaInPAs, and the like, that can
absorb the optical radiation produced in the active layer 34 may be
used for the current blocking regions 141A and 141B.
[0296] FIG. 21 shows a modification of the laser diode 140 of FIG.
20.
[0297] Referring to FIG. 21, the laser diode 140 includes an n-type
GaAs layer 143A on the current blocking region 141A and an n-type
GaAs layer 143B on the current blocking region 141B, while the
region 141A or 141B itself is formed of an undoped GaInNAs that
absorbs the optical radiation produced in the active layer 34. In
this case, the n-type GaAs layer 143A or 143B acts as the current
blocking structure while the GaInNAs region 141A or 141B forms a
lateral optical waveguide. Further, the optical waveguide region
141A and the current blocking region 143A thereon or the optical
waveguide region 141B and the current blocking region 143B thereon
may be interchanged.
[0298] As noted already, the current confinement structure of FIG.
20 or 21 is applicable to any of the laser diode structures 10-130
described heretofore with various embodiments.
[0299] Further, the present invention is applicable also to DFB
laser diodes or DBR laser diodes that use a diffraction corrugation
in place of the mirrors M1 and M2.
[0300] Further, the present invention is not limited to the
embodiments described heretofore, but various variations and
modifications may be made without departing from the scope of the
invention.
* * * * *