U.S. patent application number 09/828218 was filed with the patent office on 2002-05-02 for semiconductor laser device.
Invention is credited to Miyashita, Motoharu, Ono, Ken-Ichi, Sasaki, Motoko.
Application Number | 20020051475 09/828218 |
Document ID | / |
Family ID | 18806688 |
Filed Date | 2002-05-02 |
United States Patent
Application |
20020051475 |
Kind Code |
A1 |
Miyashita, Motoharu ; et
al. |
May 2, 2002 |
SEMICONDUCTOR LASER DEVICE
Abstract
To provide a semiconductor laser device exhibiting a reduced
threshold current with less deterioration in temperature properties
in current-optical output performance and excellent beam
properties, in the semiconductor laser device according to the
present invention, a current blocking layer 22 made of n-AlInP
having a stripe-shaped opening 24 is disposed on a first upper clad
layer 20, the first upper clad layer 20 and the current blocking
layer 22 facing the opening 24 respectively are covered by a buffer
layer 26 made of p-Al.sub.0.5Ga.sub.0.5As and a second upper clad
layer 28 made of p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub- .0.5P is
disposed via the buffer layer 26, to prevent lattice defect
formation in the crystal layer grown on the surface of the current
blocking layer 22 facing the opening 24.
Inventors: |
Miyashita, Motoharu; (Tokyo,
JP) ; Sasaki, Motoko; (Tokyo, JP) ; Ono,
Ken-Ichi; (Tokyo, JP) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
700 THIRTEENTH ST. NW
SUITE 300
WASHINGTON
DC
20005-3960
US
|
Family ID: |
18806688 |
Appl. No.: |
09/828218 |
Filed: |
April 9, 2001 |
Current U.S.
Class: |
372/46.01 |
Current CPC
Class: |
H01S 5/2231 20130101;
H01S 5/2202 20130101; H01S 5/2218 20130101; H01S 5/32325 20130101;
H01S 5/221 20130101 |
Class at
Publication: |
372/46 |
International
Class: |
H01S 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 30, 2000 |
JP |
2000-330117 |
Claims
What is claimed is:
1. A semiconductor laser device comprising: a semiconductor
substrate of a first conductivity type; a first clad layer of a
first conductivity type made of a III-V group compound
semiconductor disposed on the semiconductor substrate; an active
layer made of a III-V group compound semiconductor having a smaller
band gap than the first clad layer, disposed on the first clad
layer; a first second-clad layer of a second conductivity type made
of a III-V group compound semiconductor having a larger band gap
than the active layer, disposed on the active layer; a current
blocking layer of a first conductivity type made of a III-V group
compound semiconductor having a larger band gap than the active
layer, disposed on the first second-clad layer and having a
stripe-shaped opening to be a current path; a buffer layer of a
second conductivity type made of a III-V group compound
semiconductor having a larger band gap than the active layer,
disposed on the surface of the current blocking layer facing the
opening; and a second second-clad layer of a second conductivity
type made of a III-V group compound semiconductor having a larger
band gap than the active layer, disposed on the first second-clad
layer facing the opening and the current blocking layer via the
buffer layer.
2. The semiconductor laser device according to claim 1, wherein the
current blocking layer, the buffer layer and the second second-clad
layer are made of an AlInP material, an AlGaAs material and an
AlGaInP material, respectively.
3. The semiconductor laser device according to claim 2, wherein a
protective layer made of a GaInP material is further disposed
between the surface of the current blocking layer except the
surface facing the opening and the second second-clad layer.
4. A semiconductor laser device comprising: a semiconductor
substrate of a first conductivity type; a first clad layer of a
first conductivity type made of a III-V group compound
semiconductor disposed on the semiconductor substrate; an active
layer made of a III-V group compound semiconductor having a smaller
band gap than the first clad layer, disposed on the first clad
layer; a first second-clad layer of a second conductivity type made
of a III-V group compound semiconductor having a larger band gap
than the active layer, disposed on the active layer; a current
blocking layer of a first conductivity type made of an AlInP
material having a larger band gap than the active layer, disposed
on the first second-clad layer and having a stripe-shaped opening
to be a current path; a protective layer made of a GaInP material
disposed on the surface of the current blocking layer except the
surface facing the opening; and a second second-clad layer of a
second conductivity type made of an AlGaInP material having a
larger band gap than the active layer, disposed on the current
blocking layer via the protective layer and the first second-clad
layer facing the opening.
5. A process for manufacturing a semiconductor laser device
including the steps of: depositing a first clad layer of a first
conductivity type made of a III-V group compound semiconductor, an
active layer made of a III-V group compound semiconductor having a
smaller band gap than the first clad layer, a first second-clad
layer of a second conductivity type made of a III-V group compound
semiconductor having a larger band gap than the active layer, and a
current blocking layer of a first conductivity type made of a III-V
group compound semiconductor having a larger band gap than the
active layer on the first second-clad layer in order, on a
semiconductor substrate of a first conductivity type; forming a
stripe-shaped opening penetrating the current blocking layer;
forming a buffer layer of a second conductivity type made of a
III-V group compound semiconductor having a larger band gap than
the active layer on the surface of the current blocking layer
facing the opening; and forming a second second-clad layer of a
second conductivity type made of a III-V group compound
semiconductor having a larger band gap than the active layer on the
first second-clad layer facing the opening and the current blocking
layer via the buffer layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a semiconductor laser device and a
manufacturing method thereof. In particular, it relates to a
semiconductor laser device used to process optical information and
a manufacturing method thereof.
[0003] 2. Description of the Related Art
[0004] A semiconductor laser device for optical information
processing has conventionally employed a gain guided structure
using a GaAs current blocking layer. Recently, there has been,
however, developed a semiconductor laser device which employs a
real refractive index guided structure using an AlInP layer as a
current blocking layer to reduce an operating current.
[0005] A real refractive index guided structure may reduce an
optical absorption loss in a current blocking layer resulting in
not only a reduced threshold current but also an improved luminous
efficiency, therefore a reduced operating current.
[0006] This technical trend has been driven for developing
semiconductor laser devices having a higher output. Conventional
optical information processing involves only reading as in, for
example, DVD-ROM, which does not require very high output. Recent
optical information processing involves, however, not simply
reading but also writing on a recording medium as in, for example,
DVD-RW or DVD-R, which necessarily requires a higher output. It has
been, therefore, required that an internal loss is minimized to
reduce an operating current for improving temperature properties of
the semiconductor laser device and thus reliability under a high
output.
[0007] FIG. 14 is a cross-sectional view of a conventional SAS
(Self-Aligned Structure) type of red semiconductor laser diode
(hereinafter, referred to as a "redLD") described in Electronics
Letters, Vol. 33, No. 14 (1997), pp.1223-5.
[0008] In FIG. 14, reference numeral 100 denotes a red LD, 102 an
n-type GaAs substrate (hereinafter, n-type and p-type are denoted
as "n-" and "p-", respectively), 104 an n-GaAs buffer layer, 106 a
lower clad layer made of
n-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P, and 108 an active
layer of an MQW structure made of GaInP/AlGaInP where GaInP is a
material for a well layer and AlGaInP is a material for a barrier
layer.
[0009] In this figure, reference numeral 110 denotes a first upper
clad layer made of p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P, 112
a current blocking layer made of n-AlInP, 114 a stripe-shaped
opening to be a current channel in the current blocking layer 112,
116 a second upper clad layer made of
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P, 118 a p-GaAs contact
layer, 120 a p-electrode, and 122 an n-electrode.
[0010] There will be described a process for manufacturing this
semiconductor laser device 100.
[0011] FIGS. 15, 16 and 17 are cross-sectional views of a
conventional red LD in individual manufacturing steps.
[0012] First, on an n-GaAs substrate 102 are sequentially deposited
an n-GaAs layer to be a buffer layer 104, an
n-(Al.sub.0.7Ga.sub.0.3).sub.0.- 5In.sub.0.5 layer to be a lower
clad layer 106, a GaInP/AlGaInP MQW layer to be an active layer
108, a p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a
first upper clad layer 110 and an n-AlInP layer to be a current
blocking layer 112, by primary epitaxial growth based on crystal
growth such as MOCVD. For dopants, silicon is used as an n-type
dopant while zinc is used as a p-type dopant. The result of this
step is shown in FIG. 15.
[0013] Then, a resist pattern 126 is formed on the surface of the
n-AlInP layer to be a current blocking layer 112 by a
photolithographic process, and a stripe-shaped opening 114 to be a
current path is formed in the n-AlInP layer to be the current
blocking layer 112 by wet etching. The result of this step is shown
in FIG. 16.
[0014] After removing the resist pattern 126, a
p-(Al.sub.0.7Ga.sub.0.3).s- ub.0.5In.sub.0.5P layer to be a second
upper clad layer 116 is formed on the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a first
upper clad layer 110 facing the opening 114 and the n-AlInP layer
to be a current blocking layer 112 by secondary epitaxial growth
based on crystal growth such as MOCVD. The result of this step is
shown in FIG. 17.
[0015] Then, a p-GaAs layer to be a contact layer 118 is formed on
the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a
second upper clad layer 116.
[0016] In this process, a crystal growth temperature is about
650.degree. C. to 750.degree. C. A crystal growth temperature as
low as possible is used to prevent a p-type dopant Zn from
diffusing from the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P
layer as a first upper clad layer 110 to the MQW layer to be an
active layer 108 as much as possible.
[0017] Then, a p-electrode 120 and an n-electrode 122 are formed on
the surface of the p-GaAs layer to be a contact layer 110 and the
rear surface of the n-GaAs substrate 102, respectively.
[0018] A conventional red LD 100 has a configuration as described
above. When forming the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P
layer to be a second upper clad layer 116 on the n-AlInP layer to
be a current blocking layer 112 in the manufacturing process for
the red LD 100 as illustrated in FIG. 17, lattice defects may
frequently develop on the surface facing the opening 114 in the
n-AlInP layer to be a current blocking layer 112, leading to
increase in an internal loss, deterioration in temperature
properties and poor reliability in the red LD 100.
[0019] A technique for preventing lattice defects in crystal growth
has been described in Proceedings of the Tenth International
Conference on Metalorganic Vapor Phase Epitaxy (2000), p. 82. In
the report, an (Al.sub.0.7Ga.sub.0.3).sub.0.51In.sub.0.49P layer is
deposited on a GaAs substrate which is a (100) facet misoriented by
10.degree. toward [011] direction. Then, on the layer is formed an
Al.sub.0.51In.sub.0.49P layer having a grooved structure whose side
wall is a (111) A facet, and with Ga.sub.0.51In.sub.0.49P as a
marker sandwiched in between, an
(Al.sub.0.7Ga.sub.0.3).sub.0.51In.sub.0.49P layer is formed above
the (Al.sub.0.7Ga.sub.0.3).sub.0.51In.sub.0.49P layer parallel to
the GaAs substrate exposed in the bottom of the grooved structure
and above the Al.sub.0.51In.sub.0.49P layer having a (111) A facet,
during which development of lattice defects is studied using the
then substrate temperature as a parameter.
[0020] According to the report, crystal growth was caused at
substrate temperatures of 720.degree. C., 760.degree. C. and
800.degree. C. It was found that lattice defects developed in a
crystal layer growing on a (111) A facet at a substrate temperature
of 720.degree. C. or 760.degree. C., while crystal growth at a
substrate temperature of 800.degree. C. reduced lattice defects in
the (Al.sub.0.7Ga.sub.0.3).sub.0.51In.sub.0.49- P layer on a (111)
A facet.
[0021] However, for a red LD, a crystal growth temperature of
800.degree. C. may cause diffusion of the p-type dopant Zn from the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer as a first upper
clad layer 110 to the MQW layer to be an active layer 108, leading
to deterioration in temperature properties or reliability in
current-optical output performance.
[0022] Besides the prior art described above, JP-B 2842465 has
disclosed an SAS type semiconductor laser where on the surface of a
current blocking layer made of AlGaAs material having a
stripe-shaped opening is deposited a protective layer made of an
AlGaAs material with small aluminum content, on which a p-AlGaAs
material is deposited as a p-clad layer, but has not described that
on a current blocking layer made of an AlInP material are formed a
capping layer made of a GaInP material and a
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer.
SUMMARY OF THE INVENTION
[0023] The present invention has been made to solve the above
problem in the art, and an objective of this invention is to
provide a reliable semiconductor laser device exhibiting a reduced
threshold current with less deterioration in temperature properties
in current-optical output performance.
[0024] A semiconductor laser device according to the present
invention comprises: a semiconductor substrate of a first
conductivity type; a first clad layer of a first conductivity type
made of a III-V group compound semiconductor disposed on the
semiconductor substrate; an active layer made of a III-V group
compound semiconductor having a smaller band gap than the first
clad layer, disposed on the first clad layer; a first second-clad
layer of a second conductivity type made of a III-V group compound
semiconductor having a larger band gap than the active layer,
disposed on the active layer; a current blocking layer of a first
conductivity type made of a III-V group compound semiconductor
having a larger band gap than the active layer, disposed on the
first second-clad layer and having a stripe-shaped opening to be a
current path; a buffer layer of a second conductivity type made of
a III-V group compound semiconductor having a larger band gap than
the active layer, disposed on the surface of the current blocking
layer facing the opening; and a second second-clad layer of a
second conductivity type made of a III-V group compound
semiconductor having a larger band gap than the active layer,
disposed on the first second-clad layer facing the opening and the
current blocking layer via the buffer layer.
[0025] Accordingly, a semiconductor laser device according to the
present invention is advantageous in that lattice defects in the
second second-clad layer disposed on the surface of the current
blocking layer facing the opening via the buffer layer may reduce
and it can be prevented from a second conductivity type of dopant
diffusing from the first second-clad layer to the active layer.
Therefore the construction according to the present invention makes
it possible to reduce deterioration in temperature properties in
current-optical output performance, and consequently to improve
reliability of the semiconductor laser device.
[0026] Another objective of this invention is to lead to
improvement in reliability of a semiconductor laser device with a
red LD.
[0027] A semiconductor laser device according to the present
invention comprises: a semiconductor substrate of a first
conductivity type; a first clad layer of a first conductivity type
made of a III-V group compound semiconductor disposed on the
semiconductor substrate; an active layer made of a III-V group
compound semiconductor having a smaller band gap than the first
clad layer, disposed on the first clad layer; a first second-clad
layer of a second conductivity type made of a III-V group compound
semiconductor having a larger band gap than the active layer,
disposed on the active layer; a current blocking layer of a first
conductivity type made of an AlInP material having a larger band
gap than the active layer, disposed on the first second-clad layer
and having a stripe-shaped opening to be a current path; a
protective layer made of a GaInP material disposed on the surface
of the current blocking layer except the surface facing the
opening; and a second second-clad layer of a second conductivity
type made of an AlGaInP material having a larger band gap than the
active layer, disposed on the current blocking layer via the
protective layer and the first second-clad layer facing the
opening.
[0028] Accordingly, a semiconductor laser device according to the
present invention is advantageous that because the current blocking
layer made of the AlInP material is protected by the GaInP material
not containing Al the second second-clad layer made of the AlGaInP
material whose composition tends to be deviated is disposed on the
crystal facet with a reduced amount of oxide film. Consequently
reducing a risk of lattice defect formation leads to improvement in
reliability of a semiconductor laser device with a red LD.
[0029] A further objective of this invention is to provide a
process for manufacturing a reliable semiconductor laser device
exhibiting a reduced threshold current with less deterioration in
temperature properties in current-optical output performance by
simple steps.
[0030] A process for manufacturing a semiconductor laser device
according to the present invention includes the steps of:
depositing a first clad layer of a first conductivity type made of
a III-V group compound semiconductor, an active layer made of a
III-V group compound semiconductor having a smaller band gap than
the first clad layer, a first second-clad layer of a second
conductivity type made of a III-V group compound semiconductor
having a larger band gap than the active layer, and a current
blocking layer of a first conductivity type made of a III-V group
compound semiconductor having a larger band gap than the active
layer on the first second-clad layer in order, on a semiconductor
substrate of a first conductivity type; forming a stripe-shaped
opening penetrating the current blocking layer; forming a buffer
layer of a second conductivity type made of a III-V group compound
semiconductor having a larger band gap than the active layer on the
surface of the current blocking layer facing the opening; and
forming a second second-clad layer of a second conductivity type
made of a III-V group compound semiconductor having a larger band
gap than the active layer on the first second-clad layer facing the
opening and the current blocking layer via the buffer layer.
[0031] Accordingly, a process for manufacturing a semiconductor
laser device according to the present invention is advantageous
that because the crystal of the second second-clad layer may grow
with reduced lattice defects on the surface of the current blocking
layer facing the opening via the buffer layer even at a common
substrate temperature and such a common substrate temperature may
reduce diffusion of a second conductivity type of dopant from the
first second-clad layer to the active layer during crystal growth,
a reliable semiconductor laser device with reduced deterioration in
temperature properties in current-optical output performance can be
manufactured by simple steps. Consequently, a semiconductor laser
device exhibiting excellent laser properties can be produced with a
low price.
[0032] Other objects and advantages of the invention will become
apparent from the detailed description given hereinafter. It should
be understood, however, that the detailed description and specific
embodiments are given by way of illustration only since various
changes and modifications within the scope of the invention will
become apparent to those skilled in the art from this detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a cross-sectional view of the red LD according to
the present invention.
[0034] FIG. 2 is a cross-sectional view of the MQW structure in the
active layer according to the present invention.
[0035] FIG. 3 is a schematic view of an energy band for the MQW in
the active layer according to the present invention.
[0036] FIG. 4 is a cross-sectional view of the red LD in a
manufacturing step according to the present invention.
[0037] FIG. 5 is a cross-sectional view of the red LD in a
manufacturing step according to the present invention.
[0038] FIG. 6 is a cross-sectional view of the red LD in a
manufacturing step according to the present invention.
[0039] FIG. 7 is a view of a transmission electron microscope (TEM)
photograph of a conventional red LD.
[0040] FIG. 8 is a view of a transmission electron microscope (TEM)
photograph of a red LD according to the present invention.
[0041] FIG. 9 is a cross-sectional view of the red LD according to
the present invention.
[0042] FIG. 10 is a cross-sectional view of the red LD in a
manufacturing step according to the present invention.
[0043] FIG. 11 is a cross-sectional view of the red LD in a
manufacturing step according to the present invention.
[0044] FIG. 12 is a cross-sectional view of the red LD in a
manufacturing step according to the present invention.
[0045] FIG. 13 is a cross-sectional view of the red LD according to
the present invention.
[0046] FIG. 14 is a cross-sectional view of the conventional red
LD.
[0047] FIG. 15 is a cross-sectional view of the conventional red LD
in a manufacturing step.
[0048] FIG. 16 is a cross-sectional view of the conventional red LD
in a manufacturing step.
[0049] FIG. 17 is a cross-sectional view of the conventional red LD
in a manufacturing step.
In all figures, the substantially same elements are given the same
reference number.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0050] First Embodiment
[0051] There will be described, as an example, an SAS type red LD
used for information processing.
[0052] In a red LD according to Embodiment 1, a current blocking
layer made of n-AlInP having a stripe-shaped opening is disposed on
a first upper clad layer, a buffer layer made of
p-Al.sub.0.5Ga.sub.0.5As covers the first upper clad layer and the
current blocking layer facing the opening, and a second upper clad
layer made of p-(Al.sub.0.7Ga.sub.0.3).s- ub.0.5In.sub.0.5P is
disposed on the buffer layer to prevent lattice defects from
developing in a crystal layer grown on the surface of the current
blocking layer facing the opening.
[0053] FIG. 1 is a cross-sectional view of the red LD according to
Embodiment 1 of this invention.
[0054] In FIG. 1, reference numeral 10 denotes a red LD, 12 an
n-GaAs substrate; 14 an n-GaAs first buffer layer disposed on the
main surface of the substrate 12; 16 a lower clad layer as a first
clad layer made of n-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P
disposed on the first buffer layer 14; and 18 an MQW structure of
active layer made of GaInP/AlGaInP disposed on the lower clad layer
16 where GaInP is a material for a well layer and AlGaInP is a
material for a barrier layer.
[0055] Then, reference numeral 20 denotes a first upper clad layer
as a first second-clad layer made of
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.- 5P disposed on the
active layer 18; 22 a current blocking layer made of
n-Al.sub.0.5In.sub.0.5P disposed on the first upper clad layer 20;
and 24 a stripe-shaped opening to be a current channel formed in
the current blocking layer 22. The longitudinal direction of the
opening 24 is an output direction of a laser beam and the surface
facing the opening 24 in the current blocking layer 22 is a (111) A
facet.
[0056] Then, reference numeral 26 denotes a second buffer layer as
a buffer layer made of, for example, p-Al.sub.0.5Ga.sub.0.5As. The
Al content in the second buffer layer 26 is adjusted such that a
band gap in the second buffer layer 26 is larger than that in the
active layer 18, for avoiding absorption of a laser beam from the
active layer 18. The material for the second buffer layer 26 is,
therefore, selected in the light of the band gap in the active
layer 18.
[0057] Since a wavelength of the active layer 18 is 650 nm to 660
nm, an Al content x in p-Al.sub.xGa.sub.1-xAs of the second buffer
layer 26 is 0.4 to 1, preferably 0.5 to 0.7.
[0058] The second buffer layer 26 covers the surface of the exposed
first upper clad layer 20 facing the opening 24 , the (111) A facet
of the current blocking layer 22 facing the opening 24 and the
upper surface of the current blocking layer 22.
[0059] The thickness of the second buffer layer 26 must be 10 nm or
more. For example, it is about 10 nm to 100 nm for an Al content
x=0.5.
[0060] Then, reference numeral 28 denotes a second upper clad layer
as a second second-clad layer made of
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0- .5P disposed on the
surface of the current blocking layer 22; 30 a contact layer made
of p-GaAs disposed on the second upper clad layer 28; 32 a
p-electrode disposed on the surface of the contact layer; and 34 an
n-electrode disposed on the rear surface of the substrate 12.
[0061] FIG. 2 is a cross-sectional view of an MQW structure in the
active layer 18 and FIG. 3 is a schematic view of an energy band
for the MQW in the active layer 18.
[0062] In FIGS. 2 and 3, reference numeral 18a denotes an optical
guide layer, adjacent to which is formed a quantum well layer 18b
inside of the active layer 18. A barrier layer 18c is deposited,
which is sandwiched by the quantum well layers 18b. The thickness
of the quantum well layer 18b and the barrier layer 18c is, for
example, about 6 nm.
[0063] There will be described a process for manufacturing a red LD
10.
[0064] FIGS. 4, 5 and 6 are cross-sectional views of the red LD in
individual manufacturing steps according to Embodiment 1.
[0065] On an n-GaAs substrate 12 are sequentially deposited an
n-GaAs layer to be a first buffer layer 14, an
n-(Al.sub.0.7Ga.sub.0.3).sub.0.5I- n.sub.0.5 layer to be a lower
clad layer 16, a GaInP/AlGaInP MQW layer to be an active layer 18,
a p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a first
upper clad layer 20 and an n-Al.sub.0.5In.sub.0.5P layer to be a
current blocking layer 22, by primary epitaxial growth based on
crystal growth such as MOCVD. For a dopant, an n-type dopant is
silicon while a p-type dopant is zinc. The result of this step is
shown in FIG. 4.
[0066] Then, a resist pattern 38 is formed on the surface of the
n-Al.sub.0.5In.sub.0.5P layer to be a current blocking layer 22 by
a photolithographic process, and a stripe-shaped opening 24 to be a
current path is formed in the n-Al.sub.0.5In.sub.0.5P layer to be
the current blocking layer 22 by wet etching. The result of this
step is illustrated in FIG. 5.
[0067] After removing the resist pattern 38, a
p-Al.sub.0.5Ga.sub.0.5As layer to be a second buffer layer 26 is
formed on the surface of the exposed
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a first
upper clad layer 20 facing the opening 24, the (111) A facet facing
the opening 24 in the n-Al.sub.0.5In.sub.0.5P layer to be a current
blocking layer 22 and the upper surface of the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.- sub.0.5P layer by secondary
epitaxial growth based on crystal growth such as MOCVD. On the
second buffer layer 26 is formed a
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a second
upper clad layer 28. The result of this step is shown in FIG.
6.
[0068] Then, a p-GaAs layer to be a contact layer 30 is formed on
the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a
second upper clad layer 28.
[0069] In this secondary epitaxial growth, a crystal growth
temperature is about 650.degree. C. to 750.degree. C. A crystal
growth temperature as low as possible is selected to prevent a
p-type dopant Zn from diffusing from the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer as a first upper
clad layer 20 to the MQW layer to be an active layer 18 as much as
possible.
[0070] Then, a p-electrode 32 and an n-electrode 34 are formed on
the surface of the p-GaAs layer to be a contact layer 30 and the
rear surface of the n-GaAs substrate 12, respectively, to provide a
red laser LD 10 shown in FIG. 1.
[0071] There will be described operation of the red LD 10.
[0072] When applying a forward direction voltage between the
n-electrode 34 and the p-electrode 32, current flow is blocked by a
depletion layer generated by a p-n junction between the current
blocking layer 22 and the second upper clad layer 28 to reduce a
current and to make the current flow through the opening 24 to the
active layer 18.
[0073] A current higher than a given threshold to the active layer
18 causes recombination of electrons and positive holes in the
active layer 18, whereby a laser beam is emitted.
[0074] The lower clad layer 16, the first upper clad layer 20 and
the second upper clad layer 28 have a larger band gap than that of
the active layer 18. The refractive indices of lower clad layer 16,
the first upper clad layer 20 and the second upper clad layer 28
are smaller than that of the active layer 18, so that the laser
beam is enclosed between the lower clad layer 16 and the first and
the second upper clad layers 20, 28.
[0075] The band gap of the current blocking layer 22 is larger than
those of the first upper clad layer 20 and the second upper clad
layer 28, while the real refractive index of the current blocking
layer 22 is smaller than those of the first upper clad layer 20 and
the second upper clad layer 28, so that diffusion in a horizontal
lateral direction of the laser beam is inhibited by the current
blocking layer 22.
[0076] Thus, this red LD 10 has a configuration such that there are
real refractive index differences in both vertical and horizontal
directions at the emission point of the laser beam, whereby the
laser may be effectively enclosed around the emission point.
[0077] There will be discussed the reason why in a conventional red
LD 100, lattice defects may be frequently formed on the surface
facing the opening 114 in the n-AlInP layer to be a current
blocking layer 112 when forming the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a second
upper clad layer 116 on the current blocking layer 112.
[0078] FIG. 7 is a view of a transmission electron microscope (TEM)
photograph of a conventional red LD, where the photograph shows an
area around the opening 114 in a cross section perpendicular to a
light emission direction of the red LD 100.
[0079] FIG. 7 shows that many lattice defects are formed on a
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer grown on the
(111) A facet facing the opening 114 in the n-Al.sub.0.5In.sub.0.5P
layer to be a current blocking layer 112 when forming the
p-(Al.sub.0.7Ga.sub.0.3).sub.- 0.5In.sub.0.5P layer to be a second
upper clad layer 116 directly on the current blocking layer 112.
Thus, the lattice defects may develop due to the following
reason.
[0080] The surface of the opening 114 in the
n-Al.sub.0.5In.sub.0.5P layer to be a current blocking layer 112
formed by wet etching is a (111) A facet. On a (111) A facet,
crystal growth is difficult and furthermore, in the
n-Al.sub.0.5In.sub.0.5P layer to be a current blocking layer 112,
an Al content is high, which means higher susceptibility to
oxidation. Crystal defects may, therefore, develop in crystal
growth on the (111) A facet of the n-Al.sub.0.5In.sub.0.5P layer.
Furthermore, p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P is such a
complex material that deviation in a composition may generate
distortion causing crystal defects.
[0081] Thus, crystal of p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P
in which composition tends to be deviated is grown on the (111) A
facet, on which crystal growth is difficult, of the
n-Al.sub.0.5In.sub.0.5P layer which is likely to form an oxide
film. Many lattice defects may be, therefore, formed in the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer grown on the
(111) A facet of the current blocking layer 112 facing the opening
114. In particular, when growing crystal of
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P which tends to form
lattice defects due to, for example, composition deviation, crystal
defects may be formed from the initial stage in crystal growth.
[0082] In the red LD 10 according to Embodiment 1, during the
initial stage of secondary epitaxial growth before crystal-growing
of the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a
second upper clad layer 28, the layer of p-Al.sub.0.5Ga.sub.0.5As
which does not generate lattice mismatch due to composition
deviation is formed as the second buffer layer 26 on the (111) A
facet facing the opening 24 in the n-Al.sub.0.5In.sub.0.5P layer to
be a current blocking layer 22 which is most susceptible to lattice
mismatch, on which is then formed the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a second
upper clad layer 28. This can inhibit development of lattice
defects during the initial stage of growth to reduce crystal
defects in the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to
be a second upper clad layer 28.
[0083] FIG. 8 shows a view of a transmission electron microscope
(TEM) photograph of the red LD according to Embodiment 1, where the
photograph shows an area around the opening 24 in a cross section
perpendicular to a light emission direction of the red LD 10.
[0084] As seen in FIG. 8, it may be found that there are no lattice
defects in the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer
grown on the (111) A facet facing the opening 24 in the
n-Al.sub.0.5In.sub.0.5P layer to be a current blocking layer 22 by
forming the p-Al.sub.0.5Ga.sub.0.5As layer as the second buffer
layer 26 and the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer
to be a second upper clad layer 28 on the current blocking layer
22, resulting in good crystal growth.
[0085] As described above, in the red LD 10 according to Embodiment
1, crystal defects in the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a second
upper clad layer 28 may be reduced by forming, as the initial
growth layer, the p-Al.sub.0.5Ga.sub.0.5As layer as the second
buffer layer 26 on the surface of the exposed
p-(Al.sub.0.7Ga.sub.0.3).su- b.0.5In.sub.0.5P layer to be a first
upper clad layer 110 facing the opening 24, on the (111) A facet
facing the opening 24 in the n-Al.sub.0.5In.sub.0.5P layer to be a
current blocking layer 22 and on the upper surface of the
n-Al.sub.0.5In.sub.0.5P layer and forming the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a second
upper clad layer 28 on the second buffer layer 26, by secondary
epitaxial growth. It may reduce an internal loss in the red LD 10,
reduce deterioration in temperature properties in current-optical
output performance of the red LD 10 and finally improve reliability
of the red LD.
[0086] A substrate temperature during this crystal growth is
650.degree. C. to 750.degree. C. which is within a common
temperature range for crystal growth. It may reduce diffusion of a
p-type dopant Zn from the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer as the first
upper clad layer 20 to the MQW layer to be an active layer 18. It
may, therefore, prevent Zn from diffusing into the active layer in
the first upper clad layer 20 and may reduce deterioration of
temperature properties in current-optical output performance and
reliability.
[0087] Furthermore, the band gap of the p-Al.sub.0.5Ga.sub.0.5As
layer as the second buffer layer 26 is adjusted to be larger than
that of the active layer 18. A laser beam from the active layer 18
may not be, therefore, absorbed and laser properties may not be
deteriorated.
[0088] Second Embodiment
[0089] In a red LD according to Embodiment 2, a current blocking
layer made of n-AlInP having a stripe-shaped opening is disposed on
a first upper clad layer; a cap layer made of
n-Ga.sub.0.5In.sub.0.5P is disposed on the upper main surface of
the current blocking layer; and a
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P as a second upper clad
layer is disposed on the exposed first upper clad layer facing the
opening, the surface facing the opening of the current blocking
layer and the cap layer, to inhibit lattice defects formed in the
p-(Al.sub.0.7Ga.sub.0.3).- sub.0.5In.sub.0.5P as the second upper
clad layer.
[0090] FIG. 9 is a cross-sectional view of the red LD according to
Embodiment 2 of this invention.
[0091] In FIG. 9, reference numeral 40 denotes a red LD; and 42 a
cap layer as a protective layer made of n-Ga.sub.0.5In.sub.0.5P.
The same symbols as those in FIG. 1 have the same or equivalent
meanings, which is applied to Embodiments later.
[0092] In the red LD 40, on the (100) facet which is the upper
surface of the current blocking layer 22 is formed the cap layer
42, and the opening 24 penetrates the cap layer 42 and the current
blocking layer 22. The second upper clad layer is disposed on the
exposed first upper clad layer 20 facing the opening 24, the (111)
A facet of the current blocking layer 22 facing the opening 24 and
the cap layer 42.
[0093] There will be described a process for manufacturing a red LD
40.
[0094] FIGS. 10, 11 and 12 are cross-sectional views of a red LD in
individual manufacturing steps according to Embodiment 2.
[0095] On an n-GaAs substrate 12 are sequentially formed an n-GaAs
layer to be a first buffer layer 14, an
n-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.- 0.5 layer to be a lower
clad layer 16, a GaInP/AlGaInP MQW layer to be an active layer 18,
a p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a first
upper clad layer 20, an n-Al.sub.0.5In.sub.0.5P layer to be a
current blocking layer 22, and an n-Ga.sub.0.5In.sub.0.5P layer to
be a cap layer 42, by primary epitaxial growth based on crystal
growth such as MOCVD. For a dopant, an n-type dopant is silicon
while a p-type dopant is zinc. The results of this step are shown
in FIG. 10.
[0096] Then, a resist pattern 38 is formed on the surface of the
n-Ga.sub.0.5In.sub.0.5P layer to be a cap layer 42 by a
photolithographic process, and a stripe-shaped opening 24 to be a
current path is formed in the n-Ga.sub.0.5In.sub.0.5P layer to be a
cap layer 42 and the n-Al.sub.0.5In.sub.0.5P layer to be the
current blocking layer 22 by wet etching. The results of this step
are illustrated in FIG. 11.
[0097] After removing the resist pattern 38, a
p-(Al.sub.0.7Ga.sub.0.3).su- b.0.5In.sub.0.5P layer to be a second
upper clad layer 28 is formed on the surface of the exposed
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a first
upper clad layer 20 facing the opening 24, the (111) A facet facing
the opening 24 in the n-Al.sub.0.5In.sub.0.5P layer to be a current
blocking layer 22 and the upper surface of the
n-Ga.sub.0.5In.sub.0.5P layer to be a cap layer 42, by secondary
epitaxial growth based on crystal growth such as MOCVD. The results
of this step are shown in FIG. 12.
[0098] Then, a p-GaAs layer to be a contact layer 30 is formed on
the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a
second upper clad layer 28.
[0099] In this secondary epitaxial growth, a crystal growth
temperature is about 650.degree. C. to 750.degree. C. A crystal
growth temperature as low as possible is selected to prevent a
p-type dopant Zn from diffusing from the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer as a first upper
clad layer 20 to the MQW layer to be an active layer 18 as much as
possible.
[0100] Then, a p-electrode 32 and an n-electrode 34 are formed on
the surface of the p-GaAs layer to be a contact layer 30 and the
rear surface of the n-GaAs substrate 12, respectively, to provide a
red laser LD 40 shown in FIG. 9.
[0101] In the red LD 40, the cap layer 42 made of
n-Ga.sub.0.5In.sub.0.5P which is antioxidative because of absence
of Al is disposed on the n-Al.sub.0.5In.sub.0.5P layer to be a
current blocking layer 22, via which is formed the second upper
clad layer 28 made of p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P.
It may reduce crystal defects in the second upper clad layer 28
which is susceptible to lattice defects due to its tendency to
composition deviation to provide a reliable red LD 40.
[0102] Third Embodiment
[0103] In a red LD according to Embodiment 3, a current blocking
layer made of n-AlInP having a stripe-shaped opening is disposed on
a first upper clad layer; a cap layer made of
n-Ga.sub.0.5In.sub.0.5P is disposed on the upper main surface of
the current blocking layer; and the exposed first upper clad layer
facing the opening, the surface facing the opening of the current
blocking layer and the cap layer are covered by a
p-Al.sub.0.5Ga.sub.0.5As buffer layer, on which is then disposed a
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P as a second upper clad
layer, to inhibit lattice defects formed in the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5- In.sub.0.5P as the second clad
layer.
[0104] FIG. 13 is a cross-sectional view of the red LD according to
Embodiment 3 of this invention.
[0105] In FIG. 13, reference numeral 46 denotes a red LD according
to Embodiment 3; and 42 a cap layer as a protective layer made of
n-Ga.sub.0.5In.sub.0.5P.
[0106] The red LD 46 has the configuration as described for the red
LD 10 in Embodiment 1, except that the n-Ga.sub.0.5In.sub.0.5P as a
cap layer 42 is disposed on the n-Al.sub.0.5In.sub.0.5P layer to be
a current blocking layer 22 by primary epitaxial growth.
[0107] There will be described the reason why the
n-Ga.sub.0.5In.sub.0.5P layer as a cap layer 42 is formed. The
n-Al.sub.0.5In.sub.0.5P layer to be a current blocking layer 22 is
susceptible to oxidization due to its higher Al contents. The
surface of the n-Al.sub.0.5In.sub.0.5P layer to be a current
blocking layer 22 is a crystal facet in which crystal growth is
easier than the (111) A facet in the n-Al.sub.0.5In.sub.0.5P layer
facing the opening 24. Nonetheless, an oxide film formed may cause
lattice defects during epitaxial growth of
p-(Al.sub.0.7Ga.sub.0.3).sub.0- .5In.sub.0.5P as the second upper
clad layer.
[0108] The n-Ga.sub.0.5In.sub.0.5P layer as the cap layer 42 is,
therefore, disposed on the (100) facet in the
n-Al.sub.0.5In.sub.0.5P layer to be a current blocking layer 22 to
prevent the formation of oxide film and inhibit the development of
lattice defects during epitaxial growth of
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P as the second upper
clad layer. Furthermore, on the (111) A facet facing the opening 24
in the n-Al.sub.0.5In.sub.0.5P layer to be a current blocking layer
22 is formed the p-Al.sub.0.5Ga.sub.0.5As layer as a second buffer
layer 26, on which is then formed the
p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to be a second
upper clad layer 28. This can inhibit development of lattice
defects during the initial stage of growth to reduce crystal
defects in the p-(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P layer to
be a second upper clad layer 28.
[0109] The red LD 46, therefore, exhibits higher laser reliability
and yield than the red LD 10.
[0110] Although the conductivity type of the cap layer 42 has been
n-type in Embodiments 2 and 3, it may be p-type or of higher
resistance, which may provide similar effects.
[0111] In these Embodiments, the clad layer may, of course, have a
different composition.
[0112] There may be provided the following effects by a
semiconductor laser device according to this invention or a
manufacturing process thereof which has the above configuration or
steps.
[0113] A semiconductor laser device according to this invention
comprises a first conductivity type of semiconductor substrate; a
first conductivity type of first clad layer made of a III-V group
compound semiconductor disposed on the semiconductor substrate; an
active layer made of a III-V group compound semiconductor having a
smaller band gap than the first clad layer, disposed on the first
clad layer; a second conductivity type of first second-clad layer
made of a III-V group compound semiconductor having a larger band
gap than the active layer, disposed on the active layer; a first
conductivity type of current blocking layer made of a III-V group
compound semiconductor having a larger band gap than the active
layer, disposed on the first second-clad layer and having a
stripe-shaped opening to be a current path; a second conductivity
type of buffer layer made of a III-V group compound semiconductor
having a larger band gap than the active layer, disposed on the
surface of the current blocking layer facing the opening; and a
second conductivity type of second second-clad layer made of a
III-V group compound semiconductor having a larger band gap than
the active layer, disposed on the first second-clad layer facing
the opening and the current blocking layer via the buffer layer. In
this semiconductor laser device, lattice defects in the second
second-clad layer disposed on the surface of the current blocking
layer facing the opening via the buffer layer may reduce, and it
can be prevented from a second conductivity type of dopant
diffusing from the first second-clad layer to the active layer,
thus the construction according to the present invention making it
possible to reduce deterioration in temperature properties in
current-optical output performance, and consequently to improve
reliability of the semiconductor laser device.
[0114] In the semiconductor laser device wherein the current
blocking layer, the buffer layer and the second second-clad layer
are made of an AlInP material, an AlGaAs material and an AlGaInP
material, respectively, lattice defects may be effectively reduced
in the second second-clad layer disposed on the surface of the
current blocking layer via the buffer layer, leading to improvement
in reliability of the semiconductor laser device with a red LD.
[0115] In the semiconductor laser device wherein a protective layer
made of a GaInP material is further disposed between the surface of
the current blocking layer except the surface facing the opening
and the second second-clad layer, a risk of lattice defect
formation may be reduced in the second second-clad layer made of an
AlGaInP material whose composition tends to be deviated, even on
the main surface of the current blocking layer except the surface
facing the opening, and may allow us to provide a semiconductor
laser device with a red LD with a higher yield.
[0116] A semiconductor laser device according to this invention
comprises a first conductivity type of semiconductor substrate; a
first conductivity type of first clad layer made of a III-V group
compound semiconductor disposed on the semiconductor substrate; an
active layer made of a III-V group compound semiconductor having a
smaller band gap than the first clad layer, disposed on the first
clad layer; a second conductivity type of first second-clad layer
made of a III-V group compound semiconductor having a larger band
gap than the active layer, disposed on the active layer; a first
conductivity type of current blocking layer made of an AlInP
material having a larger band gap than the active layer, disposed
on the first second-clad layer and having a stripe-shaped opening
to be a current path; a protective layer made of a GaInP material
disposed on the surface of the current blocking layer except the
surface facing the opening; and a second conductivity type of
second second-clad layer made of an AlGaInP material having a
larger band gap than the active layer, disposed on the current
blocking layer via the protective layer and the first second-clad
layer facing the opening. In this device, the current blocking
layer made of the AlInP material is protected by the GaInP material
not containing Al and the second second-clad layer made of the
AlGaInP material whose composition tends to be deviated is disposed
on the crystal facet with a reduced amount of oxide film to further
reduce a risk of lattice defect formation, leading to improvement
in reliability of a semiconductor laser device with a red LD.
[0117] A process for manufacturing a semiconductor laser device
according to this invention also includes the steps of depositing a
first conductivity type of first clad layer made of a III-V group
compound semiconductor, an active layer made of a III-V group
compound semiconductor having a smaller band gap than the first
clad layer, a second conductivity type of first second-clad layer
made of a III-V group compound semiconductor having a larger band
gap than the active layer, and a first conductivity type of current
blocking layer made of a III-V group compound semiconductor having
a larger band gap than the active layer on the first second-clad
layer in order, on a first conductivity type of semiconductor
substrate; forming a stripe-shaped opening penetrating the current
blocking layer; forming a second conductivity type of buffer layer
made of a III-V group compound semiconductor having a larger band
gap than the active layer on the surface of the current blocking
layer facing the opening; and forming a second conductivity type of
second second-clad layer made of a III-V group compound
semiconductor having a larger band gap than the active layer on the
first second-clad layer facing the opening and the current blocking
layer via the buffer layer, and may allow the crystal of the second
second-clad layer to grow with reduced lattice defects on the
surface of the current blocking layer facing the opening via the
buffer layer, even at a common substrate temperature. Such a common
substrate temperature may reduce diffusion of a second conductivity
type of dopant from the first second-clad layer to the active layer
during crystal growth.
[0118] Accordingly, a process for manufacturing a semiconductor
laser device according to this invention may allow us to
manufacture a reliable semiconductor laser device with reduced
deterioration in temperature properties in current-optical output
performance by simple steps and consequently to provide, with a low
price, a semiconductor laser device exhibiting excellent laser
properties.
[0119] While the presently preferred embodiments of the present
invention have been shown and described. It is to be understood
these disclosures are for the purpose of illustration and that
various changes and modifications may be made without departing
from the scope of the invention as set forth in the appended
claims.
[0120] The entire disclosure of a Japanese Patent Application No.
2000-330117, filed on Oct. 30, 2000 including specification,
claims, drawings and summary, on which the Convention priority of
the present application is based, are incorporated herein by
reference in its entirety.
* * * * *