U.S. patent application number 10/641462 was filed with the patent office on 2004-02-19 for semiconductor laser device and method of fabricating the same.
This patent application is currently assigned to KABUSHIKI KAISHI TOSHIBA. Invention is credited to Gen-Ei, Koichi, Okada, Makoto.
Application Number | 20040032895 10/641462 |
Document ID | / |
Family ID | 18588638 |
Filed Date | 2004-02-19 |
United States Patent
Application |
20040032895 |
Kind Code |
A1 |
Okada, Makoto ; et
al. |
February 19, 2004 |
Semiconductor laser device and method of fabricating the same
Abstract
A monolithic two-wavelength semiconductor laser device includes
a front end face film 19 on a resonator front end face 18, and a
high-reflectivity end face film 22 as a multilayered film on a
resonator rear end face 21. The front end face film 19 is formed
using a low-refractive-index material, and the film thickness is so
set that the reflectivity is 20%. The high-reflectivity end face
film 22 is formed by alternately stacking thin films of low- and
high-refractive-index materials, and the film thickness is so set
that the reflectivity is 80%. The film thickness of each of these
two end face films is calculated by an optical length
d=(1/4+j).times..lambda.m by using a mean value
.lambda.m=(.lambda.1+.lambda.2)/2 of the oscillation wavelengths of
the two semiconductor laser diodes. This makes it possible to
obtain an end face film having a desired reflectivity and capable
of being formed at once, and to fabricate a two-wavelength
semiconductor laser device having high reliability, meeting the
required performance, and also having high productivity.
Inventors: |
Okada, Makoto;
(Ichikawa-Shi, JP) ; Gen-Ei, Koichi;
(Ichikawa-Shi, JP) |
Correspondence
Address: |
HOGAN & HARTSON L.L.P.
500 S. GRAND AVENUE
SUITE 1900
LOS ANGELES
CA
90071-2611
US
|
Assignee: |
KABUSHIKI KAISHI TOSHIBA
|
Family ID: |
18588638 |
Appl. No.: |
10/641462 |
Filed: |
August 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10641462 |
Aug 14, 2003 |
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09808267 |
Mar 14, 2001 |
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6628689 |
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Current U.S.
Class: |
372/50.1 |
Current CPC
Class: |
H01S 5/0287 20130101;
H01S 5/4031 20130101; H01S 5/4087 20130101 |
Class at
Publication: |
372/50 |
International
Class: |
H01S 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2000 |
JP |
2000-69820 |
Claims
What is claimed is:
1. A semiconductor laser device comprising: a substrate; a first
laser element portion formed on said substrate to oscillate laser
light having a first wavelength; a second laser element portion
formed on said substrate to oscillate laser light having a second
wavelength; a front end face film formed at once on front end faces
of said first and second laser element portions and having a
uniform film thickness; and a rear end face film formed at once on
rear end faces of said first and second laser element portions,
having a uniform film thickness, and comprising a plurality of thin
films, wherein the film thickness of said front end face film and
said plurality of thin films of said rear end face film have an
optical length d=(1/4+j).times..lambda.(j=0, 1, 2, . . . ) with
respect to a mean wavelength .lambda. of the first and second
wavelengths.
2. A device according to claim 1, wherein said front end face film
has a reflectivity of 3 to 37%, and said rear end face film has a
reflectivity of not less than-75%.
3. A device according to claim 1, wherein said front end face film
is made of a low-refractive-index material having a refractive
index n<1.8, and said rear end face film comprises stacked
layers of thin films made of a low-refractive-index material having
a refractive index n<1.8 and thin films made of a
high-refractive-index material having a refractive index
n>1.9.
4. A device according to claim 1, wherein said front end face film
is made of Al.sub.2O.sub.3, and said rear end face film comprises
stacked layers of thin films made of a low-refractive-index
material selected from the group consisting of Al.sub.2O.sub.3 and
SiO.sub.2 and thin films made of a high-refractive-index material
selected from the group consisting of SiN.sub.4 and Si.
5. A device according to claim 3, wherein said front end face film
is made of Al.sub.2O.sub.3, and said rear end face film comprises
stacked layers of thin films made of a low-refractive-index
material selected from the group consisting of Al.sub.2O.sub.3 and
SiO.sub.2 and thin films made of a high-refractive-index material
selected from the group consisting of SiN.sub.4 and Si.
6. A semiconductor laser device fabrication method comprising the
steps of: forming, on a substrate, a first laser element portion
which oscillates laser light having a first wavelength; forming, on
said substrate, a second laser element portion which oscillates
laser light having a second wavelength; forming a front end face
film having a uniform film thickness at once on front end faces of
said first and second laser element portions by using ECR
sputtering; and forming a rear end face film having a uniform film
thickness and comprising a plurality of thin films at once on rear
end faces of said first and second laser element portions by using
ECR sputtering.
7. A method according to claim 6, wherein the step of forming said
front end face film comprises forming a film having a reflectivity
of 3 to 37%, and the step of forming said rear end face film
comprises forming a film having a reflectivity of not less than
75%.
8. A method according to claim 6, wherein the step of forming said
front end face film comprises forming a film by using a
low-refractive-index material having a refractive index n<1.8,
and the step of forming said rear end face film comprises forming a
film which comprises stacked layers of thin films made of a
low-refractive-index material having a refractive index n<1.8
and thin films made of a high-refractive-index material having a
refractive index n>1.9.
9. A method according to claim 6, wherein the step of forming said
front end face film comprises forming an Al.sub.2O.sub.3 film, and
the step of forming said rear end face film comprises forming
stacked layers of thin films made of a low-refractive-index
material selected from the group consisting of Al.sub.2O.sub.3 and
SiO.sub.2 and thin films made of a high-refractive-index material
selected from the group consisting of SiN.sub.4 and Si.
10. A method according to claim 8, wherein the step of forming said
front end face film comprises forming an Al.sub.2O.sub.3 film, and
the step of forming said rear end face film comprises forming
stacked layers of thin films made of a low-refractive-index
material selected from the group consisting of Al.sub.2O.sub.3 and
SiO.sub.2 and thin films made of a high-refractive-index material
selected from the group consisting of SiN.sub.4 and Si.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of priority under 35USC
.sctn.119 to Japanese Patent Application No. 2000-69820, filed on
Mar. 14, 2000, the entire contents of which are incorporated by
reference herein.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a two-wavelength laser
device which includes a front end face film and a high-reflectivity
multilayered film and performs two-wavelength oscillation.
[0003] Optical disk systems currently put into practical use are
roughly classified into a system which records and reproduces data
into and from a compact disc and a DVD system which performs data
recording and reproduction at higher density. An optical
semiconductor laser used for a compact disc recording medium has an
oscillation wavelength of 780 nm. An optical semiconductor laser
used in the DVD system has an oscillation wavelength of 650 nm. To
obtain a high optical output, each of these optical semiconductor
lasers has a front end face anti-reflectivity film and a rear end
face high-reflectivity film on its end faces, thereby efficiently
extracting light, emitted from the rear surface of a resonator,
from the front surface. The thicknesses of these front end face
anti-reflectivity film and rear end face high-reflectivity film are
calculated on the basis of the oscillation wavelength of each
laser.
[0004] Recently, disk apparatuses including a high-density
recording medium such as a DVD in addition to a CD-R, CD-RW, and
the like have appeared. Some disk apparatuses of this type
incorporate both an optical semiconductor laser having an
oscillation wavelength of 780 nm and an optical semiconductor laser
having an oscillation wavelength of 650 nm. However, since optical
systems are required to shrink as disk apparatuses are
miniaturized, two-wavelength lasers including two resonators having
the above-mentioned two oscillation wavelengths in a single crystal
structure are most often used.
[0005] In this two-wavelength laser, however, the film thicknesses
of the front end face anti-reflectivity film and rear end face
high-reflectivity film must be respectively matched with their
wavelengths .lambda.. This introduces inconveniences to the
fabrication steps. FIGS. 32 and 33 show end face film formation
steps relevant to the present invention. As shown in FIG. 32,
semiconductor laser diodes having an oscillation frequency of 650
nm and semiconductor laser diodes having an oscillation frequency
of 780 nm are alternately formed on a single chip. The front end
faces of the 650-nm laser diodes 51 are exposed, and their other
portions and the 780-nm laser diodes are covered with a mask 52. A
single-layer reflecting film 54 is formed by sputtering on a laser
emission portion 53 of each exposed end face. The film thickness of
this single-layer reflecting film 54 is calculated on the basis of
an oscillation frequency of 650 nm. Subsequently, as shown in FIG.
33, the mask 52 is moved to expose laser emission regions 56 at the
front end faces of the 780-nm laser diodes 55, in order to form a
thin film having a predetermined film thickness on these front end
face emission regions 56. After that, a single-layer reflecting
film having a film thickness calculated on the basis of an
oscillation frequency of 780 nm is formed on the exposed
portions.
[0006] In the above fabrication steps, the spacing between the two
semiconductor lasers is set to around 100 .mu.m in accordance with
the effective dimensions of these optical semiconductor elements
and the requirements of an optical system into which these optical
semiconductor elements are incorporated. Therefore, the fabrication
method which forms end face films by using the mask 52 is
inefficient because the method requires highly accurate
microfabrication. The working efficiency is also low because thin
film formation is performed for each semiconductor laser. The
working efficiency is similarly low when an end face
high-reflectivity film is formed on the rear end face of each
semiconductor laser. Furthermore, the mask 52 is very difficult to
move since the planarity of the element surface is disturbed by the
multilayered thin films already formed.
[0007] As a method of forming thin films without using any
shielding masks, a thin film formation method using optical CVD or
the like described in patent gazette (Patent No. 2862037) is used.
However, this method has the following problems. In the method
using optical CVD, as shown in FIG. 34, a light amount control ND
filter 62 for controlling the thin film growth rate is placed
between a light source and the end face of a laser diode 61 on
which a thin film is to be formed. Light emission regions 53 and 56
having the different oscillation wavelengths as described above are
arranged with fine intervals between them. Therefore, lights 63
passing through the ND filter 62 must exactly irradiate desired
light emission regions 53 and 56. Accordingly, the ND filter 62 and
the light emission regions 53 and 56 require an extremely high
level of positional adjustment. Any adjustment difference produces
an error in thin film formation by a change in the light amount,
and this greatly lowers the productivity.
[0008] Also, in the two formation steps described above, the
structures of jigs and the mechanism of a reaction tank inside the
film fabrication apparatus are elaborated in the process of forming
films on optical semiconductor lasers. This degrades the
flexibility of the apparatus.
SUMMARY OF THE INVENTION
[0009] It is, therefore, an object of the present invention to
provide a two-wavelength semiconductor laser device having high
reliability, meeting the necessary performance, and capable of
forming a high-productivity end face reflecting film at one
time.
[0010] A semiconductor laser device of the present invention is
characterized by comprising a substrate, a first laser element
portion formed on the substrate to oscillate laser light having a
first wavelength, a second laser element portion formed on the
substrate to oscillate laser light having a second wavelength, a
front end face film formed at once on front end faces of the first
and second laser element portions and having a uniform film
thickness, and a rear end face film formed at once on rear end
faces of the first and second laser element portions, having a
uniform film thickness, and comprising a plurality of thin films,
wherein the film thickness of the front end face film and the
plurality of thin films of the rear end face film have an optical
length d=(1/4+j).times..lambda. (j=0, 1, 2, . . . ) with respect to
a mean wavelength .lambda. of the first and second wavelengths.
This device is characterized in that the front end face film has a
reflectivity of 3 to 37%, and the rear end face film has a
reflectivity of not less than 75%. The device is also characterized
in that the front end face film is made of a low-refractive-index
material having a refractive index n<1.8, and the rear end face
film comprises stacked layers of thin films made of a
low-refractive-index material having a refractive index n<1.8
and thin films made of a high-refractive-index material having a
refractive index n>1.9. Furthermore, the front end face film is
made of Al.sub.2O.sub.3, and the rear end face film comprises
stacked layers of thin films made of Al.sub.2O.sub.3 or SiO.sub.2
as a low-refractive-index material and thin films made of SiN.sub.4
or Si as a high-refractive-index material.
[0011] A semiconductor laser device fabrication method of the
present invention comprises the steps of forming, on a substrate, a
first laser element portion which oscillates laser light having a
first wavelength, forming, on the substrate, a second laser element
portion which oscillates laser light having a second wavelength,
forming a front end face film having a uniform film thickness at
once on front end faces of the first and second laser element
portions by using ECR sputtering, and forming a rear end face film
having a uniform film thickness and comprising a plurality of thin
films at once on rear end faces of the first and second laser
element portions by using ECR sputtering.
[0012] The present invention can provide a semiconductor laser
device having high reliability, meeting the necessary performance,
and also having high productivity. It is also possible to provide a
semiconductor laser device fabrication method capable of forming an
end face film at once and thereby capable of reducing the number of
fabrication steps and saving the space of the film formation
apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic view showing the structure of a
two-wavelength semiconductor laser of the present invention;
[0014] FIG. 2 is a sectional view showing the stacked structure of
a laser diode having an oscillation wavelength of 650 nm according
to the first embodiment of the present invention;
[0015] FIG. 3 is a view showing the energy bandgap of the laser
diode having an oscillation wavelength of 650 nm according to the
first embodiment of the present invention;
[0016] FIG. 4 is a sectional view showing the stacked structure of
a laser diode having an oscillation wavelength of 780 nm according
to the first embodiment of the present invention;
[0017] FIG. 5 is a view showing the energy bandgap of the laser
diode having an oscillation wavelength of 780 nm according to the
first embodiment of the present invention;
[0018] FIG. 6 is a view showing the end face film structure of the
two-wavelength semiconductor laser according to the first
embodiment of the present invention;
[0019] FIG. 7 is a graph showing changes in reflectivity of a front
end face film at different wavelengths according to the first
embodiment of the present invention;
[0020] FIG. 8 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the first
embodiment of the present invention;
[0021] FIG. 9 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the first
embodiment of the present invention;
[0022] FIG. 10 is a graph showing changes in reflectivity of the
high-reflectivity end face film at different wavelengths according
to the first embodiment of the present invention;
[0023] FIG. 11 is a graph showing changes in reflectivity for
different wavelengths of the high-reflectivity end face film
according to the first embodiment of the present invention;
[0024] FIG. 12 is a graph showing changes in reflectivity for
different wavelengths of the high-reflectivity end face film
according to the first embodiment of the present invention;
[0025] FIG. 13 is a view showing the end face film structure of a
two-wavelength semiconductor laser according to the second
embodiment of the present invention;
[0026] FIG. 14 is a sectional view showing the stacked structure of
a laser diode having an oscillation wavelength of 780 nm according
to the second embodiment of the present invention;
[0027] FIG. 15 is a view showing the energy bandgap of the laser
diode having an oscillation wavelength of 780 nm according to the
second embodiment of the present invention;
[0028] FIG. 16 is a graph showing changes in reflectivity of a
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0029] FIG. 17 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0030] FIG. 18 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0031] FIG. 19 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0032] FIG. 20 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0033] FIG. 21 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0034] FIG. 22 is a graph showing changes in reflectivity of the
high-reflectivity end face film at different wavelengths according
to the second embodiment of the present invention;
[0035] FIG. 23 is a graph showing changes in reflectivity for
different wavelengths of the high-reflectivity end face film
according to the second embodiment of the present invention;
[0036] FIG. 24 is a graph showing changes in reflectivity for
different wavelengths of the high-reflectivity end face film
according to the second embodiment of the present invention;
[0037] FIG. 25 is a view showing the end face film structure of a
two-wavelength semiconductor laser according to the third
embodiment of the present invention;
[0038] FIG. 26 is a graph showing changes in reflectivity of a
front end face film at different wavelengths according to the third
embodiment of the present invention;
[0039] FIG. 27 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the third
embodiment of the present invention;
[0040] FIG. 28 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0041] FIG. 29 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0042] FIG. 30 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0043] FIG. 31 is a graph showing changes in reflectivity of the
front end face film at different wavelengths according to the
second embodiment of the present invention;
[0044] FIG. 32 is a view showing end face film formation steps
relevant to the present invention;
[0045] FIG. 33 is a view showing end face film formation steps
relevant to the present invention; and
[0046] FIG. 34 is a view showing end face film formation steps
relevant to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0047] In a monolithic two-wavelength semiconductor laser device
including two oscillation wavelength regions, i.e., an oscillation
wavelength .lambda.1 of 650 nm and an oscillation wavelength
.lambda.2 of 730 nm on a single chip, when a laser diode having the
oscillation wavelength .lambda.1 is used for a DVD-ROM and a laser
diode having the oscillation wavelength .lambda.2 is used for a
CD-ROM, the present invention can obtain a light oscillation output
of 10 mW at a temperature of a maximum of 70.degree. C. equally for
these two laser diodes. For this purpose, end face films of the two
laser diodes are characterized by having a film thickness
calculated by using a mean value .lambda.m=(.lambda.1+.lambda.2-
)/2 of the two oscillation wavelengths as a design numerical value.
Since the end face films of the two laser diodes have the same film
thickness, these films can be formed on the two laser diodes at
once. Accordingly, it is possible to provide a high-productivity
monolithic two-wavelength laser device by simplifying the
fabrication steps, e.g., obviating the need for dedicated jig sets
corresponding to film formation in different wavelength bands.
[0048] Embodiments of the present invention will be described below
with reference to the accompanying drawings.
[0049] A two-wavelength semiconductor device according to the first
embodiment of the present invention will be described. FIG. 1 is a
schematic view showing the two-wavelength semiconductor laser
device according to the first embodiment. A 650-nm band laser diode
and a 780-nm band laser diode are formed on the same substrate. A
laser emission region 1 of the 650-nm band laser diode and a laser
emission region 2 of the 780-nm band laser diode are physically
separated by wet etching using an acidic solution or by dry gas
etching or the like. Since this facilitates independently driving
the two laser diodes, this two-wavelength semiconductor laser
device functions as a CD or DVD laser light source optimum for a
limited space. That is, it is possible to prolong the operating
life by eliminating the influence of heat generation which the two
laser diodes have on each other, and to save energy by eliminating
any electrical leak.
[0050] FIG. 2 shows the stacked structure of the 650-nm band laser
diode having the laser emission region 1. An n-GaAs buffer layer 4,
n-InGaAlP first cladding layer 5, an InGaAl light guide layer 6, an
InGaAl/InGaAlP multiple quantum well active layer 7, an InGaAl
light guide layer 8, a p-InGaAlP second cladding layer 9, and an
InGaP etching stop layer 10 are sequentially formed on an n-GaAs
substrate 3. A ridge-shaped p-InGaAlP third cladding layer 11 is
formed on the etching stop layer 10. An n-GaAs current blocking
layer 12 is formed on the two sides of this third cladding layer
11, and a p-InGaP easy current passing layer 13 is formed on top of
the third cladding layer 11. A p-GaAs contact layer 14 is formed on
the current blocking layer 12 and on the easy current passing layer
13. A p-type electrode 15 is formed on the contact layer 14, and an
n-type electrode 16 is formed on the lower surface of the n-GaAs
substrate 3. The optical semiconductor laser of this embodiment has
an SBR (Selective Buried Ridge) structure by which laser diodes
capable of maintaining a single transverse mode at high output can
be formed with high productivity by setting the ridge width to 3 to
6 .mu.m or less. This allows a condenser lens or the like to
converge oscillated laser light to a narrow stop area on an optical
disk. Hence, this laser diode is suitable as a high-output laser
diode for optical disks.
[0051] FIG. 3 shows the Al compositions near the multiple quantum
well active layer 7 of the 650-nm band laser diode. The abscissa
indicates the individual layers, and the ordinate indicates the Al
compositions. The Al composition of the cladding layers 5, 9, and
11 is 0.7. The Al composition of barrier layers in the light guide
layers 6 and 8 and the active layer 7 is 0.5. The Al composition of
well layers in the active layer 7 is 0.15. A well layer thickness
L.sub.w is 3 to 8 nm, a barrier layer thickness L.sub.b is 2 to 5
nm, a light guide layer thickness is 10 to 40 .mu.m, and the
divergence angle is 20 to 25.degree.. As a consequence, an optical
output having a CW oscillation of 30 mw or more can be stably
obtained.
[0052] FIG. 4 shows the stacked structure of the 780-nm band laser
diode having the laser emission region 2. An n-GaAs buffer layer 4,
an n-InGaAlP first cladding layer 5, an InGaAl light guide layer 6,
a bulk structure GaAlAs active layer 17, an InGaAl light guide
layer 8, a p-InGaAlP second cladding layer 9, and an InGaP etching
stop layer 10 are sequentially formed on an n-GaAs substrate 3. A
ridge-shaped p-InGaAlP third cladding layer 11 is formed on the
etchings top layer 10. An n-GaAs current blocking layer 12 is
formed on the two sides of this third cladding layer 11, and a
p-InGaP easy current passing layer 13 is formed on top of the third
cladding layer 11. A p-GaAs contact layer 14 is formed on the
current blocking layer 12 and on the easy current passing layer 13.
A p-type electrode 15 is formed on the contact layer 14, and an
n-type electrode 16 is formed on the lower surface of the n-GaAs
substrate 3.
[0053] FIG. 5 shows the Al compositions near the active layer 17 of
the 780-nm band laser diode. The active layer has a single-layer
well structure having an Al composition of 0.1 to 0.2 and a layer
thickness of 0.01 to 1 .mu.m. This structure can reduce the bandgap
discontinuity and obtain high productivity. Also, by adjusting the
Al composition and active layer thickness within the above ranges,
it is possible to obtain a 780-nm oscillation laser diode having a
horizontal divergence angle=70 to 100 and a vertical divergence
angle=200 to 300 suited to optical disks, and also having high
reliability.
[0054] As described above, InGaAlP is used as the cladding layers
in both the 650-nm and the 780-nm band laser diodes of the
two-wavelength laser according to this embodiment. Therefore, the
ridges of these two elements can be simultaneously formed by
patterning, and the directions of the two laser beams can be
matched with high accuracy substantially equal to the ridge mask
patterns.
[0055] FIG. 6 shows the end face film structure as the
characteristic feature of the present invention. A resonator front
end face 18 of a semiconductor laser device 20 has a front end face
film 19 having a reflectivity of 20%. A resonator rear end face 21
has a multilayered, high-reflectivity end face film 22 having a
reflectivity of 80%. The combination of the reflectivities of these
end face films 19 and 22 has high reliability and can obtain a
monitor current required for a DVD-ROM laser. The first embodiment
is characterized in that the film thicknesses of the front end face
film 19 and the high-reflectivity end face film 22 is calculated on
the basis of the mean value of the oscillation wavelengths of the
two laser diodes.
[0056] The front end face film 19 is made of a low-refractive-index
material having a refractive index n<1.8, preferably 1.7 or
less. By the use of this low-refractive-index material, a
single-layer film having a relatively large film thickness and high
controllability can be obtained, and high productivity can be
obtained. Al.sub.2O.sub.3 is preferred as this low-refractive-index
material. Since a linear expansion coefficient of 6.6 of
Al.sub.2O.sub.3 is close to a linear expansion coefficient of 7.7
of GaAs, the Al.sub.2O.sub.3 front end face film 19 readily gets
intimate with and is highly adhesive to a device containing GaAs as
its main constituent substance. The film thickness of this front
end face film 19 is calculated by an optical film thickness
d=(1/4+j).times..lambda.m (j=0, 1, 2, . . . ) by using the mean
oscillation wavelength .lambda.m=(.lambda.1+.lambda.2)/2 of the
laser diode having the oscillation wavelength .lambda.1=780 nm and
the laser diode having the oscillation wavelength .lambda.2=650 nm,
i.e., .lambda.m=715 nm. By the use of the mean wavelength, the
front end face film 19 having a uniform film thickness and a
reflectivity of 20% can be formed on the two front end faces of the
650-nm oscillation laser diode and the 780-nm oscillation laser
diode. This front end face film 19 is formed at once by ECR
(Electron Cyclotron Resonance) sputtering. ECR sputtering is
proposed by Japanese Patent Laid-Open No. 9-162496 and reference
(Kiyotake Tanaka et al., Manuscripts for The 44th Applied Physics
Related Joint Lecture Meeting, 31-NG-7, 1997). This method can
simplify the fabrication steps and obviates the need for a
conventional large-scaled film formation system. Also, since the
film is formed at one time, film formation which reduces damage to
the laser emission regions is possible. Consequently, a highly
reliable semiconductor laser device can be provided.
[0057] The resonator rear end face 21 of the semiconductor laser
device has the high-reflectivity end face film 22. This
high-reflectivity end face film 22 is formed by sequentially
stacking, from the resonator rear end face 21, a first film 23 made
of the same material as the front end face film 19, a second film
24 made of a high-refractive-index material having a refractive
index of 2.0 or more, a third film 25 made of a
low-refractive-index material having a refractive index of 1.7 or
less, a fourth film 26 made of a high-refractive-index material
having a refractive index of 2.0 or more, and a fifth film 27 using
the low-refractive-index material of the third film 25. The
reflectivity of this high-reflectivity end face film 22 is 75 to
85%, preferably 80%. The materials of the high-reflectivity end
face film 22 are preferably Al.sub.2O.sub.3, SiN.sub.2, SiO.sub.2,
and Si. These materials are stacked in the order of
Al.sub.2O.sub.3, SiN.sub.2, SiO.sub.2, Si, and SiO.sub.2 from the
resonator rear end face. The refractive indices of Al.sub.2O.sub.3
and SiO.sub.2 as low-refractive-index materials are 1.7 and 1.5,
respectively. The refractive indices of Si and SiN.sub.2 as
high-refractive-index materials are 4.5 and 2.0, respectively. The
film thicknesses of these low-refractive-index material film and
high-refractive-index material films are calculated by the optical
film thickness d=(1/4+j).times.(j=0, 1, 2, . . . ) by using the
means oscillation wavelength .lambda.m=(.lambda.1+.lambda.2)/2 of
the laser diode oscillation wavelengths .lambda.1=780 nm and
.lambda.2=650 nm, i.e., .lambda.m=715 nm. By the use of the mean
wavelength, the high-reflectivity end face film 22 having a uniform
film thickness can be formed at one time on the two end faces of
the 650-nm oscillation laser diode and the 780-nm oscillation laser
diode. By setting the number of thin-film stacked layers of the
high-reflectivity end face film 22 to 5 or less, problems caused by
an increase in the number of stacked layers can be eliminated. That
is, it is possible to reduce stress between the high-reflectivity
end face film 22 and a semiconductor layer or between the thin-film
layers in the high-reflectivity end face film 22, to prevent film
peeling, and to prevent deterioration of the device end face. Also,
the high-reflectivity end face film 22 obtains a desired
reflectivity of 80% by adding one Si layer having light absorbing
properties and yet having a high refractive index. The film
thickness of the outermost fifth film 27 is so adjusted that the
added reflectivity is 80%.
[0058] The method of forming the high-reflectivity end face film 22
of this embodiment uses ECR sputtering, as in the formation of the
front end face film 19. By this method, a film of a desired one of
thin-film materials Al.sub.2O.sub.3, SiO.sub.2, Si, and SiN.sub.4
can be formed by using two targets, i.e., Al and Si, and
appropriately switching material gases O.sub.2 and N.sub.2
necessary in the film formation. Compared to a process which forms
films by using dedicated targets for individual materials, the
productivity can be dramatically improved.
[0059] The combination of a reflectivity of 20% of the front end
face film 19 and a reflectivity of 80% of the high-reflectivity end
face film 22 is highly reliable and makes it possible to obtain a
monitor current required for a DVD-ROM laser. Also, even when the
film thicknesses of the front end face film 19 and the
high-reflectivity end face film 22 more or less vary, their
reflectivities do not deviate from their predetermined values.
Therefore, end face films having very high productivity can be
provided. On the other hand, if the film thicknesses of the front
end face film 19 and the high-reflectivity end face film 22 are not
calculated by using the mean wavelength, i.e., if they are
calculated using the oscillation wavelengths .lambda.1=650 nm and
.lambda.2=780 nm, the reflectivities vary with variations in the
film thicknesses. Accordingly, no reliable semiconductor laser
device can be provided. This will be verified below.
[0060] FIGS. 7 to 9 show the results of trial calculations of
reflectivities as functions of variations in the film thickness of
the front end face film 19. FIG. 7 shows changes in reflectivities
when the film thickness is calculated on the basis of a design
wavelength of 715 nm to obtain a reflectivity R=20%. When the film
thickness has no variation, a reflectivity R.sub.650 of the 650-nm
laser diode is 19%, and a reflectivity R.sub.780 of the 780-nm
laser diode is 22%. Even when the film thickness has a variation of
.+-.5% from the calculated value, the reflectivities fall within
the range of 15% to 25%. Therefore, appropriate reflectivities are
obtained even when the film thickness calculated on the basis of
the design central wavelength of 715 nm has a variation of
.+-.5%.
[0061] FIG. 8 shows the reflectivity as a function of variations in
the film thickness when the design wavelength .lambda.1=650 nm is
used to calculate the film thickness for obtaining a reflectivity
R=20%. When the film thickness variation is .+-.5%, the
reflectivity R.sub.650 of the semiconductor laser having an
oscillation wavelength of 650 nm falls within the practical range
of 15% to 25%. However, if the film thickness varies from the
desired value to negative values, the reflectivity R.sub.780 of the
semiconductor laser having an oscillation wavelength of 780 nm
exceeds the practical range of 15% to 25%.
[0062] FIG. 9 shows the reflectivity as a function of variations in
the film thickness when the design wavelength .lambda.2=780 nm is
used to calculate the film thickness for obtaining a reflectivity
R=20%. When the film thickness variation is .+-.5%, the
reflectivity R.sub.780 of the semiconductor laser having an
oscillation wavelength of 780 nm falls within the practical range
of 15% to 25%. However, if the film thickness varies from the
desired value to positive values, the reflectivity R.sub.650 of the
semiconductor laser having an oscillation wavelength of 650 nm
exceeds the practical range of 15% to 25%.
[0063] From the foregoing, the front end face film 19 having a film
thickness calculated by using the mean wavelength, i.e., the design
wavelength .lambda.m=715, has an appropriate reflectivity even when
the film thickness varies.
[0064] The high-reflectivity end face film 22 will be similarly
verified below. This high-reflectivity end face film 22 is a
multilayered film having five layers. Therefore, the possibility
that reflectivity differences produced by film thickness errors of
the individual thin films add up to make the overall reflectivity
difficult to control cannot be ignored. FIGS. 10 to 12 show the
reflectivity as a function of variations in the film thickness of
the high-reflectivity end face film 22.
[0065] FIG. 10 shows the reflectivity when the average wavelength,
i.e., the design wavelength .lambda.m=715 nm is used to calculate a
film thickness for obtaining a reflectivity of approximately 80%.
When the film thickness has no variation, the reflectivity
R.sub.650 of the 650-nm laser diode is 80%, and the reflectivity
R.sub.780 of the 780-nm laser diode is 79%. That is, the
reflectivities of these two semiconductor lasers fall within the
practical range of 80.+-..+-.5%. Even when the film thickness has a
variation of .+-.5% from the calculated value, the reflectivities
fall within the range of 80.+-.5%. Therefore, appropriate
reflectivities are obtained even when the film thickness calculated
on the basis of the design wavelength of 715 nm has a variation of
.+-.5%.
[0066] FIG. 11 shows the reflectivity as a function of variations
in the film thickness when the design wavelength .lambda.1=650 nm
is used to calculate the film thickness for obtaining a
reflectivity of approximately 80%. When the film thickness
variation is .+-.5%, the reflectivity R.sub.650 of the
semiconductor laser having an oscillation wavelength of 650 nm
exceeds the practical range of 80.+-.5%. Also, if the film
thickness varies from the desired value to negative values, the
reflectivity R.sub.780 of the semiconductor laser having an
oscillation wavelength of 780 nm exceeds the practical range of
80.+-.5%.
[0067] FIG. 12 shows the reflectivity as a function of variations
in the film thickness when the design wavelength .lambda.2=780 nm
is used to calculate the film thickness for obtaining a
reflectivity of approximately 80%. When the film thickness
variation is .+-.5%, the reflectivity R.sub.780 of the
semiconductor laser having an oscillation wavelength of 780 nm
falls within the practical range of 80.+-.5%. However, if the film
thickness varies from the desired value to positive values, the
reflectivity R.sub.650 of the semiconductor laser having an
oscillation wavelength of 650 nm exceeds the practical range of
80.+-.5%.
[0068] Accordingly, the high-reflectivity end face film 22 having
the film thickness calculated by using the design wavelength
.lambda.m=715 has a reflectivity within the practical range even
when the film thickness varies.
[0069] From the foregoing, when the end face film thickness is
calculated using the mean value .lambda.m=715 nm, an end face film
having a uniform film thickness and a desired reflectivity can be
formed on the two semiconductor laser diodes at once. In addition,
even when the film thickness of the end face film varies .+-.5%
from the calculated value, the two laser diodes can have
reflectivities within the practical range. A two-wavelength
semiconductor laser device having an end face film with the above
film thickness can regularly oscillate 10 to 20 mW at a temperature
of 70.degree. C., and has a small light absorption and low
loss.
[0070] The second embodiment will be described below. FIG. 13 shows
the structure of an end face film of a two-wavelength semiconductor
laser according to the second embodiment. This second embodiment
differs from the first embodiment in that a mean value .lambda.m
(.lambda.1+.lambda.2)/2, i.e., .lambda.m=715 nm of the oscillation
wavelengths of two semiconductor laser diodes is used to obtain a
film thickness by which the reflectivity of a front end face film
40 is a few % to 10% and the reflectivity of a high-reflectivity
end face film 30 is 90% or more. The second embodiment also differs
from the first embodiment in that the high-reflectivity end face
film 30 has a stacked structure including nine low- and
high-refractive-index films. Since the end face films have film
thicknesses calculated by using the mean wavelength, a
semiconductor laser device optimum as a light source for a DVD-ROM
and for a CD-R of a double speed or higher can be provided.
[0071] Similar to the first embodiment, the two-wavelength lasers
according to the second embodiment are formed on a single device,
and their laser emission regions are physically separated by wet
etching using an acidic solution or by dry gas etching or the like.
The stacked structure of a laser diode having an oscillation
wavelength of 650 nm is analogous to the structure of the first
embodiment, i.e., has an InGaAl/InGaAlP multiple quantum well
active layer. Also, this structure is an SBR (Selectively Buried
Ridge) structure by which lasers capable of maintaining a single
transverse mode at high output can be formed with high productivity
by setting the ridge width to 3 to 6 .mu.m or less. Since this
stacked structure is identical with the first embodiment, a
detailed description thereof will be omitted. FIG. 14 shows the
stacked structure of a laser diode having an oscillation wavelength
of 780 nm. In FIG. 14, the same reference numerals as in the first
embodiment denote the same parts. An n-GaAs buffer layer 4, an
n-InGaAlP first cladding layer 5, an InGaAl light guide layer 6, a
GaAlAS multiple quantum well active layer 42, an AlGaAs light guide
layer 8, a p-InGaAlP second cladding layer 9, and an InGaP etching
stop layer 10 are sequentially formed on an n-GaAs substrate 3. A
ridge-shaped p-InGaAlP third cladding layer 11 is formed on the
etching stop layer 10. An n-GaAs current blocking layer 12 is
formed on the two sides of this third cladding layer 11, and a
p-InGaP easy current passing layer 13 is formed on top of the third
cladding layer 11. A p-GaAs contact layer 14 is formed on the
current blocking layer 12 and on the easy current passing layer 13.
A p-type electrode 15 is formed on the contact layer 14, and an
n-type electrode 16 is formed on the lower surface of the n-GaAs
substrate 3. The structure of the second embodiment is an SBR
structure. Different from the first embodiment in which the active
layer has a bulk structure, the active layer of this second
embodiment has an AlGaAs multiple quantum well structure. Well
layers in the active layer are Al.sub.x1Ga.sub.1-x1As, and barrier
layers and light guide layers are Al.sub.x2Ga.sub.1-x2As. FIG. 15
shows the Al compositions of the multiple quantum well structure.
The Al composition of the well layers is 0.15, and the Al
composition of the barrier layers and light guide layers is 0.5. A
well layer thickness L.sub.w is 3 to 8 nm, and a barrier layer
thickness L.sub.b is 2 to 5 nm. These parameters can be properly
adjusted. As a consequence, it is possible to obtain a laser diode
suitable for optical disks, which has a continuous oscillation of
30 mw or more, a horizontal divergence angle of 7.degree. to
10.degree., and a vertical divergence angle of 20.degree. to
25.degree..
[0072] The end face film structure as the characteristic feature of
the present invention will be described below. The structure is
shown in FIG. 13 in which the same reference numerals as in the
first embodiment denote the same parts. A resonator front end face
18 of each of the two semiconductor lasers has the front end face
film 40 having a reflectivity of a few % to 10%. A resonator rear
end face 21 of each laser has the high-reflectivity end face film
30 having a reflectivity of 90% or more. The combination of the
reflectivities of these end face films allows the 780-nm band laser
diode to perform high-output laser oscillation at 30 mW or more and
to be used as a light source for a CD-R of a double speed or
higher. Also, the 650-nm laser diode can be used as an optimum
light source for a DVD-ROM.
[0073] As in the first embodiment, the front end face film 40 is
made of a low-refractive-index material having a refractive index
n<1.8, preferably 1.7 or less, and an example is
Al.sub.2O.sub.3. By the use of this low-refractive-index material,
a single-layer film having a relatively large film thickness and
high controllability can be obtained, and the productivity also
improves. A linear expansion coefficient of 6.6 of Al.sub.2O.sub.3
is close to a linear expansion coefficient of 7.7 of GaAs.
Therefore, this Al.sub.2O.sub.3 front end face film has high
intimacy and strong adhesion as a thin film in contact with a laser
diode end face containing GaAs as its main constituent substance.
The film thickness of this front end face film 40 is calculated by
d (film thickness)=(1/4+j).times..lambda.m (j=0, 1, 2, . . . ) by
using the mean value .lambda.m=(.lambda.1+.lambda.2)/2 of the
oscillation wavelengths of the two laser diodes, i.e.,
.lambda.m=715 nm. A semiconductor laser device having the above
film thickness has a reflectivity of a few % to 10% at the front
end faces of both the 650-nm oscillation laser diode and the 780-nm
oscillation laser diode. Compared to the first embodiment,
therefore, a larger amount of laser light can be extracted from
these front end faces, and this reduces the load on the laser
diodes. Also, the front end face film 40 can be formed on the
resonator front end face 18 at once by ECR sputtering. This method
allows film formation which reduces damage to the laser emission
regions. Consequently, a highly reliable semiconductor laser device
can be provided.
[0074] The resonator rear end face 21 has the high-reflectivity end
face film 30 having a reflectivity of 90% or more. This
high-reflectivity end face film 30 includes nine layers formed in
the following order from the resonator rear end face 21: a first
film 31 using a low-refractivity-index material having a refractive
index of 1.7 or less, a second film 32 using a
high-refractive-index material having a refractive index of 2.0 or
more, a third film 33 using a low-refractive-index material, a
fourth film 34 using the same high-refractive-index material as the
second film, a fifth film 35 using the same low-refractive-index
material as the third film, a sixth film 36 using a
high-refractive-index material, a seventh film 37 using the same
low-refractive-index material as the third film, an eighth film 38
using the same high-refractive-index material as the sixth film,
and a ninth film 39 using the same low-refractive-index material as
the third film. The materials are preferably Al.sub.2O.sub.3,
SiN.sub.2, SiO.sub.2, and Si. These materials are stacked in the
order of Al.sub.2O.sub.3, SiN, SiO.sub.2, SiN, SiO.sub.2, Si,
SiO.sub.2, Si, and SiO.sub.2 from the semiconductor element. As in
the case of the front end face film 40, the film thicknesses of
these low-refractive-index material films and high-refractive-index
material films are calculated by the optical film thickness
d=(1/4+j).times.(j=0, 1, 2, . . . ) by using the mean oscillation
wavelength .lambda.m=(.lambda.1+.lambda.2)/2 of the laser diode
oscillation wavelengths .lambda.1=780 nm and .lambda.2=650 nm,
i.e., .lambda.m=715 nm. The outermost ninth film 39 is formed to
protect the films up to the eighth film 38 against chemical
changes. The film thickness of this outermost ninth film 39 is so
calculated that the ninth film 39 functions as a total reflection
film in order to maintain the reflectivities up to the eighth
layer. The mean wavelength of 715 nm is used in the
calculation.
[0075] In the second embodiment, a film having a reflectivity of
90% or more is formed by setting the total number of thin-film
stacked layers to 9 or less and forming the sixth film 36 and the
eighth film 38 by using Si which absorbs light.
[0076] A desired film thickness of the high-reflectivity end face
film 30 is obtained by using two, Al and Si targets by ECR
sputtering, and appropriately switching O.sub.2 and N.sub.2 as
material gases necessary in the film formation. Compared to a
process which forms films by using dedicated targets for individual
materials, the productivity can be dramatically improved. The
combination of the front end face film 40 having a reflectivity of
a few % to 10% and the high-reflectivity end face film 30 having a
reflectivity of 90% or more can provide a 650-nm and laser diode
optimum as a light source for a DVD-ROM and a 780-nm band laser
diode optimum as a light source for a CD-R of a double speed or
higher. Also, even when the film thickness of the high-reflectivity
end face film 30 and the film thickness of the
low-refractivity-index film 40 more or less vary, their
reflectivities do not deviate from their predetermined values.
Therefore, end face films having very high productivity can be
provided.
[0077] As in the first embodiment, the front end face film 40 and
the high-reflectivity end face film 30 have film thicknesses
calculated by using the mean wavelength. Hence, desired
reflectivities are obtained even if these film thicknesses more or
less vary. This will be explained below.
[0078] FIGS. 16 to 21 show the results of trial calculations of the
reflectivity as a function of variations in the film thickness of
the front end face film 40. FIGS. 16 and 17 show the reflectivity
when the film thickness is calculated on the basis of the design
wavelength of 715 nm. The film thickness is so calculated that the
reflectivity R=6% in the laser diode having the oscillation
wavelength .lambda.1=650 nm and the reflectivity R=10% in the laser
diode having the oscillation wavelength .lambda.2=780 nm. Even when
this is the cases a front end face film 40 having a uniform film
thickness can be formed at one time. When the desired film
thickness is obtained, a reflectivity R of the 650-nm semiconductor
laser is 6% (FIG. 16), and a reflectivity R.sub.780 of the 780-nm
semiconductor laser is 11% (FIG. 17). When the film thickness has a
variation of .+-.5%, the reflectivity of the 650-nm semiconductor
laser falls within the practical range of
3%.ltoreq.R.sub.650.ltoreq.10% (FIG. 16), and the reflectivity of
the 780-nm semiconductor laser falls within the practical range of
5% .ltoreq.R.sub.780.ltoreq.15% (FIG. 17). Therefore, appropriate
reflectivities are obtained even when the film thickness calculated
on the basis of the design wavelength of 715 nm has a variation of
.+-.5%.
[0079] FIGS. 18 and 19 show the reflectivity as a function of
variations in the film thickness when the design wavelength
.lambda.1=650 nm is used. When the film thickness variation is
.+-.5%, the reflectivity of the semiconductor laser diode having an
oscillation wavelength of 650 nm exceeds the practical range of
3%.ltoreq.R.sub.650.ltoreq.10% at the reflectivity R=6%, if the
film thickness varies to negative values (FIG. 18). Also, the
reflectivity of the semiconductor laser diode having an oscillation
wavelength of 780 nm exceeds the practical range of
5%.ltoreq.R.sub.780.ltoreq.15% at the reflectivity R=10%, if the
film thickness varies to negative values (FIG. 19).
[0080] FIGS. 20 and 21 show the reflectivity as a function of
variations in the film thickness when the oscillation wavelength
.lambda.2=780 nm is used. When the film thickness variation is
.+-.5%, the reflectivity of the semiconductor laser having an
oscillation wavelength of 650 nm exceeds the practical range of
3%.ltoreq.R.sub.650.ltoreq.10% at the reflectivity R=6%, if the
film thickness varies to positive values (FIG. 20). Also, the
reflectivity of the semiconductor laser having an oscillation
wavelength of 780 nm exceeds the practical range of
5%.ltoreq.R.sub.780.ltoreq.15% at the reflectivity R=10%, if the
film thickness varies to positive values (FIG. 21).
[0081] From the foregoing, when the front end face film 40 has the
film thickness calculated by using the mean wavelength, a desired
reflectivity is obtained even if the film thickness more or less
varies. However, if the film thickness is not calculated by using
the mean wavelength, the reflectivity exceeds the practical range
as the film thickness varies.
[0082] The high-reflectivity end face film 30 will be similarly
verified below. This high-reflectivity end face film 30 is a
multilayered film having nine layers. Therefore, the possibility
that reflectivity differences produced by film thickness errors of
the individual thin films add up to make the overall reflectivity
difficult to control cannot be ignored. FIGS. 22 to 24 show
reflectivities as functions of variations in the film thickness of
the high-reflectivity end face film 30.
[0083] FIG. 22 shows changes in reflectivities with variations in
the film thickness when the design wavelength .lambda.m=715 nm is
used to calculate a film thickness for obtaining the reflectivity
R.gtoreq.90%. When the desired film thickness is obtained, the
reflectivity R.sub.650 of the 650-nm semiconductor laser diode is
95%, and a reflectivity R.sub.780 of the 780-nm semiconductor laser
diode is 97%. Even when the film thickness has a variation of
.+-.5% from the calculated value, the reflectivities of these two
semiconductor lasers fall within the practical range of 90 to 100%.
Therefore, reflectivities within the practical range are obtained
even when the film thickness calculated on the basis of the design
wavelength of 715 nm has a variation of .+-.5%.
[0084] FIG. 23 shows the reflectivity as a function of variations
in the film thickness when the design wavelength .lambda.1=650 nm
is used to calculate the film thickness for obtaining the
reflectivity R.gtoreq.90%. When the film thickness variation is
.+-.5%, the reflectivity of the semiconductor laser diode having an
oscillation wavelength of 650 nm falls within the practical range
of 90%.ltoreq.R.sub.650.ltoreq.100%. However, if the film thickness
varies from the desired value to negative values, the reflectivity
of the semiconductor laser diode having an oscillation wavelength
of 780 nm exceeds the practical range of
90%.ltoreq.R.sub.780.ltoreq.100%.
[0085] FIG. 24 shows reflectivities as functions of variations in
the film thickness when .lambda.2=780 nm is used as a design
wavelength. When the film thickness variation is .+-.5%, the
reflectivity of the semiconductor laser diode having an oscillation
wavelength of 780 nm falls within the practical range of
90%.ltoreq.R.sub.780.ltoreq.100%. However, the reflectivity of the
semiconductor laser diode having an oscillation wavelength of 650
nm falls outside the practical range of
90%.ltoreq.R.sub.650.ltoreq.100%. Accordingly, no reflectivities
within the practical range can be obtained by the film thickness
calculated by using .lambda.2=780 nm.
[0086] From the foregoing, when the high-reflectivity end face film
30 has the film thickness calculated using the mean wavelength,
desired reflectivities can be obtained even if the film thickness
more or less varies. However, if the film thickness is not
calculated by using the mean wavelength, the reflectivity exceeds
the practical range as the film thickness varies.
[0087] As described above, since each end face film has the film
thickness calculated by using the mean value
.lambda.m=(.lambda.1+.lambda.2)/2 of the oscillation wavelengths,
i.e., .lambda.m=715 nm, the end face film can have reflectivity
within the practical range even if the film thickness varies. In
addition, the end face film can be formed at once by using EECR
sputtering on the two semiconductor lasers having the oscillation
wavelength .lambda.1 and the oscillation wavelength .lambda.2.
Furthermore, it is readily possible to obtain a laser diode having
a front end face reflectivity of 10% and a rear end face
reflectivity of 90% or more.
[0088] The third embodiment will be described below.
[0089] FIG. 25 shows the end face structure of a semiconductor
laser according to the third embodiment. This third embodiment
differs from the second embodiment in that a mean value
.lambda.m=(.lambda.1+.lambda.2)/2 of the oscillation wavelengths of
two semiconductor lasers having oscillation wavelengths of 650 and
780 nm, i.e., .lambda.m=715 nm, is used to obtain a film thickness
by which a front end face film 41 having a reflectivity of
8%.ltoreq.R.ltoreq.20% or 29%.ltoreq.R.ltoreq.32% is formed. When
the film thickness of the end face film satisfies the above
reflectivity, a two-wavelength monolithic laser diode having
various advantages can be provided. For example, when the front end
face film 41 has a reflectivity of 8%.ltoreq.R.ltoreq.20%,
reflected optical noise from an object to be Irradiated with a
laser can be reduced in a 650-nm band laser diode structure for a
DVD-ROM. In a 780-nm band laser diode structure requiring high
output so as to be used for a CD-R, interference by emitted light
can be suppressed. When this front end face film 41 has a
reflectivity of 29%.ltoreq.R.ltoreq.32%, the influence of reflected
light from an optical disk ca be further reduced.
[0090] The stacked structures of the two semiconductor laser diodes
having oscillation wavelengths of 650 and 780 nm are analogous to
those of the second embodiment. That is, these two semiconductor
laser diodes have an SBR structure, and an active layer has a
multiple quantum well structure. In the 650-nm semiconductor laser
diode, an InGaAl/InGaAlP material is used in the active layer. In
the 780-nm semiconductor laser diode, an AlGaAs-based material is
used in the active layer. A detailed description of the stacked
structures of these two diodes will be omitted. The structure of a
high-reflectivity end face film 30 of the semiconductor laser
diodes is also a stacked structure identical with that of the
second embodiment. This stacked structure includes nine layers of
thin films made of low-refractive-index materials and thin films
made of high-refractive-index materials. Si which absorbs light and
has a high refractive index is used in a sixth layer 36 and an
eighth layer 38.
[0091] As in the second embodiment, the front end face film 41 is
made of a low-refractive-index material having a refractive index
of 1.7 or less, and an example is Al.sub.2O.sub.3. The film
thickness of this front end face film 41 is so set that the
reflectivity is 8%.ltoreq.R.ltoreq.20% or 29%.ltoreq.R.ltoreq.32%.
When this reflectivity is 8%.ltoreq.R.ltoreq.20%, the 650-nm
oscillation laser diode has a reflectivity of 8%, and the 780-nm
oscillation laser diode has a reflectivity of 20%. Since the front
end face film of the two semiconductor lasers has a uniform film
thickness, this front end face film can be formed at once by ECR
sputtering. By this film formation method, it is possible to reduce
damage to laser emission regions and provide a highly reliable
device.
[0092] FIGS. 26 to 28 show the reflectivity as a function of
variations in the film thickness of the front end face film 41
having a reflectivity of 8%.ltoreq.R.ltoreq.20%.
[0093] FIG. 26 shows a case in which this front end face film 41
has a film thickness calculated by using the mean wavelength
.lambda.m=715 as a design wavelength. When the desired film
thickness is obtained, a reflectivity R.sub.650 of the 650-nm
semiconductor laser is 20%, and a reflectivity R.sub.780 of the
780-nm semiconductor laser is 8%. When the film thickness has a
variation of .+-.5% from the calculated value, the reflectivity of
the 650-nm semiconductor laser falls within the practical range of
15%.ltoreq.R.sub.650.ltoreq.25%, and the reflectivity of the 780-nm
semiconductor laser falls within the practical range of
3%.ltoreq.R.sub.780.ltoreq.13%. Therefore, appropriate
reflectivities are obtained even when the film thickness calculated
on the basis of the design central wavelength of 715 nm has a
variation of .+-.5%.
[0094] FIG. 27 shows the reflectivity as a function of variations
in the film thickness when the film thickness is so set that the
reflectivity is 8%.ltoreq.R.ltoreq.20% at the design wavelength
.lambda.1=650 nm. When the film thickness variation is .+-.5%, the
reflectivity of the semiconductor laser having an oscillation
wavelength of 650 nm falls outside the practical range of
15%.ltoreq.R.ltoreq.25%. Also, the reflectivity of the
semiconductor laser having an oscillation wavelength of 780 nm
falls outside the practical range of 3%.ltoreq.R.sub.780.ltoreq-
.13%.
[0095] FIG. 28 shows the reflectivity as a function of variations
in the film thickness when the film thickness is so set that the
reflectivity is 8%.ltoreq.R.ltoreq.20% at the design wavelength
.lambda.2=780 nm. When the film thickness variation is .+-.5%, the
reflectivity of the semiconductor laser having an oscillation
wavelength of 780 nm exceeds the practical range of
3%.ltoreq.R.ltoreq.13%. If the film thickness varies from the
desired value to negative values, the reflectivity of the
semiconductor laser having an oscillation wavelength of 650 nm
exceeds the practical range of 15%.ltoreq.R.sub.650.ltoreq.25%.
[0096] From the foregoing, when having the film thickness
calculated by using the mean wavelength .lambda.m=715 nm, the two
semiconductor lasers can obtain reflectivities of
8%.ltoreq.R.ltoreq.20%. Also, even if this film thickness more or
less varies, the reflectivities fall within the practical
ranges.
[0097] FIGS. 29 to 31 show the reflectivity as a function of
variations in the film thickness of the front end face film 41
having a reflectivity of 29%.ltoreq.R.ltoreq.32%.
[0098] FIG. 29 shows the reflectivity as a function of variations
in the film thickness calculated by using the mean wavelength
.lambda.m=715 nm as a design wavelength. When this film thickness
has no variation, the reflectivity R.sub.650 of the 650-nm
semiconductor laser diode is 29%, and the reflectivity R.sub.780 of
the 780-nm semiconductor laser diode is 97%. When the film
thickness has a variation of .+-.5% from the calculated value, the
reflectivity of the 650-nm semiconductor laser diode falls within
the practical range of 24%.ltoreq.R.sub.650.ltoreq.37%- , and the
reflectivity of the 780-nm semiconductor laser diode falls within
the practical range of 24%.ltoreq.R.sub.780.ltoreq.37%. Therefore,
appropriate reflectivities are obtained even when the film
thickness calculated on the basis of the design central wavelength
of 715 nm has a variation of 5%.
[0099] FIG. 30 shows the reflectivity as a function of variations
in the film thickness calculated by using the design wavelength
.lambda.1=650 nm. When the film thickness variation is .+-.5%, the
reflectivity of the semiconductor laser having an oscillation
wavelength of 650 nm falls within the practical range of
24%.ltoreq.R.sub.650.ltoreq.37%. However, if the film thickness
varies from the desired value to negative values, the reflectivity
of the semiconductor laser having an oscillation wavelength of 780
nm falls outside the practical range of
24%.ltoreq.R.sub.780.ltoreq.37%.
[0100] FIG. 31 shows the reflectivity as a function of variations
in the film thickness calculated by using the design wavelength
.lambda.2=780 nm. When the film thickness variation is .+-.5%, the
reflectivity of the semiconductor laser having an oscillation
wavelength of 780 nm falls within the practical range of
24%.ltoreq.R.sub.780.ltoreq.37%. However, if the film thickness
varies from the desired value to positive values, the reflectivity
of the semiconductor laser having an oscillation wavelength of 650
nm falls outside the practical range of
24%.ltoreq.R.sub.650.ltoreq.37%.
[0101] From the foregoing, when having the film thickness
calculated by using the mean wavelength .lambda.m=715 nm, the two
semiconductor lasers can obtain reflectivities of
29%.ltoreq.R.ltoreq.32%. Also, even if this film thickness more or
less varies, the reflectivities fall within the practical
ranges.
[0102] As described above, each end face film has the film
thickness calculated by using the mean value
.lambda.m=(.lambda.1+.lambda.2)/2 of the oscillation wavelengths,
i.e., .lambda.m=715 nm. Therefore, the end face film can be formed
at once on the two-wavelength semiconductor laser including the two
semiconductor lasers having the oscillation wavelength .lambda.1
and the oscillation wavelength .lambda.2. Also, it is readily
possible to obtain a laser diode having high reflectivities, i.e.,
a front end face reflectivity of 8%.ltoreq.R.ltoreq.20% or
29%.ltoreq.R.ltoreq.32% and a rear end face reflectivity of 90% or
more. Furthermore, even if the film thickness of each end face film
varies .+-.5% from the calculated value, the two laser diodes can
have reflectivities within the practical ranges.
[0103] In the semiconductor laser diodes according to the first to
third embodiments, the stacked structures are not limited to those
described above, and another structure can also be used. The
materials of these stacked structures are also not limited to those
of the above embodiments, so some other material can be used. The
materials of the front end face film and the high-reflectivity end
face film are also not limited to those of the above embodiments,
and another material can be used. The numbers of stacked layers of
the front end face film and the high-reflectivity end face film are
also not limited to those of the above embodiments and can be
properly changed.
[0104] As described above, in a monolithic two-wavelength
semiconductor laser device, the film thickness of an end face film
of the individual semiconductor laser diodes is calculated by using
the mean value of the oscillation wavelengths of these
semiconductor laser diodes. Therefore, an end face film having a
uniform film thickness and a desired reflectivity can be obtained.
Additionally, the fabrication steps can be simplified because the
end face film can be formed at once. It is also possible to provide
a two-wavelength semiconductor laser device having high
reliability, meeting the required performance, and also having high
productivity.
[0105] A semiconductor laser device of the present invention can
provide an end face film having a desired reflectivity and capable
of being formed at once. This makes it possible to provide a
two-wavelength semiconductor laser device having high reliability,
meeting the necessary performance, and also having high
productivity.
* * * * *