U.S. patent application number 14/489899 was filed with the patent office on 2015-01-01 for semiconductor laser.
The applicant listed for this patent is OCLARO JAPAN, INC.. Invention is credited to Haruki FUKAI, Masato HAGIMOTO, Satoshi KAWANAKA, Tsutomu KIYOSUMI, Shinji SASAKI.
Application Number | 20150003483 14/489899 |
Document ID | / |
Family ID | 48779938 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150003483 |
Kind Code |
A1 |
HAGIMOTO; Masato ; et
al. |
January 1, 2015 |
SEMICONDUCTOR LASER
Abstract
An aluminium gallium indium phosphide (AlGaInP)-based
semiconductor laser device is provided. On a main surface of a
semiconductor substrate formed of n-type GaAs (gallium arsenide),
from the bottom layer, an n-type buffer layer, an n-type cladding
layer formed of an AlGaInP-based semiconductor containing silicon
(Si) as a dopant, an active layer, a p-type cladding layer formed
of an AlGaInP-based semiconductor containing magnesium (Mg) or zinc
(Zn) as a dopant, an etching stopper layer, and a p-type contact
layer are formed. Here, when an Al composition ratio x of the
AlGaInP-based semiconductor is taken as a composition ratio of Al
and Ga defined as (Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P, a
composition of the n-type cladding layer is expressed as
(Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P (0.9<xn<1) and a
composition of the p-type cladding layer is expressed as
(Al.sub.xpGa.sub.1-xp).sub.0.5In.sub.0.5P (0.9<xp.ltoreq.1), and
xn and xp satisfy a relationship of xn<xp.
Inventors: |
HAGIMOTO; Masato; (Saku,
JP) ; FUKAI; Haruki; (Komoro, JP) ; KIYOSUMI;
Tsutomu; (Ueda, JP) ; SASAKI; Shinji; (Miyota,
JP) ; KAWANAKA; Satoshi; (Komoro, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OCLARO JAPAN, INC. |
Yokohama-shi |
|
JP |
|
|
Family ID: |
48779938 |
Appl. No.: |
14/489899 |
Filed: |
September 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13714508 |
Dec 14, 2012 |
|
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|
14489899 |
|
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Current U.S.
Class: |
372/44.01 |
Current CPC
Class: |
H01S 5/0021 20130101;
H01S 5/3211 20130101; H01S 5/34326 20130101; H01S 5/3063 20130101;
H01S 5/2004 20130101; H01S 5/3213 20130101; B82Y 20/00 20130101;
H01S 5/3013 20130101; H01S 5/209 20130101; H01S 2301/18 20130101;
H01S 5/22 20130101; H01S 5/0224 20130101; H01S 5/02212 20130101;
H01S 5/2031 20130101; H01S 5/305 20130101 |
Class at
Publication: |
372/44.01 |
International
Class: |
H01S 5/32 20060101
H01S005/32 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 12, 2012 |
JP |
2012-4283 |
Claims
1. A semiconductor laser device comprising a semiconductor laser,
wherein the semiconductor laser includes: an n-type cladding layer
having a composition of (Al.sub.xnGa.sub.1-xn).sub.0.5In.sub.0.5P
where 0.9<xn<1; a p-type cladding layer having a composition
of (Al.sub.xpGa.sub.1-xp).sub.0.5In.sub.0.5P where
0.9<xp.ltoreq.1; an active layer provided between the n-type
cladding layer and the p-type cladding layer, and wherein an Al
composition ratio xn of the n-type cladding layer and an Al
composition ratio xp of the p-type cladding layer satisfy a
relationship of xn.ltoreq.xp.
2. The semiconductor laser device according to claim 1, wherein a
difference between the Al composition ratio xp of the p-type
cladding layer and the Al composition ratio xn of the n-type
cladding layer satisfies a relationship of
0.ltoreq.xp-xn.ltoreq.0.08.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a divisional of U.S. patent
application Ser. No. 13/714,508, filed Dec. 14, 2012, which claims
priority from Japanese Patent Application No. 2012-004283 filed on
Jan. 12, 2012, the content of which is hereby incorporated by
reference into this application.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to a semiconductor laser. More
particularly, the present invention relates to technique
effectively applied to a semiconductor laser using an aluminium
gallium indium phosphide (AlGaInP)-based semiconductor.
BACKGROUND OF THE INVENTION
[0003] Japanese Patent Application Laid-Open Publication No.
2002-217495 (Patent Document 1) discloses technique of suppressing
impurity diffusion to an active layer and also increasing
characteristic temperature and modulation frequency for a
semiconductor laser in which at least an n-type cladding layer, a
bottom optical wave guiding layer, an active layer, a top optical
guiding layer and a p-type cladding layer are stacked on a
semiconductor substrate.
[0004] More specifically, by using Al.sub.0.5In.sub.0.5P which is
lattice-matched to a gallium arsenide (GaAs) substrate and having
the largest bandgap among AlGaInP-based semiconductors to the
p-type cladding layer and the n-type cladding layer to obtain a
bandgap difference between the active layer and the cladding
layers, electron overflow from the active layer to the p-type
cladding layer is suppressed. Also, diffusion of a dopant (zinc
(Zn), selenium (Se)) doped to the p-type cladding layer and the
n-type cladding layer at a high concentration, i.e.,
1.times.10.sup.18 cm.sup.-3 into the active layer is suppressed by
providing an undoped layer between the active layer and the p-type
cladding layer and between the active layer and the n-type cladding
layer, respectively.
[0005] Japanese Patent Application Laid-Open Publication No.
2006-120968 (Patent Document 2) discloses technique of improving
efficiency and temperature characteristics of a semiconductor laser
having an active layer between an n-type cladding layer and a
p-type cladding layer. More specifically, according to Patent
Document 2, generation of misfit dislocation is suppressed by
forming the p-type cladding layer and the n-type cladding layer
with lattice-aligned Al.sub.0.5In.sub.0.5P and introducing a
strained layer for inhibiting overflow of electrons between the
active layer and the p-type cladding layer as well as making the
thickness of the strained layer smaller than or equal to a critical
thickness.
[0006] Japanese Patent Application Laid-Open Publication No.
2011-023493 (Patent Document 3) discloses technique of reducing
catastrophic optical damage (COD) at end facets and also improving
stability of output beam for an AlGaInP-based semiconductor laser
of a horizontal cavity type having a lasing wavelength shorter than
650 nm. More specifically, according to Patent Document 3,
dissipation of an optical waveguide structure near end facets
occurring when a window structure is formed by diffusion of an
impurity such as Zn near end facets is suppressed by forming the
optical waveguide layer excluding a well layer with
(Al.sub.xGa.sub.1-x)InP where x>0.66. In addition, in the
semiconductor laser, the n-type cladding layer and the p-type
cladding layer are formed of Al.sub.0.51In.sub.0.49P or
(Al.sub.0.9Ga.sub.0.1).sub.0.51In.sub.0.49P.
[0007] Note that, a band gap of an AlGaInP-based semiconductor is
2.3 eV when x=0.7 in (Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P.
However, it is reported that the bandgap is 2.35 eV when x=1 and it
is the largest value among AlGaInP-based semiconductors (see D. P.
Bour, R. S. Geels, D. W. Treat, T. L. Paoli, F. Ponce, R. L.
Thornton, B. S. Krusor, R. D. Bringans, D. F. Welch (1994).
"Strained GaxIn1-xP/(AlGa)0.5In0.5P heterostructures and
quantum-well laser diodes", IEEE Journal of Quantum
Electronics--IEEE J QUANTUM ELECTRON, vol. 30, no. 2, pp. 593-607
(Non-Patent Document 3)).
SUMMARY OF THE INVENTION
[0008] Application of a red semiconductor laser used as a light
source for DVDs to small-size projectors laser displays such as a
red light source has been advanced.
[0009] When using a red semiconductor laser as a light source for
projectors, high-temperature operation and high-optical output
power operation for corresponding to improvements in luminance of
projectors or improvements in luminosity factor by shortening the
wave length are required. In addition, the red semiconductor laser
has been required to correspond to high-temperature operation
eyeing usages for mobile devices and in-car usages.
[0010] However, semiconductor lasers have problems of difficulties
in achieving good high-temperature and high-optical output power
characteristics due to significant influences of electron overflow
into a p-type cladding layer from an active layer caused by a
temperature increase in a vicinity of the active layer upon
high-temperature and high-optical output power operation.
[0011] Existing lasing wavelengths of red semiconductor lasers for
DVDs are around 660 nm. In comparison, lasing wavelengths around
640 nm are required of red semiconductor lasers for displays.
However, the shorter the lasing wavelengths are, the smaller the
bandgap difference between the active layer and the p-type cladding
layer is and the more significant carrier overflow upon
high-temperature operation is. Therefore, achieving improvements in
high-temperature characteristics is a problem.
[0012] Generally, one of the characteristics of AlGaInP-based
materials is that the larger the Al composition ratio is, the
larger the bandgap is and the lower the refractive index is.
Therefore, in view of high-temperature characteristics, it is
preferable to make the Al composition ratio of the cladding layer
as large as possible. However, a problem in reliability occurs such
that the larger the Al composition ratio is, the lower the
crystallinity is and the more significant diffusion of dopant is.
Accordingly, the Al composition ratio of the cladding layer has
been set at about 0.6 to 0.7 for the red semiconductor lasers
having lasing wavelengths around 660 nm, in consideration of
high-temperature characteristics and reliability.
[0013] However, as the carrier overflow at high temperature is more
significant in the red semiconductor lasers having lasing
wavelengths around 640 nm, it is required to further improve
high-temperature characteristics and thus it is desirable to make
the Al composition of the cladding layer ratio as large as
possible. Therefore, when AlInP having the largest Al composition
ratio is used as material of the cladding layer, the bandgap
difference between the active layer and the cladding layer is
maximum and also the refractive index of the cladding layer is
minimum and thus there is a merit that optical confinement in the
active layer can be large.
[0014] As the technique disclosed in Patent Document 1 described
above, technique of improving temperature characteristics by using
AlInP in the p-type cladding layer and the n-type cladding layer
and of preventing dopant diffusion by providing undoped layers
between the active layer and the p-type cladding layer and between
the active layer and the n-type cladding layer has been known.
[0015] However, as described below, the inventors of the present
invention have examined reliability of a red semiconductor laser
device using a lasing wavelength of 640 nm in which AlInP is used
in a p-type cladding layer and an n-type cladding layer and the
reliability was thousand-hour scale as a result. Although the
reliability is at a practical level for general usages of
semiconductor lasers, it has been found out that using AlInP in the
p-type cladding layer and the n-type cladding layer cannot
sufficiently correspond to high requirements in reliability, i.e.,
ten thousand hours or longer that is required of light sources for
laser displays.
[0016] A preferred aim of the present invention is to provide
technique capable of achieving both improvements in
high-temperature characteristics and improvements in reliability of
semiconductor laser devices using AlGaInP-based semiconductors.
[0017] The above and other preferred aims and novel characteristics
of the present invention will be apparent from the description of
the present specification and the accompanying drawings.
[0018] The typical ones of the inventions disclosed in the present
application will be briefly described as follows.
[0019] A semiconductor laser of a preferred embodiment of the
present invention is provided with a semiconductor laser including:
an n-type cladding layer having a composition of
(Al.sub.xnGa.sub.1-xn).sub.0.5In.sub.0.5P where 0.9<xn<1; a
p-type cladding layer having a composition of
(Al.sub.xpGa.sub.1-xp).sub.0.5In.sub.0.5P where 0.9<xp.ltoreq.1;
and an active layer provided between the n-type cladding layer and
the p-type cladding layer, in which a relationship of an Al
composition ratio xn of the n-type cladding layer and an Al
composition ratio xp of the p-type cladding layer satisfies
xn<xp.
[0020] The effects obtained by typical aspects of the present
invention will be briefly described below.
[0021] Both improvements in high-temperature characteristics and
improvements in reliability of semiconductor laser devices using
AlGaInP can be achieved.
BRIEF DESCRIPTIONS OF THE DRAWINGS
[0022] FIG. 1 is a cross-sectional view illustrating a
configuration of a main part of a semiconductor laser device
according to a first embodiment of the present invention;
[0023] FIG. 2 is a graph illustrating a result of calculating
optical confinement .GAMMA. into an active layer where a
composition of a p-type cladding layer is fixed and an Al
composition ratio of an n-type cladding layer is changed;
[0024] FIG. 3 is a graph illustrating a calculation result of an
FFP horizontal half width;
[0025] FIG. 4 is a graph illustrating I-L shapes at high
temperature of the first embodiment and a comparative example;
[0026] FIG. 5 is graph illustrating lifetime test results of the
first embodiment and the comparative example;
[0027] FIG. 6 is a graph illustrating measurement results of
photoluminescence wavelengths from active layers of the first
embodiment and the comparative example;
[0028] FIG. 7 is a graph illustrating a relationship of a Al
composition ratio and fluctuations of an FFP horizontal half width
and a relationship of the Al composition ratio and a kink level of
the n-type cladding layer;
[0029] FIG. 8 is a broken perspective view of main parts
illustrating a whole configuration of the semiconductor laser
device according to the first embodiment of the present
invention;
[0030] FIG. 9 is a broken perspective view of main parts
illustrating a whole configuration of a semiconductor laser device
according to a second embodiment of the present invention; and
[0031] FIG. 10 is a cross-sectional view illustrating a
configuration of a main part of a semiconductor laser device
according to a third embodiment of the present invention.
DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
[0032] Hereinafter, embodiments of the present invention will be
described in detail with reference to the accompanying drawings.
Note that components having the same function are denoted by the
same reference symbols throughout the drawings for describing the
embodiment, and the repetitive description thereof will be
omitted.
First Embodiment
[0033] A first embodiment is applied to a red semiconductor laser
having a lasing wavelength of 640 nm. FIG. 1 is a cross-sectional
view illustrating a configuration of a main part (laser chip) in
the semiconductor laser according to the present embodiment.
[0034] As illustrated in FIG. 1, a laser chip 10A includes a
semiconductor substrate 10 formed of n-type GaAs (gallium
arsenide). On a main surface of the semiconductor substrate 10, an
n-type buffer layer 11, an n-type cladding layer 12, an active
layer 13, a p-type cladding layer 14, an etching stopper layer 15,
and a p-type contact layer 17 are formed in this order from the
bottom.
[0035] The n-type buffer layer 11 is formed of GaAs containing Si
(silicon) as a dopant. The n-type cladding layer 12 is formed of
AlGaInP (aluminium gallium indium phosphide) containing Si
(silicon) as a dopant. Here, when taking an Al composition ratio
"x" (Al composition ratio of Al and Ga in a compound semiconductor
defined by (Al.sub.xGa.sub.1-xn).sub.0.5In.sub.0.5P) of the n-type
cladding layer as "xn", xn satisfies 0.9<xn<1, where xn=0.95,
i.e., (Al.sub.0.95Ga.sub.0.05).sub.0.5In.sub.0.5P in the present
embodiment.
[0036] In addition, a Si concentration of the n-type cladding layer
12 is 3.times.10.sup.17 cm.sup.-3 and a thickness of the n-type
cladding layer 12 is 2.5 .mu.m. When the dopant concentration of
the n-type cladding layer 12 is low, an increase in series
resistance is posed and when the dopant concentration is high, a
lowering of slope efficiency due to an increase in internal loss is
posed. Therefore, the dopant concentration which makes it possible
to prevent both an increase in series resistance and a lowering of
slope efficiency is preferable to be set within a range from
1.times.10.sup.17 cm.sup.-3 to 6.times.10.sup.17 cm.sup.-3.
[0037] The active layer 13 is formed in a multi quantum well (MQW)
structure in which an optical guiding layer 13a formed of AlGaInP,
a well layer 13b formed of GaInP or AlGaInP, a barrier layer 13c
formed of AlGaInP, and a well layer 13d formed of GaInP or AlGaInP,
and an optical guiding layer 13e formed of AlGaInP are stacked. A
lowering in reliability of the active layer 13 due to dopant
diffusion into the well layers 13b and 13d can be prevented by
making the above-mentioned layers 13a to 13e undoped,
respectively.
[0038] For efficiently confining light in the well layers 13b and
13d, thicknesses of the optical guiding layers 13a and 13e are 20
to 150 nm and Al composition ratios x of the optical guiding layers
13a and 13e are 0.4 to 0.8. For example, to obtain a beam spread
angle in a direction perpendicular to the active layer 13 at
18.degree. (full width at half maximum), the thickness is set at 25
nm and the Al composition ratio x is set at 0.7.
[0039] The well layers 13b and 13d can be given various lasing
wavelengths and oscillation modes by changing their thicknesses
within a range smaller than or equal to a critical thickness,
compositions and strains. When the well layers are unstrained or
compressive strain is given, the thickness is preferably 3 to 6 nm
and the Al composition ratio x is preferably 0 to 0.15. For
example, to set the lasing wavelength at 640 nm, the thickness is 5
nm and the Al composition ratio x is 0.1 and a compressive strain
of +0.8% is given. In this case, oscillation is generated in a TE
mode in which electric-field components of the laser beam are
vibrated in a direction parallel to the active layer 13.
[0040] A thickness of the barrier layer 13c is 5 to 10 nm and a
composition of the barrier layer 13c is the same as the optical
guiding layers 13a and 13c. Note that, the composition of the
barrier layer 13c may be different from that of the optical guiding
layers 13a and 13c and a structure in which a plurality of the
barrier layers 13c sandwich the well layers 13b and 13d may be
used.
[0041] Note that, while a MQW structure in which the number of the
well layers is two has been used here, a single quantum well (SQW)
structure in which the number of the well layers is one may be
used. When using the single quantum well (SQW) structure, since the
volume of the well layer is reduced, it is easier to increase the
carrier density inside the well layer and a threshold gain can be
obtained even when the injection current is small; therefore, there
is an advantage that a threshold current for laser oscillation can
be largely reduced.
[0042] The p-type cladding layer 14 is formed of AlGaInP containing
Mg (magnesium) or Zn (zinc) as a dopant. Here, when taking an Al
composition ratio x of the p-type cladding layer 14 as xp, xp
satisfies 0.9<xp.ltoreq.1. In the present embodiment, xp=1, that
is, Al.sub.0.5In.sub.0.5P.
[0043] A preferable dopant concentration of the p-type cladding
layer 14 is 1.times.10.sup.18 cm.sup.-3. In addition, a preferable
thickness is 1.25 .mu.m to be smaller than a thickness of the
n-type cladding layer 12. To improve high-temperature
characteristics and to reduce serial resistance, it is preferable
to make the dopant concentration of the p-type cladding layer 14 as
high as possible. However, when the Al composition ratio x is high,
the dopant is prone to diffuse. Therefore, to prevent degradation
of characteristics and lowering of reliability, it is preferable to
set the dopant concentration within a range of 6.times.10.sup.17 to
1.3.times.10.sup.18 cm.sup.-3.
[0044] When using an asymmetric structure in which the Al
composition ratio xp of the p-type cladding layer 14 is higher than
the Al composition ratio xn of the n-type cladding layer 12 like
the present embodiment, the optical distribution is biased toward
the n-type cladding layer 12 side. Therefore, there is an effect of
improving the slope efficiency when setting the dopant
concentration of the n-type cladding layer 12 lower than that of
the p-type cladding layer 14. In addition, as to the type of the
dopant, it is preferable to use Mg which is less prone to diffuse
than Zn. Moreover, although the thickness of the p-type cladding
layer 14 is preferable to be as small as possible for reducing
series resistance, lowering of the slope efficiency is significant
when the thickness is too small.
[0045] A ridge portion (ridge waveguide) 16 extending in a stripe
pattern along a direction perpendicular to the paper sheet is
formed to the p-type cladding layer 14. A height of the ridge
portion 16 is 1 .mu.m and a width of the ridge portion 16 is 2
.mu.m. The ridge portion 16 is oscillated in a single mode (single
transverse mode). The p-type contact layer 17 formed of GaAs
containing Zn as a dopant is formed to an upper portion of the
ridge portion 16.
[0046] In addition, the etching stopper layer 15 is provided to the
p-type cladding layer 14 at a height that is about 0.25 .mu.m from
the active layer 13. The etching stopper layer 15 is a layer for
stopping etching in the middle of forming the ridge portion 16 by
etching the p-type cladding layer 14. The etching stopper layer 15
is formed of an AlGaInP layer having a different Al composition
ratio x than the p-type cladding layer 14 for giving an etching
selectivity with respect to the p-type cladding layer 14. The
etching stopper layer 15 of the present embodiment is formed of,
for example, GaInP in which the Al composition ratio x is zero, and
has a thickness of 3 nm. Note that, when etching conditions of the
p-type cladding layer 14 can be controlled well, the etching
stopper layer 15 may be omitted.
[0047] A passivation film 18 formed of an insulating film such as
silicon oxide film, silicon nitride film, or aluminum oxide film is
formed to each of sidewalls of the ridge portion 16 and the p-type
contact layer 17 and planar portions (upper surfaces of the etching
stopper layer 15) in a vicinity of both sides of the ridge portion
16.
[0048] Each of the semiconductor layers formed on the main surface
of the semiconductor substrate 10 is deposited by metal organic
chemical vapor deposition (MOCVD). In addition, the passivation
film 18 is formed by depositing an insulating film on the
semiconductor substrate 10 to which the p-type contact layer 17 is
formed by CVD and then selectively etching only the insulating film
at an upper portion of the p-type contact layer 17 so that an upper
surface of the p-type contact layer 17 is exposed.
[0049] A p-side electrode 20 to be electrically connected to the
p-type contact layer 17 is formed to an upper portion of the
passivation film 18. On the other side of the p-side electrode 20,
an n-side electrode 21 is formed to a rear surface of the
semiconductor substrate 10. Each of the p-side electrode 20 and the
n-side electrode 21 includes an Au film and is formed of a metal
film capable of making an ohmic contact.
[0050] The Al composition ratio xp of AlInP forming the p-type
cladding layer 14 is preferable to be as high as possible in view
of high-temperature characteristics and thus it is set at 1 at the
maximum in the present embodiment. Then, on that basis, the Al
composition ratio xn of AlGaInP forming the n-type cladding layer
12 is set at 0.95.
[0051] In this manner, the semiconductor laser device of the
present embodiment has an asymmetric structure in which the Al
composition ratio x differs in the n-type cladding layer 12 and the
p-type cladding layer 14 and the Al composition ratio x of the
p-type cladding layer 14 is higher than that of the n-type cladding
layer 12. Therefore, penetration of light is biased toward the
n-type cladding layer 12 side where the refractive index is larger.
There are advantages as follows.
[0052] More specifically, there is an effect of improving the slope
efficiency because, when the penetration of light into the p-type
cladding layer 14 is reduced, it is possible to reduce light
absorption in the p-type contact layer 17 which is formed of GaAs
that is an absorber of light having a wavelength of 640 nm. In
addition, as the present embodiment, when the dopant concentration
of the n-type cladding layer 12 is made lower than that of the
p-type cladding layer 14, more light is distributed in the n-type
cladding layer 12 having a lower dopant concentration and thus it
is possible to reduce internal loss occurring due to free-carrier
overflow in the cladding layers.
[0053] Reduction of the light penetration into the p-type cladding
layer 14 means enabling thickness reduction of the p-type cladding
layer 14. When the p-type cladding layer 14 is thinner, an effect
of reducing heat generation amount by a reduction in series
resistance and an effect of improving exhaust heat are achieved and
thus high-temperature characteristics are improved. Further, when
the light distribution is biased toward the n-type cladding layer
12 side, it is necessary to ensure a refractive index difference by
making the thickness of the p-type cladding layer 14 under the
etching stopper 15 thinner for ensuring a refractive index
difference in a direction horizontal to the active layer 13
(ensuring a same level of FFP horizontal half width). However, by
reducing the thickness of the p-type cladding layer 14 under the
ridge portion 16, reactive current which does not contribute to
laser oscillation is reduced and thus threshold current can be
reduced.
[0054] On the other hand, in the asymmetric structure in which the
Al composition ratio x differs in the n-type cladding layer 12 and
the p-type cladding layer 14, when the Al composition ratio
difference between the n-type cladding layer 12 and the p-type
cladding layer 14 is increased, due to variations in compositions,
the waveguide mode may be unstable and/or variations in
characteristics may be increased. Further, when the Al composition
ratio difference is extremely large so that a point of the maximum
light intensity is positioned at a boundary of the active layer 13
and the cladding layers, even a fundamental mode is cut off and
laser oscillation is not permitted. Therefore, there is an upper
limit in the Al composition ratio difference between the n-type
cladding layer 12 and the p-type cladding layer 14.
[0055] Accordingly, the upper limit in the Al composition ratio
difference of the n-type cladding layer 12 and the p-type cladding
layer 14 was examined by a calculation. FIG. 2 illustrates a result
of a calculation of optical confinement coefficient .GAMMA. of
optical confinement into the active layer 13 where the composition
of the p-type cladding layer 14 is fixed to Al.sub.0.5In.sub.0.5P
and the Al composition ratio xn of the n-type cladding layer 12 is
varied based on the structure of the present embodiment described
above.
[0056] The smaller the Al composition ratio xn of the n-type
cladding layer 12, the smaller the optical confinement coefficient
.GAMMA.. When the Al composition ratio xn is 0.9, light does not
enter the active layer 13 in a non-ridge portion and thus laser
oscillation is not permitted. Therefore, it is understood that the
Al composition difference of the n-type cladding layer 12 and the
p-type cladding layer 14 is required to be smaller than 0.1. Here,
when a lowering rate of optical confinement having the Al
composition ratio x=0.92 is converted into threshold current, the
lowering rate corresponds to a lowering by 20% or more. An increase
in threshold current poses an increase in reactive current and it
lowers high-temperature characteristics and reliability. Therefore,
it is preferable to set the increased amount to 20% or smaller,
that is, it is preferable to set the Al composition ratio
difference to 0.08 or smaller.
[0057] The calculation result explained above means that light is
not permitted to enter the ridge portion 16 abruptly as the Al
composition ratio xn of the n-type cladding layer 12 is decreased
and it largely affects the FFP in the horizontal direction among
other characteristics values. Accordingly, influence on the beam
spread angle in the FFP horizontal direction given by the Al
composition ratio difference was further examined. FIG. 3
illustrates a calculation result of FFP horizontal half width.
[0058] When the inventors of the present invention confirmed
variations in Al composition ratio x among wafers, it was found
that there was a variation of about .+-.0.01 from a target value.
Accordingly, assuming that a variation is .+-.0.01 in mass
production, the influence on the FFP horizontal half width is
illustrated with the case of shifting the Al composition ratio x by
+0.01 and the case of shifting the Al composition ratio x by -0.01
together in FIG. 3.
[0059] As illustrated in FIG. 3, the smaller the Al composition
ratio xn of the n-type cladding layer 12, that is, the larger the
Al composition ratio difference from the p-type cladding layer 14,
the larger the influence on the FFP horizontal half width. In
addition, in accordance with customer's demands, that is,
restrictions in optical design, it is necessary to suppress the FFP
horizontal half width from 6 to 12 degrees, that is, a range within
6 degrees. To achieve that, in consideration of fluctuations in the
Al composition ratio x in mass production, the Al composition ratio
difference of the n-type cladding layer 12 and the p-type cladding
layer 14 is preferable to be smaller than or equal to 0.08.
[0060] To confirm the effects of the present embodiment, the
inventors of the present invention made a semiconductor laser and
carried out an evaluation of characteristics and reliability on the
semiconductor laser. A wafer was cleaved with a cavity length of
2500 .mu.m and a passivation film is used at end facets. To
evaluate characteristics and reliability, a chip (laser chip) to
which the semiconductor laser is formed was mounted on a sub-mount
in the junction-down manner and then the sub-mount was loaded on a
stem and the chip was hermetic sealed with a cap. Also, as a
comparative example of the present embodiment, a semiconductor
laser in which an n-type cladding layer and a p-type cladding layer
are formed with Al.sub.0.5In.sub.0.5P was made.
[0061] FIG. 4 illustrates high-temperature I-L shapes of the
present embodiment and the comparative example. The measurement
conditions are 50.degree. C. and CW. In the present embodiment, the
threshold current was smaller than the comparative example by about
10% and the optical output power was larger than the comparative
example by 10% or more. In addition, although a kink was generated
in the comparative example, a kink was not found in the present
embodiment and thus the I-L characteristic is good in the present
embodiment. The improvements in the high-temperature
characteristics in the present embodiment is considered to be
achieved by an effect of an improvement in crystallinity of the
active layer, which will be described in detail below, in addition
to the effects of a reduction in heat generation amount and an
improvement in slope efficiency by a reduction in threshold
current.
[0062] FIG. 5 illustrates results of lifetime test of the present
embodiment and the comparative example. Test conditions are
50.degree. C. and 150 mW CW-APC. In the comparative example, an
increased ratio of operation current after 1500 hours was 2.5 to
4%. On the contrary, an increased ratio of operation current after
1500 hours was 0 to 1% in the present embodiment. A reason of this
result is considered as follows.
[0063] Generally, as Al is prone to be oxidized in property, when
AlInP having a high Al composition ratio x is made by crystal
growth in MOCVD, oxygen and/or moisture in the air is absorbed on
AlInP and it makes crystallinity to degrade easily. In addition,
degradation in crystallinity in the n-type cladding layer
influences quality of the active layer which will be grown on the
n-type cladding layer. Therefore, when AlInP is used in the n-type
cladding layer like the comparative example, a reason of the
lowering in reliability occurred is considered such that
dislocations threading the active layer were increased due to
degradation in crystallinity in the n-type cladding layer and
multiplication of dislocations became more prone to occur when the
laser element was operated for a long time. On the other hand, in
the present embodiment setting the Al composition ratio xn of the
n-type cladding layer at 0.95, it is considered such that
multiplication of dislocations in the active layer was suppressed
by an improvement in crystallinity in the n-type cladding
layer.
[0064] FIG. 6 illustrates measurement results of photoluminescence
wavelengths from the active layers. While a full width at half
maximum of the comparative example described above was 18.3 nm, a
full width at half maximum of the present embodiment was 11.7 nm
and this is smaller than that of the comparative example. The full
width at half maximum indicates quality of the active layer and the
result means that the composition ratio or thickness has less
fluctuations in the photoluminescence-observed region having a
diameter of about 2.5 mm. As to a thickness in the wafer plane upon
finishing crystal growth, fluctuations were suppressed to 1% or
less in both the present embodiment and the comparative example.
Therefore, the change in the full width at half maximum is supposed
to be caused by a fluctuation in elemental composition ratio.
[0065] More specifically, in the comparative example, it is
considered that Al and In which are group III elements locally
formed Al-rich regions and/or In-rich regions so that fluctuations
in composition ratio occur. Meanwhile, in the present embodiment,
it is considered that introduction of Ga brought an effect of
averaging fluctuations in composition ratio. More specifically, it
is supposed that Ga is easy to be mixed in the local Al-rich
regions and/or In-rich regions as Ga has an atomic radius between
those of Al and In and thus local Al-rich regions and/or In-rich
regions are easier to dissipate as a result. When the local Al-rich
regions are dissipated, regions in which local absorption of oxygen
and/or moisture often occurs are reduced. Therefore, it is supposed
that dislocations by concentration of dislocations and further
multiplication of dislocations could be suppressed and it resulted
in the high reliability. That is, by using AlGaInP in which a
minute amount of Ga was added in the n-type cladding layer, the
crystallinity of the n-type cladding layer was improved and it was
lead to the improvement in crystallinity in the active layer formed
on the n-type cladding layer and thus the effects of improvements
in characteristics and reliability were exhibited.
[0066] As a modification example of the present embodiment, the
n-type cladding layer may be formed of
(Al.sub.0.92Ga.sub.0.08).sub.0.5In.sub.0.5P (Al composition ratio
xn=0.92) and the p-type cladding layer may be formed of
Al.sub.0.5In.sub.0.5P (Al composition ratio xp=1). In this case,
the Al composition ratio difference between the cladding layers are
larger and thus variations in FFP horizontal half width among
wafers will be large in mass production. However, as described in
the calculation example in FIG. 3 above, the present modification
example is the lower limit that does not pose a problem in view of
the standard of the FFP horizontal half width.
[0067] When the Al composition ratio difference between the n-type
cladding layer and the p-type cladding layer is large, as described
above, there is a merit of high-temperature characteristics because
the p-type cladding layer can be made thinner etc. However, on the
other hand, there is such a tradeoff relationship that there is a
demerit of an increase in characteristics variations when the Al
composition ratio difference is too large.
[0068] FIG. 7 illustrates a relation of an Al composition ratio xn
of the n-type cladding layer and a variation of FFP horizontal half
width and a relation of an Al composition ratio xn of the n-type
cladding layer and a kink level (optical output power value in
which a kink occurs). Here, the variation of FFP horizontal half
width is a value calculated from the calculation result in FIG. 3
and the value is obtained by dividing a difference between the half
widths of the variations .+-.0.01 of the Al composition ratio (xn)
by a half width at the center. In addition, the kink level is
obtained from actual measured values of the present embodiment and
the comparative examples.
[0069] A lowering of kink level and an increase in variation of FFP
horizontal half width pose an increase in a cost due to a lowering
of yield. Therefore, to consider mass production, a difference
(xp-xn) between the Al composition ratio (xp) of the p-type
cladding layer and the Al composition ratio (xn) of the n-type
cladding layer is preferable to be 0.02 or larger.
[0070] Also, as a modification example of the present embodiment, a
configuration in which As is contained in the active layer can be
used. For example, an optical guiding layer is formed of AlGaAs and
a well layer is formed with GaAs and a thickness of the well layer
is 6 nm. In this case, a near infrared light around 0.83 .mu.m
lasing wavelength can be obtained. Generally, near infrared and
infrared semiconductor lasers of 0.7 nm to 1 .mu.m lasing
wavelength have a cladding layer formed of AlGaAs. On the contrary,
when the cladding layer is formed of an AlGaInP-based material, a
bandgap difference between the cladding layer and the active layer
can be large and thus an improvement in high-temperature
characteristics is possible.
[0071] FIG. 8 is a broken perspective view of main parts
illustrating a whole configuration of the semiconductor laser of
the present embodiment. The semiconductor laser includes: a stem 30
in a disk-like shape formed of a Fe (iron) alloy and having a
diameter of about 5.6 mm and a thickness of about 1.2 mm; and a
package (sealing container) having a cap 31 for covering an upper
surface of the stem 30. An outer perimeter of a bottom portion of
the cap 31 is fixed to the upper surface of the stem 30. In
addition, a round hole 33 to which a glass plate 32 permeable to
laser beam is joined is provided to a center portion of the upper
surface of the cap 31.
[0072] A heat sink 34 formed of a metal having a good heat
conductivity is mounted in a vicinity of a center of the upper
surface of the stem 30 covered by the cap 31. The heat sink 34 is
joined to the upper surface of the stem via a brazing filler metal
(not illustrated) and a submount 35 is fixed to the whole surface
of the upper surface of the stem 30 via solder (not illustrated).
On the other side, to the lower surface of the stem 30, three leads
40, 41 and 42 are attached.
[0073] A laser chip 10A illustrated in FIG. 1 is mounted to a
chip-mounting surface of the submount 35 in the junction-down
manner.
[0074] The submount 35 both works as a heat dissipation plate for
dissipating heat generated upon light emission of a laser beam to
the outside of the laser chip 10A and a supporting substrate for
supporting the laser chip 10A.
[0075] To the chip-mounting surface of the submount 35, a submount
electrode (not illustrated) electrically connected to the p-side
electrode 20 (see FIG. 1) of the laser chip 10A is formed and an
end of an Au wire 36 is bonded to a surface of the submount
electrode. On the other side, an end of an Au wire 37 is bonded to
a surface of the n-side electrode 21 of the laser chip 10A.
[0076] A laser beam exits from both end facets (upper end facet and
lower end facet in FIG. 8) of the laser chip 10A mounted on the
submount 35. Thus, the submount 35 supporting the laser chip 10A is
fixed to the heat sink 34, so that the chip-mounting surface faces
a direction perpendicular to the upper surface of the stem 30. The
laser beam (forward light) exited from the upper end facet of the
laser chip 10A exits to the outside through the round hole 33 of
the cap 31.
Second Embodiment
[0077] In a second embodiment, in addition to an improvement in
high-temperature characteristics and an improvement in
crystallinity described in the first embodiment, an effect of an
improvement in humidity resistance according to a high-temperature
and high-humidity test will be described.
[0078] In a semiconductor laser of the second embodiment, an n-type
cladding layer is formed of
(Al.sub.0.91Ga.sub.0.09).sub.0.5In.sub.0.5P (Al composition ratio
xn=0.91) and a p-type cladding layer is formed of
(Al.sub.0.96Ga.sub.0.04).sub.0.5In.sub.0.5P (Al composition ratio
xp=0.96) and the other configuration of the semiconductor laser is
the same as that of the first embodiment.
[0079] Normally, a semiconductor laser is housed in a package
hermetic sealed with a cap and the inside of the cap is maintained
in atmosphere like dry air in which moisture is eliminated as much
as possible. However, if implementing open-packaging of the
semiconductor laser is possible, there are great merits in
downsizing and price reduction.
[0080] Accordingly, as illustrated in FIG. 9, the laser chip 10A to
which the semiconductor laser having the configuration described
above was mounted on the submount 35 in the junction-down manner
and subjected to a high-temperature and high-humidity test
(85.degree. C., 85% RH) without a hermetic sealing with a cap. In
addition, the chip of the comparative example used in the first
embodiment was mounted on the submount in the junction-down manner
and subjected to a high-temperature and high-humidity test
(85.degree. C., 85% RH) without a hermetic sealing with a cap.
[0081] As a result, in the comparative example, there was no
problem occurred until 1000 hours but there was a lowering in
characteristics among the laser elements (two in 22 laser elements
tested) at 1500 hours. On the other hand, in the present
embodiment, no laser element had a lowering in characteristics even
after 1500 hours (zero in 22 laser elements tested).
[0082] As a result of a visual inspection of the laser element of
the comparative example which had a lowering in characteristics,
there was a discoloration in the p-type cladding layer at the end
facet portions. That is, a reason of the lowering in
characteristics in the high-temperature and high-humidity test is
considered as corrosion by a reaction of Al in the semiconductor
crystal and moisture. On the other hand, in the present embodiment,
it is considered that introduction of a minute amount of Ga in
semiconductor crystal and increased the resistance to corrosion
reaction.
[0083] As described above, the upper surface of the p-type cladding
layer is covered with the passivation film and the p-side electrode
and thus it is considered to be difficult for moisture to intrude
from the upper surface of the semiconductor substrate. However,
since the end facets are covered only by the passivation film, it
is easy for moisture to intrude as compared with the upper surface.
A conceivable countermeasure is that the passivation film is formed
of a film having a high humidity resistance such as a silicon
nitride film. However, there are restrictions in the selection of
the passivation film at the end facets in view of reliability and
thus desirable characteristics and reliability may not be achieved
when the specie of the passivation film is limited. Therefore,
increasing the resistance of the semiconductor crystal itself is
effective in achieving both good characteristics and good
reliability.
Third Embodiment
[0084] In a third embodiment, the present invention is applied to a
broad-area red semiconductor laser. FIG. 10 is a cross-sectional
view illustrating a configuration of a main part (laser chip) of a
semiconductor laser device of the present embodiment.
[0085] Generally, a broad-area semiconductor laser achieves higher
optical output power and higher heat generation than a ridge
semiconductor laser. Therefore, it is more effective to use an
asymmetric structure in which an n-type cladding layer and a p-type
cladding layer have different Al composition ratios.
[0086] In the present embodiment, the n-type cladding layer 12 is
formed of (Al.sub.0.95Ga.sub.0.05).sub.0.5In.sub.0.5P (Al
composition ratio xn=0.95) and the p-type cladding layer 14 is
formed of Al.sub.0.5In.sub.0.5P (Al composition ratio xp=1). The
active layer 13 is formed of a single quantum well (SQW) structure
including an optical guiding layer 13a formed of AlGaInP, a well
layer 13b formed of GaInP or AlGaInP and an optical guiding layer
13e formed of AlGaInP.
[0087] As to the well layer 13b, various lasing wavelengths and
oscillation modes can be given by changing its thickness within a
range smaller than or equal to a critical thickness, composition
and strain. When a tensile strain is given, the thickness is
preferably 6 to 18 nm and the Al composition ratio x is preferably
0 to 0.15. For example, to obtain a lasing wavelength of 640 nm,
the thickness of the well layer 13b is set to 15 nm, the Al
composition ratio x of the well layer 13b is x=0, i.e., GaInP is
used, and the tensile strain is given at -0.8%. In this case, the
laser is oscillated in a TM mode in which components of the
electric field of the laser beam are vibrated in a direction
perpendicular to the active layer 13.
[0088] In addition, a width of a light emitting portion E can be 3
to 200 .mu.m. When the width of the light emitting portion E is set
like this, the laser is oscillated in a multimode. The larger the
width of the light emitting portion R is, the larger the optical
output power is obtained; however, as a threshold current is
increased, a heat generation amount is increased and
high-temperature characteristics are degraded. For example, to
obtain an optical output power of 500 mW, the width of the light
emitting portion E is set at 50 .mu.m. Formation of the light
emitting portion E may be done in a method of forming a ridge or a
method of opening a contact window for current injection.
Fourth Embodiment
[0089] In a fourth embodiment, in addition to the effect of
improving humidity resistance described in the second embodiment,
an effect of improving ability of mass production will be
described.
[0090] In a semiconductor laser of the present embodiment, an
n-type cladding layer is formed of
(Al.sub.0.95Ga.sub.0.05).sub.0.5In.sub.0.5P (Al composition ratio
xn=0.95) and a p-type cladding layer is formed of
(Al.sub.0.95Ga.sub.0.05).sub.0.5In.sub.0.5P (Al composition ratio
xp=0.95). Except for this point, the semiconductor laser of the
present embodiment has the same configuration as the first
embodiment. That is, in the semiconductor laser of the present
embodiment, the Al composition ratio xp of the p-type cladding
layer and the Al composition ratio xn of the n-type cladding layer
are the same (xp=xn).
[0091] When the cladding layers are formed by MOCVD, a composition
ratio of Al and Ga is adjusted by a flow rate of an organic metal
gas. However, when the Al composition ratio is high like the
present embodiment, since a flow rate of Ga is little and it is
close to a limitation in accuracy of a flow rate meter, variations
in composition is large among wafers (wafers with different growth
batches). According to a confirmation of variations in the
composition ratio of Al and Ga (Al composition ratio x) among
wafers, there were variations of about .+-.0.01 found with respect
to a target value.
[0092] The result means that, in the case of the second embodiment,
an actually formed n-type cladding layer may have an Al composition
ratio xn=0.9 to 0.92 while a target value is xn=0.91, and an
actually formed p-type cladding layer may have an Al composition
ratio xp=0.95 to 0.97 while a target value is xp=0.96. Therefore,
when the Al composition ratio x differs in the p-type cladding
layer and n-type cladding layer like the second embodiment, not
only variations among wafers but also variations in composition
among the cladding layers will occur, increasing variations in
characteristics among semiconductor lasers and posing a yield
lowering.
[0093] On the other hand, in the case of the fourth embodiment,
since the n-type cladding layer and p-type cladding layer can be
successively formed with setting flow rate ratios of Al and Ga
identical, there is little difference in the Al composition ratio
between the n-type cladding layer and the p-type cladding layer and
thus what should be considered is only variations among wafers.
Therefore, as compared with the semiconductor laser in which the Al
composition ratio x differs in the p-type cladding layer and n-type
cladding layer, variations in characteristics can be suppressed and
thus an improvement in yield and an improvement in ability of mass
production can be achieved.
[0094] Also, as a modification example of the present embodiment,
the n-type cladding layer can be formed of
(Al.sub.0.91Ga.sub.0.09).sub.0.5In.sub.0.5P (Al composition ratio
xn=0.91) and a p-type cladding layer is formed of
(Al.sub.0.91Ga.sub.0.09).sub.0.5In.sub.0.5P (Al composition ratio
xp=0.91). When the Al composition ratio xp of the p-type cladding
layer is reduced, high-temperature characteristics are lowered but
humidity resistance is improved. This case is on the assumption
that the usage environment is severe and thus the Al composition
ratio x can be set within the range of the present invention in
accordance with a required usage.
[0095] While the first to fourth embodiments of the present
invention made by the inventors have been concretely described in
the foregoing, to obtain the effects of the present invention in
view of high-temperature characteristics, reliability and ability
of mass production, the Al composition ratio (xp) of the p-type
cladding layer is preferable to be in the range of
0.9<xp.ltoreq.1 and the Al composition ratio (xn) of the n-type
cladding layer is preferable to be in the range of 0.9<xn<1
and also a difference between xp and xn is preferable to be in the
range of 0.ltoreq.xp-xn.ltoreq.0.08.
[0096] Further, it is needless to say that the present invention is
not limited to the foregoing embodiments and various modifications
and alterations can be made within the scope of the present
invention.
[0097] While the present invention has been applied to an
AlGaInP-based red semiconductor laser having a lasing wavelength of
640 nm for example in the embodiments described above, since an
AlGaInP-based semiconductor laser can achieve a lasing wavelength
of 0.6 .mu.m to 0.7 .mu.m by changing only the design of the active
layer, the present invention can be applied to an AlGaInP-based
semiconductor laser which is oscillated by a lasing wavelength of
0.6 .mu.m to 0.7 .mu.m.
[0098] In addition, since a lasing wavelength of 0.7 .mu.m to 1
.mu.m can be achieved when As is contained in the active layer, the
present invention can be applied to near-infrared and infrared
semiconductor lasers which are oscillated by a lasing wavelength of
0.7 nm to 1 .mu.m in which an AlGaInP-based material is used in a
cladding layer and also As is contained in an active layer.
[0099] Moreover, the present invention can be applied to an array
laser in which a plurality of semiconductor lasers are arrayed or a
multi-beam laser.
[0100] The present invention is applicable to AlGaInP-based
semiconductor lasers.
* * * * *