U.S. patent application number 09/777822 was filed with the patent office on 2001-08-09 for high-power semiconductor laser device having current confinement structure and index-guides structure and oscillating in transverse mode.
Invention is credited to Fukunaga, Toshiaki.
Application Number | 20010012308 09/777822 |
Document ID | / |
Family ID | 18555374 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012308 |
Kind Code |
A1 |
Fukunaga, Toshiaki |
August 9, 2001 |
High-power semiconductor laser device having current confinement
structure and index-guides structure and oscillating in transverse
mode
Abstract
In a semiconductor laser device: an active layer; a first upper
cladding layer of a first conductive type; a current confinement
layer of a second conductive type; a second upper cladding layer of
the first conductive type; and a contact layer of the first
conductive type are formed above a GaN layer of the second
conductive type. In the semiconductor laser device: a groove is
formed through the full thickness of the current confinement layer
so as to form an index-guided structure; the active layer is a
single or multiple quantum well active layer formed by alternately
forming at least one In.sub.x1Ga.sub.1-x1N well and a plurality of
In.sub.x2Ga.sub.1-x2N barriers, where 0.ltoreq.x2<x1<0.5; the
current confinement layer has a superlattice structure formed with
Ga.sub.1-z4Al.sub.z4N barriers and GaN wells, where 0<z4<1;
the second upper cladding layer is formed over the current
confinement layer so as to cover the groove; and the contact layer
is formed on the entire upper surface of the second upper cladding
layer.
Inventors: |
Fukunaga, Toshiaki;
(Kaisei-machi, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN
MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3202
US
|
Family ID: |
18555374 |
Appl. No.: |
09/777822 |
Filed: |
February 7, 2001 |
Current U.S.
Class: |
372/45.01 |
Current CPC
Class: |
B82Y 20/00 20130101;
H01S 5/0213 20130101; H01S 5/2231 20130101; H01S 5/2228 20130101;
H01S 5/0655 20130101; H01S 5/204 20130101; H01S 2304/12 20130101;
H01S 5/3216 20130101; H01S 5/34333 20130101 |
Class at
Publication: |
372/45 |
International
Class: |
H01S 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 8, 2000 |
JP |
030349/2000 |
Claims
What is claimed is:
1. A semiconductor laser device comprising: a GaN layer of a first
conductive type; an active layer; a first upper cladding layer of a
second conductive type; a current confinement layer of said first
conductive type; a second upper cladding layer of said second
conductive type; and a GaN contact layer of said second conductive
type; wherein said active layer, said first upper cladding layer,
said current confinement layer, said second upper cladding layer,
and said contact layer are formed above said GaN layer; a groove is
formed through a full thickness of said current confinement layer
so as to form an index-guided structure; said active layer is a
single or multiple quantum well active layer formed by alternately
forming at least one In.sub.x1Ga.sub.1-x1N well and a plurality of
In.sub.x2Ga.sub.1-x2N barriers, where 0.ltoreq.x2<x1<0.5;
said current confinement layer has a superlattice structure formed
with Ga.sub.1-z4Al.sub.z4N barriers and GaN wells, where
0<z4<1; said second upper cladding layer is formed over said
current confinement layer so as to cover said groove; and said GaN
contact layer is formed on an entire upper surface of said second
upper cladding layer.
2. A semiconductor laser device according to claim 1, wherein said
Ga.sub.1-z4Al.sub.z4N barriers in said current confinement layer
are doped with a dopant of said first conductive type.
3. A semiconductor laser device according to claim 1, wherein said
Ga.sub.1-z4Al.sub.z4N barriers and said GaN wells in said current
confinement layer are doped with a dopant of said first conductive
type.
4. A semiconductor laser device according to claim 1, wherein said
groove has a width equal to or greater than 1 micrometer, and
smaller than 3 micrometers, and a difference between an equivalent
refractive index of a portion of said active layer under said
groove for light in a propagation mode in a thickness direction and
an equivalent refractive index of another portion of said active
layer under said current confinement layer other than said groove
for said light is in a range of 0.001 to 0.007.
5. A semiconductor laser device according to claim 1, wherein said
groove has a width equal to or greater than 3 micrometers, and a
difference between an equivalent refractive index of a portion of
said active layer under said groove for light in a propagation mode
in a thickness direction and an equivalent refractive index of
another portion of said active layer under said current confinement
layer other than said groove for said light is in a range of 0.001
to 0.02.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a semiconductor laser
device having an index-guided structure.
[0003] 2. Description of the Related Art
[0004] S. Nakamura et al. ("InGaN/GaN/AlGaN-Based Laser Diodes
Grown on GaN Substrates with a Fundamental Transverse Mode,"
Japanese Journal of Applied Physics, vol. 37 (1998) L1020-L1022)
disclose a short-wavelength semiconductor laser device which emits
laser light in the 410 nm band.
[0005] This semiconductor laser device is formed as follows. First,
a GaN substrate is formed by forming a first GaN layer on a
sapphire substrate, selectively growing a second GaN layer by using
a SiO.sub.2 mask, and removing an excessive portion of the second
GaN layer above the top surface of the SiO.sub.2 mask. Then, an
n-type GaN buffer layer, an n-type InGaN crack preventing layer, an
n-type AlGaN/n-type GaN modulation-doped superlattice cladding
layer, an n-type GaN optical waveguide layer, an n-type InGaN/InGaN
multiple quantum well active layer, a p-type AlGaN carrier block
layer, a p-type GaN optical waveguide layer, a p-type AlGaN/GaN
modulation-doped superlattice cladding layer, and a p-type GaN
contact layer are formed on the above GaN substrate. In addition,
an index-guided structure is realized by forming a ridge structure
having a width of about 2 micrometers. However, since it is very
difficult to control the etching depth, the maximum output power in
the fundamental transverse mode is at most about 30 mW. In the
above semiconductor laser device, the contact area between the p
electrode and the p-type GaN contact layer is small, and therefore
the contact resistance and heat generation are great. Therefore, it
is difficult to increase the output power.
[0006] In addition, as disclosed in Japanese Unexamined Patent
Publication, No. 9 (1997)-307190, in the conventional GaN-based
index-guided semiconductor laser devices, the index-guided
structure is realized by the difference in the refractive index
between an AlGaN current confinement layer and a cladding layer.
However, when a difference between equivalent refractive indexes is
increased to a large value in order to obtain a high quality laser
beam by current confinement using the AlGaN current confinement
layer, the relative composition of aluminum in the AlGaN current
confinement layer becomes greater than that in the cladding layer.
Therefore, it is difficult to form the AlGaN current confinement
layer with a sufficient thickness.
[0007] In order to solve the above problem, Japanese Unexamined
Patent Publication, No. 11(1999)-204882 discloses a semiconductor
laser device having a ridge-type index-guided structure realized by
an AlGaN current confinement layer, and the current confinement
layer is realized by a thick superlattice structure. In this
semiconductor laser device, an attempt to decrease the contact
resistance between the electrode and the contact layer is made in
order to avoid the aforementioned problem of the heat generation
due to the contact resistance. However, since the index-guided
structure is realized by forming the ridge, the contact area is
small, and therefore the contact resistance cannot be sufficiently
decreased. In addition, since the stripe area should be formed
corresponding to an undefective region of the GaN layer, and the
undefective region has a width of about 2 micrometers, the maximum
possible width of the stripe area is about 2 micrometers.
Therefore, it is difficult to realize a wide-stripe high-power
semiconductor laser device.
SUMMARY OF THE INVENTION
[0008] The object of the present invention is to provide a
semiconductor laser device which can oscillate in a fundamental
transverse mode even when output power is high, and output a
high-quality Gaussian laser beam.
[0009] According to the present invention, there is provided a
semiconductor laser device comprising a GaN layer of a first
conductive type; an active layer; a first upper cladding layer of a
second conductive type; a current confinement layer of the first
conductive type; a second upper cladding layer of the second
conductive type; and a GaN contact layer of the second conductive
type. In the semiconductor laser device, the active layer, the
first upper cladding layer, the current confinement layer, the
second upper cladding layer, and the GaN contact layer are formed
above the GaN layer; a groove is formed through the full thickness
of the current confinement layer so as to form an index-guided
structure; the active layer is a single or multiple quantum well
active layer formed by alternately forming at least one
In.sub.x1Ga.sub.1-x1N well and a plurality of In.sub.x2Ga.sub.1-x2N
barriers, where 0.ltoreq.x2<x1<0.5; the current confinement
layer has a superlattice structure formed with
Ga.sub.1-z4Al.sub.z4N barriers and GaN wells, where 0<z4<1;
the second upper cladding layer is formed over the current
confinement layer so as to cover the groove; and the GaN contact
layer is formed on the entire upper surface of the second upper
cladding layer. In the active layer, the In.sub.x2Ga.sub.1-x2N
barriers are arranged in both of the outermost layers of the single
or multiple quantum well active layer.
[0010] Due to the above construction, the semiconductor laser
device according to the present invention can oscillate in a
fundamental transverse mode, and output a high-quality Gaussian
laser beam even when output power is high.
[0011] In particular, since the active layer is a single or
multiple quantum well active layer formed by alternately forming at
least one In.sub.x1Ga.sub.1-x1N well and a plurality of
In.sub.x2Ga.sub.1-x2N barriers, the probability of occurrence of a
crystal defect can be reduced, and the semiconductor laser device
according to the present invention can generate a reliable
short-wavelength laser beam. Further, when the active layer is a
multiple quantum well active layer, the characteristics of the
semiconductor laser device can be improved. For example, the
threshold current can be reduced.
[0012] When an aluminum-rich GaAlN material is used in a layered
structure made of GaN-based materials, the lattice mismatch occurs,
and it is difficult to obtain a highly reliable, high-quality
semiconductor laser device. However, since, according to the
present invention, the current confinement layer has a superlattice
structure formed with Ga.sub.1-z4Al.sub.z4N barriers and GaN wells,
and 0<z4<1, it is possible to form the current confinement
layer with a thickness equal to or greater than a critical
thickness, i.e., the thickness of the current confinement layer can
be sufficiently increased so that a desired difference in the
equivalent refractive index can be achieved.
[0013] Further, since the second upper cladding layer is formed
over the current confinement layer so as to cover the groove, and
the contact layer is formed on the entire upper surface of the
second upper cladding layer, the contact area between the second
upper cladding layer and the contact layer can be increased, and
the contact resistance can be reduced. Therefore, the emission
efficiency can be increased, and the threshold current can be
reduced. In particular, when the output power is high, it is
possible to reduce heat generation in and near the electrode. Thus,
it is possible to prevent deterioration of the semiconductor layers
and the electrode due to the heat generation.
[0014] Since the index-guided structure is realized by the internal
confinement structure, the width of the groove can be adjusted by
etching with high accuracy, and therefore a desired stripe width
can be realized. Thus, the semiconductor laser device according to
the present invention can generate a high-quality laser beam.
[0015] Preferably, the semiconductor laser device according to the
present invention may also have one or any possible combination of
the following additional features (i) to (iv).
[0016] (i) The Ga.sub.1-z4Al.sub.z4N barriers in the current
confinement layer may be doped with a dopant of the first
conductive type.
[0017] (ii) The Ga.sub.1-z4Al.sub.z4N barriers and the GaN wells in
the current confinement layer may be doped with a dopant of the
first conductive type.
[0018] In either of the cases (i) and (ii), a desired difference
between the equivalent refractive index of the portion of the
active layer under the groove and the equivalent refractive index
of the other portion of the active layer under the current
confinement layer other than the groove can be obtained, and
therefore a high-quality laser beam can be obtained.
[0019] (iii) The groove may have a width equal to or greater than 1
micrometer, and smaller than 3 micrometers, and the difference
between the equivalent refractive index of the portion of the
active layer under the groove for light in a propagation mode in
the thickness direction and the equivalent refractive index of the
other portion of the active layer under the current confinement
layer (other than the groove) for the light in the propagation mode
in the thickness direction may be in a range of 0.001 to 0.007.
[0020] In this case, the semiconductor laser device according to
the present invention can oscillate in the fundamental transverse
mode which is controlled with high accuracy.
[0021] (iv) The groove may have a width equal to or greater than 3
micrometers, and the difference between the equivalent refractive
index of the portion of the active layer under the groove for light
in a propagation mode in the thickness direction and the equivalent
refractive index of the other portion of the active layer under the
current confinement layer (other than the groove) for the light in
the propagation mode in the thickness direction may be in a range
of 0.001 to 0.02.
[0022] In this case, it is possible to avoid the instability of the
transverse modes due to the higher-mode oscillation.
[0023] The first conductive type is different in carrier polarity
from the second conductive type. That is, when the first conductive
type is n type, the second conductive type is p type.
DESCRIPTION OF THE DRAWING
[0024] FIG. 1 is a cross-sectional view of the semiconductor laser
device as the first embodiment of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENT
[0025] An embodiment of the present invention and its variations
are explained in detail below with reference to the drawing.
[0026] FIG. 1 is a cross-sectional view of the semiconductor laser
device as an embodiment of the present invention.
[0027] As illustrated in FIG. 1, a GaN buffer layer 12 having a
thickness of about 20 nm is formed on a (0001) C face of a sapphire
substrate 11 at a temperature of 500.degree. C. by organometallic
vapor phase epitaxy. Then, a GaN layer 13 having a thickness of
about 2 micrometers is formed on the GaN buffer layer 12 at a
temperature of 1,050.degree. C. Next, a SiO.sub.2 layer 14 (not
shown) is formed on the GaN layer 13, and a resist 15 is applied to
the SiO.sub.2 layer 14 (not shown). Then, stripe areas of the
SiO.sub.2 layer 14 are removed by using conventional lithography,
where the stripe areas are oriented in the <1100> direction
and spaced with intervals of about 10 micrometers, and each have a
width of about 7 micrometers. Thereafter, the exposed stripe areas
of the GaN buffer layer 12 and the GaN layer 13 are removed to the
depth of the upper surface of the sapphire substrate 11 by dry
etching using a chlorine gas as an etchant and the remaining
portions of the SiO.sub.2 layer 14 and the resist 15 as a mask. At
this time, the sapphire substrate 11 may also be etched. Then, the
SiO.sub.2 layer 14 and the resist 15 are removed, so that stripe
grooves are formed between the remaining portions of the GaN buffer
layer 12 and the GaN layer 13. Next, a GaN layer 16 having a
thickness of about 20 micrometers is formed by selective growth.
Due to growth in the lateral directions, the above stripe grooves
between the remaining portions of the GaN buffer layer 12 and the
GaN layer 13 are filled with the GaN layer 16, the remaining
portions of the GaN buffer layer 12 and the GaN layer 13 are
covered with the GaN layer 16, and finally the surface of the GaN
layer 16 is planarized.
[0028] Subsequently, an n-type GaN contact layer 17, an n-type
Ga.sub.1-z1Al.sub.z1N/GaN superlattice lower cladding layer 18, an
n-type or i-type (intrinsic) Ga.sub.1-z2Al.sub.z2N optical
waveguide layer 19, a Si-doped
In.sub.x2Ga.sub.1-x2N/In.sub.x1Ga.sub.1-x1N multiple quantum well
active layer 20 (0.5>x1>x2.gtoreq.0), an p-type
Ga.sub.1-z3Al.sub.z3N carrier block layer 21, an n-type or i-type
Ga.sub.1-z2Al.sub.z2N optical waveguide layer 22, a p-type Ga.sub.
1-z1Al.sub.z1N/GaN superlattice first upper cladding layer 23, and
an n-type Ga.sub.1-z4Al.sub.z4N/GaN superlattice current
confinement layer 24 are formed. Thereafter, a SiO.sub.2 layer 25
(not shown) is formed, and a resist 26 is applied to the SiO.sub.2
layer 25 (not shown). Then, stripe areas of the SiO.sub.2 layer 25
are removed by using conventional lithography, where the stripe
areas each have a width of 2 micrometers (as indicated by the
reference number 31). Then, the exposed stripe areas of the n-type
Ga.sub.1-z4Al.sub.z4N/GaN superlattice current confinement layer 24
are etched to a mid-thickness of the p-type
Ga.sub.1-z1Al.sub.z1N/GaN superlattice first upper cladding layer
23 by using a chlorine gas as an etchant and the remaining portions
of the SiO.sub.2 layer 25 and the resist 26 as a mask so as to form
a groove. After the remaining portions of the SiO.sub.2 layer 25
and the resist 26 are removed, a p-type Ga.sub.1-z1Al.sub.z1N/GaN
superlattice second upper cladding layer 27 and a p-type GaN
contact layer 28 are formed. In the formation of the above layers,
trimethyl gallium (TMG), trimethyl indium (TMI), trimethyl aluminum
(TMA), and ammonia are used as raw materials, silane gas is used as
an n-type dopant gas, and cycropentadienyl magnesium (Cp2Mg) is
used as a p-type dopant gas.
[0029] In the above construction, the compositions and the
thicknesses of the p-type Ga.sub.1-z1Al.sub.z1N/GaN superlattice
first upper cladding layer 23 and the n-type
Ga.sub.1-z4Al.sub.z4N/GaN superlattice current confinement layer 24
are arranged such that the fundamental transverse mode is
achieved.
[0030] It is preferable that the above groove formed in the n-type
Ga.sub.1-z4Al.sub.z4N/GaN superlattice current confinement layer 24
is located above a region of the GaN layer 16 which is not likely
to contain a defect. The first regions of the GaN layer 16 above
the remaining areas of the GaN layer 13 are likely to contain a
defect through the thickness of the GaN layer 16, and the second
regions of the GaN layer 16 located approximately midway between
the remaining areas of the GaN layer 13 are likely to contain a
defect since the second regions are finally filled by the selective
growth in the lateral directions. Therefore, it is preferable that
the above groove formed in the n-type Ga.sub.1-z4Al.sub.z4N/GaN
superlattice current confinement layer 24 is not located right
above the first and second regions of the GaN layer 16, as
illustrated in FIG. 1.
[0031] Next, in order to enable contact with n electrodes 29, both
sides of the above index-guided structure are etched to a
mid-thickness of the n-type GaN contact layer 17 by
photolithography and dry etching. Then, the lower surface of the
sapphire substrate 11 is polished, and the n electrodes 29 and p
electrode 30 are formed by conventional lithography and
evaporation. Thereafter, end surfaces of the resonant cavity are
formed by cleaving the layered materials, and a high-reflection
coating (not shown) and a low-reflection coating (not shown) are
laid on the end surfaces of the resonant cavity, respectively.
Then, the construction of FIG. 1 is formed into a chip.
[0032] In the above construction, the compositions of the AlGaN
layers are arranged such that 1>z4>z1>z2.gtoreq.0 and
0.4>z3>z2. In this case, when the equivalent refractive index
of the region including the cross section A-A' illustrated in FIG.
1 is denoted by n.sub.A, and the equivalent refractive index of the
region including the cross section B-B' illustrated in FIG. 1 is
denoted by n.sub.B, it is possible to control the difference
n.sub.B-n.sub.A in the equivalent refractive index so that
7.times.10.sup.-3>n.sub.B-n.sub.A>1.times.10.sup.-3.
[0033] Although only the Ga.sub.1-z4Al.sub.z4N barriers in the
Ga.sub. 1-z4Al.sub.z4N/GaN superlattice current confinement layer
24 are doped with the n type dopant in the above embodiment, the
GaN wells in the Ga.sub.1-z4Al.sub.z4N/GaN superlattice current
confinement layer 24 may also be doped with an n type dopant.
[0034] Each layer in the construction of the above embodiment may
be formed by molecular beam epitaxy using a solid or gas raw
material.
[0035] The conductivity type of the GaN contact layer 17 may be
inverted. In this case, the conductivity types of all of the other
layers in the above construction should be inverted
accordingly.
[0036] The oscillation wavelength of the semiconductor laser device
as the above embodiment can be controlled within the range between
380 and 550 nm.
[0037] Although the sapphire substrate 11 is used in the above
embodiment, the substrate may be made of one of SiC, ZnO,
LiGaO.sub.2, LiAlO.sub.2, GaAs, GaP, Ge, and Si.
[0038] In the semiconductor laser device as the above embodiment,
the sapphire substrate 11 and the GaN layer 16 remain as
constituents of the semiconductor laser device, and portions of the
upper surfaces of the n-type GaN contact layer 17 are exposed in
order to enable contact with the n electrodes 29. However,
alternatively, the sapphire substrate 11 and the GaN layer 16 may
be removed so as to expose the lower surface of the n-type GaN
contact layer 17, and then the other layers above the n-type GaN
contact layer 17 may be formed, where the n-type GaN contact layer
17 is used as a substrate. In this case, an n electrode can be
formed on the lower surface of the n-type GaN contact layer 17.
[0039] Although the semiconductor laser device as the above
embodiment is arranged so as to oscillate in the fundamental
transverse mode, the stripe groove formed in the current
confinement layer may have a width of 3 micrometers or more. In
this case, the semiconductor laser device according to the present
invention can be used with a wavelength conversion element or a
fiber laser, where the semiconductor laser device according to the
present invention functions as a low-noise wide-stripe
semiconductor laser device which excites the wavelength conversion
element or the fiber laser.
[0040] When the present invention is applied to a wide-stripe
semiconductor laser device, it is preferable to use the GaN
substrates disclosed in Japanese patent applications, Nos.
2000-4940, 11(1999)-285146, 11(1999)-289069, and 11(1999)-292112,
which are assigned to the assignee of the present patent
application. In each of the GaN substrates disclosed in the above
Japanese patent applications, the probability of occurrence of a
defect through the thickness of the substrate is low, and the
probability of occurrence of a defect is low in a wide area of the
substrate. Therefore, the reliability of the semiconductor laser
device can be further increased by use of the above substrates.
[0041] The semiconductor laser device according to the present
invention can be used as a light source in the fields of
high-speed, information processing, image processing,
communications, laser measurement, medicine, printing, and the
like.
* * * * *