U.S. patent application number 12/836170 was filed with the patent office on 2011-09-22 for group-iii nitride semiconductor laser device, and method for fabricating group-iii nitride semiconductor laser device.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Masahiro ADACHI, Yohei ENYA, Takatoshi IKEGAMI, Koji KATAYAMA, Takashi KYONO, Takao NAKAMURA, Takamichi SUMITOMO, Shimpei TAKAGI, Shinji TOKUYAMA, Masaki UENO, Yusuke YOSHIZUMI.
Application Number | 20110228804 12/836170 |
Document ID | / |
Family ID | 44304136 |
Filed Date | 2011-09-22 |
United States Patent
Application |
20110228804 |
Kind Code |
A1 |
YOSHIZUMI; Yusuke ; et
al. |
September 22, 2011 |
GROUP-III NITRIDE SEMICONDUCTOR LASER DEVICE, AND METHOD FOR
FABRICATING GROUP-III NITRIDE SEMICONDUCTOR LASER DEVICE
Abstract
Provided is a group-III nitride semiconductor laser device with
a laser cavity of high lasing yield, on a semipolar surface of a
support base in which the c-axis of a hexagonal group-III nitride
is tilted toward the m-axis. First and second fractured faces 27,
29 to form the laser cavity intersect with an m-n plane. The
group-III nitride semiconductor laser device 11 has a laser
waveguide extending in a direction of an intersecting line between
the m-n plane and the semipolar surface 17a. For this reason, it is
feasible to make use of emission by a band transition enabling the
low threshold current. In a laser structure 13, a first surface 13a
is opposite to a second surface 13b. The first and second fractured
faces 27, 29 extend from an edge 13c of the first surface 13a to an
edge 13d of the second surface 13b. The fractured faces are not
formed by dry etching and are different from
conventionally-employed cleaved facets such as c-planes, m-planes,
or a-planes.
Inventors: |
YOSHIZUMI; Yusuke;
(Itami-shi, JP) ; TAKAGI; Shimpei; (Osaka-shi,
JP) ; ENYA; Yohei; (Itami-shi, JP) ; KYONO;
Takashi; (Itami-shi, JP) ; ADACHI; Masahiro;
(Osaka-shi, JP) ; UENO; Masaki; (Itami-shi,
JP) ; SUMITOMO; Takamichi; (Itami-shi, JP) ;
TOKUYAMA; Shinji; (Osaka-shi, JP) ; KATAYAMA;
Koji; (Osaka-shi, JP) ; NAKAMURA; Takao;
(Itami-shi, JP) ; IKEGAMI; Takatoshi; (Itami-shi,
JP) |
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
Osaka-shi
JP
|
Family ID: |
44304136 |
Appl. No.: |
12/836170 |
Filed: |
July 14, 2010 |
Current U.S.
Class: |
372/44.011 ;
257/E33.003; 438/33; 438/46 |
Current CPC
Class: |
H01S 5/0014 20130101;
H01S 5/320275 20190801; H01S 5/0202 20130101; H01S 5/2201 20130101;
H01S 5/2031 20130101; H01S 5/3213 20130101; H01S 2301/14 20130101;
H01S 5/0287 20130101; B82Y 20/00 20130101; H01S 5/2009 20130101;
H01S 5/34333 20130101 |
Class at
Publication: |
372/44.011 ;
438/33; 438/46; 257/E33.003 |
International
Class: |
H01S 5/30 20060101
H01S005/30; H01L 33/02 20100101 H01L033/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 18, 2010 |
JP |
2010-008181 |
Claims
1. A group-III nitride semiconductor laser device comprising: a
laser structure including a support base and a semiconductor
region, the support base comprising a hexagonal group-III nitride
semiconductor and having a semipolar principal surface, the
semiconductor region being provided on the semipolar principal
surface of the support base; and an electrode being provided on the
semiconductor region of the laser structure, the semiconductor
region including a first cladding layer of a first conductivity
type gallium nitride-based semiconductor, a second cladding layer
of a second conductivity type gallium nitride-based semiconductor,
and an active layer, the active layer being provided between the
first cladding layer and the second cladding layer, the first
cladding layer, the second cladding layer, and the active layer
being arranged along a normal axis to the semipolar principal
surface, the active layer including a gallium nitride-based
semiconductor layer, a c-axis of the hexagonal group-III nitride
semiconductor of the support base tilting at a finite angle ALPHA
with respect to the normal axis toward an m-axis of the hexagonal
group-III nitride semiconductor, the angle ALPHA falling within a
range of not less than 45.degree. and not more than 80.degree. or
within a range of not less than 100.degree. and not more than
135.degree., the laser structure including first and second
fractured faces, the first and second fractured faces intersecting
with an m-n plane defined by the m-axis of the hexagonal group-III
nitride semiconductor and the normal axis, a laser cavity of the
group-III nitride semiconductor laser device including the first
and second fractured faces, the laser structure including first and
second surfaces, the first surface is opposite to the second
surface, each of the first and second fractured faces extending
from an edge of the first surface to an edge of the second surface,
an end face of the support base and an end face of the
semiconductor region being exposed in each of the first and second
fractured faces, and the first and second fractured faces including
a region such that an angle between this region and a plane
indicated by plane index (-1, 0, 1, L) or (1, 0, -1, -L) falls
within a range of not less than -5.degree. and not more than
+5.degree., with L as an integer number not less than 4.
2. The group-III nitride semiconductor laser device according to
claim 1, wherein the first and second fractured faces can include a
region such that an angle formed by this region and the
arrangements of N atom --Ga atom extending toward a direction
tilting at an angle of 70.53.degree. in the direction opposite to
the direction of the m-axis of the hexagonal group-III nitride
semiconductor with respect to the direction of the c-axis of the
hexagonal group-III nitride semiconductor, falls within a range of
not less than -10.degree. and not more than +10.degree..
3. The group-III nitride semiconductor laser device according to
claim 1, wherein a part of the first and second fractured faces
that is included in the active layer can include a part of or the
whole of an region such that an angle between this region and the
plane indicated by plane index (-1, 0, 1, L) or (1, 0, -1, -L)
falls within a range of not less than -5.degree. and not more than
+5.degree..
4. The group-III nitride semiconductor laser device according to
claim 1, wherein a part of the first and second fractured faces
that is included in the active layer can include a part of or the
whole of an region such that an angle formed by this region and the
arrangements of N atom --Ga atom extending toward a direction
tilting at an angle of 70.53.degree. in the direction opposite to
the direction of the m-axis of the hexagonal group-III nitride
semiconductor with respect to the direction of the c-axis of the
hexagonal group-III nitride semiconductor, falls within a range of
not less than -10.degree. and not more than +10.degree..
5. The group-III nitride semiconductor laser device according to
claim 1, wherein the angle ALPHA falls within a range of not less
than 63.degree. and not more than 80.degree. or within a range of
not less than 100.degree. and not more than 117.degree..
6. The group-III nitride semiconductor laser device according to
claim 1, wherein a thickness of the support base is not more than
400 .mu.m.
7. The group-III nitride semiconductor laser device according to
claim 1, wherein a thickness of the support base is not less than
50 .mu.m and not more than 100 .mu.m.
8. The group-III nitride semiconductor laser device according to
claim 1, wherein laser light from the active layer is polarized in
a direction of an a-axis of the hexagonal group-III nitride
semiconductor.
9. The group-III nitride semiconductor laser device according to
claim 1, wherein light in an LED mode of the group-III nitride
semiconductor laser device includes a polarization component I2 in
a direction indicated by a projection of the c-axis of the
hexagonal group-III nitride semiconductor onto the principal
surface, and a polarization component I1 in the direction of an
a-axis of the hexagonal group-III nitride semiconductor, and
wherein the polarization component I1 is greater than the
polarization component I2.
10. The group-III nitride semiconductor laser device according to
claim 1, wherein the semipolar principal surface is slightly tilted
in a range of not less than -4.degree. and not more than +4.degree.
with respect to any one of {20-21} plane, {10-11} plane, {20-2-1}
plane, and {10-1-1} plane.
11. The group-III nitride semiconductor laser device according to
claim 1, wherein the semipolar principal surface is any one of
{20-21} plane, {10-11} plane, {20-2-1} plane, and {10-1-1}
plane.
12. The group-III nitride semiconductor laser device according to
claim 1, wherein a stacking fault density of the support base is
not more than 1.times.10.sup.4 cm.sup.-1.
13. The group-III nitride semiconductor laser device according to
claim 1, wherein the support base comprises any one of GaN, AlGaN,
AlN, InGaN, and InAlGaN.
14. The group-III nitride semiconductor laser device according to
claim 1, further comprising a dielectric multilayer film provided
on at least one of the first and second fractured faces.
15. The group-III nitride semiconductor laser device according to
claim 1, wherein the active layer includes a light emitting region
provided so as to generate light at a wavelength of not less than
360 nm and not more than 600 nm.
16. The group-III nitride semiconductor laser device according to
claim 1, wherein the active layer includes a quantum well structure
provided so as to generate light at a wavelength of not less than
430 nm and not more than 550 nm.
17. A method for fabricating a group-III nitride semiconductor
laser device, the method comprising the steps of: preparing a
substrate of a hexagonal group-III nitride semiconductor, the
substrate having a semipolar principal surface; forming a substrate
product that has a laser structure, an anode electrode and a
cathode electrode, the laser structure including the substrate and
a semiconductor region, the semiconductor region being formed on
the semipolar principal surface; scribing a first surface of the
substrate product in part in a direction of an a-axis of the
hexagonal group-III nitride semiconductor; and carrying out breakup
of the substrate product by press against a second surface of the
substrate product, to form another substrate product and a laser
bar, the first surface being opposite to the second surface, the
semiconductor region being located between the first surface and
the substrate, the laser bar having first and second end faces, the
first and second end faces being formed by the breakup, and the
first and second end faces extending from the first surface to the
second surface, the first and second end faces constituting a laser
cavity of the group-III nitride semiconductor laser device, the
anode electrode and the cathode electrode being formed on the laser
structure, the semiconductor region comprising a first cladding
layer of a first conductivity type gallium nitride-based
semiconductor, a second cladding layer of a second conductivity
type gallium nitride-based semiconductor and an active layer, the
active layer being provided between the first cladding layer and
the second cladding layer, the first cladding layer, the second
cladding layer, and the active layer being arranged along a normal
axis to the semipolar principal surface, the active layer
comprising a gallium nitride-based semiconductor layer, a c-axis of
the hexagonal group-III nitride semiconductor of the substrate
tilting at an angle ALPHA with respect to the normal axis toward an
m-axis of the hexagonal group-III nitride semiconductor, the angle
ALPHA falling within a range of not less than 45.degree. and not
more than 80.degree. or within a range of not less than 100.degree.
and not more than 135.degree., the first and second end faces
intersecting with an m-n plane defined by the m-axis of the
hexagonal group-III nitride semiconductor and the normal axis, and
the first and second end faces including a region such that an
angle between this region and a plane indicated by plane index (-1,
0, 1, L) or (1, 0, -1, -L) falls within a range of not less than
-5.degree. and not more than +5.degree., with L as an integer
number not less than 4.
18. The method according to claim 17, wherein the first and second
end faces can include a region such that an angle formed by this
region and the arrangements of N atom --Ga atom extending toward a
direction tilting at an angle of 70.53.degree. in the direction
opposite to the direction of the m-axis of the hexagonal group-III
nitride semiconductor with respect to the direction of the c-axis
of the hexagonal group-III nitride semiconductor, falls within a
range of not less than -10.degree. and not more than
+10.degree..
19. The method according to claim 17, wherein a part of the first
and second end faces that is included in the active layer can
include a part of or the whole of a region such that an angle
between this region and the plane indicated by plane index (-1, 0,
1, L) or (1, 0, -1, -L) falls within a range of not less than
-5.degree. and not more than +5.degree..
20. The method according to claim 17, wherein a part of the first
and second end faces that is included in the active layer can
include a part of or the whole of a region such that an angle
formed by this region and the arrangements of N atom --Ga atom
extending toward a direction tilting at an angle of 70.53.degree.
in the direction opposite to the direction of the m-axis of the
hexagonal group-III nitride semiconductor with respect to the
direction of the c-axis of the hexagonal group-III nitride
semiconductor, falls within a range of not less than -10.degree.
and not more than +10.degree..
21. The method according to claim 17, wherein the angle ALPHA falls
within a range of not less than 63.degree. and not more than
80.degree. or within a range of not less than 100.degree. and not
more than 117.degree..
22. The method according to claim 17, wherein the step of forming
the substrate product comprises performing processing such as
slicing or grinding of the substrate so that a thickness of the
substrate becomes not more than 400 .mu.m, and wherein the second
surface is one of the following: a processed surface formed by the
processing; and a surface including an electrode formed on the
processed surface.
23. The method according to claim 17, wherein the step of forming
the substrate product comprises polishing the substrate so that a
thickness of the substrate becomes not less than 50 .mu.m and not
more than 100 .mu.m, and wherein the second surface is one of the
following: a polished surface formed by the polishing; and a
surface including an electrode formed on the polished surface.
24. The method according to claim 17, wherein the scribing is
carried out using a laser scriber, and wherein the scribing forms a
scribed groove, and a length of the scribed groove is shorter than
a length of an intersecting line between the first surface and an
a-n plane defined by the normal axis and the a-axis of the
hexagonal group-III nitride semiconductor.
25. The method according to claim 17, wherein the semipolar
principal surface is any one of {20-21} plane, {10-11} plane,
{20-2-1} plane, and {10-1-1} plane.
26. The method according to claim 17, wherein the substrate
comprises any one of GaN, AlGaN, AlN, InGaN, and InAlGaN.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a group-III nitride
semiconductor laser device, and a method for fabricating the
group-III nitride semiconductor laser device.
[0003] 2. Related Background Art
[0004] Non-patent Document 1 (Jpn. J. Appl. Phys. Vol. 35, (1996)
L74-L76) describes a semiconductor laser made on a c-plane sapphire
substrate. The mirror faces of the semiconductor laser are formed
by dry etching. The Document 1 shows micrographs of the laser
cavity mirror faces of the laser, and describes that the roughness
of the end faces is about 50 nm.
[0005] Non-patent Document 2 (Appl. Phys. Express 1 (2008) 091102)
describes a semiconductor laser formed on a (11-22) plane GaN
substrate. The mirror faces of the semiconductor laser are formed
by dry etching.
[0006] Non-patent Document 3 (Jpn. J. Appl. Phys. Vol. 46, (2007)
L789) describes a gallium nitride (GaN)-based semiconductor laser.
It proposes generation of laser light polarized in an off direction
of the c-axis of the substrate, in order to use m-plane cleaved
facets for the laser cavity. Specifically, this Document describes
increase of the well thickness on a non-polar surface and decrease
of the well thickness on a semipolar surface.
SUMMARY OF THE INVENTION
[0007] The band structure of the GaN-based semiconductor has some
possible transitions capable of lasing. According to the inventor's
knowledge, it is considered that in the group-III nitride
semiconductor laser device using the semipolar-plane support base
the c-axis of which tilts toward the m-axis, the threshold current
can be lowered when the laser waveguide extends along a plane
defined by the c-axis and the m-axis. When the laser waveguide
extends in this orientation, a mode with the smallest transition
energy (difference between conduction band energy and valence band
energy) among the above possible transitions becomes capable of
lasing; when this mode becomes capable of lasing, the threshold
current can be reduced.
[0008] However, this orientation of the laser waveguide does not
allow use of the conventional cleaved facets such as c-planes,
a-planes, or m-planes for the laser cavity mirrors. For this
reason, the laser cavity mirrors have been made heretofore by
forming dry-etched facets of semiconductor layers by reactive ion
etching (RIE). What is now desired is an improvement (that is to
say, the development of laser cavity of high lasing yield) in the
laser cavity mirrors, which have been formed by RIE, in terms of
perpendicularity to the laser waveguide, flatness of the dry-etched
facets, or ion damage. It becomes a heavy burden to find process
conditions for obtaining excellent dry-etched faces in the current
technical level.
[0009] As far as the inventor knows, in a single group-III nitride
semiconductor laser device formed on the semipolar surface, no one
has succeeded heretofore in achieving both of the laser waveguide,
which extends in the tilting direction (off direction) of the
c-axis, and the end faces for laser cavity mirrors formed without
use of dry etching.
[0010] The present invention has been accomplished in view of the
above-described circumstances. It is an object of the present
invention to provide a group-III nitride semiconductor laser device
with a laser cavity of high lasing yield, on the semipolar surface
of a support base that tilts with respect to the c-axis toward the
m-axis of a hexagonal group-III nitride. It is another object of
the present invention to provide a method for fabricating the
group-III nitride semiconductor laser device.
[0011] A group-III nitride semiconductor laser device according to
one aspect of the present invention comprises: a laser structure
including a support base and a semiconductor region, the support
base comprising a hexagonal group-III nitride semiconductor and
having a semipolar principal surface, the semiconductor region
being provided on the semipolar principal surface of the support
base; and an electrode being provided on the semiconductor region
of the laser structure, the semiconductor region including a first
cladding layer of a first conductivity type gallium nitride-based
semiconductor, a second cladding layer of a second conductivity
type gallium nitride-based semiconductor, and an active layer, the
active layer being provided between the first cladding layer and
the second cladding layer, the first cladding layer, the second
cladding layer, and the active layer being arranged along a normal
axis to the semipolar principal surface, the active layer including
a gallium nitride-based semiconductor layer, a c-axis of the
hexagonal group-III nitride semiconductor of the support base
tilting at a finite angle ALPHA with respect to the normal axis
toward an m-axis of the hexagonal group-III nitride semiconductor,
the angle ALPHA falling within a range of not less than 45.degree.
and not more than 80.degree. or within a range of not less than
100.degree. and not more than 135.degree., the laser structure
including first and second fractured faces, the first and second
fractured faces intersecting with an m-n plane defined by the
m-axis of the hexagonal group-III nitride semiconductor and the
normal axis, a laser cavity of the group-III nitride semiconductor
laser device including the first and second fractured faces, the
laser structure including first and second surfaces, the first
surface is opposite to the second surface, each of the first and
second fractured faces extending from an edge of the first surface
to an edge of the second surface, an end face of the support base
and an end face of the semiconductor region being exposed in each
of the first and second fractured faces, and the first and second
fractured faces including a region such that an angle between this
region and a plane indicated by plane index (-1, 0, 1, L) or (1, 0,
-1, -L) falls within a range of not less than -5.degree. and not
more than +5.degree., with L as an integer number not less than 4.
Therefore, the first and second fractured faces forming the laser
cavity mirrors include the region of the plane index such as
mentioned above. Thus, these laser cavity mirrors has flatness and
perpendicularity, and the lasing yield of the laser cavity can be
improved.
[0012] In this group-III nitride semiconductor laser device, the
first and second fractured faces can include a region such that an
angle formed by this region and the arrangements of N atom --Ga
atom extending toward a direction tilting at an angle of
70.53.degree. in the direction opposite to the direction of the
m-axis of the hexagonal group-III nitride semiconductor with
respect to the direction of the c-axis of the hexagonal group-III
nitride semiconductor, falls within a range of not less than
-10.degree. and not more than +10.degree.. Therefore, even when the
first and second fractured faces included in the laser cavity
include the region such that the angle formed by this region and
the arrangements of N atom --Ga atom of the hexagonal group-III
nitride semiconductor of the support base falls within a range of
not less than -10.degree. and not more than +10.degree., the first
and second fractured faces have flatness and perpendicularity as a
laser cavity mirror, and thus, the lasing yield of the laser cavity
can be improved.
[0013] In this group-III nitride semiconductor laser device, a part
of the first and second fractured faces that is included in the
active layer can include a part of or the whole of an region such
that an angle between this region and the plane indicated by plane
index (-1, 0, 1, L) or (1, 0, -1, -L) falls within a range of not
less than -5.degree. and not more than +5.degree.. Therefore, the
part that is at least included in the active layer on the first and
second fractured faces forming the laser cavity mirrors includes
the region of the plane index such as above. Thus, these laser
cavity mirrors have flatness and perpendicularity, and the lasing
yield of the laser cavity can be improved.
[0014] In this group-III nitride semiconductor laser device, a part
of the first and second fractured faces that is included in the
active layer can include a part of or the whole of an region such
that an angle formed by this region and the arrangements of N atom
--Ga atom extending toward a direction tilting at an angle of
70.53.degree. in the direction opposite to the direction of the
m-axis of the hexagonal group-III nitride semiconductor with
respect to the direction of the c-axis of the hexagonal group-III
nitride semiconductor, falls within a range of not less than
-10.degree. and not more than +10.degree.. Even when the part that
is at least included in the active layer on the first and second
fractured faces forming the laser cavity mirrors includes the
region such that the angle formed by this region and the
arrangements of N atom --Ga atom of the hexagonal group-III nitride
semiconductor of the support base falls within a range of not less
than -10.degree. and not more than +10.degree., the first and
second fractured faces have flatness and perpendicularity as a
laser cavity mirror, and thus, the lasing yield of the laser cavity
can be improved.
[0015] In this group-III nitride semiconductor laser device, the
angle ALPHA falls within a range of not less than 63.degree. and
not more than 80.degree. or within a range of not less than
100.degree. and not more than 117.degree.. In this group-III
nitride semiconductor laser device, when the angle ALPHA is in a
range of not less than 63.degree. and not more than 80.degree. or
in a range of not less than 100.degree. and not more than
117.degree., it is going to be more likely that the end face formed
by the press will be almost perpendicular to the principal surface
of the substrate. Furthermore, when the angle is in a range of more
than 80.degree. and less than 100.degree., it might result in
failing to achieve desired flatness and perpendicularity.
[0016] In this group-III nitride semiconductor laser device, a
thickness of the support base is not more than 400 .mu.m. This
group-III nitride semiconductor laser device can be used to obtain
a good-quality fractured face for a laser cavity.
[0017] In this group-III nitride semiconductor laser device, a
thickness of the support base is not less than 50 .mu.m and not
more than 100 .mu.m. When the thickness is not less than 50 .mu.m,
handling becomes easier, and production yield becomes higher. When
the thickness is in a range of not more than 100 .mu.m, it can be
used to obtain a good-quality fractured face for a laser
cavity.
[0018] In this group-III nitride semiconductor laser device, laser
light from the active layer is polarized in a direction of an
a-axis of the hexagonal group-III nitride semiconductor. In this
group-III nitride semiconductor laser device, a band transition
allowing for implementation of a low threshold current has
polarized nature.
[0019] In this group-III nitride semiconductor laser device, light
in an LED mode of the group-III nitride semiconductor laser device
includes a polarization component I2 in a direction indicated by a
projection of the c-axis of the hexagonal group-III nitride
semiconductor onto the principal surface, and a polarization
component I1 in the direction of an a-axis of the hexagonal
group-III nitride semiconductor, and the polarization component I1
is greater than the polarization component I2. In this group-III
nitride semiconductor laser device, using the laser cavity of the
group-III nitride semiconductor laser device, the device can be
lased to emit light in a mode with large emission intensity in the
LED mode.
[0020] In this group-III nitride semiconductor laser device, the
semipolar principal surface is slightly tilted in a range of not
less than -4.degree. and not more than +4.degree. with respect to
any one of {20-21} plane, {10-11} plane, {20-2-1} plane, and
{10-1-1} plane. In this group-III nitride semiconductor laser
device, when the slight tilt surface tilts from these typical
semipolar surfaces, it is also feasible to provide the first and
second end faces with flatness and perpendicularity enough to
construct the laser cavity of the group-III nitride semiconductor
laser device.
[0021] In this group-III nitride semiconductor laser device, the
semipolar principal surface is any one of {20-21} plane, {10-11}
plane, {20-2-1} plane, and {10-1-1} plane. In this group-III
nitride semiconductor laser device, these typical semipolar
surfaces can provide the first and second end faces with flatness
and perpendicularity enough to construct the laser cavity of the
group-III nitride semiconductor laser device.
[0022] In this group-III nitride semiconductor laser device, a
stacking fault density of the support base is not more than
1.times.10.sup.4 cm.sup.-1. In this group-III nitride semiconductor
laser device, since the stacking fault density is in a range of not
more than 1.times.10.sup.4 cm.sup.-1, the flatness and/or
perpendicularity of the fractured face is less likely to be
disturbed for a certain accidental reason.
[0023] In this group-III nitride semiconductor laser device, the
support base comprises any one of GaN, AlGaN, AlN, InGaN, and
InAlGaN. In this group-III nitride semiconductor laser device, when
the substrate used comprises one of these GaN-based semiconductors,
it becomes feasible to obtain the first and second end faces
applicable to the cavity. Use of an AlN substrate or AlGaN
substrate allows for increase in polarization degree and for
enhancement of optical confinement by virtue of low refractive
index. Use of an InGaN substrate allows for decrease in the degree
of lattice mismatch between the substrate and the light emitting
layer and for improvement in crystal quality.
[0024] This group-III nitride semiconductor laser device further
comprises a dielectric multilayer film provided on at least one of
the first and second fractured faces. In this group-III nitride
semiconductor laser device, an end face coat is also applicable to
the fractured faces. The end face coat allows for adjustment of
reflectance.
[0025] In this group-III nitride semiconductor laser device, the
active layer includes a light emitting region provided so as to
generate light at a wavelength of not less than 360 nm and not more
than 600 nm. Since this group-III nitride semiconductor laser
device makes use of the semipolar surface, the resultant device is
the group-III nitride semiconductor laser device making efficient
use of polarization in the LED mode, and achieves a low threshold
current.
[0026] In this group-III nitride semiconductor laser device, the
active layer includes a quantum well structure provided so as to
generate light at a wavelength of not less than 430 nm and not more
than 550 nm. Since this group-III nitride semiconductor laser
device makes use of the semipolar surface, it allows for increase
in quantum efficiency through decrease of the piezoelectric field
and improvement in crystal quality of the light emitting layer
region, and it is thus suitably applicable to generation of light
at the wavelength of not less than 430 nm and not more than 550
nm.
[0027] A method for fabricating a group-III nitride semiconductor
laser device according to one aspect of the present invention
comprises the steps of: preparing a substrate of a hexagonal
group-III nitride semiconductor, the substrate having a semipolar
principal surface; forming a substrate product that has a laser
structure, an anode electrode and a cathode electrode, the laser
structure including the substrate and a semiconductor region, the
semiconductor region being formed on the semipolar principal
surface; scribing a first surface of the substrate product in part
in a direction of an a-axis of the hexagonal group-III nitride
semiconductor; and carrying out breakup of the substrate product by
press against a second surface of the substrate product, to form
another substrate product and a laser bar, the first surface being
opposite to the second surface, the semiconductor region being
located between the first surface and the substrate, the laser bar
having first and second end faces, the first and second end faces
being formed by the breakup, and the first and second end faces
extending from the first surface to the second surface, the first
and second end faces constituting a laser cavity of the group-III
nitride semiconductor laser device, the anode electrode and the
cathode electrode being formed on the laser structure, the
semiconductor region comprising a first cladding layer of a first
conductivity type gallium nitride-based semiconductor, a second
cladding layer of a second conductivity type gallium nitride-based
semiconductor and an active layer, the active layer being provided
between the first cladding layer and the second cladding layer, the
first cladding layer, the second cladding layer, and the active
layer being arranged along a normal axis to the semipolar principal
surface, the active layer comprising a gallium nitride-based
semiconductor layer, a c-axis of the hexagonal group-III nitride
semiconductor of the substrate tilting at an angle ALPHA with
respect to the normal axis toward an m-axis of the hexagonal
group-III nitride semiconductor, the angle ALPHA falling within a
range of not less than 45.degree. and not more than 80.degree. or
within a range of not less than 100.degree. and not more than
135.degree., the first and second end faces intersecting with an
m-n plane defined by the m-axis of the hexagonal group-III nitride
semiconductor and the normal axis, and the first and second end
faces including a region such that an angle between this region and
a plane indicated by plane index (-1, 0, 1, L) or (1, 0, -1, -L)
falls within a range of not less than -5.degree. and not more than
+5.degree., with L as an integer number not less than 4. Therefore,
the first and second end faces forming the laser cavity mirrors
include the region of the plane index such as mentioned above.
Thus, these laser cavity mirrors have flatness and
perpendicularity, and the lasing yield of the laser cavity can be
improved.
[0028] In this method, the first and second end faces can include a
region such that an angle formed by this region and the
arrangements of N atom --Ga atom extending toward a direction
tilting at an angle of 70.53.degree. in the direction opposite to
the direction of the m-axis of the hexagonal group-III nitride
semiconductor with respect to the direction of the c-axis of the
hexagonal group-III nitride semiconductor, falls within a range of
not less than -10.degree. and not more than +10.degree.. Therefore,
even when the first and second end faces included in the laser
cavity include the region such that the angle formed by this region
and the arrangements of N atom --Ga atom of the hexagonal group-III
nitride semiconductor of the support base falls within a range of
not less than -10.degree. and not more than +10.degree., the first
and second end faces have flatness and perpendicularity as a laser
cavity mirror, and thus, the lasing yield of the laser cavity can
be improved.
[0029] In this method, a part of the first and second end faces
that is included in the active layer can include a part of or the
whole of a region such that an angle between this region and the
plane indicated by plane index (-1, 0, 1, L) or (1, 0, -1, -L)
falls within a range of not less than -5.degree. and not more than
+5.degree.. Therefore, the part that is at least included in the
active layer on the first and second end faces forming the laser
cavity mirrors includes the region of the plane index such as
above. Thus, these laser cavity mirrors have flatness and
perpendicularity, and the lasing yield of the laser cavity can be
improved.
[0030] In this method, a part of the first and second end faces
that is included in the active layer can include a part of or the
whole of a region such that an angle formed by this region and the
arrangements of N atom --Ga atom extending toward a direction
tilting at an angle of 70.53.degree. in the direction opposite to
the direction of the m-axis of the hexagonal group-III nitride
semiconductor with respect to the direction of the c-axis of the
hexagonal group-III nitride semiconductor, falls within a range of
not less than -10.degree. and not more than +10.degree.. Even when
the part that is at least included in the active layer on the first
and second end faces forming the laser cavity mirrors includes the
region such that the angle formed by this region and the
arrangements of N atom --Ga atom of the hexagonal group-III nitride
semiconductor of the support base falls within a range of not less
than -10.degree. and not more than +10.degree., the first and
second end faces have flatness and perpendicularity as a laser
cavity mirror, and thus, the lasing yield of the laser cavity can
be improved.
[0031] In this method, the angle ALPHA falls within a range of not
less than 63.degree. and not more than 80.degree. or within a range
of not less than 100.degree. and not more than 117.degree.. In this
group-III nitride semiconductor laser device, when the angle ALPHA
is in a range of not less than 63.degree. and not more than
80.degree. or in a range of not less than 100.degree. and not more
than 117.degree., it is going to be more likely that the end face
formed by the press will be almost perpendicular to the principal
surface of the substrate. Furthermore, when the angle is in a range
of more than 80.degree. and less than 100.degree., it might result
in failing to achieve desired flatness and perpendicularity.
[0032] In this method, the step of forming the substrate product
comprises performing processing such as slicing or grinding of the
substrate so that a thickness of the substrate becomes not more
than 400 .mu.m, and the second surface is one of the following: a
processed surface formed by the processing; and a surface including
an electrode formed on the processed surface. This group-III
nitride semiconductor laser device can be used to obtain a
good-quality end face for a laser cavity.
[0033] In this method, the step of forming the substrate product
comprises polishing the substrate so that a thickness of the
substrate becomes not less than 50 .mu.m and not more than 100
.mu.m, and the second surface is one of the following: a polished
surface formed by the polishing; and a surface including an
electrode formed on the polished surface. When the thickness is not
less than 50 .mu.m, handling becomes easier, and production yield
becomes higher. When the thickness is in a range of not more than
100 .mu.m, it can be used to obtain a good-quality end face for a
laser cavity.
[0034] In this method, the scribing is carried out using a laser
scriber, and the scribing forms a scribed groove, and a length of
the scribed groove is shorter than a length of an intersecting line
between the first surface and an a-n plane defined by the normal
axis and the a-axis of the hexagonal group-III nitride
semiconductor. According to this method, the other substrate
product and the laser bar are formed by fracture of the substrate
product. This fracture is brought about by using the scribed groove
shorter than a fracture line of the laser bar.
[0035] In this method, the semipolar principal surface is any one
of {20-21} plane, {10-11} plane, {20-2-1} plane, and {10-1-1}
plane. According to this method, these typical semipolar surfaces
can provide the first and second end faces with flatness and
perpendicularity enough to construct the laser cavity of the
group-III nitride semiconductor laser device.
[0036] In this method, the substrate comprises any one of GaN,
AlGaN, AlN, InGaN, and InAlGaN. According to this method, when the
substrate used comprises one of these GaN-based semiconductors, it
becomes feasible to obtain the first and second end faces
applicable to the cavity. Use of an AlN substrate or AlGaN
substrate allows for increase in polarization degree and for
enhancement of optical confinement by virtue of low refractive
index. Use of an InGaN substrate allows for decrease in the degree
of lattice mismatch between the substrate and the light emitting
layer and for improvement in crystal quality.
[0037] The above objects and the other objects, features, and
advantages of the present invention can more readily become
apparent in view of the following detailed description of the
preferred embodiments of the present invention proceeding with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a drawing schematically showing a structure of a
group-III nitride semiconductor laser device according to an
embodiment of the present invention.
[0039] FIG. 2 is a drawing showing a band structure in an active
layer in the group-III nitride semiconductor laser device.
[0040] FIG. 3 is a drawing showing polarization of emission in the
active layer of the group-III nitride semiconductor laser
device.
[0041] FIG. 4 is a drawing showing a relation between an end face
of the group-III nitride semiconductor laser device and an m-plane
of the active layer.
[0042] FIG. 5 is a flowchart showing major steps in a method for
fabricating the group-III nitride semiconductor laser device
according to the embodiment.
[0043] FIG. 6 is a drawing schematically showing major steps in the
method for fabricating the group-III nitride semiconductor laser
device according to the embodiment.
[0044] FIG. 7 is a drawing showing a scanning electron microscope
image of a cavity end face, along with a {20-21} plane in crystal
lattices.
[0045] FIG. 8 is a drawing showing a structure of a laser diode
shown in Example 1.
[0046] FIG. 9 is a drawing showing a relation of determined
polarization degree .rho. versus threshold current density.
[0047] FIG. 10 is a drawing showing a relation of tilt angles of
the c-axis toward the m-axis of GaN substrate versus lasing
yield.
[0048] FIG. 11 is a drawing showing a relation of stacking fault
density versus lasing yield.
[0049] FIG. 12 is a drawing showing a relation of substrate
thickness versus lasing yield.
[0050] FIG. 13 is a drawing showing angles between (20-21) plane
and other plane orientations (indices).
[0051] FIG. 14 is a drawing showing atomic arrangements in (20-21)
plane, (-101-6) plane, and (-1016) plane.
[0052] FIG. 15 is a drawing showing atomic arrangements in (20-21)
plane, (-101-7) plane, and (-1017) plane.
[0053] FIG. 16 is a drawing showing atomic arrangements in (20-21)
plane, (-101-8) plane, and (-1018) plane.
[0054] FIG. 17 is a drawing showing atomic arrangements of GaN.
[0055] FIG. 18 is a drawing showing a relation between off angle of
GaN substrate, lasing yield, and plane index.
[0056] FIG. 19 is a drawing showing a relation between plane index,
angle formed by plane index and the m-plane, angle between the
principal surface and the c-plane.
[0057] FIG. 20 is a drawing showing a relation between off angle of
GaN substrate, lasing yield, and off angle of GaN substrate having
a range of 70.53.+-.10.degree..
[0058] FIG. 21 is a drawing showing a relation between tilt angle
of the c-axis with respect to the principal surface of the
substrate and polarization degree.
[0059] FIG. 22 is a drawing showing a relation between current
density and polarization degree.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0060] The expertise of the present invention can be readily
understood in view of the following detailed description with
reference to the accompanying drawings provided by way of
illustration only. The following will describe embodiments of the
group-III nitride semiconductor laser device and the method for
fabricating the group-III nitride semiconductor laser device
according to the present invention, with reference to the
accompanying drawings. The same portions will be denoted by the
same reference symbols if possible.
[0061] FIG. 1 is a drawing schematically showing a structure of a
group-III nitride semiconductor laser device according to an
embodiment of the present invention. The group-III nitride
semiconductor laser device 11 has a gain-guiding type structure,
but embodiments of the present invention are not limited to the
gain-guiding type structure. The group-III nitride semiconductor
laser device 11 has a laser structure 13 and an electrode 15. The
laser structure 13 includes a support base 17 and a semiconductor
region 19. The support base 17 comprises a hexagonal group-III
nitride semiconductor and has a semipolar principal surface 17a and
a back surface 17b. The semiconductor region 19 is provided on the
semipolar principal surface 17a of the support base 17. The
electrode 15 is provided on the semiconductor region 19 of the
laser structure 13. The semiconductor region 19 includes a first
cladding layer 21, a second cladding layer 23 and an active layer
25. The first cladding layer 21 comprises a first conductivity type
gallium nitride based semiconductor, e.g., n-type AlGaN, n-type
InAlGaN, or the like. The second cladding layer 23 comprises a
second conductivity type GaN-based semiconductor, e.g., p-type
AlGaN, p-type InAlGaN, or the like. The active layer 25 is provided
between the first cladding layer 21 and the second cladding layer
23. The active layer 25 includes gallium nitride based
semiconductor layers, and the gallium nitride based semiconductor
layers are, for example, well layers 25a. The active layer 25
includes barrier layers 25b of a gallium nitride based
semiconductor, and the well layers 25a and the barrier layers 25b
are alternately arranged. The well layers 25a comprise, for
example, of InGaN or the like and the barrier layers 25b, for
example, GaN, InGaN, or the like. The active layer 25 can include a
quantum well structure provided so as to generate light at the
wavelength of not less than 360 nm and not more than 600 nm, and
making use of the semipolar surface is suitably applicable to
generation of light at the wavelength of not less than 430 nm and
not more than 550 nm. The first cladding layer 21, the second
cladding layer 23, and the active layer 25 are arranged along an
axis NX normal to the semipolar principal surface 17a. In the
group-III nitride semiconductor laser device 11, the laser
structure 13 includes a first fractured face 27 and a second
fractured face 29, which intersect with an m-n plane defined by the
normal axis NX and the m-axis of the hexagonal group-III nitride
semiconductor.
[0062] Referring to FIG. 1, an orthogonal coordinate system S and a
crystal coordinate system CR are depicted. The normal axis NX is
directed along a direction of the Z-axis of the orthogonal
coordinate system S. The semipolar principal surface 17a extends in
parallel with a predetermined plane defined by the X-axis and the
Y-axis of the orthogonal coordinate system S. In FIG. 1, a typical
c-plane Sc is also depicted. The c-axis of the hexagonal group-III
nitride semiconductor of the support base 17 tilts at an angle
ALPHA with respect to the normal axis NX toward the m-axis of the
hexagonal group-III nitride semiconductor.
[0063] The group-III nitride semiconductor laser device 11 further
has an insulating film 31. The insulating film 31 covers a surface
19a of the semiconductor region 19 of the laser structure 13, and
the semiconductor region 19 is located between the insulating film
31 and the support base 17. The support base 17 comprises a
hexagonal group-III nitride semiconductor. The insulating film 31
has an opening 31a, and the opening 31a extends in a direction of
an intersecting line LIX between the surface 19a of the
semiconductor region 19 and the foregoing m-n plane, and has, for
example, a stripe shape. The electrode 15 is in contact with the
surface 19a of the semiconductor region 19 (e.g., a contact layer
33 of the second conductivity type) through the opening 31a, and
extends in the direction of the foregoing intersecting line LIX. In
the group-III nitride semiconductor laser device 11, a laser
waveguide includes the first cladding layer 21, the second cladding
layer 23 and the active layer 25, and extends in the direction of
the foregoing intersecting line LIX.
[0064] In the group-III nitride semiconductor laser device 11, the
first fractured face 27 and the second fractured face 29 intersect
with the m-n plane defined by the m-axis of the hexagonal group-III
nitride semiconductor and the normal axis NX. A laser cavity of the
group-III nitride semiconductor laser device 11 includes the first
and second fractured faces 27 and 29, and the laser waveguide
extends from one of the first fractured face 27 and the second
fractured face 29 to the other. The laser structure 13 includes a
first surface 13a and a second surface 13b, and the first surface
13a is opposite to the second surface 13b. The first and second
fractured faces 27, 29 extend from an edge 13c of the first surface
13a to an edge 13d of the second surface 13b. The first and second
fractured faces 27, 29 are different from the conventional cleaved
facets like c-planes, m-planes, or a-planes.
[0065] The first and second fractured faces 27, 29 include a region
(hereinafter referred to as region R) such that an angle between
the region R and the plane indicated by plane index (-1, 0, 1, L)
or (1, 0, -1, -L) falls within a range of not less than -5.degree.
and not more than +5.degree., with L as an integer number not less
than 4. A part of the first and second fractured faces 27, 29 that
is included in the active layer 25 can include a part of or the
whole of the region R mentioned above. As the first and second
fractured faces 27, 29 included in the laser cavity include the
region of such a plane index, the first and second fractured faces
27, 29 have flatness and perpendicularity as a laser cavity mirror,
and thus, the lasing yield of the laser cavity can be improved.
[0066] The first and second fractured faces 27, 29 can also include
a region such that an angle formed by this region and the
arrangements (arrangements extending along the direction of vector
NX) of N atom --Ga atom extending toward the direction tilting at
an angle of 70.53.degree. in the direction opposite to the
direction of the m-axis of the hexagonal group-III nitride
semiconductor of the support base 17 with respect to the direction
of the c-axis (vector VC) of the hexagonal group-III nitride
semiconductor of the support base 17 falls within a range of not
less than -10.degree. and not more than +10.degree. (see FIG. 17).
A part of the first and second fractured faces 27, 29 that is
included in the active layer 25 can include a part of or the whole
of the region mentioned above such that an angle formed by this
region and the above arrangements (arrangements extending along the
direction of vector NX) of N atom --Ga atom of the support base 17
falls within a range of not less than -10.degree. and not more than
+10.degree.. Especially, an angle formed by the region R and the
arrangements of N atom --Ga atom of the support base 17 extending
along the vector NX can fall within a range of not less than
-10.degree. and not more than +10.degree.. Even if the first and
second fractured faces 27, 29 included in the laser cavity include
a region such that an angle formed by this region and the
arrangements of N atom --Ga atom of the support base 17 extending
along the vector NX is in a range of not less than -10.degree. and
not more than +10.degree., the first and second fractured faces 27,
29 have flatness and perpendicularity as a laser cavity mirror, and
thus, the lasing yield of the laser cavity can be improved.
[0067] The table shown in FIG. 19 can be seen as indicating a
relation between an angle (angle ALPHA) formed by the semipolar
principal surface 17a of the support base 17 and the c-plane (plane
Sc) of the hexagonal group-III nitride semiconductor of the support
base 17, and plane indices of planes which are orthogonal to the
semipolar principal surface 17a and extend along the first and
second fractured faces 27, 29. Referring to FIG. 19, in the present
embodiment, the angle (angle ALPHA) between the semipolar principal
surface 17a of the support base 17 and the c-plane (plane Sc) of
the hexagonal group-III nitride semiconductor of the support base
17 is in a range of not less than 64.84.+-.5.degree. and not more
than 79.37.+-.5.degree.. As can be seen, even when the angle (angle
ALPHA) between the semipolar principal surface 17a of the support
base 17 and the c-plane (plane Sc) of the hexagonal group-III
nitride semiconductor of the support base 17 is in a range of not
less than 64.84.+-.5.degree. and not more than 79.37.+-.5.degree.,
the first and second fractured faces 27, 29 have flatness and
perpendicularity as a laser cavity mirror, and thus, the lasing
yield of the laser cavity can be improved.
[0068] In this group-III nitride semiconductor laser device 11, the
first and second fractured faces 27, 29 that form the laser cavity
intersect with the m-n plane. This allows for provision of the
laser waveguide extending in the direction of the intersecting line
between the m-n plane and the semipolar surface 17a. For this
reason, the group-III nitride semiconductor laser device 11 has the
laser cavity enabling a low threshold current.
[0069] The group-III nitride semiconductor laser device 11 includes
an n-side optical guide layer 35 and a p-side optical guide layer
37. The n-side optical guide layer 35 includes a first portion 35a
and a second portion 35b, and the n-side optical guide layer 35
comprises, for example, of GaN, InGaN, or the like. The p-side
optical guide layer 37 includes a first portion 37a and a second
portion 37b, and the p-side optical guide layer 37 comprises, for
example, of GaN, InGaN, or the like. A carrier block layer 39 is
provided, for example, between the first portion 37a and the second
portion 37b. Another electrode 41 is provided on the back surface
17b of the support base 17, and the electrode 41 covers, for
example, the back surface 17b of the support base 17.
[0070] FIG. 2 is a drawing showing a band structure in the active
layer in the group-III nitride semiconductor laser device. FIG. 3
is a drawing showing polarization of emission from the active layer
25 of the group-III nitride semiconductor laser device 11. FIG. 4
is a schematic cross sectional view taken along a plane defined by
the c-axis and the m-axis. With reference to Part (a) of FIG. 2,
three possible transitions between the conduction band and valence
bands in the vicinity of F point of the band structure BAND are
shown. The energy difference between band A and band B is
relatively small. An emission by transition Ea between the
conduction band and band A is polarized in the a-axis direction,
and an emission by transition Eb between the conduction band and
band B is polarized in a direction of the c-axis projected onto the
principal surface. Concerning lasing, a threshold of transition Ea
is smaller than a threshold of transition Eb.
[0071] With reference to Part (b) of FIG. 2, there are shown
spectra of light in the LED mode in the group-III nitride
semiconductor laser device 11. The light in the LED mode includes a
polarization component I1 in the direction of the a-axis of the
hexagonal group-III nitride semiconductor, and a polarization
component I2 in the direction of the projected c-axis of the
hexagonal group-III nitride semiconductor onto the principal
surface, and the polarization component I1 is larger than the
polarization component 12. Polarization degree .rho. is defined by
(I1-I2)/(I1+I2). The laser cavity of the group-III nitride
semiconductor laser device 11 enables the device to emit a laser
beam in the mode that has large emission intensity in the LED
mode.
[0072] As shown in FIG. 3, the device may be further provided with
dielectric multilayer film 43a, 43b on at least one of the first
and second fractured faces 27, 29 or on the respective faces. An
end face coating is also applicable to the fractured faces 27, 29.
The end face coating allows adjustment of their reflectance.
[0073] As shown in Part (b) of FIG. 3, the laser light L from the
active layer 25 is polarized in the direction of the a-axis of the
hexagonal group-III nitride semiconductor. In this group-III
nitride semiconductor laser device 11, a band transition allowing
for implementation of a low threshold current has polarized nature.
The first and second fractured faces 27, 29 for the laser cavity
are different from the conventional cleaved facets like c-planes,
m-planes, or a-planes. But, the first and second fractured faces
27, 29 have flatness and perpendicularity as mirrors for laser
cavity. For this reason, by using the first and second fractured
faces 27, 29 and the laser waveguide extending between these
fractured faces 27, 29, as shown in Part (b) of FIG. 3, it becomes
feasible to achieve low-threshold lasing through the use of the
emission by transition Ea stronger than the emission by transition
Eb that is polarized in the direction indicated by the c-axis
projected onto the principal surface.
[0074] In the group-III nitride semiconductor laser device 11, an
end face 17c of the support base 17 and an end face 19c of the
semiconductor region 19 are exposed in each of the first and second
fractured faces 27, 29, and the end face 17c and the end face 19c
are covered with the dielectric multilayer film 43a. An angle BETA
between an m-axis vector MA of the active layer 25 and a vector NA
normal to the end face 17c of the support base 17, and an end face
25c in the active layer 25 has a component (BETA).sub.1 defined on
a first plane S1, which is defined by the c-axis and m-axis of the
group-III nitride semiconductor, and a component (BETA).sub.2
defined on a second plane S2 (which is not shown but is referred to
as "S2" for easier understanding), which is perpendicular to the
first plane S1 (which is not shown but is referred to as"S1" for
easier understanding) and the normal axis NX. The component
(BETA).sub.1 is preferably in a range of not less than
(ALPHA-5).degree. and not more than (ALPHA+5).degree. in the first
plane S1 defined by the c-axis and m-axis of the group-III nitride
semiconductor. This angular range is shown as an angle between a
typical m-plane S.sub.M and a reference plane F.sub.A in FIG. 4.
The typical m-plane S.sub.M is depicted from the inside to the
outside of the laser structure in FIG. 4, for easier understanding.
The reference plane F.sub.A extends along the end face 25c of the
active layer 25. This group-III nitride semiconductor laser device
11 has the end faces in which the angle BETA taken from one of the
c-axis and the m-axis to the other satisfies the aforementioned
perpendicularity. The component (BETA).sub.2 is preferably in a
range of not less than -5.degree. and not more than +5.degree. on
the second plane S2. Here,
BETA.sup.2=(BETA).sub.1.sup.2+(BETA).sub.2.sup.2. The end faces
(the fractured faces 27, 29) of the group-III nitride semiconductor
laser device 11 satisfy the aforementioned perpendicularity as to
the in-plane angle defined in the plane that is perpendicular to
the normal axis NX to the semipolar surface 17a.
[0075] Referring again to FIG. 1, in the group-III nitride
semiconductor laser device 11, the thickness DSUB of the support
base 17 is preferably not more than 400 .mu.m. This group-III
nitride semiconductor laser device can provide good-quality
fractured faces for the laser cavity. In the group-III nitride
semiconductor laser device 11, the thickness DSUB of the support
base 17 is more preferably not less than 50 .mu.m and not more than
100 .mu.m. This group-III nitride semiconductor laser device 11 can
be provided good-quality fractured faces more preferred for the
laser cavity. Furthermore, its handling becomes easier and the
production yield can be improved.
[0076] In the group-III nitride semiconductor laser device 11, the
angle ALPHA between the normal axis NX and the c-axis of the
hexagonal group-III nitride semiconductor is preferably not less
than 45.degree. and preferably not more than 80.degree., and the
angle ALPHA is preferably not less than 100.degree. and preferably
not more than 135.degree.. When the angle is in a range of less
than 45.degree. and in a range of more than 135.degree., the end
faces made by press are highly likely to be comprised of m-planes.
When the angle is in a range of more than 80.degree. and less than
100.degree., it could result in failing to achieve desired flatness
and perpendicularity.
[0077] In the group-III nitride semiconductor laser device 11, more
preferably, the angle ALPHA between the normal axis. NX and the
c-axis of the hexagonal group-III nitride semiconductor is not less
than 63.degree. and not more than 80.degree.. Furthermore, the
angle ALPHA is particularly preferably not less than 100.degree.
and not more than 117.degree.. When the angle is in a range of less
than 63.degree. and in a range of more than 117.degree., an m-plane
can be formed in part of an end face made by press. When the angle
is in a range of more than 80.degree. and less than 100.degree., it
could result in failing to achieve desired flatness and
perpendicularity.
[0078] The semipolar principal surface 17a can be any one of
{20-21} plane, {10-11} plane, {20-2-1} plane and {10-1-1} plane.
Furthermore, a surface with a slight tilt in a range of not less
than -4.degree. and not more than +4.degree. with respect to these
planes may also be applied as the principal surface. On the
semipolar surface 17a of one of these typical planes, it is
feasible to provide the first and second end faces (fractured faces
27, 29) with flatness and perpendicularity enough to construct the
laser cavity of the group-III nitride semiconductor laser device
11. Furthermore, end faces with sufficient flatness and
perpendicularity are obtained in an angular range across these
typical plane orientations.
[0079] In the group-III nitride semiconductor laser device 11, the
stacking fault density of the support base 17 can be not more than
1.times.10.sup.4 cm.sup.-1. Since the stacking fault density is not
more than 1.times.10.sup.4 cm.sup.-1, the flatness and/or
perpendicularity of the fractured faces is less likely to be
disturbed for a certain accidental reason. The support base 17 can
comprise any one of GaN, AlN, AlGaN, InGaN, and InAlGaN. When the
substrate of any one of these GaN-based semiconductors is used, the
end faces (fractured faces 27, 29) applicable to the cavity can be
obtained. When an AlN or AlGaN substrate is used, it is feasible to
increase the polarization degree and to enhance optical confinement
by virtue of low refractive index. When an InGaN substrate is used,
it is feasible to decrease degree of the lattice mismatch between
the substrate and the light emitting layer and to improve crystal
quality.
[0080] FIG. 5 is a drawing showing major steps in a method for
fabricating the group-III nitride semiconductor laser device
according to the present embodiment. With reference to Part (a) of
FIG. 6, a substrate 51 is shown. In step S101, the substrate 51 is
prepared for fabrication of the group-III nitride semiconductor
laser device. The c-axis (vector VC) of the hexagonal group-III
nitride semiconductor of the substrate 51 tilts at an angle ALPHA
with respect to the normal axis NX toward the m-axis (vector VM) of
the hexagonal group-III nitride semiconductor. Accordingly, the
substrate 51 has a semipolar principal surface 51a of the hexagonal
group-III nitride semiconductor.
[0081] In step S102, a substrate product SP is formed. In Part (a)
of FIG. 6, the substrate product SP is depicted as a member of a
nearly disklike shape, but the shape of the substrate product SP is
not limited thereto. For obtaining the substrate product SP, step
S103 is first performed to form a laser structure 55. The laser
structure 55 includes a semiconductor region 53 and the substrate
51, and in step S103, the semiconductor region 53 is grown on the
semipolar principal surface 51a. For forming the semiconductor
region 53, a first conductivity type GaN-based semiconductor region
57, a light emitting layer 59, and a second conductivity type
GaN-based semiconductor region 61 are grown sequentially on the
semipolar principal surface 51 a. The GaN-based semiconductor
region 57 can include, for example, an n-type cladding layer, and
the GaN-based semiconductor region 61 can include, for example, a
p-type cladding layer. The light emitting layer 59 is provided
between the GaN-based semiconductor region 57 and the GaN-based
semiconductor region 61, and can include an active layer, optical
guide layers, an electron block layer, and so on. The GaN-based
semiconductor region 57, the light emitting layer 59, and the
second conductivity type GaN-based semiconductor region 61 are
arranged in the direction of the normal axis NX to the semipolar
principal surface 51a. These semiconductor layers are epitaxially
grown thereon. The surface of the semiconductor region 53 is
covered with an insulating film 54. The insulating film 54
comprises, for example, of silicon oxide. The insulating film 54
has an opening 54a. The opening 54a has, for example, a stripe
shape.
[0082] Step S104 is carried out to form an anode electrode 58a and
a cathode electrode 58b on the laser structure 55. Before forming
the electrode on the back surface of the substrate 51, the back
surface of the substrate used in crystal growth is polished to form
a substrate product SP in desired thickness DSUB. In formation of
the electrodes, for example, the anode electrode 58a is formed on
the semiconductor region 53, and the cathode electrode 58b is
formed on the back surface (polished surface) 51b of the substrate
51. The anode electrode 58a extends in the X-axis direction, and
the cathode electrode 58b covers the entire area of the back
surface 51b. After these steps, the substrate product SP is
obtained. The substrate product SP includes a first surface 63a,
and a second surface 63b located opposite thereto. The
semiconductor region 53 is located between the first surface 63a
and the substrate 51.
[0083] Step S105 is carried out, as shown in Part (b) of FIG. 6, to
scribe the first surface 63a of the substrate product SP. This
scribing step is carried out with a laser scriber 10a. This
scribing step forms scribed grooves 65a. In Part (b) of FIG. 6,
five scribed grooves are already formed, and formation of a scribed
groove 65b is in progress with laser beam LB. The length of the
scribed grooves 65a is shorter than the length of an intersecting
line MS between the first surface 63a and an a-n plane defined by
the normal axis NX and the a-axis of the hexagonal group-III
nitride semiconductor, and the laser beam LB is applied to a part
of the intersecting line MS. By the application with the laser beam
LB, grooves extending in the specific direction and reaching the
semiconductor region are formed in the first surface 63a. The
scribed grooves 65a can be formed, for example, in an edge of the
substrate product SP.
[0084] As shown in Part (c) of FIG. 6, step S106 is carried out to
implement breakup of the substrate product SP by press against the
second surface 63b of the substrate product SP, thereby forming a
substrate product SP1 and a laser bar LB1. The press is carried out
with a breaking device, such as, a blade 69. The blade 69 includes
an edge 69a extending in one direction, and at least two blade
faces 69b and 69c that are formed to define the edge 69a. The
pressing onto the substrate product SP1 is carried out on a support
device 71. The support device 71 includes a support table 71a and a
recess 71b, and the recess 71b extends in one direction. The recess
71b is formed in the support table 71a. The orientation and
position of the scribed groove 65a of the substrate product SP1 are
aligned with the extending direction of the recess 71b of the
support device 71 to position the substrate product SP1 to the
recess 71b on the support device 71. The orientation of the edge of
the breaking device is aligned with the extending direction of the
recess 71b, and the edge of the breaking device is pressed against
the substrate product SP1 from a direction intersecting with the
second surface 63b. The intersecting direction is preferably an
approximately vertical direction to the second surface 63b. This
implements the breakup of the substrate product SP to form the
substrate product SP1 and laser bar LB1. The press results in
forming the laser bar LB1 with first and second end faces 67a and
67b, and these end faces 67a and 67b have perpendicularity and
flatness enough to make at least a part of the light emitting layer
applicable to mirrors for the laser cavity of the semiconductor
laser.
[0085] The laser bar LB1 thus formed has the first and second end
faces 67a, 67b formed by the aforementioned breakup, and each of
the end faces 67a, 67b extends from the first surface 63a to the
second surface 63b. The end faces 67a, 67b form the laser cavity of
the group-III nitride semiconductor laser device, and intersect
with the XZ plane. This XZ plane corresponds to the m-n plane
defined by the normal axis NX and the m-axis of the hexagonal
group-III nitride semiconductor.
[0086] The first and second end faces 67a, 67b include a region
(hereinafter referred to as region R1) such that an angle between
the region R1 and the plane indicated by plane index (-1, 0, 1, L)
or (1, 0, -1, -L) falls within a range of not less than -5.degree.
and not more than +5.degree., with L as an integer number not less
than 4. A part of the first and second end faces 67a, 67b that is
included in the active layer of the light emitting layer 59 can
include a part of or the whole of the region R1 mentioned above. As
the first and second end faces 67a, 67b included in the laser
cavity include the region of such a plane index, the first and
second end faces 67a, 67b have flatness and perpendicularity as a
laser cavity mirror, and thus, the lasing yield of the laser cavity
can be improved.
[0087] The first and second end faces 67a, 67b can also include a
region such that an angle formed by this region and the
arrangements (arrangements extending along the direction of vector
NX) of N atom --Ga atom extending toward the direction tilting at
an angle of 70.53.degree. in the direction opposite to the
direction of the m-axis of the hexagonal group-III nitride
semiconductor of the substrate 51 with respect to the direction of
the c-axis (vector VC) of the hexagonal group-III nitride
semiconductor of the substrate 51, falls within a range of not less
than -10.degree. and not more than +10.degree. (see FIG. 17). A
part of the first and second end faces 67a, 67b that is included in
the active layer of the light emitting layer 59 can include a part
of or the whole of the region mentioned above such that an angle
formed by this region and the above arrangements (arrangements
extending along the direction of vector NX) of N atom --Ga atom of
the substrate 51 falls within a range of not less than -10.degree.
and not more than +10.degree.. Especially, an angle formed by the
region R1 and the arrangements of N atom --Ga atom of the substrate
51 extending along the vector NX can fall within a range of not
less than -10.degree. and not more than +10.degree.. Even if the
first and second end faces 67a, 67b included in the laser cavity
include a region such that an angle formed by this region and the
arrangements of N atom --Ga atom of the substrate 51 extending
along the vector NX is in a range of not less than -10.degree. and
not more than +10.degree., the first and second end faces 67a, 67b
have flatness and perpendicularity as a laser cavity mirror, and
thus, the lasing yield of the laser cavity can be improved.
[0088] The table shown in FIG. 19 can be seen as indicating a
relation between an angle (angle ALPHA) formed by the semipolar
principal surface 51a of the substrate 51 and the c-plane (plane
Sc) of the hexagonal group-III nitride semiconductor of the
substrate 51, and plane indices of planes which are orthogonal to
the semipolar principal surface 51a and extend along the first and
second end faces 67a, 67b. Referring to FIG. 19, in the present
embodiment, the angle (angle ALPHA) between the semipolar principal
surface 51a of the substrate 51 and the c-plane (plane Sc) of the
hexagonal group-III nitride semiconductor of the substrate 51 is in
a range of not less than 64.84.+-.5.degree. and not more than
79.37.+-.5.degree.. As can be seen, even when the angle (angle
ALPHA) between the semipolar principal surface 51a of the substrate
51 and the c-plane (plane Sc) of the hexagonal group-III nitride
semiconductor of the substrate 51 is in a range of not less than
64.84.+-.5.degree. and not more than 79.37.+-.5.degree., the first
and second end faces 67a, 67b have flatness and perpendicularity as
a laser cavity mirror, and thus, the lasing yield of the laser
cavity can be improved.
[0089] By use of this method, the first surface 63a of the
substrate product SP is scribed in the direction of the a-axis of
the hexagonal group-III nitride semiconductor, and thereafter the
breakup of the substrate product SP is carried out by press against
the second surface 63b of the substrate product SP, thereby forming
the new substrate product SP1 and the laser bar LB1. This method
allows the formation of the first and second end faces 67a, 67b,
which intersect with the m-n plane, in the laser bar LB1. This end
face forming method provides the first and second end faces 67a,
67b with flatness and perpendicularity enough to construct the
laser cavity of the group-III nitride semiconductor laser
device.
[0090] In this method, the laser waveguide thus formed extends in
the direction of tilt of the c-axis of the hexagonal group-III
nitride. The end faces of the laser cavity mirror allowing for
provision of this laser waveguide are formed without use of
dry-etching.
[0091] This method involves the fracturing of the substrate product
SP1, thereby forming the new substrate product SP1 and the laser
bar LB1. In Step S107, the breakup is repeatedly carried out by
press to produce a number of laser bars. This fracture propagates
along the scribed grooves 65a shorter than a fracture line BREAK of
the laser bar LB1.
[0092] In Step S108, dielectric multilayer films is formed on the
end faces 67a, 67b of the laser bar LB1 to form a laser bar
product. In Step S109, this laser bar product is separated into
individual semiconductor laser dies.
[0093] In the fabrication method according to the present
embodiment, the angle ALPHA can be in a range of not less than
45.degree. and not more than 80.degree. and in a range of not less
than 100.degree. and not more than 135.degree.. When the angle is
in a range of less than 45.degree. and in a range of more than
135.degree., the end face made by press becomes highly likely to be
comprised of an m-plane. When the angle is in a range of more than
80.degree. and less than 100.degree., it may result in failing to
achieve desired flatness and perpendicularity. More preferably, the
angle ALPHA can be in a range of not less than 63.degree. and not
more than 80.degree. and in a range of not less than 100.degree.
and not more than 117.degree.. When the angle is in a range of less
than 45.degree. and in a range of more than 135.degree., an m-plane
can be formed in part of an end face formed by press. When the
angle is in a range of more than 80.degree. and less than
100.degree., it may result in failing to achieve desired flatness
and perpendicularity. The semipolar principal surface 51a can be
any one of {20-21} plane, {10-11} plane, {20-2-1} plane, and
{10-1-1} plane. Furthermore, a surface slightly tilted in a range
of not less than -4.degree. and not more than +4.degree. from the
above planes is also used as the principal surface. On these
typical semipolar surfaces, it is feasible to provide the end faces
for the laser cavity with flatness and perpendicularity enough to
construct the laser cavity of the group-III nitride semiconductor
laser device.
[0094] The substrate 51 can be made of any one of GaN, AlN, AlGaN,
InGaN, and InAlGaN. When any one of these GaN-based semiconductors
is used for the substrate, it is feasible to obtain the end faces
applicable to the laser cavity. The substrate 51 is preferably made
of GaN.
[0095] In the step S104 of forming the substrate product SP, the
semiconductor substrate used in crystal growth can be one subjected
to processing such as slicing or grinding so that the substrate
thickness becomes not more than 400 .mu.m, whereby the second
surface 63b of the semiconductor substrate becomes a processed
surface formed by polishing. In this substrate thickness, the end
faces 67a, 67b can be formed in good yield, and are provided with
flatness and perpendicularity enough to construct the laser cavity
of the group-III nitride semiconductor laser device or without ion
damage. More preferably, the second surface 63b can be is a
polished surface formed by polishing, and the thickness of the
polished substrate is not more than 100 .mu.m. For facilitating to
handle the substrate product SP, the substrate thickness is
preferably not less than 50 .mu.m.
[0096] In the production method of the laser end faces according to
the present embodiment, the angle BETA explained with reference to
FIG. 3 can be also defined in the laser bar LB1. In the laser bar
LB1, the component (BETA).sub.1 of the angle BETA is preferably in
a range of not less than (ALPHA-5).degree. and not more than
(ALPHA+5).degree. on a first plane (plane corresponding to the
first plane S1 in the description with reference to FIG. 3) defined
by the c-axis and m-axis of the group-III nitride semiconductor.
The end faces 67a, 67b of the laser bar LB1 satisfy the
aforementioned perpendicularity as to the angle component of the
angle BETA taken from one of the c-axis and the m-axis to the
other. The component (BETA).sub.2 of the angle BETA is preferably
in a range of not less than -5.degree. and not more than +5.degree.
on a second plane (plane corresponding to the second plane S2 shown
in FIG. 3). These end faces 67a, 67b of the laser bar LB1 also
satisfy the aforementioned perpendicularity as to the angle
component of the angle BETA defined on the plane perpendicular to
the normal axis NX to the semipolar surface 51a.
[0097] The end faces 67a, 67b are formed by break by press against
the plurality of GaN-based semiconductor layers epitaxially grown
on the semipolar surface 51a. Since they are epitaxial films on the
semipolar surface 51a, each of the end faces 67a, 67b are not
cleaved facets each having a low plane index like c-planes,
m-planes, or a-planes which have been used heretofore for the
conventional laser cavity mirrors. However, through the break of
the stack of epitaxial films on the semipolar surface 51a, the end
faces 67a, 67b have flatness and perpendicularity applicable as
laser cavity mirrors.
EXAMPLE 1
[0098] A GaN substrate with a semipolar surface is prepared, and
perpendicularity of a fractured face is observed as described
below. The above substrate used has a {20-21}-plane GaN substrate
formed by cutting a (0001) GaN ingot, thickly grown by HYPE, at the
angle of 75.degree. to the m-axis. The principal surface of the GaN
substrate is mirror-finished, and the back surface has pear-skin
which is finished by grinding. The thickness of the substrate is
370 .mu.m.
[0099] On the back side in the pear-skin finish, a marking line is
drawn, with a diamond pen, perpendicularly to the direction of the
c-axis projected on the principal surface of the substrate, and
thereafter the substrate is fractured by press. For observing the
perpendicularity of the resultant fractured face, the substrate is
observed from the a-plane direction with a scanning electron
microscope.
[0100] Part (a) of FIG. 7 shows a scanning electron microscope
image of the fractured face observed from the a-plane direction,
and the fractured face is shown as the right end face. As seen from
the image, the fractured face has flatness and perpendicularity to
the semipolar principal surface.
EXAMPLE 2
[0101] It is found in Example 1 that in the GaN substrate having
the semipolar {20-21} plane, the fractured face is obtained by
pressing the substrate after drawing the marking line perpendicular
to the projected direction of the c-axis onto the principal surface
of the substrate, and has the flatness and perpendicularity to the
principal surface of the substrate. For estimating applicability of
this fractured face to the laser cavity, a laser diode shown in
FIG. 8 is grown by organometallic vapor phase epitaxy as described
below. The raw materials used are as follows: trimethyl gallium
(TMGa); trimethyl aluminum (TMAl); trimethyl indium (TMIn); ammonia
(NH.sub.3); and silane (SiH.sub.4). A substrate 71 is prepared. A
GaN substrate is prepared as the substrate 71, and the GaN
substrate is cut with a wafer slicing apparatus at an angle in a
range of 0.degree. to 90.degree. to the m-axis from a (0001) GaN
ingot thickly grown by HYPE, in such a manner that the angle ALPHA
of the c-axis tilted toward the m-axis has a desired off angle in a
range of 0.degree. to 90.degree.. For example, when the substrate
is formed by cutting at the angle of 75.degree., the resultant
substrate is prepared as a GaN substrate having a {20-21}-plane,
and it is represented by reference symbol 71a in the hexagonal
crystal lattice shown in Part (b) of FIG. 7.
[0102] Before the growth, the substrate is observed by the
cathodoluminescence method in order to estimate the stacking fault
density of the substrate. In the cathodoluminescence, an emission
process of carriers excited by an electron beam is observed and in
a stacking fault, non-radiative recombination of carriers occurs in
the vicinity thereof, so that the stacking fault is expected be
observed as a dark line. The stacking fault density is defined as a
density (line density) per unit length of dark lines observed. The
cathodoluminescence method of nondestructive measurement is applied
herein in order to estimate the stacking fault density, but it is
also possible to use destructive measurement, such as a
transmission electron microscope. When a cross section of a sample
is observed from the a-axis direction with the transmission
electron microscope, a defect extending in the m-axis direction
from the substrate toward the sample surface indicates a stacking
fault contained in the support base, and the line density of
stacking faults can be determined in the same manner as in the
cathodoluminescence method.
[0103] The above substrate 71 is placed on a susceptor in a
reactor, and the epitaxial layers are grown in the following growth
procedure. First, an n-type GaN 72 is grown thereon and its the
thickness is 1000 nm. Next, an n-type InAlGaN cladding layer 73 is
grown thereon and its thickness is 1200 nm. Thereafter, an n-type
GaN guide layer 74a and an undoped InGaN guide layer 74b are grown,
their thickness are 200 nm and 65 nm, respectively, and then a
three-cycle MQW 75 constituted by GaN 15 nm thick/InGaN 3 nm thick
is grown thereon. Subsequently grown thereon are an undoped InGaN
guide layer 76a of the thickness of 65 nm, a p-type AlGaN block
layer 77 of the thickness of 20 nm, and a p-type GaN guide layer
76b of the thickness of 200 nm. Then, a p-type InAlGaN cladding
layer 77 is grown thereon, and its thickness is 400 nm. Finally, a
p-type GaN contact layer 78 is grown thereon and its thickness is
50 nm.
[0104] An insulating film 79 of SiO.sub.2 is deposited on the
contact layer 78 and then photolithography and wet etching
processes are applied to form a stripe window having the width of
10 .mu.m in the insulating film 79. In this step, two types of
contact windows are formed in two stripe directions, respectively.
These laser stripes are formed in the following directions: (1)
M-direction (direction of the contact window extending along the
predetermined plane defined by the c-axis and the m-axis); and (2)
A-direction: <11-20> direction.
[0105] After the formation of the stripe window, a p-side electrode
80a of Ni/Au and a pad electrode of Ti/Al are made by vapor
deposition. Next, the back surface of the GaN substrate (GaN wafer)
is polished using diamond slurry to produce a substrate product
with the mirror-polished back surface. Then, the thickness of the
thus formed substrate product is measured with a contact film
thickness meter. The measurement of substrate thickness may also be
carried out with a microscope from the observation of a cross
section of a prepared sample. The microscope applicable herein can
be an optical microscope or a scanning electron microscope. An
n-side electrode 80b of Ti/Al/Ti/Au is formed by vapor deposition
on the back surface (polished surface) of the GaN substrate (GaN
wafer).
[0106] The laser cavity mirrors for these two types of laser
stripes are produced with a laser scriber that uses the YAG laser
at the wavelength of 355 nm. When the break is implemented with the
laser scriber, the laser chip yield can be improved as compared
with break implemented using a diamond scriber. The conditions for
formation of the scribed grooves are as follows: laser beam output
power of 100 mW; scanning speed of 5 mm/s. The scribed grooves thus
formed each has, for example, the length of 30 .mu.m, the width of
10 .mu.m, and the depth of 40 .mu.m. The scribed grooves are formed
by applying the laser beam through the aperture of the insulating
film of the substrate directly to the epitaxially grown surface at
the pitch of 800 .mu.m. The cavity length is 600 .mu.m.
[0107] The laser cavity mirrors are made through fracture by use of
a blade. A laser bar is produced by break by press against the back
side of the substrate. More specifically, Parts (b) and (c) of FIG.
7 show relations between crystal orientations and fractured faces,
for the {20-21}-plane GaN substrate. Part (b) of FIG. 7 shows the
laser stripe that is provided to extend (1) in the M-direction, and
shows end faces 81a, 81b for the laser cavity along with the
semipolar surface 71a. The end faces 81a, 81b are approximately
perpendicular to the semipolar surface 71a, but are different from
the conventional cleaved facets like the hitherto used c-planes,
m-planes, or a-planes. Part (c) of FIG. 7 shows the laser stripe
that is provided to extend (2) in the <11-20> direction, and
shows end faces 81c, 81d for the laser cavity along with the
semipolar surface 71a. The end faces 81c 81d are approximately
perpendicular to the semipolar surface 71a and are composed of
a-planes.
[0108] The fractured faces made by break are observed with a
scanning electron microscope, and no prominent unevenness is
observed in each of (1) and (2). From this result, the flatness
(magnitude of unevenness) of the fractured faces can be not more
than 20 nm. Furthermore, the perpendicularity of the fractured
faces to the surface of the sample can be within a range of not
less than -5.degree. and not more than +5.degree..
[0109] The end faces of the laser bar are coated with a dielectric
multilayer film by vacuum vapor deposition. The dielectric
multilayer film comprises an alternate stack of SiO.sub.2 and
TiO.sub.2. Each thickness thereof is adjusted in a range of 50 to
100 nm and is designed so that the center wavelength of reflectance
falls within a range of 500 to 530 nm. The reflecting surface on
one side has an alternate stack of ten cycles and a designed value
of reflectance of about 95%, and the reflecting surface on the
other side has an alternate stack of six cycles and a designed
value of reflectance of about 80%.
[0110] The devices thus formed are operated by current injection to
make their evaluation at room temperature. A pulsed power source is
used as a power supply for the operation by current injection, and
supplies pulses with the pulse width of 500 ns and the duty ratio
of 0.1%, and the operation by current injection is implemented
through probing needles that are in contact with the surface
electrodes. In light output measurement, an emission from the end
face of the laser bar is detected with a photodiode to obtain a
current-light output characteristic (I-L characteristic). In
measurement of emission wavelength, the emission from the end face
of the laser bar is supplied through an optical fiber to a spectrum
analyzer of a detector to measure a spectrum thereof. In estimation
of a polarization, the emission from the laser bar is made to pass
through a polarizing plate by rotation, thereby determining the
polarization state. In observation of LED-mode emission, an optical
fiber is aligned to the front surface side of the laser bar to
measure optical emission from the front surface.
[0111] The polarization in the laser beam is measured for every
laser device, and it is found that the laser beam is polarized in
the a-axis direction. The lasing wavelength is in a range of
500-530 nm.
[0112] The polarization state in the LED mode (i.e., spontaneous
emission) is measured for every laser device. The current density
is 7.4 A/cm.sup.2. When the polarization component in the a-axis
direction is referred to as I1, and the polarization component in
the direction of the projected m-axis onto the principal surface is
referred to as I2, the polarization degree p is defined as
(I1-I2)/(I1+I2). The relation between determined polarization
degree p and minimum of threshold current density is investigated,
and the result obtained is shown in FIG. 9. As seen from FIG. 9,
the threshold current density demonstrates a significant decrease
in the laser (1) with the laser stripe along the M-direction when
the polarization degree is positive. Namely, it is seen that when
the polarization degree is positive (I1>I2) and the waveguide is
provided along an off direction, the threshold current density is
significantly decreased. The data shown in FIG. 9 is as
follows.
TABLE-US-00001 Polarization Threshold current, Threshold current
degree, (M-direction stripe), (<11-20> stripe); 0.08, 64, 20;
0.05, 18, 42; 0.15, 9, 48; 0.276, 7, 52; 0.4, 6
[0113] In order to obtain a high polarization degree, the GaN
substrate having a principal surface slightly tilting with respect
to the {20-21} plane is prepared by adjusting an angle at which the
GaN substrate is cut, and a relation between polarization degree
and tilt angle of the principal surface of this GaN substrate with
respect to the c-axis is investigated, and the result obtained is
shown in FIG. 21. When the principal surface of the GaN substrate
is the {20-21} plane, the angle between this principal surface and
the c-plane is about 75.degree.. However, it can be seen that the
polarization degree becomes higher, as this angle becomes larger
and nears 90.degree.. That is, in order to obtain a high
polarization degree, it is efficient to use a substrate with a
principal surface such that an angle between this principal surface
of the GaN substrate and the c-plane is as close as possible to
90.degree. and this principal surface slightly tilts with respect
to the {20-21} plane. The data shown in FIG. 21 is as follows.
TABLE-US-00002 Substrate angle polarization degree, polarization
degree; 71, 0.12; 73, 0.18; 75, 0.276; 77, 0.31; 79, 0.40
[0114] In addition, in order to confirm the stability of the
polarization degree of laser emitted by the group-III nitride
semiconductor laser device which includes the GaN substrate having
a principal surface of the {20-21} plane, the current density
dependence of the polarization degree is investigated by increasing
the current density to 0.74 kA/cm.sup.2, and the result obtained is
shown in FIG. 22. Even if the current density is increased, the
polarization degree is almost constant and not changed. Therefore,
even if the current density is close to the threshold current
density of lasing, the polarization degree is not lowered, and it
is efficient for lasing of low threshold.
[0115] The relation between the tilt angle (the off angle) of the
c-axis of the GaN substrate toward the m-axis, and lasing yield is
investigated, and the result thus obtained is shown in FIG. 10. In
the present example, the lasing yield is defined as (the number of
laser chips)/(the number of measured chips). FIG. 10 is a plot for
substrates, having the stacking fault density of substrate of not
more than 1.times.10.sup.4 (cm.sup.-1), on which lasers with the
laser stripe along (1) the M-direction are formed. As seen from
FIG. 10, the lasing yield is extremely low in the off angles of not
more than 45.degree.. The observation of the end faces with an
optical microscope finds that an m-plane is formed in almost all
chips at the tilt angles smaller than 45.degree., resulting in
failure in achieving perpendicularity. The observation also finds
that when the off angle is in a range of not less than 63.degree.
and not more than 80.degree., the perpendicularity is improved and
the lasing yield increases to 50% or more. From these experimental
results, the optimum range of off angle of the GaN substrate is not
less than 63.degree. and not more than 80.degree.. The same result
is also obtained in a range of not less than 100.degree. and not
more than 117.degree., which is an angular range to provide
crystallographically equivalent end faces. The data shown in FIG.
10 is as follows.
TABLE-US-00003 Tilt angle, Yield; 10, 0.1; 43, 0.2; 58, 50; 63, 65;
66, 80; 71, 85; 75, 80; 79, 75; 85, 45; 90, 35
[0116] A relation between off angle of the GaN substrate and plane
index of a surface (corresponding to the regions R1, R2) orthogonal
both to a surface including the c-axis and the m-axis of the
hexagonal group-III nitride semiconductor of the GaN substrate and
to the principal surface of the GaN substrate is shown in FIG. 18.
A graph shown in FIG. 18 is identical to a graph shown in FIG. 10.
As shown in FIG. 18, when the off angle of the GaN substrate is in
a range of not less than 45.degree. and not more than 80.degree., a
plane index of a surface corresponding to the regions R1, R2 is any
one of (-1, 0, 1, 2), (-1, 0, 1, 3), (-1, 0, 1, 4), (-1, 0, 1, 5),
(-1, 0, 1, 6), (-1, 0, 1, 7), (-1, 0, 1, 8), (-1, 0, 1, 9), (-1, 0,
1, 10). On the contrary, when the off angle of the GaN substrate is
in a range of not less than 63.degree. and not more than
80.degree., a plane index of a surface corresponding to the regions
R1, R2 is any one of (-1, 0, 1, 4), (-1, 0, 1, 5), (-1, 0, 1, 6),
(-1, 0, 1, 7), (-1, 0, 1, 8), (-1, 0, 1, 9), (-1, 0, 1, 10). As the
surface forming the laser cavity mirror includes the regions R1, R2
having such a plane index, this laser cavity mirror has flatness
and perpendicularity. According to FIG. 18, the lasing yield of the
laser cavity can be improved up to not less than 50%. In addition,
the off angle of the GaN substrate shown in FIG. 18 corresponds to
the angle between the principal surface and the c-plane shown in
FIG. 19, and plane indices shown in FIG. 18 correspond to plane
indices shown in FIG. 19.
[0117] In addition, both of the graph shown in FIG. 10 and the
notable description of a range of the off angle of the GaN
substrate (a range of 70.53.+-.10) are shown in FIG. 20. As shown
in FIG. 20, when the off angle of GaN substrate is in a range of
70.53.+-.10, that is, in a range of not less than 60.53.degree. and
not more than 80.53, a plane index of a surface corresponding to
the regions R1, R2 is any one of (-1, 0, 1, 4), (-1, 0, 1, 5), (-1,
0, 1, 6), (-1, 0, 1, 7), (-1, 0, 1, 8), (-1, 0, 1, 9), (-1, 0, 1,
10). As the surface forming the laser cavity mirror includes the
regions R1, R2 having such a plane index, this laser cavity mirror
has flatness and perpendicularity. According to FIG. 20, the lasing
yield of the laser cavity can be improved up to not less than
50%.
[0118] Indicated below is the result of investigation of embodiment
of the breaking in the step S106 on the angle between the principal
surface of the GaN substrate and the c-plane of the hexagonal
group-III nitride semiconductor of the GaN substrate. When the
angle between the principal surface of the GaN substrate and the
c-plane of the hexagonal group-III nitride semiconductor of the GaN
substrate is 0.degree., the substrate product breaks at the m-plane
with good yield by the breaking in the step S106. When the angle
between the principal surface of the GaN substrate and the c-plane
of the hexagonal group-III nitride semiconductor of the GaN
substrate is in a range of more than 0.degree. and less than
45.degree., the substrate product breaks at the m-plane by the
breaking in the step S106. When the angle between the principal
surface of the GaN substrate and the c-plane of the hexagonal
group-III nitride semiconductor of the GaN substrate is in a range
of not less than 45.degree. and less than 63.degree., some
substrate products break at the m-plane by the breaking in the step
S106, and others break perpendicularly on the principal surface by
the breaking in the step S106. When the angle between the principal
surface of the GaN substrate and the c-plane of the hexagonal
group-III nitride semiconductor of the GaN substrate is in a range
of not less than 63.degree. and not more than 80.degree., many
substrate products break perpendicularly on the principal surface
by the breaking in the step S106. The end face (corresponding to
such as the end face 63a) formed by this breaking includes high
index (-1, 0, 1, L) plane perpendicular to the principal surface, L
including an integer number in a range of not less than 4 and not
more than 10. When the angle between the principal surface of the
GaN substrate and the c-plane of the hexagonal group-III nitride
semiconductor of the GaN substrate is in a range of more than
80.degree. and less than 90.degree., the substrate product does not
easily break by the breaking in the step S106, and when the angle
between the principal surface of the GaN substrate and the c-plane
of the hexagonal group-III nitride semiconductor of the GaN
substrate is 90.degree., the substrate product breaks at the
c-plane with good yield by the breaking in the step S106.
[0119] The relation between stacking fault density and lasing yield
is investigated and the result obtained is shown in FIG. 11. The
definition of lasing yield is the same as above. As seen from FIG.
11, the lasing yield is suddenly decreased with the stacking fault
density over 1.times.10.sup.4 (cm.sup.-1). The observation of the
end face state with an optical microscope shows that devices in the
sample group categorized as decreased lasing yield exhibits the
significant unevenness of the end faces, so that no flat fractured
faces are obtained. The reason therefor is that a difference in
easiness of fracture depends on the existence of stacking faults.
From this result, the stacking fault density in the substrate is
not more than 1.times.10.sup.4 (cm.sup.-1).
[0120] The data shown in FIG. 11 is as follows.
TABLE-US-00004 Stacking fault density (cm.sup.-1), Yield; 500, 80;
1000, 75; 4000, 70; 8000, 65; 10000, 20; 50000, 2
[0121] The relation between substrate thickness and lasing yield is
investigated, and the result obtained is shown in FIG. 12. The
definition of lasing yield is the same as above. FIG. 12 is a plot
for laser devices in which the stacking fault density of the
substrate is not more than 1.times.10.sup.4 (cm.sup.-1) and the
laser stripe extends along (1) the M-direction. From FIG. 12, the
lasing yield is high when the substrate thickness is smaller than
100 .mu.m and larger than 50 .mu.m. When the substrate thickness is
larger than 100 .mu.m, the perpendicularity of fractured faces
becomes deteriorated. When the thickness is smaller than 50 .mu.m,
handling of substrates does not become easy and the semiconductor
chips become easy to break. From these, the optimum thickness of
the substrate is in a range of not less than 50 .mu.m and not more
than 100 .mu.m. The data shown in FIG. 12 is as follows.
TABLE-US-00005 Substrate thickness, Yield; 48, 10; 80, 65; 90, 70;
110, 45; 150, 48; 200, 30; 400, 20
EXAMPLE 3
[0122] In Example 2, the plurality of epitaxial films for the
semiconductor laser are grown on the GaN substrate having the
{20-21} surface. As described above, the end faces for the optical
cavity are formed through the formation of scribed grooves and by
press. In order to find candidates for these end faces, plane
orientations different from the a-plane and making an angle near
90.degree. with respect to the (20-21) plane are obtained by
calculation. With reference to FIG. 13, the following angles and
plane orientations have angles near 90.degree. with respect to the
(20-21) plane.
TABLE-US-00006 Specific plane index, Angle to {20-21} plane;
(-1016), 92.46.degree.; (-1017), 90.10.degree.; (-1018),
88.29.degree.
[0123] FIG. 14 is a drawing showing atomic arrangements in the
(20-21) plane, (-101-6) plane, and (-1016) plane. FIG. 15 is a
drawing showing atomic arrangements in the (20-21) plane, (-101-7)
plane, and (-1017) plane. FIG. 16 is a drawing showing atomic
arrangements in the (20-21) plane, (-101-8) plane, and (-1018)
plane. As shown in FIGS. 14 to 16, local atom arrangements
indicated by arrows show configurations of atoms with charge
neutrality, and electrically neutral arrangements of atoms appear
periodically. The reason why near-vertical faces with respect to
the grown surface are obtained can be that creation of fractured
faces is considered to be relatively stable because of the periodic
appearance of the neutral atomic configurations in terms of
charge.
[0124] According to various experiments containing the
above-described Examples 1 to 3, the angle ALPHA can be in a range
of not less than 45.degree. and not more than 80.degree. or in a
range of not less than 100.degree. and not more than 135.degree..
In order to improve the laser chip yield, the angle ALPHA can be in
a range of not less than 63.degree. and not more than 80.degree. or
in a range of not less than 100.degree. and not more than
117.degree.. The typical semipolar principal surface can be any one
of {20-21} plane, {10-11} plane, {20-2-1} plane, and {10-1-1}
plane. Furthermore, the principal surface can be a slight tilt
surface from these semipolar surfaces. For example, the semipolar
principal surface can be a slight tilt surface off in a range of
not less than -4.degree. and not more than +4.degree. toward the
m-plane with respect to any one of {20-21} plane, {10-11} plane,
{20-2-1} plane, and {10-1-1} plane.
[0125] As described above, the present embodiment provides the
group-III nitride semiconductor laser device with the laser cavity
enabling the low threshold current, on the semipolar surface of the
support base that tilts with respect to the c-axis of the hexagonal
group-III nitride toward the m-axis. The present embodiment also
provides the method for fabricating the group-III nitride
semiconductor laser device.
[0126] Having described and illustrated the principle of the
invention in a preferred embodiment thereof, it is appreciated by
those having skill in the art that the invention can be modified in
arrangement and detail without departing from such principles. The
present invention is not limited to the specific configurations
disclosed in the embodiment. We therefore claim all modifications
and variations coming within the spirit and scope of the following
claims.
* * * * *