U.S. patent application number 10/751959 was filed with the patent office on 2004-07-15 for nitride semiconductor laser and method of fabricating the same.
This patent application is currently assigned to PIONEER CORPORATION. Invention is credited to Chikuma, Kiyofumi, Ota, Hiroyuki, Tanaka, Toshiyuki.
Application Number | 20040137655 10/751959 |
Document ID | / |
Family ID | 14993027 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040137655 |
Kind Code |
A1 |
Chikuma, Kiyofumi ; et
al. |
July 15, 2004 |
Nitride semiconductor laser and method of fabricating the same
Abstract
A method for fabricating a nitride semiconductor laser device
having crystal layers each made of a group III nitride
semiconductor (Al.sub.xGa.sub.1-x).sub.1-yIn.sub.yN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) layered in order on a
ground layer (Al.sub.x'Ga.sub.1-x').sub.1-y'In.sub.y'N
(0.ltoreq.x'.ltoreq.1, 0.ltoreq.y'.ltoreq.1). The method including
a step of forming a plurality of crystal layers each made of group
III nitride semiconductor on a ground layer formed on a substrate
such as sapphire; a step of applying a light beam from the
substrate side toward the interface between the substrate and the
ground layer thereby forming the decomposed-matter area of a
nitride semiconductor; a step of separating the ground layer
carrying the crystal layers from the substrate along the
decomposed-matter area; and a step of cleaving the ground layer
thereby forming a cleavage plane of the crystal layers.
Inventors: |
Chikuma, Kiyofumi;
(Tsurugashima-shi, JP) ; Ota, Hiroyuki;
(Tsurugashima-shi, JP) ; Tanaka, Toshiyuki;
(Tsurugashima-shi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
PIONEER CORPORATION
|
Family ID: |
14993027 |
Appl. No.: |
10/751959 |
Filed: |
January 7, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10751959 |
Jan 7, 2004 |
|
|
|
09567024 |
May 9, 2000 |
|
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|
6711192 |
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Current U.S.
Class: |
438/22 |
Current CPC
Class: |
H01S 5/0202 20130101;
H01S 5/32341 20130101; H01S 5/0217 20130101; H01L 2224/48463
20130101; H01S 5/22 20130101; H01S 2304/04 20130101; H01S 5/0215
20130101 |
Class at
Publication: |
438/022 |
International
Class: |
H01L 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 10, 1999 |
JP |
11-128769 |
Claims
What is claimed is:
1. A method for fabricating a nitride semiconductor laser device
having crystal layers each made of a group III nitride
semiconductor (Al.sub.xGa.sub.1-x).sub.1-yIn.sub.yN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) layered in order on a
ground layer (Al.sub.x'Ga.sub.1-x').sub.1-y'In.sub.y'N
(0.ltoreq.x'.ltoreq.1, 0.ltoreq.y'.ltoreq.1), the method comprising
the steps of: forming a plurality of crystal layers each made of
group III nitride semiconductor on a ground layer formed on a
substrate, the crystal layers including an active layer; applying a
light beam from the substrate side toward the interface between the
substrate and the ground layer thereby forming the
decomposed-matter area of a nitride semiconductor; separating the
ground layer with the crystal layers thereon from the substrate
along the decomposed-matter area; and cleaving the ground layer
thereby forming a cleavage plane of the crystal layers.
2. A method for fabricating a nitride semiconductor laser device
according to claim 1, wherein the substrate is made of
sapphire.
3. A method for fabricating a nitride semiconductor laser device
according to claim 1, wherein the wavelength of the light beam is
selected from wavelengths passing through the substrate and
absorbed by the ground layer in the vicinity of the interface.
4. A method for fabricating a nitride semiconductor laser device
according to claim 1 further comprising, between said step of
forming the crystal layers and said step of applying the light beam
toward the interface, a step of bonding a cleavable second
substrate onto a surface of the crystal layers in such a manner
that a cleavage plane of the second substrate substantially
coincides with a cleavage plane of the crystal layers of the
nitride semiconductor.
5. A method for fabricating a nitride semiconductor laser device
according to claim 1, wherein the cleavable second substrate is
made of a semiconductor single-crystal material.
6. A method for fabricating a nitride semiconductor laser device
according to claim 5, wherein the semiconductor single-crystal
material is selected from a group consisting of GaAs, InP and
Si.
7. A method for fabricating a nitride semiconductor laser device
according to claim 1, wherein, in the step of applying the light
beam toward the interface, the light beam is applied uniformly or
entirely onto the interface between the substrate and the ground
layer.
8. A method for fabricating a nitride semiconductor laser device
according to claim 1, wherein, in the step of applying the light
beam toward the interface, the interface between the substrate and
the ground layer is scanned with a spot or line of the light
beam.
9. A method for fabricating a nitride semiconductor laser device
according to claim 1 further comprising a step of forming a
waveguide extending along a direction normal to the cleavage plane
of the nitride semiconductor.
10. A method for fabricating a nitride semiconductor laser device
according to claim 1, wherein the crystal layers of the nitrid
semiconductor are formed by metal-organic chemical vapor d
position.
11. A method for fabricating a nitride semiconductor laser device
according to claim 1, wherein, in the step of applying the light
beam toward the interface, the light beam Is an ultraviolet ray
generated from a frequency quadrupled YAG laser.
12. A nitride semiconductor laser device having successively grown
crystal layers each made of a group III nitride semiconductor
(Al.sub.xGa.sub.1-x).sub.1-yIn.sub.yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1) comprising: a ground layer made of group III
nitride semiconductor (Al.sub.x'Ga.sub.1-x').sub.1-y'In.sub.y'N
(0.ltoreq.x'.ltoreq.1, 0.ltoreq.y'.ltoreq.1); a plurality of
crystal layers each made of group III nitride semiconductor formed
on the ground layer; a cleavable substrate bonded onto a surface of
the crystal layers opposite to the ground layer.
13. A nitride semiconductor laser device according to claim 12,
wherein the device further comprises a heat sink bonded onto the
ground layer.
14. A nitride semiconductor laser device according to claim 12,
wherein the device further comprises a heat sink bonded onto the
cleavable substrate.
15. A nitride semiconductor laser device according to claim 12,
wherein the cleavable substrate has a cleavage plane coinciding
with a cleavage plane of the crystal layers of the nitride
semiconductor.
16. A nitride semiconductor laser device according to claim 12,
wherein the device further comprises a waveguide extending along a
direction normal to the cleavage plane of th nitride
semiconductor.
17. A nitride semiconductor laser device according to claim 12,
wherein the cleavable substrate is made of a semiconductor
single-crystal material.
18. A nitride semiconductor laser device according to claim 17,
wherein the semiconductor single-crystal material is selected from
a group consisting of GaAs, InP and Si.
19. A nitride semiconductor laser device according to claim 12,
wherein the cleavable substrate is made of an electrically
conductive material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a group III nitride
semiconductor device (hereafter also referred to as a device
simply) and, particularly to a fabrication method of a
semiconductor laser device using the same material system.
[0003] 2. Description of the Related Art
[0004] A laser device needs to have a resonator consisting of a
pair of flat parallel mirror facets for its operation. For example,
in the case of the manufacture of a conventional laser device
(Fabry-Perot type) using a semiconductor crystal material such as
GaAs, the cleavage nature of GaAs crystal i.e., the substrate
crystal is utilized for the fabrication of the mirror facets.
[0005] In the case of a group III nitride semiconductor device, it
is inevitable to perform the epitaxial-growth of the crystal film
onto a dissimilar substrate such as sapphire, SiC or the like,
because a nitride bulk crystal is extremely expensive to be used in
practice although it could be manufactured.
[0006] SiC is not frequently used as a substrate for the nitride
devices, because SiC substrates are also expensive and a nitride
film on the SiC substrate easily cracks due to the difference in
thermal expansion coefficient therebetween. Thus sapphire is
commonly used as a substrate for the group III nitride
semiconductor laser devices. In the case of epitaxial growth of
nitrides on a sapphire substrate, a high quality single-crystal
film is obtained on a C-face i.e., (0001) plane of sapphire, or on
an A-face, i.e., (11{overscore (2)}0) plane (hereafter referred to
as (11-20) plane) of sapphire.
[0007] The mirror facets may be formed by an etching process such
as reactive ion etching (RIE) instead of cleavage, because it is
hard to split the sapphire substrate to laser bars in comparison
with the GaAs substrate having been used so far for semiconductor
laser devices.
[0008] Reactive ion etching is mainly used as a method for
obtaining the mirror facets of the nitride semiconductor laser on
the sapphire substrate at present. However, the resultant device
with the mirror facets formed by the reactive ion etching method
has a disadvantage that the far-field pattern of its emitted light
exhibits multiple spots. The mass-production-type GaN laser with
the cleaved mirror facets is studied again in view of overcoming
the multiple spots phenomenon in the far field pattern as mentioned
above.
[0009] It is a matter of course that the cleavage cannot be
preferably performed on sapphire in mass production. Therefore, the
following method have been used. First, after forming a thick
ground layer of GaN film e.g., at approximately 200 .mu.m thickness
on a sapphire substrate, the backside of the sapphire substrate of
the obtained wafer is ground or lapped to remove the sapphire
portion, so that the GaN substrate is obtained. Next, the epitaxial
growth of laser structure is preformed on the GaN substrate. From
the obtained wafer, laser devices may be fabricated.
[0010] However, the conventional method of lapping the back side
the sapphire substrate backside as described above requires many
steps, and is complicated. As a result, the method invites a very
low yield of the group III nitride semiconductor devices. Such a
method is not suitable for mass production.
[0011] Although sapphire does not have a definite cleavage plane
like a Si or GaAs wafer, a C-face sapphire is fairly easily split
along its 1{overscore (1)}00) plane (hereafter referred to as
(1-100) plane), and also an A-face sapphire can be easily parted
along its (1{overscore (1)}02) plane (hereafter referred to as
(1-102) plane), so called R-plane, considerably close to the
cleavage of ordinary crystal. It is considered that the formation
of the mirror facets of nitride semiconductor lasers on a sapphire
substrate may be achieved through following methods: First is a
method of growing nitride semiconductor layers on a C-face sapphire
substrate and then splitting the wafer along (1-100) plane of the
sapphire substrate. Second is a method of growing nitride
semiconductor layers on an A-face sapphire substrate and then
splitting the wafer along (1-102) plane of the sapphire
substrate.
[0012] As to the first method of mirror facet formation applied to
the device grown on a C-face sapphire substrate, there are problems
that a sapphire substrate cannot be split unless the substrate is
made thin enough by lapping down the backside of the substrate and
that it does not have high reproducibility. These problems are
caused by the fact that (1-100) plane of sapphire is not an
explicit cleavage plane. Since sapphire is very hard crystal, it
cannot be split exactly along a line notched on its surface unless
it is made thin enough, and the thickness of the sapphire substrate
should be reduced to approximately 100 .mu.m in order to obtain
mirror facets practical for laser devices. When lapping the
backside of a wafer on which a device structure is already formed,
the wafer is warped or distorted due to the difference between
thermal expansion coefficients of sapphire and nitrides or due to
the residual stress caused by lapping process. When the back of a
device wafer is lapped, the wafer is thereby apt to fracture during
the process. This is very disadvantageous for mass production. The
(1-100) plane of sapphire is not a cleavage plane. Therefore, in
many cases GaN is split along in a direction slightly deviated from
the cleavage plane thereof, the fracture surface consists of many
facts of (1-100) planes of GaN, each of which is the cleavage
plane, forming a stepwise appearance. The stepwise appearance
causes degradation of the reflectivity and perturbation of the wave
front of emitted light and, thereby deteriorates the quality of
mirror facets for optical resonance of a laser device.
[0013] Whereas, the second method of mirror facet forming method
applied to the device formed on an A-face sapphire substrate has a
problem that the quality of the fracture plane of GaN is not
sufficient.
[0014] Since the sapphire substrate can be easily split along its
cleavage plane (1-102), so called R-plane, it is possible to cleave
the sapphire having a thickness of 250 to 350 .mu.m normally used
as a substrate. However, as shown in FIG. 1, when forming a laser
structure on the A-face of a sapphire substrate and parting
sapphire along its R-plane as depicted by the arrow in the figure,
fine striations are formed on the side surface of GaN layers. This
is caused by the following reason that the laser wafer splits along
the R-plane of the sapphire since a major part of the wafer is made
of sapphire. The R-plane of sapphire tilts by an angle of
2.4.degree. from (1-100) plane of the grown GaN as shown in FIG. 2,
after a propagating crack along the sapphire's R-plane reaches at
the sapphire-GaN interface, the crack still propagates into GaN
still along the R-plane of sapphire up to a certain depth. However,
GaN tends to crack on its crystallographic cleavage plane (1-100).
Therefore, a plurality of (1-100) facets of GaN are formed in such
a stepwise manner that the striations appear on the fracture plane
of GaN as shown in FIG. 1.
[0015] As a result, in the case of the A-face sapphire substrate,
the quality of fracture plane is not very good though it is
reproducible.
OBJECT AND SUMMARY OF THE INVENTION
[0016] Therefore, an object of the present invention is to provide
a group III nitride-semiconductor laser having high-quality mirror
facets for a laser structure and a method of fabricating the laser
device with high reproducibility.
[0017] A fabrication method according to the present invention is a
method for producing a nitride semiconductor laser device having
crystal layers each made of a group III nitride semiconductor
(Al.sub.xGa.sub.1-x).sub.1- -yIn.sub.yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1), layered in order on a ground layer
(Al.sub.x'Ga.sub.1-x').sub.1-y'In.sub.y'N (0.ltoreq.x'.ltoreq.1,
0.ltoreq.y'.ltoreq.1), the method comprising the steps of:
[0018] forming a plurality of crystal layers each made of group III
nitride semiconductor on a ground layer formed on a substrate, the
crystal layers including an active layer;
[0019] applying a light beam from the substrate side toward the
interface between the substrate and the ground layer thereby
forming the decomposed-matter area of a nitride semiconductor;
[0020] separating the ground layer with the crystal layers thereon
from the substrate along the decomposed-matter area; and
[0021] cleaving the ground layer thereby forming a cleavage plane
of the crystal layers for a laser resonator.
[0022] In an aspect of the fabrication method according to the
invention, the wavelength of said light beam is selected from
wavelengths passing through the substrate and absorbed by the
ground layer in the vicinity of the interface.
[0023] In another aspect of the fabrication method according to the
invention, the method further comprises, between said step of
forming the crystal layers and said step of applying the light beam
toward the interface, a step of bonding a cleavable second
substrate onto a surface of the crystal layers in such a manner
that a cleavage plane of the second substrate substantially
coincides with a cleavage plane of the crystal layers of the
nitride semiconductor.
[0024] As to a further aspect of the fabrication method according
to the invention, in the step of applying the light beam toward the
interface, the light beam is applied uniformly or entirely onto the
interface between the substrate and the ground layer.
[0025] As to a still further aspect of the fabrication method
according to the invention, in the step of applying the light beam
toward the interface, the interface between the substrate and the
ground layer is scanned with a spot or line of the light beam.
[0026] In another aspect of the fabrication method according to the
invention, the method further comprises a step of forming a
waveguide extending along a direction normal to the cleavage plane
of the nitride semiconductor.
[0027] In a further aspect of the fabrication method according to
the invention, the crystal layers of the nitride semiconductor are
formed by metal-organic chemical vapor deposition.
[0028] As to a still further aspect of the fabrication method
according to the invention, in the step of applying the light beam
toward the interface, the light beam is an ultraviolet ray
generated from a frequency quadrupled YAG laser.
[0029] In addition, a nitride semiconductor laser device according
to the present invention having successively grown crystal layers
each made of a group III nitride semiconductor
(Al.sub.xGa.sub.1-x).sub.1-yIn.sub.yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1) comprises:
[0030] a ground layer made of group III nitride semiconductor
(Al.sub.x'Ga.sub.1-x').sub.1-y'In.sub.y'N (0.ltoreq.x'.ltoreq.1,
0.ltoreq.y'.ltoreq.1);
[0031] a plurality of crystal layers each made of group III nitride
semiconductor formed on the ground layer;
[0032] a cleavable substrate bonded onto a surface of the crystal
layers opposite to the ground layer.
[0033] In another aspect of the nitride semiconductor laser device
according to the invention, the device further comprises a heat
sink bonded onto the ground layer.
[0034] In a further aspect of the nitride semiconductor laser
device according to the invention, the device further comprises a
heat sink bonded onto the cleavable substrate.
[0035] In a still further aspect of the nitride semiconductor laser
device according to the invention, the cleavable substrate has a
cleavage plane coinciding with a cleavage plane of the crystal
layers of the nitride semiconductor.
[0036] In another aspect of the nitride semiconductor laser device
according to the invention, the device further comprises a
waveguide extending along a direction normal to the cleavage plane
of the nitride semiconductor.
[0037] In a further aspect of the nitride semiconductor laser
device according to the invention, the cleavable substrate is made
of semiconductor single-crystal such as GaAs.
[0038] In a still further aspect of the nitride semiconductor laser
device according to the invention, the cleavable substrate is made
of an electrically conductive material.
[0039] According to the present invention, it is possible to obtain
high-quality mirror facets by untying the crystal bond between the
sapphire substrate and the ground layer of GaN crystal and
separating the substrate and the ground layer and thereby,
fabricating the laser device with high reproducibility.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a schematic perspective view showing the fractured
plane of a GaN crystal layer formed on a sapphire substrate;
[0041] FIG. 2 is a schematic perspective view showing the lattice
plane of a GaN crystal layer formed on a sapphire substrate;
[0042] FIG. 3 is a schematic sectional view of a group III
nitride-semiconductor laser device of an embodiment according to
the present invention;
[0043] FIG. 4 is an enlarged schematic sectional view of a group
III nitride-semiconductor laser device of an embodiment according
to the present invention, which is seen from the mirror facet for
optical resonance;
[0044] FIGS. 5 and 6 are schematic sectional views each showing a
portion of a wafer for the semiconductor laser device at each
fabricating step of an embodiment of the present invention;
[0045] FIGS. 7 to 9 are schematic perspective views each showing a
portion of a wafer for the semiconductor laser device at each
fabricating step of an embodiment of the present invention;
[0046] FIG. 10 is an enlarged schematic sectional view showing a
wafer in the semiconductor-laser fabricating step of an embodiment
of the present invention;
[0047] FIGS. 11 to 16 are schematic perspective views each showing
a wafer in the semiconductor-laser fabricating step of an
embodiment of the present invention;
[0048] FIG. 17 is a schematic sectional view of a group III
nitride-semiconductor laser device of another embodiment according
to the present invention;
[0049] FIG. 18 is an enlarged schematic sectional view of a group
III nitride-semiconductor laser device of another embodiment
according to the present invention, which is seen from the mirror
facet for optical resonance;
[0050] FIG. 19 is a schematic sectional view showing a portion of a
GaAs substrate for the semiconductor laser device at each
fabricating step of another embodiment of the present
invention;
[0051] FIGS. 20 to 23 are schematic perspective views each showing
a wafer in the semiconductor-laser fabricating step of another
embodiment of the present invention; and
[0052] FIGS. 24 and 25 are enlarged schematic sectional views each
showing a wafer in the semiconductor-laser fabricating step of an
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0053] Embodiments of group III nitride semiconductor laser devices
according to the present invention are described below by referring
to the accompanying drawings.
[0054] FIG. 3 generally shows an embodiment of the group III
nitride semiconductor laser device of a refractive index guided
type according to the invention. This device is constructed with a
laser body 100, a support substrate 200 bonded onto the laser body
100, and a chip carrier 10 bonded onto the laser body 100 serving
as a heat sink. The chip carrier 10 is made of an electrically
conductive material. The laser body 100 comprises a ground layer
103 made of group III nitride semiconductor
(Al.sub.x'Ga.sub.1-x').sub.1-y'In.sub.y'N (0.ltoreq.x'.ltoreq.1,
0.ltoreq.y'.ltoreq.1), crystal layers 104 to 110 each made of a
group III nitride semiconductor
(Al.sub.xGa.sub.1-x).sub.1-yIn.sub.yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1) successively grown in order on the ground
layer, and an electrode layer 113. The crystal layers include an
active layer. The support substrate 200 is a cleavable or partable
substrate made of an electrically conductive material or preferably
semiconductor single-crystal such as GaAs, InP, Si, or the like.
The support substrate 200 is bonded to a surface of the crystal
layers opposite to the ground layer 103 via the electrode layer
113. A cleavage plane of the support substrate 200 coincides with a
cleavage plane of the crystal layers of the nitride semiconductor.
Namely, the support substrate 200 is bonded to the surface of the
crystal layers in such a manner that the cleavage plane of the
second substrate substantially coincides with a cleavage plane of
the crystal layers of the nitride semiconductor. The surface of the
ground layer 103 of the laser body 100 is bonded onto the chip
carrier 10 via which the device is electrically connected to an
external electrode. The laser body 100 has a ridge waveguide
extending along a direction normal to the cleavage plane of the
nitride semiconductor layers 103 to 110 (which is the direction
normal to the drawing).
[0055] As shown in FIG. 4, the laser body 100 of the semiconductor
laser device is constituted of the ground layer 103 i.e., n-type
GaN layer 103, an n-type Al.sub.0.1Ga.sub.0.9N layer 104, an n-type
GaN layer 105, an active layer 106 mainly containing InGaN, a
p-type Al.sub.0.2Ga.sub.0.8N layer 107, a p-type GaN layer 108, a
p-type Al.sub.0.1Ga.sub.0.9N layer 109, and a p-type GaN contact
layer 110 which are layered on the ground layer 103 in this order.
A ridge stripe portion 118 is formed in the p-type
Al.sub.0.1Ga.sub.0.9N layer 109 and the p-type GaN contact layer
110 so as to extend along a direction normal to the cleavage plane
of the nitride semiconductor layers. The top of the laser body 100
is covered with and protected by an insulating film 111 made of
SiO.sub.2 except a contact window to the p-type GaN contact layer
110 of the ridge stripe portion 118. The insulating film 111 is
covered with the p-side electrode layer 113. The n-type GaN ground
layer 103 is connected to the chip carrier 10. The p-side electrode
113 connected through a slit of the insulating film 111 to the
p-type GaN contact layer 110 is connected to the support substrate
200 via a metal film.
[0056] The semiconductor laser device emits light by recombining
electrons with holes in the active layer 106. The n-type GaN layer
105 and p-type GaN layer 108 serve as guiding layers. Light
generated in the active layer 106 is guided in the guiding layers
105 and 108. Electrons and holes are effectively confined into the
active layer 106 by setting band gaps of the guiding layers 105 and
108 to values larger than that of the active layer 106. The p-type
Al.sub.0.2Ga.sub.0.8N layer 107 serves as a barrier layer for
further enhancing the confinement of carriers (particularly,
electrons), the n-type Al.sub.0.1Ga.sub.0.9N layer 104 and the
p-type Al.sub.0.1Ga.sub.0.9N layer 109 serve as cladding layers
respectively each formed to have refractive indexes lower than
those of the guiding layers 105 and 108. The wave-guiding in the
lateral direction is performed by the difference between refractive
indexes of the cladding layer and the guiding layer. The ridge
stripe portion 118 is formed in order to produce a
lateral-directional step in effective refractive index by changing
the thickness of the cladding layer 109, thereby confining the
generated light in the lateral direction.
[0057] The device structure shown in FIGS. 3 and 4 is fabricated in
the following fabricating steps in which layered structure for a
laser device is formed through the metal-organic chemical vapor
deposition (MOCVD) on an A-face sapphire substrate whose both sides
are mirror-finished.
[0058] <Preparation of a Laser Wafer>
[0059] FIG. 5 shows a sectional view of a target laser wafer
prepared through the following steps in which crystal layers for a
GaN semiconductor laser structure are grown on a sapphire
substrate.
[0060] First, a sapphire substrate 101 is set into an MOCVD reactor
and held for 10 minutes in a hydrogen-gas flow at a pressure of 300
Torr and a temperature of 1050.degree. C. to thermally clean the
surface of the sapphire substrate 101. Thereafter, the temperature
of the sapphire substrate 101 is lowered to 600.degree. C., and
ammonia (NH.sub.3) which is a nitrogen precursor and TMA (trimethyl
aluminium) which is an Al precursor are introduced into the reactor
to deposit a buffer layer 102 made of AlN up to a thickness of 20
nm. The GaN (or AlN) layer 102 formed at a low temperature acts as
a buffer layer to ensure a growth of GaN film on the sapphire
substrate which is a dissimilar material to GaN.
[0061] Subsequently, the supply of TMA is stopped, the temperature
of the sapphire substrate 101 on which the buffer layer 102 is
formed is raised to 1050.degree. C. again while flowing only
NH.sub.3, and trimethyl gallium is introduced to form the n-type
GaN ground layer 103 on the buffer layer 102. In this case,
Me-SiH.sub.3 (methyl silane) is added into a growth atmosphere gas
as the precursor of Si which serves as an n-type impurity.
[0062] When the n-type GaN ground layer 103 is grown up to
approximately 4 .mu.m, TMA is introduced to form the n-type AlGaN
cladding layer 104. When the n-type AlGaN cladding layer 104 is
grown up to approximately 0.5 .mu.m, the supply of TMA is stopped
to grow the n-type GaN guiding layer 105 up to 0.1 .mu.m. When
growth of the n-type GaN guiding layer 105 is completed, the supply
of TMG and Me-SiH.sub.3 is stopped, and lowering of temperature is
started to set the substrate temperature at 750.degree. C.
[0063] When the substrate temperature reaches to 750.degree. C.,
carrier gas is switched from hydrogen to nitrogen. When the
gas-flow state is stabilized, TMG, TMI, and Me-SiH.sub.3 are
introduced for growing a barrier layer in the active layer 106.
Subsequently, the supply of Me-SiH.sub.3 is stopped and then the
flow rate of TMI is increased so that a well layer having an In
composition ratio greater than that of the barrier layer is grown
on the barrier layer. The growths of the barrier layer and the well
layer are repeated in pairs in accordance with the number of wells
in the designed multiple quantum well structure. In this way, the
active layer 106 of the multiple quantum well structure is
formed.
[0064] When the growth of active layer is finished, the supply of
TMG, TMI, and Me-SiH.sub.3 is stopped, and the carrier gas is
switched from nitrogen to hydrogen. When the gas-flow is
stabilized, the substrate temperature is raised to 1050.degree. C.
again and TMG, TMA, and Et-CP.sub.2Mg (ethyl cyclopentadienyl
magnesium) as the precursor of Mg which serves as a p-type impurity
are introduced to form the p-type AlGaN layer 107 on the active
layer 106 up to 0.01 .mu.m. Then, the supply of TMA is stopped to
grow the p-type GaN guiding layer 108 up to 0.1 .mu.m and TMA is
introduced again to grow the p-type AlGaN cladding layer 109 up to
0.5 .mu.m. Moreover, the p-type GaN contact layer 110 is grown on
the layer 109 up to 0.1 .mu.m. Thereafter, the supply of TMG and
Et-CP.sub.2Mg is stopped and temperature lowering is started. When
the substrate temperature reaches 400 .degree. C., the supply of
NH.sub.3 is also stopped. When the substrate temperature reaches
room temperature, the sapphire substrate 101 is taken out of the
reactor.
[0065] The obtained wafer is set into a heat treatment furnace to
apply heat treatment for the p-type conversion.
[0066] In this way, the laser wafer shown in FIG. 5 is
prepared.
[0067] <Formation of Ridge Waveguide>
[0068] A ridge waveguide is formed as the index-guided type
structure on the prepared laser wafer through the following
steps:
[0069] As shown in FIG. 6, a mask 115 having a plurality of slits
parallel to each other is formed on the surface of the p-type GaN
contact layer 110, and the exposed area of the nitride
semiconductor layer is partially etched by reactive ion etching
(RIE).
[0070] In this case, as shown in FIG. 7, the etching is performed
down to a depth where the p-type AlGaN cladding layer 109 is
slightly left to form a recessed portion 201. Then, the mask 115 is
removed to form narrow ridge structures 118 of 5 .mu.m-wide
extending parallel to each other. FIG. 7 shows two narrow ridge
structures 118.
[0071] An SiO.sub.2 protective film 111 is deposited on the wafer
by sputtering as shown in FIG. 8.
[0072] Thereafter, a plurality of 3 .mu.m-wide window portions 113a
for n-type electrodes are formed at the tops of the ridge
structures 118 in the SiO.sub.2 protective film 111 by a standard
photolithographic technique.
[0073] Nickel (Ni) with a thickness of 50 nm and subsequently Gold
(Au) with a thickness of 200 nm are evaporated onto the SiO.sub.2
protective film 111 and the portion where the p-type GaN contact
layer 110 is exposed to form the p-side electrode 113. Thus, the
device structures each shown in FIG. 10 are formed on the device
wafer.
[0074] <Bonding of a Cleavable Substrate to the Wafer>
[0075] Subsequently, as shown in FIG. 11, a GaAs single-crystal
substrate 200 is bonded onto the p-side electrode 113 at the ridge
waveguide side of the wafer as to be connected electrically to the
laser structure. In this bonding step, the GaAs substrate 200 is
aligned to the GaN laser structure in such a manner that the
crystallographic orientation of the GaAs crystal substrate is set
to be parallel to that of the GaN crystal layers, so that the
cleavage of the GaAs crystal will coincide with the GaN cleavage
plane in the next cleaving step wherein the desired laser resonator
plane is formed by the cleavage of GaN crystal. A GaAs
single-crystal substrate of p-type conductivity is used in this
case. A Ti--Au thin film and Au--Sn thin film are previously formed
in order by evaporation onto the surface of GaAs single-crystal
substrate to be contacted to the p-side electrode 113 of the GaN
crystal layer. The GaAs surface with the metal films and the
electrode 113 are brought into contact, and then pressurized to
achieve the bonding of both the substrates.
[0076] <Irradiation of Light from Sapphire Side to the
Wafer>
[0077] Subsequently, as shown in FIG. 12, the bonded wafer is
irradiated from the backside of and through the sapphire substrate
101 to the ground layer 103 with a focused ultraviolet ray
generated by a short-wavelength high output laser such as a
frequency quadrupled YAG laser (with 266 nm wavelength), a KrF
excimer laser (with 248 nm wavelength) or the like. The UV light
beam may be applied uniformly onto the entire interface between the
sapphire substrate 101 and the ground layer 103.
[0078] Whereas the sapphire substrate is almost transparent at 248
nm which is the wavelength of the laser beam used for the above UV
irradiation, GaN of the ground layer absorbs the irradiation beam
with a small penetration depth because it has an absorption edge of
365 nm. Moreover, because of a large lattice mismatch (15%) present
between the sapphire substrate and the GaN layer, extremely dense
defects are present in the GaN crystal nearby the interface and
thereby, absorbed light is almost converted to heat. The
temperature of an area of GaN nearby the sapphire substrate rapidly
rises and thus, GaN is decomposed into gallium and nitrogen.
Therefore, a decomposed-matter area 150 of the nitride
semiconductor is produced at the interface region in the ground
layer 103.
[0079] The decomposed-matter area 150 is provided for the purpose
of promoting the crystal separation of the sapphire substrate 101
from the ground layer 103 of GaN and AlN. The sapphire substrate is
used only for the manufacture of device. The wavelength of applied
laser beam is selected from wavelengths absorbed by a GaN crystal
layer and passing through the sapphire substrate. Therefore, as for
the irradiated interface area in the GaN ground layer 103, direct
crystal bonds between sapphire 101 and GaN 103 are disconnected.
Thus the GaN ground layer 103 may be readily separated from the
sapphire substrate 101 along the decomposed-matter area 150.
[0080] <Separation of Sapphire and Laser Wafers>
[0081] After that, the sapphire substrate 101 is slightly heated to
separate the ground layer 103 with other crystal layers thereon
from the sapphire substrate 101.
[0082] By this heating step, as shown in FIG. 13, the sapphire
substrate 101 is removed from the lamination i.e., the laser wafer
of the bonded laser body 100 and the support substrate 200, because
atomic bonds between gallium and nitrogen are lost within the
decomposed-matter area 150.
[0083] After the removal of the sapphire substrate 101, the exposed
surface of the ground layer 103 is cleaned by dipping the laser
wafer into a dilute hydrochloric acid solution or the like to
remove residual metallic Ga therefrom.
[0084] Ti with a thickness of 50 nm and Au with a thickness 200 nm
are successively evaporated onto the exposed surface of the laser
wafer to form an n-side electrode 102.
[0085] The GaAs support substrate 200 may be thinned by lapping to
facilitate to cleave the laser wafer. Ti/Au electrode is evaporated
onto the surface of GaAs of the laser wafer.
[0086] <Cleaving of the Ground Layer>
[0087] As shown in FIG. 14, in the case of the laser wafer of the
bonded laser body 100 and the support substrate 200, the support
substrate 200 is cleaved together with the ground layer 103 along
the lines perpendicular to the ridge waveguide extending direction
at an interval P which corresponds to the length of the final
device.
[0088] At this cleaving step, the scribing (so-called notching
operation) may be performed on the surface of the support substrate
200 by using a diamond point. As a result, a plurality of laser
bars 300 are obtained.
[0089] <Reflection Coating on Laser Bar Side Surface>
[0090] As shown in FIG. 15, dielectric multilayer reflection
coatings 302 are formed on both the fracture planes (cleavage
planes) 301 of each laser bar 300 by a sputtering system or the
like.
[0091] <Formation of Laser-Chips From Laser Bar>
[0092] As shown in FIG. 16, individual laser chips are obtained by
further splitting the laser bar by means of second cleavage along
the direction parallel to the ridge waveguide extending
direction.
[0093] <Assembling of Laser-Chip>
[0094] Each laser-chip of the bonded laser body 100 and support
substrate 200 is bonded via a Ti--Au thin film onto a chip carrier
10 serving as a heat sink in such a manner that the ground layer
103 of the laser body 100 is electrically connected to the chip
carrier.
[0095] As described above, the laser structure formed on the A-face
of a sapphire substrate is disclosed. In addition, the laser
structure of ridge waveguide type may be formed on the C-face of
the sapphire substrate.
[0096] <Secondary Embodiment>
[0097] The second embodiment to be fabricated is a group III
nitride semiconductor laser device of a gain-guided type.
[0098] FIGS. 17 and 18 shows the gain-guided type group III
nitride-semiconductor laser device of the second embodiment. The
members of the device in those figures are the same as ones of the
first embodiment shown in FIGS. 3 and 4.
[0099] This device of the second embodiment is constructed with a
laser body 100, a support substrate 200 bonded onto the laser body
100, and an electrically conductive chip carrier 10 serving as a
heat sink bonded onto the support substrate 200. The laser body 100
comprises crystal layers 104 to 110 each made of a group III
nitride semiconductor (Al.sub.xGa.sub.1-x).sub.1-yIn.sub.yN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) successively grown in
order on the ground layer 103. An electrode layer 113a is formed on
the n-type GaN ground layer 103 through which the device is
electrically connected to an external electrode. Whereas, the
p-type GaN contact layer 110 of the device is electrically
connected through a stripe window 213a to the support substrate 200
and the chip carrier 10. A window film 213 made of GaAs oxide is
formed between the laser body 100 and the support substrate 200.
The GaAs oxide window film 213 is provided with the stripe window
213a of GaAs extending along a direction normal to the cleavage
plane of the nitride semiconductor layers. The GaAs stripe 213a
serves as an electric current passage between the laser body 100
and the support substrate 200. It is preferable that a joining
metal film is provided between the GaAs oxide window film 213 and
the p-type GaN contact layer 110 to bond the body 100 and the
support substrate 200.
[0100] As shown in FIG. 18, the body 100 of the laser device
includes a plurality of crystal layers from the n-type GaN layer
103 and the p-type GaN contact layer 110 which are layered as the
same order as shown in FIG. 4. In this laser device, only the GaAs
stripe 213a provides the electric current to the active
semiconductor layer instead of the ridge waveguide formed in the
cladding layer so that the device is the gain-guided type. The GaAs
oxide film makes insulation between the laser body 100 and the
support substrate 200 expect the GaAs stripe 213a.
[0101] The device structure shown in FIGS. 17 and 18 is fabricated
in a similar manner as the first embodiment in which layered
structure for the device is formed through the metal-organic
chemical vapor deposition (MOCVD) on an A-face sapphire
substrate.
[0102] First, the laser wafer with the GaN semiconductor laser
structure shown in FIG. 5 is prepared on the basis of the sapphire
substrate.
[0103] Whereas, a GaAs oxide window film 213 with a plurality of
GaAs stripes 213a is formed on a GaAs single-crystal substrate 200
i.e., support substrate as shown in FIG. 19. The interval of GaAs
stripes 213a is approximately 200 .mu.m and the width of each
stripe is approximately 2 to 5 .mu.m for example. Those film and
stripes may be formed in the following steps 1) to 6):
[0104] 1) coating the surface of GaAs single-crystal substrate 200
with a photoresist;
[0105] 2) irradiate pertinent light through a mask having stripe
windows onto the photoresist layer;
[0106] 3) developing the photoresist layer on the substrate;
[0107] 4) settling the remaining photoresist down to the substrate
to form given striped photoresist masks;
[0108] 5) oxidizing the exposed surface of the GaAs substrate other
than the stripe photoresist masks to form a GaAs oxide window film
213 with a plurality of GaAs stripes 213a. There are defined plural
GaAs stripes 213a beneath the stripe photoresist masks; and
[0109] 6) removing the stripe photoresist masks from the
substrate.
[0110] In this way, the GaAs oxide window film 213 with a plurality
of GaAs stripes 213a is formed on the GaAs support substrate.
[0111] Subsequently, as shown in FIG. 20, the GaAs support
substrate of GaAs single-crystal 200 is bonded onto the p-type GaN
contact layer 110 of the wafer via the GaAs oxide window film 213
with a plurality of GaAs stripes 213a as to be connected
electrically to the laser structure. In this bonding step, the GaAs
substrate 200 is aligned to the GaN laser structure in such a
manner that the crystallographic orientation of the GaAs crystal
substrate is parallel to or coincides with that of the GaN crystal
layers, so that there will be appearance of the GaAs cleavage
surface or fractured plane matched for the GaN one in the next
cleaving step wherein a given laser resonator consists of the GaN
cleavage surface of the crystal layer.
[0112] A thin metal film such as In, Ni or the like may be
previously formed by evaporation on at least one of contacting
surfaces of GaAs oxide film 213 on the substrate 200 and the p-type
GaN contact layer 110 of the wafer in order that both the
substrates come contact with each other via the thin metal film
sandwiched between the p-type GaN contact layer 110 and the GaAs
oxide window film 213. In this case, as shown in FIG. 24, it is
preferable that the joining thin metal films 222a and 222b are
provided on the GaAs single-crystal substrate 200 and the p-type
GaN contact layer 110 respectively in which slits 223 with an about
10 .mu.m width are formed along the edges of the GaAs stripe 213a
in the GaAs oxide film 213. That is, as shown in FIG. 25, the slits
223 define a narrowed current path CP of metal from the GaAs
substrate 200 through the GaAs stripe 213a to the p-type GaN
contact layer 110, when the joining thin metal films 222a and 222b
are fused in the next step.
[0113] Anyway, the bonding surface of the support substrate is made
contact with the surface of cladding layer opposite to the ground
layer 103 with respect to the active layer of the laser wafer while
being pressurized and heated, and then close adhesion of both the
substrates is achieved.
[0114] Subsequently, as shown in FIG. 21, an ultraviolet ray is
irradiated through the sapphire substrate 101 to the ground layer
103 by using a short-wavelength high output laser device. Namely
the UV radiation is performed from the backside of sapphire
substrate while being converged by a converging lens. Since GaN
absorbs UV light, the temperature of the area of GaN nearby the
sapphire substrate suddenly rises and thus, GaN is decomposed into
gallium and nitrogen, so that the decomposed-matter area 150 of the
nitride semiconductor is produced along the light trace.
[0115] After that, the sapphire substrate 101 carrying the GaN
layers is slightly heated and then, as shown in FIG. 22, the
sapphire substrate 101 is removed from the lamination i.e., laser
wafer of the bonded laser body 100 and support substrate 200 at the
boundary of the decomposed-matter area 150 of the ground layer
103.
[0116] Then, an n-side electrode layer 103a is formed on the
exposed surface of the GaN ground layer 103 of the laser body
100.
[0117] After that, the cleaving step, the reflection layer
formation step and the assembling step are preformed in turn and
then the semiconductor laser device as shown in FIGS. 17 and
18.
[0118] According to the present invention, it is possible to
utilize the natural cleavage plane of nitride semiconductor for
fabricating the resonator of the device by removing the substrate
for the crystal growth. An atomically flat mirror facet is easily
obtained, thereby reducing the optical scattering loss. As a
result, continuous oscillation of laser is achieved and the same
time a long life of the laser device is obtained in practical.
* * * * *