U.S. patent application number 11/928185 was filed with the patent office on 2008-03-20 for nitride-based semiconductor device and method of fabricating the same.
Invention is credited to Masayuki Hata, Yasuhiko Nomura, Tadao Toda, Tsutomu Yamaguchi.
Application Number | 20080067541 11/928185 |
Document ID | / |
Family ID | 28449238 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080067541 |
Kind Code |
A1 |
Toda; Tadao ; et
al. |
March 20, 2008 |
NITRIDE-BASED SEMICONDUCTOR DEVICE AND METHOD OF FABRICATING THE
SAME
Abstract
A method of fabricating a nitride-based semiconductor device
capable of reducing contact resistance between a nitrogen face of a
nitride-based semiconductor substrate or the like and an electrode
is provided. This method of fabricating a nitride-based
semiconductor device comprises steps of etching the back surface of
a first semiconductor layer consisting of either an n-type
nitride-based semiconductor layer or a nitride-based semiconductor
substrate having a wurtzite structure and thereafter forming an
n-side electrode on the etched back surface of the first
semiconductor layer.
Inventors: |
Toda; Tadao; (Souraku-gun,
JP) ; Yamaguchi; Tsutomu; (Nara-shi, JP) ;
Hata; Masayuki; (Kadoma-shi, JP) ; Nomura;
Yasuhiko; (Moriguchi-shi, JP) |
Correspondence
Address: |
MOTS LAW, PLLC
1001 PENNSYLVANIA AVE. N.W.
SOUTH, SUITE 600
WASHINGTON
DC
20004
US
|
Family ID: |
28449238 |
Appl. No.: |
11/928185 |
Filed: |
October 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11607896 |
Dec 4, 2006 |
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11928185 |
Oct 30, 2007 |
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11114193 |
Apr 26, 2005 |
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11607896 |
Dec 4, 2006 |
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10936499 |
Sep 9, 2004 |
6890779 |
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11114193 |
Apr 26, 2005 |
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10394260 |
Mar 24, 2003 |
6791120 |
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10936499 |
Sep 9, 2004 |
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Current U.S.
Class: |
257/103 ;
257/E21.172; 257/E21.407 |
Current CPC
Class: |
H01L 29/66462 20130101;
H01L 33/14 20130101; H01L 33/32 20130101; H01S 5/32341 20130101;
H01S 5/2201 20130101; H01L 21/28575 20130101; H01L 33/0093
20200501; H01S 5/305 20130101; H01L 33/40 20130101; H01S 2304/04
20130101; H01S 5/34333 20130101; H01L 29/2003 20130101; B82Y 20/00
20130101; H01L 33/007 20130101; H01S 5/04252 20190801 |
Class at
Publication: |
257/103 ;
257/E21.172 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2002 |
JP |
JP2002-85085 |
Claims
1-27. (canceled)
28. A nitride-based semiconductor device comprising: a substrate
made from an n-type nitride-based semiconductor formed by using
growth on a substrate for growth; and an n-side electrode formed on
a back surface of said substrate, wherein a dislocation density is
not more than 1.times.10.sup.9 cm.sup.-2 in the vicinity of the
interface between said substrate and said n-side electrode.
29. The nitride based semiconductor device according to claim 28,
wherein said substrate is thinned by processing the back surface
thereof.
30. (canceled)
31. (canceled)
32. (canceled)
33. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a nitride-based
semiconductor device and a method of fabricating the same, and more
particularly, it relates to a nitride-based semiconductor device
having an electrode and a method of fabricating the same.
[0003] 2. Description of the Background Art
[0004] A nitride-based semiconductor laser device has recently been
expected as a light source for an advanced large capacity optical
disk, and actively developed.
[0005] In general, an insulating sapphire substrate is employed for
forming a nitride-based semiconductor laser device. When a
nitride-based semiconductor layer is formed on the sapphire
substrate, however, a large number of defects (dislocations) are
disadvantageously formed in the nitride-based semiconductor layer
due to large difference between the lattice constants of the
sapphire substrate and the nitride-based semiconductor layer.
Consequently, the characteristics of the nitride-based
semiconductor laser device are disadvantageously reduced.
[0006] In this regard, a nitride-based semiconductor laser device
employing a nitride-based semiconductor substrate such as a GaN
substrate having small difference in lattice constant with respect
to a nitride-based semiconductor layer is proposed in general.
[0007] FIG. 7 is a sectional view showing a conventional
nitride-based semiconductor laser device employing an n-type GaN
substrate 101. Referring to FIG. 7, nitride-based semiconductor
layers (102 to 110) are grown on a Ga face ((HKLM) plane: M denotes
a positive integer) to be improved in crystallinity in a process of
fabricating the conventional nitride-based semiconductor laser
device. A nitrogen face ((HKL-M) plane: M denotes a positive
integer) of the n-type GaN substrate 101 having a wurtzite
structure is employed as the back surface, so that an n-side
electrode 112 is formed on this back surface of the n-type GaN
substrate 101. The fabrication process for the conventional
nitride-based semiconductor laser device is now described in
detail.
[0008] As shown in FIG. 7, an n-type layer 102 consisting of n-type
GaN having a thickness of about 3 .mu.m, an n-type buffer layer 103
consisting of n-type In.sub.0.05Ga.sub.0.95N having a thickness of
about 100 nm, an n-type cladding layer 104 consisting of n-type
Al.sub.0.05Ga.sub.0.95N having a thickness of about 400 nm, an
n-type light guide layer 105 consisting of n-type GaN having a
thickness of about 70 nm an MQW (multiple quantum well) active
layer 106 having an MQW structure, a p-type layer 107 consisting of
p-type Al.sub.0.2Ga.sub.0.8N having a thickness of about 200 nm, a
p-type light guide layer 108 consisting of p-type GaN having a
thickness of about 70 nm, a p-type cladding layer 109 consisting of
p-type Al.sub.0.05Ga.sub.0.95N having a thickness of about 400 nm
and a p-type contact layer 110 consisting of p-type GaN having a
thickness of about 100 nm are successively formed on the upper
surface (Ga face) of the n-type GaN substrate 101 having a
thickness of about 300 .mu.m to about 500 .mu.m.
[0009] Then, a p-side electrode 111 is formed on a prescribed
region of the upper surface of the p-type contact layer 110. The
back surface of the n-type GaN substrate 101 is polished until the
thickness of the n-type GaN substrate 101 reaches a prescribed
level of about 100 .mu.m, and an n-side electrode 112 to thereafter
formed on the back surface (nitrogen face) of the n-type GaN
substrate 101. Finally, the n-type GaN substrate 101 and the layers
102 to 110 are cleft thereby performing element isolation and
forming a cavity facet. Thus, the conventional nitride-based
semiconductor laser device shown in FIG. 7 is completed.
[0010] In the conventional nitride-based semiconductor laser device
shown in FIG. 7, however the n-type GaN substrate 101 is so hard
that it is difficult to excellently perform device isolation and
form the cavity facet by cleavage. In order to cope with such
inconvenience, a method of mechanically polishing the back surface
of the n-type GaN substrate 101 before the cleavage step for
reducing irregularity on the back surface thereby excellently
performing element isolation and forming the cavity facet Is
proposed. This method is disclosed in Japanese Patent Laying-Open
No. 2002-26438, for example.
[0011] In the aforementioned conventional method disclosed in
Japanese Patent Laying-Open No. 2002-26438, however, stress is
applied in the vicinity of the back surface of the n-type GaN
substrate 101 when the back surface of the n-type GaN substrate 101
is mechanically polished. Therefore, microscopic defects such as
cracks are disadvantageously formed in the vicinity of the back
surface of the n-type GaN substrate 101. Consequently, contact
resistance between the n-type GaN substrate 101 and the n-side
electrode 112 formed on the back surface (nitrogen face) thereof is
disadvantageously increased.
[0012] Further, the nitrogen face of the n-type GaN substrate 101
is so easily oxidized that the contact resistance between the
n-type GaN substrate 101 and the n-side electrode 112 formed on the
back surface (nitrogen face) thereof is disadvantageously increased
also by this.
SUMMARY OF THE INVENTION
[0013] An object of the present invention is to provide a method of
fabricating a nitride-based semiconductor device capable of
reducing contact resistance between the back surface of a
nitride-based semiconductor substrate or the like and an
electrode.
[0014] Another object of the present invention is to reduce the
number of defects in the vicinity of the back surface of the
nitride-based semiconductor substrate or the like in the
aforementioned method of fabricating a nitride-based semiconductor
device.
[0015] Still another object of the present invention is to provide
a nitride-based semiconductor device capable of reducing contact
resistance between the back surface of a nitride-based
semiconductor substrate or the like and an electrode.
[0016] In order to attain the aforementioned objects, a method of
fabricating a nitride-based semiconductor device according to a
first aspect of the present invention comprises steps of etching
the back surface of a first semiconductor layer consisting of
either an n-type nitride-based semiconductor layer or a
nitride-based semiconductor substrate having a wurtzite structure
and thereafter forming an n-side electrode on the etched back
surface of the first semiconductor layer.
[0017] In the method of fabricating a nitride-based semiconductor
device according to the first aspect, the back surface of the first
semiconductor layer consisting of either an n-type nitride-based
semiconductor layer or a nitride-based semiconductor substrate
having a wurtzite structure is etched as hereinabove described,
whereby a region including defects in the vicinity of the back
surface of the first semiconductor layer resulting from a polishing
step or the like can be removed for reducing the number of defects
in the vicinity of the back surface of the first semiconductor
layer. Thus, an electron carrier concentration can be inhibited
from reduction resulting from trap of electron carriers by defects,
so that the electron carrier concentration can be increased on the
back surface of the first semiconductor layer. Consequently,
contact resistance between the first semiconductor layer and the
n-side electrode can be reduced. Further, the back surface of the
first semiconductor layer is so etched that flatness thereof can be
improved as compared with that of a mechanically polished back
surface. Thus, the n-side electrode formed on the back surface of
the first semiconductor layer can also be improved in flatness,
whereby adhesion between the n-side electrode and a radiator base
can be improved when the former is mounted on the latter.
Consequently, excellent reliability can be attained. Further, the
n-side electrode formed on the back surface of the first
semiconductor layer can be so improved in flatness that wire
bondability with respect to the n-side electrode can be improved
when the n-side electrode is wire-bonded.
[0018] In the aforementioned method of fabricating a nitride-based
semiconductor device according to the first aspect, the back
surface of the first semiconductor layer preferably includes a
nitrogen face of the first semiconductor layer. The term "nitrogen
face" denotes a wide concept indicating not only a complete
nitrogen face but also a surface mainly formed by a nitrogen face.
More specifically, the term nitrogen face includes a surface having
a nitrogen face of at least 50% in the present invention. When
formed by a nitrogen face, the back surface of the first
semiconductor layer is so easily oxidized that the oxidized portion
of the back surface can be removed by etching. Thus, contact
resistance between the first semiconductor layer and the n-side
electrode can be further reduced.
[0019] In the aforementioned method of fabricating a nitride-based
semiconductor device according to the first aspect, the etching
step preferably includes a step of etching the back surface of the
first semiconductor layer by dry etching. According to this
structure the back surface of the first semiconductor layer can be
easily improved in flatness and the number of defects can be
reduced in the vicinity of the back surface due to the dry
etching.
[0020] In the aforementioned method of fabricating a nitride-based
semiconductor device including the step of etching the back surface
of the first semiconductor layer by dry etching, the step of
etching the back surface of the first semiconductor layer by dry
etching preferably includes a step of etching the back surface of
the first semiconductor layer by reactive ion etching with Cl.sub.2
gas and BCl.sub.3 gas. According to this structure, the back
surface of the first semiconductor layer can be easily improved in
flatness and the number of defects can be easily reduced in the
vicinity of the back surface. In this case, the ratio of the flow
rate of BCl.sub.3 gas to the flow rate of Cl.sub.2 gas in the step
of etching the back surface of the first semiconductor layer by the
reactive ion etching is preferably at least 30% and not more than
70%. It has been experimentally confirmed that the back surface of
the first semiconductor layer can be improved in flatness in this
range of the ratio of the flow rate of BCl.sub.3 gas to that of
Cl.sub.2 gas, and hence the back surface of the first semiconductor
layer can be reliably improved in flatness by setting the ratio
within this range.
[0021] In the aforementioned method of fabricating a nitride-based
semiconductor device including the step of etching the back surface
of the first semiconductor layer by dry etching, the etching depth
and the etching time in the step of etching the back surface of the
first semiconductor layer by dry etching are preferably
proportional to each other. According to this structure, the
etching depth can be accurately controlled by adjusting the etching
time.
[0022] In the aforementioned method of fabricating a nitride-based
semiconductor device according to the first aspect, the etching
step preferably includes a step of etching the back surface of the
first semiconductor layer thereby converting the back surface of
the first semiconductor layer to a mirror surface. According to
this structure, the back surface of the first semiconductor layer
can be further improved in flatness.
[0023] The aforementioned method of fabricating a nitride-based
semiconductor device according to the first aspect preferably
further comprises a step of performing heat treatment after the
step of forming the n-side electrode. According to this structure,
contact resistance between the first semiconductor layer and the
n-side electrode can be further reduced.
[0024] In the aforementioned method of fabricating nitride-based
semiconductor device according to the first aspect, the etching
step preferably includes a step of etching the back surface of the
first semiconductor layer by a thickness of at least about 1 .mu.m.
According to this structure, a region including defects in the
vicinity of the back surface of the first semiconductor layer
resulting from a polishing step or the like can be so sufficiently
removed that the number of defects can be further reduced in the
vicinity of the back surface of the first semiconductor layer.
[0025] In the aforementioned method of fabricating a nitride-based
semiconductor device according to the first aspect, the first
semiconductor layer may include the n-type nitride-based
semiconductor layer or the nitride-based semiconductor substrate
consisting of at least one material selected from a group
consisting of GaN, BN, AlN, InN and TlN. Further, the n-side
electrode may include an Al film.
[0026] In the aforementioned method of fabricating a nitride-based
semiconductor device according to the first aspect, the
nitride-based semiconductor device is preferably a nitride-based
semiconductor light-emitting device. According to this structure,
contact resistance between the first semiconductor layer and the
n-side electrode can be reduced in the nitride based semiconductor
light-emitting device, whereby the nitride-based semiconductor
light-emitting device can attain excellent emissivity.
[0027] The aforementioned method of fabricating a nitride-based
semiconductor device according to the first aspect preferably
further comprises a step of dipping a nitrogen face of the etched
first semiconductor layer in a solution containing at least one of
chlorine, fluorine, bromine, iodine, sulfur and ammonium in advance
of the step of forming the n-side electrode. According to this
structure, residues resulting from etching can be easily removed
from the back surface of the first semiconductor layer. Thus,
contact resistance between the first semiconductor layer and the
n-side electrode can be further reduced. In this case, the method
of fabricating a nitride-based semiconductor device further
comprises a step of performing hydrochloric acid treatment on the
back surface of the first semiconductor layer with an HCl solution
in advance of the step of forming the n-side electrode. According
to this structure, chlorine-based residues adhering to the back
surface of the first semiconductor layer due to the etching can be
easily removed.
[0028] The aforementioned method of fabricating a nitride-based
semiconductor device according to the first aspect preferably
further comprises a step of polishing the back surface of the first
semiconductor layer in advance of the etching step. Also, when
polishing the back surface of the first semiconductor layer, the
back surface of the first semiconductor layer can be improved in
flatness and the number of defects resulting from polishing can be
reduced in the vicinity of the back surface through the etching
step following the polishing step.
[0029] In the aforementioned method of fabricating a nitride-based
semiconductor device according to the first aspect, the etching
step preferably includes a step of etching the back surface of the
first semiconductor layer by wet etching. According to this
structure, the back surface of the first semiconductor layer can be
easily improved in flatness and the number of defects can be easily
reduced in the vicinity of the back surface due to the wet etching.
In this case, the step of etching the back surface of the first
semiconductor layer by wet etching preferably includes a step of
etching the back surface of the first semiconductor layer with at
least one etchant selected from a group consisting of aqua regia,
KOH and K.sub.2S.sub.2O.sub.8. Further, the step of etching the
back surface of the first semiconductor layer by wet etching
preferably includes a step of etching the back surface of the first
semiconductor layer while increasing the temperature to about
120.degree. C. According to this structure, the etching rate can be
increased to about 10 times that in wet etching carried out under
the room temperature.
[0030] A method of fabricating a nitride-based semiconductor device
according to a second aspect of the present invention comprises
steps of etching a nitrogen face of a first semiconductor layer
consisting of either an n-type nitride-based semiconductor layer or
a nitride-based semiconductor substrate having a wurtzite structure
by dry etching and thereafter forming an n-side electrode on the
etched nitrogen face of the first semiconductor layer.
[0031] In the method of fabricating a nitride-based semiconductor
device according to the second aspect, the nitrogen face of the
first semiconductor layer consisting of either an n-type
nitride-based semiconductor layer or a nitride-based semiconductor
substrate having a wurtzite structure is etched by dry etching as
hereinabove described, whereby a region including defects in the
vicinity of the first semiconductor layer resulting from a
polishing step or the like can be so reduced that the number of
defects can be reduced in the vicinity of the nitrogen face of the
first semiconductor layer. Thus, reduction of an electron carrier
concentration resulting from trap of electron carriers by defects
can be suppressed, whereby the electron carrier concentration can
be increased in the nitrogen face of the first semiconductor layer.
Consequently, contact resistance between the first semiconductor
layer and the n-side electrode can be reduced. Further, the
nitrogen face of the first semiconductor layer is so etched by dry
etching that flatness thereof can be improved as compared with that
of a mechanically polished nitrogen face. Thus, the n-side
electrode formed on the nitrogen face of the first semiconductor
layer can also be improved in flatness, whereby adhesion between
the n-side electrode and a radiator base can be improved when the
former is mounted on the latter. Consequently, high radiability can
be attained. Further, the n-side electrode formed on the nitrogen
face of the first semiconductor layer can be so improved in
flatness that wire bondability with respect to the n-side electrode
can be improved when the n-side electrode is wire-bonded.
[0032] A nitride-based semiconductor device according to a third
aspect of the present invention is formed through steps of etching
the back surface of a first semiconductor layer consisting of
either an n-type nitride-based semiconductor layer or a
nitride-based semiconductor substrate having a wurtzite structure
and thereafter forming an n-side electrode on the etched back
surface of the first semiconductor layer.
[0033] In the nitride-based semiconductor device according to the
third aspect, a region including defects in the vicinity of the
first semiconductor layer resulting from a polishing step or the
like can be removed by etching the back surface of the first
semiconductor layer consisting of either an n-type nitride-based
semiconductor layer or a nitride-based semiconductor substrate
having a wurtzite structure as hereinabove described, whereby the
number of defects can be reduced in the vicinity of the
back-surface of the first semiconductor layer. Thus, an electron
carrier concentration can be inhibited from reduction resulting
from trap of electron carriers by defects, whereby the electron
carrier concentration can be increased on the back surface of the
first semiconductor layer. Consequently, contact resistance between
the first semiconductor layer and the n-side electrode can be
reduced. Further, the back surface of the first semiconductor layer
is so etched that flatness thereof can be improved as compared with
that of a mechanically polished back surface. Thus, the n-side
electrode formed the back surface of the first semiconductor layer
can also be improved flatness, whereby adhesion between the n-side
electrode and a radiator base can be improved when the former is
mounted on the latter. Further, the n-side electrode formed on the
back surface of the first semiconductor layer can be so improved in
flatness that wire bondability with respect to the n-side electrode
can be improved when the n-side electrode is wire-bonded.
[0034] A nitride-based semiconductor device according to a fourth
aspect of the present invention comprises a first semiconductor
layer consisting of either an n-type nitride-based semiconductor
layer or a nitride-based semiconductor substrate having a wurtzite
structure and an n-side electrode formed on the back surface of the
first semiconductor layer, while contact resistance between the
n-side electrode and the first semiconductor layer is not more than
0.05 .OMEGA.cm.sup.2.
[0035] In the nitride-based semiconductor device according to the
fourth aspect, the contact resistance between the n-side electrode
and the first semiconductor layer is set to not more than 0.05
.OMEGA.cm.sup.2, so that the nitride-based semiconductor device can
attain excellent device characteristics by reducing the contact
resistance between the n-side electrode and the first semiconductor
layer.
[0036] In the aforementioned nitride-based semiconductor device
according to the fourth aspect, an electron carrier concentration
is preferably at least 1.times.10.sup.17 cm.sup.-3 in the vicinity
of the interface between the first semiconductor layer and the
n-side electrode. According to this structure, the nitride-based
semiconductor device can easily reduce the contact resistance
between the n-side electrode and the first semiconductor layer.
[0037] In the aforementioned nitride-based semiconductor device
according to the fourth aspect, a dislocation density is preferably
not more than 1.times.10.sup.9 cm.sup.-2 in the vicinity of the
interface between the first semiconductor layer and the n-side
electrode. According to this structure, the number of defects
(dislocations) can be reduced in the vicinity of the interface
between the first semiconductor layer and the n-side electrode,
whereby the contact resistance can be reduced in the interface
between the first semiconductor layer and the n-side electrode.
[0038] In the-aforementioned nitride-based semiconductor device
according to the fourth aspect, the back surface of the first
semiconductor layer preferably includes a nitrogen face of the
first semiconductor layer.
[0039] In the aforementioned nitride-based semiconductor device
according to the fourth aspect, the first semiconductor layer may
include the n-type nitride-based semiconductor layer or the
nitride-based semiconductor substrate consisting of at least one
material selected from a group consisting of GaN, BN, AlN, InN and
TlN. Further, the n-side electrode may include an Al film.
[0040] In the aforementioned nitride-based semiconductor device
according to the fourth aspect, the nitride-based semiconductor
device is preferably a nitride based semiconductor light-emitting
device. According to this structure contact resistance between the
first semiconductor layer and the n-side electrode can be reduced
in the nitride-based semiconductor light-emitting device, whereby
the nitride-based semiconductor light-emitting device can attain
excellent emissivity.
[0041] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIGS. 1 to 3 are sectional views for illustrating a process
of fabricating a nitride-based semiconductor laser device according
to an embodiment of the present invention;
[0043] FIG. 4 is an enlarged sectional view in the step shown in
FIG. 3;
[0044] FIG. 5 is a perspective view for illustrating the process of
fabricating a nitride-based semiconductor laser device according to
the embodiment of the present invention;
[0045] FIG. 6 is a graph showing change of an etching rate in a
case of varying the etching gas ratio in RIE; and
[0046] FIG. 7 is a sectional view showing a conventional
nitride-based semiconductor laser device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0047] An embodiment of the present invention is now described with
reference to the drawings.
[0048] A process of fabricating a nitride-based semiconductor laser
device according to the embodiment is described with reference to
FIGS. 1 to 5. According to this embodiment an oxygen-doped n-type
GaN substrate 1 having a wurtzite structure is formed by a method
disclosed in Japanese Patent Laying-Open No. 2000-44400, for
example. More specifically, an oxygen-doped n-type GaN layer is
formed on a GaAs substrate (not shown) by HVPE with a thickness of
about 120 .mu.m to about 400 .mu.m. Thereafter the GaAs substrate
is removed thereby obtaining the n-type GaN substrate 1 shown in
FIG. 1. The n-type GaN substrate 1 has a substrate carrier
concentration of 5.times.10.sup.18 cm.sup.-3 according to Hall
effect measurement. The impurity concentration of the n-type GaN
substrate 1 according to SIMS (secondary ton mass spectroscopy)
analysis is 1.times.10.sup.19 cm.sup.-3. The n-type GaN substrate 1
is an example of the first semiconductor layers in the present
invention.
[0049] An n-type buffer layer 2 consisting of n-type GaN having a
thickness of about 5 .mu.m, an n-type cladding layer 3 consisting
of n-type Al.sub.0.08Ga.sub.0.92N having a thickness of about 1
.mu.m, an MQW active layer 4 consisting of InGaN, a p-type cladding
layer 5 consisting of p-type Al.sub.0.08Ga.sub.0.92N having a
thickness of about 0.28 .mu.m and a p-type contact layer 6
consisting of p-type GaN having a thickness of about 70 nm are
successively formed on the upper surface (Ga face), i.e., the
(0001) plane of the n-type GaN substrate 1 by atmospheric pressure
MOCVD under a pressure of about 1 atm (about 100 kPa).
[0050] The MQW active layer 4 is formed by alternately stacking
four barrier layers of GaN each having a thickness of about 20 nm
and three well layers of In.sub.0.15Ga.sub.0.85N each having a
thickness of about 3.5 nm. Ga(CH.sub.3).sub.3, In(CH.sub.3).sub.3,
Al(CH.sub.3).sub.3 and NH.sub.3 are employed as material gases, and
H.sub.2 and N.sub.2 are employed as carrier gases. According to
this embodiment, the quantities of these material gases are varied
for adjusting the compositions of the layers 2 to 6. SiH.sub.4 gas
(Si) is employed as the n-type dopant for the n-type buffer layer 2
and the n-type cladding layer 3. Cp.sub.2Mg gas (Mg) is employed as
the p-type dopant for the p-type cladding layer 5 and the p-type
contact layer 6.
[0051] Then, the p-type contact layer 6 and the p-type cladding
layer 5 are partially etched through photolithography and etching,
thereby forming projecting portions (ridge portions) of about 2
.mu.m in thickness consisting of projecting portions of the p-type
cladding layer 5 and p-type contact layers 6, as shown in FIG. 2.
Then, p-side electrodes 7 consisting of Pt films having a thickness
of about 1 nm, Pd films having a thickness of about 10 nm and Ni
films having a thickness of about 300 nm in ascending order are
formed on the upper surfaces of the p-type contact layers 6. Thus,
a nitride-based semiconductor laser device structure 20 to formed
to include a region formed with a plurality of elements as shown in
FIG. 2.
[0052] Thereafter the back surface (nitrogen face) of the n-type
GaN substrate 1 is mechanically polished as shown in FIGS. 3 and 4.
A mechanical polisher 30 employed for this polishing step is formed
by a glass plate 11 having a flat surface, a holder 12 supported to
be vertically movable and rotatable along arrow R and a buff 13, as
shown in FIG. 3. An abrasive (not shown) consisting of diamond,
silicon oxide or alumina having particle roughness of about 0.2
.mu.m to about 1 .mu.m to arranged on the buff 13. This abrasive
can particularly excellently polish the back surface of the n-type
GaN substrate 1 if the particle roughness thereof is in the range
of about 0.2 .mu.m to about 0.5 .mu.m. The nitride-based
semiconductor laser device structure 20 is mounted on the lower
surface of the holder 12 at an interval-through wax 14 not to
directly come into contact with the holder 12, as shown in FIGS. 3
and 4. Thus, the nitride-based semiconductor laser device structure
20 is prevented from breaking in mechanical polishing. A flat
polishing plate made of metal may be used instead of the glass
plate 11.
[0053] The mechanical polisher 30 shown in FIG. 3 is employed for
polishing the back surface (nitrogen face) of the n-type GaN
substrate 1 so that the thickness of the n-type GaN substrate 1
reaches about 120 .mu.m to about 180 .mu.m. More specifically, the
back surface (see FIG. 4) of the n-type GaN substrate 1 of the
nitride-based semiconductor laser device structure 20 mounted on
the lower surface of the holder 12 is pressed against the upper
surface of the buff 13 provided with the abrasive with a constant
load. The holder 12 is rotated along arrow R while feeding water or
oil to the buff 13 (see FIG. 3). Thus, the back surface of the
n-type GaN substrate 1 is polished until the thickness of the
n-type GaN substrate 1 reaches about 120 .mu.m to about 180 .mu.m.
The n-type GaN substrate 1 is worked so that the thickness thereof
is in the range of about 120 .mu.m to about 180 .mu.m, since a
cleavage step described later can be excellently carried out when
the thickness of the n-type GaN substrate 1 to within this
range.
[0054] According to this embodiment, the back surface (nitrogen
face) of the n-type GaN substrate 1 is thereafter etched for about
20 minutes by reactive ion etching (RIE). This etching is carried
out under conditions of a Cl.sub.2 gas flow rate of 10 sccm, a
BCl.sub.3 gas flow rate of 5 sccm, an etching pressure of about 3.3
Pa and RF power of 200 W (0.63 W/cm.sup.2) under the room
temperature. Thus, the back surface (nitrogen face) of the n-type
GaN substrate 1 is removed by a thickness of about 1 .mu.m.
Consequently, a region, including defects resulting from the
aforementioned mechanical polishing, in the vicinity of the back
surface of the n-type GaN substrate 1 can be removed. Further, the
back surface of the n-type GaN substrate 1 can be worked into a
flatter mirror surface as compared with that worked by only
mechanical polishing. The mirror surface is defined as a surface
state allowing excellent visual confirmation of a reflected image
of the back surface of the n-type GaN substrate 1.
[0055] In order to confirm the effect of the aforementioned
etching, the defect (dislocation) density on the back surface of
the n-type GaN substrate 1 was measured before and after etching by
TEM (transmission electron microscope) analysis. Consequently, it
has been proved that the defect density, which was at least
1.times.10.sup.10 cm.sup.-2 before etching, was reduced to below
1.times.10.sup.6 cm.sup.-2 after the etching. Further, the electron
carrier concentration in the vicinity of the back surface of the
etched n-type GaN substrate 1 was measured with an electrochemical
C-V profiler. Consequently, the electron carrier concentration In
the vicinity of the back surface of the etched n-type GaN substrate
1 was at least 1.0.times.10.sup.18 cm.sup.-3. Thus, it has been
recognized that the electron carrier concentration in the vicinity
of the back surface can be set to a level substantially identical
to the substrate carrier concentration (5.times.10.sup.18
cm.sup.-3) of the n-type GaN substrate 1.
[0056] Under the aforementioned etching conditions, the etching
time and the etching depth are proportional to each other.
Therefore, the etching depth can be accurately controlled by
adjusting the etching time. The etching rate and the surface state
vary with the composition of etching gases. FIG. 6 is a graph
showing change of the etching rate upon variation of the etching
gas ratio in RIE. In this case, the Cl.sub.2 gas flow rate was
fixed to 10 scam and the BCl.sub.3 gas flow rate was varied for
measuring the etching rate. Consequently, it has been proved that
the etched surface is converted to a flat mirror surface when the
ratio of the BCl.sub.3 gas flow rate to the Cl.sub.2 gas flow rate
is in the range of at least 30% and not more than 70%, as shown in
FIG. 6. When the ratio of the BCl.sub.3 gas flow rate to the
Cl.sub.2 gas flow rate was less than 5% or in excess of 85%, the
etched surface was damaged in flatness and clouded.
[0057] After the aforementioned etching step, the nitride-based
semiconductor laser device structure 20 is dipped in an HCl
solution (concentration: 10%) under the room temperature for 1
minute thereby performing hydrochloric acid treatment. Thus,
chlorine-based residues adhering to the back surface of the n-type
GaN substrate 1 in the RIB step are removed.
[0058] Thereafter an n-side electrode 8 consisting of an Al film
having a thickness of 6 nm, an Si film having a thickness of 2 nm,
an Ni film having a thickness of 10 nm and an Au film having a
thickness of 300 nm successively from a side closer to the back
surface of the n-type GaN substrate 1 is formed on the back surface
(nitrogen face) of the n-type GaN substrate 1 of the nitride-based
semiconductor laser device structure 20 by sputtering or vacuum
deposition.
[0059] Finally, elements are isolated and a cavity facet is formed
by cleavage, thereby completing the nitride-based semiconductor
laser device according to this embodiment as shown in FIG. 5.
[0060] In the process of fabricating a nitride-based semiconductor
laser device according to this embodiment, the back surface
(nitrogen face) of the n-type GaN substrate 1 is etched by RIE as
hereinabove described, whereby the region, including defects
resulting from the polishing step, in the vicinity of the back
surface of the n-type GaN substrate 1 can be removed. Thus, the
electron carrier concentration can be inhibited from reduction
resulting from trap of electron carriers by defects. When formed by
a nitrogen face, the back surface of the n-type GaN substrate 1 is
easily oxidized and hence the oxidized part can be removed by
etching. Consequently, the contact resistance between the n-type
GaN substrate 1 and the n-side electrode 8 can be reduced. Contact
resistance between the n-type GaN substrate 1 and the n-side
electrode 8 of a nitride-based semiconductor laser device actually
prepared according to this embodiment measured by a TLM
(transmission line model) method was not more than
2.0.times.10.sup.-4 .OMEGA.cm.sup.2. When the n-side electrode 8
was formed on the back surface (nitrogen face) of the n-type GaN
substrate 1 and the structure was heat-treated in a nitrogen gas
atmosphere of 500.degree. C. for 10 minutes, the contact resistance
was further reduced to 1.0.times.10.sup.-5 .OMEGA.cm.sup.2.
[0061] In the process of fabricating a nitride-based semiconductor
laser device according to this embodiment, the back surface of the
n-type GaN substrate 1 is etched by RIB as hereinabove described,
whereby the back surface of the n-type GaN substrate 1 can be
further improved in flatness as compared with a mechanically
polished back surface. Thus, the n-side electrode 8 formed on the
back surface of the n-type GaN substrate 1 can be also improved in
flatness. When the nitride-based semiconductor laser device is
mounted in a junction-down system, wire bondability with respect to
the n-side electrode 8 can be improved. When the n-side electrode 8
is mounted on a radiator base (submount), adhesion between the
n-side electrode 8 and the radiator base can be improved for
attaining excellent reliability.
[0062] In order to confirm the effect of the present invention
etching the back surface (nitrogen face) of the n-type GaN
substrate 1 by RIE in more detail, an experiment was performed as
shown in Table 1. TABLE-US-00001 TABLE 1 Electron Contact Carrier
Resis- Concen- Sam- Method of Forming Electrode tance tration ple
(Back Surface Treatment Condition) (.OMEGA. cm.sup.2) (cm.sup.-3) 1
Polishing Back Surface of GaN Substrate 20 2.0 .times. 10.sup.16
.fwdarw.Formation of n-Side Electrode 2 Polishing Back Surface of
GaN Substrate 0.1 5.0 .times. 10.sup.16 .fwdarw. Hydrochloric Acid
Treatment .fwdarw. Formation of n-Side Electrode 3 Polishing Back
Surface of GaN Substrate 0.05 1.0 .times. 10.sup.17 .fwdarw.Etching
by 0.5 .mu.m by RIE (Cl.sub.2 Gas).fwdarw. Formation of n-Side
Electrode 4 Polishing Back Surface of GaN Substrate 7.0 .times. 7.1
.times. 10.sup.17 .fwdarw. Etching by 1 .mu.m by RIE (Cl.sub.2
Gas).fwdarw. 10.sup.-4 Formation of n-Side Electrode 5 Polishing
Back Surface of GaN Substrate 3.0 .times. 1.7 .times. 10.sup.18
.fwdarw.Etching by 1 .mu.m by RIE (Cl.sub.2 Gas + BCl.sub.3
10.sup.-4 Gas).fwdarw.Formation of n-Side Electrode 6 Polishing
Back Surface of GaN Substrate 2.0 .times. 2.5 .times. 10.sup.18
.fwdarw.Etching by 1 .mu.m by RIE (Cl.sub.2 Gas + BCl.sub.3
10.sup.-4 Gas) .fwdarw. Hydrochloric Acid Treatment .fwdarw.
Formation of n-Side Electrode 7 Polishing Back Surface of GaN
Substrate 1.0 .times. 5.0 .times. 10.sup.18 .fwdarw.Etching by 1
.mu.m by RIE (Cl.sub.2 Gas + BCl.sub.3 10.sup.-5 Gas) .fwdarw.
Hydrochloric Acid Treatment .fwdarw. Formation of n-Side Electrode
.fwdarw. Heat Treatment
[0063] Referring to Table 1, various nitrogen face (back surface)
treatments were performed on samples 1 to 7 consisting of n-type
GaN substrates having a wurtzite structure, for thereafter
measuring electron carrier concentrations in the vicinity of the
back surfaces of the n-type GaN substrates with an electrochemical
V-C measured concentration profiler. After this measurement of the
electron carrier concentrations, n-side electrodes were formed on
the back surfaces of the n-type GaN substrates of the samples 1 to
7 for measuring contact resistance values between the n-type GaN
substrates and the n-side electrodes by the TLM method.
[0064] The n-side electrodes of the samples 1 to 7 were formed by
Al films, Si films, Ni films and Au films, similarly to the n-side
electrode 8 according to the aforementioned embodiment. Substrate
polishing, etching by RIE and hydrochloric acid treatment were
performed under conditions similar to those employed in the
aforementioned embodiment. The sample 6 was prepared through the
fabrication process according to the aforementioned embodiment.
[0065] In the inventive samples 3 to 7 prepared by etching the back
surfaces of the n-type GaN substrates by RIE, contact resistance
values were remarkably reduced as compared with the sample 1
prepared by a method similar to the prior art. More specifically,
the sample 1 exhibited contact resistance of 20 .OMEGA.cm.sup.2,
while the inventive samples 3 to 7 exhibited contact resistance of
not more than 0.05 .OMEGA.cm.sup.2, conceivably for the following
reason: In the inventive samples 3 to 7, regions including defects
resulting from mechanical polishing in the vicinity of the back
surfaces of the n-type GaN substrates were conceivably removed by
RIE. Therefore, the electron carrier concentrations were inhibited
from reduction resulting from defects in the vicinity of the back
surfaces of the n-type GaN substrates in the inventive samples 3 to
7.
[0066] Further, the inventive samples 3 to 7 exhibited higher
electron carrier concentrations in the vicinity of the back
surfaces of the n-type GaN substrates as compared with the sample 1
corresponding to the prior art. More specifically, the sample 1
corresponding to the prior art exhibited an electron carrier
concentration of 2.0.times.10.sup.16 cm.sup.-3, while the inventive
samples 3 to 7 exhibited electron carrier concentrations of at
least 1.0.times.10.sup.17 cm.sup.3.
[0067] In the sample 4 prepared by removing the back surface of the
n-type GaN substrate by a thickness of about 1 .mu.m by RIE with
Cl.sub.2 gas, it was possible to attain lower contact resistance
than the sample 3 prepared by removing the back surface of the
n-type GaN substrate by a thickness of about 0.5 .mu.m by RIB with
Cl.sub.2 gas. This is conceivably because it was not possible to
sufficiently remove the region including defects resulting from
mechanical polishing in the vicinity of the back surface of the
n-type GaN substrate by removing the back surface of the n-type GaN
substrate by the thickness of about 0.5 .mu.m. When defect
(dislocation) densities of the back surfaces of the n-type GaN
substrates were measured by TEM analysis in these samples the
sample 3 exhibited a defect density of 1.times.10.sup.9 cm.sup.-2.
In the sample 4, on the other hand, no defects were observed in the
field of view and the defect density was not more than
1.times.10.sup.6 cm.sup.-2. Thus, it is preferable to remove the
back surface of the n-type GaN substrate by a thickness of at least
about 1.0 .mu.m by RIE.
[0068] In the sample 5 subjected to RIE with Cl.sub.2 gas and
BCl.sub.3 gas, contact resistance was further reduced as compared
with the sample 4 prepared by etching the back surface of the
n-type GaN substrate by RIE with only Cl.sub.2 gas.
[0069] In the samples 6 and 7, corresponding to the aforementioned
embodiment, prepared by etching the back surfaces of the n-type GaN
substrates by RIE with Cl.sub.2 gas and BCl.sub.3 gas and
thereafter performing hydrochloric acid treatment and the sample 7
further heat-treated in a nitrogen atmosphere of 500.degree. C. for
10 minutes, it was possible to obtain lower contact resistance
values as compared with the sample 5 subjected to neither
hydrochloric acid treatment nor heat treatment. Comparing the
samples 6 and 7 with each other, it has been proved that the
contact resistance between the n-type GaN substrate and the n-side
electrode can be further reduced and the electron carrier
concentration in the vicinity of the back surface of the n-type GaN
substrate can be further improved by heat treatment.
[0070] In the sample 2 dipped in an HCl solution of 10% in
concentration for about 10 minutes (hydrochloric acid treatment)
without RIE, it was possible to obtain lower contact resistance
than the sample 1 corresponding to the prior art subjected to no
hydrochloric acid treatment. More specifically, the sample 1
exhibited contact resistance of 20 .OMEGA.cm.sup.2, while the
sample 2 exhibited contact resistance of 0.1 .OMEGA.cm.sup.2. This
is conceivably because the back surface of the n-type GaN substrate
was cleaned by the hydrochloric acid treatment.
[0071] If oxygen is employed as the n-type dopant for the n-type
GaN substrate, crystallinity is reduced when the oxygen dose is
increased to increase the carrier concentration in order to reduce
the contact resistance. According to the present invention,
however, the contact resistance can be reduced also with the oxygen
dose (substrate carrier concentration: 5.times.10.sup.18 cm.sup.-3)
for the n-type GaN substrate 1 according to the aforementioned
embodiment.
[0072] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
[0073] For example, while the above embodiment has been described
with reference to the case of forming a nitride-based semiconductor
laser device with the n-type GaN substrate 1, the present invention
is not restricted to this but an n-type nitride-based semiconductor
substrate or a nitride-based semiconductor layer having a wurtzite
structure may alternatively be employed. For example, a
nitride-based semiconductor substrate or a nitride-based
semiconductor layer consisting of BN (boron nitride), AlN (aluminum
nitride), InN (indium nitride) or TlN (thallium nitride) is
conceivable. The nitride-based semiconductor substrate or the
nitride-based semiconductor layer may consist of ternary or
quaternary nitride-based semiconductor thereof.
[0074] While the back surface (nitrogen face) of the n-type GaN
substrate 1 is etched by RIE in the aforementioned embodiment, the
present invention is not restricted to this but other dry etching
may alternatively be employed. For example, reactive ion beam
etching, radical etching, or plasma etching may be employed.
[0075] While the back surface (nitrogen face) of the n-type GaN
substrate 1 is etched by RIB with Cl.sub.2 gas and BCl.sub.3 gas in
the aforementioned embodiment, the present invention is not
restricted to this but other etching gases may alternatively be
employed. For example, a gas mixture of Cl.sub.2 and SiCl.sub.4, a
gas mixture of Cl.sub.2 and CF.sub.4 or Cl.sub.2 gas may be
employed.
[0076] While the nitride-based semiconductor laser device structure
20 is dipped in the HCl solution (hydrochloric acid treatment)
after the etching by RIE thereby removing the chlorine-based
residues adhering to the back surface of the n-type GaN substrate 1
in the aforementioned embodiment, the present invention is not
restricted to this but the nitride-based semiconductor layer device
structure 20 may alternatively be dipped in another solution
containing at least one of chlorine, fluorine, bromine, iodine,
sulfur and ammonia.
[0077] While the back surface (nitrogen face) of the n-type GaN
substrate 1 is mechanically polished after growing the layers 2 to
6 on the upper surface (Ga face) of the n-type GaN substrate 1 in
the aforementioned embodiment, the present invention is not
restricted to this but the back surface (nitrogen face) of the
n-type GaN substrate 1 may alternatively be previously mechanically
polished to a prescribed thickness for thereafter forming the
layers 2 to 6 on the upper surface (Ga face) of the n-type GaN
substrate 1. Further alternatively, the nitrogen face of the n-type
GaN substrate 1 may not be mechanically polished.
[0078] While the n-type dopant and the p-type dopant for the layers
2 to 6 are prepared from Si and Mg respectively in the
aforementioned embodiment, the present invention is not restricted
to this but another n- or p-type dopant may alternatively be
employed. For example, Se, Ge or the like may be employed as the
n-type dopant. Further, Be or Zn may be employed as the p-type
dopant.
[0079] While the layers 2 to 6 are formed on the n-type GaN
substrate 1 by atmospheric pressure MOCVD in the aforementioned
embodiment, the present invention is not restricted to this but the
layers 2 to 6 may alternatively be formed by another growth method.
For example, the layers 2 to 6 may be formed by low-pressure
MOCVD.
[0080] While the n-type buffer layer 2 is formed on the n-type GaN
substrate 1 in the aforementioned embodiment, the present invention
is not restricted to this but no n-type buffer layer 2 may be
formed. In this case, the fabrication process can be simplified
although the layers 3 to 6 are slightly reduced in
crystallinity.
[0081] While the n-side electrode 8 is formed by the Al, Si, Ni and
Au films in the aforementioned embodiment, the present invention is
not restricted to this but the n-side electrode 8 may alternatively
be formed by another electrode structure such as that consisting of
a Ti film having a thickness of 10 nm and an Al film having a
thickness of 500 nm, an Al film having a thickness of 6 nm, an Ni
film having a thickness of 10 nm and an Au film having a thickness
of 300 nm or an AlSi film having a thickness of 10 nm, a Zn film
having a thickness of 300 nm and an Au film having a thickness of
100 nm, for example.
[0082] While a ridge structure is employed as a current narrowing
structure or a transverse light confinement structure in the
aforementioned embodiment, the present invention is not restricted
to this but a current may alternatively be narrowed by an embedded
structure employing a high-resistance blocking layer or an n-type
blocking layer. Further alternatively, a light absorption layer may
be formed by ton implantation or the like as a current narrowing
layer or a transverse light confinement structure.
[0083] While the present invention is applied to a nitride-based
semiconductor laser device in the aforementioned embodiment, the
present invention is not restricted to this but may be applied to a
semiconductor device employing an n-type nitride-based
semiconductor layer or a nitride-based semiconductor substrate
having a wurtzite structure. For example, the present invention may
be applied to a MESFET (metal semiconductor field-effect
transistor), a HEMT (high electron mobility transistor), a
light-emitting diode (LED) device or a VCSEL (vertical cavity
surface emitting laser) device requiring surface flatness, for
example.
[0084] While the p- and n-side electrodes 7 and 8 have prescribed
thicknesses in the aforementioned embodiment, the present invention
is not restricted to this but the p- and n-side electrodes 7 and 8
may alternatively have other thicknesses. For example, the
electrodes 7 and 8 may be reduced in thickness to have
translucency, for employing the semiconductor laser device as a
VCSEL device or an LED device. In particular, the contact
resistance of the n-side electrode 8 can be sufficiently reduced
according to the present invention also when the n-side electrode 8
is formed with a small thickness to have translucency.
[0085] While the back surface (nitrogen face) of the n-type GaN
substrate 1 is dry-etched by RIE in the aforementioned embodiment,
the present invention is not restricted to this but the back
surface (nitrogen face) of the n-type GaN substrate 1 may
alternatively be wet-etched. In this case, aqua regia, KOH or
K.sub.2S.sub.2O .sub.8 is employed as the wet etchant. For example,
the nitrogen face forming the back surface of the n-type GaN
substrate 1 may be wet-etched under the room temperature with KOH
of 0.1 mol in concentration. When the temperature is increased to
about 120.degree. C. in this case, the etching rate can be
increased to about 10 times as compared with that under the room
temperature.
[0086] While the back surface, consisting of the nitrogen face, of
the n-type GaN substrate 1 is dry-etched by RIE in the
aforementioned embodiment, the present invention is not restricted
to this but the back surface of the n-type GaN substrate 1 may
alternatively be wet-etched when the back surface consists of a Ga
face. In this case, aqua regia, KOH or K.sub.2S.sub.2O.sub.8 is
employed as the wet etchant. For example, the Ga face forming the
back surface of the n-type GaN substrate 1 may be wet-etched in KOH
of 0.1 mol in concentration with a mercury lamp of 365 nm under the
room temperature. When the temperature is increased to about
120.degree. C. in this case, the etching rate can be increased to
about 10 times as compared with that under the room
temperature.
[0087] While the n-type GaN Just substrate 1 having the back
surface entirely formed by a nitrogen face is employed in the
aforementioned embodiment, the present invention is not restricted
to this but an n-type GaN misoriented substrate having a back
surface partially including a Ga face may alternatively be
employed. Such back surface of the n-type GaN misoriented substrate
is also included in the nitrogen face according to the present
invention.
* * * * *