U.S. patent application number 13/781568 was filed with the patent office on 2013-09-05 for gallium nitride substrate and optical device using the same.
This patent application is currently assigned to Hitachi Cable, Ltd.. The applicant listed for this patent is HITACHI CABLE, LTD.. Invention is credited to Shunsuke YAMAMOTO.
Application Number | 20130230447 13/781568 |
Document ID | / |
Family ID | 49042952 |
Filed Date | 2013-09-05 |
United States Patent
Application |
20130230447 |
Kind Code |
A1 |
YAMAMOTO; Shunsuke |
September 5, 2013 |
GALLIUM NITRIDE SUBSTRATE AND OPTICAL DEVICE USING THE SAME
Abstract
A gallium nitride substrate that a GaL.alpha./CK.alpha. peak
intensity ratio in EDX spectrum is not less than 2. The EDX
spectrum is obtained in energy dispersive X-ray microanalysis (EDX)
of a surface of the gallium nitride substrate using a scanning
electron microscope (SEM) at an accelerating voltage of 3 kV.
Inventors: |
YAMAMOTO; Shunsuke; (Mito,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HITACHI CABLE, LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
Hitachi Cable, Ltd.
Tokyo
JP
|
Family ID: |
49042952 |
Appl. No.: |
13/781568 |
Filed: |
February 28, 2013 |
Current U.S.
Class: |
423/409 |
Current CPC
Class: |
H01L 21/30625 20130101;
H01L 33/0075 20130101; H01S 5/32341 20130101; H01L 29/2003
20130101; H01S 5/0206 20130101; C30B 29/406 20130101; C30B 25/00
20130101; H01L 21/30612 20130101; H01L 33/32 20130101; H01L 22/12
20130101; C01B 21/0632 20130101; G01N 23/2252 20130101; H01L
21/02057 20130101 |
Class at
Publication: |
423/409 |
International
Class: |
C01B 21/06 20060101
C01B021/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 2, 2012 |
JP |
2012-047203 |
Claims
1. A gallium nitride substrate, wherein a GaL.alpha./CK.alpha. peak
intensity ratio in EDX spectrum is not less than 2, the EDX
spectrum being obtained in energy dispersive X-ray microanalysis
(EDX) of a surface of the gallium nitride substrate using a
scanning electron microscope (SEM) at an accelerating voltage of 3
kV.
2. The gallium nitride substrate according to claim 1, wherein the
GaL.alpha./CK.alpha. peak intensity ratio in the EDX spectrum is
not less than 3.
3. An optical device, comprising: a device structure formed on the
gallium nitride substrate according to claim 1.
4. The optical device according to claim 3, wherein the
GaL.alpha./CK.alpha. peak intensity ratio in the EDX spectrum is
not less than 3.
Description
[0001] The present application is based on Japanese patent
application No. 2012-047203 filed on Mar. 2, 2012, the entire
contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a gallium nitride substrate and an
optical device using the gallium nitride substrate.
[0004] 2. Description of the Related Art
[0005] GaN-based semiconductor crystals such as gallium nitride
(GaN) has attracted attention as a material of optical devices such
as light emitting diode (LED) which emits high-intensity blue light
or long-life laser diode (LD) which emits blue light.
[0006] Bulk crystal growth of the GaN-based semiconductor crystals
is difficult, and accordingly, it is difficult to produce a large
single crystal GaN with high quality. However, in recent years, a
method of manufacturing a GaN-based semiconductor crystal has been
proposed, using a DEEP (Dislocation Elimination by the Epi-growth
with Inverted-Pyramidal Pits) method or a VAS (Void-Assisted
Separation) method, etc., and also using GaN free-standing
substrate in which a GaN single crystal is grown on a heterogeneous
substrate by a HVPE (Hydride Vapor Phase Epitaxy) method.
[0007] In the DEEP method, a patterned mask of SiN, etc., is formed
on a GaAs substrate which is removable by etching, a GaN layer is
the formed thereon, plural pits surrounded by facet planes are
intentionally formed on a crystal surface and dislocations are
accumulated at a bottom of the pits to reduce dislocation in other
regions. The GaAs substrate is then removed, thereby obtaining a
GaN free-standing substrate with reduced dislocation (see, e.g.,
JP-A-2003-165799).
[0008] In the VAS method, a GaN layer is grown on a substrate of
sapphire, etc., via a GaN substrate with voids and a TiN thin film
having a mesh structure, thereby allowing separation of the GaN
substrate and reduction of dislocation at the same time (see, e.g.,
JP-A-2004-269313).
[0009] The GaN free-standing substrate obtained by the
above-mentioned methods is flattened by grinding and polishing
front and back surfaces of a substrate epitaxially grown by the
HVPE method. Subsequently, an outer periphery of the substrate is
shaped in order to have a circular shape with a given diameter.
Then, after removing processing strain by wet-etching, etc., the
substrate is cleaned and a GaN mirror wafer is thus obtained.
[0010] A known method of polishing the GaN substrate is, e.g.,
disclosed in JP-A-2001-322899. In JP-A-2001-322899, after the GaN
substrate is fixed to a substrate-attaching board using a wax, both
front and back surfaces of the GaN substrate are polished by loose
abrasive supplied onto the surface plate. Diamond is used as the
loose abrasive by taking into consideration hardness of the GaN
substrate.
SUMMARY OF THE INVENTION
[0011] However, crystal quality of an epitaxial growth layer is
poor is case of epitaxial growth using the GaN substrate polished
by the method of JP-A-2001-322899 and an optical device using such
a substrate has a problem that emission intensity decreases, which
causes failure and a decrease in a yield. As a result of intensive
examination of this problem, it was found that the diamond used as
the loose abrasive is embedded into and remains on the surface of
the GaN substrate when the GaN substrate is polished and crystal
quality of the epitaxial growth layer deteriorates due to a carbon
component of the diamond. It is also found that the wax used when
polishing remains and the crystal quality deteriorates due to a
carbon component of the wax.
[0012] Accordingly, it is an object of the invention to provide a
gallium nitride substrate that an amount of residual carbon on a
substrate surface is small. It is another object of the invention
to provide an optical device that is formed using the gallium
nitride substrate and has excellent emission intensity.
(1) According to one embodiment of the invention, a gallium nitride
substrate wherein a GaL.alpha./CK.alpha. peak intensity ratio in
EDX spectrum is not less than 2, the EDX spectrum being obtained in
energy dispersive X-ray microanalysis (EDX) of a surface of the
gallium nitride substrate using a scanning electron microscope
(SEM) at an accelerating voltage of 3 kV.
[0013] In the above embodiment (1) of the invention, the following
modifications and changes can be made.
[0014] (i) The GaL.alpha./CK.alpha. peak intensity ratio in the EDX
spectrum is not less than 3.
(2) According to another embodiment of the invention, an optical
device comprises:
[0015] a device structure formed on the gallium nitride substrate
according to the embodiment (1).
[0016] In the above embodiment (2) of the invention, the following
modifications and changes can be made.
[0017] (ii) The GaL.alpha./CK.alpha. peak intensity ratio in the
EDX spectrum is not less than 3.
POINTS OF THE INVENTION
[0018] According to one embodiment of the invention, a gallium
nitride(GaN) substrate is constructed so as to satisfy the
GaL.alpha./CK.alpha. peak intensity ratio of not less than 2. Based
on the GaL.alpha./CK.alpha. peak intensity ratio which is
calculated as an amount of C (carbon) with respect to Ga, an
increase or decrease in the amount of residual carbon can be
determined so as to evaluate the amount of residual carbon on the
surface of the GaN substrate. By satisfying the above
GaL.alpha./CK.alpha. peak intensity ratio, it is possible to obtain
the gallium nitride substrate with the reduced amount of residual
carbon on the surface thereof. Thus, the crystalline quality of an
epitaxial layer grown on the gallium nitride substrate can be
enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Next, the present invention will be explained in more detail
in conjunction with appended drawings, wherein:
[0020] FIG. 1 is a diagram illustrating EDX spectrum of a GaN
substrate at an accelerating voltage of 3 kV;
[0021] FIG. 2 is a diagram illustrating a correlative relationship
between an accelerating voltage (Eb) and electron penetration depth
(Re);
[0022] FIG. 3 is a schematic cross sectional view showing an HVPE
apparatus for manufacturing a gallium nitride substrate in an
embodiment of the present invention; and
[0023] FIG. 4 is a schematic cross sectional view showing an
optical device in the embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] When a GaN substrate is polished, carbon derived from
diamond or wax remains on the substrate surface, as described
above. The residual carbon deteriorates crystal quality of an
epitaxial growth layer to be epitaxially grown. Therefore, the
present inventors measured an amount of residual carbon on a
substrate surface and intensively examined a relation between the
amount of carbon and a decrease in emission intensity of an optical
device to be formed. In detail, the surface of the GaN substrate
was measured in energy dispersive X-ray microanalysis (EDX) and an
amount of carbon was calculated from a GaL.alpha./CK.alpha. peak
intensity ratio in the obtained EDX spectrum. Then, influence of
the amount of carbon on an increase or decrease in emission
intensity was evaluated. As a result, it was found that, when the
GaL.alpha./CK.alpha. peak intensity ratio is greater than a
predetermined value and the amount of residual carbon near the
surface the GaN substrate decreases, good crystal quality of the
epitaxial growth layer is obtained and also the emission intensity
of the optical device can be improved, and the invention was made
based on the findings.
[0025] GaN Substrate
[0026] When a surface of a gallium nitride substrate (GaN
substrate) in the present embodiment is measured by a scanning
electron microscope (SEM) at an accelerating voltage of 3 kV in
energy dispersive X-ray microanalysis (EDX), a GaL.alpha./CK.alpha.
peak intensity ratio in EDX spectrum obtained by the EDX is not
less than 2.
[0027] The SEM is a device for observing a surface profile of a
sample by irradiating an electron beam, which is a focused electron
beam (electron) emitted from an electron source (electron gun), and
scanning the surface of the sample to detect secondary electrons
emitted from the surface of the sample. In the EDX, a
characteristic X-ray generated from the surface of the sample at
the time of the electron-beam-scanning by the SEM is measured to
identify elements contained on the surface of the sample. In
addition, the number of counts/second (peak intensity) of the
characteristic X-ray with given energy is measured to evaluate the
content of a specific element. The EDX spectrum obtained by the EDX
is as shown in, e.g., FIG. 1 (EDX spectrum of a GaN substrate in
below-described Examples at an accelerating voltage of 3 kV). In
FIG. 1, the horizontal axis indicates energy of the characteristic
X-ray and the vertical axis indicates the number of counts/second
[cps (count per second)] of the characteristic X-ray at such
energy. From FIG. 1, an approximate content of an element
constituting the GaN substrate is understood from a level of peak
(peak intensity). In the present embodiment, in order to evaluate
the amount of residual carbon on the surface of the GaN substrate,
an increase or decrease in the amount of residual carbon is
determined based on a GaL.alpha./CK.alpha. peak intensity ratio
which is calculated as an amount of C (carbon) with respect to Ga.
Here, in the GaN substrate of the present embodiment, the
GaL.alpha./CK.alpha. peak intensity ratio is not less than 2, which
indicates that C with respect to Ga is not more than a
predetermined ratio.
[0028] Meanwhile, in the SEM, the electron beam (electron) is
focused at a predetermined accelerating voltage. Here, the electron
penetrates deeper from the sample surface when the accelerating
voltage is higher and this allows information about a deeper region
from the sample surface to be obtained. In other words, in the SEM,
it is possible to obtain information about a region at a
predetermined depth from the sample surface by adjusting the
accelerating voltage to control electron penetration depth.
[0029] Here, a relation between an accelerating voltage of the SEM
and a depth of the sample (GaN substrate) to be measured will be
described. The electron penetration depth depends on an
accelerating voltage of irradiated electron and an atomic weight,
atomic number and density of a measurement sample, and is
calculated from the following formula (I) (see, e.g., JPn. J. ApPl.
Phys. Vol. 40 (2001) PP. 476-479).
Re = 0.0276 A .rho. Z 0.889 E b 1.67 ( .mu.m ) ( 1 )
##EQU00001##
[0030] FIG. 2 shows a correlative relationship between an
accelerating voltage (Eb) and electron penetration depth (Re) in
the above formula (1) in case that a GaN substrate is measured.
FIG. 2 shows that the electron penetration depth increases with an
increase in the accelerating voltage. In other words, the electron
penetration depth is smaller (shallower) when the accelerating
voltage is lower and this allows information about a region closer
to the sample surface to be obtained. For example, Re is 0.09 .mu.m
when Eb is 3 kV, and Re is 0.20 .mu.m when Eb is 5 kV.
[0031] It is preferable to scan at a low accelerating voltage in
the present embodiment since the amount of carbon on the surface of
the GaN substrate is evaluated. In this regard, however, when the
accelerating voltage is low, the number of detectable
characteristic X-rays of elements decreases and also intensity of
the characteristic X-ray to be detected is low, which results in
that it takes very long time to measure. Therefore, in the present
embodiment, the amount of carbon on the surface of the GaN
substrate is evaluated by EDX using the SEM at an accelerating
voltage of 3 kV.
[0032] In the gallium nitride substrate of the present embodiment,
the GaL.alpha./CK.alpha. peak intensity ratio in the EDX spectrum
is not less than 2 in energy dispersive X-ray microanalysis (EDX)
using the SEM at an accelerating voltage of 3 kV. By this
configuration, it is possible to obtain the gallium nitride
substrate in which the amount of residual carbon on the surface
thereof is small. Therefore, in case of crystal growth using this
gallium nitride substrate as a base substrate, crystal quality of
the epitaxial growth layer to be obtained can be improved.
[0033] In the above-mentioned gallium nitride substrate, the
GaL.alpha./CK.alpha. peak intensity ratio in the EDX spectrum is
preferably not less than 3. By such a configuration, it is possible
to further reduce the amount of residual carbon on the surface of
the gallium nitride substrate and it is thus possible to further
improve the crystal quality of the epitaxial growth layer.
[0034] In an optical device formed using the gallium nitride
substrate of the present embodiment, since the crystal quality of
the epitaxial growth layer to be crystal-grown is good, emission
intensity is high.
[0035] Method of Manufacturing Gallium Nitride Substrate
[0036] A method of manufacturing such a gallium nitride substrate
includes a step of forming a gallium nitride substrate (GaN
substrate), a step of grinding/polishing the gallium nitride
substrate, a step of boiling and cleaning the gallium nitride
substrate at a predetermined temperature and a step of wet-etching
the gallium nitride substrate at a predetermined temperature. In
the present embodiment, the GaN substrate is formed by the VAS
method.
[0037] Firstly, a GaN base layer is grown on a sapphire substrate
by a MOVPE method. A metal Ti thin film is deposited on the GaN
base layer. Subsequently, by heat treatment in a mixture stream of
ammonium and hydrogen gas, the metal Ti thin film is nitride to
turn into a TiN thin film having a mesh structure and also the GaN
base layer is etched to form voids thereon, thereby forming a
void-containing substrate.
[0038] Following this, a GaN crystal is grown on the
void-containing substrate by the hydride vapor phase epitaxy (HYPE)
method using GaCl and NH.sub.3 as raw materials. In the HVPE
method, a crystal growth rate is high and it is possible to easily
grow a thick GaN crystal film. For growing a crystal by the HVPE
method, an HVPE apparatus as shown in FIG. 3 is used.
[0039] The HVPE apparatus has a reaction tube 12 and a heater 11
provided therearound. The reaction tube 12 has a substrate holder
17 for placing a void-containing substrate 18, reaction gas inlet
tubes 13 and 15 opening near the void-containing substrate 18, an
etching gas inlet tube 14 opening near the void-containing
substrate 18 and an exhaust outlet 21. A raw material deposition
chamber 20 having a Ga metal 16 therein is provided on the reaction
gas inlet tube 15.
[0040] NH.sub.3 is supplied to the reaction gas inlet tube 13 and
HCl gas is supplied to the reaction gas inlet tube 15. The reaction
gases are supplied together with a carrier gas such as H.sub.2 or
N.sub.2. In the reaction gas inlet tube 15, the Ga metal 16 housed
in the raw material deposition chamber 20 is reacted with HCl and
GaCl is thereby produced. In other words, GaCl and NH.sub.3 are
supplied from the reaction gas inlet tubes 13 and 15 to the
void-containing substrate 18. GaCl is reacted with NH.sub.3 and a
GaN crystal is thereby vapor-grown on the void-containing substrate
18. The HCl gas for etching is supplied from the etching gas inlet
tube 14 to the void-containing substrate 18. The HCl gas is
supplied continuously during a crystal growth process or is
supplied between crystal growth processes in order to make
individual initial nuclei large.
[0041] The thick GaN film is naturally separated from the sapphire
substrate at the voids in the course of temperature drop after the
crystal growth and the GaN substrate (GaN free-standing substrate)
is thereby obtained.
[0042] Subsequently, the GaN substrate is attached and fixed to a
ceramic plate by a wax and the back surface of the GaN substrate is
ground/polished to improve flatness of the GaN substrate. Likewise,
the front surface (growth face) of the GaN substrate is
ground/polished. Diamond slurry which is abrasive grain is embedded
into the surface of the GaN substrate in this process. Meanwhile,
the wax is removed by heating but slightly remains on the surface
of the substrate. In other words, a carbon component is attached to
and remains on the surface of the GaN substrate in the
grinding/polishing process.
[0043] Following this, the polished GaN substrate is boiled and
cleaned at a predetermined temperature. The residual wax on the
surface of the GaN substrate is removed by this cleaning process,
thereby reducing the carbon component on the substrate surface.
Temperature for boiling and cleaning is preferably not less than
40.degree. C. By setting the temperature to not less than
40.degree. C., reactivity of a cleaning agent used is improved and
it is possible to dissolve the wax containing the carbon component
and thus to enhance removal thereof. In other words, it is possible
to appropriately remove the carbon component and thus to increase
the GaL.alpha./CK.alpha. peak intensity ratio in the EDX spectrum.
The cleaning agent to be used is not specifically limited but is
preferably isopropyl alcohol (IPA) which can appropriately remove
the carbon component derived from the wax.
[0044] Furthermore, the polished GaN substrate is wet-etched at a
predetermined temperature. Processing strain on the GaN substrate
is removed by the wet-etching process. In addition, the residual
wax which could not be completely removed in the cleaning process
is removed, together with the diamond slurry embedded into the
surface of the GaN substrate, by the wet-etching process, thereby
reducing the amount of residual carbon on the substrate surface. In
the etching process, the etching is preferably carried out at not
less than 77.degree. C. by heating etchant. Etching at a relatively
high temperature improves etching reactivity and thus allows
etching treatment time to be shortened. In addition, it is possible
to dissolve the wax and thus to appropriately remove the carbon
component.
[0045] Method of Manufacturing Optical Device
[0046] Next, a method of manufacturing an optical device in which
the GaN substrate obtained as described above is used to
manufacture the optical device will be described.
[0047] A nitride semiconductor crystal such as InGaN is epitaxially
grown on the surface of the above GaN substrate by the MOVPE
method. In the present embodiment, since the amount of the residual
carbon component on the surface of the GaN substrate is small,
crystal quality of the nitride semiconductor crystal to be grown is
good. In addition, good crystal quality provides high emission
intensity, reduces failures caused by a decrease in emission
intensity and allows a yield to be improved.
[0048] Although the gallium nitride substrate formed by the VAS
method has been described in the embodiment, the invention is not
limited thereto and is applicable to a gallium nitride substrate
formed by the DEEP method, etc., in the same manner.
EXAMPLES
[0049] Gallium nitrides substrate and optical devices in Examples
of the invention were manufactured by the following method under
the following conditions. These Examples are the illustrative
gallium nitride substrate and optical device of the invention and
the invention is not limited to these Examples.
Example 1
[0050] In Example 1, a GaN single crystal was grown by the VAS
method to make a GaN substrate.
[0051] Firstly, a void-containing substrate was prepared. For
making the void-containing substrate, a 500 nm-thick GaN base layer
was formed on a sapphire substrate (3.5 inches in diameter) by the
MOVPE method, etc., a 30 nm-thick Ti layer was deposited on a
surface thereof, and subsequently, heat treatment (at a temperature
of 1000.degree. C.) was carried out in a mixture gas of H.sub.2 and
NH.sub.3 for 30 minutes to form voids in the GaN layer while
converting the Ti layer into TiN having a mesh structure.
[0052] The void-containing substrate was placed on the substrate
holder 17 in the HVPE apparatus shown in FIG. 3, and was heated in
the reaction tube 12 at atmospheric pressure so as to have a
substrate temperature of 1050.degree. C. The initial nucleation
conditions were as follows: 5.times.10.sup.-2 atm of NH.sub.3 gas
was introduced together with 6.times.10.sup.-1 atm of N.sub.2 gas
as a carrier gas from the reaction gas inlet tube 13,
5.times.10.sup.-3 atm of GaCl gas was introduced together with
2.0.times.10.sup.-1 atm of N.sub.2 gas and 1.0.times.10.sup.-1 atm
of H.sub.2 gas as carrier gases from the reaction gas inlet tube 15
and a crystal was grown for 20 minutes.
[0053] After the initial nucleation, the crystal was grown under
the same conditions as the initial nucleation conditions except
that the partial pressure of GaCl gas was set to be
1.5.times.10.sup.-2 atm and the partial pressure of N.sub.2 gas as
the carrier gas of NH.sub.3 gas was set to be 5.85.times.10.sup.-1
atm. The crystal was then grown until the entire GaN crystal
becomes 900 thereby obtaining the GaN crystal. The thick GaN film
was naturally separated from the sapphire substrate in the course
of temperature drop after the growth of the GaN crystal, thereby
obtaining a free-standing GaN substrate.
[0054] Subsequently, the surface of the GaN substrate was attached
and fixed to a ceramic plate using a wax. After that, the back
surface of the GaN substrate was ground by a horizontal surface
grinding machine. The conditions for grinding the back surface were
as follows: grinding stone used--metal bond #800; diameter of
grinding stone--150 mm; rotation speed of grinding stone--2000 rpm;
feeding speed of grinding stone--0.1 .mu.m/second; and grinding
time--30 minutes. Furthermore, the back surface of the GaN
substrate was polished by a high speed single-surface precision
lapping machine. The conditions for mechanical polishing of N-polar
surface were as follows: rotation speed of surface plate--200 rpm;
pressure--0.25 MPa; polishing solution--diamond slurry (loose
abrasive) having a grain diameter of 3 .mu.m; feed rate of
polishing solution--0.3 L/min; and polishing time--20 minutes.
Then, the ceramic plate to which the GaN substrate is attached was
heated by a hot plate to melt the wax, thereby separating the GaN
substrate.
[0055] In addition, the front surface which is another surface of
the GaN substrate was ground/polished in the same manner as the
back surface. The grinding conditions were as follows: grinding
stone used--metal bond #800; diameter of grinding stone--200 mm;
rotation speed of grinding stone--2500 rpm; feeding speed of
grinding stone--0.1 .mu.m/second; and grinding time--30 minutes.
The polishing conditions were as follows: rotation speed of surface
plate--200 rpm; pressure--0.30 MPa; polishing solution--diamond
slurry (loose abrasive) having a grain diameter of 1 .mu.m; feed
rate of polishing solution--0.30 L/min; and polishing time--20
minutes. The ground and polished GaN substrate then had a thickness
of 400 .mu.m.
[0056] Subsequently, the outer diameter process was performed on
the GaN substrate by an outer diameter processing machine so as to
have a diameter of 76.2 mm (3 inches).
[0057] Next, for the purpose of removing the wax attached to the
surface of the GaN substrate, the substrate was boiled and cleaned
for 30 minutes using WA (isopropyl alcohol). During the cleaning,
the cleaning temperature was set to 41.degree. C. In addition, for
the purpose of removing processing strain on the GaN substrate and
the carbon component derived from the diamond slurry embedded into
the substrate surface, wet-etching was carried out by immersing the
GaN substrate in a 25% NH.sub.4OH solution. The wet-etching was
carried out for 90 minutes at an etching temperature of 77.degree.
C. The cleaning condition and the wet-etching condition of the GaN
substrate are shown in Table 1.
TABLE-US-00001 TABLE 1 IPA boiling-cleaning Wet-etching temperature
temperature Example 1 41 77 Example 2 44 78 Example 3 47 79 Example
4 50 80 Example 5 53 81 Example 6 56 82 Example 7 59 83 Example 8
62 84 Example 9 65 85 Example 10 68 86 Example 11 71 87 Example 12
74 88 Example 13 77 89 Example 14 80 90 Comparative Example 1 20 70
Comparative Example 2 23 71 Comparative Example 3 26 72 Comparative
Example 4 29 73 Comparative Example 5 32 74 Comparative Example 6
35 75 Comparative Example 7 38 76
[0058] Lastly, the GaN substrate was washed with pure water and was
dried by a nitrogen gun, thereby obtaining a GaN substrate of
Example 1.
Examples 2 to 14 and Comparative Examples 1 to 7
[0059] GaN substrates in Examples 2 to 14 and Comparative Examples
1 to 7 were made under the same conditions as Example 1 except that
the cleaning condition (cleaning temperature) and the wet-etching
condition (etching temperature) of Example 1 were changed to those
shown in Table 1.
[0060] EDX measurement was performed on the surfaces of the GaN
substrates obtained in Examples 1 to 14 and Comparative Examples 1
to 7, and the amount of residual carbon on the surface of the GaN
substrate was each evaluated. In detail, using VE-9800S
(manufactured by KEYENCE CORPORATION) as a scanning electron
microscope (SEM) and GENESIS2000 (manufactured by EDAX Inc.) as an
EDX spectrum detector, EDX spectrum at the center of the GaN
substrate was measured at a characteristic x-ray takeoff angle of
16.28.degree.. The measurement was performed while changing the
accelerating voltage of the SEM from 3 kV, 5 kV to 8 kV. The
electron penetration depths at respective accelerating voltages
calculated from the formula (I) are respectively 0.09 .mu.m, 0.20
.mu.m and 0.45 .mu.m. Then, in order to measure the amount of
carbon near the surface of the GaN substrate, a ratio of GaLa peak
intensity (about 1.100 keV) to CK.alpha. peak intensity (about
0.266 keV) in EDX spectrum was examined. The results thereof are
shown in Table 2.
TABLE-US-00002 TABLE 2 GaL.alpha./CK.alpha. in EDX spectrum
Accelerating Accelerating Accelerating voltage voltage voltage 3 kV
5 kV 8 kV Example 1 2.0 31.1 41.3 Example 2 2.2 32.7 40.6 Example 3
2.4 31.7 39.7 Example 4 2.6 33.7 42.9 Example 5 2.8 32.4 38.1
Example 6 3.0 33.8 39.0 Example 7 4.3 32.7 41.5 Example 8 5.4 31.5
38.6 Example 9 7.0 33.6 39.0 Example 10 8.9 32.9 41.1 Example 11
10.3 31.9 40.6 Example 12 11.5 32.8 39.9 Example 13 12.9 31.9 39.2
Example 14 14.1 32.8 40.1 Comparative 0.8 33.2 39.7 Example 1
Comparative 0.9 31.0 42.5 Example 2 Comparative 1.1 31.8 41.6
Example 3 Comparative 1.3 32.0 38.4 Example 4 Comparative 1.5 32.5
40.6 Example 5 Comparative 1.7 31.4 42.8 Example 6 Comparative 1.9
33.7 38.5 Example 7
[0061] From Table 2, it was confirmed that, when the accelerating
voltage of the SEM is 3 kV, the GaL.alpha./CK.alpha., peak
intensity ratio increases with an increase in the IPA
boiling-cleaning temperature and NH.sub.4OH wet-etching temperature
and carbon near the substrate surface is removed. On the other
hand, when the accelerating voltage of the SEM was 5 kV and 8 kV,
the GaL.alpha./CK.alpha. peak intensity ratio in the EDX spectrum
hardly changed. This is because the penetration depth of electron
beam into the surface of the GaN substrate is too far and variation
in carbon level near the substrate surface is not observed.
Therefore, an appropriate accelerating voltage is considered to be
3 kV in order to examine the variation in carbon level near the
surface.
[0062] Following this, optical devices were manufactured using the
GaN substrates obtained in Examples and Comparative Examples, and
crystal quality was evaluated by measuring emission intensity
thereof.
[0063] A H.sub.2 carrier gas, ammonium, trimethylgallium and
trimethylindium were supplied onto a Ga-polar surface (front
surface) of the GaN substrate at a substrate temperature of
1020.degree. C. by the MOVPE method, thereby growing a structure of
the epitaxial film shown in FIG. 4. In detail, a GaN buffer layer 2
(2500 nm in thickness), an InGaN barrier layer (about 8 nm in
thickness) and an InGaN well layer (about 5 nm in thickness) were
alternately laminated six times on the GaN substrate 1 (400 .mu.m
in thickness) of Example 1, and a multiple quantum well layer 3
formed by growing the InGaN barrier layer (about 8 nm in thickness)
and a GaN cap layer 4 (about 30 nm in thickness) were further
laminated thereon, thereby making an optical device 10.
[0064] Photoluminescence peak intensity corresponding to a band gap
of an InGaN quantum well layer at the center of the GaN substrate
was measured on the obtained optical device by a photoluminescence
measurement system RPM 2000 (manufactured by Accent). The
photoluminescence measurement conditions were as follows: laser
light source--He--Cd laser with a wavelength of 325 nm; width of
light receiving slit--0.1 mm; and measurement-wavelength
range--367.9 nm to 432.4 nm. Emission intensity of the GaN
substrate was examined. The examination results are shown in Table
3.
TABLE-US-00003 TABLE 3 Photoluminescence emission intensity Example
1 1.515 Example 2 1.547 Example 3 1.493 Example 4 1.525 Example 5
1.563 Example 6 3.152 Example 7 3.045 Example 8 2.997 Example 9
3.078 Example 10 2.965 Example 11 3.036 Example 12 3.058 Example 13
2.987 Example 14 3.015 Comparative Example 1 0.543 Comparative
Example 2 0.589 Comparative Example 3 0.478 Comparative Example 4
0.552 Comparative Example 5 0.513 Comparative Example 6 0.492
Comparative Example 7 0.524
[0065] According to Table 3, in the optical devices of Examples 1
to 5 in which a peak intensity ratio is not less than 2 and less
than 3, the photoluminescence emission intensity is 1.493 to 1.563
Volt/mW. In addition, in the optical devices of Examples 6 to 14 in
which a peak intensity ratio is not less than 3, the emission
intensity is 2.965 to 3.152 Volt/mW. On the other hand, in the
optical devices of Comparative Examples 1 to 7 in which a peak
intensity ratio is less than 2, the emission intensity is 0.478 to
0.589 Volt/mW. In other words, emission intensity is lower in
Comparative Examples 1 to 7 than in Examples 1 to 14.
[0066] In Comparative Examples 1 to 7, the GaN substrate, of which
GaL.alpha./CK.alpha. peak intensity ratio at an accelerating
voltage of 3 kV is less than 2 and in which the amount of residual
carbon on the surface is large, is used and it is thus considered
that the crystal quality deteriorates at the time of growing the
nitride semiconductor crystal and emission intensity decreases. In
contrast, the GaN substrate in which the peak intensity ratio is
not less than 2.0 is used in Examples 1 to 14 and it is thus
considered that the amount of residual carbon component on the
surface is small and the crystal quality of the nitride
semiconductor crystal to be grown is good. Especially in Examples 6
to 14 in which the peak intensity ratio is not less than 3.0, the
amount of residual carbon is smaller and it is thus considered that
the crystal quality of the epitaxial growth layer is better. As a
result of having good crystal quality, a decrease in emission
intensity was suppressed in the optical devices of Examples 1 to
14, thereby obtaining large emission intensity.
[0067] Although the invention has been described with respect to
the specific embodiment for complete and clear disclosure, the
appended claims are not to be therefore limited but are to be
construed as embodying all modifications and alternative
constructions that may occur to one skilled in the art which fairly
fall within the basic teaching herein set forth.
* * * * *