U.S. patent application number 10/810309 was filed with the patent office on 2005-01-13 for semiconductor apparatus, method for growing nitride semiconductor and method for producing semiconductor apparatus.
This patent application is currently assigned to Kyocera Corporation. Invention is credited to Akasaki, Isamu, Amano, Hiroshi, Kamiyama, Satoshi, Matsuda, Toshiya, Yasuda, Takanori.
Application Number | 20050006635 10/810309 |
Document ID | / |
Family ID | 32966791 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050006635 |
Kind Code |
A1 |
Akasaki, Isamu ; et
al. |
January 13, 2005 |
Semiconductor apparatus, method for growing nitride semiconductor
and method for producing semiconductor apparatus
Abstract
A semiconductor apparatus includes a substrate made of a
diboride single crystal expressed by a chemical formula XB.sub.2,
in which X includes at least one of Tl, Zr, Nb and Hf, a
semiconductor buffer layer formed on a principal surface of the
substrate and made of Al.sub.yGa.sub.1-yN (0<y.ltoreq.1), and a
nitride semiconductor layer which is formed on the semiconductor
buffer layer and which includes at least one kind or plural kinds
selected from among 13 group elements and As.
Inventors: |
Akasaki, Isamu; (Nagoya-shi,
JP) ; Amano, Hiroshi; (Nagoya-shi, JP) ;
Kamiyama, Satoshi; (Nagoya-shi, JP) ; Yasuda,
Takanori; (Soraku-gun, JP) ; Matsuda, Toshiya;
(Soraku-gun, JP) |
Correspondence
Address: |
HOGAN & HARTSON L.L.P.
500 S. GRAND AVENUE
SUITE 1900
LOS ANGELES
CA
90071-2611
US
|
Assignee: |
Kyocera Corporation
|
Family ID: |
32966791 |
Appl. No.: |
10/810309 |
Filed: |
March 26, 2004 |
Current U.S.
Class: |
257/11 ;
257/E21.119; 257/E21.136 |
Current CPC
Class: |
H01L 33/007 20130101;
H01L 21/0254 20130101; H01L 21/02458 20130101; H01L 21/02433
20130101; C30B 25/183 20130101; H01L 21/0242 20130101; H01L 21/2205
20130101; H01L 21/02546 20130101; C30B 25/02 20130101; C30B 29/403
20130101; H01L 21/0262 20130101 |
Class at
Publication: |
257/011 |
International
Class: |
H01L 029/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2003 |
JP |
P2003-085941 |
Mar 26, 2003 |
JP |
P2003-085942 |
Mar 26, 2003 |
JP |
P2003-085955 |
May 23, 2003 |
JP |
P2003-146965 |
May 28, 2003 |
JP |
P2003-151697 |
Claims
What is claimed is:
1. A semiconductor apparatus comprising: a substrate made of a
diboride single crystal expressed by a chemical formula XB.sub.2,
in which X includes at least one of Ti, Zr, Nb and Hf; a
semiconductor buffer layer formed on a principal surface of the
substrate and made of Al.sub.yGa.sub.1-yN (0<y.ltoreq.1); and a
nitride semiconductor layer formed on the semiconductor buffer
layer, including at least one kind or plural kinds selected from
among 13 group elements and As.
2. A semiconductor apparatus comprising: a substrate made of a
diboride single crystal expressed by a chemical formula XB.sub.2,
in which X includes at least one of Ti, Zr, Nb and Hf; a
semiconductor buffer layer formed on a principal surface of the
substrate and made of (AlN).sub.x(GaN).sub.1-x (0<x.ltoreq.1);
and a nitride semiconductor layer formed on the semiconductor
buffer layer, including at least one kind or plural kinds selected
from among 13 group elements and As.
3. The semiconductor apparatus of claim 1, wherein the substrate is
of ZrB.sub.2 or TiB.sub.2.
4. The semiconductor apparatus of claim 2, wherein the substrate is
of ZrB.sub.2 or TiB.sub.2.
5. The semiconductor apparatus of claim 1, wherein the substrate is
a solid solution containing one or a plurality of impurity elements
of 5 atom % or less, the one or a plurality of impurity elements
being selected from a group consisting of Ti, Cr, Hf, V, Ta and Nb
when the substrate is of ZrB.sub.2, or selected from a group
consisting of Zr, Cr, Hf, V, Ta and Nb when the substrate is of
TiB.sub.2.
6. The semiconductor apparatus of claim 2, wherein the substrate is
a solid solution containing one or a plurality of impurity elements
of 5 atom % or less, the one or a plurality of impurity elements
being selected from a group consisting of Ti, Cr, Hf, V, Ta and Nb
when the substrate is of ZrB.sub.2, or selected from a group
consisting of Zr, Cr, Hf, V, Ta and Nb when the substrate is of
TiB.sub.2.
7. The semiconductor apparatus of claim 1, wherein the
semiconductor buffer layer is AlN.
8. The semiconductor apparatus of claim 2, wherein the
semiconductor buffer layer is AlN.
9. The semiconductor apparatus of claim 7, wherein the thickness of
the semiconductor buffer layer made of AlN is 10 to 250 nm.
10. The semiconductor apparatus of claim 8, wherein the thickness
of the semiconductor buffer layer made of AlN is 10 to 250 nm.
11. The semiconductor apparatus of claim 2, wherein the thickness
of the semiconductor buffer layer made of (AlN).sub.x(GaN).sub.1-x
is within a range of 10 to 100 nm.
12. The semiconductor apparatus of claim 2, wherein x of the
semiconductor buffer layer made of (AlN).sub.x(GaN).sub.1-x is
0.1.ltoreq.x.ltoreq.1.
13. The semiconductor apparatus of claim 2, wherein x of the
semiconductor buffer layer made of (AlN).sub.x(GaN).sub.1-x is
0.4.ltoreq.x.ltoreq.0.6.
14. The semiconductor apparatus of claim 1, wherein an angle
.theta.1 formed by a normal line of the principal surface of the
substrate and a normal line of the (0001) plane of the substrate is
0.degree..ltoreq..theta.1.ltoreq.5.degree..
15. The semiconductor apparatus of claim 2, wherein an angle
.theta.1 formed by a normal line of the principal surface of the
substrate and a normal line of the (0001) plane of the substrate is
0.degree..ltoreq..theta.1.ltoreq.5.degree..
16. The semiconductor apparatus of claim 7, wherein an angle
.theta.1 formed by a normal line of the principal surface of the
substrate and a normal line of the (0001) plane of the substrate is
0.degree..ltoreq.O1.ltoreq.0.55.degree..
17. The semiconductor apparatus of claim 8, wherein an angle O1
formed by a normal line of the principal surface of the substrate
and a normal line of the (0001) plane of the substrate is
0.degree..ltoreq..theta.1.ltoreq.- 0.55.degree..
18. The semiconductor apparatus of claim 1, wherein the substrate
is eroded and removed by etching.
19. The semiconductor apparatus of claim 2, wherein the substrate
is eroded and removed by etching.
20. A method for growing a nitride semiconductor, comprising: on a
substrate of a diboride single crystal expressed by a chemical
formula XB.sub.2, in which X includes at least one of Ti, Zr, Nb
and Hf, growing Al.sub.yGa.sub.1-yN layer (0<y.ltoreq.1) from
vapor phase, and subsequently, growing a nitride semiconductor
layer including at least one kind selected from among 13 group
elements and As from vapor phase.
21. A method for growing a nitride semiconductor, comprising: on a
substrate of a diboride single crystal expressed by a chemical
formula XB.sub.2, in which X includes at least one of Ti, Zr, Nb
and Hf, growing an (AlN).sub.x(GaN).sub.1-x layer (1<x.ltoreq.1)
from vapor phase within a temperature range of more than
400.degree. C. and less than 1100.degree. C. by an MOVPE method,
and subsequently, growing a nitride semiconductor layer including
at least one kind selected from among 13 group elements and As from
vapor phase.
22. The method of claim 21, wherein the thickness of the
(AlN).sub.x(GaN).sub.1-x layer is within a range of 10 to 100
nm.
23. A method for growing a nitride semiconductor, comprising: on
the (0001) plane of a substrate of a diboride single crystal
expressed by a chemical formula XB.sub.2, in which X includes at
least one of Ti, Zr, Nb and Hf, growing an AlN layer from vapor
phase so that a deviation angle of a normal line of a surface of
the substrate from a direction of the [0001] becomes 0.55 degrees
or less, and subsequently, growing a nitride semiconductor layer
including at least one kind selected from among 13 group elements
and As from vapor phase.
24. The method of claim 23, wherein the thickness of the AlN layer
is within a range of 10 to 250 nm.
25. A method for producing a semiconductor apparatus, comprising:
eroding and removing a diboride single crystal substrate of a
semiconductor apparatus obtained by the method for growing nitride
semiconductor of claim 21 by etching.
26. A method for producing a semiconductor apparatus, comprising:
eroding and removing a diboride single crystal substrate of a
semiconductor apparatus obtained by the method for growing nitride
semiconductor of claim 22 by etching.
27. A method for producing a semiconductor apparatus, comprising:
eroding and removing a diboride single crystal substrate of a
semiconductor apparatus obtained by the method for growing nitride
semiconductor of claim 23 by etching.
28. A method for producing a semiconductor apparatus, comprising:
eroding and removing a diboride single crystal substrate of a
semiconductor apparatus obtained by the method for growing nitride
semiconductor of claim 24 by etching.
29. A method for producing a semiconductor apparatus, comprising
the steps of: carrying out crystal growth of a nitride
semiconductor layer on one principal surface of a single crystal
substrate of a hexagonal crystal symmetry having electrical
conductivity; and eroding and removing the single crystal substrate
by etching.
30. The method of claim 29, wherein the single crystal substrate is
a substrate of a diboride single crystal expressed by XB.sub.2, in
which X includes at least one of Zr and Ti.
31. The method of claim 29, wherein in growing the nitride
semiconductor layer from vapor phase, a nitride semiconductor layer
grown firstly is an Al.sub.xGa.sub.1-x layer (0<x.ltoreq.1).
32. The method of claim 29, wherein a mixed solution of at least
nitric acid and hydrofluoric acid is used for the etching.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a semiconductor apparatus and
specifically relates to a semiconductor apparatus, a method for
growing a nitride semiconductor and a method for producing a
semiconductor apparatus which are suitable for a light emitting
device and a light receiving device such as a light emitting diode
(LED), a laser diode (LD), a solar cell and a photosensor, and an
electronic device such as a transistor and a power device.
[0003] 2. Description of the Related Art
[0004] A nitride semiconductor containing Ga as a main constituent
(a GaN semiconductor) is utilized as a material for an optical
device such as a light emitting diode of blue light or violet
light, a laser diode and a photodetector. Moreover, attention is
given to the GaN semiconductor as a high-performance material for
an electronic device as well, because the GaN semiconductor is
capable of satisfying high frequency and high electric power, and
is highly reliable.
[0005] Further, a light emitting diode using the GaN semiconductor
is known (refer to Japanese Unexamined Patent Publication JP-A
4-321280 (1992), for example). An example of a structure of such a
light emitting diode is shown in FIG. 17. A GaN buffer layer 21 is
formed on a sapphire (Al.sub.2O.sub.3) substrate 20, and on the GaN
buffer layer 21 is formed a growth layer made of a GaN
semiconductor layer having a multilayer structure made by
sequentially laminating an n-GaN layer 22 of the n-type
semiconductor layer, an n-AlGaN cladding layer 23 of the n-type
semiconductor layer, an InGaN light emitting layer 24, a p-AlGaN
cladding layer 25 of the p-type semiconductor layer and a p-GaN
layer 26 of the p type semiconductor layer. In part of the growth
layer, a region from the p-GaN layer 26 to (an upper portion of the
n-GaN layer 22) is etched and removed, and thus part of the n-GaN
layer 22 is exposed. The n-type electrode 20 is formed on an upper
surface of the exposed region, and the p-type electrode 27 is
formed on an upper surface of the p-GaN layer 26 of an uppermost
layer.
[0006] Since production of a single crystal substrate of the GaN
semiconductor is difficult, there is a need to form a semiconductor
apparatus using the GaN semiconductor on a substrate made of a
different material. Although sapphire is generally used as the
substrate 20, a Si substrate, a ZnO substrate, an
MgO.Al.sub.2O.sub.3 (spinel) substrate, a SiC substrate, a GaAs
substrate and the like are tested other than the sapphire
substrate.
[0007] In the case of growing the GaN semiconductor layers on the
sapphire substrate 20, a problem is a lattice mismatch between
them. A relation of lattice constants thereof is as shown below.
GaN grows in a direction rotated through 30.degree. from an a axis
on a c plane of the sapphire substrate. Regarding sapphire, a
lattice constant a (which is described as a.sub.1 in Table 1
described later) is 4.7580 .ANG.. An interval value (which is
described as a.sub.2 in Table 1 described later when a lattice
rotates through 30.degree. is 2.747 obtained by 4.758.times.1/1.732
(a numerical value obtained by multiplying a length of the a axis
in a unit lattice of sapphire by 1/1.732 becomes a reference). On
the other hand, regarding GaN, a lattice constant a is 3.1860
.ANG..
[0008] A ratio of the lattice mismatch of GaN with reference to
sapphire becomes +15.98% (=100.times.(3.1860-2.747)/2.747). Thus,
the lattice constant of sapphire is significantly different from
the lattice constant of GaN. Consequently, a good quality crystal
cannot be obtained when GaN is directly grown on sapphire. Besides,
it is possible to consider a substrate made of a material of
another kind in the same manner.
[0009] In a prior art, in order to increase crystalline quality of
the growth layer, a buffer layer made of an AlN or GaN material
which is amorphous or polycrystalline is formed on the (0001) plane
of the sapphire substrate 20 in advance, and the GaN growth layer
is formed on the buffer layer. The buffer layer has a function of
reducing the lattice mismatch between the GaN growth layer and the
sapphire substrate and increasing crystalline quality.
[0010] Further, in the case of a semiconductor apparatus such as a
laser diode or a transistor which requires a better quality
crystal, after the GaN semiconductor layer is once grown on the
single crystal substrate 20, the single crystal substrate 20 is
eliminated, and then the semiconductor apparatus is formed. This is
because when the semiconductor apparatus is formed on a substrate
20 made of a different material from the semiconductor apparatus, a
crystal defect which results from a difference in coefficient of
thermal expansion occurs in a cooling process after crystal growth
at high temperatures of 1000.degree. C. or more.
[0011] Furthermore, in another prior art, there is also a method
for growing the GaN semiconductor layer 22 in which a mask made by
patterning a SiO.sub.2 thin film is formed and the GaN
semiconductor is grown on the mask in a lateral direction, in order
to avoid an influence by the lattice mismatch with the substrate
20.
[0012] However, since the ratio of the lattice mismatches between
the sapphire substrate 20 and the GaN layer 22 is as large as
15.98%, the GaN growth layer 22 contains dislocations whose density
is 10.sup.8 to 10.sup.11 cm.sup.-2 even when formed via the buffer
layer 21 made of an AlN or GaN material. Moreover, even a layer of
a GaN crystal laterally grown after elimination of the sapphire
substrate 20 contains dislocations whose density is 10.sup.4 to
10.sup.7 cm.sup.-2. They contain extremely large dislocations as
compared with GaAs grown on a GaAs substrate containing
dislocations whose density is 10.sup.2 cm.sup.-2 to 10.sup.7
cm.sup.-2.
[0013] The dislocation density of the GaN growth layer
significantly restricts performance of a semiconductor apparatus to
be produced from this, and moreover, there is a need to increase
the amount of additive elements in the semiconductor layer for
generation of sufficient carrier. This has a problem of
deteriorating a characteristic of a semiconductor apparatus, such
as a life, a withstand voltage, a driving voltage, consumed
electric power (operation efficiency), an operation speed or a leak
current.
[0014] Then, in a proposed art, growth of a nitride semiconductor
on a diboride single crystal substrate expressed by a chemical
formula XB.sub.2, in which X contains at least one of Ti and Zr, is
proposed.
1 TABLE 1 Coefficient of Lattice constant thermal expansion [.ANG.]
[.times.10.sup.-6/K] ZrD.sub.2 3.1696 5.9 TiD.sub.2 3.0303
Sapphire(Al.sub.2O.sub.3) a.sub.1 = 4.7580, a.sub.2 = 2.747 7.5 CaN
3.1860 5.6 AlN 3.1114 4.2 BN 2.5502 InN 3.54 5.7 SiC 3.08 4.3 GaAs
4.00 8.7 Si 3.84 2.6
[0015] Since the nitride semiconductor is formed with a good
lattice match relation with the diboride single crystal substrate
in this manner, a lattice defect in the growth layer is small, and
crystalline quality of the nitride film is extremely good.
[0016] However, in the aformentioned prior art, for example, when
GaN is crystal-grown as the nitride semiconductor on the diboride
single crystal substrate, because of a change of a growth
temperature in a growth process, B in the substrate diffuses into
the crystal-grown GaN crystal, and a nitride semiconductor GaBN
which contains a ternary 13 group (a former IIIB group element) is
generated on an interface between the GaN and the substrate. In the
case of RN, as shown in Table 1, a mismatch of lattice constants
becomes as large as approximately 20%, as compared with GaN (for
example, (2.5502-3.1696)/3.1696-19.5%). Therefore, in the case of
CaBN, which is a ternary nitride semiconductor, a difference of
lattice constants becomes significantly large as a mixed crystal
ratio of B becomes large, unlike AlGaN, which is a ternary nitride
semiconductor which the mismatch ratio of lattice constants is 2%
or less. Consequently, a lattice defect occurs on the interface
even when GaN is grown on the diboride single crystal substrate as
described above, and a good quality crystal cannot be obtained.
[0017] In recent years, research and development of a nitride
semiconductor which contains at least one selected from among B,
Al, Ca, In and Tl have become active, and applied technique has
rapidly developed. Then, at present, a light emitting diode of
green, blue and ultraviolet, a laser diode of blue and violet, and
the like using the nitride semiconductor is put to practical
use.
[0018] In specific, (InN).sub.x(GaN).sub.1-x whose band gap covers
from red to violet and (AlN).sub.x(GaN).sub.1-xN whose band gap
covers from violet to ultraviolet are positioned as main materials
among III group nitride semiconductors, because the former makes it
possible to realize a device emitting light of blue green, blue,
violet or the like which was not realized before, and the latter
makes it possible to expect application as a light source for
measurement, sterilization or excitation.
[0019] The III group nitride semiconductor is grown from vapor
phase on a single crystal substrate made of sapphire, SiC, GaAs, Si
or the like by a MOVPE (Metalorganic Vapor Phase Epitaxy)
method.
[0020] The III group nitride semiconductor is a hexagonal symmetry,
and a-axis lattice constants of InN, GaN and AlN are 0.311 nm,
0.319 nm and 0.354 nm, respectively. Moreover, respective lattice
constants of (InN).sub.x(GaN).sub.1-x and (AlN).sub.x(GaN).sub.1-x
are values in a middle range of the respective aforementioned
lattice constants of InN, GaN and AlN depending on x.
[0021] However, since intervals between atoms that sapphire, SiC,
GaAs and Si should achieve lattice matches with the III group
nitride semiconductor are 0.275 nm, 0.308 nm, 0.400 nm and 0.384
nm, respectively (refer to Table 1), and substrates which
completely achieve lattice matches are not obtained.
[0022] In the meantime, in another prior art a technique on a
low-temperature buffer layer is proposed (refer to Japanese
Examined Patent Publication JP-B2 4-15200 (1992) and Japanese
Patent 3026087, for example).
[0023] It is possible to grow a good quality crystal on the lattice
mismatching substrates described above by using these technique,
however, penetration dislocations of approximately 10.sup.8
cm.sup.-2 to 10.sup.11 cm.sup.-2 still exitss. Moreover,
differences in the coefficient of thermal expansion between the
above single crystal substrates and the nitride semiconductor are
large, and a difference in contraction amounts after crystal growth
at a high temperature of approximately 1000.degree. C. causes
cracks.
[0024] As a method for solving these problems, in another prior art
a technique of growing a nitride semiconductor on the (0001) plane
of a ZrB.sub.2 single crystal substrate is proposed (refer to
Japanese Unexamined Patent Publication JP-A 2002-43223).
[0025] The ZrB.sub.2 single crystal substrate is a hexagonal
symmetry, and a lattice constant of an a axis is 0.317 nm (refer to
table 1), which completely achieves a lattice match with 0.26 of x
of (AlN).sub.x(GaN).sub.1-x. Moreover, the coefficient of thermal
expansion is 5.9.times.10.sup.6K.sup.-1, which is a close value to
5.6.times.10.sup.-6K.sup.-1 of that of GaN.
[0026] Further, the ZrB.sub.2 single crystal substrate, whose
resistivity is as small as 4.6 .eta..OMEGA..multidot.cm, is
electrically conductive. On the other hand, a sapphire substrate
110 generally used as a substrate up to now is insulative, and
therefore, a light emitting diode formed on the sapphire substrate
has a structure such that two electrodes 101 and 109 are disposed
on the same plane side (the upper side of the substrate 110 in FIG.
18) as shown in FIG. 18.
[0027] The structure shown in FIG. 18 is a structure in which a
low-temperature buffer layer 107, the n-type contact layer 106, the
n-type cladding layer 105, a light emitting layer 104, the p-type
cladding layer 103 and the p-type contact layer 102 are
sequentially laminated on a sapphire substrate 110, and
furthermore, a p electrode 101 is formed thereon. Moreover, an n
electrode 109 is formed on an exposed surface of the n-type contact
layer 106.
[0028] As described above, in the last one or two years, research
and development of the technique of growing the nitride
semiconductor on the ZrB.sub.2 single crystal substrate have
progressed.
[0029] According to "Abstr. 13th Int. Conf. Crystal Growth, August
2001, 02a-SB2-20", a technique of enabling growth of GaN on the
(0001) plane of the ZrB.sub.2 single crystal substrate by an MBE
method is proposed.
[0030] However, this technique has a problem that it is inferior in
mass production because it uses the MBE method.
[0031] Further, a technique of growing GaN on the (0001) plane of
the ZrB.sub.2 single crystal substrate by using an AlN buffer layer
by the MOVPE method is also proposed (refer to Ext. Abstr. (62nd
Autumn Meet. 2001); Japan Society of Applied Physics, 12p
R-14).
[0032] Further, in the prior art, a GaN film grown on the (0001)
plane of a ZrB.sub.2 single crystal substrate by an MOVPE method
has a problem that a surface shape thereof tends to become uneven
as shown in FIG. 13.
[0033] As described above, in the case of growing a nitride
semiconductor on the (0001) plane of the ZrB.sub.2 single crystal
substrate by using an AlN buffer layer, it is desired to reduce
unevenness appearing on a surface shape thereof.
[0034] However, according to both the techniques, in the GaN film
grown on the (0001) plane of the ZrB.sub.2 single crystal
substrate, a rocking curve half value width of (0002) plane omega
scan by an X-ray diffraction method, which becomes an indicator of
evaluation of quality, is approximately 1000 seconds, which is not
sufficiently good (refer to "Study on the crystal growth and
properties of group-III nitride semiconductors on ZrB.sub.2
substrate by metalorganic vapor phase epitaxy" master's thesis
written by Yohei Yukawa, graduate school of Meijo University,
2001).
[0035] Furthermore, since AlN is insulative, when the light
emitting diode or the like with the structure as shown in FIG. 8 as
described later is produced, resistance from the nitride
semiconductor layer to the substrate becomes high, and an operation
voltage becomes high.
[0036] Still further, a band gap of AlN is as large as 6.2 eV, and
therefore, it is difficult to decrease resistance by doping.
[0037] In the aforementioned prior art, when the nitride
semiconductor is grown on the (0001) plane of the ZrB.sub.2 single
crystal substrate, resistance from the nitride semiconductor layer
to the substrate becomes high, and the operation voltage becomes
high.
[0038] In recent years, a nitride semiconductor such as gallium
nitride (GaN), indium nitride (InN) or aluminum nitride (AlN) is
used as a material for an optical device such as a light emitting
diode of blue light or violet light, a laser diode or a
photodetector, because the nitride semiconductor is a compound
semiconductor of direct transition type, and has a wide band
gap.
[0039] Further, since the nitride semiconductor is capable of
satisfying high frequency or high electric power, and is highly
reliable, the nitride semiconductor is noted as a high-performance
material for an electronic device.
[0040] Up to now, there is no substrate that achieves a lattice
match with the nitride semiconductor. In a conventional art, the
nitride semiconductor is grown by the use of a substrate made of a
material, such as a sapphire substrate different kind from nitride
semiconductor.
[0041] However, for example, regarding the sapphire substrate and
GaN, a ratio of lattice mismatch is 13.8% and a difference in the
coefficient of thermal expansion is 3.2.times.10.sup.-6/K, and
there is a problem resulting from the mismatch such that
dislocations of 10.sup.8 to 10.sup.10 cm.sup.-2 arises in the GaN
film because of a crystal defect caused on an interface between the
sapphire substrate and the GaN film.
[0042] Further, because of the defect and thermal distortion, the
GaN film is warped, and crystalline quality is significantly
deteriorated.
[0043] Furthermore, considering production of a device such as a
laser diode, the nitride semiconductor is formed on a substrate
made of a material of different kind from the nitride semiconductor
such as GaN and the like, and therefore, there arises such a
problem that, in the case of forming a reflection surface of a
laser resonator, cleaved planes of the substrate made of a material
and the nitride semiconductor are different, and that formation by
cleavage is difficult. Accordingly, a good quality nitride
semiconductor substrate has been expected.
[0044] However, regarding the nitride semiconductor such as GaN, a
melting point is high and a dissociation pressure of nitride is
high at the melting point, and therefore, production of a bulk
single crystal is difficult. Consequently, as described before, for
example, by growing a thick film of GaN on the sapphire substrate
and then separating the sapphire substrate, which is made of a
material of different kind from GaN, and the GaN thick film, the
nitride semiconductor apparatus is produced.
[0045] However, in the step of separating the substrate and the
nitride semiconductor in the aforementioned production method, a
method of abrading the substrate arises a problem that stress from
the nitride semiconductor thick film becomes large as the substrate
becomes thin, and that the stress acts on the substrate, thereby
worsening a warp thereof and causing cracks.
[0046] In the meantime, as prior art another separation method,
there is, for example, a method of separating by locally
irradiating the interface between the sapphire substrate and the
GaN thick film with a laser light beam and subliming the interface
(a laser lift off method).
[0047] However, according to this method, only a small part of the
interface is separated because an area irradiated is small, and
stress concentrates on a small attaching part, with the result that
cracks are caused. Moreover, since the area irradiated is small, a
time period for treatment is long.
[0048] By the way, as a method for reducing dislocations caused in
the nitride semiconductor thick film, and ELO growth (Epitaxial
Lateral Overgrowth) method is proposed.
[0049] The conventional ELO growth method is exemplified in FIG.
19.
[0050] After an AlN buffer layer 321 is grown on a sapphire
substrate 320 as shown in FIG. 19A, a first GaN layer 322 is grown
as shown in FIG. 19B.
[0051] After that, a SiO.sub.2 film 325 is formed on the first GaN
layer 322 as shown in FIG. 19C, and a slit line of SiO.sub.2 is
formed in the [11-20] direction in mask treatment as shown in FIG.
19D.
[0052] Then, a second GaN layer 323 is grown again as shown in FIG.
19E.
[0053] Finally, the sapphire substrate 320 is separated as shown in
FIG. 19F. The second GaN layer 323 grows from a slit window, and
completely fills in the SiO.sub.2 line to become a flat film,
because a speed of a longitudinal growth in the [1-100] direction
is faster than a speed of growth in the [0001] direction. Although
dislocation curves in a longitudinal direction in accordance with
the longitudinal growth, and penetration dislocations on the
SiO.sub.2 line can be reduced, the penetration dislocations
concentrate on a part of the SiO.sub.2 slit window. Therefore, in
order to produce a device by selecting a region with small
penetration dislocation, it is only necessary to carry out mask
treatment on the SiO.sub.2 line.
[0054] However, since SiO.sub.2 is filled in, it is difficult to
carry out mask treatment on SiO.sub.2. Moreover, since curved
dislocations concentrate on a central portion on the SiO.sub.2
line, there arises a problem of inclination of crystal orientation
in a horizontal direction of the substrate, for example.
Furthermore, since crystal growth is carried out while SiO.sub.2 is
contained, diffusion of Si and oxygen atoms occurs. In addition,
the ELO growth method needs a complicated production process, and
therefore, brings about cost increase.
[0055] As described above, according to the conventional production
methods, when the substrate made of a material of different kind
from nitride semiconductor and the nitride semiconductor thick film
are separated, stress resulting from differences in lattice
constants and the coefficient of thermal expansion causes a warp
and cracks on the produced nitride semiconductor apparatus.
Moreover, the production process is complicated in the ELO growth
that reduces dislocations, and it is difficult to keep away from a
portion on the which penetration dislocation density concentrates,
and to carry out the mask treatment on contained SiO.sub.2.
Furthermore, there arises a problem of inclination of crystal
orientation in a horizontal direction of the substrate because of
curved dislocation, for example.
SUMMARY OF THE INVENTION
[0056] Accordingly, the present invention was made in consideration
of the aforementioned problems, and an object thereof is to provide
excellent semiconductor apparatus and production methods with a
small lattice defect by which a good characteristic can be
expected.
[0057] Another object of the invention is to provide a
semiconductor apparatus and production methods which, for example,
at a time of growing the nitride semiconductor on the (0001) plane
of a ZrB.sub.2 single crystal substrate, makes electric resistance
from the nitride semiconductor to the substrate to be small and
increases crystalline quality of the nitride semiconductor
grown.
[0058] Still another object of the invention is to provide a
semiconductor apparatus and production methods to reduce unevenness
appearing on surface shape thereof in the case of growing a nitride
semiconductor on the (0001) plane of the diboride single crystal
substrate by using an AlN buffer layer, it is desired to reduce
unevenness appearing on a surface shape thereof.
[0059] Still another object of the invention thereof is to, in a
nitride semiconductor apparatus, avoid a warp and cracks of the
apparatus and decrease dislocation density, and to provide a
nitride semiconductor apparatus and production methods which are
uniform.
[0060] The invention provides a semiconductor apparatus
comprising:
[0061] a substrate made of a diboride single crystal expressed by a
chemical formula XB.sub.2, in which X includes at least one of Ti,
Zr, Nb and Hf;
[0062] a semiconductor buffer layer formed on a principal surface
of the substrate and made of Al.sub.yGa.sub.1-yN (0<y.ltoreq.1);
and
[0063] a nitride semiconductor layer formed on the semiconductor
buffer layer, including at least one kind or plural kinds selected
from among 13 group elements and As.
[0064] Further, the invention provides a semiconductor apparatus
comprising:
[0065] a substrate made of a diboride single crystal expressed by a
chemical formula XB.sub.2, in which X includes at least one of Ti,
Zr, Nb and Hf;
[0066] a semiconductor buffer layer formed on a principal surface
of the substrate and made of (AlN).sub.x(GaN).sub.1-x
(0<x.ltoreq.1); and
[0067] a nitride semiconductor layer formed on the semiconductor
buffer layer, including at least one kind or plural kinds selected
from among 13 group elements and As.
[0068] 0.1.ltoreq.x.ltoreq.1.0 is preferable.
[0069] Consequently, diffusion of B, which is a main element
contained in a diboride single crystal substrate, and formation of
a nitride semiconductor containing B on an interface between the
substrate and a nitride semiconductor are avoided, and it is
possible to obtain a nitride semiconductor with a small crystal
defect, which is good quality and excellent.
[0070] In the invention, the substrate if of ZrB.sub.2 or
TiB.sub.2.
[0071] In the invention, the substrate is a solid solution
containing one or a plurality of impurity elements of 5 atom % or
less, the one or a plurality of impurity elements being selected
from a group consisting of Ti, Cr, Hf, V, Ta and Nb when the
substrate is of ZrB.sub.2, or selected from a group consisting of
Zr, Cr, Hf, V, Ta and Nb when the substrate is of TiB.sub.2.
[0072] In the invention, the thickness of the semiconductor buffer
layer made of (AlN).sub.x(GaN).sub.1-x is within a range of 10 to
100 nm.
[0073] In the invention, x of the semiconductor buffer layer made
of (AlN).sub.x(GaN).sub.1-x is 0.1.ltoreq.x.ltoreq.1.
[0074] In the invention, x of the semiconductor buffer layer made
of (AlN).sub.x(GaN).sub.1-x is 0.4.ltoreq.x.ltoreq.0.6.
[0075] In the invention, an angle .theta.1 formed by a normal line
of the principal surface of the substrate and a normal line of the
(0001) plane of the substrate is
0.degree..ltoreq..theta.1.ltoreq.5.degree..
[0076] Further, the invention provides a method for growing a
nitride semiconductor, comprising:
[0077] on a substrate of a diboride single crystal expressed by a
chemical formula XB.sub.2, in which X includes at least one of Ti,
Zr, Nb and Hf, growing Al.sub.yGa.sub.1-yN layer (0<y.ltoreq.1)
from vapor phase, and subsequently, growing a nitride semiconductor
layer including at least one kind selected from among 13 group
elements and As from vapor phase.
[0078] Further, the invention provides a method for growing a
nitride semiconductor, comprising:
[0079] on a substrate of a diboride single crystal expressed by a
chemical formula XB.sub.2, in which X includes at least one of Ti,
Zr, Nb and Hf, growing an (AlN).sub.x(GaN.sub.1-x layer
(0<x.ltoreq.1) from vapor phase within a temperature range of
more than 400.degree. C. and less than 1100.degree. C. by an MOVPE
method, and subsequently, growing a nitride semiconductor layer
including at least one kind selected from among 13 group elements
and As from vapor phase.
[0080] In the invention, the thickness of the
(AlN).sub.x(GaN).sub.1-x layer is within a range of 10 to 100
nm.
[0081] According to the invention, as in the aforementioned
structure, as a result of growing an (AlN).sub.x(GaN).sub.1-x layer
(0<x.ltoreq.1) from vapor phase on the diboride single crystal
substrate by an MOVPE method, and preferably, defining the
thickness of this layer within a range of 10 nm to 100 nm,
resistivity of (AlN).sub.x(GaN).sub.1-x becomes lower than that of
AlN, with the result that resistance from the nitride semiconductor
to the substrate is decreased, and quality is increased because an
a-axis lattice constant of (AlN).sub.x(GaN).sub.1-x is closer to an
a-axis lattice constant of the (0001) plane of the ZrB.sub.2 single
crystal than AlN. 0.1.ltoreq.x.ltoreq.1.0 is preferable. When x is
less than 0.1, an (AlN).sub.x(GaN).sub.1-x layer formed on the
substrate made of XB.sub.2 becomes to be exfoliated easily.
[0082] Further, it is possible to decrease resistance of
(AlN).sub.x(GaN).sub.1-x by doping, with the result that it is
possible to reduce resistance from the nitride semiconductor to the
substrate.
[0083] As described above, according to the invention, in the case
of growing a nitride semiconductor layer which contains at least
one selected from among B, Al, Ga, In and Ti on the (0001) plane of
the diboride single crystal substrate expressed by a chemical
formula XB.sub.2, in which X contains at least one of Ti, Zr, Nb
and Hf, by the MOVPE method, by growing the
(AlN).sub.x(GaN).sub.1-x layer (0<x.ltoreq.1) between the
diboride single crystal substrate and the nitride semiconductor
layer at 400.degree. C. to 1100.degree. C., and preferably, growing
to film thickness of 10 nm to 100 nm, crystalline quality of the
nitride semiconductor is increased as apparent from that a rocking
curve half value width of (0002) plane omega scan by an x-ray
diffraction method is a value smaller than 1000 seconds. Therefore,
according to the invention, a characteristic and the yield of a
device such as a light emitting diode produced on the ZrB.sub.2
single crystal substrate are increased.
[0084] Further, according to the invention, serial resistance is
decreased because resistivity of (AlN).sub.x(GaN).sub.1-x is lower
than that of AlN. Furthermore, it is possible to decrease
resistance of (AlN).sub.x(GaN).sub.1-x by doping, so that it is
possible to reduce resistance from the nitride semiconductor to the
substrate. Consequently, it is possible to decrease a driving
voltage of a light emitting diode of type shown in FIG. 3, which
makes it possible to obtain a large number from one wafer.
[0085] In the invention, the semiconductor buffer layer is AlN.
[0086] In the invention, the thickness of the semiconductor buffer
layer made of AlN is 10 to 250 nm.
[0087] In the invention, an angle .theta.1 formed by a normal line
of the principal surface of the substrate and a normal line of the
(0001) plane of the substrate is
0.degree..ltoreq..theta.1.ltoreq.0.55.degree..
[0088] Further, the invention provides a method for growing a
nitride semiconductor, comprising:
[0089] on the (0001) plane of a substrate of a diboride single
crystal expressed by a chemical formula XB.sub.2, in which X
includes at least one of Ti, Zr, Nb and Hf, growing an AlN layer
from vapor phase so that a deviation angle of a normal line of a
surface of the substrate from a direction of the [0001] becomes
0.55 degrees or less, and subsequently, growing a nitride
semiconductor layer including at least one kind selected from among
13 group elements and As from vapor phase.
[0090] In the invention, the thickness of the AlN layer is within a
range of 10 to 250 nm.
[0091] As described above, according to the invention, in the case
of growing the nitride semiconductor layer containing at least one
selected from among B, Al, Ga, In and Ti on the (0001) plane of the
diboride single crystal substrate expressed by the chemical formula
XB.sub.2, in which X contains at least one of Ti, Zr, Nb and Hf, by
the MOVPE method, by using a method of growing an AlN layer of 10
nm to 100 nm on the (0001) plane of the diboride single crystal
substrate at 800.degree. C. or less and thereafter growing the
nitride semiconductor layer containing at least one selected from
among B, Al, Ga, In and Ti, wherein a substrate in which a
deviation angle of a normal line on a surface of the diboride
single crystal substrate from a [0001] direction of diboride
crystal is 0.55.degree. degrees or less is used, it is possible to
grow a nitride semiconductor layer which has a smooth surface.
[0092] As mentioned above, in a constitution that the buffer layer
is made of AlN, the deviation angle .theta.1 is selected less than
0.55.degree.. Thereby, it is possible to prevent an undesirable
scaly asperity from being formed on a surface of the nitride
semiconductor layer formed on the buffer layer.
[0093] The thickness of the semiconductor buffer layer made of AlN
is selected in a range of 10 nm to 250 nm. In the case of less than
10 nm, the effect of the semiconductor buffer layer is insufficient
and the scaly asperity exists on the surface of the nitride
semiconductor layer formed on the semiconductor buffer layer. In
the case of larger than 250 nm, film quality of the nitride
semiconductor layer formed on the semiconductor buffer layer
becomes bad, and a crystal layer as the nitride semiconductor layer
formed on the semiconductor buffer layer, which crystal layer is
formed above a crystal plane of the substrate via the semiconductor
buffer layer, degrades.
[0094] Therefore, by the use of the invention, a device such as a
light emitting diode produced on the ZrB.sub.2 single crystal
substrate can be produced on a smooth plane, and a characteristic
and the yield thereof are increased. The thickness of the
semiconductor buffer layer is so selected as to become thinner than
the thickness of a crystal layer made of nitride semiconductor
formed on the semiconductor buffer layer first.
[0095] In the invention, the substrate is eroded and removed by
etching.
[0096] Further, the invention provides a method for producing a
semiconductor apparatus, comprising:
[0097] eroding and removing a diboride single crystal substrate of
a semiconductor apparatus obtained by the aforementioned method for
growing nitride semiconductors by etching.
[0098] Further, the invention provides a method for producing a
semiconductor apparatus, comprising the steps of:
[0099] carrying out crystal growth of a nitride semiconductor layer
on one principal surface of a single crystal substrate of a
hexagonal crystal symmetry having electrical conductivity; and
[0100] eroding and removing the single crystal substrate by
etching.
[0101] In the invention, the single crystal substrate is a
substrate of a diboride single crystal expressed by XB.sub.2, in
which X includes at least one of Zr and Ti.
[0102] In the invention, in growing the nitride semiconductor layer
from vapor phase, a nitride semiconductor layer grown firstly is an
Al.sub.xGa.sub.1-xN layer (0<x.ltoreq.1).
[0103] In the invention, a mixed solution of at least nitric acid
and hydrofluoric acid is used for the etching.
[0104] As described above, according to a method for producing a
semiconductor apparatus of the invention, by going through the step
of carrying out crystal growth of a nitride semiconductor layer on
one principal surface of a single crystal substrate of a hexagonal
symmetry structure having electrical conductivity, and the step of
eroding and removing the single crystal substrate by etching, it is
possible to avoid a warp and cracks of the semiconductor apparatus,
decrease dislocation density, curve dislocation, prevent
inclination of crystal orientation in a horizontal direction of the
semiconductor apparatus, and thereby provide a nitride
semiconductor apparatus in which in-plane uniformity of dislocation
density is good.
BRIEF DESCRIPTION OF THE DRAWINGS
[0105] Other and further objects, features, and advantages of the
invention will be more explicit from the following detailed
description taken with reference to the drawings wherein:
[0106] FIG. 1 is a sectional view schematically explaining a
semiconductor apparatus of the invention;
[0107] FIG. 2 is a sectional view showing a diboride single crystal
substrate (chemical formula XB.sub.2) of the semiconductor
apparatus shown in FIG. 1;
[0108] FIGS. 3A and 3B are crystal structural views of XB.sub.2
respectively;
[0109] FIGS. 4A and 4B are crystal structural views of XB.sub.2
respectively;
[0110] FIG. 5 is a view showing a surface state B of GaN film;
[0111] FIG. 6 is a view showing a surface state A of GaN film;
[0112] FIG. 7 is a graph showing a relation between an angle formed
by a normal line of a principal surface of the substrate and a
normal line of the (0001) plane and a surface state;
[0113] FIG. 8 is a schematically sectional view of the
semiconductor apparatus in which a nitride semiconductor layer is
formed on a ZrB.sub.2 single crystal substrate;
[0114] FIG. 9 is a view showing of photographs of surfaces of GaN
films;
[0115] FIG. 10 is a line view showing a relation between film
thickness of (AlN).sub.x(GaN).sub.1-x and X-ray half value
width;
[0116] FIG. 11 is a schematically sectional view of the
semiconductor apparatus in which a nitride semiconductor layer is
formed on a ZrB.sub.2 single crystal substrate;
[0117] FIG. 12 is a line view showing a relation between an off
angle of a substrate surface and surface state;
[0118] FIG. 13 is a view showing a surface state (surface state B)
of GaN film;
[0119] FIG. 14 is a view showing a surface state (surface state A)
of GaN film according to the invention;
[0120] FIGS. 15A thorough 15F are process views showing a method
for producing a semiconductor substrate of the invention;
[0121] FIG. 16 is a crystal structural view of diaboride single
crystal described as ZrB.sub.2;
[0122] FIG. 17 is a sectional view explaining a conventional
semiconductor apparatus;
[0123] FIG. 18 is a schematically sectional view of the
semiconductor apparatus in which a nitride semiconductor layer is
formed on a sapphire substrate; and
[0124] FIGS. 19A through 19F are process views showing a method for
producing a conventional semiconductor substrate.
DETAILED DESCRIPTION
[0125] Now referring to the drawings, preferred embodiments of the
invention are described below.
[0126] FIG. 1 is a sectional view showing a semiconductor apparatus
according to one embodiment of the invention. FIG. 2 is a sectional
view showing a diboride single crystal substrate (chemical formula
XB.sub.2) 10 of semiconductor apparatus shown in FIG. 1. A normal
line of a principal surface 34 of the substrate 10 is inclined with
respect to a crystal axis 32 which is a [0001] axis perpendicular
to a (0001) plane 31 of the substrate 10, by an angle .theta.1,
which is larger than or equal to 0.degree. and less than or equal
to 5.degree. (0.degree..ltoreq..theta- .1.ltoreq.5.degree.). That
is, it is preferable that the diboride single crystal substrate
(chemical formula XB.sub.2) 10 of the present invention is the
substrate 10 such that the (0001) plane 31 or a plane obtained by
inclining the (0001) plane 31 by 0.degree. or more and 5.degree. or
less in an arbitrary direction is defined as the principal surface
34. In order to make crystalline quality of a nitride semiconductor
layer 11 through 16 grown on the substrate 10 to be fine, and
obtain a semiconductor apparatus which has a more excellent
characteristic, an angle .theta.1 formed by a normal line 33 of a
principal surface 34 of the substrate and a normal line 32 of the
(0001) plane 31 is set to 0.degree. or more and less than
1.7.degree. (0.degree..ltoreq..theta.1.lt- oreq.1.7.degree.).
Preferably, the angle is set to 0.degree. or more and less than
0.7.degree. (0.degree..ltoreq..theta.1<0.7.degree.). Further, a
(01-10) plane, a (11-20) plane, a (01-12) plane and so on, other
than the (0001) plane, can also be used as the growth principal
plane. Symbol `-` of `-1` and `-2` by Miller index expression
represents an inverse (bar) symbol, and the same applies in the
following description.
[0127] In specific, TiB.sub.2 and ZrB.sub.2 wherein X is Ti and Zr
have differences in lattice constant of 2% or less in any
composition of the nitride semiconductor made of Al, Ga and N which
are elements constituting the semiconductor buffer layer. For
example, in a semiconductor apparatus in which the nitride
semiconductor is AlN and the substrate is constituted by ZrB.sub.2,
referring to Table 1 mentioned above, the difference in lattice
constant is (3.1696-3.1114)/3.1114=1.9%. Thus, they become a
combination of extremely high matching. The lattice constant of the
nitride semiconductor containing Al or Ga and N, namely,
Al.sub.yGa.sub.1-yN (0<y.ltoreq.1), is in a range of 3.1114 to
3.1860 with reference to Table 1 mentioned above. According to the
invention, it is enough that at least one of the elements of Ti and
Zr on diaboride single crystal substrate is contained, and both the
elements of Ti and Zr may be contained.
[0128] A crystal structure of XB.sub.2 is a hexagonal symmetry
structure referred to as an AlB.sub.2 structure as shown in FIGS.
3A, 3B. This structure is similar to a wurtzite structure of a GaN
crystal shown in FIGS. 4A, 4B. In specific, regarding a match
relation of crystal lattice between the (0001) plane of an XB.sub.2
crystal of Ti or Zr and GaN or AlN, as shown in Table 1, a
difference in lattice constant between TiB.sub.2 or ZrB.sub.2 and
GaN or AlN is 2% or less in any combination, and therefore, they
can be considered as a combination of extremely high matching.
[0129] For forming semiconductor buffer layer in crystal growth, a
molecular beam epitaxy (MBE) method, a metalorganic vapor phase
epitaxy (MOCVD) method, a hydride vapor epitaxy (HVPE) method, a
sublimation method and the like are used. Moreover, it is possible
to appropriately combine the aforementioned growing methods as
well. For example, it is possible to use the MBE method, by which
it is possible to grow while controlling a surface state, for
initial epitaxy growth, and use the HVPE method, by which it is
possible to grow at high speeds, for a thick GaN thin film
required.
[0130] Next, after a buffer layer 11 is formed, a nitride
semiconductor which contains a 13 group (former IIIB group) element
to be aimed is formed. Here, crystal growth of the nitride
semiconductor is carried out at growth temperatures of 700.degree.
C. to 900.degree. C. At his moment, B, which is a main element
contained in the diboride single crystal substrate 10, diffuses
from a substrate side into the nitride semiconductor of the buffer
layer 11.
[0131] In the invention, a semiconductor buffer layer composed of
at least AlGaN is used as the buffer layer 11. In the nitride
semiconductor, an interatomic distance of AlN is smaller than
interatomic distances of InN and GaN. Therefore, crystal bond in
AlN is stronger than those in InN and GaN, and diffusion of B from
the diboride single crystal substrate in AlN is less than in InN
and GaN.
[0132] Further, as shown in aforementioned Table 1, InN and InGaN
largely mismatch with the diboride single crystal substrate in
lattice constants, and therefore, in the case of using InN and
InGaN as the buffer layers and carrying out crystal growth directly
on the substrate, a lattice defect or the like occurs. On the
contrary, in the invention, it is preferable since AlGaN matches
with the diboride single crystal substrate in lattice
constants.
[0133] Further, in specific, the nitride semiconductor containing
the 13 group element contains one or more selected from among Ga,
Al, In, H and Ti and moreover may contain As which is the 15 group
element.
[0134] Regarding the diboride single crystal, assume one or more
kinds of impurity elements selected from among Cr, Hf, V, Ta and
Nb, which are 4 group to 6 group (former IVA group to VIA group)
elements, are solid solutions of 5 atom % or less. This is because
when the impurity elements are more than 5 atom %, a value of
physical property shown in Table 1 and a value of specific
resistance of the substrate vary, which is not preferable. It is
possible by containing Cr of 5 atom % or less to expect an effect
of inhibiting growth of crystal grains of the nitride semiconductor
layer, and therefore, it is preferable for forming a good layer
without occurrence of cracks or the like.
[0135] In this way, according to the invention, it is possible to
avoid diffusion of B, which is a main element contained in the
diboride single crystal substrate, and formation of a nitride
semiconductor containing B on an interface between the substrate
and the nitride semiconductor, and it is possible to obtain a good
quality nitride semiconductor with a small lattice defect, and
thus, it is possible to obtain a semiconductor device which has an
excellent characteristic.
[0136] Besides, a nitride semiconductor apparatus (a light emitting
diode) which contains the 13 group element shown in FIG. 1 will be
described as an embodiment of the invention.
[0137] A GaN layer is grown on the (0001) plane of a substrate 10
of ZrB.sub.2 by the molecular beam epitaxy (MBE) method. On a
ZrB.sub.2 single crystal substrate 10 of the (0001) plane
orientation, AlGaN of a buffer layer 11, which is a semiconductor
buffer layer, and crystal growth of nitride semiconductor 12
through 16 to be aimed is carried out on the buffer layer 11 by the
MBE method in order. In high vacuum, a temperature of the ZrB.sub.2
substrate 10 is increased to 800.degree. C. and an Al molecular
beam, a Ga molecular beam and active nitrogen supplied from a high
frequency excitation plasma cell are supplied to start crystal
growth.
[0138] Here, one conductive type, which is one of p type and n
type, of semiconductor contact layer 12 on the buffer layer 11 is
made of n-GaN, for example. The one conductive type of
semiconductor contact layer 12 contains approximately
1.times.10.sup.17 to 10.sup.19 atoms/cm.sup.3 of one conductive
type of semiconductor impurity, such as silicon. Furthermore, a one
conductive type of semiconductor layer 13 on the layer 12 is a
cladding layer made of n-AlGaN, for example. Moreover, a one
conductive type of semiconductor layer 13 contains approximately
1.times.10.sup.16 to 10.sup.19 atoms/cm.sup.3 of one conductive
type of semiconductor impurity, such as silicon.
[0139] A light emitting layer 14 is formed on the layer 13 and made
of GaN, InGaN or the like. Here, the light emitting layer 14 may
have a quantum well structure, a quantum thin line structure of a
quantum dot structure.
[0140] A reverse conductive type, which is the other of p type and
n type, of semiconductor layer 15 is formed on the layer 14 and is
a cladding layer made of p-AlGaN or the like, and contains
approximately 1.times.10.sup.16 to 10.sup.19 atoms/cm.sup.3 of
impurity which changes into a reverse conductive type, such as Mg
or Zn. Here, this layer may contain a small amount of one or more
selected from among In, P, As and the like.
[0141] A reverse conductive type of semiconductor contact layer 16
is formed on the layer 15 is made of ZrB.sub.2, for example, and
contains approximately 1.times.10.sup.19 to 10.sup.20
atoms/cm.sup.3 of impurity which changes into a reverse conductive
type, such as Mg or Zn. A portion from the layer 16 to an upper
region of the layer 12 is partly etched and removed.
[0142] Thereafter, one conductive type of electrode 18 on the layer
12 is made of one or more selected from among Au, Al, Cr, Ti and
Ni. Moreover, a reverse conductive type of electrode 17 on the
layer 16 is made of one or more selected from among Au, Al, Cr, Ti
and Ni as well.
[0143] In this way, this embodiment also realizes an excellent
semiconductor apparatus with a small lattice defect, by which a
good characteristic can be expected. Here, a layer structure of the
semiconductor apparatus is not restricted to the structure shown in
FIG. 1, and it may be a structure such that nitride semiconductor
layers 112 to 117 are formed on one principal surface of a
substrate 118 such as FIG. 8 described-after, one electrode 111 is
formed on the nitride semiconductor layer 112 and another electrode
119 is formed on another principal surface of the substrate
118.
[0144] Next, in the respective embodiments in FIGS. 1 to 4, the
result of a study of the principal surface 34 of the substrate 10
for preferably growing the nitride semiconductor layer and a most
suitable crystal plane 31 will be described.
[0145] Step 1A:
[0146] Firstly, several kinds of ZrB.sub.2 single crystal
substrates 10 which have different off angles (angles .theta.1
formed by the normal line 33 of the principal surface 34 of the
substrate 10 and the normal line 32 of the (0001) plane 31) were
prepared. The surface of ZrB.sub.2 was cleansed by the use of an
alkali solvent.
[0147] Step 2A:
[0148] Before the nitride semiconductor was grown, the substrate 10
was heated for three minutes in a hydrogen (H.sub.2) atmosphere (1
air pressure), and annealed at 1150.degree. C. for one minute.
[0149] Step 3A:
[0150] After that, temperature was decreased for five minutes, and
an AlGaN layer serving as a semiconductor buffer layer 11 was
grown. A growth temperature was 850.degree. C. and film thickness
was 20 nm at that moment. Further, ammonia (NH.sub.3),
trimethylaluminum (TMAI) and trimethylgallium (TMGa) were used as
source gases, the amounts of supplied NH.sub.3, TMAI and TMGa were
0.07 mol/min, 8 .mu.mol/min and 11 .mu.mol/min, respectively, and 7
slm of H.sub.2 was flown as a carrier gas. NH.sub.3 was supplied
from one minute before supply of TMA.
[0151] Step 4A:
[0152] Next, the temperature was increased to 1150.degree. C., and
GaN serving as the nitride semiconductor layer 12 was grown to
thickness of approximately 3 .mu.m. NH.sub.3 and TMGa were used as
source gases, and the amount of supplied TMGa and NH.sub.3 were 44
.mu.mol/min and 0.07 mol/min, respectively. Moreover, 3 slm of
H.sub.2 was flown as a carrier gas.
[0153] In microscopic observation of surfaces of the GaN films 12
after growth, surfaces with much unevenness as shown in FIG. 5
(surface state B) and surfaces of smooth states so shown in FIG. 6
(surface state A) were observed, respectively.
[0154] A relation between the off angles of the ZrB.sub.2 single
crystal substrates 10 and the surface states of the grown films is
shown in FIG. 7. Here, an angle O2 of deviation from a [0001]
crystal axis 32 of the normal line 33 of the surface 34 of the
substrate 10 in the [10 10] direction, an angle O3 of deviation of
the same in the [11-20] direction, and the sum of squares of these
deviation angles (-O2.sup.2+.theta.3.sup.- 2) are shown by the
lines 36, 37 and 38, respectively. All substrates that the sums of
squares of the deviation angles were less than 0.7.degree. kept
good surface states of the surface state A.
[0155] Regarding substrates that the sums of squares of the
deviation angles were 0.7.degree. or more and less than
1.7.degree., both the surface state A and the surface state B were
observed. It can be concluded that this results from variation of
operations in the growth experiment and states of the device, and
it is believed that the surface state A can be reproduced by
reducing the variation. In the case of substrates that the sums of
squares of the deviation angles were 1.7.degree. or more, almost
all of the substrates kept the surface state B.
[0156] From these results, it became clear that in order to grow a
nitride semiconductor layer containing one or more selected from
among 13 group elements in a suitable crystal state and thus obtain
a semiconductor apparatus which is excellent in a characteristic
such as the efficiency of light emission, it is more desirable to
make an angle .theta.1 formed by the normal line 33 of the
principal surface 34 of the substrate 10 and the normal line 32 of
the (0001) plane 31 to be 0.degree. or more and less than
1.7.degree., and that in the case of forming a good quality nitride
semiconductor layer, a crystal angle of the principal surface of
the substrate has the aforementioned allowance range.
[0157] Although (1) a GaN growth layer is formed by the use of a
ZrH.sub.2 substrate in the aforementioned embodiment, it is also
possible to form a nitride semiconductor layer containing a 13
group element, such as a GaN growth layer, on (2) a single crystal
substrate made of TiB.sub.2 or (3) a single crystal substrate
formed of solid solutions of ZrB.sub.2 and TiB.sub.2 in the same
manner as described above, and it is possible to appropriately
change and embody the invention within the scope of the
invention.
[0158] The steps in the invention will be sequentially described
below.
[0159] FIG. 8 is a sectional view showing a semiconductor apparatus
of a light emitting diode according to another embodiment of the
invention.
[0160] According to the invention, a diboride single crystal
substrate 118 expressed by a chemical formula XB.sub.2 is used, in
which X contains at least one of Ti, Zr, Nb and Hf.
[0161] This substrate 118 can be to a ZrB.sub.2 substrate, a
TiB.sub.2 substrate or a Zr.sub.xTi.sub.1-xB.sub.2 substrate,
however, a method for carrying out vapor phase growth on the
ZrB.sub.2 substrate by the MOVPE method will be described in this
embodiment.
[0162] Step 1B:
[0163] The ZrB.sub.2 substrate 118 is cleansed by the use of an
alkali solvent.
[0164] Step 2B:
[0165] Before growth of a nitride semiconductor, the ZrB.sub.2
substrate 118 is heated for three minutes in a hydrogen (H.sub.2)
atmosphere (1 air pressure), and annealed at a temperature of
1150.degree. C. for one minute.
[0166] Step 3B:
[0167] After that, temperature was decreased for about five
minutes, and an (AlN).sub.x(GaN).sub.1-x buffer layer 117 is
deposited.
[0168] It is good to set a growth temperature T within a
temperature range of 400.degree. C.<T<1100.degree. C. at this
moment, and then grow the (AlN).sub.x(GaN).sub.1-x layer
(0<x.ltoreq.1.0) from vapor phase as the buffer layer 117.
[0169] Further, it is good to set thickness of the
(AlN).sub.x(GaN).sub.1-- x layer 117 within a range of 10 to 100
nm. According to such vapor phase growth, used source gases are
ammonia (NH.sub.2), trimethylaluminum (TMA) and trimethylgallium
(TMG), the amounts of supplied TMA and TMG are, for example, 7
.mu.mol/min and 11 .mu.mol/min, respectively, and 7 slm of H.sub.2
is flown as a carrier gas. 0.07 mol/min of NH.sub.3 is supplied
from one minute before supply of TMA and TMG.
[0170] Step 4B:
[0171] Next, the substrate is heated to 1150.degree. C., for
example, and an n-GaN contact layer 116 is grown to approximately 3
.mu.m by the MOVPE method, for example. Used source gases are
NH.sub.3 and TMG, and the amounts of supplied TMG and NH.sub.3 are,
for example, 44 .mu.mol/min and 0.07 mol/min, respectively. 3 slm
of H.sub.2 is flown a carrier gas.
[0172] Further, a cladding layer 115 made of n-AlGaN, a light
emitting layer 134 made of GaN, InGaN or the like, a cladding layer
113 made of p-AlGaN, a contact layer 112 made of p-GaN and a
p-electrode 111 are formed, and an n-electrode 119 is formed on a
back surface of the substrate 118.
[0173] In the case of using the ZrB.sub.2 single crystal substrate
118, since such a structure that one electrode 119 is arranged on
the back surface of the substrate 110 is possible, there is an
advantage that device area can be reduced.
[0174] Next, an experimental example which was carried out by the
inventors will be described.
[0175] Although the aforementioned steps 1B to 4B were executed
sequentially, according to the example, a deposition temperature
was changed among 400.degree. C., 725.degree. C., 850.degree. C.
and 1100.degree. C. and film thickness was changed among 10 nm, 20
nm, 50 nm and 80 nm at step 3B, whereby various kinds of samples
were produced.
[0176] Source gases were ammonia (NH.sub.3), trimethylaluminum
(TMA) and trimethylgallium (TMG), the supply amounts of TMA and TMG
were 7 .mu.mol/min and 11 .mu.mol/min, respectively, and 7 slm of
H.sub.2 was flown as a carrier gas. 0.07 mol/min of NH.sub.3 was
supplied from one minute before supply of TMA and TMG. Next, at
step 4B, the substrate was heated to 1150.degree. C. and GaN was
grown to approximately 3 .mu.m. Used source gases were NH.sub.3 and
TMG, and the amounts of supplied TMG and NH.sub.3 were 44
.mu.mol/min and 0.07 mol/min, respectively. 3 slm of H.sub.2 was
flown as a carrier gas.
[0177] The results of the experiment on the various kinds of
samples obtained by changing the deposition temperature and the
film thickness at step 3B as described above are shown in FIG.
9.
[0178] FIG. 9 shows photographs of surfaces of GaN films in the
case of growing the (AlN).sub.x(GaN).sub.1-x layer 117 at
temperatures of 400.degree. C. or less, 850.degree. C., and
1100.degree. C. or more.
[0179] Further, a value of x, and a rocking curve half value width
of (0002) plane omega scan by an X-ray diffraction method are shown
on the right of description of the temperature given in each of the
FIG. 9 (refer to Table 2).
[0180] The value of x is data obtained by measuring the
(AlN).sub.x(GaN).sub.1-x films grown under the same condition, by
an EDX method.
2TABLE 2 Half Deposition value Working example/ temperature T width
Comparative example (.degree. C.) x (seconds) (1) Comparative
example 1 400 or less 0.8 -- (2) Working example 1 725 0.6 760 (3)
Working example 2 850 0.5 596 (4) Working example 3 925 0.4 -- (5)
Comparative example 2 1100 or more 0.25 873
[0181] As apparent from these results, the GaN film was not formed
at 400.degree. C. or less, and the GaN film had a hexagonal surface
shape at 1100.degree. C. or more. The GaN films had smooth surfaces
at 850.degree. and 725.degree. C. Here, the rocking curve half
value width was 1000 seconds or less in every case that the growth
temperature T was more than 400.degree. C. and less than
1100.degree. C.
[0182] A relation between film thickness of the
(AlN).sub.x(GaN).sub.1-x layer deposited at 850.degree. C. and the
rocking curve half value width of the (0002) plane omega scan by
the X-ray diffraction method is shown in FIG. 10. In a case where
the layer was grown in a setting such that the film thickness was
less than 10 nm, a surface state thereof became the same as that at
deposition temperatures 400.degree. C. or less. It can be read from
the graph that the half value width becomes 1000 seconds or less
between the film thickness 10 nm and 100 nm.
[0183] Further, for comparison, in a comparative example 3, after
the AlN layer was deposited on the ZrB.sub.2 substrate at
600.degree. C., GaN was grown to approximately 3 .mu.m at
1150.degree. C. Source gases used at the time of growing the AlN
layer were NH.sub.3 and TMA. The amounts of supplied TMA and
NH.sub.3 as the sources were 3.5 .mu.mol/min and 0.07 mol/min,
respectively, and 2 slm of H.sub.2 was flown as a carrier gas.
Conditions other than the above were the same as those of the
(AlN).sub.x(GaN).sub.1-x layer. The rocking curve half value width
of the (0002) plane omega scan by X-ray diffraction was
approximately 1000 seconds.
[0184] Further, as a comparative example 4, instead of growing the
(AlN).sub.x(GaN).sub.1-x layer (0<x<1.0) from vapor phase,
the GaN layer was grown at 400.degree. C., and thereafter,
according to step 4B, the temperature was increased to 1150.degree.
C. to grow the GaN layer to approximately 3 .mu.m by the MOVPE
method.
[0185] According to such vapor phase growth of the GaN layer at a
deposition temperature 400.degree. C., conditions were the same as
those of the AlN layer except that the source was replaced from TMA
to TMG.
[0186] As a result of production in this manner in comparative
example 3, the GaN film exfoliated from the ZrB.sub.2 substrate
right away after growth.
[0187] Here, the invention is not restricted to the above
embodiment, and can be changed and improved in various manners
within the scope of the invention. For example, although the
ZrB.sub.2 substrate was used as the diboride single crystal
substrate, it was confirmed by an experiment that, in place of the
above substrate, a substrate such that a chemical formula is
XB.sub.2 and X is Ti, Nb or Hf or a combination thereof also takes
the operations and effects of the invention.
[0188] According to the invention, a diboride single crystal
substrate 218 expressed by a chemical formula XB.sub.2 is used, in
which X contains at least one of Ti, Zr, Nb and Hf.
[0189] This substrate 218 can be to a ZrB.sub.2 substrate, a
TiB.sub.2 substrate or a Zr.sub.xTi.sub.1-xB.sub.2 substrate,
however, a method for carrying out vapor phase growth on the
ZrB.sub.2 substrate 218 by the MOVPE method will be described in
this embodiment.
[0190] Step 1C:
[0191] The ZrB.sub.2 substrate 218 is cleansed by the use of an
alkali solvent.
[0192] Step 2C:
[0193] Before growth of a nitride semiconductor layer 217, the
ZrB.sub.2 substrate 218 is heated for three minutes in a hydrogen
(H.sub.2) atmosphere (1 air pressure), and annealed at a
temperature of 1150.degree. C. for one minute.
[0194] Step 3C:
[0195] After that, temperature is decrease for about five minutes,
and an AlN layer 217 which is low temperature buffer layer is
deposited.
[0196] At this moment, it is good to set a growth temperature T
within a temperature range of 800.degree. C. or less, and grow the
AlN layer 217 from vapor phase. Here, in the present embodiment,
the temperature is set to 600.degree. C. Further, it is good to set
the thickness of the AlN layer 217 within a range of 10 nm to 250
nm. Here, in the embodiment, the thickness is set to 20 nm.
[0197] According to such vapor phase growth, ammonia (NH.sub.3),
trimethylaluminum (TMA) and trimethylgallium (TMG) are used as
source gases, the amounts of supplied NH.sub.3 and TMA are, for
example, 0.07 mol/min and 3.5 .mu.mol/min, respectively, and 4 slm
of H.sub.2 is flown as a carrier gas. NH.sub.3 is supplied from one
minute before supply of TMA.
[0198] Step 4C:
[0199] Next, the substrate is heated to 1150.degree. C., for
example, and a p-GaN contact layer 216 is grown to approximately 3
.mu.m by the MOVPE method, for example. Used source gases are
NH.sub.3 and TMG, and the amounts of supplied TMG and NH.sub.3 are,
for example, 44 .mu.mol/min and 0.07 mol/min, respectively. 3 slm
of H.sub.2 is flown a carrier gas.
[0200] A cladding layer 215 made of n-GaN, a light emitting layer
214 made of InN, InGaN or the like, a cladding layer 213 made of
p-GaN, a contact layer 312 made of p-GaN are formed on the p-type
contact layer 216 in this order, and a p electrode 211 are further
formed. An n electrode 219 is formed on a back surface of the
substrate 218.
[0201] In observation of surfaces of the GaN films 216 after
growth, an uneven surface as shown in FIG. 13 (surface state H) and
a smooth surface as shown in FIG. 14 (surface state A) are
observed.
[0202] Then, a relation between an off angle .theta.1 of the
ZrB.sub.2 single crystal substrate 218 and a surface state of the
grown film is shown in FIG. 12. Here, an angle .theta.4 of
deviation of a normal line 33 (refer to aforementioned FIG. 12) on
a surface of a substrate 218 from a [0001] crystal axis 32 in the
[10-10] direction, an angle .theta.5 of deviation of the same in
the [11-20] direction, and the sum of squares
(-.theta.4.sup.2.vertline.O5.sup.2) thereof are shown by lines 236,
237 and 238 in FIG. 12, respectively. All substrates that the sums
of squares of the off angle are 0.35 degrees or less keep the
surface state A. In the case of substrates that the sum of squares
of the off angles are between 0.35 degree and 0.55 degrees, both
the surface state A and the surface state B are observed. It can be
concluded that this results from variation of operations in the
growth experiment and states of the device, and it is believed that
the surface state A can be reproduced by reducing the variation.
All substrates that the sums of squares of the off angles are 0.55
degrees or more keep the surface state B.
[0203] Here, the invention is not restricted to the above
embodiment, and can be changed and improved in various manners
within the scope of the invention. For example, although the
ZrB.sub.2 substrate was used as the diboride single crystal
substrate 218, it was confirmed by an experiment that, in place of
the above substrate, a substrate such that a chemical formula is
XB.sub.2 and X is Ti, Nb or Hf or a combination thereof also takes
the operations and effects of the invention.
[0204] FIG. 15 is a process view showing a method for producing a
nitride semiconductor device according to one embodiment of the
invention, and FIG. 16 shows a crystal structure of a single
crystal substrate 310 which has a hexagonal crystal symmetry.
[0205] Firstly, the step of crystal-growing a nitride semiconductor
layer 313 on one principal surface of the single crystal substrate
310 will be described.
[0206] The substrate 310 used in the invention is a diboride single
crystal substrate having a metallic or semimetallic characteristic
whose crystal structure is a hexagonal symmetry structure.
[0207] Preferably, in the case of using a diboride single crystal
substrate expressed by, for example, ZrB.sub.2 as the single
crystal substrate 310 is shown in FIG. 16, it is good to make the
(0001) plane to be the principal surface of the substrate.
[0208] Further, as compared with on insulating material, this
single crystal substrate 310 has the same level of electric
conductivity as semimetal because it is a high electric
conductor.
[0209] Regarding ZrB.sub.2 whose lattice constant a is 3.170 .ANG.,
as compared with GaN of wurtzite structure (a lattice constant:
a=3.189 .ANG.), a ration of a lattice mismatch thereof is 0.60%
(-3.189-3.170)/3.170), which is small, and a difference in the
coefficient of thermal expansion is 2.7.times.10.sup.-6/K, and
therefore, it achieves an extremely highly matching combination,
with the result that it is possible to obtain a good quality
nitride semiconductor such that a lattice defect is small and
stress between the substrate and the nitride semiconductor is
small.
[0210] In crystal growth of the buffer layer 311, a molecular beam
epitaxy (MBE) method, a metalorganic vapor Phase epitaxy (MOCVD)
method, a hydride vapor phase epitaxy (HVPE) method, a sublimation
method and the like are used.
[0211] Further, these growth methods may be used in combination.
For example, it is possible to use the MBE method or the MOCVD
method, by which it is possible to grow while controlling a surface
state, for initial epitaxy growth, and use the HVPE method, by
which it is possible to grow at high speeds, for a thick GaN thin
film 311 required.
[0212] Firstly, a buffer layer 311 is formed, and then, a nitride
semiconductor 312 and 313 of required layer structure are formed.
The buffer layer 311 and the nitride semiconductor 312 and 313 are
grown from vapor phase at growth temperatures of 600.degree. C. to
1100.degree. C.
[0213] According to such crystal growth, the substrate 310 is
heated by thermal conduction from a susceptor heated by a heater or
the like. Thermal energy from the susceptor is thermally conducted
to the single crystal substrate (ZrB.sub.2 substrate) 310 by
thermal contact. Moreover, thermal energy is emitted as thermal
radiation (infrared radiation) from the surface of the susceptor,
other than by the thermal contact. The single crystal substrate 310
made of a metallic or semimetallic substrate absorbs the thermal
radiation, and is heated. Being heated, Zr and B diffuse a little
into the nitride semiconductor 312 and 313.
[0214] Next, the step of eroding and removing the single crystal
substrate 310 by etching will be described.
[0215] By applying resist coat treatment to the whole surface of
the grown GaN thick film 313 by the use of a photoresist 314, and
etching with an etching solution having a selective etching
characteristic together with the ZrB.sub.2 substrate 310, the
ZrB.sub.2 substrate 310 is completely eroded. After that, by
exfoliating the photoresist 314 by the use of an exfoliation
solution, a semiconductor apparatus made of a GaN thick film 311,
312 and 313 are obtained.
[0216] Thus, according to the method for producing the
semiconductor apparatus of the invention, by going through the
steps as mentioned above, it is possible to produce a nitride
semiconductor apparatus with low dislocation density and without
curving dislocation, and it is possible to obtain a high quality
nitride semiconductor without inclining crystal orientation in the
horizontal direction of the substrate and without producing a
dislocation concentrating point within the plane of the
semiconductor apparatus.
[0217] Next, an embodiment of the invention such that a GaN
apparatus is obtained by the use of a ZrB.sub.2 single crystal
substrate 310 will be described.
[0218] In the process shown in FIG. 15, steps A to F are executed
sequentially.
[0219] Step 1D:
[0220] An example of forming and growing a buffer layer 311 made of
GaN on a substrate 310 of the (0001) plane of ZrB.sub.2 by using
both the MOCVD method and the HVPE method is shown.
[0221] On the ZrB.sub.2 single crystal substrate 310 of the (0001)
plane orientation, a buffer layer 311 is grown from vapor phase by
the MOCVD method.
[0222] As shown in FIG. 15A, the substrate 310 of the (0001) plane
of ZrB.sub.2 is set in an MOCVD growth furnace, and the temperature
of the ZrB.sub.2 substrate 310 is increased to 800.degree. C. in a
high vacuum to start crystal growth. After that, the temperature is
decreased to 600.degree. C. to grow the buffer layer 311. The
buffer layer 311 is made of AlGaN, whose raw materials are
trimethylaluminum (hereinafter referred to as TMA),
trimethylgallium (hereinafter referred to as TMG) and ammonia gas,
and grown to thickness of 0.1 .mu.m.
[0223] Step 2D:
[0224] As shown in FIG. 15B, the temperature of the substrate 310
is increased to 1050.degree. C., and a first semiconductor layer
312 is grown by the MOCVD method.
[0225] The first semiconductor layer 312 is made of GaN, whose raw
materials are TMG and ammonia gas, and grown to thickness of 1
.mu.m.
[0226] Step 3D:
[0227] The substrate 312 grown as FIG. 15B is taken out and moved
into an HVPE furnace, where a second semiconductor layer 313 is
grown as shown in FIG. 15C.
[0228] The HVPE method is appropriate for production of a thick
film of thickness of several score .mu.m to several hundred .mu.m,
because a growth speed in this method is higher than that in the
MOCVD method. The second semiconductor layer 313 is made of GaN,
whose raw sources are gallium chloride and ammonia gas, and grown
to 300 .mu.m.
[0229] Step 4D:
[0230] Then, the substrate grown as shown in FIG. 15C is taken out,
and a surface of the second GaN layer 313 is spin-coated by a
photoresist 314 and subjected to baking treatment as shown in FIG.
15D.
[0231] Step 5D:
[0232] After that, as shown in FIG. 15E, the ZrB.sub.2 substrate
310 is completely eroded by an etching solution composed of nitric
acid and hydrofluoric acid.
[0233] The etching solution composed of nitric acid and
hydrofluoric acid is excellent in selectivity, because a ratio of
an etching rate of GaN and an etching rate of ZrB.sub.2 is 100
times or more.
[0234] Step 6D:
[0235] After that, the photoresist 314 is exfoliated by an
exfoliation solution as shown in FIG. 15F, whereby a GaN
semiconductor apparatus formed by a GaN thick film 313 can be
obtained.
[0236] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The present embodiment are therefore to be considered in
all respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than by the
foregoing description and all changes which come within the meaning
and the range of equivalency of the claims are therefore intended
to be embraced therein.
* * * * *