U.S. patent application number 11/382254 was filed with the patent office on 2006-08-31 for group iii nitride based semiconductor substrate and process for manufacture thereof.
Invention is credited to Yuichi Oshima, Masatomo Shibata, Akira Usui.
Application Number | 20060191467 11/382254 |
Document ID | / |
Family ID | 28449231 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060191467 |
Kind Code |
A1 |
Usui; Akira ; et
al. |
August 31, 2006 |
GROUP III NITRIDE BASED SEMICONDUCTOR SUBSTRATE AND PROCESS FOR
MANUFACTURE THEREOF
Abstract
To provide a semiconductor substrate of a group III nitride with
a little warp, this invention provides a process comprising such
steps of: epitaxial-growing a GaN layer 33 with a GaN low
temperature grown buffer layer 32 upon a sapphire substrate 31;
removing the sapphire substrate 31, the GaN buffer layer 32 and a
small portion of the GaN layer 33 from the substrate taken out of a
growth reactor to obtain a self-supporting GaN substrate 35; and
after that, heat-treating the GaN substrate 35 by putting it into
an electric furnace under the NH.sub.3 atmosphere at: 1200.degree.
C. for 24 hours; which leads to a marked reduction of the warp of
the self-supporting GaN substrate 35 such that dislocation
densities of its obverse and reverse surface are 4.times.10.sup.7
cm.sup.-2 and 8.times.10.sup.5 cm.sup.-2, and thereby such a low
ratio of dislocation densities of 50 is well-controlled.
Inventors: |
Usui; Akira; (Tokyo, JP)
; Shibata; Masatomo; (Tsuchiura-shi, JP) ; Oshima;
Yuichi; (Tsuchiura-shi, JP) |
Correspondence
Address: |
HAYES, SOLOWAY P.C.
3450 E. SUNRISE DRIVE, SUITE 140
TUCSON
AZ
85718
US
|
Family ID: |
28449231 |
Appl. No.: |
11/382254 |
Filed: |
May 8, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10395766 |
Mar 24, 2003 |
|
|
|
11382254 |
May 8, 2006 |
|
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Current U.S.
Class: |
117/2 ; 117/3;
257/E21.108; 257/E21.131 |
Current CPC
Class: |
H01L 21/02458 20130101;
H01L 21/0265 20130101; H01L 21/0254 20130101; H01L 21/0242
20130101; H01L 21/02664 20130101; Y10T 428/265 20150115; C30B 33/00
20130101; C30B 29/406 20130101; C30B 29/403 20130101; H01L 21/02639
20130101; H01L 21/0262 20130101 |
Class at
Publication: |
117/002 ;
117/003 |
International
Class: |
H01L 21/322 20060101
H01L021/322; C30B 15/14 20060101 C30B015/14 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2002 |
JP |
2002-084963 |
Claims
1. A group III nitride based semiconductor substrate which is a
self-supporting substrate; wherein when a dislocation density of a
surface thereof on a side of a lower dislocation density is
designated n, and a dislocation density of a surface thereof on a
side of a higher dislocation density is designated n.sub.2, its
ratio of n.sub.2/n.sub.1 is less than 750.
2. A group III nitride based semiconductor substrate claimed in
claim 1, wherein n, is not higher than 1.times.10.sup.8
cm.sup.-2.
3. A group III nitride based semiconductor substrate which is a
self-supporting substrate; wherein when an edge dislocation density
of a surface thereof on a side of a lower edge dislocation density
is designated m.sub.1 and an edge dislocation density of a surface
thereof on a side of a higher edge dislocation density is
designated m.sub.2, its ratio of m.sub.2/m.sub.1 is less than
1000.
4. A group III nitride based semiconductor substrate claimed in
claim 3, wherein ml is not higher than 5.times.10.sup.7
cm.sup.-2.
5. A group III nitride based semiconductor substrate claimed in any
one of claims 1-4, wherein a thickness thereof is not less than 30
.mu.m but not greater than 1 mm.
6. A group III nitride based semiconductor substrate claimed in any
one of claims 1-5, wherein it comprises a layer made of GaN or
AlGaN.
7. A process for manufacturing a group III nitride based
semiconductor substrate; which comprises the steps of: forming a
group III nitride based semiconductor layer on top of a substrate
of a different material; separating said substrate of the different
material from said group III nitride based semiconductor layer; and
applying a treatment to reduce a dislocation density onto a surface
of said group III nitride based semiconductor layer which lies on a
side from which said substrate of the different material has been
separated.
8. A process for manufacturing a group III nitride based
semiconductor substrate; which comprises the step of forming a
group III nitride based semiconductor layer on top of a substrate
of a different material by epitaxial growth, and thereafter
separating said substrate of the different material from said group
III nitride based semiconductor layer; wherein a heat treatment is
carried out at a temperature not lower than 1150.degree. C. either
during the growth of said group III nitride based semiconductor
layer or after the growth of said group III nitride based
semiconductor layer.
9. A process for manufacturing a group III nitride based
semiconductor substrate according to claim 8, which further
comprises the step of applying a treatment to reduce a dislocation
density onto a surface of said group III nitride based
semiconductor layer which lies on a side from which said substrate
of the different material has been separated.
10. A process for manufacturing a group III nitride based
semiconductor substrate according to claim 7 or 9, wherein said
treatment to reduce a dislocation density comprises the step of
removing a region of said group III nitride based semiconductor
layer to a thickness not less than 100 Am from a side from which
said substrate of the different material has been separated.
11. A process for manufacturing a group III nitride based
semiconductor substrate according to claim 7, 9 or 10, wherein said
treatment to reduce a dislocation density comprises the step of
applying a heat treatment onto said group III nitride based
semiconductor layer at a temperature not lower than 1150.degree. C.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a group III nitride based
semiconductor substrate and a process for manufacture thereof.
BACKGROUND TO THE INVENTION
[0002] Since the nitride semiconductor material is known to have a
sufficiently large band gap and besides its inter-band transition
is of direct transition type, many investigations for utilizing the
nitride semiconductor material in the short wavelength light
emission device are underway. Furthermore as its saturation drift
velocity of electrons is high and the two-dimensional carrier gas
is available in their hetero-junction, the nitride semiconductor
material is also regarded to be highly applicable to the electron
device.
[0003] The nitride semiconductor layer to constitute these devices
can be obtained by epitaxial growth on a base substrate with the
vapor phase deposition method such as metal-organic vapor phase
epitaxy (MOVPE) method, molecular beam epitaxy (MBE) method or
hydride vapor phase epitaxy (HVPE) method. However, there is not
any base substrate that has a lattice constant matching with that
of this nitride semiconductor layer, and, therefore, a growth layer
of good quality is hard to acquire and the nitride semiconductor
layer obtained tends to contain numerous crystal defects. Because
these crystal defects are a very factor to hinder the improvement
of device performance, variety of approaches to decrease the
crystal defects within the nitride semiconductor layer have been so
far examined.
[0004] As one of the methods to obtain group III element nitride
based crystals containing a relatively small number of crystal
defects, there is known a method wherein a low temperature
deposition buffer layer is formed on a substrate of a different
material such as sapphire and thereon an epitaxial growth layer is
formed. In the crystal growth method using a low temperature
deposition buffer layer, deposition of AlN or GaN onto a sapphire
substrate or such is first applied around 500.degree. C. to form an
amorphous film or a continuous film containing in part,
poly-crystals. By heating this deposition up to about 1000.degree.
C., a part of the deposition is evaporated away and the remains are
converted into crystals to form crystal nuclei of high density.
Application of those as nuclei for crystal growth leads to GaN
layer of relatively high crystalline quality. Nevertheless, even
using the method comprising the step of forming the low temperature
deposition buffer layer, it still contains a considerable number of
crystal defects such as threading dislocations and vacant pipes,
and, thus, its crystalline quality is insufficient to provide such
high performance devices as currently required.
[0005] Alternatively, another technique in which a GaN substrate is
used as a substrate for crystal growth and thereon a semiconductor
multi-layered film for constructing a device section is formed has
been extensively studied. Such a GaN substrate for crystal growth
is referred to as a self-supporting GaN substrate, hereinafter.
Among techniques to prepare a self-supporting GaN substrate, the
ELO (Epitaxial Lateral Overgrowth) technique is widely known. The
ELO is a technique in which a mask layer having stripe openings is
formed on a base substrate and, the lateral growth is initiated
from the openings to attain a GaN layer with a few dislocations. In
Japanese Patent Application Laid-open No. 251253/1999, it is
proposed that a GaN layer is formed on a sapphire substrate using
this ELO technique, and thereafter the sapphire substrate is
removed by etching or such to prepare a self-supporting GaN
substrate.
[0006] Meanwhile, the FIELO (Facet-Initiated Epitaxial Lateral
Overgrowth) technique (A. Usui et al., Jpn. J. Appl. Phys., Vol. 36
(1997) pp. L899-L902) has been developed as one of the techniques
progressed from the ELO technique. This technique shares common
ground with the ELO in the point of carrying out the selective
growth using a silicon oxide mask, but differs from the ELO in the
point of forming facets, thereat in mask opening sections.
Formation of facets changes the propagation direction of
dislocations and, thus, reduces the number of threading
dislocations that reach the top of the epitaxial growth layer. With
this method, a self-supporting GaN substrate of high quality having
a relatively small number of crystal defects can be obtained by the
process where a thick GaN layer is grown upon a base substrate of,
for instance, sapphire, and subsequently the base substrate is
removed from that.
BRIEF SUMMARY OF THE INVENTION
OBJECT OF THE INVENTION
[0007] However, for the self-supporting GaN substrate fabricated in
such a method, there still remain problems to be solved. The utmost
problem is the occurrence of the warp. For instance, the
self-supporting GaN substrate from which the sapphire substrate is
removed is known to bow inwards like a concave, with the growth
face topside. The radius of curvature of this bow may reach to the
level of several tens cm or so. If this warp is severe, when it is
used as a substrate on which a layered structure for the device is
grown with a MOVPE apparatus or such, the substrate cannot adhere
to its substrate holder and thereby the temperature distribution is
generated, which makes the uniform distribution of composition and
dopant density impossible to achieve. Further, because it becomes
difficult to conduct lithography uniformly thereon, a yield for
devices falls a great deal. Naturally, the smaller the extent of
the bow is, the better it is, and it is desirable to make the
radius of curvature not less than 1 m.
[0008] In light of the above problems, an object of the present
invention is to provide a self-supporting substrate of group III
nitride based semiconductor with a lessened bowing.
SUMMARY OF THE INVENTION
[0009] The studies by the present inventors revealed that the warp
of the self-supporting substrate can be attributed to the variety
of the dislocation density in the substrate, that is to say, the
dislocation density (in particular, the density of edge
dislocation) averaged over for the obverse surface of substrate
differs from that for the reverse surface. In other words, the
greater the difference between the dislocation densities of one
surface of the substrate and of the other surface is, the more
severe the degree of the bowing is. Accordingly, in order to reduce
the warp, it becomes particularly important to control this
distribution of dislocation densities.
[0010] An observation that a density gradient of dislocation or a
difference in the edge dislocation density between one surface and
the other surface results in the warp of the substrate may be
explained in the following way. In a hexagonal crystal of GaN, when
crystal grains are present with a high density, slight variations
in orientation of crystal grains are induced by its lattice
mismatch with the substrate of a different material, and thus, it
may lead to numerous edge dislocations generated on their
boundaries. A nearly linear relationship is found out between the
edge dislocation density and the grain size, and also the following
relationship equation is found to exist between the size d.sub.0 of
this crystal grain and the amount .epsilon. of the strain
accumulated inside of the substrate: .epsilon.=.DELTA./d.sub.0 (1)
wherein, .DELTA. is almost equal to the Burger's vector of the edge
dislocation. Therefore, assuming that there is difference between
the dislocation densities on one surface and the other surface of a
substrate, there is variety in the amount of the strain inside of
the substrate, which brings about the generation of the warp.
[0011] In practice, when a self-supporting GaN substrate is
fabricated by growing a GaN layer on a substrate of a different
material by epitaxial growth and thereafter removing the substrate
of the different material, the edge dislocation density on the
interface between the substrate and the GaN layer becomes as high
as 10.sup.9 to 10.sup.11 cm.sup.-2 due to the lattice mismatch.
Even with such crystals, the dislocation density on the top surface
of the GaN layer may be lessened to such a low level as
10.sup.5-10.sup.7 cm.sup.-2 by various techniques of reducing the
number of dislocations such as means of lateral growth or thick
film growth. As for such a substrate warp, it is normally observed
that the edge dislocation density is of about 10.sup.9 cm.sup.-2
for one surface and of about 10.sup.6 cm.sup.-2 for the other
surface, respectively. In the case of a self-supporting GaN
substrate with a thickness of 200 .mu.m, the warp becomes very
severe with a radius of curvature of 20 cm or the like so that it
is difficult to present such a substrate for the device application
as it is. Nevertheless if the level of the edge dislocation density
for the surface having the higher density side is reduced to
10.sup.7 cm.sup.-2 or so, a marked improvement is made in respect
of the bow, with the radius of curvature for the warp reaching to
10 m or so, and the substrate suitable for the device application
can be obtained.
[0012] The reasons why the warp of the substrate can be suppressed
specifically through the control of the edge dislocation density
has been so far described, but the warp of the substrate can be
similarly suppressed with effect through the control of the total
dislocation density including the edge dislocation density.
[0013] Accordingly, the present invention is based on the view
mentioned above; thereby the warp of the substrate is suppressed
through the control of the total dislocation density and more
particularly through the control of the edge dislocation
density.
[0014] The present invention provides a group III nitride based
semiconductor substrate which is a self-supporting substrate;
wherein
[0015] when a dislocation density of a surface thereof on a side of
a lower dislocation density is designated n.sub.1 and a dislocation
density of a surface thereof on a side of a higher dislocation
density is designated n.sub.2, its ratio of n.sub.2/n.sub.1 is less
than 750.
[0016] The present invention makes a marked improvement in respect
of the warp of a substrate. Since its effect for reducing the warp
is given stably, excellent stability for process may be also
gained.
[0017] In the group III nitride based semiconductor substrate
according to this invention, n.sub.1 may be set preferably not
greater than 1.times.10.sup.8 cm.sup.-2 and more preferably not
greater than 1.times.10.sup.7 cm.sup.-2. This will achieve the
suppression of the warp with effect, while realizing excellent
crystalline quality.
[0018] The present invention provides further a group III nitride
based semiconductor substrate which is a self-supporting substrate;
wherein
[0019] when an edge dislocation density of a surface thereof on a
side of a lower edge dislocation density is designated m.sub.1 and
an edge dislocation density of a surface thereof on a side of a
higher edge dislocation density is designated m.sub.2, its ratio of
m.sub.2/m.sub.1 is less than 1000.
[0020] This aspect of the present invention makes a marked
improvement in respect of the warp of a substrate. Since its effect
for reducing the warp is given stably, excellent stability for
process may be also gained.
[0021] In the group III nitride based semiconductor substrate
according to the present invention, m.sub.1 may be set preferably
not greater than 5.times.10.sup.7 cm.sup.-2 and more preferably not
greater than 5.times.10.sup.6 cm.sup.-2. This can achieve the
suppression of the warp with effect, while realizing excellent
crystalline quality.
[0022] Further, the present invention provides a process for
manufacturing a group III nitride based semiconductor substrate;
which comprises the steps of:
[0023] forming a group III nitride based semiconductor layer on top
of a substrate of a different material;
[0024] separating said substrate of the different material from
said group III nitride based semiconductor layer; and
[0025] applying a treatment to reduce a dislocation density onto a
surface of said group III nitride based semiconductor layer which
lies on a side from which said substrate of the different material
has been separated.
[0026] Furthermore, the present invention provides a process for
manufacturing a group III nitride based semiconductor substrate;
which comprises the step of
[0027] forming a group III nitride based semiconductor layer on top
of a substrate of a different material by epitaxial growth, and
thereafter separating said substrate of the different material from
said group III nitride based semiconductor layer; wherein
[0028] a heat treatment is carried out at a temperature not lower
than 1150.degree. C. either during the growth of said group III
nitride based semiconductor layer or after the growth of said group
III nitride based semiconductor layer. This process for
manufacturing may further comprise the step of applying a treatment
to reduce a dislocation density onto a surface of said group III
nitride based semiconductor layer which lies on a side from which
said substrate of the different material has been separated.
[0029] According to the process for manufacturing afore-mentioned,
a self-supporting group III nitride based semiconductor substrate
which is markedly improved in respect of the warp of the substrate
can stably obtained.
[0030] In these processes for manufacturing a group III nitride
based semiconductor substrate according to the present invention,
they may have the constitution wherein said treatment to reduce a
dislocation density comprises the step of removing a region of said
group III nitride based semiconductor layer to a thickness not less
than 100 .mu.m from a side from which said substrate of the
different material has been separated.
[0031] Further, in these processes for manufacturing a group III
nitride based semiconductor substrate according to the present
invention, they may have the constitution wherein said treatment to
reduce a dislocation density comprises the step of applying a heat
treatment onto said group III nitride based semiconductor layer at
a temperature not lower than 1150.degree. C. In this way, the
dislocation densities can be decreased with effect. Herein, the
duration for the treatment is preferably set 10 minutes or longer.
Further, in view of the aim for reducing the number of dislocation
densities stably, the heat treatment is more preferably conducted
at a temperature not lower than 1200.degree. C.
[0032] Besides, the dislocation density and the edge dislocation
density as used in the present invention imply the density averaged
over in a specific plane. For instance, in the case of group III
nitride based semiconductor substrates fabricated by masked growth,
with some methods of growing, there are occasions the dislocation
density varies within a surface of a substrate. Even if such a
variety in the in-plane distribution of dislocation density is
present, the warp of the substrate can be reduced with effect by
making the average dislocation density and the average edge
dislocation density take the values within the respective ranges
described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIGS. 1(a)-(d) are a series of cross-sectional views
illustrating the steps of one example of a process for
manufacturing a self-supporting GaN substrate according to the
present invention.
[0034] FIG. 2 is a schematic view showing a hydride vapor phase
epitaxy apparatus used for the GaN growth as described in the
Examples.
[0035] FIGS. 3(a) & (b) are a series of cross-sectional views
illustrating the steps of another example of a process for
manufacturing a self-supporting GaN substrate according to the
present invention.
[0036] FIGS. 4(a)(c) are a series of cross-sectional views
illustrating the steps of another example of a process for
manufacturing a self-supporting GaN substrate according to the
present invention.
[0037] FIGS. 5(a)-(c) are a series of cross-sectional views
illustrating the steps of another example of a process for
manufacturing a self-supporting GaN substrate according to the
present invention.
[0038] FIG. 6 is a diagram in explaining one example of a
temperature profile employed in a process for manufacturing a
self-supporting GaN substrate according to the present
invention.
[0039] FIG. 7 is a diagram in explaining another example of a
temperature profile employed in a process for manufacturing a
self-supporting GaN substrate according to the present
invention.
[0040] FIG. 8 is a plot showing the dependence of the radius of
curvature of the substrate on the ratio of the total dislocation
densities observed on the obverse surface and the reverse surface
in the self-supporting GaN substrate.
[0041] FIG. 9 is a plot showing the dependence of the radius of
curvature of the substrate on the ratio of the edge dislocation
densities observed on the obverse surface and the reverse surface
in the self-supporting GaN substrate.
DETAILED DESCRIPTION OF THE INVENTION
[0042] A "self-supporting" substrate as used in the present
invention denotes any substrate that can maintain its own shape and
has enough mechanical strength not to cause any inconvenience in
handling. To have such a strength, a thickness of a self-supporting
substrate is set to be preferably not less than 30 .mu.m and more
preferably not less than 50 .mu.m. Further, taking such a factor as
easiness of the cleavage after the device formation into
consideration, the thickness of a self-supporting substrate is set
to be preferably not greater than 1 mm and more preferably not
greater than 300 .mu.m. If the substrate is unduly thick, its
cleavage becomes difficult to make, bringing about roughness on the
cleaved facet. As a result, when applied to, for example, a
semiconductor laser or such, there may arise a problem of
degradation of the device formation resulting from the reflection
loss.
[0043] For a group III nitride based semiconductor in the present
invention, there can be given a semiconductor expressed by
In.sub.xGa.sub.yAl.sub.1-x-yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, 0.ltoreq.x+y.ltoreq.1). Among semiconductors
of this sort, GaN, AlGaN and the like are preferably employed, as
they well satisfy the qualities required for the substrate
materials, including mechanical strength and manufacturing
stability.
[0044] In the present invention, the warp of the substrate is
suppressed through a reduction of the dislocation density in the
self-supporting substrate, especially the edge dislocation density
therein. For characterizing dislocations, the terms "edge" and
"screw" are generally used. The edge dislocation and the screw
dislocation represent cases in which the Burger's vector b is
perpendicular and parallel to the running direction of the
dislocation line, respectively In the case that the dislocation has
a mixture of characters of edge and screw, in other words, the
Burger's vector b is oblique to the dislocation line, it is
designated the "mixed dislocation". Now, there are occasions,
within a line of a dislocation pattern, the direction of the
dislocation line changes with respect to the direction of the
Burger's vector b. The dislocation does not necessarily run in a
straight line and often bends. As an extreme example, assuming the
case wherein a dislocation line is formed as a ring, its segments
running parallel to the Burger's vector b belong to screw
dislocations, while its segments running perpendicular to the
Burger's vector b belong to edge dislocations. The "edge
dislocation" as used in the present invention includes such a case,
that is, only a part of the dislocation belongs to the edge
dislocations.
[0045] The character of the dislocations may be identified, for
instance, by using transmission electron microscopy (TEM), When the
normal vector g (referred to as "diffraction vector", hereinafter)
to the selected diffraction plane of lattice is perpendicular to
the Burger's vector b of the dislocation line, in other words, the
inner product of those vectors is zero (the diffraction vector gthe
Burger's vector b=0 ), the contrast for the dislocation vanishes
out in the TEM observations. Making the use of this, the character
(edge, screw or mixed) for unknown dislocations can be
determined.
[0046] Further, the identification of the dislocation character may
be also made from the result of observations for the shape and the
depth of the etch pit which is formed through selective etching
applied thereto with a solution of chemicals.
[0047] A self-supporting group III nitride based semiconductor
substrate according to the present invention can be formed by
growing a layer of group III nitride based semiconductor on a
substrate of a different material by FIELO or pendeo-epitaxy and
thereafter removing the substrate of the different material. When
fabricated in such a method, a substrate with a low dislocation
density on its surface can be stably manufactured. The FIELO is a
method wherein a mask having a plurality of openings is first
formed and, then, while forming a facet structure by setting the
openings as its growth region, a GaN layer is grown by vapor phase
deposition. GaN crystals grown from neighboring openings coalesce
so that the propagation direction of dislocations are changed to
the direction parallel to the substrate and a GaN layer with a low
dislocation density on its surface can be attained. By separating
this GaN layer from the substrate of the different material, a
self-supporting GaN substrate of high quality can be obtained.
Meanwhile, in the pendeo-epitaxy, a low temperature grown buffer
layer made of Al.sub.xGa.sub.1-xN (0.ltoreq.x.ltoreq.1) is first
formed on a substrate of a different material and then a first
crystalline layer made of Al.sub.yGa.sub.1-yN (0.ltoreq.y.ltoreq.1)
is formed thereon. After a mask having a plurality of stripe
openings is formed on this crystalline layer, etching is performed
to form the first Al.sub.yGa.sub.1-yN layer patterned into
stripe-shape. Next, by using this as an origin crystals of
Al.sub.zGa.sub.1-zN (0.ltoreq.z.ltoreq.1) are grown with vapor
phase deposition to form a second single crystalline layer made of
a thick film of Al.sub.zGa.sub.1-zN. After that, by separating the
second single crystalline layer form the substrate of the different
material, a self-supporting substrate of high quality can be
obtained.
[0048] However, when a substrate is fabricated by these methods,
although its dislocations are reduced and excellent crystalline
quality is attained, the degree of its warp shows a tendency to
rise further. While dislocations on the surface for device
formation are certainly reduced with effect, numerous dislocations
on the surface opposite to the surface for device formation remain
as they are. Therefore, a ratio of the dislocation density of the
obverse surface of the substrate to that of the reverse surface
becomes all the more greater than that for conventional one by the
very attempt to reduce dislocations. When applied to such a
substrate with reduced dislocations, the present invention can
demonstrate marked effects and can suppress the warp effectively,
while achieving excellent crystalline quality.
EXAMPLES
[0049] With reference to examples, the present invention is further
explained in details below. The terms, the "total dislocation
density" and the "edge dislocation density" as used in Examples
imply the respective densities averaged over either the obverse
surface or reverse surface of the substrate.
Example 1
[0050] In the present example, a self-supporting GaN substrate was
fabricated by growing a GaN epitaxial layer on a sapphire substrate
with the afore-mentioned FIELO and thereafter removing the sapphire
substrate and the evaluation thereof was made. Referring to FIG. 1,
a process for manufacturing a self-supporting GaN substrate
according to the present example is explained below.
[0051] First, as shown in FIG. 1(a), using a sapphire substrate 11,
a GaN epitaxial layer 12 was grown, and a silicon oxide mask 13
having stripe openings was formed thereon, and then, the substrate
was set into a HVPE deposition apparatus as shown in FIG. 2. This
apparatus enables GaCl which is a halide of a group III element to
carry onto the substrate 24, and GaCl itself was formed, thereon,
by the reaction of Ga metal 21 with HCl that was supplied together
with a carrier gas of H.sub.2 or N.sub.2 through a supply tube 22.
In the substrate area thereof, GaCl and NH.sub.3 that was supplied
through a supply tube 23 were mixed, and, by reaction of those, GaN
was formed on a substrate 24 by vapor phase deposition. The
temperature of the substrate area was set to be at 1000.degree. C.
by heating with an electric furnace 25. Further, the partial
pressures of GaCl and NH.sub.3 for the source materials were
5.times.10.sup.-3 atm and 0.3 atm, respectively. Under these
conditions, the growth rate thereof was approximately 50 .mu.m/h.
Doping was carried out by supplying SiH.sub.2Cl.sub.2 as a doping
source gas for the substrate area through a doping gas supply tube
26 and a Si-doped GaN layer 14 with a thickness of approximately
350 .mu.m was grown, as shown in FIG. 1(b).
[0052] Now, this substrate was taken out of a reactor and, as shown
in FIG. 1(c), the sapphire substrate as well as a portion of the
GaN layer within a thickness of several .mu.m or so were removed
from the thick layer and thereby a self-supporting GaN substrate 15
was obtained. As a method of removing a sapphire substrate, it is
possible to employ, for instance, means of mechanical polishing or
etching with a strong basic or strong acidic chemical. Further,
physical etching with charged particle beam or a neutral particle
beam may be also used. In addition, the sapphire substrate can be
removed by applying thereto an ultraviolet laser beam which can
transmit through the sapphire substrate but is absorbed by GaN and
thereby melting its portion close to the interface.
[0053] The examination of the dislocation densities in this
self-supporting GaN substrate 15 showed that the density for the
surface from which the sapphire substrate had been removed was
valued at 5.times.10.sup.9 cm.sup.-2, while the density for the
growth front face was valued at 1.times.10.sup.6 cm.sup.-2. Among
them, the densities of edge dislocations alone for the rear and the
front were valued at 4.5.times.10.sup.9 cm.sup.-2 and
3.5.times.10.sup.5 cm.sup.-2, respectively. Herein, for the
determination of the dislocation densities, when the dislocation
density exceeded 10.sup.8 cm.sup.-2, in particular, as for the
substrate reverse surface, the transmission electron microscopy
(TEM) observations for its lower surface and vertical section were
performed, but when the dislocation density did not exceed that,
selective etching with a chemical solution was first applied
thereto and the number of etch pits formed thereby was counted,
using either an optical microscope or a scanning electron
microscope The shapes of etch pits can be roughly classified into
two groups and a group having respective shallow pits correspond to
edge dislocations. This classification can be verified as follows.
When a dark-field image is taken for a vertical section of a sample
showing an etch pit using the transmission electron microscopy, the
Burger's vector of the dislocation can determined on the basis of
the relationship between the g vector of the electron beam and the
direction of the dislocation line therein, and, with this result,
the character of that dislocation can be identified.
[0054] The measurement of the warp of the fabricated
self-supporting GaN substrate 15 indicated that its radius of
curvature was valued 30 cm. For the method for this warp
measurement, good accurate results can be easily gained, for
example, by the X-ray rocking curve measurement. In short, the
change in Bragg angle .theta..sub.B is monitored, while the sample
with a warp is moved by x in the transverse direction, and the
radius of curvature .rho. can be given by the relationship
equation: (1/.rho.)=d.theta..sub.B/dx.
[0055] Next, as shown in FIG. 1(d), from the surface from which the
sapphire substrate had been removed, a region 16 up to about 150
.mu.m thickness was removed, and thereby a self-supporting GaN
substrate 17 was obtained. For this removal, molten KOH was used.
Because this etchant can etch only N-plane selectively, it can be
conveniently used especially for etching of the reverse surface
(the side opposite to the growth front face) of the GaN layer.
Moreover without selectivity for etching, it is still possible to
apply chemical etching onto GaN reverse surface, if a protective
film of SiO.sub.2 or such is formed over the GaN obverse surface,
and besides the removal can be made even by mechanical
polishing.
[0056] As a result, in the self-supporting GaN substrate 17, the
total dislocation density and the edge dislocation density for the
surface from which the sapphire substrate had been removed
decreased to 5.times.10.sup.7 cm.sup.-2 and 3.times.10.sup.7
cm.sup.-2 respectively. When measured the warp of this substrate,
it measured a radius of curvature of 5 m, showing a marked
improvement with respect to the warp. On this substrate, a layered
structure for an InGaN based laser was grown and the laser was
fabricated by way of trial. As the lessened warp did not adversely
affect uniformity of exposure at the step of lithography, the
production yield increased a great deal.
Example 2
[0057] In the present example, a self-supporting GaN substrate was
fabricated by growing a GaN epitaxial layer on a sapphire substrate
with the afore-mentioned ELO technique (S. Nakamura et al., MRS
Internet. J. Nitride Semicond. Res., 4S1, G1. 1 (1999)), and
thereafter removing the sapphire substrate and the evaluation
thereof was made. Referring to FIG. 4, a process for manufacturing
a self-supporting GaN substrate according to the present example is
explained below.
[0058] First, using a sapphire substrate 41, a thin GaN layer 42
was epitaxially grown and thereon a silicon oxide mask 43 having
stripe openings in the [1 -1 0 0] direction of GaN was formed, and
then, by the MOVPE method using trimethylgallium (TMGa) and
NH.sub.3 as the main source material, a flat GaN layer 44 was grown
to a thickness of 10 .mu.m, as shown in FIG. 4(b).
[0059] Next, this substrate was set into the afore-mentioned HVPE
growth apparatus shown in FIG. 2. The temperature of the substrate
area was set to be at 1000.degree. C. by heating with the electric
furnace 25. Further, the partial pressures of GaCl and NH.sub.3 for
the source materials were 5.times.10.sup.-3 atm and 0.3 atm,
respectively. Under these conditions, the growth rate thereof was
approximately 50 .mu.m/h. Further, Doping was carried out by
supplying SiH.sub.2Cl.sub.2 as a doping source gas for the
substrate area through the doping gas supply tube 26 and a Si-doped
GaN layer 45 with a thickness of approximately 350 .mu.m was grown,
as shown in FIG. 4(c). After that, this substrate was taken out of
the reactor and, in a similar manner to that shown in FIG. 1(c),
the sapphire substrate as well as a GaN layer with a thickness of
several .mu.m or so were removed from the thick layer and thereby a
GaN layer 45 in the form of a self-supporting substrate was
obtained. As a method of removing a sapphire substrate, it is
possible to employ, for instance, means of mechanical polishing or
etching with a strong basic or strong acidic chemical. Further,
physical etching with s charged particle beam or a neutral particle
beam can be also used. In addition, the sapphire substrate can be
removed by applying thereto an ultraviolet laser beam which can
transmit through the sapphire substrate but is absorbed by GaN and
thereby melting its portion close to the interface. The examination
of the dislocation densities in this GaN layer 45 showed that the
density for the surface from which the sapphire substrate had been
removed was valued at 1.5.times.10.sup.9 cm.sup.2, while the
density for the growth front face was valued at 2.times.10.sup.6
cm.sup.-2. Among them, the densities of edge dislocations alone for
the rear and the front were valued at 1.times.10.sup.9 cm.sup.-2
and 1.times.10.sup.6 cm.sup.-2, respectively.
[0060] The measurement of the warp of the fabricated GaN layer 45
(self-supporting GaN substrate) indicated that its radius of
curvature was valued 1 m.
[0061] Next, in a similar manner to that shown in FIG. 1(d), from
the surface from which the sapphire substrate had been removed, a
region was removed up to about 150 .mu.m thickness. For this
removal, molten KOH was used. Because this etchant can etch only
N-plane selectively, it can be conveniently used especially for
etching of the reverse surface (the side opposite to the growth
front face) of the GaN layer. Moreover without selectivity for
etching, it is still possible to apply chemical etching onto GaN
reverse surface, if a protective film of SiO.sub.2 or such is
formed over the GaN obverse surface, and besides the removal can be
made even by mechanical polishing.
[0062] As a result, in the GaN layer 45, the total dislocation
density and the edge dislocation density for the surface from which
the sapphire substrate had been removed decreased to
5.times.10.sup.8cm.sup.2 and 2.5.times.10.sup.8 cm.sup.-2,
respectively. When measured the warp of this substrate, it measured
a radius of curvature of 3 m, showing a marked improvement with
respect to the warp. On this substrate, a layered structure for an
InGaN based laser was grown and the laser was fabricated by way of
trial. As the lessened warp did not adversely affect uniformity of
exposure at the step of lithography, the production yield increased
a great deal.
Example 3
[0063] In the present example, a self-supporting GaN substrate was
fabricated by growing a GaN epitaxial layer on a sapphire substrate
with a technique called PENDEO (T. S. Zheleva, MRS Internet. J.
Nitride Semicond. Res., 4S1, G3. 38 (1999)), and thereafter
removing the sapphire substrate and the evaluation thereof was
made. Referring to FIG. 5, a process for manufacturing a
self-supporting GaN substrate according to the present example is
explained below.
[0064] First, using a sapphire substrate 51, upon a thin GaN layer
52 a silicon oxide mask 53 having stripe openings in the [1 -1 0 0]
direction of GaN was formed, and thereafter, by means of dry
etching or such, some parts of the GaN epitaxial layer 52 and some
parts 54 of the sapphire substrate therein were etched, as shown in
FIG. 5(b). Next, by the MOVPE method using trimethylgallium (TMGa)
and NH.sub.3 as the main source material, a flat GaN layer 55 was
grown to a thickness of 10 .mu.m, as shown in FIG. 5(c). Parts of
dry etched sections remained as gap space.
[0065] This substrate was set into the HVPE growth apparatus shown
in FIG. 2. The temperature of the substrate area in the apparatus
was set to be at 1000.degree. C. by heating with an electric
furnace 25. Further, the partial pressures of GaCl and NH.sub.3 for
the source materials were 5.times.10.sup.-3 atm and 0.3 atm,
respectively. Under these conditions, the growth rate thereof was
approximately 50 .mu.m/h. Further, Doping was carried out by
supplying SiH.sub.2Cl.sub.2 as a doping source gas for the
substrate area through the doping gas supply tube 26 and a Si-doped
GaN layer 45 with a thickness of approximately 350 .mu.m was grown,
as shown in FIG. 5(c).
[0066] This substrate was taken out of the reactor and, in a
similar manner to that shown in FIG. 1(c), the sapphire substrate
as well as a GaN layer with a thickness of several .mu.m or so were
removed from the thick layer and thereby a GaN layer 56 in the form
of a self-supporting substrate was obtained. As a method of
removing a sapphire substrate, it is possible to employ, for
instance, means of mechanical polishing or etching with a strong
basic or strong acidic chemical. Further, physical etching with s
charged particle beam or a neutral particle beam can be also used.
In addition, the sapphire substrate can be removed by applying
thereto an ultraviolet laser beam which can transmit through the
sapphire substrate but is absorbed by GaN and thereby melting its
portion close to the interface. The examination of the dislocation
densities in this GaN layer 56 showed that the density for the
surface from which the sapphire substrate had been removed was
valued at 3.times.10.sup.9 cm.sup.-2, while the density for the
growth front face was valued at 3.times.10.sup.6 cm.sup.-2. Among
them, the densities of edge dislocations alone for the rear and the
front were valued at 2.4.times.10.sup.9 cm.sup.-2 and
1.2.times.10.sup.6 cm.sup.-2, respectively.
[0067] The measurement of the warp of the fabricated GaN layer 56
in the form of a self-supporting substrate indicated that its
radius of curvature was valued 80 cm.
[0068] Next, in a similar manner to that shown in FIG. 1(d), from
the surface from which the sapphire substrate had been removed, a
region was removed up to about 150 .mu.m thickness. For this
removal, molten KOH was used. Because this etchant can etch only
N-plane selectively, it can be conveniently used especially for
etching of the reverse surface (the side opposite to the growth
front face) of the GaN layer. Moreover, without selectivity for
etching, it is still possible to apply chemical etching onto GaN
reverse surface, if a protective film of SiO.sub.2 or such is
formed over the GaN obverse surface, and besides the removal can be
made even by mechanical polishing.
[0069] As a result, in the GaN layer 56, the total dislocation
density and the edge dislocation density for the surface from which
the sapphire substrate had been removed decreased to
3.5.times.10.sup.8 cm.sup.-2 and 1.times.10.sup.8 cm.sup.-2,
respectively. When measured the warp of this substrate, it measured
a radius of curvature of 4 m, showing a marked improvement with
respect to the warp.
[0070] On this substrate, a layered structure for an InGaN based
laser was grown and the laser was fabricated by way of trial. As
the lessened warp did not adversely affect uniformity of exposure
at the step of lithography, the production yield increased a great
deal.
Example 4
[0071] In the present example, the dislocation densities of the
surfaces of a self-supporting substrate were controlled by a heat
treatment. Referring to FIG. 3, a process for manufacturing a
self-supporting GaN substrate according to the present example is
explained below.
[0072] First, using a sapphire substrate 31, a GaN layer 33 was
formed on a GaN low temperature growth buffer layer 32 with the
afore-mentioned HVPE growth apparatus of FIG. 2 (FIG. 3(a)). In the
substrate area inside of the apparatus, GaCl and NH.sub.3 that was
supplied through a supply tube 23 were mixed, and, while
interacting, formed GaN on a substrate 24 by vapor deposition. The
temperature of the substrate area was set to be at 1000.degree. C.
using an electric furnace 25. Further, the partial pressures of
GaCl and NH.sub.3, both of which were the source gases, were
5.times.10.sup.-3 atm and 0.3 atm, respectively. Under these
conditions, the growth rate thereof was approximately 50 .mu.m/h.
Further, Doping was carried out by supplying SiH.sub.2Cl.sub.2 as a
doping source gas for the substrate area through the doping gas
supply tube 26. In this way, a Si-doped GaN layer 33 with a
thickness of approximately 200 .mu.m was epitaxially grown.
[0073] After that, This substrate was taken out of the reactor, and
the sapphire substrate 31, the GaN low temperature grown buffer
layer 32 and a small portion of the GaN layer 33 were removed (FIG.
3(b)). Therefore, the GaN layer 33 shown in FIG. 3(a) were, in FIG.
3(b), divided into a self-supporting GaN substrate 35 and a GaN
layer 34 that had been removed from the self-supporting GaN
substrate. The GaN layer 34 to be removed was set to be several
tens .mu.m or so in thickness.
[0074] As a method of removing the sapphire substrate 31, it is
possible to employ, for instance, means of mechanical polishing or
etching with a strong basic or strong acidic chemical. Further,
physical etching with s charged particle beam or neutral particle
beam can be also used. In addition, the sapphire substrate can be
removed by applying thereto an ultraviolet laser beam which can
transmit through the sapphire substrate but is absorbed by GaN and
thereby melting its portion close to the interface.
[0075] The examination of the dislocation densities in the
self-supporting GaN substrate 35 obtained in the process described
above showed that the density for the surface from which the
sapphire substrate had been removed was valued at 9.times.10.sup.9
cm.sup.-2, while the density for the growth front face was valued
at 1.times.10.sup.7 cm.sup.-2. Among them, the densities of edge
dislocations alone for the rear and the front were valued at
7.times.10.sup.9 cm.sup.-2 and 5.times.10.sup.6 cm.sup.-2,
respectively. The measurement of the warp of the self-supporting
GaN substrate 35 indicated that its radius of curvature was valued
at as a large value as 90 cm.
[0076] This self-supporting GaN substrate 35 was put into an
electric furnace and a heat treatment was carried out under the
NH.sub.3 atmosphere at 1200.degree. C. for 24 hours. The NH.sub.3
atmosphere was selected for preventing decomposition during the
heat treatment, but if the sample could be sealed well, NH.sub.3
supply was not necessarily required. After the heat treatment, the
dislocation densities were again examined, and it was found that
the dislocation density for the surface from which the sapphire
substrate had been removed became 4.times.10.sup.7 cm.sup.-2, while
the density for the growth front face became 8.times.10.sup.5
cm.sup.-2, showing a marked improvement in dislocation densities.
Among them, the densities of edge dislocations alone for the rear
and the front were valued at 1.times.10.sup.7 cm.sup.-2 and
3.times.10.sup.5 cm.sup.-2, respectively. When measured the warp of
the self-supporting GaN substrate 35 after the heat treatment, it
measured a radius of curvature of 6 m, showing a marked improvement
with respect to the warp.
[0077] On this substrate, a layered structure for an InGaN based
laser was grown and the laser was fabricated by way of trial. As
the lessened warp did not adversely affect uniformity of exposure
at the step of lithography, the production yield increased a great
deal.
Example 5
[0078] In the present example, the dislocation densities of the
surfaces of a self-supporting substrate were controlled by a step
of heat treatment adding in the midst of epitaxial growth. A
process for manufacturing a self-supporting GaN substrate according
to the present example is explained below.
[0079] In the present example, using a sapphire C-plane substrate,
a GaN layer was grown by the step shown in FIG. 3 with the
afore-mentioned HVPE apparatus of FIG. 2. Hereat, the growth of the
GaN layer 33 and a heat treatment were carried out according to the
temperature sequence shown in FIG. 6. During the growth, the
partial pressures of GaCl and NH.sub.3 were set to be
5.times.10.sup.-3 atm and 0.3 atm, respectively.
[0080] First, the temperature in a furnace was set at 1200.degree.
C. and thermal cleaning of the sapphire substrate was conducted in
H.sub.2 gas flow. Next, the temperature in the furnace was lowered
to 500 .degree. C. and a deposition of a GaN low temperature grown
buffer layer 32 was made. After that, the temperature in the
furnace was raised to 1000.degree. C. and a GaN layer was grown to
a thickness of 50 .mu.m. Hereat, the Ga source supply was stopped
once and a heat treatment was performed. That is, the temperature
in the furnace was raised to 1400.degree. C. under the NH.sub.3
atmosphere and was kept for 10 minutes. Following that, the
temperature in the furnace was lowered to 500.degree. C. and kept
for 5 minutes. After this sequence of a heat treatment was
completed, the temperature in the furnace was again raised to
1000.degree. C. Subsequently, a GaN layer was grown further as
thick as 150 .mu.m and, thus, a GaN layer 33 with a total film
thickness of 200 .mu.m was obtained.
[0081] After that, this substrate was taken out of a reactor and,
the sapphire substrate 31, the GaN low temperature grown buffer
layer 32 and a small portion of the GaN layer 33 were removed (FIG.
3(b)). Here, the GaN layer 33 shown in FIG. 3(a) were, in FIG.
3(b), divided into a self-supporting GaN substrate 35 and a GaN
layer 34 that had been removed from the self-supporting GaN
substrate. The GaN layer 34 to be removed was set to be several
tens .mu.m or so in thickness. As a method of removing the sapphire
substrate 31, one of the afore-mentioned methods can be
employed.
[0082] The examination of the dislocation densities in the
self-supporting GaN substrate 35 obtained in the process described
above showed that the density for the surface from which the
sapphire substrate had been removed was valued at 4.times.10.sup.7
cm.sup.-2, while the density for the growth front face was valued
at 5.times.10.sup.6 cm.sup.-2. Among them, the densities of edge
dislocations alone for the rear and the front were valued at
1.5.times.10.sup.7 cm.sup.-2 and 2.times.10.sup.6 cm.sup.-2,
respectively. The measurement of the warp of the GaN layer
indicated that its radius of curvature was valued at 7 m. When the
layer was grown without performing the heat treatment step, the
density for the surface from which the sapphire substrate had been
removed was valued at 9.times.19 cm.sup.-2, while the density for
the growth front face was valued at 1.times.10.sup.7 cm.sup.-2.
Among them, the densities of edge dislocations alone for the rear
and the front were valued at 7.times.10.sup.9 cm.sup.-2 and
5.times.10.sup.6 cm.sup.-2, respectively. The measurement of the
warp of this GaN layer indicated that it is a substrate having a
severe warp with its radius of curvature of 90 cm, and, thus,
confirmed that a marked improvement with respect to the warp was
certainly made by an addition of the step of a heat treatment.
Example 6
[0083] In the present example, the dislocation densities of the
surfaces of a self-supporting substrate were controlled with a
higher accuracy by a plurality of steps of heat treatment adding in
the midst of epitaxial growth. A process for manufacturing a
self-supporting GaN substrate according to the present example is
explained below.
[0084] In the present example, using a sapphire C-plane substrate,
a GaN layer was grown by the step shown in FIG. 3 with the
afore-mentioned HVPE apparatus of FIG. 2. Hereat, the growth of the
GaN layer 33 and heat treatments were carried out according to the
temperature sequence shown in FIG. 7. During the growth, the
partial pressures of GaCl and NH.sub.3 were set to be
5.times.10.sup.-3 atm and 0.3 atm, respectively.
[0085] First, the temperature in a furnace was set at 1200.degree.
C. and thermal cleaning of the sapphire substrate was conducted in
H.sub.2 gas flow. Next, the temperature in the furnace was lowered
to 500.degree. C. and a deposition of a GaN low temperature grown
buffer layer 32 was made. After that, the temperature in the
furnace was raised to 1000.degree. C. and a GaN layer was grown to
a thickness of 25 .mu.m. Hereat, the Ga source supply was stopped
once and a heat treatment was performed. That is, the temperature
in the furnace was raised to 1400.degree. C. under the NH.sub.3
atmosphere and was kept for 10 minutes. Following that, the
temperature in the furnace was lowered to 500.degree. C. and kept
for 5 minutes. After this sequence of a heat treatment was
completed, the temperature in the furnace was again raised to
1000.degree. C. Hereafter, every time the GaN layer was grown
further to add a thickness of 25 .mu.m, a growth interruption and a
subsequent heat treatment were similarly carried out, and a GaN
layer 33 with a total film thickness of 200 .mu.m was obtained.
[0086] After that, this substrate was taken out of a reactor and,
the sapphire substrate 31, the GaN low temperature grown buffer
layer 32 and a small portion of the GaN layer 33 were removed (FIG.
3(b)). Here, the GaN layer 33 shown in FIG. 3(a) were, in FIG.
3(b), divided into a self-supporting GaN substrate 35 and a GaN
layer 34 that had been removed from the self-supporting GaN
substrate. The GaN layer 34 to be removed was set to be several
tens .mu.m or so in thickness. As a method of removing the sapphire
substrate 31, one of the afore-mentioned methods can be
employed.
[0087] The examination of the dislocation densities in the
self-supporting GaN substrate 35 obtained in the process described
above showed that the density for the surface from which the
sapphire substrate had been removed was valued at 2.times.10.sup.7
cm.sup.-2, while the density for the growth front face was valued
at 4.times.10.sup.6 cm.sup.-2. Among them, the densities of edge
dislocations alone for the rear and the front were valued at
9.times.10.sup.6 cm.sup.-2 and 1.5.times.10.sup.6 cm.sup.-2,
respectively. The measurement of the warp of the GaN layer
indicated that its radius of curvature was valued at 10 m. When the
layer was grown without performing any heat treatment steps, the
density for the surface from which the sapphire substrate had been
removed was valued at 9.times.10.sup.9 cm.sup.-2, while the density
for the growth front face was valued at 1.times.10.sup.7 cm.sup.-2.
Among them, the densities of edge dislocations alone for the rear
and the front were valued at 7.times.10.sup.9 cm.sup.-2 and
5.times.10.sup.6 cm.sup.-2, respectively. The measurement of the
warp of this GaN layer indicated that it is a substrate having a
severe warp with its radius of curvature of 90 cm, and, thus,
confirmed that a marked improvement with respect to the warp was
certainly made by an addition of the steps of heat treatment.
Example 7
[0088] In the present example, the dislocation densities of the
surfaces of a self-supporting substrate were controlled by
conducting such a heat treatment that on applying the heat
treatment to the self-supporting substrate, there was employed a
method wherein its face for device formation was covered with a
mask. A process for manufacturing a self-supporting GaN substrate
according to the present example is explained below.
[0089] In the present example, using a sapphire C-plane substrate,
a GaN layer was grown by the step shown in FIG. 3 with the
afore-mentioned HVPE apparatus of FIG. 2. First, upon a sapphire
substrate 31, a GaN low temperature grown buffer layer 32 was
formed. Subsequently, a GaN layer 33 was grown as follows. Firstly,
in the apparatus of FIG. 2, the temperature of the substrate area
was set to be at 1000.degree. C. by heating with an electric
furnace 25, and the partial pressures of the substrate area for
GaCl and NH.sub.3, both of which were the source gases, were set to
be 5.times.10.sup.-3 atm and 0.3 atm, respectively. Under these
conditions, the growth rate thereof was approximately 50 .mu.m/h.
Further, Doping was carried out by supplying SiH.sub.2Cl.sub.2 as a
doping source gas for the substrate area through the doping gas
supply tube 26. In this way, a Si-doped GaN layer 33 with a
thickness of approximately 200 .mu.m was grown.
[0090] After that, this substrate was taken out of a reactor and,
the sapphire substrate 31, the GaN low temperature grown buffer
layer 32 and a small portion of the GaN layer were removed (FIG.
3(b)). Here, the GaN layer 33 shown in FIG. 3(a) were, in FIG.
3(b), divided into a self-supporting GaN substrate 35 and a GaN
layer 34 that had been removed from the self-supporting GaN
substrate. The GaN layer 34 to be removed was set to be several
tens .mu.m or so in thickness. As a method of removing the sapphire
substrate 31, one of the afore-mentioned methods can be
employed.
[0091] The examination of the dislocation densities in the
self-supporting GaN substrate 35 obtained in the steps described
above showed that the density for the surface from which the
sapphire substrate had been removed was valued at 9.times.10.sup.9
cm.sup.-2, while the density for the growth front face was valued
at 1.times.10.sup.7 cm.sup.-2. Among them, the densities of edge
dislocations alone for the rear and the front were valued at
7.times.10.sup.9 cm.sup.-2 and 5.times.10.sup.6 cm.sup.-2,
respectively. The measurement of the warp of this GaN layer
indicated that it is a substrate having a large warp with its
radius of curvature of 90 cm, Next, the entire surface of this
self-supporting GaN substrate 35 was covered with a SiO.sub.2 film.
The deposition of the film was made by the CVD (Chemical Vapor
Deposition). This could prevent decomposition of the GaN substrate
even when a heat treatment was carried out at a considerably high
temperature. Subsequently, this self-supporting GaN substrate 35
was put into an electric furnace and a heat treatment was carried
out in the air at 1600.degree. C. for 2 hours. After the heat
treatment, the dislocation densities were again examined, and it
was found that the density for the surface from which the sapphire
substrate had been removed became 6.times.10 .sup.7 cm.sup.-2,
while the density for the growth front face became 9.times.10.sup.5
cm.sup.-2, showing a marked improvement in dislocation densities.
Among them, the densities of edge dislocations alone for the rear
and the front were valued at 4.times.10.sup.7 cm.sup.-2 and
3.5.times.10.sup.5 cm.sup.-2, respectively. When measured the warp
of the self-supporting GaN substrate 35 after the heat treatment,
it measured a radius of curvature of 3.5 m, showing a marked
improvement with respect to the warp. On this substrate, a layered
structure for an InGaN based laser was grown and the laser was
fabricated by way of trial. As the lessened warp did not adversely
affect uniformity of exposure at the step of lithography, the
production yield increased a great deal.
[0092] The results of the examples mentioned above are summarized
in Table 1 and Table 2. In addition, the dependences of the radius
of curvature of the substrate on the ratio of dislocation density
of the obverse surface to that of the reverse surface in the
self-supporting GaN substrate obtained are shown in FIG. 8 and FIG.
9. As seen in the results of FIG. 8 and FIG. 9, the radius of
curvature sharply increases and, thus, the degree of the warp
decreases, when the ratio of the total dislocation densities is
less than 750 (a line in FIG. 8 indicating a ratio of the total
dislocation densities of 750) and when the ratio of the edge
dislocation densities is less than 1000 (a line in FIG. 9
indicating a ratio of the edge dislocation densities of 1000).
TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4
Without With Without With Without With Without With Treatment
Treatment Treatment Treatment Treatment Treatment Treatment
Treatment Density of Rear face 5 .times. 10.sup.9 5 .times.
10.sup.7 1.5 .times. 10.sup.9 5 .times. 10.sup.8 3 .times. 10.sup.9
3.5 .times. 10.sup.9 9 .times. 10.sup.9 4 .times. 10.sup.7
Dislocation Growth 1 .times. 10.sup.6 1 .times. 10.sup.6 2 .times.
10.sup.6 2 .times. 10.sup.6 3 .times. 10.sup.6 3 .times. 10.sup.6 1
.times. 10.sup.7 8 .times. 10.sup.5 (cm.sup.-2) front face Density
of Rear face 4.5 .times. 10.sup.9 3 .times. 10.sup.7 1 .times.
10.sup.9 2.5 .times. 10.sup.8 2.4 .times. 10.sup.9 1 .times.
10.sup.8 7 .times. 10.sup.9 1 .times. 10.sup.7 Edge Growth 3.5
.times. 10.sup.5 3.5 .times. 10.sup.5 1 .times. 10.sup.6 1 .times.
10.sup.6 1.2 .times. 10.sup.5 1.2 .times. 10.sup.5 5 .times.
10.sup.6 3 .times. 10.sup.5 Dislocation front face (cm.sup.-2)
Ratio of Total 5000 50 750 250 1000 120 900 50 Dislocation
Densities: n.sub.2/n.sub.1 Ratio of Edge 13000 86 1000 250 2000 83
1400 33 Dislocation Densities: m.sub.2/m.sub.1 Radius of 0.3 5 1 3
0.8 4 0.9 6 Curvature (m)
[0093] TABLE-US-00002 TABLE 2 Example 5 Example 6 Without With
Without With Treat- Treat- Treat- Treat- ment ment ment ment
Density of Rear face 9 .times. 10.sup.9 4 .times. 10.sup.7 9
.times. 10.sup.9 2 .times. 10.sup.7 Dislocation Growth 1 .times.
10.sup.7 5 .times. 10.sup.6 1 .times. 10.sup.7 4 .times. 10.sup.6
(cm.sup.-2) front face Density of Rear face 7 .times. 10.sup.9 1.5
.times. 10.sup.7 7 .times. 10.sup.9 9 .times. 10.sup.6 Edge Growth
5 .times. 10.sup.6 2 .times. 10.sup.6 5 .times. 10.sup.6 1.5
.times. 10.sup.6 Dislocation front face (cm.sup.-2) Ratio of Total
900 8 900 5 Dislocation Densities: n.sub.2/n.sub.1 Ratio of Edge
1400 7.5 1400 6 Dislocation Densities: m.sub.2/m.sub.1 Radius of
0.9 7 0.9 10 Curvature (m) Example 7 Without With Treatment
Treatment Density of Rear face 9 .times. 10.sup.9 6 .times.
10.sup.7 Dislocation Growth 1 .times. 10.sup.7 9 .times. 10.sup.5
(cm.sup.-2) front face Density of Rear face 7 .times. 10.sup.9 4
.times. 10.sup.7 Edge Growth 5 .times. 10.sup.6 3.5 .times.
10.sup.5 Dislocation front face (cm.sup.-2) Ratio of Total 900 67
Dislocation Densities: n.sub.2/n.sub.1 Ratio of Edge 1400 114
Dislocation Densities: m.sub.2/m.sub.1 Radius of 0.9 3.5 Curvature
(m)
[0094] While preferred embodiments have been described by referring
the examples, it is to be understood by those skilled in the art
that the foregoing description is intended to illustrate the
present invention and that various changes and modifications in the
combination of the process of the examples herein described may be
made without departing from the spirit and scope of the invention,
which variation and modification fall into the technical scope of
this invention. For instance, in Example 5 or Example 6, after
removing the sapphire substrate, an additional treatment to reduce
dislocations may be carried out. For example, the step of removing
a region to a thickness of 100 .mu.m or more from the side from
which the substrate of the different material has been separated
may be performed. Alternatively, the step of conducting a heat
treatment of the self-supporting GaN substrate at a temperature not
lower than 1150.degree. C. may be performed.
[0095] Further, while examples of a process for manufacturing a
self-supporting GaN substrate are given in Examples, the present
invention can be applied to a self-supporting AlGaN substrate.
SUMMARY OF DISCLOSURE
[0096] As set forth above, in the present invention, because the
dislocation densities in the substrate, especially the edge
dislocation densities therein are well controlled, a
self-supporting group III nitride based semiconductor substrate
having a lessened warp can be stably obtained.
[0097] The application of substrate of the present invention
enables high-yield production of light emission devices and
electron devices in accordance with design.
* * * * *