U.S. patent application number 09/758287 was filed with the patent office on 2001-06-28 for method for growing nitride semiconductor crystals, nitride semiconductor device, and method for fabricating the same.
Invention is credited to Ishida, Masahiro, Itoh, Kunio.
Application Number | 20010005023 09/758287 |
Document ID | / |
Family ID | 26479024 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005023 |
Kind Code |
A1 |
Itoh, Kunio ; et
al. |
June 28, 2001 |
Method for growing nitride semiconductor crystals, nitride
semiconductor device, and method for fabricating the same
Abstract
A method for growing nitride semiconductor crystals according to
the present invention includes the steps of: a) forming a first
metal single crystal layer on a substrate; b) forming a metal
nitride single crystal layer by nitrifying the first metal single
crystal layer; and c) epitaxially growing a first nitride
semiconductor layer on the metal nitride single crystal layer.
Inventors: |
Itoh, Kunio; (Kyoto, JP)
; Ishida, Masahiro; (Osaka, JP) |
Correspondence
Address: |
NIXON PEABODY, LLP
8180 GREENSBORO DRIVE
SUITE 800
MCLEAN
VA
22102
US
|
Family ID: |
26479024 |
Appl. No.: |
09/758287 |
Filed: |
January 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09758287 |
Jan 12, 2001 |
|
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09317604 |
May 25, 1999 |
|
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6218207 |
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Current U.S.
Class: |
257/103 ;
257/E21.134 |
Current CPC
Class: |
H01L 21/02614 20130101;
H01L 21/02381 20130101; H01L 21/02516 20130101; H01L 21/02433
20130101; H01L 21/02458 20130101; H01L 21/0254 20130101 |
Class at
Publication: |
257/103 |
International
Class: |
H01L 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 1998 |
JP |
10-148997 |
May 29, 1998 |
JP |
10-148998 |
Claims
In the claims:
1. A nitride semiconductor device comprising: a single crystal
substrate; a metal nitride single crystal layer formed by
nitrifying a metal single crystal layer on the single crystal
substrate; a semiconductor multilayer structure including a first
nitride semiconductor layer epitaxially grown on the metal nitride
single crystal layer; and a pair of electrodes for applying a
voltage to the semiconductor multilayer structure.
2. A nitride semiconductor device comprising: a single crystal
substrate with conductivity; a metal nitride single crystal layer
formed by nitrifying a metal single crystal layer on the single
crystal substrate; a semiconductor multilayer structure including a
first nitride semiconductor layer epitaxially grown on the metal
nitride single crystal layer; and a pair of electrodes formed to
face each other on respective surfaces of the single crystal
substrate and the semiconductor multilayer structure, which are
interposed between the surfaces.
3. The device of claim 1, further comprising a first metal single
crystal layer on the single crystal substrate, and wherein the
metal nitride crystal layer is formed by nitrifying a second metal
single crystal layer epitaxially grown on the first metal single
crystal layer.
4. The device of claim 1, wherein the single crystal substrate
comprises a metal diffused layer in which metal atoms have diffused
from the metal nitride single crystal layer.
5. The device of claim 1, wherein the single crystal substrate
comprises a metal diffused layer in which metal atoms have diffused
from the first metal single crystal layer.
6. The device of claim 1, wherein the single crystal substrate is
made of Si.sub.1-s-tGe.sub.sC.sub.t(where 0.ltoreq.s, t.ltoreq.1
and 0.ltoreq.s+t.ltoreq.1).
7. The device of claim 1, wherein the single crystal substrate is
made of A.sub.1-uB.sub.u, where 0<u<1, A is one of Al, Ga and
In and B is one of As, P and Sb.
8. The device if claim 1, wherein the single crystal substrate is
made of a material selected from the group consisting of: sapphire;
spinel; magnesium oxide; zinc oxide; chromium oxide; lithium
niobium oxide; lithium tantalum oxide; and lithium gallium
oxide.
9. The device of claim 5, wherein the first metal single crystal
layer is made of Au or an alloy containing Au.
10. The device of claim 1, wherein the metal nitride single crystal
layer is made of Al.sub.1-x-yGa.sub.xIn.sub.yN (where 0.ltoreq.x,
y.ltoreq.1 and 0.ltoreq.x+y<1).
11. The device of claim 10, wherein the single crystal substrate is
a single crystal substrate of silicon, of which the principal
surface is (III) plane, and wherein the metal nitride single layer
is formed on the (III) plane and the principal surface of the metal
nitride single crystal layer is (0001) plane.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method for growing
nitride semiconductor crystals, a nitride semiconductor device and
a method for fabricating the same.
[0002] Nitride semiconductors such as GaN, InN and AlN are
materials suitably used for blue-light-emitting semiconductor laser
devices and numerous types of semiconductor devices, e.g.,
transistors operating at a high speed at an elevated
temperature.
[0003] Various methods have been suggested to form a single crystal
layer of a nitride semiconductor suitable for these semiconductor
devices.
[0004] For example, according to a conventional technique, a
nitride semiconductor layer (e.g., an AlN layer) is directly
deposited on a single crystal substrate of sapphire
(Al.sub.2O.sub.3) or Si by a metalorganic vapor phase epitaxy
(abbreviated to "MOVPE" and also called a "metalorganic chemical
vapor deposition (MOCVD)") process. The nitride semiconductor layer
formed by this method, however, has poor surface morphology and is
likely to crack, resulting in a lower yield. Thus, this method has
not been put into practice. Cracking is probably caused due to a
thermal stress resulting from a difference in thermal expansion
coefficient between a single crystal substrate and a nitride
semiconductor layer during the process of lowering the deposition
temperature of the nitride semiconductor layer (about 1000.degree.
C. for AlN) to room temperature.
[0005] Another technique of forming a single crystalline nitride
semiconductor layer was developed later as disclosed in Japanese
Laid-Open Publications Nos. 4-297023 and 7-312350. According to
this technique, an amorphous or polycrystalline nitride
semiconductor layer (i.e., a GaN or Ga.sub.1-aAl.sub.aN (where
0<a.ltoreq.1) layer) is once formed on a single crystal
substrate of sapphire or silicon at a relatively low temperature by
an MOVPE process. Thereafter, the nitride semiconductor layer is
heated to form a partially single crystalline buffer layer and then
nitride semiconductor layers for a semiconductor device are
epitaxially grown on the buffer layer.
[0006] A light-emitting device disclosed in Japanese Laid-Open
Publication No. 6-177423 is known as an exemplary semiconductor
device using a nitride semiconductor layer formed on a buffer
layer. As shown in FIG. 14, this light-emitting device 900
includes: a buffer layer 95 of polycrystalline or amorphous GaN or
Ga.sub.1-aAl.sub.aN (where 0<a.ltoreq.1); an n-type
Ga.sub.1-bAl.sub.bN (where 0.ltoreq.b<1) cladding layer 96; an
n-type In.sub.xGa.sub.1-xN (where 0<x<0.5) active layer 97;
and a p-type Ga.sub.1-cAl.sub.cN (where 0.ltoreq.c<1) cladding
layer 98, which are stacked in this order on a sapphire substrate
92.
[0007] The crystal growing technique for the buffer layer 95 is
also disclosed in Japanese Laid-Open Publications Nos. 4-297023 and
7-312350 identified above. Specifically, according to the method
disclosed in these references, GaN or Ga.sub.1-aAl.sub.aN (where
0<a.ltoreq.1) crystals are grown at a temperature ranging from
200.degree. C. to 900.degree. C., both inclusive, by an MOVPE
process to form the buffer layer 95. In accordance with this
method, part of the buffer layer 95 is turned into single crystals
during a process of raising the temperature after the buffer layer
95 of polycrystalline Ga.sub.1-aAl.sub.aN (where 0<a.ltoreq.1)
has been deposited on the sapphire substrate 92 at a low
temperature and before a nitride semiconductor crystal layer, e.g.,
the n-type Ga.sub.1-bAl.sub.bN (where 0.ltoreq.b<1) cladding
layer 96, is deposited at a temperature of about 1000.degree.
C.
[0008] The present inventors minutely analyzed the cross-section of
nitride semiconductor crystals, which had been grown on a sapphire
substrate at a low temperature by the conventional technique, using
a transmission electron microscope. As a result, we found that the
nitride semiconductor crystal layer, which had been formed by the
prior art crystal growing technique, had a lot of dislocations and
that the lifetime of a semiconductor device including such a
nitride semiconductor layer was short.
[0009] In the conventional method for fabricating a semiconductor
device, it seems to be only a small region of the buffer layer 95
within a plane of the sapphire substrate 92 that is turned into
single crystals during the temperature raising process before the
nitride semiconductor crystal layers are grown. Thus, it is
considered that, in the remaining region of the buffer layer 95
that is not turned into single crystals, the polycrystals have
poorly aligned orientations to generate a large number of
dislocations (or other defects) in the interface between the
sapphire substrate 92 and the buffer layer 95. And such
dislocations would grow to reach the nitride semiconductor crystal
layers (i.e., the cladding layer 96, active layer 97 and cladding
layer 98 in this case). We found that the density of dislocations
in the nitride semiconductor crystal layers was as high as 10.sup.9
cm.sup.-2, thus adversely shortening the life of the semiconductor
device.
[0010] Still another technique of forming an AlN buffer layer by
nitrifying (in this specification, to "nitrify" means "to combine
with nitrogen or its compounds") the surface of a sapphire single
crystal substrate was suggested in Japanese Laid-Open Publication
No. 63-178516, for example. In accordance with this technique,
however, the buffer layer is also likely to crack or a lot of
dislocations are also created in the buffer layer as in the prior
art method just described. Thus, this technique has not been put
into practice, either.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is providing a method for
growing nitride semiconductor crystals with the number of
dislocations created in a nitride semiconductor crystal layer
reduced, a highly reliable semiconductor device with a longer
lifetime, and a method for fabricating the same.
[0012] A method for growing nitride semiconductor crystals
according to the present invention includes the steps of: a)
forming a first metal single crystal layer on a substrate; b)
forming a metal nitride single crystal layer by nitrifying the
first metal single crystal layer; and c) epitaxially growing a
first nitride semiconductor layer on the metal nitride single
crystal layer.
[0013] The present invention also provides a method for fabricating
a nitride semiconductor device including a semiconductor multilayer
structure and a pair of electrodes for applying a voltage to the
semiconductor multilayer structure. In this method, the step of
forming the semiconductor multilayer structure includes the step of
epitaxially growing the first nitride semiconductor layer by the
method of the present invention for growing nitride semiconductor
crystals.
[0014] A nitride semiconductor device according to the present
invention includes: a single crystal substrate; a metal nitride
single crystal layer formed by nitrifying a metal single crystal
layer on the single crystal substrate; a semiconductor multilayer
structure including a first nitride semiconductor layer epitaxially
grown on the metal nitride single crystal layer; and a pair of
electrodes for applying a voltage to the semiconductor multilayer
structure.
[0015] Another nitride semiconductor device according to the
present invention includes: a single crystal substrate with
conductivity; a metal nitride single crystal layer formed by
nitrifying a metal single crystal layer on the single crystal
substrate; a semiconductor multilayer structure including a first
nitride semiconductor layer epitaxially grown on the metal nitride
single crystal layer; and a pair of electrodes formed to face each
other on respective surfaces of the single crystal substrate and
the semiconductor multilayer structure, which are interposed
between the surfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIGS. 1A, 1B and 1C are cross-sectional views illustrating a
method for growing nitride semiconductor crystals according to an
exemplary embodiment of the present invention.
[0017] FIGS. 2A, 2B and 2C are cross-sectional views illustrating a
method for growing nitride semiconductor crystals according to
another embodiment of the present invention.
[0018] FIGS. 3A, 3B and 3C are cross-sectional views illustrating a
method for growing nitride semiconductor crystals according to
still another embodiment of the present invention.
[0019] FIGS. 4A, 4B and 4C are cross-sectional views illustrating a
method for growing nitride semiconductor crystals according to yet
another embodiment of the present invention.
[0020] FIGS. 5A, 5B, 5C and 5D are cross-sectional views
illustrating a method for growing nitride semiconductor crystals
according to yet another embodiment of the present invention.
[0021] FIGS. 6A, 6B, 6C and 6D are cross-sectional views
illustrating a method for growing nitride semiconductor crystals
according to yet another embodiment of the present invention.
[0022] FIG. 7 is a cross-sectional view schematically illustrating
a light-emitting device 100 in a first specific example of a first
embodiment according to the present invention.
[0023] FIGS. 8A, 8B and 8C are cross-sectional views schematically
illustrating a method for fabricating the light-emitting device 100
shown in FIG. 7.
[0024] FIG. 9 is a graph illustrating respective relationships
between operating time and variation in operating current for the
light-emitting devices of the present invention and a conventional
light-emitting device.
[0025] FIG. 10 is a cross-sectional view schematically illustrating
a light-emitting device 200 in a first specific example of a second
embodiment according to the present invention.
[0026] FIGS. 11A, 11B and 11C are cross-sectional views
schematically illustrating a method for fabricating the
light-emitting device 200 shown in FIG. 10.
[0027] FIG. 12 is a cross-sectional view schematically illustrating
a light-emitting device 300 in a second specific example of the
second embodiment according to the present invention.
[0028] FIG. 13 is a cross-sectional view schematically illustrating
a light-emitting device 400 in a third specific example of the
second embodiment according to the present invention.
[0029] FIG. 14 is a cross-sectional view schematically illustrating
a conventional light-emitting device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] In a method for growing nitride semiconductor crystals
according to the present invention, first, a metal single crystal
layer is formed on a substrate, and then a metal nitride single
crystal layer is formed by nitrifying the metal single crystal
layer. Thereafter, nitride semiconductor layers are epitaxially
grown on the resulting metal nitride single crystal layer. In the
nitrification process step, the metal single crystal layer need not
be nitrified entirely. Alternatively, only part of the metal single
crystal layer may be nitrified and then nitride semiconductor
layers may be epitaxially grown on a metal nitride single crystal
layer formed around the surface of the metal single crystal layer.
As a further alternative, another metal single crystal layer,
different from the metal single crystal layer to be nitrified, may
be formed on the substrate and then the metal single crystal layer
to be nitrified may be formed thereon.
[0031] The metal nitride single crystal layer, on which nitride
semiconductor layers are to be epitaxially grown, functions as a
conventional buffer layer, thus improving the crystallinity of the
nitride semiconductor layers. Such a buffer layer, made of a metal
nitride formed by nitrifying the metal single crystals, is a single
crystal layer with a much smaller number of defects than a
polycrystalline layer or layer formed by turning part of a
polycrystalline layer into single crystals in the prior art.
Accordingly, nitride semiconductor layers with a low density of
dislocations may be deposited thereon by an epitaxy process.
[0032] In addition, compared to a conventional crystal-growing
technique, the creation of cracks in the metal nitride single
crystal layer or the nitride semiconductor layers formed thereon
can be suppressed or virtually prevented. The creation of cracks
can be suppressed probably by the following mechanism. Firstly, in
accordance with the thermal hysteresis during the process steps of
forming the metal single crystal layer and nitrifying the metal
single crystal layer, thermal stress, which is caused between the
metal nitride single crystal layer and the substrate or the metal
single crystal layer can be reduced. Secondly, since the
interfacial state between the metal single crystal layer or the
metal nitride single crystal layer and the substrate is different
from that resulting from the conventional technique, the stress can
be relaxed, or the generation of the stress can be suppressed.
[0033] The metal single crystal layer may be formed by a known
technique. For example, the metal single crystal layer may be
epitaxially grown on a single crystal substrate by an ionized
cluster beam (ICB) process or a sputtering technique. Methods for
growing a metal single crystal layer by an ICB process, ICB
apparatuses and growth conditions are disclosed, for example, by H.
Inokawa et al., Jpn. J. Appl. Phys. 24 (1985), pp. L173-L174, I.
Yamada et al., J. Appl. Phys. 56 (1986), pp. 2746-2750 and K.
Yamada, edited by Japan Surface Science Association, "Thin-film
Designing with Ion beams", Section 5.5, pp. 90-95, Kyoritsu
Shuppan, 1991. A method for growing a metal single crystal layer by
a sputtering technique is described, for example, by S. Yokoyama et
al., Jpn. J. Appl. Phys. 32 (1993), pp. L283-L286. According to the
ICB process, in particular, a metal single crystal layer of
excellent quality can be formed (the interfacial state between the
metal single crystal layer and the single crystal substrate would
also be good). In addition, in accordance with the ICB process, a
metal single crystal layer can be epitaxially grown on a single
crystal substrate with a relatively large lattice mismatch (e.g.,
about 25% or more). The documents cited above are hereby
incorporated by reference as those disclosing a method for
epitaxially growing a metal single crystal layer, which is
applicable to the embodiments of the present invention.
[0034] According to a method for epitaxially growing a metal single
crystal layer on a single crystal substrate, various types of
single crystal substrates may be used. The single crystal substrate
may either be a dielectric (insulator) or have electrical
conductivity (semiconductor or conductor). If a conductive
substrate is used, then the structure of the semiconductor device
can be advantageously simplified. This point will be detailed in
describing embodiments of a method for fabricating a semiconductor
device.
[0035] If a nitride semiconductor device is fabricated in
accordance with the method for growing nitride semiconductor
crystals according to the present invention, the creation of cracks
in a nitride semiconductor layer can be suppressed and the density
of defects in the layer can be reduced. Accordingly, a highly
reliable nitride semiconductor device with a long lifetime can be
fabricated.
[0036] Hereinafter, a method for growing nitride semiconductor
crystals according to an exemplary embodiment of the present
invention will be described with reference to FIGS. 1A through 6.
In all the drawings referred to in the following description,
components with similar basic functions will be identified by the
same reference numeral for the sake of simplicity
[0037] FIGS. 1A, 1B and 1C are cross-sectional views illustrating
respective process steps for growing nitride semiconductor crystals
in an exemplary embodiment of the present invention.
[0038] First, as shown in FIG. 1A, a metal single crystal layer 24
is formed on a substrate 22. For example, a single crystal
substrate is used as the substrate 22 and the metal single crystal
layer 24 is epitaxially grown on the single crystal substrate 22 by
an ICB process, which may be carried out as disclosed in the
documents cited above. For instance, the ICB process may be
performed at room temperature within an ambient at a pressure of
about 1.times.10.sup.-9 Torr (i.e., about 1.4.times.10.sup.-7 Pa)
or less. Before this epitaxial growth process step is performed, a
process step of cleaning the surface of the single crystal
substrate 22 may be carried out.
[0039] The single crystal substrate 22 may be made of: insulator
single crystals of sapphire, spinel, magnesium oxide, zinc oxide,
chromium oxide, lithium niobium oxide, lithium tantalum oxide or
lithium gallium oxide; semiconductor single crystals represented by
Si.sub.1-s-tGe.sub.sC.sub.t (where 0.ltoreq.s, t.ltoreq.1 and
0.ltoreq.s+t.ltoreq.1) or A.sub.1-uB.sub.u (where 0<u<1, A is
one of Al, Ga and In and B is one of As, P and Sb); or metal single
crystals of hafnium, for example. The metal single crystal layer 24
to be epitaxially grown on the single crystal substrate 22 may be
made of Al.sub.1-x-yGa.sub.xIn.sub.y (where 0.ltoreq.x, y.ltoreq.1
and 0.ltoreq.x+y<1).
[0040] Next, as shown in FIG. 1B, the metal single crystal layer 24
is nitrified, thereby forming the metal nitride single crystal
layer 25. This nitrification process step may be performed by
heating the metal single crystal layer 24 within an ambient of a
compound containing nitrogen. The compound containing nitrogen is
preferably hydrazine (N.sub.2H.sub.4) or ammonium (NH.sub.3).
Hydrazine is particularly preferable, because hydrazine has higher
nitrification ability than ammonium and can shorten the
nitrification time or lower the nitrification temperature.
[0041] The nitrification temperature can be appropriately set
depending on the necessity. However, the upper limit of the
nitrification temperature is preferably lower than the melting
point of the metal single crystal layer 24. This is because if the
metal single crystal layer 24 is heated at a temperature equal to
or higher than the melting point thereof for a long time, then the
metal single crystal layer 24 melts and the crystal structure
thereof collapses. In such a situation, the metal nitride layer,
formed by the nitrification, is sometimes a non-single crystal
layer or a crystal layer with a large number of dislocations.
Accordingly, in order to form a metal nitride single crystal layer
of good quality, the metal single crystal layer is preferably
nitrified at a temperature lower than the melting point of the
metal single crystal layer by about 100.degree. C. or more. The
nitrification temperature has no particular lower limit. However,
since the nitrification reaction of a metal is an Arrhenius-type
reaction, the higher the nitrification temperature, the shorter the
time taken to nitrify the metal single crystal layer. For example,
in nitrifying a metal single crystal layer of
Al.sub.1-x-yGa.sub.xIn.sub.y, the nitrification temperature is
preferably about 200.degree. C. or more within the hydrazine
ambient or about 400.degree. C. or more within the ammonium
ambient. If the nitrification temperature is set at such a value, a
metal single crystal layer with a thickness of several tens
nanometers can be nitrified within several tens minutes. Since the
metal nitride single crystal layer 25, which is formed by
nitrifying the metal single crystal layer 24, is thicker than the
original metal single crystal layer 24, the thickness of the layer
25 is emphasized in FIG. 1B.
[0042] Then, a nitride semiconductor layer 26 is epitaxially grown
on the resulting metal nitride layer 25 by a known technique. For
example, a layer made of a nitride represented as
Al.sub.1-s-tGa.sub.sIn.sub.tN (where 0.ltoreq.s, t.ltoreq.1 and
0.ltoreq.s+t.ltoreq.1) may be epitaxially grown as the nitride
semiconductor layer 26. Naturally, the composition of the nitride
semiconductor layer 26 may be different from that of the metal
nitride layer 25. Since the metal nitride single crystal layer 25
has a small number of dislocations, the nitride semiconductor layer
26, which is epitaxially grown thereon, is also a single crystal
layer with a small number of dislocations. In addition, compared to
a conventional crystal-growing technique, the creation of cracks in
the metal nitride single crystal layer 25 or the nitride
semiconductor layer 26 formed thereon can be suppressed or
virtually prevented. For example, according to the conventional
crystal-growing technique, if a GaN layer is epitaxially grown on
an AlN buffer layer deposited on an Si single crystal substrate at
a high temperature (e.g., about 1000.degree. C.), a distance
between cracks, which are generated in the AlN buffer layer and the
GaN layer, is about 20 .mu.m on average. On the other hand, if the
GaN layer is epitaxially grown on an AlN layer obtained by
nitrifying an Al metal single crystal layer, a distance between
cracks is about 2 mm to 30 mm on average. An average distance
between cracks, which are generated in the nitride semiconductor
layer formed according to the crystal-growing method of the present
invention, is about 10 mm or more. Therefore, according to the
crystal-growing method of the present invention, semiconductor
devices can be fabricated with a good yield.
[0043] Another exemplary embodiment of a method for growing nitride
semiconductor crystals is illustrated in FIGS. 2A, 2B and 2C. This
embodiment is different from the embodiment shown in FIGS. 1A, 1B
and 1C in the nitrification process step shown in FIG. 2B.
Specifically, in the step shown in FIG. 2B, the metal single
crystal layer 24 is nitrified and metal atoms diffuse from the
metal single crystal layer 24 into the substrate 22 to form a metal
diffused layer 22a within the surface of the substrate 22 (i.e.,
the interface between the metal nitride single crystal layer 25 and
the substrate 22).
[0044] The probability of diffusion of the metal atoms is dependent
on the combination of materials for the single crystal substrate 22
and the metal single crystal layer 24. For example, if the single
crystal substrate 22 is made of silicon or a semiconductor
represented as A.sub.1-uB.sub.u (where 0<u<1, A is one of Al,
Ga and In and B is one of As, P and Sb) and the metal single
crystal layer 24 is made of Al or an alloy containing Al, more
specifically, Al.sub.1-x-yGa.sub.xIn.sub.- y, then Al atoms are
likely to diffuse into the substrate 22 to form the metal diffused
layer 22a easily. For example, if Al is used to form the metal
single crystal layer 24 and the resultant Al single crystal layer
is nitrified at about 550.degree.C. for about an hour, then a metal
diffused layer 22a with a thickness of about 1 nm is obtained.
[0045] It is considered that this metal diffused layer 22a improves
the adhesion between the substrate 22 and the metal nitride single
crystal layer 25 and relaxes a stress resulting from a difference
in thermal expansion coefficient therebetween. In addition, the
metal diffused layer 22a can also reduce the thermal contact
resistance between the substrate 22 and the multilayer structure
formed thereon. Furthermore, when the single crystal substrate 22
and the metal nitride single crystal layer 25 both have
conductivity, the metal diffused layer 22a can constitute an ohmic
contact therebetween.
[0046] Still another exemplary embodiment of a method for growing
nitride semiconductor crystals is illustrated in FIGS. 3A, 3B and
3C. This embodiment is different from the embodiment shown in FIGS.
1A, 1B and 1C in the nitrification process step shown in FIG. 3B.
Specifically, in the step shown in FIG. 3B, only a part of the
metal single crystal layer 24 is nitrified to form the metal
nitride single crystal layer 25. The thickness of that part of the
metal single crystal layer 24 to be nitrified can be controlled by
adjusting the nitrification time, for example.
[0047] By partially leaving the metal single crystals 24 between
the single crystal substrate 22 and the metal nitride single
crystal layer 25 without completely nitrifying the metal single
crystal layer 24, the thermal contact resistance between the
substrate 22 and the multilayer structure formed thereon can be
reduced. In addition, a stress created between the single crystal
substrate 22 and the metal nitride single crystal layer 25 can be
relaxed by the metal single crystal layer 24. This is probably
because the elastic modulus of a metal is generally lower than that
of a nitride of the metal.
[0048] Yet another exemplary embodiment of a method for growing
nitride semiconductor crystals is illustrated in FIGS. 4A, 4B and
4C. This embodiment is different from the embodiment shown in FIGS.
1A, 1B and 1C in the nitrification process step shown in FIG. 4B.
Specifically, in the step shown in FIG. 4B, only a part of the
metal single crystal layer 24 is nitrified to form the metal
nitride single crystal layer 25, and metal atoms diffuse from the
metal single crystal layer 24 into the substrate 22 to form a metal
diffused layer 22a within the surface of the substrate 22 (i.e.,
the interface between the metal single crystal layer 24 and the
substrate 22). As already described for the embodiment shown in
FIGS. 2A, 2B and 2C, the metal diffused layer 22a is formed
sometimes easily but sometimes not, dependent on the combination of
materials for the single crystal substrate 22 and the metal single
crystal layer 24. By using the above-exemplified combination of
materials and controlling the thickness of that part of the metal
single crystal layer 24 to be nitrified, the structure shown in
FIG. 4B can be obtained. As already described for the embodiment
shown in FIGS. 3A, 3B and 3C, the thickness of that part of the
metal single crystal layer 24 to be nitrified can be controlled by
adjusting the nitrification time, for example.
[0049] Yet another exemplary embodiment of a method for growing
nitride semiconductor crystals is illustrated in FIGS. 5A, 5B, 5C
and 5D. This embodiment is different from the foregoing embodiments
in that an additional metal single crystal layer 23 is formed
before the metal single crystal layer 24 to be nitrified is formed
over the substrate 22 as shown in FIG. 5A.
[0050] The metal single crystal layer 23, as well as the metal
single crystal layer 24 of the foregoing embodiments, may be formed
by a known technique. For example, a single crystal substrate is
prepared as the substrate 22 and the metal single crystal layer 23
is epitaxially grown thereon by an ICB process, for example. A
metal material for the metal single crystal layer 23 is preferably
Au or an alloy containing Au (e.g., an alloy of Au and Ge). By
forming this additional metal single crystal layer 23, the thermal
contact resistance between the substrate 22 and the multilayer
structure formed thereon can be reduced.
[0051] If the single crystal substrate 22 is made of a
semiconductor represented as Si.sub.1-s-tGe.sub.sC.sub.t (where
0.ltoreq.s, t.ltoreq.1 and 0.ltoreq.s+t.ltoreq.1) or
Al.sub.1-uB.sub.u (where 0<u<1, A is one of Al, Ga and In and
B is one of As, P and Sb) where the metal single crystal layer 23
is made of Au or an alloy containing Au, then metal atoms
constituting the metal single crystal layer 23 diffuse into the
single crystal substrate 22 to form a metal diffused layer therein
during the process step of nitrifying the metal single crystal
layer 24. As a result, not only thermal contact resistance but also
electrical contact resistance can be reduced between the single
crystal substrate 22 and the semiconductor multilayer structure
formed thereon. In this case, part of the atoms constituting the
metal single crystal layer 23 may be diffused. Alternatively, as
shown in FIGS. 6A, 6B, 6C and 6D, a metal diffused layer 22a may be
formed by diffusing all the atoms constituting the metal single
crystal layer 23 into the single crystal substrate 22 during the
step of forming the metal nitride single crystal layer 25 through
the nitrification of the metal single crystal layer 24. According
to this method, the metal single crystal layer 23 disappears (see
FIG. 6C). In order to eliminate the metal single crystal layer 23
through diffusion, the thickness of the metal single crystal layer
23 is preferably about 3 nm or less. Also, the temperature and time
for the process step of nitrifying the metal single crystal layer
24 may be set based on the degree of diffusion of the metal single
crystal layer 23. For example, if atoms in the metal single crystal
layer 23 should be continuously diffused after the nitrification
reaction of the metal single crystal layer 24 is over, heating may
be continued.
[0052] This embodiment may be combined with any of the foregoing
embodiments. For example, if the metal single crystal layer 23 is
made of Au or an alloy containing Au and the single crystal
substrate 22 is made of a semiconductor represented as
Si.sub.1-s-tGe.sub.sC.sub.t or A.sub.1-uB.sub.u, then metal atoms
diffuse from the metal single crystal layer 23 into the single
crystal substrate 22 to form the metal diffused layer 22a as in the
embodiment shown in FIG. 2B. In this case, if the metal single
crystal layer 23 is sufficiently thin (e.g., about 3 nm or less),
then all the metal atoms constituting the metal single crystal
layer 23 diffuse into the single crystal substrate 22 and
substantially no metal single crystal layer 23 is left. In this
structure, the AlN single crystal layer 25 with satisfactorily
aligned crystal orientations is formed on the sapphire substrate
22. Accordingly, the density of defects or dislocations can be
reduced both in the interface between the sapphire substrate 22 and
the AlN single crystal layer 25 and in the n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active layer 27,
p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type GaN
contact layer 29, which are stacked thereon. In this structure,
increase in resistance due to a Schottky barrier generated by a
semiconductor/metal interface can be prevented, since there is no
semiconductor/metal interface.
[0053] Also, after the metal single crystal layers 23 and 24 have
been formed, only a part of the metal single crystal layer 24 may
be nitrified as described for the embodiment shown in FIG. 3B.
Furthermore, after the metal single crystal layers 23 and 24 have
been formed, the metal diffused layer 22a may be formed and only a
part of the metal single crystal layer 24 may be nitrified to leave
the metal single crystal layer 24 between the metal nitride single
crystal layer 25 and the metal single crystal layer 23 as described
for the embodiment shown in FIG. 4B. In any combination, the
effects of the embodiment shown in FIG. 2B, 3B or 4B can be
additionally attained.
[0054] Since a nitride semiconductor layer having (0001) principal
surface is generally used in a semiconductor device, it is
preferable to form a metal nitride layer having (0001) principal
surface so that a nitride semiconductor layer having (0001)
principal surface can be epitaxially grown on the metal nitride
layer. More specifically, when a single crystal substrate, made of
semiconductor single crystals represented by
Si.sub.1-s-tGe.sub.sC.sub.t (where 0.ltoreq.s, t.ltoreq.1 and
0.ltoreq.s+t.ltoreq.1) or A.sub.1-uB.sub.u (where 0<u<1, A is
one of Al, Ga and In and B is one of As, P and Sb), is used, an
Al.sub.1-x-yGa.sub.xIn.sub.yN (where 0.ltoreq.x, y.ltoreq.1 and
0.ltoreq.x+y<1) single crystal layer having (0001) principal
surface can be obtained by nitrifying an
Al.sub.1-x-yGa.sub.xIn.sub.y layer having (111) principal surface
epitaxially grown on (111) plane of the single crystal substrate
made of Si.sub.1-s-tGe.sub.sC.sub.t or A.sub.1-uB.sub.u.
[0055] Hereinafter, specific embodiments of fabricating a
semiconductor device in accordance with the foregoing method for
growing nitride semiconductor crystals will be described. In the
following illustrative embodiments, a light-emitting device (i.e.,
a semiconductor laser diode) is fabricated as an exemplary
semiconductor device. However, the present invention is not limited
to those specific embodiments, but is applicable to various other
semiconductor devices like a field effect transistor (FET).
EMBODIMENT 1
[0056] In a first exemplary embodiment of the present invention, a
light-emitting device 100 is formed using a substrate with no
conductivity.
Specific Example 1-1
[0057] As shown in FIG. 7, the light-emitting device 100 includes:
an AlN single crystal layer 25 (thickness: 10 nm); an n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26 (thickness: 1 .mu.m); a
multiple quantum well (MQW) active layer 27; a p-type
Ga.sub.0.9Al.sub.0.1N cladding layer 28 (thickness: 0.5 .mu.m); and
a p-type GaN contact layer 29 (thickness: 0.1 .mu.m), which are
stacked in this order on a sapphire substrate 22. The MQW active
layer 27 is formed by alternately stacking ten pairs of undoped
In.sub.0.2Ga.sub.0.8N layers (thickness: 5 nm) and undoped GaN
layers (thickness: 5 nm). Of these layers, the lowermost undoped
GaN layer is in contact with the n-type Ga.sub.0.9Al.sub.0.1N
cladding layer 26.
[0058] This semiconductor multilayer structure, including the
cladding layer 26, active layer 27, cladding layer 28 and contact
layer 29, which are formed on the AlN single crystal layer 25 has
been subjected to a mesa-etching process. Through this etching
process, a pair of electrodes 32a and 32b for applying a voltage to
the semiconductor multilayer structure are formed on the contact
layer 29 and on the cladding layer 26, respectively.
[0059] In this structure, the AlN single crystal layer 25 with
satisfactorily aligned crystal orientations is formed on the
sapphire substrate 22. Accordingly, the density of defects or
dislocations can be reduced both in the interface between the
sapphire substrate 22 and the AlN single crystal layer 25 and in
the n-type Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active
layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type
GaN contact layer 29, which are stacked thereon.
[0060] The light-emitting device 100 may be fabricated in
accordance with the crystal-growing method shown in FIGS. 1A
through 1C. A method for fabricating this light-emitting device 100
will be described with reference to FIGS. 8A, 8B and 8C.
[0061] First, as shown in FIG. 8A, an Al single crystal layer 24 is
deposited on a sapphire substrate 22 by an ICB process. Next, as
shown in FIG. 8B, the Al single crystal layer 24 is nitrified to be
an AlN single crystal layer 25. The nitrification may be performed
by reacting nitrogen components, which are included in a nitrogen
compound such as hydrazine or ammonium contained in an appropriate
carrier gas (e.g., H.sub.2 gas), with the Al single crystal layer
24 while the temperature of the sapphire substrate 22 is kept at
550.degree. C., which is about 100.degree. C. lower than the
melting point of Al single crystals (i.e., 660.degree. C.).
[0062] Thereafter, as shown in FIG. 8C, an n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26 doped with Si, an MQW
active layer 27, a p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28
doped with Mg and a p-type GaN contact layer 29 doped with Mg are
stacked in this order on the AlN single crystal layer 25 by an
MOVPE process. In this process step, crystals for the n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26, p-type
Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type GaN contact
layer 29 are grown at 1000.degree. C., while crystals for the MQW
active layer 27 are grown at 800.degree. C.
[0063] The resulting semiconductor multilayer structure, including
the respective layers 26, 27, 28 and 29, is partially etched,
thereby exposing the n-type Ga.sub.0.9Al.sub.0.1N cladding layer
26. Finally, respective ohmic electrodes 32a and 32b are formed on
the p-type GaN contact layer 29 and on the n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26 to complete the
light-emitting device 100. The electrode 32a may be formed of, for
example, Ni/Au, and the electrode 32b may be formed of, for
example, Ti/Au by an electron beam deposition method.
[0064] In this structure, since the AlN single crystal layer 25 is
formed by nitrifying the Al single crystal layer 24, the AlN single
crystal layer 25 can be formed over the entire surface of the
sapphire substrate 22. Accordingly, the crystallinity of the n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active layer 27,
p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type GaN
contact layer 29, which are stacked on the AlN single crystal layer
25, can be improved.
[0065] The cross section of the light-emitting device 100 according
to the first specific example of the first embodiment was observed
with a transmission electron microscope (TEM). As a result, the
density of defects or dislocations in the n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active layer 27,
p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and GaN contact
layer 29 was 1.0.times.10.sup.5/cm.sup.2, which is about {fraction
(1/10,000)}compared to a conventional light-emitting device.
Specific Example 1-2
[0066] A light-emitting device according to a second specific
example of the first embodiment includes an Al.sub.0.9Ga.sub.0.1N
single crystal layer 25 (thickness: 5 nm), the n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active layer 27,
p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type GaN
contact layer 29, which are stacked in this order on an
MgAl.sub.2O.sub.4 (spinel) substrate 22 as shown in FIG. 7.
[0067] In this structure, the Al.sub.0.9Ga.sub.0.1N single crystal
layer 25 with satisfactorily aligned crystal orientations is formed
on the spinel substrate 22. Accordingly, the density of defects or
dislocations can be reduced in both the interface between the
spinel substrate 22 and the Al.sub.0.9Ga.sub.0.1N single crystal
layer 25, and the n-type Ga.sub.0.9Al.sub.0.1N cladding layer 26,
MQW active layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28
and p-type GaN contact layer 29, which are stacked thereon.
[0068] A method for fabricating this light-emitting device 100 will
be described with reference to FIGS. 8A, 8B and 8C again.
[0069] First, as shown in FIG. 8A, an Al.sub.0.9Ga.sub.0.1 alloy
single crystal layer 24 is deposited to be 5 nm thick on a spinel
substrate 22 by an ICB process. Next, as shown in FIG. 8B, the
Al.sub.0.9Ga.sub.0.1 alloy single crystal layer 24 is nitrified to
be an Al.sub.0.9Ga.sub.0.1N single crystal layer 25. The
nitrification may be performed by reacting nitrogen components,
which are included in a nitrogen compound such as hydrazine or
ammonium contained in an appropriate carrier gas (e.g., H.sub.2
gas), with the Al.sub.0.9Ga.sub.0.1 alloy single crystal layer 24
while the temperature of the spinel substrate 22 is kept at
500.degree. C.
[0070] Thereafter, as shown in FIG. 8C, n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26 doped with Si, MQW active
layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 doped with
Mg and p-type GaN contact layer 29 doped with Mg are stacked in
this order on the Al.sub.0.9Ga.sub.0.1N single crystal layer 25 as
in the first specific example. The resulting semiconductor
multilayer structure, including the respective layers 26, 27, 28
and 29, is partially etched, thereby exposing the n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26. Finally, respective ohmic
electrodes 32a and 32b are formed on the p-type GaN contact layer
29 and on the n-type Ga.sub.0.9Al.sub.0.1N cladding layer 26.
[0071] In this structure, since the Al.sub.0.9Ga.sub.0.1N single
crystal layer 25 is formed by nitrifying the Al.sub.0.9Ga.sub.0.1
alloy single crystal layer 24, the Al.sub.0.9Ga.sub.0.1N single
crystal layer 25 can be formed over the entire surface of the
spinel substrate 22. Accordingly, the crystallinity of the n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active layer 27,
p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type GaN
contact layer 29, which are stacked on the Al.sub.0.9Ga.sub.0.1N
single crystal layer 25, can be improved.
[0072] The cross section of the light-emitting device 100 according
to the second specific example of the first embodiment was observed
with a TEM. As a result, the density of defects or dislocations in
the n-type Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active
layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and GaN
contact layer 29 was 1.0.times.10.sup.5/cm.sup.2, which is about
{fraction (1/10,000)} compared to a conventional light-emitting
device.
[0073] FIG. 9 illustrates respective lifetimes of the
light-emitting devices 100 according to the first and second
specific examples of the first embodiment (hereinafter, identified
by E1 and E2, respectively) and a conventional light-emitting
device C, which are all operated at a temperature of 70.degree. C.
with an optical output of 5 mW. In FIG. 9, the curves E1, E2 and C
indicate respective relationships between the operating time and a
variation in operating current per unit time of the light-emitting
devices E1, E2 and C. In FIG. 9, as the variation
.DELTA.I/.DELTA.t, which is a variation of the operating current
with the operating time, comes closer to 1, a light-emitting device
deteriorates to a lesser degree and can operate for a longer time.
As shown in FIG. 9, in the light-emitting devices E1 and E2 of the
present invention, .DELTA.I/.DELTA.t is still close to 1 even after
these devices have been operated for 10,000 hours. In contrast,
after the conventional light-emitting device C has been operated
for 5,000 hours, .DELTA.I/.DELTA.t greatly deviates from 1.
Accordingly, the light-emitting devices E1 and E2 of the present
invention have much longer lifetimes, and are a lot more reliable,
than the conventional light-emitting device C. It should be noted
that the oscillation wavelengths of these light-emitting devices
were all 420 nm.
[0074] In the foregoing specific examples, the same effects can be
attained if the sapphire or spinel substrate 22 is replaced with a
single crystal substrate of MgO, ZnO, Cr.sub.2O.sub.3, LiNbO.sub.3,
LiTaO.sub.3 or LiGaO.sub.2.
[0075] As described above, the first embodiment of the present
invention provides a semiconductor device with reduced defects or
dislocations in the interface between an insulating single crystal
substrate and a nitride semiconductor crystal layer, a longer
lifetime and higher reliability and a method for fabricating the
same.
[0076] In the foregoing illustrative embodiment, a method for
fabricating a light-emitting device in accordance with the nitride
semiconductor crystal-growing method shown in FIGS. 1A through 1C
has been described. Alternatively, any of the other crystal-growing
methods shown in FIGS. 2A through 6D is also applicable. According
to any of these methods, the same effects as those of the first
embodiment can be attained.
EMBODIMENT 2
[0077] In a second exemplary embodiment of the present invention, a
light-emitting device 200 is formed using a substrate with
conductivity, which includes a semiconductor substrate and a
conductor substrate made of a metal, for example.
Specific Example 2-1
[0078] As shown in FIG. 10, the light-emitting device 200 according
to a first specific example of the second embodiment includes: an
Al single crystal layer 24 (thickness: 8 nm); an AlN single crystal
layer 25 (thickness: 2 nm); an n-type Ga.sub.0.9Al.sub.0.1N
cladding layer 26 (thickness: 1 .mu.m); an MQW active layer 27; a
p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 (thickness: 0.5
.mu.m); and a p-type GaN contact layer 29 (thickness: 0.1 .mu.m),
which are stacked in this order on an n-type Si substrate 22. The
MQW active layer 27 is formed by alternately stacking twenty pairs
of undoped In.sub.0.2Ga.sub.0.8N layers (thickness: 5 nm) and
undoped GaN layers (thickness: 5 nm). Of these layers, the
lowermost undoped GaN layer is in contact with the n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26.
[0079] A pair of electrodes 32a and 32b for applying a voltage to
the semiconductor multilayer structure, including the n-type
cladding layer 26, active layer 27, p-type cladding layer 28 and
contact layer 29, which are formed on the AlN single crystal layer
25, are formed on the contact layer 29 and on the Si single crystal
substrate 22, respectively, so as to face each other.
[0080] In this structure, the Al single crystal layer 24 with
satisfactorily aligned crystal orientations is formed on the n-type
Si single crystal substrate 22, and the AlN single crystal layer 25
is formed thereon. Accordingly, the density of defects or
dislocations can be reduced in both the interface between the
n-type Si single crystal substrate 22 and the Al single crystal
layer 24, and the n-type Ga.sub.0.9Al.sub.0.1N cladding layer 26,
MQW active layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28
and p-type GaN contact layer 29, which are stacked thereon. Also,
heat generated in the MQW active layer 27 can be directly
dissipated through the n-type Si single crystal substrate 22.
Furthermore, since an electrode can be formed on the back of the
n-type Si single crystal substrate 22, an increased number of
light-emitting devices can be formed per substrate 22 compared to a
conventional structure. That is to say, a light-emitting device can
be fabricated at a lower cost.
[0081] The light-emitting device 200 may be fabricated in
accordance with the crystal-growing method shown in FIGS. 3A
through 3C. A method for fabricating this light-emitting device 200
will be described with reference to FIGS. 11A, 11B and 11C.
[0082] First, as shown in FIG. 11A, an Al single crystal layer 24
is deposited to be 10 nm thick on an n-type Si single crystal
substrate 22 by an ICB process. Next, as shown in FIG. 8B, part of
the Al single crystal layer 24 is nitrified to the depth of 2 nm as
measured from the surface thereof, thereby turning that part into
an AlN single crystal layer 25 with the thickness of 2 nm. The
nitrification may be performed by reacting nitrogen components,
which are included in a nitrogen compound such as hydrazine or
ammonium contained in an appropriate carrier gas (e.g., H.sub.2
gas), with the Al single crystal layer 24 while the temperature of
the n-type Si single crystal substrate 22 is kept at 550.degree.
C., which is about 100.degree. C. lower than the melting point of
Al single crystals (i.e., 660.degree. C.).
[0083] Thereafter, as shown in FIG. 11C, n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26 doped with Si, MQW active
layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 doped with
Mg and p-type GaN contact layer 29 doped with Mg are stacked in
this order on the AlN single crystal layer 25 by an MOVPE process.
In this process step, crystals for the n-type Ga.sub.0.9Al.sub.0.1N
cladding layer 26, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28
and p-type GaN contact layer 29 are grown at 1000.degree. C., while
crystals for the MQW active layer 27 are grown at 800.degree.
C.
[0084] Finally, respective ohmic electrodes 32a and 32b are formed
to face each other on the p-type GaN contact layer 29 and on the
n-type Si single crystal substrate 22, thereby completing the
light-emitting device 200. The ohmic electrode 32b may be formed
of, for example, Al, Ti or Pt, with an optional annealing step at
about 300.degree. C. to about 400.degree. C. The ohmic electrode
32b may be formed as described in Embodiment 1 of the present
invention.
[0085] In this structure, since the AlN single crystal layer 25 is
formed by nitrifying the Al single crystal layer 24, the Al and AlN
single crystal layers 24 and 25 can be formed over the entire
surface of the n-type Si single crystal substrate 22. Accordingly,
the crystallinity of the n-type Ga.sub.0.9Al.sub.0.1N cladding
layer 26, MQW active layer 27, p-type Ga.sub.0.9Al.sub.0.1N
cladding layer 28 and p-type GaN contact layer 29, which are
stacked on the AlN single crystal layer 25, can be improved.
[0086] The cross section of the light-emitting device 200 according
to the first specific example of the second embodiment was observed
with a TEM. As a result, the density of defects or dislocations in
the n-type Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active
layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type
GaN contact layer 29 was 1.0.times.10.sup.5/cm.sup.2, which is
about {fraction (1/10,000)} compared to a conventional
light-emitting device.
Specific Example 2-2
[0087] A light-emitting device 300 according to a second specific
example of the second embodiment includes Al.sub.0.9Ga.sub.0.1N
single crystal layer 25 (thickness: 5 nm), n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active layer 27,
p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type GaN
contact layer 29, which are stacked in this order on an n-type GaAs
substrate 22 as shown in FIG. 12.
[0088] In this structure, the Al.sub.0.9Ga.sub.0.1N single crystal
layer 25 with satisfactorily aligned crystal orientations is formed
on the n-type GaAs substrate 22. Accordingly, the density of
defects or dislocations can be reduced in both the interface
between the n-type GaAs substrate 22 and the Al.sub.0.9Ga.sub.0.1N
single crystal layer 25 and the n-type Ga.sub.0.9Al.sub.0.1N
cladding layer 26, MQW active layer 27, p-type
Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type GaN contact
layer 29, which are stacked thereon. Also, heat generated in the
MQW active layer 27 can be directly dissipated through the n-type
GaAs substrate 22. Furthermore, since an electrode can be formed on
the back of the n-type GaAs substrate 22, an increased number of
light-emitting devices can be formed per substrate 22 compared to a
conventional structure. That is to say, a light-emitting device can
be fabricated at a lower cost.
[0089] The light-emitting device 300 is fabricated in accordance
with the crystal-growing method shown in FIGS. 1A through 1C. A
method for fabricating this light-emitting device 300 will be
described with reference to FIGS. 8A, 8B and 8C again.
[0090] First, as shown in FIG. 8A, an Al.sub.0.9Ga.sub.0.1 alloy
single crystal layer 24 is deposited to be 5 nm thick on an n-type
GaAs substrate 22 by an ICB process. Next, as shown in FIG. 8B, the
Al.sub.0.9Ga.sub.0.1 alloy single crystal layer 24 is nitrified to
be an Al.sub.0.9Ga.sub.0.1N single crystal layer 25. The
nitrification may be performed by reacting nitrogen components,
which are included in a nitrogen compound such as hydrazine or
ammonium contained in an appropriate carrier gas (e.g., H.sub.2
gas), with the Al.sub.0.9Ga.sub.0.1 alloy single crystal layer 24
while the temperature of the n-type GaAs substrate 22 is kept at
500.degree. C. Thereafter, as shown in FIG. 8C, n-type
Ga.sub.0.9Al.sub.0.1N cladding layer 26 doped with Si, MQW active
layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 doped with
Mg and p-type GaN contact layer 29 doped with Mg are stacked in
this order on the Al.sub.0.9Ga.sub.0.1N single crystal layer 25 as
in the first specific example.
[0091] Finally, respective ohmic electrodes 32a and 32b are formed
to each face other on the p-type GaN contact layer 29 and the
n-type GaAs substrate 22, thereby completing the light-emitting
device 300.
[0092] In this structure, since the Al.sub.0.9Ga.sub.0.1N single
crystal layer 25 is formed by nitrifying the Al.sub.0.9Ga.sub.0.1
alloy single crystal layer 24, the Al.sub.0.9Ga.sub.0.1N single
crystal layer 25 can be formed over the entire surface of the
n-type GaAs substrate 22. Accordingly, the crystallinity of the
n-type Ga.sub.0.9Al.sub.0.1N cladding layer 26, MQW active layer
27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28 and p-type GaN
contact layer 29, which are stacked on the Al.sub.0.9Ga.sub.0.1N
single crystal layer 25, can be improved.
[0093] The cross section of the light-emitting device 300 according
to the second specific example of the second embodiment was
observed with a TEM. As a result, the density of defects or
dislocations in the n-type Ga.sub.0.9Al.sub.0.1N cladding layer 26,
MQW active layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding layer 28
and GaN contact layer 29 was 1.0.times.10.sup.5/cm.sup.2, which is
about {fraction (1/10,000)}compared to a conventional
light-emitting device.
[0094] Respective lifetimes of the light-emitting devices 200 and
300 according to the first and second specific examples of the
second embodiment, which were both operated at a temperature of
70.degree. C. with an optical output of 5 mW, are substantially the
same as those of the light-emitting devices E1 and E2 shown in FIG.
9. That is to say, in the light-emitting devices 200 and 300 of the
second embodiment, .DELTA.I/.DELTA.t is still close to 1 even after
these devices have been operated for 10,000 hours. In contrast,
after the conventional light-emitting device C has been operated
for 5,000 hours, .DELTA.I/.DELTA.t greatly deviates from 1.
Accordingly, the light-emitting devices 200 and 300 of the second
embodiment also have much longer lifetimes, and are a lot more
reliable, than the conventional light-emitting device C. It should
be noted that the oscillation wavelengths of these light-emitting
devices were all 420 nm.
[0095] In the foregoing specific examples of the second embodiment,
the same effects are attained if the n-type Si single crystal
substrate or n-type GaAs substrate 22 is replaced with a
semiconductor single crystal substrate with conductivity such as
n-type GaAs substrate or n-type SiC substrate. Also, a p-type
semiconductor single crystal substrate with conductivity or a
conductor single crystal substrate made of a metal such as hafnium
may be used instead. Among various metals, hafnium single crystals
are preferable, because the lattice constant of hafnium single
crystals is close to that of nitride semiconductor single
crystals.
[0096] As described above, according to the second embodiment of
the present invention, a metal single crystal layer and a nitride
semiconductor single crystal layer are formed in this order on a
conductive single crystal substrate and then semiconductor layers
are formed thereon. Accordingly, heat radiation can be improved and
the density of defects or dislocations in the semiconductor layers
can be reduced. Furthermore, since an electrode can be formed on
the back of the conductive single crystal substrate, semiconductor
devices can be fabricated at a lower cost.
Specific Example 2-3
[0097] A light-emitting device 400 according to a third specific
example of the second embodiment includes AlN single crystal layer
25 (thickness: 5 nm), n-type Ga.sub.0.9Al.sub.0.1N cladding layer
26, MQW active layer 27, p-type Ga.sub.0.9Al.sub.0.1N cladding
layer 28 and p-type GaN contact layer 29, which are stacked in this
order on an n-type GaAs substrate 22 as shown in FIG. 13. In
addition, a metal diffused layer 22a is formed within the n-type
GaAs substrate 22 in the vicinity of the surface thereof closer to
the AlN single crystal layer 25. The metal diffused layer 22a is
formed by diffusing an alloy containing Au.
[0098] A pair of electrodes 32a and 32b for applying a voltage to
the semiconductor multilayer structure, including the n-type
cladding layer 26, active layer 27, p-type cladding layer 28 and
contact layer 29, which are formed on the AlN single crystal layer
25, are formed on the contact layer 29 and on the n-type GaAs
substrate 22, respectively, so as to face each other.
[0099] The light-emitting device 400 is fabricated in accordance
with the crystal-growing method shown in FIGS. 6A through 6D.
[0100] First, an n-type GaAs single crystal substrate 22, of which
the principal surface is (111) plane, is prepared. On the (111)
plane of the single crystal substrate 22, a metal single crystal
layer 23 of an Au/Ge alloy is epitaxially grown to be about 1 nm
thick by an ICB process as shown in FIG. 6A. The principal surface
of the resulting AuGe single crystal layer 23 is also (111)
plane.
[0101] Next, an Al single crystal layer 24 doped with Si at about
10.sup.18 cm.sup.-3 is epitaxially grown to be about 20 nm thick on
the (111) plane of the AuGe single crystal layer 23 by an ICB
process. The principal surface of the resulting Al single crystal
layer 24 is also (111) plane.
[0102] In epitaxially growing the metal single crystal layers 23
and 24 by an ICB process, an ICB apparatus, which includes source
gas supplies (i.e., AuGe and Si-doped Al source gas supplies) for
supplying the respective sources for the metal single crystal
layers 23 and 24 within a single chamber and can control the flow
rates of these source gases from the source gas supplies using a
shutter, for example, is preferably used. If such an ICB apparatus
is used, a high-purity film can be deposited, since there is no
need to take out the specimens from the chamber (i.e., without
breaking the vacuum within the chamber or causing leakage). The
epitaxy of the metal single crystal layers 23 and 24 by the ICB
process may be performed at room temperature, for example.
[0103] Then, while the Al single crystal layer 24 is nitrified,
AuGe atoms are diffused from the AuGe single crystal layer 23 into
the n-type GaAs substrate 22. In this process step, the GaAs single
crystal substrate 22 is heated up to a temperature lower than the
respective melting points of the GaAs single crystal substrate 22
itself and the Al single crystal layer 24 (e.g., 550.degree. C.)
and a nitrogen-containing compound gas is supplied into the
chamber. The nitrogen-containing compound is preferably hydrazine
or ammonium. In particular, since hydrazine has high nitrification
ability, hydrazine is preferable in view of the productivity. This
is because the nitrification time can be shortened and the
nitrification temperature can be lowered in such a case. By
nitrifying the Al single crystal layer 24 with a thickness of 20 nm
for about 10 to about 15 minutes, the AlN single crystal layer 25
is formed. The principal surface of the resulting AlN single
crystal layer 25 is (0001) plane. Since the thickness increases as
a result of the nitrification, the AlN layer 25 is illustrated in
FIG. 6B as being thicker than the Al layer 24. Also, in this
process step, AuGe atoms diffuse into the n-type GaAs substrate 22
to form an AuGe diffused layer 22a as shown in FIG. 6C. To diffuse
the AuGe atoms, the heating temperature and time may be adequately
adjusted during the nitrification process step. Even after the
nitrification reaction is substantially over, heating may be
continued for the diffusion purpose only.
[0104] Thereafter, a cladding layer 26 of n-type
Ga.sub.0.9Al.sub.0.1N single crystals is epitaxially grown on the
AlN single crystal layer 25 by an ICB process as shown in FIG. 6D.
Subsequently, a double heterojunction semiconductor multilayer
structure, including the n-type cladding layer 26, active layer 27,
p-type cladding layer 28 and contact layer 29, is formed thereon by
epitaxy as in the foregoing embodiments. And then electrodes 32a
and 32b are formed to face each other on the contact layer 29 and
on the n-type GaAs substrate 22, respectively. As a result, the
light-emitting device 400 shown in FIG. 13 is completed. The
crystal structure of the semiconductor multilayer structure is
hexagonal, and a laser device can be formed with crystals cleaved
on the facets of the cavity.
[0105] In the AuGe diffused layer 22a, which is formed within the
n-type GaAs substrate 22 in the vicinity of the surface thereof
closer to the n-type AlN layer 25, Ge atoms function as donors.
Accordingly, the Ge atoms decrease the electrical resistance in the
interface between the n-type GaAs substrate 22 and the n-type AlN
layer 25. Thus, this light-emitting device 400, in which the pair
of electrodes 32a and 32b are provided with the conductive
substrate 22 and the semiconductor multilayer structure interposed
therebetween, can have its operating voltage reduced.
[0106] In this third specific example of the second embodiment, if
a p-type GaAs substrate is used as the conductive single crystal
substrate 22 and AuNi single crystal layer and Mg-doped (e.g.,
about 0.5 mol %) Al single crystal layer are used as respective
metal single crystal layers 23 and 24, then a p-type AlN layer 25
can be formed over the p-type GaAs substrate 22. Accordingly, a
light-emitting device can be fabricated with a reversed arrangement
of conductivity types in the double heterostructure. In this
reversed arrangement, an AuNi diffused layer 22a is formed within
the p-type GaAs substrate 22 in the vicinity of the surface thereof
closer to the p-type AlN layer 25. Ni atoms in the AuNi diffused
layer 22a function as acceptors, thus decreasing the electrical
resistance in the interface between the p-type GaAs substrate 22
and the p-type AlN layer 25. As a result, the light-emitting device
can operate at a lower voltage.
[0107] In this specific example, the metal single crystal layer 23
may be made of element Au or any other alloy containing Au.
[0108] According to the inventive method for growing nitride
semiconductor crystals, a nitride semiconductor layer is
epitaxially grown on a metal nitride single crystal layer obtained
by nitrifying a metal single crystal layer. Therefore, a nitride
semiconductor layer is obtained with a much smaller number of
dislocations or defects than that formed by a conventional crystal
growing method.
[0109] In addition, a highly reliable nitride semiconductor device
with a longer lifetime can be obtained by fabricating the
semiconductor device using the method for growing nitride
semiconductor crystals according to the present invention. The
inventive method for growing nitride semiconductor crystals is
advantageously applicable to a method for fabricating a blue light
emitting laser diode.
[0110] While the present invention has been described in a
preferred embodiment, it will be apparent to those skilled in the
art that the disclosed invention may be modified in numerous ways
and may assume many embodiments other than that specifically set
out and described above. Accordingly, it is intended by the
appended claims to cover all modifications of the invention which
fall within the true spirit and scope of the invention.
* * * * *