U.S. patent number RE36,747 [Application Number 08/844,386] was granted by the patent office on 2000-06-27 for light-emitting device of gallium nitride compound semiconductor.
This patent grant is currently assigned to Kabushiki Kaisha Toyota Chuo Kenkyusho, Toyoda Gosei Co., Ltd. Invention is credited to Masafumi Hashimoto, Masahiro Kotaki, Katsuhide Manabe, Makoto Tamaki.
United States Patent |
RE36,747 |
Manabe , et al. |
June 27, 2000 |
Light-emitting device of gallium nitride compound semiconductor
Abstract
A light-emitting diode of GaN compound semiconductor emits a
blue light from a plane rather than dots for improved luminous
intensity. This diode includes a first electrode associated with a
high-carrier density n.sup.+ layer and a second electrode
associated with a high-impurity density .[.i.sub.H -layer.].
.Iadd.H-layer.Iaddend.. These electrodes are made up of a first Ni
layer (110 .ANG. thick), a second Ni layer (1000 .ANG. thick), an
Al layer (1500 .ANG. thick), a Ti layer (1000 .ANG. thick), and a
third Ni layer (2500 .ANG. thick). The Ni layers of dual structure
permit a buffer layer to be formed between them. This buffer layer
prevents the Ni layer from peeling. The direct contact of the Ni
layer with GaN lowers a drive voltage for light emission and
increases luminous intensity.
Inventors: |
Manabe; Katsuhide (Inazawa,
JP), Kotaki; Masahiro (Inazawa, JP),
Tamaki; Makoto (Inazawa, JP), Hashimoto; Masafumi
(Nagoya, JP) |
Assignee: |
Toyoda Gosei Co., Ltd
(Aichi-ken, JP)
Kabushiki Kaisha Toyota Chuo Kenkyusho (Aichi-ken,
JP)
|
Family
ID: |
16722426 |
Appl.
No.: |
08/844,386 |
Filed: |
April 18, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
006301 |
Jan 22, 1993 |
05408120 |
Apr 18, 1995 |
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Foreign Application Priority Data
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Jul 23, 1992 [JP] |
|
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4-218595 |
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Current U.S.
Class: |
257/431; 257/103;
257/11; 257/189; 257/21; 257/22; 257/453; 257/613; 257/615;
257/745; 257/76; 257/766; 257/94 |
Current CPC
Class: |
H01L
33/0037 (20130101); H01L 33/38 (20130101); H01L
33/40 (20130101); H01L 33/32 (20130101); H01L
33/382 (20130101) |
Current International
Class: |
H01L
33/00 (20060101); H01L 027/14 (); H01L 031/00 ();
H01L 029/00 (); H01L 049/00 () |
Field of
Search: |
;257/431,103,76,94,189,200,201,86,613,615,21,22,11,453,745,766 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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277567 |
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Aug 1988 |
|
EP |
|
0444630 |
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Sep 1991 |
|
EP |
|
2738329 |
|
Mar 1978 |
|
DE |
|
3046018 |
|
Sep 1981 |
|
DE |
|
54-071589 |
|
Aug 1979 |
|
JP |
|
54-071590 |
|
Aug 1979 |
|
JP |
|
56-059699 |
|
May 1981 |
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JP |
|
57-087184 |
|
May 1982 |
|
JP |
|
57-153479 |
|
Sep 1982 |
|
JP |
|
57-046669 |
|
Oct 1982 |
|
JP |
|
58-012381 |
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Jan 1983 |
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JP |
|
58-046686 |
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Mar 1983 |
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JP |
|
228776 |
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Dec 1984 |
|
JP |
|
61-007671 |
|
Jan 1986 |
|
JP |
|
62-119196 |
|
May 1987 |
|
JP |
|
63-188977 |
|
Aug 1988 |
|
JP |
|
2-081483 |
|
Mar 1990 |
|
JP |
|
2-081484 |
|
Mar 1990 |
|
JP |
|
2-081482 |
|
Mar 1990 |
|
JP |
|
2275682 |
|
Sep 1990 |
|
JP |
|
H3-183173 |
|
Aug 1991 |
|
JP |
|
H4-68579 |
|
Mar 1992 |
|
JP |
|
468579 |
|
Mar 1992 |
|
JP |
|
3183173 |
|
Aug 1997 |
|
JP |
|
1589351 |
|
May 1981 |
|
GB |
|
Other References
Boulou et al., "Light Emitting Diodes Based on GaN," Philips Tech.
Rev. 37, 237-240 No. 9/10, 1977. .
Patent Abstract of Japan, vol. 5, No. 145 (E-74)(817) re
JP-A-5679482, Sep. 1981. .
Patent Abstract of Japan, vol. 4 No. 36 (E-3) re JP-A-55009442,
Jul. 1978. .
U.S. application Ser. No. 7,926,022, Jan. 1994, Manabe et al. .
U.S. application Ser. No. 7,708,883, Apr. 1993, Kotaki et al. .
U.S. application Ser. No. 7,781,913, Jan. 1994, Kotaki et
al..
|
Primary Examiner: Arroyo; Teresa M.
Attorney, Agent or Firm: Pillsbury Madison & Sutro
LLP
Claims
What is claimed is:
1. A light-emitting device of gallium nitride compound
semiconductor material comprising:
an n-layer of n-type gallium nitride compound semiconductor
material (Al.sub.x Ga.sub.1-x N, x.gtoreq.0); and
.[.an i-layer of i-type.]. .Iadd.a p-type impurity doped layer
.Iaddend.gallium nitride compound semiconductor material (Al.sub.x
Ga.sub.1-x N, x.gtoreq.0) .[.doped with a p-type impurity.].;
wherein a first electrode layer including Ni is formed in contact
with said .[.i-layer.]. .Iadd.p-type impurity doped layer
.Iaddend.and functions as an electrode .[.therefore.].
.Iadd.therefor.Iaddend.; and
wherein said first electrode layer is a multi-layer structure
having a first Ni layer of predetermined thickness formed over said
.[.i-layer.]. .Iadd.p-type impurity doped layer.Iaddend., a second
Ni layer which is thicker than said first Ni layer and formed
thereon, an Al layer formed over said second Ni layer, a Ti layer
formed over said Al layer, and a third Ni layer which is thicker
than said first Ni layer formed over said Ti layer.
2. A light-emitting device of gallium nitride compound
semiconductor material comprising:
an n-layer of n-type gallium nitride compound semiconductor
.Iadd.material .Iaddend.(Al.sub.x Ga.sub.1-x N, x.gtoreq.0)
.[.material.].; and
.[.an i-layer of i-type.]. .Iadd.a p-type impurity doped layer
.Iaddend.gallium nitride compound semiconductor .Iadd.material
.Iaddend.(Al.sub.x Ga.sub.1-x N, x.gtoreq.0) .[.material doped with
a p-type impurity.].;
wherein each of said n-layer and said .[.i-layer.]. .Iadd.p-type
impurity doped layer .Iaddend.include respective electrodes formed
on a same relative surface, the electrode for said .[.i-layer.].
.Iadd.p-type impurity doped layer .Iaddend.being composed of at
least one layer with each said at least one layer being made of one
of Ni, Ag, Ti, an alloy including Ni, an alloy including Ag, and an
alloy including Ti; and
wherein the electrode for said .[.i-layer.]. .Iadd.p-type impurity
doped layer .Iaddend.has an over layer formed thereon which is made
of one of Al and an alloy containing Al.
3. A light-emitting device of gallium nitride compound
semiconductor, comprising:
at least two layers of gallium nitride compound semiconductor
material (Al.sub.x Ga.sub.1-x N, x.gtoreq.0);
a first electrode layer for one layer of said at least two layers;
and
a second electrode layer for another of said at least two
layers;
said first and second electrode layers provide an improved luminous
intensity of said light-emitting device;
wherein at least one layer of said first and second electrode
layers includes a contact layer made of one of Ni, Ag, an alloy
including Ni, an alloy including Ag, and an alloy including Ti,
said contact layer being directly contacted with any of said at
least two layers of gallium nitride compound semiconductor material
(Al.sub.x Ga.sub.1-x N, x.gtoreq.0).
4. A light-emitting device of gallium nitride compound
semiconductor according to claim 3, wherein said contact layer of
said first electrode layer is uniformly formed on a light emitting
surface of said one layer, said one layer being .[.an i-layer of
semi-insulation doped with a p-type impurity.]. .Iadd.p-type
impurity doped layer .Iaddend.and said another layer being an
n-layer with n-type conduction.
5. A light-emitting device of gallium nitride compound
semiconductor according to claim 3, wherein said second electrode
layer includes a contact layer made of one of Ni, Ag, an alloy
including Ni, an alloy including Ag, and an alloy including Ti,
said contact layer being directly contacted with said another layer
and said another layer being an n-layer with n-type conduction.
6. A light-emitting device of gallium nitride compound
semiconductor according to claim 3, wherein said first electrode
layer is a multi-layer structure having a first Ni layer of
predetermined thickness formed over said one layer, a second Ni
layer which is thicker than said first Ni layer and formed thereon,
an Al layer formed over said second Ni layer, a Ti layer formed
over said Al layer, and a third Ni layer which is thicker than said
first Ni layer formed over said Ti layer.
7. A light-emitting device of gallium nitride compound
semiconductor according to claim 3, wherein at least one layer of
said first and second electrode layers has an over layer which is
made of one of Ni, Ag, Ti, an alloy including Ni, an alloy
including Ag, and an alloy including Ti.
8. A light-emitting device of gallium nitride compound
semiconductor material according to claim 4, wherein at least one
layer of said first and second electrode layers has an over layer
which is made of one of Ni, Ag, Ti, an alloy including Ni, an alloy
including Ag, and an alloy including Ti.
9. A light-emitting device of gallium nitride compound
semiconductor according to claim 3, wherein at least one layer of
said first and second electrode layers has over layer formed
thereon which is made of one of Al and an alloy containing Al.
10. A light-emitting device of gallium nitride compound
semiconductor according to claim 4, wherein at least one layer of
said first and second electrode layers has over layer formed
thereon which is made of one of Al and an alloy containing Al.
11. A light-emitting device of gallium nitride compound
semiconductor according to claim 7, wherein at least one layer of
said first and second electrode layers has over layer formed
thereon which is made of one of Al and an alloy containing Al.
12. A light-emitting device of gallium nitride compound
semiconductor according to claim 8, wherein at least one layer of
said first and second electrode layers has over layer formed
thereon which is made of one of Al and an alloy containing Al.
13. A light-emitting device of gallium nitride compound
semiconductor according to claim 4, wherein said second electrode
layer is made of one of Al and an alloy containing Al, and said
first electrode has an over layer which is made of one of Ni, Ag,
Ti, an alloy including Ni, an alloy including Ag, and an alloy
including Ti.
14. A light-emitting device of gallium nitride compound
semiconductor material according to claim 4, wherein said first
electrode layer is a multi-layer structure having a first Ni layer
of predetermined thickness formed over said i-layer, a second Ni
layer which is thicker than said first Ni layer and formed thereon,
an Al layer formed over said second Ni layer, a Ti layer formed
over said Al layer, and a third Ni layer which is thicker than said
first Ni layer formed over said Ti layer. .Iadd.15. A
light-emitting device of gallium nitride compound semiconductor,
comprising:
a first layer of gallium nitride compound semiconductor material
(Al.sub.x Ga.sub.1-x N, x.gtoreq.0) doped with p-type impurity;
a second layer of n-type gallium nitride compound semiconductor
material (Al.sub.x Ga.sub.1-x N, x.gtoreq.0);
a first electrode layer for said first layer;
a second electrode layer for said second layer; and
wherein said first electrode layer is made of at least one of Ni,
Ag, an alloy including Ni, an alloy including Ag, and an alloy
including Ti and said second electrode layer is made of at least
one of Al, Ti, an alloy
including Al, and an alloy including Ti. .Iaddend..Iadd.16. A
light-emitting device of gallium nitride compound semiconductor
according to claim 15, wherein said first and second layers are
formed on a buffer layer and said buffer layer is formed on a
sapphire substrate. .Iaddend..Iadd.17. A light-emitting device of
gallium nitride compound semiconductor according to claim 15,
wherein said second layer is gallium nitride (GaN) of low
resistivity doped with silicon (Si) for uniform flow
of current through said first layer. .Iaddend..Iadd.18. A
light-emitting device of gallium nitride compound semiconductor
according to claim 17, wherein said first electrode layer is made
of one of Ni and an alloy including Ni and second electrode layer
is made of one of Al and an alloy including Al. .Iaddend..Iadd.19.
A light-emitting device of gallium nitride compound semiconductor
according to claim 15, wherein said first electrode layer further
comprises a multi-layer structure having at least one over layer
made of metal different from metal of a layer under said over
layer. .Iaddend..Iadd.20. A light-emitting device of gallium
nitride compound semiconductor according to claim 15, wherein said
first electrode layer is uniformly formed on a light emitting
surface of said first layer.
.Iaddend..Iadd.21. A light-emitting device of gallium nitride
compound semiconductor according to claim 15, wherein at least one
layer of said first and second electrode layers further comprises
at least one over layer made of one of Ni, Ti, an alloy including
Ni, and an alloy including Ti. .Iaddend..Iadd.22. A light-emitting
device of gallium nitride compound semiconductor according to claim
15, wherein said second electrode layer further comprises at least
one over layer made of one of Al and an alloy containing Al.
.Iaddend..Iadd.23. A light-emitting device of gallium nitride
compound semiconductor according to claim 18, wherein said first
electrode layer further comprises at least one over layer made of
metal
excluding Ni. .Iaddend..Iadd.24. A light-emitting device of gallium
nitride compound semiconductor, comprising:
a first layer of gallium nitride compound semiconductor material
(Al.sub.x Ga.sub.1-x N, x.gtoreq.0) doped with p-type impurity;
a second layer of n-type gallium nitride compound semiconductor
material (Al.sub.x Ga.sub.1-x N, x.gtoreq.0);
a first electrode layer for said first layer;
a second electrode layer for said second layer; and
wherein said first electrode layer is made of at least one of Ni
and an alloy including Ni and said second electrode layer is made
of Al, Ti, an
alloy including Al, and an alloy including Ti. .Iaddend..Iadd.25. A
light-emitting device of gallium nitride compound semiconductor
according to claim 24, wherein said first electrode layer has a
multi-layer structure comprising at least one over layer made of
metal different from metal of a layer under said over layer.
.Iaddend..Iadd.26. A light-emitting device of gallium nitride
compound semiconductor according to claim 3, wherein said improved
luminous intensity is achieved by reduction of driving voltage for
supplying a predetermined current. .Iaddend..Iadd.27. A
light-emitting device of gallium nitride compound semiconductor
according to claim 3, wherein said at least one layer of said first
and second electrode layers further comprises at least one over
layer made of a metal excluding a metal of said contact layer.
.Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-emitting device of gallium
nitride compound semiconductor which emits a blue light.
2. Description of the Prior Art
Among the conventional light-emitting diodes which emit a blue
light is the gallium nitride compound semiconductor. It attracts
attention because of its high luminous efficiency resulting from
the direct transition and its ability to emit a blue light, one of
the three primary colors of light.
The light-emitting diode of gallium nitride compound semiconductor
is made up of a sapphire substrate, an n-layer grown on the
substrate from a GaN compound semiconductor of n-type conduction,
with or without a buffer layer of aluminum nitride interposed
between them, and .[.an i-layer.]. .Iadd.a p-type impurity doped
layer .Iaddend.grown on the n-layer from a GaN compound
semiconductor which is made .[.i-type.]. by doping with a p-type
impurity. (Japanese Patent Laid-open Nos. 119196/1987 and
188977/1988)
It is known that the above-mentioned light-emitting diode will be
improved in luminous intensity when the .[.i-layer.]. .Iadd.p-type
impurity doped layer .Iaddend.is provided with an electrode of
large area because light emission takes place directly under or
near the .[.i-layer.]. .Iadd.p-type impurity doped
layer.Iaddend..
Much has been reported on the study of crystal growth for
light-emitting diodes of GaN compound semiconductors. However, only
a little has been reported on the process of producing such
light-emitting diodes. This is true particularly of the electrode
for the .[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.in
a light-emitting diode .[.having a MIS (metal insulator
semiconductor) structure.].. It has been disclosed only in Japanese
Patent Laid-open No. 46669/1982, and nothing has so far been
discussed about how the electrode for the .[.i-layer.].
.Iadd.p-type impurity doped layer .Iaddend.is associated with light
emission.
The electrode for the i-layer has the layer structure as shown in
vertical section in FIG. 8 which is a reproduction from the
Japanese patent just given above. Referring to FIG. 8, there is
shown a light-emitting diode 60, which has an electrode 67 for the
.[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.and an
electrode 68 for the n-layer. The electrode 67 is formed from
nickel deposited on an aluminum substrate deposited directly on the
.[.i-layer.]. .Iadd.p-type impurity doped layer.Iaddend.. The
electrode 68 is also formed from nickel deposited on an aluminum
substrate deposited in a hole penetrating the .[.i-layer.].
.Iadd.p-type impurity doped layer.Iaddend..
A disadvantage of forming the electrode on aluminum in direct
contact with the .[.i-layer.]. .Iadd.p-type impurity doped layer
.Iaddend.is that light is emitted from coarse dots rather than a
uniform plane, as shown in FIG. 5(a). The resulting light-emitting
diode does not have increased luminous intensity despite its large
light-emitting area.
SUMMARY OF THE INVENTION
The present invention was completed to address the above-mentioned
problem. It is an object of the present invention to provide a
light-emitting diode of GaN compound semiconductor which emits a
blue light from a plane, rather than dots, to improve luminous
intensity.
The present invention is embodied in a light-emitting device of
gallium nitride compound semiconductor having an n-layer of n-type
gallium nitride compound semiconductor (Al.sub.x Ga.sub.1-x N,
x.gtoreq.0) and .[.an i-layer of i-type.]. .Iadd.a p-type impurity
doped layer of .Iaddend.gallium nitride compound semiconductor
(Al.sub.x Ga.sub.1-x N, x.gtoreq.0) .[.doped with a p-type
impurity.]., characterized in that said .[.i-layer.]. .Iadd.p-type
impurity doped layer .Iaddend.has a Ni layer in contact therewith
which functions as an electrode therefor.
The present invention is also embodied in a light-emitting device
of gallium nitride compound semiconductor as defined above, wherein
the n-layer has a Ni layer in contact therewith which functions as
an electrode therefor.
The present invention is also embodied in a light-emitting device
of gallium nitride compound semiconductor material as defined
above, wherein the electrode for the .[.i-layer.]. .Iadd.p-type
impurity doped layer .Iaddend.is of multi-layer structure composed
of a first Ni layer (which is thin), a second Ni layer (which is
thicker than the first Ni layer), an Al layer, a Ti layer, and a
third Ni layer (which is thick), all of which are arranged upward
in the order mentioned.
The present invention is also embodied in a light-emitting device
of gallium nitride compound semiconductor material having an
n-layer of n-type gallium nitride compound semiconductor (Al.sub.x
Ga.sub.1-x N, x.gtoreq.0) material and .[.an i-layer of i-type.].
.Iadd.a p-type impurity doped layer of .Iaddend.gallium nitride
compound semiconductor (Al.sub.x Ga.sub.1-x N, x.gtoreq.0) material
.[.doped with a p-type impurity.]., characterized in that the
n-layer and .[.i-layer.]. .Iadd.p-type impurity doped layer
.Iaddend.have their respective electrodes on the same surface, with
the electrode for the n-layer being made of Al or an alloy
containing Al, and the electrode for the .[.i-layer.]. .Iadd.p-type
impurity doped layer .Iaddend.being made of Ni, Ag, or Ti, or an
alloy containing any of them.
According to the present invention, the electrode for the
.[.i-layer.].
.Iadd.p-type impurity doped layer .Iaddend.is in contact with the
.[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.through a
Ni layer. This structure permits the light-emitting device to emit
light from a plane rather than dots, which leads to improved
luminous intensity. In addition, it decreases the driving voltage,
alleviating thermal degradation and improving reliability.
In the present invention, the nickel electrode for the n-layer only
slightly increases the driving voltage for light emission and it
poses no problems (normally associated with a decrease in luminous
intensity) even when it is made in the same structure as the
electrode for the .[.i-layer.]. .Iadd.p-type impurity doped
layer.Iaddend.. Making the electrodes for both the n-layer and
.[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.from
nickel simplifies the production of the light-emitting diode.
According to the present invention, the electrode is of multi-layer
structure composed of a first Ni layer (which is thin), a second Ni
layer (which is thicker than the first Ni layer), an Al layer, a Ti
layer, and a third Ni layer (which is thick). This produces the
following two effects.
(1) Forming a first Ni layer (which is thin) directly on the
.[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.and a
second Ni layer (which is thick) subsequently permits a thermal
stress buffer layer to be formed between the two Ni layers, and it
prevents the peeling of the Ni layers due to thermal expansion and
contraction at the time of soldering and reflowing.
(2) The formation of an Al layer, a Ti layer, and a third Ni layer
on the second Ni layer permits the electrode to be connected by
soldering.
According to the present invention, the n-layer and .[.i-layer.].
.Iadd.p-type impurity doped layer .Iaddend.have their respective
electrodes on the same surface, with the electrode for the n-layer
being made of Al or an alloy containing Al, and the electrode for
the .[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.having
a lower layer made of Ni, Ag, or Ti, or an alloy containing any of
them and a higher layer made of Al or an alloy containing Al. This
structure permits the light-emitting diode to emit light from a
plane rather than dots.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical sectional view showing the structure of the
light-emitting diode pertaining to first embodiment of the present
invention.
FIGS. 2(A) to 4(C) are vertical sectional views showing the steps
of producing the light-emitting diode pertaining to the first
embodiment of the present invention.
FIGS. 5(A) to 5(d) are photomicrographs showing the metal surface
structure (as the light-emitting pattern) of each substrate metal
of the electrode for the i-layer.
FIG. 6 is a vertical sectional view showing the structure of the
light-emitting diode pertaining to a second embodiment of the
present invention.
FIG. 7 is a vertical sectional view showing the structure of the
light-emitting diode pertaining to yet a further embodiment of the
present invention.
FIG. 8 is a vertical sectional view showing the structure of a
conventional light-emitting diode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Example 1
The present invention will be described in more detail with
reference to a first disclosed embodiment. FIG. 1 shows a vertical
section of a light-emitting diode 10 pertaining to the present
invention. It has a sapphire substrate 1, on which there are
successively formed a buffer layer 2 of AlN (500 .ANG. thick), a
high-carrier density n.sup.+ -layer 3 of GaN (2.2 .mu.m thick), a
low-carrier density n-layer 4 of GaN (1.5 .mu.m thick), an
.[.i-layer.]. .Iadd.a p-type impurity doped layer .Iaddend.5 of GaN
(0.1 .mu.m thick), an electrode 7 of aluminum, and an electrode 8
of aluminum (in contact with the high-carrier density n.sup.+
-layer 3).
This light-emitting diode 10 is produced by the steps which are
explained below with reference to FIGS. 2(A) to 4(C).
The entire process was carried out using NH.sub.3, H.sub.2 (carrier
gas), trimethyl gallium Ga(CH.sub.3).sub.3 (TMG for short),
trimethyl aluminum Al(CH.sub.3).sub.3 (TMA for short), silane
SiH.sub.4 and diethyl zinc (DEZ for short).
Firstly, sapphire substrate 1 of single crystal (with the a-plane
(i.e., {1120} as the principal plane) was cleaned by washing with
an organic solvent and by subsequent heat treatment. Then, it was
placed on the susceptor in the reaction chamber for metal-organic
vaporphase epitaxy (MOVPE). H.sub.2 was fed to the reaction chamber
under normal pressure at a flow rate of 2 L/min to perform vapor
phase etching on the sapphire substrate 1 at 1100.degree. C.
With the temperature lowered to 400.degree. C., the reaction
chamber was supplied with H.sub.2, NH.sub.3, and TMA at a flow rate
of 20 L/min, 10 L/min, and 1.8.times.10.sup.-5 mol/min,
respectively, to form the buffer layer 2 of AlN (500 .ANG.
thick).
With the temperature of the sapphire substrate 1 kept at
1150.degree. C., the reaction chamber was supplied with H.sub.2,
NH.sub.3, TMG, and SiH.sub.4 (diluted to 0.86 ppm with H.sub.2) at
a flow rate of 20 L/min, 10 L/min, 1.7.times.10.sup.-4 mol/min, and
200 mL/min, respectively, for 30 minutes to form the high-carrier
density n.sup.+ -layer 3 of GaN (2.2 .mu.m thick), with a carrier
density of 1.5.times.10.sup.18 /cm.sup.3.
With the temperature of the sapphire substrate 1 kept at
1150.degree. C., the reaction chamber was supplied with H.sub.2,
NH.sub.3, and TMG at a flow rate of 20 L/min, 10 L/min, and
1.7.times.10.sup.-4 mol/min, respectively, for 20 minutes to form
the low-carrier density n-layer 4 of GaN (1.5 .mu.m thick), with a
carrier density of 1.times.10.sup.15 /cm.sup.3.
With the temperature of the sapphire substrate 1 kept at
900.degree. C., the reaction chamber was supplied with H.sub.2,
NH.sub.3, TMG, and DEZ at a flow rate of 20 L/min, 10 L/min,
1.7.times.10.sup.-4 mol/min, and 1.5.times.10.sup.-4 mol/min,
respectively, for 1 minute to form the .[.i-layer.]. .Iadd.p-type
impurity doped layer .Iaddend.5 of GaN (0.1 .mu.m thick).
In this way there was obtained the multi-layer structure as shown
in FIG. 2(a).
On the .[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.5
was formed the SiO.sub.2 layer 11 (2000 .ANG. thick) by sputtering,
as shown in FIG. 2(b). The SiO.sub.2 layer 11 was coated with a
photoresist 12, which was subsequently patterned by
photolithography after the configuration of the electrode for the
high-carrier density n.sup.+ -layer 3. The exposed part of the
SiO.sub.2 layer 11 was removed by etching with hydrofluoric acid,
as shown in FIG. 2(c). The exposed part of the .[.i-layer.].
.Iadd.p-type impurity doped layer .Iaddend.5, the underlying part
of the low-carrier density n-layer 4, and the underlying upper part
of the high-carrier density n.sup.+ -layer 3 were removed by dry
etching with BCl.sub.3 gas fed at a flow rate of 10 mL/min at 0.04
Torr in conjunction with a high-frequency power of 0.44 W/cm.sup.2,
followed by Ar dry etching, as shown in FIG. 3(A). The SiO.sub.2
layer 11 remaining on the .[.i-layer.]. .Iadd.p-type impurity doped
layer .Iaddend.5 was removed with the aid of hydrofluoric acid, as
shown in FIG. 3(B).
With the temperature kept at 225.degree. C. and the degree of
vacuum kept at 8.times.10.sup.-7 Torr, the sample was entirely
coated with the Ni layer 13 (3000 .ANG. thick) by vapor deposition,
as shown in FIG. 3(C). The Ni layer 13 was coated with a
photoresist 14, which was subsequently patterned by
photolithography after the configuration of the electrode for the
.[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.5.
The unmasked part of the Ni layer 13 was etched off using nitric
acid and the photoresist 14 was removed by acetone, so that the Ni
layer 13 partly remained on which the electrode for the
.[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.5 was
formed afterward, as shown in FIG. 4(A).
With the temperature kept at 225.degree. C. and the degree of
vacuum kept at 8.times.10.sup.-7 Torr, the sample was entirely
coated with the Al layer 15 (3000 .ANG. thick) by vapor deposition,
as shown in FIG. 4(B).
The Al layer 15 was coated with a photoresist 16, which was
subsequently patterned by photolithography after the configuration
of the respective electrodes for the high-carrier density n.sup.+
-layer 3 and the .[.i-layer.]. .Iadd.p-type impurity doped layer
.Iaddend.5, as shown in FIG. 4(C).
The exposed part of the Al layer 15 was etched off using nitric
acid and the remaining photoresist 16 was removed by acetone, Thus
there were formed the electrode 7 for the .[.i-layer.].
.Iadd.p-type impurity doped layer .Iaddend.5 and the electrode 8
for the high-carrier density n.sup.+ -layer 3.
In this way there was obtained the GaN light-emitting device of MIS
structure as shown in FIG. 1.
Incidentally, the undercoating layer 13 on the .[.i-layer.].
.Iadd.p-type impurity doped layer .Iaddend.5 may be formed from Ag
or Ti or an alloy thereof in place of Ni. Also, the electrode 7 for
the .[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.5 and
the electrode 8 for the high-carrier density n.sup.+ -layer 3 may
be formed from any metal such as Ti, in place of Al, which permits
ohmic contact.
The thus prepared light-emitting diode 10 was tested for luminous
intensity and drive voltage by applying current (10 mA) across the
electrodes. The results were compared with those of the
conventional one having the Al layer formed directly on the
.[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.5, which
gave a luminous intensity of 30 mcd. The results vary depending on
the metal used for the undercoating of the electrode for the
.[.i-layer.]. .Iadd.p-type impurity doped layer .Iaddend.as shown
in the table below. The data of luminous intensity and drive
voltage are given in terms of index values compared with those of a
conventional sample.
______________________________________ Undercoating Luminous Drive
Light-emitting metal intensity voltage pattern
______________________________________ Ni 1.5 0.82 FIG. 5(b) Ag 1.4
0.90 FIG. 5(c) Ti 1.05 0.95 FIG. 5(d)
______________________________________
It is noted that the light-emitting diode pertaining to the present
invention has a higher luminous intensity and a lower drive voltage
than conventional diodes.
Example 2
A light-emitting diode was prepared in the same manner as in
Example 1. As shown in FIG. 6, it is composed of a sapphire
substrate 1, a buffer layer 2 of AlN, a high-carrier density
n.sup.+ -layer 3 of GaN, a low-carrier density n-layer 4 (1.1 .mu.m
thick) having a carrier density of 1.times.10.sup.15 /cm.sup.3, a
low-impurity density .[.i.sub.L -layer.]. .Iadd.L-layer .Iaddend.51
(1.1 .mu.m thick) having a Zn density of 2.times.10.sup.18
/cm.sup.3, and a high-impurity density .[.i.sub.H -layer.].
.Iadd.H-layer .Iaddend.52 (0.2 .mu.m thick) having a Zn density of
1.times.10.sup.20 /cm.sup.3. It should be noted that the
.[.i-layer.]. .Iadd.p-impurity doped layer .Iaddend.is of dual
structure with 51 and 52.
A hole 60 was formed which penetrates the high-impurity density
.[.i.sub.H layer.]. .Iadd.H-layer .Iaddend.52, the low-impurity
density .[.i.sub.L layer.]. .Iadd.L-layer .Iaddend.51, and the
low-carrier density n-layer 4, reaching the high-carrier density
n.sup.+ -layer 3. In this hole 60 was formed an electrode 80 for
the high-carrier density n.sup.+ -layer 3. An electrode 70 was also
formed for the high-impurity density .[.i.sub.H -layer.].
.Iadd.H-layer .Iaddend.52.
The electrode 70 is composed of a first Ni layer 71 (100 .ANG.
thick), a second Ni layer 72 (1000 .ANG. thick), an Al layer 73
(1500 .ANG. thick), a Ti layer 74 (1000 .ANG. thick), and a third
Ni layer 75 (2500 .ANG. thick). The electrode 80 is also composed
of a first Ni layer 81 (100 .ANG. thick), a second Ni layer 82
(1000 .ANG. thick), an Al layer 83 (1500 .ANG. thick), a Ti layer
84 (1000 .ANG. thick), and a third Ni layer 85 (2500 .ANG.
thick).
The first Ni layer 71 (81) was formed by vacuum deposition at
225.degree. C. The second Ni layer 72 (82) was also formed by
vacuum deposition with heating. (The two steps were separated by an
interval in which the vacuum chamber was opened and the water was
conditioned at normal pressure and normal temperature.) The Al
layer 73 (83), Ti layer 74 (84), and third Ni layer 75 (85) were
formed successively by vacuum deposition. The Al layer 73 (83) and
Ti layer 74 (84) permit a solder bump to be formed on the third Ni
layer 75 (85).
The thus prepared light-emitting diode has a drive voltage for
light emission which is 0.8 times that of a conventional diode
having an aluminum electrode. In addition, it also exhibits a
luminous intensity of 150 mcd at 10 mA current, which is 1.5 times
that (100 mcd) of the conventional diode having an aluminum
electrode.
It was also found that the same result as mentioned above is
obtained even in the case where the electrode 70 for the
high-impurity density .[.i.sub.H -layer.]. .Iadd.H-layer
.Iaddend.52 is made of Ni in multi-layer structure and the
electrode 80 for the high-carrier density n.sup.+ -layer 3 is made
of aluminum in single-layer structure.
Example 3
The light-emitting diode in this example differs from that in the
previous example in that the first Ni layer 71 (81) and second Ni
layer 72 (82) are replaced by a Ni layer 710 (810) of single-layer
structure, which is 300 .ANG. thick, as shown in FIG. 7. This
difference in structure has nothing to do with its performance. The
Ni layer 710 (810) should preferably have a thickness in the range
of 50 .ANG. to 3000 .ANG.. With a thickness lower than specified,
it will be subject to attack by solder when a solder bump is
formed. With a thickness greater than specified, it causes the
light source to be localized near the electrode rather than the
center and it is liable to peeling at the time of soldering in a
solder bath.
* * * * *