U.S. patent application number 10/456475 was filed with the patent office on 2003-11-20 for light-emitting gallium nitride-based compound semiconductor device.
This patent application is currently assigned to Nichia Chemical Industries Ltd.. Invention is credited to Iwasa, Naruhito, Mukai, Takashi, Nakamura, Shuji.
Application Number | 20030216011 10/456475 |
Document ID | / |
Family ID | 27571840 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030216011 |
Kind Code |
A1 |
Nakamura, Shuji ; et
al. |
November 20, 2003 |
Light-emitting gallium nitride-based compound semiconductor
device
Abstract
A light-emitting gallium nitride-based compound semiconductor
device of a double-heterostructure. The double-heterostructure
includes a light-emitting layer formed of a low-resistivity
In.sub.xGa.sub.1-xN (0<x<1) compound semiconductor doped with
p-type and/or n-type impurity. A first clad layer is joined to one
surface of the light-emitting layer and formed of an n-type gallium
nitride-based compound semiconductor having a composition different
from the light-emitting layer. A second clad layer is joined to
another surface of the light-emitting layer and formed of a
low-resistivity, p-type gallium nitride-based compound
semiconductor having a composition different from the
light-emitting layer.
Inventors: |
Nakamura, Shuji; (Anan-shi,
JP) ; Mukai, Takashi; (Anan-shi, JP) ; Iwasa,
Naruhito; (Anan-shi, JP) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
Nichia Chemical Industries
Ltd.
|
Family ID: |
27571840 |
Appl. No.: |
10/456475 |
Filed: |
June 9, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10456475 |
Jun 9, 2003 |
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10227834 |
Aug 27, 2002 |
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10227834 |
Aug 27, 2002 |
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09516193 |
Mar 1, 2000 |
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6469323 |
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09516193 |
Mar 1, 2000 |
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09145972 |
Sep 3, 1998 |
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6078063 |
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09145972 |
Sep 3, 1998 |
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08705972 |
Aug 30, 1996 |
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5880486 |
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08705972 |
Aug 30, 1996 |
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08153153 |
Nov 17, 1993 |
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5578839 |
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Current U.S.
Class: |
438/478 ;
257/E33.049 |
Current CPC
Class: |
H01L 33/025 20130101;
H01L 33/32 20130101; H01L 33/325 20130101; H01S 5/32341 20130101;
H01L 33/0025 20130101 |
Class at
Publication: |
438/478 |
International
Class: |
H01L 021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 20, 1992 |
JP |
4-335556 |
Jan 8, 1993 |
JP |
5-18122 |
Jan 8, 1993 |
JP |
5-18123 |
Mar 5, 1993 |
JP |
5-70873 |
Mar 5, 1993 |
JP |
5-70874 |
May 17, 1993 |
JP |
5-114542 |
May 17, 1993 |
JP |
5-114543 |
May 17, 1993 |
JP |
5-114544 |
Claims
What is claimed is:
1. A light-emitting gallium nitride-based compound semiconductor
device having a double-heterostructure comprising: a light-emitting
layer having first and second major surfaces and formed of a
low-resistivity In.sub.xGa.sub.1-xN, where 0<x<1, compound
semiconductor doped with an impurity; a first clad layer joined to
said first major surface of said light-emitting layer and formed of
an n-type gallium nitride-based compound semiconductor having a
composition different from that of said compound semiconductor of
said light-emitting layer; and a second clad layer joined to said
second major surface of said light-emitting layer and formed of a
low-resistivity, p-type gallium nitride-based compound
semiconductor having a composition different from that of said
compound semiconductor of said light-emitting layer.
2. The device according to claim 1, wherein said compound
semiconductor of said light-emitting layer is of p-type, doped with
a p-type impurity.
3. The device according to claim 2, wherein said p-type impurity
comprises a Group II element.
4. The device according to claim 1, wherein said compound
semiconductor of said light-emitting layer is of n-type, doped with
at least a p-conductivity type impurity.
5. The device according to claim 3, wherein said impurity doped in
said compound semiconductor of said light-emitting layer comprises
a p-type impurity including a Group II element and an n-type
impurity including a Group IV or VI element.
6. The device according to claim 1, wherein said compound
semiconductor of said light-emitting layer is of n-type, does with
an n-type impurity.
7. The device according to claim 6, wherein said n-type impurity
comprises a Group IV or VI element.
8. The device according to claim 1, wherein said compound
semiconductor or said first clad layer is represented by the
formula: Ga.sub.yAl.sub.1-yN, where 0.ltoreq.y.ltoreq.1.
9. The device according to claim 1, wherein said compound
semiconductor of said second clad layer is represented by the
formula: Ga.sub.zAl.sub.1-zN, where 0.ltoreq.z.ltoreq.1.
10. The device according to claim 1, wherein said light-emitting
layer has a thickness of 10 .ANG. to 0.5 .mu.m.
11. The device according to claim 1, wherein said
double-heterostructure has an n-type GaN contact layer joined to
said first clad layer, and a p-type GaN contact layer joined to
said second clad layer.
12. The device according to claim 1, wherein 0<x<0.5.
13. A light-emitting gallium nitride-based compound semiconductor
device having a double-heterostructure comprising: a light-emitting
layer having first and second major surfaces and formed of a
low-resistivity In.sub.xGa.sub.1-xN, where 0<x<1, compound
semiconductor doped with a p-type impurity; a first clad layer
joined to said first major surface of said light-emitting layer and
formed of an n-type gallium nitride-based compound semiconductor
having a composition different from that of said compound
semiconductor of said light-emitting layer; and a second clad layer
joined to said second major surface of said light-emitting layer
and formed of a low-resistivity, p-type gallium nitride-based
compound semiconductor having a composition different from that of
said compound semiconductor of said light-emitting layer.
14. The device according to claim 13, wherein said p-type impurity
doped in said compound semiconductor of said light-emitting layer
comprises at least one element selected from the group consisting
of cadmium, zinc, beryllium, magnesium, calcium, strontium, and
barium.
15. The device according to claim 13, wherein said compound
semiconductor of said first clad layer is represented by a formula:
Ga.sub.yAl.sub.1-yN, where 0.ltoreq.y.ltoreq.1.
16. The device according to claim 13, wherein said compound
semiconductor of said second clad layer is represented by a
formula: Ga.sub.zAl.sub.1-zN, where 0.ltoreq.z.ltoreq.1.
17. The device according to claim 13, wherein said light-emitting
layer has a thickness of 10 .ANG. to 0.5 .mu.m.
18. The device according to claim 13, wherein said p-type impurity
doped in said compound semiconductor of said light-emitting layer
comprises zinc, and a concentration of the zinc is
1.times.10.sup.17 to 1.times.10.sup.21/cm.sup.3.
19. The device according to claim 13, wherein said p-type impurity
doped in said compound semiconductor of said second clad layer
comprises magnesium, and a concentration of the magnesium is
1.times.10.sup.18 to 1.times.10.sup.21/cm.sup.3.
20. The device according to claim 13, wherein said second clad
layer has a thickness of 0.05 .mu.m to 1.5 .mu.m.
21. The device according to claim 13, wherein said
double-heterostructure is provided on a substrate through a buffer
layer.
22. The device according to claim 13, wherein said
double-heterostructure has an n-type GaN contact layer joined to
said first clad layer, and a p-type GaN contact layer joined to
said second clad layer.
23. The device according to claim 13, wherein 0<x<0.5
24. A light-emitting gallium nitride-based compound semiconductor
device having a double-heterostructure comprising: a light-emitting
layer having first and second major surfaces and formed of a
low-resistivity, n-type In.sub.xGa.sub.1-xN, where 0<x<1,
compound semiconductor doped with at least a p-type impurity; a
first clad layer joined to said first major surface of said
light-emitting layer and formed of an n-type gallium nitride-based
compound semiconductor having a composition different from that of
said compound semiconductor of said light-emitting layer; and a
second clad layer joined to said second major surface and formed of
a low-resistivity, p-type gallium nitride-based compound
semiconductor having a composition different from that of said
compound semiconductor of said light-emitting layer.
25. The device according to claim 24, wherein said compound
semiconductor of said light-emitting layer has an electron carrier
concentration of 1.times.10.sup.17 to
5.times.10.sup.21/cm.sup.3.
26. The device according to claim 24, wherein said compound
semiconductor of said light-emitting layer is doped with not only
said p-type impurity but also an n-type impurity.
27. The device according to claim 24, wherein said p-type impurity
doped in said compound semiconductor of said light-emitting layer
comprises at least one element selected from the group consisting
of cadmium, zinc, beryllium, magnesium, calcium, strontium, and
barium.
28. The device according to claim 26, wherein said n-type impurity
doped in said compound semiconductor of said light-emitting layer
comprises at least one element selected from the group consisting
of silicon, germanium, and tin.
29. The device according to claim 24, wherein said compound
semiconductor of said first clad layer is represented by a formula:
Ga.sub.yAl.sub.1-yN, where 0.ltoreq.y.ltoreq.1.
30. The device according to claim 24, wherein said compound
semiconductor of said second clad layer is represented by a
formula: Ga.sub.zAl.sub.1-zN, where 0.ltoreq.z.ltoreq.1.
31. The device according to claim 26, wherein said p-type impurity
doped in said compound semiconductor of said light-emitting layer
comprises zinc, and said n-type impurity comprises silicon.
32. The device according to claim 24, wherein said
double-heterostructure is provided on a substrate through a buffer
layer.
33. The device according to claim 24, wherein said
double-heterostructure has an n-type GaN contact layer joined to
said first clad layer, and a p-type GaN contact layer joined to
said second clad layer.
34. The device according to claim 24, wherein 0<x<0.5.
35. A light-emitting gallium nitride-based compound semiconductor
device having a double-heterostructure comprising a light-emitting
layer having first and second major surfaces and formed of a
low-resistivity, n-type In.sub.xGa.sub.1-xN, where 0<x<1,
compound semiconductor doped with an n-type impurity; a first clad
layer joined to said first major surface of said light-emitting
layer and formed of an n-type gallium nitride-based compound
semiconductor having a composition different from that of said
compound semiconductor of said light-emitting layer; and a second
clad layer joined to said second major surface of said
light-emitting layer and formed so a low-resistivity, p-type
gallium nitride-based compound semiconductor having a composition
different from that of said compound semiconductor of said
light-emitting layer.
36. The device according to claim 35, wherein said n-type impurity
doped in said compound semiconductor of said light-emitting layer
comprises silicon or germanium.
37. The device according to claim 35, wherein said n-type impurity
doped in said compound semiconductor of said light-emitting layer
comprises silicon, and a concentration of the silicon is
1.times.10.sup.17 to 1.times.10.sup.21/cm.sup.3.
38. The device according to claim 35, wherein said compound
semiconductor of said first clad layer is represented by a formula:
Ga.sub.yAl.sub.1-yN, where 0.ltoreq.y.ltoreq.1.
39. The device according to claim 35, wherein said compound
semiconductor of said second clad layer is represented by a
formula: Ga.sub.zAl.sub.1-zN, where 0.ltoreq.z.ltoreq.1.
40. The device according to claim 35, wherein said light-emitting
layer has a thickness of 10 .ANG. to 0.5 .mu.m.
41. The device according to claim 35, wherein said compound
semiconductor of said second clad layer is doped with a p-type
impurity comprising magnesium, and a concentration of the magnesium
is 1.times.10.sup.18 to 1.times.10.sup.21/cm.sup.3.
42. The device according to claim 35, wherein said second clad
layer has a thickness of 0.05 to 1.5 .mu.m.
43. A device according to claim 35, wherein said
double-heterostructure is provided on a substrate through a buffer
layer.
44. The device according to claim 35, wherein said
double-heterostructure has an n-type GaN contact layer joined to
said first clad layer, and a p-type GaN con-tact layer joined to
said second clad layer.
45. The device according to claim 35, wherein 0<x<0.5.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a light-emitting gallium
nitride-based compound semiconductor device and, more particularly,
to a light-emitting compound semiconductor device having a
double-heterostructure capable of emitting high-power visible light
ranging from near-ultraviolet to red, as desired, by changing the
composition of a compound semiconductor constituting an active
layer (light-emitting layer).
[0003] 2. Description of the Related Art
[0004] Gallium nitride-based compound semiconductors such as
gallium nitride (GCN), gallium aluminum nitride (GaAlN), indium
gallium nitride (InGaN), and indium aluminum gallium nitride
(InAlGaN) have a direct band gap, and their band gaps change in the
range of 1.95 eV to 6 eV. For this reason, these compound
semiconductors are promising as materials for light-emitting
devices such as a light-emitting diode and a laser diode.
[0005] For example, as a light-emitting device using a gallium
nitride semiconductor, a blue light-emitting device in which a
homojunction structure is formed on a substrate normally made of
sapphire through an AlN buffer layer has been proposed. The
homojunction structure includes a light-emitting layer formed of
p-type impurity-doped GaN on an n-type GaN layer. As the p-type
impurity doped in the light-emitting layer, magnesium or zinc is
normally used. However, even when the p-type impurity is doped, the
GaN crystal has a poor quality, and remains an i-type crystal
having a high resistivity almost close to an insulator. That is,
the conventional light-emitting device is substantially of a MIS
structure. As a light-emitting device having the MIS structure,
layered structures in which Si- and Zn-doped, i-type GaAlN layers
(light-emitting layers) are formed on n-type CaAlN layers are
disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 4-10665,
4-10666, and 4-10667.
[0006] However, in the light-emitting device having the MIS
structure, both luminance and light-emitting output power are too
low to be practical.
[0007] In addition, the light-emitting device of a homojunction is
impractical because of the low power output by its nature. To
obtain a practical light-emitting device having a large output
power, it is required to realize a light-emitting device of a
single-heterostructure, and more preferably, a
double-heterostructure.
[0008] However, no light-emitting semiconductor devices of a
double-heterostructure are known, in which the
double-heterostructure is entirely formed of low-resistivity
gallium nitride-based compound semiconductors, and at the same
time, has a light-emitting layer consisting or low-resistivity,
impurity-doped InGaN.
[0009] Jpn. Pat. Appln. KOKAI Publication Nos. 4-209577, 4-236477,
and 4-236478 disclose a light-emitting device having a
double-heterostructure in which an InGaN light-emitting layer is
sandwiched between an n-type InGaAlN clad layer and a p-type
InGaAlN clad layer. However, the light-emitting layer is not doped
with an impurity, and it is not disclosed or explicitly suggested
that an impurity is doped into the light-emitting layer. In
addition, the p-type clad layer is a high-resistivity layer in
fact. A similar structure is disclosed in Jpn. Pat. Appln. KOKAI
Publication No. 64-17484.
[0010] Jpn. Pat. Appln. KOKAI Publication 4-213878 discloses a
structure in which an undoped InGaAlN light-emitting layer is
formed on an electrically conductive ZnO substrate, and a
high-resistivity InGaN layer is formed thereon.
[0011] Jpn. Pat. Appln. KOKAI Publication No. 4-68579 discloses a
double-heterostructure having a p-type GaInN clad layer formed on
an oxygen-doped, n-type GaInN light-emitting layer. However,
another clad layer consists of electrically conductive ZnO. The
oxygen is doped in the light-emitting layer to be lattice-matched
with the ZnO. The emission wavelength of the light-emitting device
having this double-heterostructure is 365 to 406 nm.
[0012] All conventional light-emitting devices are unsatisfactory
in both output power and luminance, and have no satisfactory
luminosity.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide a
double-heterostructure in which all of the light-emitting layer
(active layer) and the clad layers are formed of low-resistivity
gallium nitride-based III-V Group compound semiconductors, thereby
realizing a semiconductor device exhibiting an improved luminance
and/or light-emitting output power.
[0014] It is another object of the present invention to provide a
light-emitting device excellent in luminosity.
[0015] It is still another object of the present invention to
provide an ultraviolet to red light-emitting device having a
wavelength in the region of 365 to 620 nm.
[0016] According to the present invention, there is provided a
light-emitting gallium nitride-based compound semiconductor device
having a double-heterostructure comprising:
[0017] a light-emitting layer (active layer) having first and
second major surfaces and formed of a low-resistivity
In.sub.xGa.sub.1-xN (0<x<1) compound semiconductor doped with
an impurity;
[0018] a first clad layer joined to the first major surface of the
light-emitting layer and formed of an n-type gallium nitride-based
compound semiconductor having a composition different from that of
the compound semiconductor of the light-emitting layer; and
[0019] a second clad layer joined to the second major surface of
the light-emitting layer and formed of a low-resistivity, p-type
gallium nitride-based compound semiconductor having a composition
different from that of the compound semiconductor of the
light-emitting layer.
[0020] In the first embodiment, the compound semiconductor of the
light-emitting layer (active layer) is of p-type, doped with a
p-type impurity.
[0021] In the second embodiment, the compound semiconductor of the
light-emitting layer (active layer) remains an n-type, doped with
at least a p-type impurity.
[0022] In the third embodiment, the compound semiconductor of the
light-emitting layer (active layer) is of n-type, doped with an
n-type impurity.
[0023] In the present invention, the compound semiconductor of the
first clad layer is preferably represented by the following
formula:
Ga.sub.yA2.sub.1-yN(0.ltoreq.y.ltoreq.1)
[0024] The compound semiconductor of the second clad layer is
preferably represented by the following formula:
Ga.sub.zA2.sub.1-zN(0.ltoreq.z.ltoreq.1)
[0025] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0027] FIG. 1 is a view showing a basic structure of a
semiconductor light-emitting diode of the present invention;
[0028] FIG. 2 is a graph showing a relationship between the light
intensity and the thickness of a light-emitting layer in the
light-emitting semiconductor device of the present invention;
[0029] FIG. 3 shows a photoluminescence spectrum of a
low-resistivity, n-type In.sub.xGa.sub.1-xN light-emitting layer
according to the second embodiment of the present invention;
[0030] FIG. 4 shows a photoluminescence spectrum of an undoped
In.sub.xGa.sub.1-xN light-emitting layer;
[0031] FIG. 5 is a graph showing a relationship between a p-type
impurity concentration in the light-emitting layer and the light
intensity in the light-emitting semiconductor device according to
the second embodiment of the present invention;
[0032] FIG. 6 is a graph showing a relationship between a p-type
impurity concentration in a p-type clad layer and the light
emission characteristics in the light-emitting semiconductor device
according to the second embodiment of the present invention;
[0033] FIG. 7 is a graph showing a relationship between an electron
carrier concentration in the light-emitting layer and the light
emission characteristics in the light-emitting semiconductor device
according to the second embodiment of the present invention;
[0034] FIG. 8 is a graph showing the light emission characteristics
of the light-emitting semiconductor device according to the second
embodiment of the present invention;
[0035] FIG. 9 is a graph showing a relationship between an n-type
impurity concentration in a light-emitting layer and the light
emission characteristics in a light-emitting semiconductor device
according to the third embodiment of the present invention;
[0036] FIG. 10 is a graph showing a relationship between a p-type
impurity concentration in a p-type clad layer and the light
emission characteristics in the light-emitting semiconductor device
according to the third embodiment of the present invention;
[0037] FIG. 11 shows a structure of still another light-emitting
diode according to the present invention; and
[0038] FIG. 12 is a view showing a structure of a laser diode of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] The present invention provides a double-heterostructure in
which all of the light-emitting layer and clad layers sandwiching
the light-emitting layer are formed of low-resistivity gallium
nitride-based III-V Group compound semiconductors, and at the same
time, the light-emitting layer is formed of an impurity-doped,
low-resistivity In.sub.xGa.sub.1-xN compound semiconductor, thereby
realizing a visible light emitting semiconductor device which is
excellent in output power, luminance, and luminosity, for the first
time.
[0040] The semiconductor device of the present invention includes a
light-emitting diode (LED) and a laser diode (LD).
[0041] The present invention will be described below in detail with
reference to the accompanying drawings. The same reference numerals
denote the same parts throughout the drawings.
[0042] FIG. 1 shows a basic structure of an LED to which the
present invention is applied. As shown in FIG. 1, an LED 10 of the
present invention has a double-heterostructure 22 comprising a
light-emitting layer (active layer) 18 formed of impurity-doped,
low-resistivity (LR) In.sub.xGa.sub.1-xN, a first clad layer 16
joined to the lower surface (first major surface) of the
light-emitting layer 18 and formed of an n-type, low-resistivity
GaN-based III-V Group compound semiconductor, and a second clad
layer 20 joined to the upper surface (second major surface) of the
light-emitting layer 18 and formed of a p-type, low-resistivity
GaN-based III-V Group compound semiconductor. In.sub.xGa.sub.1-xN
co the light-emitting layer 18 is a gallium nitride-based III-V
Group compound semiconductor.
[0043] Because of the double-heterostructure, the compound
semiconductor composition (except for impurities) of the first clad
layer 16 is different from that of the light-emitting layer 18. The
compound semiconductor composition of the second clad layer 20 is
also different from that of the light-emitting layer 18. The
compound semiconductor compositions of the clad layers 16 and 20
may be the same or different.
[0044] The present inventors have made extensive studies on the
light-emitting device having all gallium nitride-based III-V Group
compound semiconductor double-heterostructure having high light
emission characteristics, and found that, when the light-emitting
layer is formed of In.sub.xGa.sub.1-xN, and the ratio x of indium
(In) is changed within the range of 0<x<1, a light-emitting
device capable of emitting visible light ranging from
near-ultraviolet to red can be obtained. The present inventors have
also found that, when an impurity is doped in In.sub.xGa.sub.1-xN
and In.sub.xGa.sub.1-xN has a low resistivity, a light-emitting
device having improved light emission characteristics, especially a
high output power, a high luminance, and a high luminosity could be
obtained.
[0045] In the light-emitting device of the present invention, when
the value of x in In.sub.xGa.sub.1-xN of the light-emitting layer
is close to 0, the device emits ultraviolet light. When the value
of x increases, the emission falls in the longer-wavelength region.
When the value of x is close to 1, the device emits red light. When
the value of x is in the range of 0<x<0.5, the light-emitting
device of the present invention emits blue to yellow light in the
wavelength range of 450 to 550 nm.
[0046] In the present invention, an impurity (also called as a
dopant) means a p- or n-type impurity, or both of them. In the
present invention, the p-type impurity includes Group II elements
such as cadmium, zinc, beryllium, magnesium, calcium, strontium,
and barium. As the p-type impurity, zinc is especially preferable.
The n-type impurity includes Group IV elements such as silicon,
germanium and tin, and Group VI elements such as selenium,
tellurium and sulfur.
[0047] In the present invention, "low-resistivity" means, when
referred to a p-type compound semiconductor, that the p-type
compound semiconductor has a resistivity of 1.times.10.sup.5
.omega..multidot.m or less, and when referred to an n-type compound
semiconductor, that the n-type compound semiconductor has a
resistivity of 10 .omega..multidot.cm or less.
[0048] Therefore, in the present invention, In.sub.xGa.sub.1-xN of
the light-emitting layer 13 includes a low-resistivity, p-type
In.sub.xGa.sub.1-xN doped with a p-type impurity (the first
embodiment to be described below in detail), a low-resistivity,
n-type In.sub.xGa.sub.1-xN doped with at least a p-type impurity
(the second embodiment to be described below in detail), or an
n-type In.sub.xGa.sub.1-xN doped with an n-type impurity (the third
embodiment to be described below in detail).
[0049] In the present invention, the first clad layer 16 is formed
of a low-resistivity n-type gallium nitride-based III-V Group
compound semiconductor. Although the n-type gallium nitride-based
III-V Group compound semiconductor tends to be of an n-type even
when undoped, it is preferable to dope an n-type impurity therein
and positively make an n-type compound semiconductor. The compound
semiconductor forming the first clad layer 16 is preferably
represented by the following formula:
Ga.sub.yAl.sub.1-yN(0.ltoreq.y.ltoreq.1)
[0050] In the present invention, the second clad layer 20 is formed
of a low-resistivity, p-type gallium nitride-based III-V Group
compound semiconductor doped with a p-type impurity. The compound
semiconductor is preferably represented by the following
formula:
Ga.sub.zAl.sub.1-zN(0.ltoreq.z.ltoreq.1)
[0051] The first, n-type clad layer 16 normally has a thickness of
0.05 to 10 .mu.m, and preferably has a thickness of 0.1 to 4 .mu.m.
An n-type gallium nitride-based compound semiconductor having a
thickness of less than 0.05 tends not to function as a clad layer.
On the other hand, when the thickness exceeds 10 .mu.m, cracks tend
to form in the layer.
[0052] The second, p-type clad layer 20 normally has a thickness of
0.05 to 1.5 .mu.m, and preferably has a thickness of 0.1 to 1
.mu.m. A p-type gallium nitride-based compound semiconductor layer
having a thickness less than 0.05 .mu.m tends to be hard to
function as a clad layer. On the other hand, when the thickness of
the layer exceeds 1.5 .mu.m, the layer tends to be difficult to be
converted into a low-resistivity layer.
[0053] In the present invention, the light-emitting layer 18
preferably has a thickness within a range such that the
light-emitting device of the present invention provides a practical
relative light intensity of 90% or more. In more detail, the
light-emitting layer 18 preferably has a thickness of 10 .ANG. to
0.5 .mu.m, and more preferably 0.01 to 0.2 .mu.m. FIG. 2 is a graph
showing a measurement result of the relative light intensities of
blue light-emitting diodes each having the structure shown in FIG.
1. Each blue light-emitting diode was prepared by forming the
light-emitting layer 18 made of low-resistivity
In.sub.0.1Ga.sub.0.9N while changing the thickness. As is apparent
from FIG. 2, when the thickness of the In.sub.xGa.sub.1-xN
light-emitting layer is 10 .ANG. to 0.5 .mu.m, the semiconductor
device exhibits a practical relative light intensity of 90% or
more. The almost same relationship between the thickness and the
relative light intensity was obtained for the low-resistivity
p-type In.sub.xGa.sub.1-xN doped with a p-type impurity, the
low-resistivity, n-type In.sub.xGa.sub.1-xN doped with at least a
p-type impurity, and the n-type In.sub.xGa.sub.1-xN doped with an
n-type impurity.
[0054] Referring back to FIG. 1, the double-heterostructure is
normally formed on a substrate 12 through an undoped buffer layer
14.
[0055] In the present invention, the substrate 12 can normally be
formed of a material such as sapphire, silicon carbide (SiC), or
zinc oxide (ZnO), and is most normally formed of sapphire.
[0056] In the present invention, the buffer layer 14 can be formed
of AlN or a gallium nitride-based compound semiconductor. The
buffer layer 14 is preferably formed of Ga.sub.mAl.sub.1-mN
(0<m.ltoreq.1). The Ga.sub.mAl.sub.1-mN allows the formation of
a gallium nitride-based compound semiconductor (first clad layer
16) having a better crystallinity thereon than on AlN. As is
disclosed in U.S. patent application Ser. No. 07/826,997 filed on
Jan. 28, 1992 by Shuji NAKAMURA and assigned to the same assignee,
the Ga.sub.mAl.sub.1-mN buffer layer is preferably formed at a
relatively low temperature of 200 to 900.degree. C., and preferably
400 to 800.degree. C. by the metaloranic chemical vapor deposition
(MOCVD) method. The buffer layer 14 preferably has substantially
the same semiconductor composition as the first clad layer 16 to be
formed thereon.
[0057] In the present invention, the buffer layer 14 normally has a
thickness of 0.002 .mu.m to 0.5 .mu.m.
[0058] In the present invention, the first clad layer 16, the
light-emitting layer 18, and the second clad layer 20, all of which
constitute the double-heterostructure, can be formed by any
suitable method. These layers are preferably sequentially formed on
the buffer layer 14 by the MOCVD. The gallium source which can be
used for the MOCVD includes trimethylgallium and triethylgallium.
The indium source includes trimethylindium and triethylindium. The
aluminum source includes trimethylaliminum and triethylaluminum.
The nitrogen source includes ammonia and hydrazine. The p-type
dopant source includes Group II compounds such as diethylcadmium,
dimethylcadmium, cyclopentadienyl-magnesium, and diethylzinc. The
n-type dopant source includes Group IV compounds such as silane,
and Group VI compounds such as hydrogen sulfide and hydrogen
selenide.
[0059] The gallium nitride-based III-V Group compound semiconductor
can be grown in the presence of the p-type impurity source and/or
the n-type impurity source by using the above gas source at a
temperature of 600.degree. C. or more, and normally 1,200.degree.
C. or less. As a carrier gas, hydrogen, nitrogen or the like can be
used.
[0060] In an as-grown state, the gallium nitride-based III-V Group
compound semiconductor doped with a p-type impurity tends to
exhibit a high resistivity and have no p-type characteristics (that
is, it is not a low-resistivity semiconductor) even if the compound
semiconductor contains the p-type impurity. Therefore, as is
disclosed in U.S. Ser. No. 07/970,145 filed on Nov. 2, 1992 by
Shuji NAKAMURA, Naruhito IWASA, and Masayuki SENOH and assigned to
the same assignee, the grown compound semiconductor is preferably
annealed at a temperature of 400.degree. C. or more, and preferably
600.degree. C. or more, for preferably one to 20 minutes or more,
or the compound semiconductor layer is preferably irradiated with
an electron beam while kept heated to a temperature of 600.degree.
C. or more. When the compound semiconductor is annealed at such a
high temperature that the compound semiconductor may be decomposed,
annealing is preferably performed in a compressed nitrogen
atmosphere to prevent the decomposition of the compound
semiconductor.
[0061] When annealing is performed, a p-type impurity in a form
bonded with hydrogen, such as Mg--H and Zn--H, is released from the
bonds with the hydrogen thermally, and the released hydrogen is
discharged from the semiconductor layer. As a result, the doped
p-type impurity appropriately functions as an acceptor to convert
the high-resistivity semiconductor into a low-resistivity p-type
semiconductor. Preferably, the annealing atmosphere does not
therefore contain a gas containing hydrogen atoms (e.g., ammonia or
hydrogen). Preferred examples of an annealing atmosphere includes
nitrogen and argon atmospheres. A nitrogen atmosphere is most
preferable.
[0062] After the double-heterostructure is formed, as shown in FIG.
1, the second clad layer 20 and the light-emitting layer 18 are
partially etched away to expose the first clad layer 16. An
n-electrode 24 is formed on the exposed surface while a p-electrode
26 is formed on the surface of the first clad layer 20. The
electrodes 24 and 26 are preferably heat-treated to achieve ohmic
contact to the semiconductor layers. Above-described annealing may
be achieved by this heat treatment.
[0063] The present invention has been generally described above.
The firs, second, and third embodiments will be individually
described below. It should be understood that unique points of the
respective embodiments will be particularly pointed out and
explained, and the above general description will be applied to
these embodiments unless otherwise specified, in the following
description.
[0064] In the first embodiment of the present invention,
low-resistivity In.sub.xGa.sub.1-xN constituting the light-emitting
layer 18 of the double-heterojunction structure shown in FIG. 1 is
of p-type, doped with a p-type impurity. Condition 0<x<0.5 is
preferable to form the light-emitting layer having a good
crystallinity and obtain a blue to yellow light-emitting device
excellent in the luminosity.
[0065] In the first embodiment, the concentration of the p-type
impurity doped in In.sub.xGa.sub.1-xN of the light-emitting layer
18 should be higher than the electron carrier concentration of a
particular, corresponding undoped In.sub.xGa.sub.1-xN (The electron
carrier concentration of an undoped InGaN varies within a range of
about 10.sup.17/cm.sup.3 to 1.times.10.sup.22/cm.sup.3, depending
on a particular growth condition used). Subject to this condition,
the p-type impurity concentration is preferably about
10.sup.17/cm.sup.3 to 1.times.10.sup.21/cm.sup.3 from the viewpoint
of light emission characteristics of the device. The most
preferable p-type impurity is zinc. As described above, the p-type
impurity-doped InGaN can be converted into a low-resistivity InGaN
by annealing (preferred) or radiating the electron beam.
[0066] In the second embodiment of the present invention, the
low-resistivity In.sub.xGa.sub.1-xN constituting the light-emitting
layer 18 of the structure shown in FIG. 1 is of n-type, doped with
at least a p-type impurity. Condition 0<x.ltoreq.0.5 is
preferable to provide the light-emitting layer having a good
crystallinity and obtain a blue to yellow light-emitting device
excellent in the luminosity. In the second embodiment, the
light-emitting layer should be subjected to the annealing treatment
described above, since it contains a p-type impurity.
[0067] In the second embodiment, when only a p-type impurity is
doped in In.sub.xGa.sub.1-xN layer 18, the concentration of the
p-type impurity should be lower than the electron concentration of
a corresponding undoped In.sub.xGa.sub.1-xN. Subject to this
condition, the p-type impurity concentration is preferably
1.times.10.sup.16/cm.sup.3 to 1.times.10.sup.22/cm.sup.3 from the
viewpoint of the light-emitting characteristics of the device.
Especially, when zinc is doped as the p-type impurity at a
concentration of 1.times.10.sup.17/cm.sup.3 to
1.times.10.sup.21/cm.sup.3, and especially
1.times.10.sup.18/cm.sup.3 to 1.times.10.sup.20/cm.sup.3, the
luminosity of the light-emitting device can be further improved and
the luminous efficacy can be further increased.
[0068] In the second embodiment, the second clad layer 20 is as
described above. However, when magnesium is doped as the p-type
impurity at a concentration of 1.times.10.sup.18/cm.sup.3 to
1.times.10.sup.21/cm.sup.3- , the luminous efficacy of the
light-emitting layer 18 can be further increased.
[0069] FIG. 3 is a diagram of the photoluminescence spectrum of a
wafer irradiated with a 10-mW laser beam from an He--Cd laser. The
wafer was prepared such that a low-resistivity
In.sub.0.14Ga.sub.0.86N layer doped with cadmium (p-type impurity)
was formed, according to the second embodiment, or a GaN layer
formed on a sapphire substrate. FIG. 4 is a diagram of the
photoluminescence spectrum of a wafer prepared following the same
procedures except that the In.sub.0.14Ga.sub.0.86N layer was not
doped with cadmium (undoped).
[0070] As can be apparent from FIG. 3, the p-type impurity-doped,
low-resistivity In.sub.0.14Ga.sub.0.86N layer of the present
invention exhibits strong blue light emission near 480 nm. As can
be apparent from FIG. 4, undoped In.sub.0.14Ga.sub.0.86N layer not
doped with a p-type impurity exhibits violet light emission near
400 nm. The same results as in FIG. 3 were obtained when zinc,
beryllium, magnesium, calcium, strontium, and/or barium was doped,
instead of Cd, according to the present invention. Thus, when the
p-type impurity is doped in InGaN according to the present
invention, the luminosity is improved.
[0071] When the p-type impurity is doped in InGaN, the
photoluminescence intensity can be greatly increased as compared to
the undoped InGaN. In the device relating to FIG. 3, blue
luminescence centers are formed in the InGaN by the p-type
impurity, thereby increasing the blue luminescence intensity FIG. 3
shows this phenomenon. In FIG. 3, a low peak appearing near 400 nm
is the inter-band emission peak of the undoped
In.sub.0.14Ga.sub.0.86N and corresponds to the peak in FIG. 4.
Therefore, in the case of FIG. 3, the luminous intensity is
increased by 20 times or more as compared to FIG. 4.
[0072] FIG. 5 is a graph obtained by measuring and plotting the
relative light intensities and the Zn concentrations of blue
light-emitting devices each having the structure of FIG. 1. Each
device was prepared such that the concentration of the p-type
impurity Mg of the second clad layer 20 was kept at
1.times.10.sup.20/cm.sup.3, while changing the Zn concentration of
the p-type impurity Zn-doped In.sub.0.1Ga.sub.0.9N of the
light-emitting layer 18. As shown in FIG. 5, the light-emitting
device exhibits a practical relative intensity of 90% or more in
the Zn concentration range of 1.times.10.sup.17/cm.sup.3 to
1.times.10.sup.21/cm.sup.3 and the highest relative light intensity
(almost 100%) in the Zn concentration range of
1.times.10.sup.18/cm.sup.3 to 1.times.10.sup.20/cm.sup.3.
[0073] FIG. 6 is a graph obtained by measuring and plotting the
relative light intensities and the Mg concentrations of blue
light-emitting devices each having the structure of FIG. 1. Each
device was prepared such that the Zn concentration of the p-type
impurity Zn-doped In.sub.0.1Ga.sub.0.9N of the light-emitting layer
18 was kept at 1.times.10.sup.20/cm.sup.3, while changing the
concentration of the p-type impurity Mg of the second clad layer
20. As shown in FIG. 6, the light intensity of the light-emitting
device tends to rapidly increase when the Mg concentration of the
clad layer 20 exceeds 1.times.10.sup.17/cm.sup.3, and the light
intensity tends to rapidly decrease when the Mg concentration
exceeds 1.times.10.sup.21/cm.sup.3. FIG. 6 clearly shows that the
light-emitting device exhibits a practical relative intensity of
90% or more (almost 100%) when the p-type impurity concentration of
the second clad layer 20 is in the range of
1.times.10.sup.18/cm.sup.3 to 1.times.10.sup.21/cm.sup.3. In FIGS.
5 and 6, the impurity concentrations were measured by a secondary
ion mass spectrometer (SIMS).
[0074] It is found that, more strictly, the electron carrier
concentration in the In.sub.xGa.sub.1-xN layer is preferably in the
range of 1.times.10.sup.17/cm.sup.3 to 5.times.10.sup.21/cm.sup.3
when at least a p-type impurity is doped in In.sub.xGa.sub.1-xN to
form an n-type In.sub.xGa.sub.1-xN light-emitting layer having a
low resistivity of 10 .OMEGA..multidot.cm or less. The electron
carrier concentration can be measured by Hall effects measurements.
When the electron carrier concentration exceeds
5.times.10.sup.21/cm.sup.3, it is difficult to obtain a
light-emitting device exhibiting a practical output power. The
electron carrier concentration is inversely proportional to the
resistivity. When the electron carrier concentration is less than
1.times.10.sup.16/cm.sup.3, InGaN tends to be high-resistivity
i-type InGaN, and the electron carrier concentration cannot be
measured. The impurity to be doped may be only a p-type impurity,
or both p- and n-type impurities. More preferably, both p- and
n-type impurities are doped. In this case, zinc as the p-type
impurity and silicon as the n-type impurity are preferably used.
Each of zinc and silicon is preferably doped at a concentration of
1.times.10.sup.17/cm.sup.3 to 1.times.21/cm.sup.3. When the
concentration of zinc is lower than that of silicon, InGaN can be
converted into preferable n-type InGaN.
[0075] When InGaN not doped with an impurity is grown, nitrogen
lattice vacancies are created to provide n-type InGaN. The residual
electron carrier concentration of this undoped n-type InGaN is
about 1.times.10.sup.17/cm.sup.3 to 1.times.10.sup.22/cm.sup.3
depending on a growth condition used. By doping a p-type impurity
serving as a luminescence center in the undoped n-type InGaN layer,
the electron carrier concentration in the n-type InGaN layer is
decreased. Therefore, when the p-type impurity is doped in InGaN
such that the electron carrier concentration is excessively
decreased, n-type InGaN is converted into high-resistivity i-type
InGaN. When the electron carrier concentration is adjusted to fall
within the above range according to the present invention, the
output power is increased. This indicates that the p-type impurity
serving as the luminescence center performs emission by forming
donor-acceptor (D-A) light-emitting pairs with the donor impurity.
The detailed mechanism has not been clarified yet. However, it is
found that, in the n-type InGaN in which both donor impurity (e.g.,
the n-type impurity or nitrogen lattice vacancy) for making some
electron carriers and the p-type impurity serving as an acceptor
impurity are present, the light intensity by the formation of the
luminescence centers is apparently increased. Since an increase in
the number of light-emitting pairs attributes to an increase in
light intensity as described, not only p-type impurity but also
n-type impurity is preferably doped in InGaN. More specifically,
when the n-type impurity (especially silicon) is dosed in InGaN
doped with the p-type impurity (especially zinc), the donor
concentration is increased, and at the same time, a constant donor
concentration with good reproducibility can be obtained, unlike in
undoped InGaN in which the electron carrier concentration varies
depending on the growth condition as described above, and in which
the donor concentration having a constant residual concentration
with good reproducibility is hardly obtained. In fact, it is found
that, by doping silicon, the electron carrier concentration is
increased from about 1.times.10.sup.18/cm.sup.3 to
2.times.10.sup.19/cm.sup.3 by one figure, and the donor
concentration is thus increased. Therefore, the amount of zinc to
be doped can be increased by the increased amount of the donor
concentration, and accordingly, the number of D-A light-emitting
pairs can be increased, thereby increasing the light intensity.
[0076] FIG. 7 is a graph obtained by measuring and plotting the
relative output powers of blue light-emitting diodes and the
elect-on carrier concentrations in the InGaN layers (measured by
Hall effects measurements after growth of the InGaN layer). The
blue light emitting diode was prepared such that an Si-dozed n-type
GaN layer was crown on the sapphire substrate, a Zn-doped n-type
In.sub.0.15Ga.sub.0.85N layer was grown thereon while changing the
Zn concentration, and an Mg-doped p-type GaN layer was grown. The
points in FIG. 7 correspond to electron carrier concentrations of
1.times.10.sup.16, 1.times.10.sup.17, 4.times.10.sup.17,
1.times.10.sup.18, 1.times.10.sup.19, 4.times.10.sup.19,
1.times.10.sup.20, 3.times.10.sup.20, 1.times.10.sup.21, and
5.times.10.sup.21/cm.sup.3 from the left, respectively.
[0077] As shown in FIG. 7, the output power of the light-emitting
device changes depending on the electron carrier concentration in
the n-type InGaN light-emitting layer. The output power starts to
rapidly increase at an electron carrier concentration of about
1.times.10.sup.16/cm.sup.3, reaches the maximum level at about
1.times.10.sup.19/cm.sup.3, slowly decreases until
5.times.10.sup.21/cm.sup.3, and rapidly decreases when the electron
carrier concentration exceeds that point. As is apparent from FIG.
7, when the electron carrier concentration in the n-type InGaN
layer is in the range of 1.times.10.sup.17/cm.sup.3 to
5.times.10.sup.21/cm.sup.3, the light-emitting device exhibits an
excellent output power.
[0078] FIG. 8 shows the light intensity when a laser beam from an
He--Cd laser was radiated on the n-type In.sub.0.15Ga.sub.0.85N
layer doped with only zinc at a concentration of
1.times.10.sup.18/cm.sup.3, and the n-type In.sub.0.15Ga.sub.0.85N
layer doped with zinc and silicon at concentrations of
1.times.10.sup.19/cm.sup.3 and 5.times.10.sup.19/cm.sup- .3,
respectively, and the photoluminescence was measured at room
temperature. The measurement result about the n-type
In.sub.0.15Ga.sub.0.85N layer doped with only zinc is represented
by a curve a, and the measurement result about the n-type
In.sub.0.15Ga.sub.0.85N layer doped with zinc and silicon is
represented by a curve b (in the curve b, measured intensity is
reduced to {fraction (1/20)}). Although the both InGaN layers
exhibit the major light-emitting peaks at 490 nm, the n-type InGaN
layer doped with both zinc and silicon exhibits a light intensity
ten times or more that of the n-type InGaN layer doped with only
zinc.
[0079] In the third embodiment of the present invention,
low-resistivity In.sub.xGa.sub.1-xN constituting the light-emitting
layer 18 of the structure of FIG. 1 is of n-type, doped with only
an n-type impurity. Condition 0<x.ltoreq.0.5 is preferable to
provide a light-emitting layer semiconductor having a good
crystallinity and obtain a blue light-emitting device excellent in
the luminosity.
[0080] In the third embodiment, the n-type impurity doped in
In.sub.xGa.sub.1-xN of the light-emitting layer 18 is preferably
silicon (Se). The concentration of the n-type impurity is
preferably 1.times.10.sup.17/cm.sup.3 to 1.times.10.sup.21/cm.sup.3
from the viewpoint of the light emission characteristics, and more
preferably 1.times.10.sup.18/cm.sup.3 to
1.times.10.sup.20/cm.sup.3.
[0081] In the third embodiment, as in the second embodiment, the
second clad layer 20 is as already described above. However, when
magnesium is used as the p-type impurity, and is doped at a
concentration of 1.times.10.sup.18/cm.sup.3 to
1.times.10.sup.21/cm.sup.3, the luminous efficacy of the
light-emitting layer 18 can be further increased.
[0082] FIG. 9 is a graph obtained by measuring and plotting the
relative light intensities and the Si concentrations of blue
light-emitting devices each having the structure of FIG. 1. Each
device was prepared such that the concentration of the p-type
impurity Mg of the second clad layer 20 was kept at
1.times.10.sup.19/cm.sup.3, while changing the Si concentration of
the n-type impurity Si-doped In.sub.0.1Ga.sub.0.9N of the
light-emitting layer 18. As shown in FIG. 9, the light-emitting
device exhibits a practical relative intensity of 90% or more in
the Si concentration range of 1.times.10.sup.17/cm.sup.3 to
1.times.10.sup.21/cm.sup.3, and the highest relative light
intensity (almost 100%) in the Si concentration range of
1.times.10.sup.18/cm.sup.3 to 1.times.10.sup.20/cm.sup.3.
[0083] FIG. 10 is a graph obtained by measuring and plotting the
relative light intensities and the Mg concentrations of blue
light-emitting devices each having the structure of FIG. 1. Each
device was prepared such that the Si concentration of the n-type
impurity Si-doped In.sub.0.1Ga.sub.0.9N of the light-emitting layer
18 was kept at 1.times.10.sup.19/cm.sup.3, while changing the
concentration of the p-type impurity Mg of the second clad layer
20. As shown in FIG. 10, the light intensity of the light-emitting
device tends to rapidly increase when the Mg concentration of the
second p-type clad layer 20 exceeds 1.times.10.sup.17/cm.sup.3, and
to rapidly decrease when the Mg concentration exceeds
1.times.10.sup.21/cm.sup.3. FIG. 10 shows that the light-emitting
device exhibits a practical relative intensity of 90% or more
(almost 100%) when the p-type impurity concentration of the second
clad layer 20 is in the range of 1.times.10.sup.18/cm.sup.3 to
1.times.10.sup.21/cm.sup.3. In FIGS. 9 and 10, the impurity
concentrations were measured by the SIMS.
[0084] In the third embodiment, the light-emitting device having
the double-heterostructure of the present invention uses inter-band
emission of the n-type InGaN layer. For this reason, the half width
of the emission peak is as narrow as about 25 nm, which is
{fraction (1/2)} or less that of the conventional homojunction
diode. In addition, the device of the present invention exhibits an
output power four times or more that of the homojunction diode.
Further, when the value of x of In.sub.xGa.sub.1-xN is changed in
the range of 0.02<x<0.5, emission within the wavelength
region of about 380 nm to 500 nm can be obtained as desired.
[0085] FIG. 11 show a structure of a more practical light-emitting
diode 30 having a double-heterostructure of the present
invention.
[0086] The light-emitting diode 30 a double-heterostructure 22
constituted by an impurity-doped In.sub.xGa.sub.1-xN light-emitting
layer 18, and two clad layers sandwiching the light-emitting layer
18, i.e., an n-type gallium nitide-based compound semiconductor
layer 16 and a p-type gallium nitride-based compound semiconductor
layer 20, as described above in detail.
[0087] A buffer layer 14 described above in detail is formed on a
substrate 20 described above in detail. An n-type GaN layer 32 is
formed on the buffer layer 14 to a thickness of, for example, 4 to
5 .mu.m, and provides a contact layer for an n-electrode which is
described below. The h-type contact layer 32 allows the formation
of a clad layer 16 having a better crystallinity, and can establish
a better ohmic contact with the n-electrode.
[0088] The double-heterostructure 22 is provided on the n-type
contact layer 32, with the clad layer 16 joined to the contact
layer 32.
[0089] A p-type GaN contact layer 34 is formed on the clad layer 20
to a thickness of, for example, 500 .ANG. to 2 .mu.m. The contact
layer 34 establishes a better ohmic contact with a p-electrode
described below, and increases the luminous efficacy so the
device.
[0090] The p-type contact layer 34 and the double-heterostructure
22 are partially etched away to expose the n-type contact layer
32.
[0091] A p-electrode is provided on the p-type contact layer 34,
and an n-electrode is provided on the exposed surface of the n-type
contact layer 32.
[0092] The light-emitting diodes embodying the present invention
have been described above. However, the present invention should
not be limited to these embodiments. The present invention
encompasses various types of light-emitting devices including a
laser diode, so far as those devices have the
double-heterostructures of the present invention.
[0093] FIG. 12 shows a structure of a laser diode 40 having a
double-heterostructure of the present invention.
[0094] The laser diode 40 has a double-heterostructure constituted
by an impurity-doped In.sub.xGa.sub.1-xN active layer 18 described
above in detail in association with the light-emitting diode, and
two clad layers sandwiching the active layer 18, i.e., an n-type
gallium nitride-based compound semiconductor layer 16 and a p-type
gallium nitride-based compound semiconductor layer 20, as described
above. A buffer layer 14 described above in detail is formed on a
substrate 12 described above in detail. An n-type gallium nitride
layer 42 is formed on the buffer layer 14, providing a contact
layer for an n-electrode described below.
[0095] The double-heterostructure 22 is provided on the n-type
gallium nitride contact layer 42, with the clad layer joined to the
contact layer 42.
[0096] A p-type GaN contact layer 44 is formed on the clad layer
20.
[0097] The p-type contact layer 44, the double heterostructure 22
and part of the n-type contact layer 42 are etched away to provide
a protruding structure as shown. A p-electrode is formed on the
p-type contact layer 44. A pair of n-electrodes 24a and 24b are
formed on the n-type GaN layer 42 to oppose each other, with the
protruding structure intervening therebetween.
[0098] For example, the substrate 12 is a sapphire substrate having
a thickness of 100 .mu.m, the buffer layer 14 is a GaN buffer layer
having a thickness of 0.02 .mu.m, and the n-type GaN contact layer
42 has a thickness of 4 .mu.m. The first clad layer 16 is an n-type
GaAlN clad layer having a thickness of 0.1 .mu.m, the second clad
layer 20 is a p-type GaAlN clad layer having a thickness of 0.1
.mu.m, and the active layer 18 is an n-type layer doped with
silicon or germanium. The p-type GaN contact layer 44 has a
thickness of 0.3 .mu.m.
[0099] The present invention will be described below with reference
to the following examples. In the examples below, a compound
semiconductor was grown by the MOCVD method. An MOCVD apparatus
used is a conventional MOCVD apparatus having a structure in which
a susceptor for mounting a substrate thereon is arranged in a
reaction vessel, and raw material gases can be supplied together
with a carrier gas toward a substrate while the substrate is
heated, thereby growing a compound semiconductor on the
substrate.
EXAMPLE 1
[0100] Cleaning of Substrate:
[0101] First, a sapphire substrate sufficiently washed was mounted
on a susceptor in an MOCVD reaction vessel, and the atmosphere in
the reaction vessel was sufficiently substituted with hydrogen.
Subsequently, while hydrogen was flown, the substrate was heated to
1,050.degree. C., and this temperature was held for 20 minutes,
thereby cleaning the sapphire substrate.
[0102] Growth of Buffer Layer:
[0103] The substrate was then cooled down to 510.degree. C. While
the substrate temperature was kept at 510.degree. C., ammonia
(NH.sub.3) as a nitrogen source, trimethylgallium (TMG) as a
gallium source, and hydrogen as a carrier gas were kept supplied at
flow rates of 4 liters (L)/min, 27.times.10.sup.-6 mol/min, and 2
L/min, respectively, toward the surface of the sapphire substrate
for one minute. Thus, a GaN buffer layer having a thickness of
about 200 .ANG. was grown on the sapphire substrate.
[0104] Growth of First Clad Layer:
[0105] After the buffer layer was formed, only the supply of TMG
was stopped, and the substrate was heated to 1,030.degree. C. While
the substrate temperature was kept at 1,030.degree. C., the flow
rate of TMG was switched to 54.times.10.sup.-6 mol/min, silane gas
(SiH.sub.4) as an n-type impurity was added at a flow rate of
2.times.10.sup.-9 mol/min, and each material gas was supplied for
60 minutes. Thus, an n-type GaN layer, doped with Si at a
concentration of 1.times.10.sup.20/cm.sup.3, having a thickness of
4 .mu.m was grown on the GaN buffer layer.
[0106] Growth of Light-Emitting Layer:
[0107] After the first clad layer was formed, the substrate was
cooled down to 800.degree. C. while flowing only the carrier gas.
While the substrate temperature was kept at 800.degree. C., the
carrier gas was switched to nitrogen at a flow rate of 2 L/m-n, and
TMG as a gallium source, trimethylindium (TMI) as an indium source,
ammonia as a nitrogen source, and diethylcadmium as a p-type
impurity source were supplied at flow rates of 2.times.10.sup.-6
mol/min, 1.times.10.sup.-5 mol/min, 4 L/min, and 2.times.10.sup.-6
mol/min, respectively, for ten minutes. Thus, an n-type
In.sub.0.14Ga.sub.0.86N layer, doped with Cd at a concentration of
1.times.10.sup.20/cm.sup.3, having a thickness of 200 .ANG. was
grown on the first clad layer.
[0108] Growth of Second Clad Layer:
[0109] After the light-emitting layer was formed, the substrate was
heated to 1,020.degree. C. while flowing only the carrier gas
nitrogen. While the substrate temperature was kept at 1,020.degree.
C., the carrier gas was switched to hydrogen, a gallium source,
TMG, a nitrogen source, ammonia, a p-type impurity source,
cyclopentadienyl-magnesium (Cp.sub.2Mg), were supplied at flow
rates of 54.times.10.sup.-6 mol/min, 4 L/min, 3.6.times.10.sup.-6
mol/min, respectively, for 15 minutes. Thus, a p-type GaN layer,
doped with Mg at a concentration of 1.times.10.sup.20/cm.sup.3,
having a thickness of 0.8 .mu.m was grown on the light-emitting
layer.
[0110] Conversion into Low-Resistivity Layer:
[0111] After the second clad layer was grown, the wafer was taken
out of the reaction vessel. The wafer was annealed under nitrogen
at a temperature of 700.degree. C. or more for 20 minutes. Thus,
the second clad layer and the light-emitting layer were converted
into low-resistivity layers.
[0112] Fabrication of LED:
[0113] The second clad layer and the light-emitting layer of the
wafer obtained above were partially etched away to expose the first
clad layer. An ohmic n-electrode was formed on the exposed surface
while an ohmic p-electrode was formed on the second clad layer. The
wafer was cut into chips each having a size of 500 .mu.m.sup.2, and
a blue light-emitting diode was fabricated by a conventional
method.
[0114] The blue light-emitting diode exhibited an output power of
300 .mu.W at 20 mA, and its emission peak wavelength was 480 nm.
The luminance of the light-emitting diode measured by a
commercially available luminance meter was 50 or more times that of
a light-emitting diode of Example 5 to be described later.
EXAMPLE 2
[0115] A blue light-emitting diode was prepared following the same
procedures as in Example 1 except that, in the growth process of a
buffer layer, trimethylaluminum (TMA) was used, instead of TMG, to
form an AlN buffer layer on a sapphire substrate at a substrate
temperature of 600.degree. C.
[0116] The blue light-emitting diode exhibited an output power of
80 .mu.W at 20 mA, and its emission peak wavelength was 480 nm. The
luminance of the light-emitting diode was about 20 times that of a
light-emitting diode of Example 5 to be described later.
EXAMPLE 3
[0117] Cleaning of a substrate and the growth of a buffer layer
were performed following the same procedures as in Example 1.
[0118] A Her the buffer layer was formed, only the TMG flow was
stopped, and the substrate was heated to 1,030.degree. C. While the
substrate temperature was kept at 1,030.degree. C., and the flow
rate of ammonia was not changed, the flow rate of TMG was switched
to 54.times.10.sup.-6 mol/min, and an aluminum source, TMA, and a
p-type impurity source, silane gas (SiH.sub.4), were added at flow
rates of 6.times.10.sup.-6 mol/min and 2.times.10.sup.-9 mol/min,
respectively, and each gas was supplied for 30 minutes. Thus, an
n-type Ga.sub.0.9Al.sub.0.1N layer (first clad layer), doped with
Si at a concentration of 1.times.10.sup.20/cm.sup.3, having a
thickness of 2 .mu.m was grown on the GaN buffer layer.
[0119] A light-emitting layer was subsequently grown following the
same procedures as in Example 1, to form a Cd-doped, n-type
In.sub.0.14Ga.sub.0.86N layer having a thickness of 200 .ANG..
[0120] After the light-emitting layer was formed, supply of all the
raw material gases was stopped, and the substrate was heated to
1,020.degree. C. While the substrate temperature was kept at
1,020.degree. C., and the flow rate of the carrier gas was not
changed, a gallium source, TMG, an aluminum source, TMA, a nitrogen
source, ammonia, and a p-type impurity source, Cp.sub.2Mg, were
supplied at flow rates of 54.times.10.sup.-6 mol/min,
6.times.10.sup.-6 mol/min, 4 L/min, and 3.6.times.10.sup.-6
mol/min, respectively, for 15 minutes. Thus, a p-type
Ga.sub.0.9Al.sub.0.1N layer (second clad layer) doped with Mg at a
concentration of 1.times.10.sup.20/cm.sup.3, having a thickness of
0.8 .mu.m was grown on the light-emitting layer.
[0121] The annealing treatment and fabrication of a diode from the
wafer were performed following the same procedures as in Example 1,
to prepare a blue light-emitting diode.
[0122] The blue light-emitting diode obtained above exhibited the
same output power, the same emission wavelength, and the same
luminance as in the diode of Example 1.
EXAMPLE 4
[0123] A blue light-emitting diode was prepared following the same
procedures as in Example 1 except that, in the growth process of a
light-emitting layer, Cp.sub.2Mg was users instead of
diethylcadmium at the same flow rate to grow an Mg-doped, p-type
In.sub.0.14Ga.sub.0.86N light-emitting layer.
[0124] The blue light-emitting layer obtained above exhibited the
same output power, the same emission wavelength, and the same
luminance as in the diode of Example 1.
EXAMPLE 5
[0125] A homojunction GaN light-emitting diode was prepared
following the same procedures as in Example 1 except that no
light-emitting InGaN layer was grown.
[0126] The light-emitting diode exhibited an output power of 50
.mu.W at 20 mA. The emission peak wavelength was 430 nm, and the
luminance was 2 milicandela (mcd).
EXAMPLE 6
[0127] A blue light-emitting diode was prepared following the same
procedures as in Example 1 except that, in the growth process of a
light-emitting layer, silane gas at a flow rate of
2.times.10.sup.-9 mol/min was used, instead of dimethylcadmlum, to
form n-type In.sub.0.14Ga.sub.0.86N light-emitting layer doped with
Si at a concentration of 1.times.10.sup.20/cm.sup.3.
[0128] The light-emitting diode exhibited an output power output of
120 .mu.W at 20 mA. The emission peak wavelength was 400 nm, and
the luminance was about {fraction (1/50)} that of the diode in
Example 1. The low luminance was due to the short wavelength of the
emission peak to lower the luminosity.
EXAMPLE 7
[0129] Cleaning of a substrate, the growth of a buffer layer, and
the growth of a first clad layer (Si-doped, n-type GaN layer) were
performed following the same procedures as in Example 1.
[0130] After the first clad layer was formed, a light-emitting
layer was grown as in Example 1 except that diethylzinc (DEZ) at a
flow rate of 1.times.10.sup.-6 mol/min was used, instead of
diethylcadmium, to form an n-type In.sub.0.15Ga.sub.0.85N layer
(light-emitting layer), doped with Zn at a concentration of
1.times.10.sup.19/cm.sup.3, having a thickness of 200 .ANG. on the
first clad layer.
[0131] A second clad layer was subsequently grown following the
same procedures as in Example 1, to form an Mg-doped, p-type GaN
layer having a thickness of 0.8 .mu.m. The annealing treatment and
fabrication of a diode from the wafer were performed following the
same procedures as in Example 1, to prepare a blue light-emitting
diode.
[0132] The light-emitting device exhibited an output power of 300
.mu.W at 20 mA. The emission peak wavelength was 480 nm, and the
luminance was 400 mcd.
EXAMPLE 8
[0133] Cleaning of a substrate and the growth of a buffer layer
were performed following the same procedures as in Example 1.
[0134] A first clad layer was grown following the same procedures
as in Example 3, to form an Si-doped, n-type Ga.sub.0.9Al.sub.0.1N
layer having a thickness of 2 .mu.m.
[0135] After the first clad layer was formed, a light-emitting
layer was grown as in Example 7, to form an n-type
In.sub.0.15Ga.sub.0.85N layer, doped with Zn at a concentration of
1.times.10.sup.19/cm.sup.3, having a thickness of 200 .ANG..
[0136] After the light-emitting layer was formed, a second clad
layer was grown as in Example 3, to form a p-type
Ga.sub.0.9Al.sub.0.1N layer, doped with Mg at a concentration of
1.times.10.sup.20/cm.sup.3, having a thickness of 0.8 .mu.m on the
light-emitting layer.
[0137] The annealing treatment of the second clad layer and
fabrication of a diode from the wafer were performed following the
same procedures as in Example 1, to prepare a blud light-emitting
diode.
[0138] The blue light-emitting diode obtained above exhibited the
same output power, the same emission peak wavelength, and the same
luminance as in the diode of Example 7.
EXAMPLE 9
[0139] A blue light-emitting diode was prepared following the same
procedures as in Example 7 except that, in the growth process of a
light-emitting layer, the flow rate of DEZ was increased, to form
an In.sub.0.15Ga.sub.0.85N light-emitting layer doped with zinc at
a concentration of 1.times.10.sup.22/cm.sup.3.
[0140] The blue light-emitting diode thus obtained exhibited an
output power of about 40% of that of the diode of Example 7.
EXAMPLE 10
[0141] A blue light-emitting diode was prepared following the same
procedures as in Example 7 except that, in the growth process of a
second clad layer, the flow rate of Cp.sub.2Mg was decreased, to
form a p-type GaN layer (second clad layer) doped with Mg at a
concentration of 1.times.10.sup.17/cm.sup.3.
[0142] The light-emitting diode exhibited an output power of about
10% of that of the diode of Example 7.
EXAMPLE 11
[0143] Cleaning of a substrate, the growth of a buffer layer, and
the growth of a first clad layer (Si-doped, n-type GaN layer) were
performed following the same procedures as in Example 1.
[0144] After the first clad layer was formed, a light-emitting
layer was grown as in Example 1 except that diethylzinc was used,
instead of diethycadimium, to form a Zn-doped, n-type
In.sub.0.15Ga.sub.0.85N layer having a thickness of 100 .ANG. on
the first clad layer. The electron carrier concentration of the
n-type In.sub.0.5Ga.sub.0.85N layer was
1.times.10.sup.19/cm.sup.3.
[0145] A second clad layer was grown following the same procedures
as in Example 1, to form an Mg-doped, p-type GaN layer. The
annealing treatment and fabrication of a diode from the wafer were
performed as in Example 1, to prepare a light emitting diode.
[0146] The light-emitting diode exhibited an output power of 400
.mu.W at 20 mA. The emission peak wavelength was 490 nm, and the
luminance was 600 mcd.
EXAMPLE 12
[0147] A blue light-emitting diode was prepared following the same
procedures as in Example 11 except that, in the growth process of a
light-emitting layer, the flow rate of DEZ gas was adjusted, to
form an n-type In.sub.0.15Ga.sub.0.85N layer (light-emitting layer)
having an electron carrier concentration of
4.times.10.sup.17/cm.sup.3.
[0148] The light-emitting diode exhibited an output power of 40
.mu.W at 20 mA. The emission peak wavelength was 490 nm.
EXAMPLE 13
[0149] A blue light-emitting diode was prepared following the same
procedures as in Example 11 except that, in the growth process of a
light-emitting layer, the flow rate of the DEZ gas was adjusted, to
form an n-type In.sub.0.15Ga.sub.0.85N layer (light-emitting layer)
having an electron carrier concentration of
1.times.10.sup.21/cm.sup.3.
[0150] The light-emitting diode exhibited an output power of 40
.mu.W at 20 mA The emission peak wavelength was 490 nm.
EXAMPLE 14
[0151] A blue light-emitting diode was prepared following the same
procedures as in Example 11 except that, in the growth process of a
light-emitting layer, the flow rate of the DEZ gas was adjusted, to
form an n-type In.sub.0.15Ga.sub.0.85N layer (light-emitting layer)
having an electron carrier concentration of
1.times.10.sup.17/cm.sup.3.
[0152] The light-emitting diode exhibited an output power of 4
.mu.W at 20 mA. The emission peak wavelength was 490 nm.
EXAMPLE 15
[0153] A blue light-emitting diode was prepared following the same
procedures as in Example 11 except that, in the growth process of a
light-emitting layer, the flow rate of DEZ gas was adjusted, to
form an n-type In.sub.0.15Ga.sub.0.85N layer having an electron
carrier concentration of 5.times.10.sup.21/cm.sup.3.
[0154] The light-emitting diode exhibited an output power of 4
.mu.W at 20 mA. The emission peak wavelength was 490 nm.
EXAMPLE 16
[0155] A buffer layer and an n-type GaN layer were formed on a
sapphire substrate following the same procedures as in Example
11.
[0156] A high-resistivity, i-type GaN layer was grown by using TMG
as a gallium source, ammonia as a nitrogen source, and DEZ as a
p-type impurity source. The i-type GaN layer was partially etched
away to expose the n-type GaN layer. An electrode was formed on the
exposed surface, and another electrode was formed on the i-type GaN
layer, thereby preparing a light-emitting diode of a MIS
structure.
[0157] The MIS structure diode exhibited a radiant power output of
1 .mu.W at 20 mA and a luminance of 1 mcd.
EXAMPLE 17
[0158] A blue light-emitting diode was prepared following the same
procedures as in Example 11 except that, in the growth process of a
light-emitting layer, silane gas as an impurity source was added,
to form an n-type In.sub.0.15Ga.sub.0.85N light-emitting layer,
doped with Zn and Si, having an electron carrier concentration of
1.times.10.sup.19/cm.sup.- 3.
[0159] The light-emitting diode exhibited an output power of 600
.mu.W at 20 mA. The emission peak wavelength was 490 nm, and the
luminance was 800 mcd.
EXAMPLE 18
[0160] Cleaning of a substrate, the growth of a buffer layer, and
the growth of a first clad layer (Si-doped GaN layer) were
performed following the same procedures as in Example 1.
[0161] After the first clad layer was formed, a light-emitting
layer was grown as in Example 1 except that silane and DEZ were
used, instead of diethylcadmium, to form an n-type
In.sub.0.14Ga.sub.0.86N layer, doped with Si and Zn, having a
thickness of 100 .ANG. on the first clad layer. The light-emitting
layer had an electron carrier concentration of
1.times.10.sup.18/cm.sup.3.
[0162] A second clad layer was grown following the same procedures
as in Example 7, to form an Mg-doped (concentration of
2.times.10.sup.20/cm.sup- .3), p-type GaN layer.
[0163] The annealing treatment and fabrication of an LED from the
wafer were performed following the same procedures as in Example
1.
[0164] The blue light-emitting diode exhibited an output power of
580 .mu.W at 20 mA. The luminance was 780 mcd, and the emission
peak wavelength was 490 nm.
EXAMPLE 19
[0165] A blue light-emitting diode was prepared following the same
procedures as in Example 18 except that, in the growth of a
light-emitting layer, the flaw rates of the silane gas and the DEZ
gas, were adjusted, to form an n-type In.sub.0.14Ga.sub.0.86N
light-emitting layer, doped with Si and Zn, having an electron
carrier concentration of 1.times.10.sup.20/cm.sup.3.
[0166] The blue light-emitting diode exhibited an output power of
590 .mu.W at 20 mA. The luminance was 790 mcd, and the emission
peak wavelength was 490 nm.
EXAMPLE 20
[0167] A blue light-emitting diode was prepared following the same
procedures as in Example 18 except that, in the growth process of a
light-emitting layer, the flow rates of the silane gas and the DEZ
gas were adjusted, to form an n-type In.sub.0.14Ga.sub.0.86N
light-emitting layer, doped with Si and Zn, having an electron
carrier concentration of 4.times.10.sup.17/cm.sup.3.
[0168] The blue light-emitting diode exhibited a radiant power
output of 60 .mu.W at 20 mA. The luminance was 80 mcd, and the
emission peak wavelength was 490 nm.
EXAMPLE 21
[0169] A blue light-emitting diode was prepared following the same
procedures as in Example 18 except that, in the growth process of a
light-emitting layer, the flow rates of the silane gas and the DEZ
gas were adjusted, to form an n-type In.sub.0.14Ga.sub.0.86N
light-emitting layer, doped with Si and Zn, having an electron
carrier concentration of 5.times.10.sup.21/cm.sup.3.
[0170] The blue light-emitting diode exhibited an output power of 6
.mu.W at 20 mA. The luminance was 10 mcd, and the emission peak
wavelength was 490 nm.
EXAMPLE 22
[0171] A green light-emitting diode was prepared following the same
procedures as in Example 18 except that, in the growth process of a
light-emitting layer, the flow rate of TMI was adjusted, to form an
Si- and Zn-doped In.sub.0.25Ga.sub.0.75N light-emitting layer.
[0172] The green light-emitting layer exhibited an output power of
500 .mu.W at 20 mA. The luminance was 1,000 mcd, and the emission
peak wavelength was 510 nm.
EXAMPLE 23
[0173] A buffer layer and an n-type GaN layer were formed a
sapphire substrate following the same procedures as in Example
11.
[0174] Using TMG as a gallium source, ammonia as a nitrogen source,
and silane and DEZ as impurity sources, an i-type GaN layer doped
with Si and Zn was formed. The i-type GaN layer was partially
etched away to expose the n-type GaN layer. An electrode was formed
on the exposed surface, and another electrode was formed on the
i-type GaN layer, thereby preparing a light-emitting diode of a MIS
structure.
[0175] The MIS structure diode exhibited an output power of 1 .mu.W
at 20 mA, and a luminance of 1 mcd.
EXAMPLE 24
[0176] Cleaning of a substrate, the growth of a buffer layer, and
the growth of a first clad layer (Si-doped, n-type GaN layer) were
performed following the same procedures as in Example 1.
[0177] After the first clad layer was formed, a light-emitting
layer was grown as in Example 1 except that an n-type impurity
source, silane, was used, instead of diethylcadmium, at an adjusted
flow rate, and growth was conducted for 5 minutes, to form an
n-type In.sub.0.15Ga.sub.0.85N light-emitting layer, doped with Si
at a concentration of 1.times.10.sup.20/cm.sup.3, having a
thickness of 100 .ANG. on the first clad layer.
[0178] Then, a second clad layer was grown as in Example 1 except
that the flow rate of Cp.sub.2Mg was adjusted, to form a p-type GaN
layer (second clad layer) doped with Mg at a concentration of
1.times.10.sup.18/cm.sup.- 3. The annealing treatment and
fabrication of a diode from the wafer were performed as in Example
1, to prepare a blue light-emitting diode.
[0179] The light-emitting diode exhibited an output power of 300
.mu.W at 20 mA. The emission peak wavelength was 405 nm.
EXAMPLE 25
[0180] A blue light-emitting diode was prepared following the same
procedures as in Example 24 except that, in the growth process of a
first clad layer, an Si-doped, n-type Ga.sub.0.9Al.sub.0.1N layer
(first clad layer) having a thickness of 2 fm was formed following
the same procedures as in Example 3, and in the growth process of a
second clad layer, a p-type Ga.sub.0.9Al.sub.0.1N layer (second
clad layer), doped with Mg at a concentration of
1.times.10.sup.18/cm.sup.3, having a thickness of 0.8 .mu.m was
formed following the same procedures as in Example 3.
[0181] The light-emitting diode exhibited the same output power and
the same emission peak wavelength as in the light-emitting diode of
Example 24.
EXAMPLE 26
[0182] A blue light-emitting diode was prepared following the same
procedures as in Example 24 except that, an the growth process of a
light-emitting layer, the flow rate of silane gas was increased, to
form an n-type In.sub.0.15Ga.sub.0.85N layer doped with Si at a
concentration of 1.times.10.sup.22/cm.sup.3.
[0183] The output of the light-emitting diode was about. 40% of
that of the diode of Example 24.
EXAMPLE 27
[0184] A blue light-emitting diode was prepared following the same
procedures as in Example 24 except that, in the growth process of a
second clad layer, the flow rate of Cp.sub.2Mg was decreased, to
form a p-type GaN layer doped with Mg at a concentration of
1.times.10.sup.17/cm.sup.3.
[0185] The output of the light-emitting diode was about 20% of that
of the diode of Example 24.
EXAMPLE 28
[0186] Cleaning of a substrate and the growth of a buffer layer
were performed following the same procedures as in Example 1.
[0187] After the buffer layer was formed, only the TMG flow was
stopped, and the substrate was heated to 1,030.degree. C. While the
substrate temperature was kept at 1,030.degree. C., and the flow
rate of ammonia was not changed, the flow rate of TMG was switched
to 54.times.10.sup.-6 mol/min, an n-type impurity source, silane,
was added at a flow rate of 2.times.10.sup.-9 mol/min, and the
growth was conducted for 60 minutes. Thus, n-type GaN layer (n-type
contact layer), doped with Si at a concentration of
1.times.10.sup.20/cm.sup.3, having a thickness of 4 .mu.m was
formed or the GaN buffer layer.
[0188] Then, an aluminum source, TMA, at an adjusted flow rate was
added, and the growth was conducted in a similar manner to that in
Example 3, to form an Si-doped n-type Ga.sub.0.8Al.sub.0.2N layer
(first clad layer) having a thickness of 0.15 .mu.m on the n-type
contact layer.
[0189] Next, a light-emitting layer was grown in the same
procedures as in Example 17, to form an n-type
In.sub.0.14Ga.sub.0.86N light-emitting layer, doped with Si and Zn,
having an electron carrier concentration of
1.times.10.sup.19/cm.sup.3 on the first clad layer.
[0190] Subsequently, a second clad layer was grown for 2 minutes in
a similar manner to that Example 3, to form an Mg-doped
Ga.sub.0.8Al.sub.0.2N layer having a thickness of 0.15 .mu.m on the
light-emitting layer.
[0191] Then, only the aluminum source flow was stopped, and the
growth was conducted for 7 minutes, to form an Mg-doped GaN layer
(p-type contact layer) having a thickness of 0.3 .mu.m on the
second clad layer.
[0192] The annealing treatment was conducted as in Example 1, to
convert the light-emitting layer, the second clad layer and the
p-type contact layer into low-resistivity layers.
[0193] From the wafer, a light-emitting diode having a structure of
FIG. 11 was fabricated.
[0194] This diode exhibited an output power of 700 .mu.W and a
luminance of 1,400 mcd. The emission peak wavelength was 490 nm.
The forward voltage was 3.3V at 20 mA.
[0195] This forward voltage was about 4V lower than that of the
diode of Example 3, 8 or 25. This lower forward voltage is due to
the better ohmic contact between the GaN contact layers and the
electrodes.
[0196] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details, and
representative devices shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *