U.S. patent application number 10/229067 was filed with the patent office on 2003-01-23 for nitride semiconductor device.
This patent application is currently assigned to Nichia Chemical Industries, Ltd.. Invention is credited to Iwasa, Naruhito, Nagahama, Shinichi, Nakamura, Shuji.
Application Number | 20030015724 10/229067 |
Document ID | / |
Family ID | 27573376 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030015724 |
Kind Code |
A1 |
Nakamura, Shuji ; et
al. |
January 23, 2003 |
Nitride semiconductor device
Abstract
A nitride semiconductor device has a nitride semiconductor layer
structure. The structure includes an active layer of a quantum well
structure containing an indium-containing nitride semiconductor. A
first nitride semiconductor layer having a band gap energy larger
than that of the active layer is provided in contact with the
active layer. A second nitride semiconductor layer having a band
gap energy smaller than that of the first layer is provided over
the first layer. Further, a third nitride semiconductor layer
having a band gap energy larger than that of the second layer is
provided over the second layer.
Inventors: |
Nakamura, Shuji;
(Tokushima-shi, JP) ; Nagahama, Shinichi;
(Tokushima-shi, JP) ; Iwasa, Naruhito;
(Komatsushima-shi, JP) |
Correspondence
Address: |
Nixon & Vanderhye P.C.
8th Floor
1100 North Glebe Road
Arlington
VA
22201-4714
US
|
Assignee: |
Nichia Chemical Industries,
Ltd.
|
Family ID: |
27573376 |
Appl. No.: |
10/229067 |
Filed: |
August 28, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10229067 |
Aug 28, 2002 |
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09293060 |
Apr 25, 2000 |
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09293060 |
Apr 25, 2000 |
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08743729 |
Nov 6, 1996 |
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5959307 |
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Current U.S.
Class: |
257/103 ;
257/E33.008 |
Current CPC
Class: |
B82Y 20/00 20130101;
H01S 5/3206 20130101; H01L 33/06 20130101; H01S 5/321 20130101;
H01S 5/3413 20130101; H01S 5/343 20130101; H01S 5/34333 20130101;
H01S 5/2009 20130101; H01S 5/2004 20130101; H01S 5/3404 20130101;
H01L 33/325 20130101; H01S 5/3201 20130101; H01L 33/02 20130101;
H01L 33/32 20130101; H01S 5/3211 20130101; H01S 5/3412 20130101;
H01S 5/2202 20130101 |
Class at
Publication: |
257/103 |
International
Class: |
H01L 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 6, 1995 |
JP |
7-287189 |
Nov 24, 1995 |
JP |
7-305279 |
Nov 24, 1995 |
JP |
7-305280 |
Nov 24, 1995 |
JP |
7-305281 |
Dec 6, 1995 |
JP |
7-317850 |
Dec 20, 1995 |
JP |
7-332056 |
Jul 16, 1996 |
JP |
8-186339 |
Aug 29, 1996 |
JP |
8-228147 |
Claims
What is claimed is:
1. A nitride semiconductor device having a nitride semiconductor
layer structure comprising: an active layer of a quantum well
structure which has a first surface and a second surface and which
comprises an indium-containing nitride semiconductor; a first
nitride semiconductor layer which is formed to adjoin the first
surface of the active layer and has a band gap energy larger than
that of the active layer; a second nitride semiconductor layer
which is formed on the first surface side of the active layer at a
location more distant from the active layer relative to the first
nitride semiconductor layer and which has a band gap energy smaller
than that of the first nitride semiconductor layer; and a third
nitride semiconductor layer which is formed on the first surface
side of the active layer at a location more distant from the active
layer relative to the second nitride semiconductor layer and which
has a band gap energy larger than that of the second nitride
semiconductor layer.
2. The device according to claim 1, wherein said first nitride
semiconductor layer has a thickness sufficiently thin such that
carriers may tunnel therethrough.
3. The device according to claim 1, wherein said first nitride
semiconductor layer has a thickness of 0.1 .mu.m or less.
4. The device according to claim 3, wherein said first nitride
semiconductor layer has a thickness of 10 angstroms or more.
5. The device according to claim 1, wherein said active layer is
doped with an impurity.
6. The device according to claim 5, wherein said impurity comprises
silicon or germanium.
7. The device according to claim 5, wherein said impurity is doped
in said at least one well layer.
8. The device according to claim 1, wherein said layer structure is
provided on a p-side of the active layer.
9. The device according to claim 8, wherein said second nitride
semiconductor layer adjoin said first nitride semiconductor
layer.
10. The device according to claim 9, wherein said third nitride
semiconductor layer adjoins said second nitride semiconductor
layer.
11. The device according to claim 1, wherein said layer structure
is provided on an n-side of the active layer.
12. The device according to claim 11, wherein said second nitride
semiconductor layer adjoins said first nitride semiconductor
layer.
13. The device according to claim 12, wherein said third nitride
semiconductor layer adjoins said second nitride semiconductor
layer.
14. A nitride semiconductor device comprising: an active layer of a
quantum well structure which has a first surface and a second
surface and which comprises an indium-containing nitride
semiconductor; a first layer which adjoins the first surface of the
active layer and has a band gap energy larger than that of the
active layer; a second layer which is formed on the first surface
side of the active layer at a location more distant from the active
layer relative to the first layer and which comprises a nitride
semiconductor containing an acceptor impurity and which has a band
gap energy smaller than that of the first layer; and a third layer
which is formed on the first surface side of the active layer at a
location more distant from the active layer relative to the second
layer and which comprises a nitride semiconductor containing an
acceptor impurity and which has a band gap energy larger than that
of the second layer.
15. The device according to claim 14, wherein said first layer has
a thickness sufficiently thin such that carriers may tunnel
therethrough.
16. The device according to claim 14, wherein said first layer has
a thickness of 0.1 .mu.m or less.
17. The device according to claim 16, wherein said first layer has
a thickness of 10 angstroms or more.
18. The device according to claim 14, wherein said active layer is
doped with an impurity.
19. The device according to claim 18, wherein said impurity
comprises silicon or germanium.
20. The device according to claim 18, wherein said impurity is
doped in said at least one well layer.
21. The device according to claim 14, wherein said second layer
adjoins said first layer.
22. The device according to claim 21, wherein said third layer
adjoins said second layer.
23. A nitride semiconductor device comprising: an active layer of a
quantum well structure which has a first surface and a second
surface and which comprises an indium-containing nitride
semiconductor, a first layer which is formed to adjoin the second
surface of the active layer and which comprises a nitride
semiconductor and which has a band gap energy larger than that of
the active layer; a second layer which is formed on the second
surface side of the active layer at a location more distant from
the active layer relative to the first layer, which comprises an
n-type nitride semiconductor, and which has a band gap energy
smaller than that of the first layer, a third layer which is formed
on the second surface side of the active layer at a location more
distant from the active layer relative to the second layer, which
comprises an n-type nitride semiconductor, and which has a band gap
energy larger than that of the second layer.
24. The device according to claim 23, wherein said first layer has
a thickness sufficiently thin such that carriers may tunnel
therethrough.
25. The device according to claim 23, wherein said first layer has
a thickness of 0.1 .mu.m or less.
26. The device according to claim 25, wherein said first layer has
a thickness of 10 angstroms or more.
27. The device according to claim 23, wherein said active layer is
doped with an impurity.
28. The device according to claim 27, wherein said impurity
comprises silicon or germanium.
29. The device according to claim 27, wherein said impurity is
doped in said at least one well layer.
30. The device according to claim 23, wherein said second layer
adjoins said first layer.
31. The device according to claim 30, wherein said third layer
adjoins said second layer.
32. A nitride semiconductor device comprising: an active layer of a
quantum well structure which has a first surface and a second
surface and which comprises an indium-containing nitride
semiconductor; a first nitride semiconductor layer structure
comprising a first p-side nitride semiconductor layer which is
formed to adjoin the first surface of the active layer and has a
band gap energy larger than that of the active layer, a second
p-side nitride semiconductor layer which is formed on the first
surface side of the active layer at a location more distant from
the active layer relative to the first p-side nitride semiconductor
layer and which has a band gap energy smaller than that of the
first p-side nitride semiconductor layer, and a third p-side
nitride semiconductor layer which is formed on the first surface
side of the active layer at a location more distant from the active
layer relative to the second p-side nitride semiconductor layer and
which has a band gap energy larger than that of the second p-side
nitride semiconductor layer; and a second nitride semiconductor
layer structure comprising a first n-side nitride semiconductor
layer which is formed to adjoin the second surface of the active
layer and has a band gap energy larger than that of the active
layer, a second n-side nitride semiconductor layer which is formed
on the second surface side of the active layer at a location more
distant from the active layer relative to the first n-side nitride
semiconductor layer and which has a band gap energy smaller than
that of the first n-side nitride semiconductor layer, and a third
n-side nitride semiconductor layer which is formed on the second
surface side of the active layer at a location more distant from
the active layer relative to the second n-side nitride
semiconductor layer and which has a band gap energy larger than
that of the second n-side nitride semiconductor layer.
33. The device according to claim 32, wherein said first p-side
nitride semiconductor layer has a thickness sufficiently thin such
that carriers may tunnel therethrough.
34. The device according to claim 32, wherein said first p-side
nitride semiconductor layer has a thickness of 0.1 .mu.m or
less.
35. The device according to claim 25, wherein said first p-side
nitride semiconductor layer has a thickness of 10 angstroms or
more.
36. The device according to claim 32, wherein said active layer is
doped with an impurity.
37. The device according to claim 36, wherein said impurity
comprises silicon or germanium.
38. The device according to claim 36, wherein said impurity is
doped in said at least one well layer.
39. The device according to claim 32, wherein said first n-side
nitride semiconductor layer has a thickness sufficiently thin such
that carriers may tunnel therethrough.
40. The device according to claim 32, wherein said first n-side
nitride semiconductor layer has a thickness of 0.1 .mu.m or
less.
41. The device according to claim 40, wherein said first n-side
nitride semiconductor layer has a thickness of 10 angstroms or
more.
42. The device according to claim 32, wherein said second p-side
nitride semiconductor layer adjoins said first p-side nitride
semiconductor layer, and said third p-side nitride semiconductor
layer adjoins said second p-side nitride semiconductor layer.
43. The device according to claim 42, wherein said second n-side
nitride semiconductor layer adjoins said first n-side nitride
semiconductor layer, and said third n-side nitride semiconductor
layer adjoins said second n-side nitride semiconductor layer.
44. A nitride semiconductor device having, on a substrate, a layer
structure comprising an n-type contact layer, a first n-type clad
layer which comprises an aluminum-containing nitride semiconductor,
a second n-type clad layer which comprises an indium-containing
nitride semiconductor or GaN, an active layer of a quantum well
structure which comprises an indium-containing nitride
semiconductor, a first p-type clad layer which comprises an
aluminum-containing nitride semiconductor, a second p-type clad
layer which comprises an indium-containing nitride or GaN, a third
p-type clad layer which comprises an aluminum-containing nitride
semiconductor, and a p-type contact layer.
45. The device according to claim 44, wherein said active layer is
doped with an impurity.
46. The device according to claim 45, wherein said impurity
comprises silicon or germanium.
47. The device according to claim 45, wherein said impurity is
doped in a well layer.
48. A nitride semiconductor device comprising: a first clad layer
comprising an n-type nitride semiconductor; an active layer of a
quantum well structure provided on the first clad layer, said
active layer comprising a nitride semiconductor containing indium
and gallium and having at least one well layer having a thickness
not greater than 70 angstroms, wherein said well layer is placed on
an underlying layer in a state lattice-mismatched with the
underlying layer and includes a plurality of indium-rich regions
and indium-poor regions; and a second clad layer which is provided
on the active layer and comprises a nitride semiconductor doped
with an acceptor impurity.
49. The device according to claim 48, wherein said active layer is
doped with an impurity.
50. The device according to claim 49, wherein said impurity
comprises silicon or germanium.
51. The device according to claim 49, wherein said impurity is
doped in said well layer.
52. A nitride semiconductor device including a first n-type layer
which comprises an n-type, aluminum-containing nitride
semiconductor or n-type gallium nitride; and a second n-type layer
which comprises an n-type, aluminum-containing nitride
semiconductor, wherein the device has a third n-type layer which
comprises an n-type, indium-containing nitride semiconductor
between the first n-type layer and the second n-type layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a nitride semiconductor
device including a light-emitting device, such as a laser diode
(LD) device or a light-emitting diode (LED), and a light receiving
device such as a solar cell, and more particularly to a nitride
semiconductor light-emitting device.
[0003] 2. Description of the Related Art
[0004] Nitride semiconductors can have a band gap energy which
ranges from 1.95 to 6.0 eV depending on their composition, and
hence they have been drawing attention as a material for such
semiconductor light-emitting devices as a light-emitting diode
(LED) device and a laser diode (LD) device. Recently, as a result
of the utilization of these nitride semiconductor materials, a
high-brightness blue LED device and green LED device have been put
to practical use. These LED devices have a double-heterostructure
having a p-n junction and both have an output power exceeding 1
mW.
[0005] A conventional LED device basically has a
double-heterostructure wherein an active layer comprising InGaN is
sandwiched between n-type and p-type clad layers each comprising
AlGaN. An n-type contact layer comprising GaN is formed on the
n-type clad layer, and a p-type contact layer comprising GaN is
formed on the p-type clad layer. This laminate structure is formed
on a substrate made of, for example, sapphire.
[0006] Basically, an LD device can have a structure similar to that
of the above-mentioned LED device. However, most of LD devices have
a separate confinement structure in which light and carrier are
separately confined. A nitride semiconductor LD device of the
separate confinement structure is disclosed in, for example,
Japanese Patent Application Laid-Open (JP-A) No. 6-21511. This
document shows a light-emitting device of the separate confinement
structure in which an InGaN active layer is sandwiched between two
light-guiding layers, i.e., n-type GaN and p-type GaN guiding
layers. A carrier confinement layer of an n-type AlGaN is formed on
the n-type light-guiding layer and another carrier confinement
layer of a p-type AlGaN is formed on the p-type light-guiding
layer.
[0007] Meanwhile, a semiconductor device of a conventional
double-heterostructure has an active layer, a first clad layer
which is formed in contact with the active layer and which has a
larger band gap energy than that of the active layer, and a second
clad layer which is formed in contact with the first clad layer and
which has a larger band gap energy than that of the first clad
layer. This structure is intended for an efficient injection of
electrons and holes into the active layer in accordance with energy
levels.
[0008] Likewise, a conventional nitride semiconductor LD device has
an active layer and clad layers disposed thereon which include, for
example, a light-guiding layer adjoined by a carrier confinement
layer (light confinement layer), each having a progressively
increased band gap energy (see, for example, the aforementioned
laid-open application).
[0009] However, it has been found that a conventional nitride
semiconductor device having an indium-containing active layer, and
particularly an LD device, of the structure mentioned above, has a
low-level light-emitting efficiency. In particular, it has been
found that the rise in the device temperature by increasing the
current supplied to the device is associated with a serious
decrease in the light-emitting efficiency.
SUMMARY OF THE INVENTION
[0010] Accordingly, an object of the present invention is to
provide a nitride semiconductor device having an active layer
comprising an indium-containing nitride semiconductor, wherein the
nitride semiconductor device has a high-level light-emitting
efficiency.
[0011] Another object of the present invention is to provide a
nitride semiconductor device which exhibits only slight reduction
in the light-emitting efficiency even when the device temperature
is raised.
[0012] In one aspect, the present invention provides a nitride
semiconductor device comprising:
[0013] an active layer of a quantum well structure which has first
and second surfaces and which comprises an indium-containing
nitride semiconductor;
[0014] a first nitride semiconductor layer which is provided in
contact with the first surface of the active layer and has a band
gap energy larger than that of the active layer;
[0015] a second nitride semiconductor layer which is provided on
the first surface side of the active layer at a location more
distant from the active layer than the first nitride semiconductor
layer and which has a band gap energy smaller than that of the
first nitride semiconductor layer, and
[0016] a third nitride semiconductor layer which is provided on the
first surface side of the active layer at a location more distant
from the active layer than the second nitride semiconductor layer
and which has a band gap energy larger than that of the second
nitride semiconductor layer.
[0017] In a second aspect, the present invention provides a nitride
semiconductor device comprising:
[0018] a first clad layer comprising an n-type nitride
semiconductor;
[0019] an active layer of a quantum well structure provided on the
first clad layer, said active layer comprising a nitride
semiconductor containing indium and gallium and having at least one
well layer having a thickness not greater than 70 angstroms,
wherein said well layer is placed on an underlying layer in a state
lattice-mismatched with the underlying layer and includes a
plurality of indium-rich regions and indium-poor regions; and
[0020] a second clad layer which is provided on the active layer
and comprises a nitride semiconductor doped with an acceptor
impurity.
[0021] In a third aspect, the present invention provides a nitride
semiconductor device comprising a first n-type layer which
comprises an n-type, aluminum-containing nitride semiconductor or
n-type gallium nitride, and a second n-type layer which comprises
an n-type, aluminum-containing nitride semiconductor, wherein the
device has a third n-type layer which comprises an n-type,
indium-containing nitride semiconductor and which is provided
between the first n-type layer and the second n-type layer.
[0022] In the nitride semiconductor device of the present
invention, the active layer is sandwiched between a layer structure
which eventually contacts with a positive electrode, and a layer
structure which eventually contacts with a negative electrode.
Hereinbelow, the side where a layer structure is provided which
eventually contacts with a positive electrode is sometimes referred
to as p-side, and the side where a layer structure is provided
which eventually contacts with a negative electrode is sometimes
referred to as n-side.
[0023] Further, in the present invention, a nitride semiconductor
in a broad sense means a nitride of a Group III element or elements
of the Periodic Table, and more specifically a nitride
semiconductor represented by In.sub.xAl.sub.yGa.sub.1-x-yN, where
0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, and
0.ltoreq.x+y.ltoreq.1.
[0024] 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
[0025] 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.
[0026] FIG. 1 illustrates energy bands corresponding to the layer
structure of a conventional LD device;
[0027] FIG. 2 is a sectional view schematically illustrating an LD
device according to a first embodiment of the present
invention;
[0028] FIG. 3 is a sectional view schematically illustrating an LD
device according to a second embodiment of the present
invention;
[0029] FIG. 4 is a sectional view schematically illustrating an LD
device according to a third embodiment of the present
invention;
[0030] FIG. 5 illustrates energy bands corresponding to the device
structure of FIG. 4; and
[0031] FIG. 6 is a sectional view schematically illustrating an LD
device according to a fourth embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] The present inventors have examined the phenomenon of the
reduction in the light-emitting efficiency which is associated with
the rise in temperature of a nitride semiconductor device having an
indium-containing active layer. As a result, they have found that
such reduction in the light-emitting efficiency is primarily caused
by the fact that an indium-containing nitride semiconductor,
particularly InGaN, is more difficult to grow in comparison with an
aluminum-containing nitride semiconductor or gallium nitride (GaN).
That is, the decomposition temperatures of InN and GaN, which
constitute InGaN, largely differ from each other, and thus InGaN
tends to be subjected to phase-separation into InN and GaN.
Therefore, an increase in indium content makes it difficult to
obtain an active layer having a uniform composition. For these
reasons, the content of indium in an InGaN semiconductor that forms
an active layer tends to be suppressed to a low level in a
conventional semiconductor device.
[0033] In the case where a light-guiding layer of GaN is formed in
contact with the InGaN active layer having such a low indium
content, the band offset between the active layer and the guiding
layer becomes extremely small. This will be explained with
reference to FIG. 1 which illustrates energy bands corresponding to
a conventional nitride semiconductor light-emitting device. As
illustrated in FIG. 1, in the case of a conventional nitride
semiconductor device, the band gap energy of the light-guiding
layers (GaN) that directly sandwich InGaN active layer is not so
large relative to the band gap energy of the active layer (InGaN)
(because of the low In content of InGaN, the InGaN composition
approaches GaN). Because of this, when the applied current is
increased and the device temperature rises, before the electrons
and the holes, which are injected into the active layer from the
n-type layer and the p-type layer, respectively, recombine to emit
light (h.nu.), the thermal energy causes the electrons and the
holes to overflow the active layer to reach, respectively, the
guiding layers (GaN) located on a side opposite to the side of the
injection, that is, the electrons reach the p-type light-guiding
layer and the holes reach the n-type light-guiding layer. As a
result, the light-emitting efficiency is low and particularly the
efficiency decreases as the temperature rises.
[0034] Therefore, in the nitride semiconductor device of the
present invention, two first layers (the first p-side layer and the
first n-side layer), which are provided to adjoin and sandwich an
active layer comprising an indium-containing nitride semiconductor,
are made of a nitride semiconductor having a larger band gap energy
than that of the active layer. Preferably, the two first layers
have a band gap energy which is larger than that of the active
layer by 0.01-4.05 eV. Because of the presence of the first layers
having such a large band gap energy, the electrons or holes, which
are injected into the active layer, do not overflow the active
layer. On each of the first layers, there is provided a second
layer (a second p-side layer or a second n-side layer) which is
preferably provided to adjoin the first layer. The second layers
have each a band gap energy smaller than that of the first layer,
but preferably larger than that of the active layer. Preferably,
the second layers have a band gap energy which is smaller than the
first layer by 0.01-4.05 eV. On each of the second layers, there is
provided a third layer (a third p-side layer or a third n-side
layer) which is preferably formed to adjoin the second layer. The
third layers have each a band gap energy larger than that of the
second layer. Preferably, the third layers have a band gap energy
which is larger than that of the second layer by 0.01-4.05 eV. The
electrons or holes which are injected from the third layer side
will be effectively injected into the second layers having a
smaller band gap energy but are unlikely to be injected into the
active layer because of the larger band gap energy of the first
layers. In the present invention, it is therefore preferable that
the first layer has a thickness sufficiently thin so that the
electrons or holes, i.e., carriers, can pass therethrough due to a
tunneling effect (tunneling). Thus, the electrons or holes will be
effectively injected from the third layers into the active layer.
As a result, in the device of the present invention, the electrons
or holes will be effectively injected from the third layers into
the active layer and will not overflow the active layer because
they are blocked by the first layers present on a side opposite to
the side of the injection, even if the device temperature
rises.
[0035] As will be apparent from the above description, the
three-layer structure, comprising the first, second and third
layers, can prevent the overflow of the carriers, i.e., the
electrons or holes, if the three-layer structure is provided on one
of the p- and n-sides of the active layer. Most preferably, the
three-layer structure is provided on both sides (p-side and n-side)
of the active layer.
[0036] The present invention will now be explained with reference
to FIGS. 2 to 6. Throughout these Figures, the same elements or
members are indicated with the same reference numerals.
[0037] FIG. 2 is a sectional view schematically illustrating an LD
device according to a first embodiment of the present invention. In
this LD device, the three-layer structure of the present invention
is provided on the p-side of an active layer.
[0038] The LD device of FIG. 2 has a nitride semiconductor laminate
structure on a substrate 11 through a buffer layer 12. The laminate
structure includes an n-type contact layer 13 on the buffer layer
12, an n-type carrier confinement layer (light confinement layer)
14, an n-type light-guiding layer 15, an active layer 16, a first
p-side nitride semiconductor layer 101 having a band gap energy
larger than that of the active layer 16, a second p-side nitride
semiconductor layer 102 having a band gap energy smaller than that
of the first p-side nitride semiconductor layer, a third p-side
nitride semiconductor layer 103 having a band gap energy larger
than that of the second p-side nitride semiconductor layer 102, and
a p-type contact layer 17. A current-restricting layer 18, which
has a contact hole 18a therein, is provided on the p-type contact
layer 17. A negative electrode 19 is provided on the exposed
surface of the n-type contact layer 13, while a positive electrode
20 is provided on the current-restricting layer 18. The positive
electrode 20 contacts with the p-type contact layer 17 through the
contact hole 18a of the current-restricting layer 18.
[0039] The substrate 11 may be made of any ordinary material
suitable for growing nitride semiconductors thereon, including
spinel (MgAl.sub.2O.sub.4), sapphire (Al.sub.2O.sub.3, including A,
R and C surfaces), SiC (including 6H, 4H and 3C), ZnS, ZnO, GaAs
and GaN.
[0040] The buffer layer 12 can be formed of AlN, GaN, AlGaN or the
like. It can be formed at a temperature not higher than 900.degree.
C. to have a thickness in the range of tens of angstroms to
hundreds of angstroms. The buffer layer is formed in order to
alleviate the lattice mismatch between the substrate 11 and an
nitride semiconductor layer to be formed thereon. Therefore, the
buffer layer 12 may be omitted in the case where a substrate is
used which has a lattice matched with that of the nitride
semiconductor or in the case where a substrate is used which has a
lattice constant approximate to that of the nitride semiconductor
or otherwise depending on the method of growing the nitride
semiconductor.
[0041] The n-type contact layer 13 is formed of a nitride
semiconductor, and preferably of GaN or In.sub.aGa.sub.1-aN
(0<a<1). (In this specification, a nitride semiconductor
represented by In.sub.aGa.sub.1-aN (0<a<1) or a similar
expression is sometimes referred to simply as InGaN.) The use of
Si-doped GaN as the n-type contact layer 13 results in an n-type
layer which has a higher carrier concentration and which
establishes a preferable ohmic contact with the negative electrode
19, thus enabling to decrease a threshold current for the laser
device. Although the thickness of the n-type contact layer 13 is
not particularly limited, the thickness usually ranges from 0.1 to
5 .mu.m.
[0042] The negative electrode formed on the surface of the n-type
contact layer 13 that is exposed by etching is preferably formed of
a metallic material such as Al, Ti, W, Cu, Zn, Sn or In and an
alloy thereof. These metallic material may establish a preferable
ohmic contact with the n-type contact layer 13.
[0043] The n-type carrier confinement layer 14 and the n-type
light-guiding layer 15 formed on the layer 14 are each formed of an
n-type nitride semiconductor. In the embodiment of FIG. 2, the
n-type carrier confinement layer 14 is preferably formed of an
aluminum-containing nitride semiconductor and more preferably of
Al.sub.bGa.sub.1-bN (0<b.ltoreq.1), while the n-type
light-guiding layer 15 is preferably formed of an indium-containing
n-type nitride semiconductor or an n-type GaN, i.e.,
In.sub.cGa.sub.1-cN (0c.ltoreq.1). The preferred thickness of the
n-type carrier confinement layer 14 usually ranges from 0.1 to 1
.mu.m, while the preferred thickness of the n-type guiding layer 15
normally ranges from 100 angstroms to 1 .mu.m.
[0044] The active layer 16 provided on the n-type light-guiding
layer 15 has a quantum well structure (i.e., a single-quantum well
(SQW) structure or a multiquantum well (MQW) structure). The
quantum well structure has a well layer or layers formed of an
indium-containing nitride semiconductor, i.e.,
In.sub.dAl.sub.eGa.sub.1-d-eN (0<d.ltoreq.1,
0.ltoreq.e.ltoreq.1, 0<d+e.ltoreq.1) which has a band gap energy
smaller than that of the n-type light-guiding layer 15 and than
that of the first p-side nitride semiconductor layer 101.
Preferably, the well layer is formed of a ternary mixed crystal
In.sub.fG.sub.1-fN (0<f<1). A ternary mixed crystal InGaN
provides a layer having a better crystallinity and therefore
enhanced light-emitting output power, relative to a quadripartite
mixed crystal.
[0045] Particularly, it is preferred that the active layer 16 is of
an MQW structure made by alternately laminating a well layer formed
of InGaN and a barrier layer formed of a nitride semiconductor
having a larger band gap energy than that of the well layer (the
MQW structure has 3 layers at minimum). In the present invention,
an MQW structure may be the one which has a well layer, as a
lowermost layer of the structure, in contact with an n-type layer
such as the n-type light-guiding layer 15, and has a well layer, as
an uppermost layer of the structure, in contact with a p-type layer
such as the first p-side layer 101, or may be the one which has a
barrier layer, as a lowermost layer of the structure, in contact
with an n-type layer such as the n-type light-guiding layer 15, and
has a barrier layer, as an uppermost layer of the structure, in
contact with a p-type layer such as the first p-side layer 101. The
nitride semiconductor that forms the barrier layer includes GaN,
AlGaN or the like. However, it is particularly preferred that the
barrier layer be formed of a ternary mixed crystal
In.sub.f,Ga.sub.1-f, N (0<f'<1 providing f'<f) as in the
case of the well layer. If the active layer 16 takes an MQW
structure made by laminating InGaN layers having different band gap
energy, a high-output power LD device can be realized which emits
light of about 365 nm to 660 nm based on the emission between
quantum levels by changing an indium mole fraction of the active
layer, and/or by changing an aluminum mole fraction of the first or
third n-side or p-side nitride semiconductor layer. Besides, the
lamination, to the well layer, of an InGaN barrier layer which has
a softer crystal than that of AlGaN and therefore can make a clad
layer, e.g., AlGaN layer, that will be superimposed thereon,
thicker and crack-free, makes it possible to realize an excellent
laser oscillation.
[0046] In the case of an MQW structure, it is particularly
desirable that the thickness of the well layer is not greater than
70 angstroms and the thickness of the barrier layer is not greater
than 150 angstroms. Meanwhile, it is particularly desirable that
the thickness of an active layer of a SQW structure formed by a
single quantum-well layer is not greater than 70 angstroms. It is
preferred that each of the well and barrier layers has a thickness
of 5 angstroms or more.
[0047] The active layer 16 may be of a type that is not doped with
an impurity or dopant (non-doped), or may be of a type which has
the well layer and/or barrier layer doped with an impurity or
dopant, i.e., an acceptor impurity and/or a donor impurity. Of the
impurity-doped active layer, a silicon-doped one is particularly
preferred. When Si is doped in the active layer, a threshold
current tends to be lowered in an LD device. The doping of Si can
be effected by adding an organosilicon gas such as
tetraethylsilane, a silicon hydride gas such as silane and/or a
silicon halide gas such as silicon tetrachloride to a raw material
gas which grows a nitride semiconductor that forms the active
layer.
[0048] The first p-side nitride semiconductor layer 101, which is
provided to adjoin the active layer 16, is formed of a nitride
semiconductor having a band gap energy larger than that of the
active layer 16 (or more strictly, its well layer). Most
preferably, the first nitride semiconductor layer is formed of an
aluminum-containing nitride semiconductor, i.e.,
In.sub.gAl.sub.hGa.sub.1-g-hN (0.ltoreq.g.ltoreq.1,
0<h.ltoreq.1, 0<g+h.ltoreq.1) and is most preferably formed
of a ternary mixed crystal Al.sub.jGa.sub.1-jN (0<j<1). (In
this specification, a nitride semiconductor represented by
Al.sub.jGa.sub.1-jN (0<j<1) or a similar expression is
sometimes referred to simply as AlGaN.)
[0049] The first p-side nitride semiconductor layer 101 is
preferably of i-type or p-type. By use of AlGaN in particular, it
is easier to obtain a p-type having a high carrier concentration.
In addition, by forming such an AlGaN layer to adjoin the active
layer 16 comprising a well layer containing InGaN, it is possible
to obtain a device having a high-level emission output power.
[0050] In the present invention, a p-type nitride semiconductor
including the one forming the active layer 16 can be obtained by
doping it with an acceptor impurity, such as Mg, Zn, C, Be, Ca or
Ba, or a mixture thereof during a crystal growing process.
Preferably the concentration of the acceptor impurity is
1.times.10.sup.17 to 1.times.10.sup.22/cm.sup.3. When the acceptor
impurity is magnesium, it is preferably doped at a concentration of
1.times.10.sup.18 to 1.times.10.sup.20/cm.sup.3, and more
preferably 1.times.10.sup.19 to 1.times.10.sup.20/cm.sup.3. In any
case, in order to obtain a p-type layer having a high-level carrier
concentration, it is desirable to conduct an annealing treatment
(heat treatment) at a temperature not lower than 400.degree. C. in
an inert gas environment after doping with an acceptor impurity.
Normally, the annealing treatment makes it possible to obtain a
carrier concentration of 1.times.10.sup.17 to
1.times.10.sup.19/cm.sup.3 in the case of an Mg-doped p-type AlGaN.
Meanwhile, the preparation of an i-type nitride semiconductor can
be conducted, for example, by growing Al.sub.jGa.sub.1-jN, where j
is not less than 0.5, without doping with an acceptor impurity.
Alternatively, an i-type nitride semiconductor can be prepared by
doping a p-type nitride semiconductor with a donor impurity in an
amount that compensates the hole carrier concentration or by doping
an n-type nitride semiconductor with an acceptor impurity in an
amount that compensates the electron carrier concentration.
[0051] It is particularly preferred that the first p-side nitride
semiconductor layer 101 be sufficiently thin so that the carriers
(hole carriers) can tunnel through it. More specifically, it is
preferred that the thickness of the first p-side nitride
semiconductor layer 101 is not greater than 0.1 .mu.m, preferably
not greater than 0.05 .mu.m (not greater than 500 angstroms) and
most preferably not greater than 0.03 .mu.m (not greater than 300
angstroms). If the thickness of the first nitride semiconductor
layer 101 is in the abovementioned range, the formation of crack in
the first p-side nitride semiconductor layer 101 is prevented and a
nitride semiconductor layer can be grown which has an excellent
crystallinity. Besides, as the proportion of Al increases and as
the thickness decreases in AlGaN, the laser oscillation thereof
becomes easier. For example, when using Al.sub.jGa.sub.1-jN, where
j is not less than 0.2, the thickness of the first p-side nitride
semiconductor layer 101 is preferably not greater than 500
angstroms. Although no lower limit is set to the thickness of the
first p-side nitride semiconductor layer 101, preferably the
thickness is not less than 10 angstroms.
[0052] The second p-side nitride semiconductor layer 102 has a band
gap energy smaller than that of the first p-side nitride
semiconductor layer 101, but preferably larger than that of the
active layer 16, and is provided at a location more distant from
the active layer relative to the first p-side nitride semiconductor
layer 101. Most preferably, the second p-side nitride semiconductor
layer 102 is formed to adjoin the first p-side nitride
semiconductor layer 101, as shown in FIG. 2. The second p-side
nitride semiconductor layer 102 is formed preferably of
In.sub.kGa.sub.1-kN (0.ltoreq.k.ltoreq.1) and most preferably of
GaN or InGaN. If the second p-side nitride semiconductor layer 102
is formed of GaN or InGaN, the resulting second p-side
semiconductor layer 102 is almost crack-free and has an excellent
crystallinity even when the layer is relatively thick. The
thickness of the second p-side nitride semiconductor layer 102 is
preferably in the range of 0.01 to 5 .mu.m and most preferably in
the range of 0.02 to 1 .mu.m, which range enables the second p-side
nitride semiconductor layer 102 to function, for example, as a
desirable light-guiding layer. Besides, the second p-side nitride
semiconductor layer 102 contains an acceptor impurity and is
preferably of a p-type.
[0053] In addition, the second p-side nitride semiconductor layer
102, particularly the one formed of InGaN or GaN, functions also as
a buffer layer which is useful for growing thereon the third p-side
nitride semiconductor layer 103, as described hereinbelow. In
comparison with AlGaN, InGaN or GaN is a softer crystal.
Accordingly, by the presence of the second p-side nitride
semiconductor layer 102, formed of InGaN or GaN, between the first
p-side nitride semiconductor layer 101 having a band gap energy
greater than that of the active layer 16 and the third p-side
nitride semiconductor layer 103, it is possible to make the third
p-side nitride semiconductor layer 103 crack-free and therefore
thicker than the first p-side nitride semiconductor layer 101.
[0054] The third p-side nitride semiconductor layer 103 has a band
gap energy larger than that of the second p-side nitride
semiconductor layer 102 and is formed at a location more distant
from the active layer relative to the second p-side nitride
semiconductor layer 102. Most preferably, the third p-side nitride
semiconductor layer 103 is formed to adjoin the second p-side
nitride semiconductor layer 102, as shown in FIG. 2. The third
p-side nitride semiconductor layer 103 is formed preferably of an
aluminum-containing nitride semiconductor, i.e.,
In.sub.mAl.sub.nGa.sub.1-m-nN (0.ltoreq.m.ltoreq.1, 0<n.ltoreq.1
, 0<m+n.ltoreq.1) and most preferably of an AlGaN ternary mixed
crystal.
[0055] The third p-side nitride semiconductor layer 103 is required
to have a band gap energy larger than that of the second p-side
nitride semiconductor layer 102. This is because the third p-side
nitride semiconductor layer 103 functions as a carrier confinement
and light confinement layer. The thickness of the third p-side
nitride semiconductor layer 103 is preferably in the range of 0.01
to 2 .mu.m and most preferably in the range of 0.05 to 1 .mu.m,
which range enables the third p-side nitride semiconductor layer
103 to function as a carrier confinement layer having an excellent
crystallinity. Besides, the third p-side nitride semiconductor
layer 103 contains an acceptor impurity and is preferably of a
p-type.
[0056] The p-type contact layer 17, provided on the third p-side
nitride semiconductor layer 103, is formed of a p-type nitride
semiconductor. Particularly, the use of InGaN or GaN, particularly
Mg-doped p-type GaN, as the p-type contact layer 17, results in a
p-type layer which has the highest carrier concentration and
establishes a good ohmic contact with the positive electrode 20,
thus enabling to decrease a threshold current.
[0057] The positive electrode 20 is preferably formed of a metallic
material including a metal having a relatively high work function
such as Ni, Pd, Ir, Rh, Pt, Ag or Au and an alloy thereof, in order
to obtain an ohmic contact.
[0058] The current-restricting layer 18 is formed of an
electrically insulating material and preferably of silicon dioxide.
This current-restricting layer 18 may be omitted.
[0059] Meanwhile, in FIG. 2, the n-type carrier confinement layer
14 is provided on the n-type contact layer 13 through a crack
preventing layer 30.
[0060] That is, an aluminum-containing nitride semiconductor tends
to form cracks in the crystal when the crystal is grown to have a
large thickness. Particularly, it is difficult to grow a thick
layer of an n-type aluminum-containing nitride semiconductor
directly on an n-type GaN or AlGaN layer without the formation of
crack. For example, it is difficult to form an n-type layer, which
is comprised of an aluminum-containing nitride semiconductor,
particularly AlGaN, and which is exemplified by the n-type carrier
confinement layer 14 requiring such a large thickness as 0.1 .mu.m
or more, on the n-type contact layer 13 formed, for example, of an
n-type GaN or the like. Therefore, firstly an n-type layer, which
is comprised of an indium-containing nitride semiconductor and
preferably of In.sub.pGa.sub.1-pN (0<p.ltoreq.1), is formed as a
crack preventing layer 30 on the n-type contact layer 13, and then
an n-type carrier confinement layer 14 is formed which is comprised
of an n-type aluminum-containing nitride semiconductor. Owing to
the presence of the crack preventing layer 30, the n-type carrier
confinement layer 14 can be grown to a desired thickness (for
example 0.1 .mu.m or more). The thickness of the crack preventing
layer 30 is preferably in the range of 100 angstroms to 0.5
.mu.m.
[0061] Accordingly, the present invention provides nitride
semiconductor device including a first n-type layer which is formed
of an n-type aluminum-containing nitride semiconductor or n-type
gallium nitride, and a second n-type layer which is formed of an
aluminum-containing n-type nitride semiconductor, wherein the
device has a third n-type layer which is formed of an
indium-containing n-type nitride semiconductor and which is
provided between the first n-type layer and the second n-type
layer. The third n-type layer may be present anywhere between the
first-n-type layer and the second n-type layer and does not need to
adjoin any of the first n-type layer and the second n-type
layer.
[0062] FIG. 3 is a sectional view schematically illustrating a
nitride semiconductor LD device according to a second embodiment of
the present invention, where the same reference numerals indicates
the same elements or members as in FIG. 2. With reference to FIG.
3, the LD device has a nitride semiconductor structure formed on
the substrate 11 through the buffer layer 12. The structure
includes the n-type contact layer 13, the crack preventing layer
30, a third n-side nitride semiconductor 203, a second n-side
nitride semiconductor layer 202, a first n-side nitride
semiconductor layer 201, the active layer 16, a p-type
light-guiding layer 31, a p-type carrier confinement layer (light
confinement layer) 32, the p-type contact layer 17 and the
current-restricting layer 18. The negative electrode 19 is
electrically connected to the n-type contact layer 13 and the
positive electrode 20 is electrically connected to the p-type
contact layer 17, as in FIG. 2.
[0063] In the LD device of FIG. 3, the first n-side nitride
semiconductor layer 201, the second n-side nitride semiconductor
layer 202 and the third n-side nitride semiconductor layer 203,
with the exception of the type of electric conduction, are
basically identical with the corresponding first p-side nitride
semiconductor layer 101, second p-side nitride semiconductor layer
102 and third p-side nitride semiconductor layer 103, respectively,
as explained with reference to FIG. 2 in terms of band gap
energies, nitride semiconductor materials used and thickness,
except for the conductivity type. In addition, what is mentioned of
the preferred material, preferred thickness and the like about the
first p-side nitride semiconductor layer 101, the second p-side
nitride semiconductor layer 102 and the third p-side nitride
semiconductor layer 103 is applicable to the first n-side nitride
semiconductor layer 201, the second n-side nitride semiconductor
layer 202 and the third n-side nitride semiconductor layer 203,
respectively.
[0064] To reiterate briefly, the first n-side nitride semiconductor
layer 201, provided to adjoin the active layer 16, is formed of a
nitride semiconductor layer having a band gap energy larger than
that of the active layer 16 (more strictly, its well layer). Most
preferably, the first n-side nitride semiconductor layer 201 is
formed of an aluminum-containing nitride semiconductor layer and
especially preferably of an AlGaN ternary mixed crystal.
[0065] The first n-side nitride semiconductor layer 201 is also
sufficiently thin so that the carriers (electron carriers) can
tunnel through it. More specifically, the thickness of the first
nitride semiconductor layer 201 is not greater than 0.1 .mu.m,
preferably not greater than 0.05 .mu.m (not greater than 500
angstroms) and most preferably not greater than 0.03 .mu.m (not
greater than 300 angstroms). It is also preferred that the
thickness of the first n-side nitride semiconductor layer 201 be
not less than 10 angstroms.
[0066] The first n-side nitride semiconductor layer 201 is
preferably of n-type or i-type.
[0067] In the present invention, although an n-type nitride
semiconductor may be obtained without doping with an impurity,
i.e., in a non-doped state, a desirable n-type can be obtained by
doping with a donor impurity such as Si, Ge, Sn, S or a combination
thereof during the growth of crystal. In this case, the
concentration of the donor impurity is preferably 1.times.10.sup.16
to 1.times.10.sup.22/cm.sup.3. In particular, Si is more preferably
doped at a concentration of 1.times.10.sup.17 to
1.times.10.sup.21/cm.sup.3. most preferably 1.times.10.sup.18 to
1.times.10.sup.20/cm.sup.3.
[0068] The second n-side nitride semiconductor layer 202 has a band
gap energy smaller than that of the first n-side nitride
semiconductor layer 201, but preferably larger than that of the
active layer 16, and is formed at a location more distant from the
active layer relative to the first n-side nitride semiconductor
layer 201. Most preferably, the second n-side nitride semiconductor
layer 202 is provided to adjoin the first n-side nitride
semiconductor layer 201, as shown in FIG. 3. The second n-side
nitride semiconductor layer 202 is formed preferably of
In.sub.kGa.sub.1-kN (0.ltoreq.k.ltoreq.1) and most preferably of
GaN or InGaN. The thickness of the second n-side nitride
semiconductor layer 202 is preferably in the range of 0.01 to 5
.mu.m and most preferably in the range of 0.02 to 1 .mu.m, which
range enables the second n-side nitride semiconductor layer 202 to
function, for example, as a desirable light-guiding layer. The
second n-side nitride semiconductor layer 202 is of an n-type. As
is explained with reference to FIG. 2, the second p-side nitride
semiconductor layer 102 functions as a buffer layer to cause the
third p-side nitride semiconductor layer 103 to grow thereon as a
relatively thick layer. Likewise, the second n-side nitride
semiconductor layer 202 functions as a buffer layer to grow the
first n-side nitride semiconductor layer 201, but the function as a
buffer layer is not so important because the first n-side nitride
semiconductor layer 201 is thin.
[0069] In order to function as a carrier confinement and light
confinement layer, the third n-side nitride semiconductor 203 also
has a band gap energy larger than that of the second n-side nitride
semiconductor layer 202 and is formed at a location more distant
from the active layer 16 relative to the second n-side nitride
semiconductor layer 202. Most preferably, the third n-side nitride
semiconductor layer 203 is formed to adjoin the second n-side
nitride semiconductor layer 202, as shown in FIG. 3. The third
n-side nitride semiconductor layer 203 is also formed preferably of
an aluminum-containing nitride semiconductor and most preferably of
an AlGaN ternary mixed crystal. The thickness of the third n-side
nitride semiconductor layer 203 is also preferably in the range of
0.01 to 2 .mu.m and most preferably in the range of 0.05 to 1
.mu.m. which range enables the third n-side nitride semiconductor
layer 203 to have an excellent crystallinity and to function as a
carrier confinement and light confinement layer. The third n-side
nitride semiconductor layer 203 is of an n-type. The third n-side
nitride semiconductor layer 203, which is preferably comprised of
an aluminum-containing nitride semiconductor, is formed on an
n-type contact layer 13, which is preferably comprised of GaN,
through the crack preventing layer 30.
[0070] The p-type light-guiding layer 31 and the p-type carrier
confinement layer (light confinement layer) 32 are each formed of a
p-type nitride semiconductor. The p-type carrier confinement layer
(light confinement layer) 32 has a band gap energy larger than that
of the p-type light-guiding layer 31 which has a band gap energy
larger than that of the active layer 16.
[0071] FIG. 4 illustrates a nitride semiconductor LD device which
has a three-layer laminate structure of the present invention on
each of p-side and n-side of an active layer and which is presently
the most preferred embodiment. Referring to FIG. 4, the LD device
has a nitride semiconductor structure on the substrate 11 through a
buffer layer 12. The structure includes the n-type contact layer
13, the crack preventing layer 30, the third n-side nitride
semiconductor layer 203, the second n-side nitride semiconductor
layer 202, the first n-side nitride semiconductor layer 201, the
active layer 16, the first p-side nitride semiconductor layer 101,
the second p-side nitride semiconductor layer 102, the third p-side
nitride semiconductor layer 103 and the p-type contact layer 17.
The current-restricting layer 18 with the contact hole 18a is
provided on the p-type contact layer 17. The negative electrode 19
is provided on the exposed surface of the n-type contact layer 13,
while the positive electrode 20 is provided on the
current-restricting layer 18. The positive electrode 20 is
connected to the p-type contact layer 17 through the contact hole
18a of the current-restricting layer 18. The elements constituting
the device of FIG. 4 are identical with those explained with
reference to FIGS. 2 and 3.
[0072] The nitride semiconductor layers constituting the device of
the present invention can be preferably grown by means of a
metalorganic vapor phase epitaxial growth (MOVPE) method. However,
the nitride semiconductor layers can also be grown by conventional
other methods including a hydride vapor phase epitaxial growth
(HDVPE) method and a molecular beam vapor phase epitaxial growth
(MBE) method.
[0073] FIG. 5 illustrates energy bands of an LD device shown in
FIG. 4 in which the active layer has an MQW structure. As
illustrated in FIG. 5, in the case of the LD device of a
double-heterostructure of the present invention, the first p-side
nitride semiconductor layer 101 and the first n-side nitride
semiconductor layer 201 are provided to adjoin the active layer 16
which comprises an indium-containing nitride semiconductor. That
is, the two first nitride semiconductor layers 101 and 201, each
having a band gap energy larger than that of the active layer 16
(more strictly, its well layer) and also larger than that of the
second n-side nitride semiconductor layer 102 and that of the
second p-side nitride semiconductor layer 202, are provided to
adjoin the active layer 16. Since these two first nitride
semiconductor layers 101 and 201 are thin and therefore they have
no function as a barrier against the carriers, the electron
carriers, which are injected into the second n-side nitride
semiconductor layer 202 from the third n-side nitride semiconductor
layer 203, and the hole carriers, which are injected into the
second p-side nitride semiconductor layer 102 from the third p-side
nitride semiconductor layer 103, can tunnel through the first
n-side nitride semiconductor layer 201 and the first p-side nitride
semiconductor layer 101, respectively, and recombine efficiently at
the active layer 16 to emit light (h.nu.).
[0074] Since the band gap energies of first nitride semiconductor
layers 101 and 201 are sufficiently large, the injected carriers
are blocked by the first nitride semiconductor layers 101 and 201
and do not overflow the active layer 16. As a result the electron
and hole carriers are effectively accumulated in the active layer
16, thus enabling an efficient emission of light, even if the
device temperature rises or the injection current density
increases. Accordingly, the nitride semiconductor device of the
present invention realizes an LD device characterized by little
reduction of the light emission efficiency even in the case where
the device temperature rises and also by a low threshold current
(density).
[0075] The present inventors have made a close study on the active
layer in the device of the present invention and particularly on
the active layer having a well layer formed of a nitride
semiconductor containing indium and gallium. As a result, they have
found that, when growing, e.g., InGaN,, the indium content does not
becomes uniform throughout the grown InGaN layer depending on
conditions, and, thus, indium-rich regions or phases and
indium-poor regions or phases are formed. Electron and hole
carriers are localized in the indium-rich regions thus formed to
emit light based on exciton or bi-exciton. That is, the indium-rich
regions constitute quantum dots or quantum boxes. In order for an
InGaN well layer to form such quantum dots or quantum boxes, it has
been found necessary that, as in the devices already explained with
reference with FIGS. 2-4, the well layer be formed on an underlying
nitride semiconductor layer such as n-type nitride semiconductor
layer (aluminum-containing nitride semiconductor layer 15 or 201)
in a state lattice-mismatched with the underlying layer and have a
thickness not greater than 70 angstroms. Such a well layer can be
conveniently provided by growing a well-forming nitride
semiconductor layer on the n-type-nitride semiconductor layer and
allowing the grown layer to stand for a short period of time,
preferably., 2 to 20 seconds before growing an additional nitride
semiconductor layer thereon. The additional layer that is to be
formed on the active layer having the well layer needs to contain
an acceptor impurity. An LD device, which has the above-mentioned
structure, has a threshold current lower than that of a usual
quantum well structure laser and can have a higher characteristic
temperature.
[0076] Accordingly, the present invention provides a nitride
semiconductor device comprising a first clad layer comprising an
n-type nitride semiconductor; an active layer of a quantum well
structure (SQW or MQW structure) provided on the first clad layer,
said active layer comprising a nitride semiconductor containing
indium and gallium and having at least one well layer having a
thickness not greater than 70 angstroms, wherein said well layer is
placed on an underlying layer in a state lattice-mismatched with
the underlying layer and includes a plurality of indium-rich
regions and indium-poor regions; and a second clad layer which is
provided on the active layer and comprises a nitride semiconductor
doped with an acceptor impurity. The underlying layer refers to the
first clad layer itself such as an n-type semiconductor layer,
e.g., the aluminum-containing nitride semiconductor layer 15 or 201
as in the devices explained with reference to FIGS. 2 to 4, a
barrier layer provided on the first clad layer, or a barrier layer
on which a well layer is formed. FIG. 6 schematically or notionally
illustrates such a device, in which the active layer has a SQW
structure for the sake of brevity. As shown in FIG. 6, a quantum
well layer (active layer) 54 provided to a thickness of 70
angstroms or less on a first clad layer comprising an n-type
nitride semiconductor layer 52 in a state lattice-mismatched with
the layer 52 is formed with InGaN in average, but constitutes
indium-rich regions 54a and gallium-rich regions (indium-poor
regions) 54b by phase separation as explained above. More
specifically, the indium-rich regions 54a and the indium-poor
regions each exist as dots or boxes which may have a size of 20 to
50 angstroms. Each indium-rich region 54a and each indium-poor
region 54b are alternately arranged substantially regularly in the
plane direction of the well layer. On the active layer 54, a second
clad layer 56 formed of a nitride semiconductor doped with an
acceptor impurity.
[0077] Naturally, it is preferable that the active layer having a
well layer, which constitutes quantum dots or boxes, constitute the
active layer 16 as explained with reference to FIGS. 2-4. The band
gap energy of the phase-separated well layer is determined by an
average semiconductor composition of such well layer, or the
composition before the phase separation.
[0078] The threshold current can be further decreased, in the case
where the active layer having a well layer or layers, which
constitute quantum dots or boxes, are doped with an acceptor
impurity and/or donor impurity.
[0079] The non-uniformity of the indium content in the plane of one
well layer means that InGaN regions, which have different band gaps
(i.e., indium-rich regions and indium-poor regions), exist in the
plane direction of a single well layer. Therefore, the electrons,
present in a conductive band, once drop into an indium-rich phase
and recombine with holes, present in a valence electron band, to
emit an energy of h.nu.. In other words, electron carriers and hole
carriers are localized in an indium-rich phase of the well layer to
form localized exciton, thus decreasing the threshold current of
laser and increasing the light emission output of laser.
[0080] Where such a well layer is doped with a donor impurity
and/or acceptor impurity, such as silicon, an additional energy
level derived from impurity is formed between a conductive band and
a valence electron band. Therefore, the electron carriers drop into
a deeper impurity-induced energy level, while the hole carriers
move to a p-type impurity-induced level to cause the recombination
of the electron carriers and hole carriers, thus emitting a smaller
energy of h.nu.. This leads us to believe that electron carriers
and hole carriers are further localized to produce further
localized exciton, which decreases the threshold current of the
laser device. In the present invention, an impurity or dopant doped
in the well layer preferably includes silicon and germanium,
particularly silicon. In particular, when silicon is doped, the
threshold current tends to be further lowered. Meanwhile, an
impurity may be doped in a barrier layer, and in an active layer of
an MQW structure, may be doped in one well layer only or in one
barrier layer only.
[0081] The present invention is explained by way of examples
below.
EXAMPLE 1
[0082] In this example, a nitride semiconductor LD device having a
structure as illustrated in FIG. 4 was fabricated.
[0083] A sufficiently cleaned spinel substrate 11
(MgAl.sub.2O.sub.4) was placed in a reaction vessel of an MOVPE
apparatus, and the gas inside the vessel was fully replaced with
hydrogen. Then, under flowing hydrogen gas, the temperature of the
substrate was raised to 1050.degree. C. to carry out the cleaning
of the substrate.
[0084] Then, the temperature was lowered to 510.degree. C. and a
GaN buffer layer 12 was grown to a thickness of about 200 angstroms
on the substrate 11 by using hydrogen as a carrier gas and ammonia
and trimethylgallium (TMG) as a raw material gas.
[0085] After the growth of the buffer layer, only TMG stream was
stopped, and the temperature was raised to 1030.degree. C. while
flowing the ammonia gas. At 1030.degree. C., TMG gas was added and,
using silane gas (SiH.sub.4) as a dopant gas, a Si-doped n-type GaN
layer, as an n-type contact layer 13, was grown to a thickness of 4
.mu.m.
[0086] Then, the temperature was lowered to 800.degree. C. and a
crack-preventing layer 30 comprised of Si-doped
In.sub.0.1Ga.sub.0.9N was grown to a thickness of 500 angstroms by
using TMG, TMI (trimethylindium) and ammonia as a raw material gas
and using silane gas as an impurity gas.
[0087] Then, the temperature was raised to 1030.degree. C. and,
using trimethylaluminum (TMA), TMG and ammonia as a raw material
gas and using silane gas as a dopant gas, a Si-doped n-type
Al.sub.0.2Ga.sub.0.8N layer, as a third n-type contact layer 203,
was grown to a-thickness of 0.5 .mu.m.
[0088] Then, the temperature was lowered to 800.degree. C. and only
TMA stream was stopped and a second n-type nitride semiconductor
layer 202 comprised of Si-doped n-type GaN was grown to a thickness
of 0.2 .mu.m.
[0089] Then, the temperature was raised to 1050.degree. C. and a
first n-type nitride semiconductor layer 201 comprised of Si-doped
Al.sub.0.1Ga.sub.0.9N was grown to a thickness of 300 angstroms by
using TMA, TMG and ammonia as a raw material gas and using silane
gas as a dopant gas.
[0090] Then, using TMG, TMI and ammonia as a raw material gas, an
active layer 16 was grown in the following way. The temperature was
kept at 800.degree. C. and a well layer comprised of non-doped
In.sub.0.2Ga.sub.0.8N was grown to a thickness of 25 angstroms.
Then, by altering the TMI molar ratio, a barrier layer comprised of
non-doped In.sub.0.01Ga.sub.0.99N was grown to a thickness of 50
angstroms at the same temperature. This series of operation was
repeated twice and such well layer were laminated to form an active
layer having a 7-layered MQW structure.
[0091] Then, the temperature was raised to 1050.degree. C. and a
first p-type nitride semiconductor layer 101 comprised of Mg-doped
p-type Al.sub.0.1Ga.sub.0.9N was grown to a thickness of 300
angstroms by using TMG, TMA , ammonia and cyclopentadienyl
magnesium (Cp.sub.2Mg).
[0092] Then, at 1050.degree. C. a second p-type nitride
semiconductor layer 102 comprised of Mg-doped p-type GaN was grown
to a thickness of 0.2 .mu.m by using TMG, ammonia and
Cp.sub.2Mg.
[0093] Then, at 1050.degree. C. a third p-type nitride
semiconductor layer 103 comprised of Mg-doped p-type
Al.sub.0.2Ga.sub.0.8N was grown to a thickness of 0.5 .mu.m by
using TMG, TMA, ammonia and cyclopentadienyl magnesium
(Cp.sub.2Mg).
[0094] Finally, at 1050.degree. C. a p-type contact layer 17
comprised of Mg-doped p-type GaN was grown to a thickness of 0.5
.mu.m.
[0095] After the reactions, the temperature was lowered to room
temperature, and the wafer was taken out of the reaction vessel.
The wafer was annealed at 700.degree. C. to further decrease the
resistance of the p-type layer. Then, from the topmost p-type
contact layer 17, etching was performed to expose the surface of
the n-type contact layer 13 in a form of a stripe. After the
etching process, a current-restricting layer 18 comprised of
silicon dioxide was formed on the p-type contact layer 17 and a
contact hole 18a was formed therein. Then, a positive electrode 20,
comprised of Ni and Au, was formed in a shape of stripe so that it
contacts with the p-type contact layer 17 through the contact hole
18a of the current-restricting layer 18. On the other hand, a
negative electrode 19, comprised of Ti and Al, was formed in a
shape of stripe.
[0096] Then, the wafer was cut to produce a bar in a direction
vertical to the stripe-shaped electrode, and the cut surface was
polished to produce a parallel mirror, which was laminated
alternatively with SiO.sub.2 and TiO.sub.2 to form a dielectric
multi-layered body. Finally, the bar was cut to produce a chip in a
stripe form of 4 .mu.m.times.600 .mu.m in a direction parallel to
the electrode and the chip was used as a laser chip. The chip, thus
obtained, was set to a heat sink and laser oscillation was
conducted at room temperature. The found laser oscillation was at a
wavelength of 400 nm, with a threshold pulse current density of 2
kA/cm.sup.2 and T.sub.0 (characteristic temperature) of 200K under
pulsed current flow (pulse width of 10 .mu.sec, duty ratio of
10%).
[0097] Next, the device of the present invention was evaluated on
the basis of the temperature dependence of the threshold current
density. The threshold current density of LD, i.e., J.sub.th, is
proportional to exp(T/T.sub.0), where T is operating temperature
(K) and T.sub.0 is characteristic temperature (K). That is, the
greater T.sub.0 is, the lower the threshold current density even at
a high temperature, thus leading to a stable operation.
[0098] In the device of Example 1, in the case where none of the
first nitride semiconductor layers 101 and 201 was formed, no laser
oscillation was observed. In the case where one of the first
nitride semiconductor layers 101 and 201 was not formed, the LD
device of the present invention exhibited J.sub.th=3 kA/cm.sup.2
and T.sub.0=100 K. The LD device of Example 1 having the first
nitride semiconductor layers 101 and 201 of Al.sub.jGa.sub.1-jN,
where j is 0.1, gave J.sub.th=2 kA/cm.sup.2 and T.sub.0=200K, as
stated hereinbefore. However, the LD device of Example 1 having the
first nitride semiconductor layers 101 and 201 of
Al.sub.jGa.sub.1-jN, where j is 0.2, gave J.sub.th=1.5 kA/cm.sup.2
and T.sub.0=300K, and the LD device of Example 1 having the first
nitride semiconductor layers 101 and 201 of Al.sub.jGa.sub.1-jN,
where j is 0.3, gave J.sub.th=1.4 kA/cm.sup.2 and T.sub.0=400K,
thereby indicating the excellent temperature characteristics of the
LD devices of the present invention.
EXAMPLE 2
[0099] An LD device of the present invention was fabricated as in
Example 1, except that the first n-side nitride semiconductor layer
201 was not grown. This LD device has the same structure as that of
the LD device of FIG. 2 so that the n-type carrier confinement
layer (light confinement layer) 14 corresponds to the third n-side
nitride semiconductor 203, while the n-type light-guiding layer 15
corresponds to the second n-side nitride semiconductor 202. This LD
device exhibited the laser oscillation of wavelength of 400 nm with
J.sub.th=3 kA/cm.sup.2 and T.sub.0=100K.
EXAMPLE 3
[0100] An LD device of the present invention was fabricated as in
Example 1, except that the first p-side nitride semiconductor layer
101 was not grown. This LD device has the same structure as that of
the LD device of FIG. 3 so that the p-type carrier confinement
layer (light confinement layer) 32 corresponds to the third p-side
nitride semiconductor 103, while the p-type light-guiding layer 31
corresponds to the second p-side nitride semiconductor 102. This LD
device exhibited the laser oscillation of wavelength of 400 nm with
J.sub.th=3 kA/cm.sup.2 as in the case of the LD of Example 2 and
gave T.sub.0=100K.
EXAMPLE 4
[0101] An LD device of the present invention was fabricated as in
Example 2, except that the active layer 16 had a single
quantum-well structure formed of a well layer comprised of
non-doped In.sub.0.2Ga.sub.0.8N having a thickness of 50 angstroms
and the first p-type nitride semiconductor layer 101 was formed of
Al.sub.0.3Ga.sub.0.7N. This LD device exhibited the laser
oscillation of wavelength of 410 nm with J.sub.th=5 kA/cm.sup.2 and
T.sub.0=50K.
EXAMPLE 5
[0102] An LD device of the present invention was fabricated as in
Example 1, except that the second n-type nitride semiconductor
layer 202 was formed of Si-doped n-type In.sub.0.01Ga.sub.0.99N and
the second p-type nitride semiconductor layer 102 was formed of
Mg-doped p-type In.sub.0.01Ga.sub.0.99N. This LD device exhibited
the same properties as those of the LD device of Example 1.
EXAMPLE 6
[0103] An LD device of the present invention was fabricated as in
Example 1, except that the well layer and barrier layer of the
active layer were each doped with silicon, as a donor impurity, at
a concentration of 1.times.10.sup.19/cm.sup.3. This device
exhibited a threshold current lower by 5% and T.sub.0 higher by
about 10% relative to the LD device of Example 1.
EXAMPLE 7
[0104] An LD device of the present invention was fabricated as in
Example 1, except that the well layer and barrier layer of the
active layer were each doped with magnesium, as an acceptor
impurity, at a concentration of 1.times.10.sup.18/cm.sup.3. This
device exhibited properties nearly the same as those of the LD
device of Example 1.
EXAMPLE 8
[0105] An LD device of the present invention was fabricated as in
Example 1, except that the well layer and barrier layer of the
active layer were each doped with silicon, as a donor impurity, at
a concentration of 1.times.10.sup.19/cm.sup.3 and with magnesium,
as an acceptor impurity, at a concentration of
1.times.10.sup.18/cm.sup.3. This LD device exhibited nearly the
same properties as those of the LD device of Example 6.
EXAMPLE 9
[0106] In this Example, a nitride semiconductor LD device having a
structure as shown in FIG. 2 was fabricated.
[0107] First, a fully washed sapphire substrate (0001 plane) 11 was
set in a reaction chamber of a MOVPE apparatus. Using TMG and
ammonia as a raw material gas, GaN was grown at 500.degree. C. on
the substrate to a thickness of 200 angstroms to form a buffer
layer 12.
[0108] Then, the temperature was raised to 1050.degree. C., and
using TMG and ammonia as a raw material gas, and silane gas as an
impurity gas, a Si-doped GaN was grown at that temperature to a
thickness of 4 .mu.m to form an n-type contact layer 13.
[0109] Thereafter, the temperature was lowered to 750.degree. C.,
and using TMG, TMI and ammonia as a raw material gas, and silane
gas as an impurity gas, an Si-doped In.sub.0.1Ga.sub.0.9N was grown
to a thickness of 500 angstrom to form a crack-preventing layer
30.
[0110] Then, the temperature was raised to 1050.degree. C., and
using triethylgallium (TEG), TMA and ammonia as a raw material gas,
and silane as an impurity gas, an Si-doped n-type
Al.sub.0.3Ga.sub.0.7N was grown to a thickness of 0.5 .mu.m to form
an n-type carrier confinement layer 14.
[0111] Then, using TMG and ammonia as a raw material gas, and
silane as an impurity gas, an Si-doped n-type GaN was grown to a
thickness of 500 angstroms to form an n-type light-guiding layer
15.
[0112] Next, an active layer 16 was grown, using TMG, TMI and
ammonia as a raw material gas. Specifically, the temperature was
maintained at 750.degree. C., and a non-doped In.sub.0.2Ga.sub.0.8N
was grown on the light-guiding layer 15 to a thickness of 25
angstroms to form a well layer. Then, a non-doped
In.sub.0.1Ga.sub.0.9N was grown on the well layer to a thickness of
50 angstroms to form a barrier layer at the same temperature by
changing the flow rate of the TMI stream. These procedures of
forming the well and barrier layers alternately were repeated 13
times in total, and finally such well layer was grown, thus forming
an active layer 16 of an MQW structure having a total thickness of
0.1 .mu.m.
[0113] After the formation of the active layer 16, the temperature
was raised to 1050.degree. C., and using TMG, TMA, ammonia as a raw
material gas, and Cp.sub.2Mg as a dopant gas, an Mg-doped
Al.sub.0.2Ga.sub.0.8N was grown to a thickness of 100 angstroms to
form a first p-type nitride semiconductor layer 101.
[0114] Then, while maintaining the temperature at 1050.degree. C.,
an Mg-doped p-type GaN was grown to a thickness of 500 angstroms to
form a second p-type nitride semiconductor layer 102, using TMG and
ammonia as a raw material gas, and Cp.sub.2Mg as a dopant gas.
[0115] Next, a third p-type nitride semiconductor layer 103 was
formed by growing an Mg-doped Al.sub.0.3Ga.sub.0.7N to a thickness
of 0.5 .mu.m, using TMG, TMA and ammonia as a raw material gas, and
Cp.sub.2Mg as a dopant gas.
[0116] Subsequently, a p-type contact layer 17 was form by growing
an Mg-doped p-type GaN to a thickness of 0.5 .mu.m, using TMG,
ammonia as a raw material gas, and Cp.sub.2Mg as a dopant gas.
[0117] After the reactions, the temperature was lowered to room
temperature, and the wafer was taken out of the reaction vessel.
Then, from the topmost p-type contact layer 17, etching was
performed to expose the surface of the n-type contact layer 13.
Then, a positive electrode 20, comprised of Ni and Au, was formed
in a shape of stripe, while a negative electrode 19, comprised of
Ti and Al, was formed in a shape of stripe. The wafer thus
processed was vertically etched in a direction normal to the
longitudinal direction of the stripe electrodes 19 and 20 to form
vertical etched surfaces, on which reflecting mirrors were formed
to prepare resonance planes, thus providing an LD device. The
device was set to a heat sink and laser oscillation was conducted
at room temperature. The found laser oscillation was at a
wavelength of 410 nm with a half bandwidth of 0.2 nm, and the
threshold current density was 2 kA/cm.sup.2.
EXAMPLE 10
[0118] An LD device was fabricated in the same manner as in Example
9, except that the third p-type nitride semiconductor 103 was
formed to a thickness of 0.1 .mu.m. This device exhibited a laser
oscillation at a wavelength of 410 nm with a half bandwidth of 0.2
nm at room temperature, and the threshold current density was 4.0
kA/cm.sup.2.
EXAMPLE 11
[0119] The same procedures were followed as in Example 9, except
that the crack-preventing layer 30 was formed to a thickens of 200
angstroms, and that a further n-type contact layer comprised of
Si-doped GaN was formed to a thickness of 0.5 .mu.m on that
crack-preventing layer before forming the first n-type clad layer
14. The LD device thus fabricated exhibited a laser oscillation at
410 nm with a half bandwidth of 0.2 nm at room temperature, and the
threshold current density was 4.0 kA/cm.sup.2.
EXAMPLE 12
[0120] The same procedures were followed as in Example 9, except
that the second clad layer 14 was formed by growing Si-doped n-type
In.sub.0.05Ga.sub.0.95N to a thickness of 500 angstroms, using TMG,
TMI and ammonia as a raw material gas, and silane as a dopant gas,
and that the second p-type nitride semiconductor layer 102 was
formed by growing Mg-doped p-type In.sub.0.01Ga.sub.0.99N to a
thickness of 500 angstroms, using TMG, TMI and ammonia as a raw
material gas, and Cp.sub.2Mg as a dopant gas. The LD device thus
fabricated exhibited a laser oscillation at 410 nm with a half
bandwidth of 0.2 nm at room temperature, and the threshold current
density was 4.0 kA/cm.sup.2.
EXAMPLE 13
[0121] The same procedures were followed as in Example 9, except
that spinel ((111)-plane) was used as the substrate 11. The
obtained wafer was processed as in Example 1, thereby obtaining an
LD device. The LD device thus fabricated exhibited a laser
oscillation at 410 nm with a half bandwidth of 0.2 nm at room
temperature, and the threshold current density was 4.0
kA/cm.sup.2.
EXAMPLE 14
[0122] The same procedures were followed as in Example 1, except
that after each well layer of In.sub.0.2Ga.sub.0.8N (average
composition) was formed, the well was allowed to stand for 5
seconds and then each barrier layer was formed, thus obtaining an
LD device. In this device, it was found that each wall layer was
phase-separated into indium-rich regions and indium-poor regions,
and each indium-rich region almost corresponded to a composition of
In.sub.0.4Ga.sub.0.6N and each indium-poor region almost
corresponded to a composition of In.sub.0.02Ga.sub.0.98N. Further,
it was confirmed by TEM sectional photograph that the indium-rich
regions and the indium-poor regions were alternately arranged
regularly in the plane direction of the well layer (refer to FIG.
6). The LD device thus fabricated exhibited a threshold current
density lower than that of the device fabricated in Example 1 by
30%, and a T.sub.0 higher than that of the device fabricated in
Example 1 by 20%.
EXAMPLE 15
[0123] The same procedures were followed as in Example 14, except
that silicon was doped in each well layer, thus fabricating an LD
device. This device exhibited a threshold current density lower
than that of the device fabricated in Example 1 by 40%, and a
T.sub.0 higher than that of the device fabricated in Example 1 by
30%.
[0124] Meanwhile, in each Example described above, an impurity
whose concentration was not particularly indicated was doped in a
preferred concentration range explained above.
[0125] The above-described examples show the most preferred
examples, where the active layer, the first nitride semiconductor
layer, the second nitride semiconductor layer and the third nitride
semiconductor layer are brought into contact. However, according to
the present invention, at least the first nitride semiconductor
layer needs to contact the active layer, and, therefore, other
nitride semiconductor layer can be inserted between the first
nitride semiconductor layer and the second nitride semiconductor or
between the second nitride semiconductor layer and the third
nitride semiconductor layer.
[0126] 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 embodiments 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.
* * * * *