U.S. patent application number 12/266002 was filed with the patent office on 2009-03-19 for nitride semiconductor device and method for fabricating the same.
This patent application is currently assigned to PANASONIC CORPORATION. Invention is credited to Yasuyuki Fukushima, Norio Ikedo, Masaaki Yuri.
Application Number | 20090072221 12/266002 |
Document ID | / |
Family ID | 37463328 |
Filed Date | 2009-03-19 |
United States Patent
Application |
20090072221 |
Kind Code |
A1 |
Ikedo; Norio ; et
al. |
March 19, 2009 |
NITRIDE SEMICONDUCTOR DEVICE AND METHOD FOR FABRICATING THE
SAME
Abstract
A nitride semiconductor device comprises: a well layer of
nitride semiconductor containing In and Ga; barrier layers of
nitride semiconductor sandwiching the well layer, containing Al and
Ga, and having a larger band gap energy than the well layer; and a
thin film layer provided between the well layer and the barrier
layer. The thin film layer is formed during lowering of the
substrate temperature after formation of the barrier layer or
during elevation of the substrate temperature after formation of
the well layer.
Inventors: |
Ikedo; Norio; (Osaka,
JP) ; Fukushima; Yasuyuki; (Osaka, JP) ; Yuri;
Masaaki; (Osaka, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, NW
WASHINGTON
DC
20005-3096
US
|
Assignee: |
PANASONIC CORPORATION
Osaka
JP
|
Family ID: |
37463328 |
Appl. No.: |
12/266002 |
Filed: |
November 6, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11437642 |
May 22, 2006 |
7456034 |
|
|
12266002 |
|
|
|
|
Current U.S.
Class: |
257/14 ;
257/E29.072 |
Current CPC
Class: |
H01L 33/06 20130101;
H01S 5/0213 20130101; B82Y 20/00 20130101; H01S 2304/04 20130101;
H01L 33/32 20130101; H01S 2301/173 20130101; H01S 5/34333
20130101 |
Class at
Publication: |
257/14 ;
257/E29.072 |
International
Class: |
H01L 29/15 20060101
H01L029/15 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2005 |
JP |
2005-152602 |
Claims
1. A nitride semiconductor device comprising: a substrate; a well
layer of nitride semiconductor provided above the substrate and
containing In and Ga; a plurality of barrier layers of nitride
semiconductor provided above the substrate so that they sandwich
the well layer to construct a quantum well, containing Al and Ga,
and having larger band gap energies than the well layer; and a thin
film layer of nitride semiconductor which is provided at least at
either a position located on one of the plurality of barrier layers
and under the well layer or a position located on the well layer
and under another one of the plurality of barrier layers, and which
has a band gap energy larger than that of the well layer and
smaller than those of the barrier layers.
2. The device of claim 1, wherein the thin film layer is provided
at a position located on one of the plurality of barrier layers and
under the well layer.
3. The device of claim 1, wherein the thin film layer is provided
at a position located on the well layer and under another one of
the plurality of barrier layers.
4. The device of claim 1, wherein the thin film layer is provided
at both of a position located on one of the plurality of barrier
layers and under the well layer and a position located on the well
layer and under another one of the plurality of barrier layers.
5. The device of claim 1, wherein a quantum well formed of the well
layer, the thin film layer, and the plurality of barrier layers is
stacked repeatedly for multiple cycles.
6. The device of claim 1, wherein the thin film layer has a
thickness of 2 nm or smaller.
7. The device of claim 1, wherein the thin film layer is made of
GaN.
8. The device of claim 1, wherein the well layer is made of
In.sub.xAl.sub.yGa.sub.1-x-yN (0<x<1, 0<y<1, and
0<x+y<1), and the barrier layer is made of
In.sub.wAl.sub.zGa.sub.1-w-zN (0.ltoreq.w<1, 0<z<1, and
0<z+w<1).
9-16. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] (a) Fields of the Invention
[0002] The present invention relates to nitride semiconductor
structures which are employed for optical devices and the like such
as light emitting diodes and semiconductor laser diodes, and to
fabrication methods of such structures.
[0003] (b) Description of Related Art
[0004] Compounds expected to be applied to visible light emitting
devices, high-temperature operable electronic devices, or the like
include nitride semiconductors indicated by AlGaInN and containing
Al, Ga, In or the like as a group III element and N as a group V
element. Semiconductor devices employing AlGaInN are being put to
practical use in the field of a blue or green light emitting diode
and a blue-violet laser diode.
[0005] In fabricating a light emitting element using this nitride
semiconductor, growing a crystal of a nitride semiconductor thin
film by a metal-organic chemical vapor deposition (MOCVD) method is
the mainstream. This technique is carried out in the following
manner. A reaction tube with a substrate of, for example, sapphire,
SiC, GaN, or Si placed therewithin is supplied with trimethyl
gallium (abbreviated hereinafter as "TMG"), trimethyl aluminum
(abbreviated hereinafter as "TMA"), trimethyl indium (abbreviated
hereinafter as "TMI") or the like as a group III material gas, and
also supplied with ammonia, hydrazine, or the like as a group V
material gas. While the temperature of the substrate is kept at a
high temperature of about 600 to 1200.degree. C., an n-type layer,
a light emitting layer, and a p-type layer are grown on the
substrate to stack nitride semiconductor layers. The growth of the
n-type layer is conducted while monosilane (SiH.sub.4) or the like
as an n-type impurity material gas is flowed with a group III
material gas, and the growth of the p-type layer is conducted while
cyclopentadienyl magnesium (Cp.sub.2Mg) or the like as a p-type
impurity material gas is flowed with a group III material gas.
[0006] After this growth step, the surfaces of the n-type layer and
the p-type layer are formed with an n-type electrode and a p-type
electrode, respectively, and the resulting substrate is separated
in chip shapes to fabricate light emitting elements.
[0007] As the material for the light emitting layer, use is made of
InGaN in which In composition is adjusted to have a desired
light-emission wavelength. This light emitting layer is sandwiched
by cladding layers with a larger band gap energy than the light
emitting layer to construct a double heterostructure, or this light
emitting layer is made of a thin film layer capable of producing a
quantum size effect to construct a quantum well structure. These
two structures have been studied actively in recent years.
[0008] The quantum well structure is constructed in the manner in
which a layer with a smaller band gap energy (a well layer) is
sandwiched by barrier layers with a large band gap energy. In the
case of using the quantum well structure for an active layer, use
is made of a single quantum well structure (SQW) having one well
layer or a multiple quantum well structure (MQW) in which the well
layer and the barrier layer are alternately formed. Of the two
structures, the MQW is conventionally fabricated by any one of
three related arts that will be shown below.
[0009] A first related art is the method disclosed in Japanese
Unexamined Patent Publication No. H10-12922. In this method, using
an MOCVD apparatus, a MQW structure is constructed by repeatedly
forming a quantum well structure composed of InGaN as a well layer
and AlGaN as a barrier layer.
[0010] A second related art is the method disclosed in Japanese
Patent No. 3304787. In this method, adjustment of thicknesses of
the barrier layers uniformizes the thicknesses of the barrier
layers after the growth of the cladding layer to prevent wavelength
shift of emitted light.
[0011] A third related art is the method disclosed in Japanese
Unexamined Patent Publication No. 2002-43618. This method is
characterized in that GaN as part of a barrier layer is formed with
the temperature elevated to the growth temperature of the barrier
layer, thereby preventing degradation of a well layer.
SUMMARY OF THE INVENTION
[0012] The MQW fabrication methods as described in three related
arts, however, have the following problems.
[0013] In the first related art, since after the growth of the
InGaN layer as the well layer, the substrate is heated to the
growth temperature of the AlGaN layer (1100.degree. C.) as the
barrier layer, the well layer may decompose during the temperature
elevation. Thus, it is difficult to form a well layer with an
excellent crystallinity.
[0014] In the second related art, after the growth of the InGaN
layer as the well layer, the substrate is heated to the growth
temperature of GaN (900.degree. C.) as the barrier layer. During
this process, the well layer decomposes, which makes it difficult
to form a well layer with a high quality.
[0015] In the third related art, after the growth of the barrier
layer (the GaN layer), TMG supply is stopped while the substrate
temperature is lowered from the growth temperature of the barrier
layer to the growth temperature of the well layer to be formed
subsequently. Thereby, decomposition of the barrier layer may
occur, which makes it difficult to maintain the barrier layer with
a high quality. This in turn affects the subsequent growth of the
well layer on this barrier layer to make it difficult to form a
high quality well layer with an excellent surface flatness. In
addition, if the barrier layer is made of a ternary or higher-order
mixed crystal such as AlGaN or AlInGaN, composition of the barrier
layer growing during the temperature elevation alters to make it
difficult to form a barrier layer with high quality. On the other
hand, if the well layer is made of a ternary or lower-order mixed
crystal such as GaN or InGaN and the barrier layer is made of a
ternary or lower-order mixed crystal such as GaN or AlGaN, strain
created at the interface between the well layer and the barrier
layer generates internal electric field within the well layer to
disadvantageously decrease the light emission efficiency.
[0016] An object of the present invention is to provide a MQW
structure with a hetero interface in which the crystallinity of a
barrier layer is improved and concurrently degradation of a well
layer is suppressed, and to provide a fabrication method of such a
structure.
[0017] A nitride semiconductor device of the present invention
comprises a substrate; a well layer of nitride semiconductor
provided above the substrate and containing In and Ga; a plurality
of barrier layers of nitride semiconductor provided above the
substrate so that they sandwich the well layer to construct a
quantum well, containing Al and Ga, and having larger band gap
energies than the well layer; and a thin film layer of nitride
semiconductor which is provided at least at either a position
located on one of the plurality of barrier layers and under the
well layer or a position located on the well layer and under
another one of the plurality of barrier layers, and which has a
band gap energy larger than that of the well layer and smaller than
those of the barrier layers.
[0018] In particular, when the thin film layer is provided at a
position located on one of the plurality of barrier layers and
under the well layer, removal of nitrogen from the barrier layer
during a formation process can be prevented to avoid degradation of
the quality of the barrier layer. Moreover, strain created between
the barrier layer and the well layer can also be reduced, so that
generation of internal electric field can be suppressed to enhance
the light emission efficiency.
[0019] With this device, when the thin film layer is provided at a
position located on the well layer and under another one of the
plurality of barrier layers, removal of nitrogen, In, and the like
from the well layer during a formation process can be prevented to
avoid degradation of the quality of the well layer. Moreover,
strain created between the barrier layer and the well layer can
also be reduced to enhance the light emission efficiency.
[0020] With this device, when the thin film layer is provided at
both of a position located on one of the plurality of barrier
layers and under the well layer and a position located on the well
layer and under another one of the plurality of barrier layers,
degradation of the qualities of the well layer and the barrier
layer can be avoided to further enhance the light emission
efficiency.
[0021] A first method for fabricating a nitride semiconductor
device according to the present invention is designed for a nitride
semiconductor device which includes a well layer provided above a
substrate, a plurality of barrier layers sandwiching the well layer
to construct a quantum well, and a first thin film layer provided
on one of the plurality of barrier layers and under the well layer.
This method comprises: the step (a) of depositing nitride
semiconductor containing Al and Ga above the substrate at a
substrate temperature T1, thereby forming one of the plurality of
barrier layers; the step (b) of depositing, on one said barrier
layer, nitride semiconductor having a smaller band gap energy than
one said barrier layer, thereby forming the first thin film layer;
the step (c) of depositing nitride semiconductor on the first thin
film layer at a substrate temperature T2 (where T1>T2) to form
the well layer, the nitride semiconductor containing In and Ga and
having a smaller band gap energy than the first thin film layer;
and the step (d) of depositing nitride semiconductor on or above
the well layer at a substrate temperature T3 (where T3>T2) to
form another one of the plurality of barrier layers, the nitride
semiconductor containing Al and Ga and having a larger band gap
energy than the first thin film layer and the well layer.
[0022] Thus, by forming the first thin film layer on the barrier
layer before formation of the well layer, removal of nitrogen and
the like from the barrier layer can be prevented in lowering the
substrate temperature to the growth temperature of the well layer.
Furthermore, the first thin film layer relaxes strain created
between the barrier layer and the well layer, whereby generation of
internal electric field can be suppressed to enhance the light
emission efficiency.
[0023] This method further comprises, after the step (c) and before
the step (d), the step (e) of depositing nitride semiconductor on
the well layer to form the second thin film layer, the nitride
semiconductor having a band gap energy larger than that of the well
layer and smaller than those of the plurality of barrier layers.
This prevents removal of nitrogen and the like from the well layer
during elevation of the substrate temperature from T2 to T3.
[0024] A second method for fabricating a nitride semiconductor
device according to the present invention is designed for a nitride
semiconductor device which includes a well layer provided above a
substrate, a plurality of barrier layers sandwiching the well layer
to construct a quantum well, and a thin film layer provided on the
well layer and under one of the plurality of barrier layers. This
method comprises: the step (a) of depositing nitride semiconductor
containing Al and Ga above the substrate at a substrate temperature
T1, thereby forming one of the plurality of barrier layers; the
step (b) of depositing nitride semiconductor on or above one said
barrier layer at a substrate temperature T2 (where T1>T2) to
form the well layer, the nitride semiconductor containing In and Ga
and having a smaller band gap energy than one said barrier layer;
the step (c) of depositing nitride semiconductor on the well layer
to form the thin film layer, the nitride semiconductor having a
band gap energy larger than that of the well layer and smaller than
those of the plurality of barrier layers; and the step (d) of
depositing nitride semiconductor on the thin film layer at a
substrate temperature T3 (where T3>T2) to form another one of
the plurality of barrier layers, the nitride semiconductor
containing Al and Ga and having a larger band gap energy than the
thin film layer and the well layer.
[0025] This method prevents removal of nitrogen and the like from
the well layer and relaxes strain placed to the upper surface side
of the well layer, which enables improvement of the light emission
efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a sectional view showing a nitride semiconductor
device with a MQW according to a first embodiment of the present
invention.
[0027] FIG. 2 is a graph showing a profile of the temperature for
growing nitride semiconductor layers constituting the MQW according
to the first embodiment.
[0028] FIG. 3 is a flow chart showing fabrication steps of the MQW
according to the first embodiment.
[0029] FIG. 4 is a sectional view showing a nitride semiconductor
device with a MQW according to a second embodiment of the present
invention.
[0030] FIG. 5 is a graph showing a profile of the temperature for
growing nitride semiconductor layers constituting the MQW according
to the second embodiment.
[0031] FIG. 6 is a sectional view showing a nitride semiconductor
device with a MQW according to a third embodiment of the present
invention.
[0032] FIG. 7 is a graph showing a profile of the temperature for
growing nitride semiconductor layers constituting the MQW according
to the third embodiment.
[0033] FIG. 8 is a sectional view showing a nitride semiconductor
device with a MQW according to a fourth embodiment of the present
invention.
[0034] FIG. 9 is a graph showing a profile of the temperature for
growing nitride semiconductor layers constituting the MQW according
to the fourth embodiment.
[0035] FIG. 10 is a sectional view showing a nitride semiconductor
device with a MQW according to a fifth embodiment of the present
invention.
[0036] FIG. 11 is a graph showing a profile of the temperature for
growing nitride semiconductor layers constituting the MQW according
to the fifth embodiment.
[0037] FIG. 12 is a graph showing the relation between the
thickness of a thin film layer and the light emission intensity
(photoluminescence (PL) intensity) in the MQW according to the
first embodiment.
[0038] FIG. 13 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW according to the first embodiment.
[0039] FIG. 14 is a graph showing the relation between the
thickness of a thin film layer and the light emission intensity (PL
intensity) in the MQW according to the second embodiment.
[0040] FIG. 15 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW according to the second embodiment.
[0041] FIG. 16 is a graph showing the relation between the
thickness of a thin film layer and the light emission intensity (PL
intensity) in the MQW according to the third embodiment.
[0042] FIG. 17 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW according to the third embodiment.
[0043] FIG. 18 is a graph showing the relation between the
thickness of a thin film layer and the light emission intensity (PL
intensity) in the MQW according to the fourth embodiment.
[0044] FIG. 19 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW according to the fourth embodiment.
[0045] FIG. 20 is a graph showing the relation between the
thickness of a thin film layer and the light emission intensity (PL
intensity) in the MQW according to the fifth embodiment.
[0046] FIG. 21 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW according to the fifth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0047] FIG. 1 is a sectional view showing a nitride semiconductor
device with a MQW according to a first embodiment of the present
invention. FIG. 1 particularly illustrates a portion of the
MQW.
[0048] Referring to FIG. 1, the nitride semiconductor device of the
first embodiment includes: a substrate 1 of sapphire; a buffer
layer 2 of GaN provided on the substrate 1; an underlying layer 3
of GaN provided on the buffer layer 2; and a MQW 80 of nitride
semiconductor provided on the underlying layer 3. The MQW 80 has a
structure in which a barrier layer of Al.sub.0.15Ga.sub.0.85N, a
well layer of Al.sub.0.02In.sub.0.02Ga.sub.0.96N provided on the
barrier layer, and a thin film layer of GaN provided on the well
layer are repeatedly stacked in this order. In the example shown in
FIG. 1, on the underlying layer 3, a barrier layer 4, a well layer
5, a thin film layer 6, a barrier layer 7, a well layer 8, a thin
film layer 9, a barrier layer 10, a well layer 11, a thin film
layer 12, and a barrier layer 13 are sequentially provided from
bottom to top. In the MQW 80, the lowermost and uppermost layers
are the barrier layers for confining carriers in the well layers.
The thin film layers 6, 9, and 12 have a thickness of, for example,
4 nm or smaller, more preferably 2 nm or smaller. The buffer layer
2 and the underlying layer 3 have thicknesses of, for example, 0.02
.mu.m and 1 .mu.m, respectively. The well layers 5, 8, and 11 have
a thickness of, for example, 2 nm. The barrier layers 4, 7, 10, and
13 have a thickness of, for example, 10 nm.
[0049] In the nitride semiconductor device of the first embodiment,
the magnitudes of the band gap energies of the barrier layer, the
thin film layer, and the well layer satisfy the barrier
layer>the thin film layer>the well layer. In particular, the
thin film layers 6, 9, and 12 have larger band gaps than the well
layers 5, 8, and 11 and thicknesses of 2 nm or smaller. Thereby,
when the MQW 80 is employed for a light emitting element such as an
LED (Light Emitting Diode) or a laser, carriers can be confined
well within the well layers 5, 8, and 11.
[0050] A characteristic of the MQW of the first embodiment is that
the thin film layer of nitride semiconductor is provided on the
well layer of nitride semiconductor containing In and Ga and under
the barrier layer.
[0051] The nitride semiconductor device with the MQW in the first
embodiment is fabricated by the following procedure using an MOCVD
method.
[0052] FIG. 2 is a graph showing a profile of the temperature for
growing the nitride semiconductor layers constituting the MQW
according to the first embodiment. FIG. 3 is a flow chart showing
fabrication steps of the MQW according to the first embodiment.
Note that the fabrication condition for the MQW of the first
embodiment which will be described below is just one example, and
temperature, pressure, and other conditions are not limited to this
example.
[0053] First, in the step 1 shown in the FIG. 3, the substrate 1 of
sapphire having been adequately cleaned is inserted into a reaction
tube of an MOCVD apparatus, and then with nitrogen and hydrogen
flowed into the reaction tube, the substrate 1 is heated at about
1100.degree. C. for ten minutes to clean the surface of the
substrate 1. During this cleaning, the pressure within the reaction
tube is set at 1013 hPa, the nitrogen flow rate is set at 7010
mL/min (=7010 sccm), and the hydrogen gas flow rate is set at 3000
mL/min.
[0054] Next, in the step 2, the temperature of the substrate 1 is
lowered to about 570.degree. C., and nitrogen, TMG, and ammonia are
flowed into the reaction tube to form the buffer layer 2 of GaN on
the substrate 1. During this formation, the pressure within the
reaction tube is set at 1013 hPa. The flow rates of nitrogen, TMG,
and ammonia are set at 15500 mL/min, 8 mL/min (=35.1 .mu.mol/min),
and 5000 mL/min, respectively.
[0055] In the step 3, while TMG supply is stopped and nitrogen and
ammonia are flowed, the temperature of the substrate 1 is elevated
to about 1150.degree. C. Under this temperature, nitrogen,
hydrogen, TMG, and ammonia are flowed into the reaction tube to
grow the underlying layer 3 of GaN on the buffer layer 2. During
this growth, the pressure within the reaction tube is set at 1013
hPa. The flow rates of nitrogen, hydrogen, TMG, and ammonia are set
at 6680 mL/min, 2080 mL/min, 19.8 mL/min (=86.8 .mu.mol/min), and
1250 mL/min, respectively.
[0056] Subsequently, in the step 4, TMG supply is stopped, and then
the temperature of the substrate 1 is lowered to about 1100.degree.
C. Under this temperature, nitrogen, hydrogen, TMG, TMA, and
ammonia are flowed into the reaction tube to grow the 10 nm-thick
barrier layer 4 of Al.sub.0.15Ga.sub.0.85N. During this growth, the
pressure within the reaction tube is set at 1013 hPa. The flow
rates of nitrogen, hydrogen, TMG, TMA, and ammonia are set at 22900
mL/min, 2708 mL/min, 3.97 mL/min (=17.4 l.mu.mol/min), 3.56 mL/min
(=3.30 .mu.mol/min), and 2500 mL/min, respectively.
[0057] In the step 5, supply of TMG, TMA, and hydrogen is stopped,
and then the temperature of the substrate 1 is lowered to about
900.degree. C. With the temperature of the substrate 1 at
900.degree. C., nitrogen, TMG, TMA, TMI, and ammonia are flowed
into the reaction tube to grow the 2 nm-thick well layer 5 of
Al.sub.0.02In.sub.0.02Ga.sub.0.96N. During this growth, the
pressure within the reaction tube is set at 1013 hPa. The flow
rates of nitrogen, hydrogen, TMG, TMA, TMI, and ammonia are set at
2494 mL/min, 6 mL/min, 1.78 mL/min (=7.80 .mu.mol/min), 0.38 mL/min
(=0.353 .mu.mol/min), 97.4 mL/min (=8.88 .mu.mol/min), and 5000
mL/min, respectively.
[0058] Then, supply of TMA and TMI is stopped, and the temperature
of the substrate 1 is elevated from 900 to 1100.degree. C. with
nitrogen, TMG, and ammonia flowed thereon. In this manner, the thin
film layer 6 of GaN is grown during elevation of temperature of the
substrate 1 (the latter half of the step 5 shown in FIG. 3). Note
that the time required to elevate the temperature of the substrate
1 is set at about 2.5 minutes.
[0059] In the step 6, with the temperature of the substrate 1 kept
at 1100.degree. C., nitrogen, hydrogen, TMG, TMA, and ammonia are
flowed into the reaction tube to grow the barrier layer 7 of
Al.sub.0.15Ga.sub.0.85N. During this growth, the pressure within
the reaction tube is set at 1013 hPa. The flow rates of nitrogen,
hydrogen, TMG, TMA, and ammonia are set at 22900 mL/min, 2708
mL/min, 3.97 mL/min (=17.4 .mu.mol/min), 3.56 mL/min (=3.30
mol/min), and 2500 mL/min. Then, supply of TMG, TMA, and hydrogen
is stopped and the temperature of the substrate 1 is lowered to
900.degree. C. This lowering is done for about seven minutes.
[0060] Thereafter, in the step 7, the well layer 8, the thin film
layer 9, the barrier layer 10, and other layers are sequentially
stacked in the same procedure as formation of the well layer 5, the
thin film layer 6, and the barrier layer 7. The MQW is thus
fabricated. By adjusting the amount of TMG supplied during
elevation of temperature of the substrate 1, the thicknesses of the
thin film layers 6, 9, and 12 made of GaN are controlled within the
range of more than 0 nm and no more than 4 nm.
[0061] In the MQW fabricated in the manner described above, the
relation between the thickness of the thin film layer and the
performance of the MQW will be now described with the result of
testing.
[0062] FIG. 12 is a graph showing the relation between the
thickness of the thin film layer and the light emission intensity
(photoluminescence (PL) intensity) in the MQW of the first
embodiment. In this test, the MQW has the structure in which three
quantum wells are stacked. The PL intensity shown in FIG. 12 was
observed when the MQW was excited by a HeCd (helium-cadmium) laser
with a wavelength of 325 nm. Note that the state in which the thin
film layer has a thickness of 0 nm means the case where the
conventional MQW was employed in this test.
[0063] As can be seen from FIG. 12, the PL intensity of the MQW
significantly varied depending on the thickness of the thin film
layer, and particularly the PL intensity was maximum when the thin
film layer had a thickness of 2 nm (20 angstrom) or smaller. From
this result, it is found that in the MQW of the first embodiment,
it is particularly preferable to have the thin film layer with a
thickness more than 0 nm and no more than 2 nm. However, for all of
the measured thicknesses, the PL intensity of the MQW of the first
embodiment greatly exceeded the PL intensity of the conventional
MQW.
[0064] FIG. 13 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW of the first embodiment. Also in this test, as in the case of
the test in FIG. 12, the MQW has the structure in which three
quantum wells are stacked. FIG. 13 plots in ordinate RMS (Root Mean
Square) of the top surface flatness of the MQW measured by an
atomic force microscope (AFM).
[0065] As can be seen from FIG. 13, the surface flatness of the MQW
significantly varied depending on the thickness of the thin film
layer. In particular, when the thin film layer had a thickness of 2
nm or smaller, the RMS value of surface flatness of the MQW became
smaller. From this result, it is found that also in regard to the
surface flatness, it is preferable to have the thin film layer with
a thickness of 2 nm or smaller. However, for all of the measured
thicknesses, the top surface of the MQW of the first embodiment was
flatter than that of the conventional MQW.
[0066] The reason why the above results were obtained is considered
as follows.
[0067] In the case where the well layer contains In, the optimal
temperature for crystal growth of the well layer is lower than that
of the barrier layer of AlGaN or the like. Because of this, if the
barrier layer is formed after formation of the well layer, the
substrate temperature should be elevated. However, since supply of
the material gas is stopped during elevation of the substrate
temperature, nitrogen and In evaporate from the well layer in the
conventional MQW to roughen the surface of the well layer and also
degrade the light emission efficiency. On the other hand, in the
MQW of the first embodiment, the thin film layer is formed during
elevation of the substrate temperature after the well layer
formation. This prevents removal of nitrogen and the like from the
well layer. From this, it is conceivable that the top surface of
the MQW in the first embodiment is made flatter than that of the
conventional MQW to provide a high PL intensity. Moreover, the
change in the composition of the well layer can also be prevented.
Furthermore, the formed thin film layer can prevent removal of
nitrogen and the like from the well layer even though the well
layer and the barrier layer are not formed at the same temperature.
This enables formation of the well layer and the barrier layer at
their optimal growth temperatures. As a result of this, the
qualities of the well layer and the barrier layer can be
improved.
[0068] In addition, since the thin film layer has a band gap energy
larger than that of the well layer and smaller than that of the
barrier layer, almost the same level of carriers as the case of
forming no thin film layer can be confined within the well
layer.
[0069] Moreover, with the MQW of the first embodiment, the formed
thin film layer can also relax strain created between the well
layer and the barrier layer as compared to the conventional MQW.
This reduces internal electric field which is generated within the
quantum well composed of the well layer, the barrier layer, and the
thin film layer, so that spatial overlap between electrons and
holes existing in the conductive band and the valence band confined
within the well layer increases to enhance the light emission
efficiency. Furthermore, relaxation of strain within the MQW
improves the controllability of peak wavelength in emitting light
by the MQW.
[0070] In addition, in the MQW of the first embodiment, the thin
film layer is formed during elevation of the substrate temperature.
Therefore, no special apparatus is required, and in addition
formation thereof is done for the same period of time and at the
same cost as the case where no thin film layer is formed.
[0071] In the result shown in FIG. 12, the PL intensity is degraded
when the thin film layer has a thickness more than 2 nm. The reason
for this is conceivably that if the thin film layer is too thick,
the thin film layer itself will serves as a barrier confining
carriers within the well layer to decrease the probability of
existence of carriers within the well layer. In the MQW of the
first embodiment, the thin film layer is formed at a temperature
range of 900 to 1100.degree. C. However, since such a temperature
is lower than the optimal condition for formation of the thin film
layer, it is conceivable that also in this regard, the thin film
layer is preferably not too thick.
[0072] The MQW of the first embodiment can be applied to a
semiconductor device such as an LED, a semiconductor laser, or a
HEMT (High Electron Mobility Transistor). For an LED, for example,
an n-type compound semiconductor layer connected to an n-side
electrode is provided below the MQW, while a p-type compound
semiconductor layer connected to a p-side electrode is provided
above the MQW. By this structure, holes injected from the p-side
electrode and electrons injected from the n-side electrode can be
recombined in the well layer, so that a high light emission
efficiency can be offered. This effect is exerted similarly by
embodiments that will henceforth be described. Alternatively,
n-type impurities are injected into a banier layer provided in the
lowermost layer of the MQW, while p-type impurities are injected
into a barrier layer provided in the uppermost layer of the MQW.
Even the device with this structure can function as an LED.
[0073] For a HEMT, source and drain electrodes coming into ohmic
contact with the barrier layer and a gate electrode coming into
Schottky contact with the barrier layer are provided above the
barrier layer as the uppermost layer of the MQW. Since the top and
bottom surfaces of the well layer functioning as a channel are
flat, the HEMT can be fabricated which has a more improved carrier
mobility than the conventional one.
[0074] In the MQW of the first embodiment,
Al.sub.0.02In.sub.0.02Ga.sub.0.96N that is a quarternary crystal is
used as the material for the well layer, but the material for the
well layer is not limited to this. As long as a mixed crystal
containing In and Ga is used as the material for the well layer,
the optimal growth temperature thereof is lower than that of a
mixed crystal containing Al and Ga. Therefore, the same effects as
the MQW of the first embodiment can be exerted. The barrier layer
may also be made of a material other than AlGaN. For example, in
the case where the thin film layer is made of GaN, even though the
well layer is formed of InGaN (a ternary crystal) and the barrier
layer is made of AlInGaN (a quarternary crystal), the same effects
as the MQW of the first embodiment can be exerted. That is to say,
it is sufficient that the well layer is made of
In.sub.xAl.sub.yGa.sub.1-x-yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, and 0<x+y<1) and the barrier layer is
made of In.sub.wAl.sub.zGa.sub.1-z-wN (0.ltoreq.w<1,
0<z<1, and 0<z+w<1). In the MQW of the first
embodiment, GaN is used as the material for the thin film layer.
Instead of this, use may be made of the material having an energy
band gap larger than that of the well layer and smaller than that
of the barrier layer.
[0075] Even if the barrier layer is undoped or n-doped, the MQW can
exert the same effects. The example shown in FIG. 1 uses a sapphire
substrate as the substrate 1, but use of another substrate such as
a SiC substrate, a ZnO substrate, a GaN substrate, or a Si
substrate can also provide the same effects as the MQW of the first
embodiment.
Second Embodiment
[0076] FIG. 4 is a sectional view showing a nitride semiconductor
device with a MQW according to a second embodiment of the present
invention. FIG. 5 is a graph showing a profile of the temperature
for growing nitride semiconductor layers constituting the MQW
according to the second embodiment.
[0077] The nitride semiconductor device of the second embodiment
has the same structure as that of the nitride semiconductor device
of the first embodiment. However, in order to make a distinction to
the nitride semiconductor device of the first embodiment, different
reference numerals from those used in FIG. 1 are retained to
nitride semiconductor layers shown in FIG. 4. Specifically, the
nitride semiconductor device of the second embodiment includes,
from bottom to top, a substrate 14, a buffer layer 15, an
underlying layer 16, a barrier layer 17, a well layer 18, a thin
film layer 19, a barrier layer 20, a well layer 21, a thin film
layer 22, a barrier layer 23, a well layer 24, a thin film layer
25, and a barrier layer 26. All the layers lying between the
barrier layer 17 and the barrier layer 26 constitute a MQW 82. The
nitride semiconductor device of the second embodiment differs from
the nitride semiconductor device of the first embodiment in the
temperature for formation of the thin film layer.
[0078] To be more specific, the nitride semiconductor device with
the MQW structure in the second embodiment is fabricated by the
following procedure using an MOCVD method. Note that the flow rates
of gases supplied and the pressure within a reaction tube applied
in the fabrication steps are set at the same values as those of the
first embodiment.
[0079] First, the substrate 14 of sapphire having been adequately
cleaned is inserted into the reaction tube of an MOCVD apparatus,
and then with nitrogen and hydrogen flowed into the reaction tube,
the substrate 14 is heated at about 1100.degree. C. for ten minutes
to clean the surface of the substrate 14.
[0080] Next, the temperature of the substrate 14 is lowered to
about 570.degree. C., and nitrogen, TMG, and ammonia are flowed
into the reaction tube to form the buffer layer 15 of GaN on the
substrate 14.
[0081] While TMG supply is stopped and nitrogen and ammonia are
flowed, the temperature of the substrate 14 is elevated to about
1150.degree. C. Under this temperature, nitrogen, hydrogen, TMG,
and ammonia are flowed into the reaction tube to grow the
underlying layer 16 of GaN on the buffer layer 15.
[0082] Subsequently, TMG supply is stopped, and then the
temperature of the substrate 14 is lowered to about 1100.degree. C.
Under this temperature, nitrogen, hydrogen, TMG, TMA, and ammonia
are flowed into the reaction tube to grow the 10 nm-thick barrier
layer 17 of Al.sub.0.15Ga.sub.0.85N.
[0083] Supply of TMG, TMA, and hydrogen is stopped, and then the
temperature of the substrate 14 is lowered to about 900.degree. C.
With the temperature of the substrate 14 at 900.degree. C.,
nitrogen, TMG, TMA, TMI, and ammonia are flowed into the reaction
tube to grow the 2 nm-thick well layer 18 of
Al.sub.0.02In.sub.0.02Ga.sub.0.96N.
[0084] Then, with the temperature of the substrate 14 kept at
900.degree. C., supply of TMA and TMI is stopped, and nitrogen,
TMG, and ammonia are flowed thereon to grow the thin film layer 19
of GaN on the well layer 18. Thereafter, supply of TMG is stopped,
and the temperature of the substrate 14 is elevated to 1100.degree.
C.
[0085] With the temperature of the substrate 14 kept at
1100.degree. C., nitrogen, hydrogen, TMG, TMA, and ammonia are
flowed into the reaction tube to grow the 10 nm-thick barrier layer
20 of Al.sub.0.15Ga.sub.0.85N. Then, supply of TMG, TMA, and
hydrogen is stopped and the temperature of the substrate 14 is
lowered to 900.degree. C.
[0086] Thereafter, the well layer 21, the thin film layer 22, the
barrier layer 23, and other layers are sequentially stacked in the
same procedure as formation of the well layer 18, the thin film
layer 19, and the barrier layer 20. The MQW 82 is thus fabricated.
By adjusting the amount of TMG supplied during elevation of
temperature of the substrate 14, the thicknesses of the thin film
layers 19, 22, and 25 made of GaN are controlled within the range
of more than 0 nm and no more than 4 nm.
[0087] FIG. 14 is a graph showing the relation between the
thickness of the thin film layer and the light emission intensity
(PL intensity) in the MQW of the second embodiment. The number of
nitride semiconductor layers stacked and the measurement condition
are identical to those of the test in FIG. 12.
[0088] As can be seen from FIG. 14, the PL intensity of the MQW of
the second embodiment significantly varied depending on the
thickness of the thin film layer, and particularly the PL intensity
was maximum when the thin film layer had a thickness of 2 nm or
smaller. From this result, it is found that in the MQW of the
second embodiment, it is particularly preferable to have the thin
film layer with a thickness more than 0 nm and no more than 2 nm.
However, for all of the measured thicknesses, the PL intensity of
the MQW of the second embodiment greatly exceeded the PL intensity
of the conventional MQW.
[0089] FIG. 15 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW of the second embodiment. Also in this test, as in the case of
the test in FIG. 14, the MQW has the structure in which three
quantum wells are stacked.
[0090] From FIG. 15, it is found that the surface flatness of the
MQW significantly varied depending on the thickness of the thin
film layer, and in particular, when the thin film layer had a
thickness of 2 nm or smaller, the RMS value of surface flatness
(surface roughness) of the MQW became smaller. From this result, it
is found that also in regard to the surface flatness, it is
preferable to have the thin film layer with a thickness of 2 nm or
smaller. However, for all of the measured thicknesses, the top
surface of the MQW of the second embodiment was flatter than that
of the conventional MQW.
[0091] From the results described above, it is found that even
though the thin film layers 19, 22, and 25 are grown at the same
temperature as those for the growth of the well layers 18, 21, and
24 like the second embodiment, the top surface of the well layer
can be made flat to provide a more improved light emission
intensity than the conventional MQW. This is conceivably because
also the method for fabricating a MQW according to the second
embodiment can prevent removal of nitrogen and the like from the
well layer and can grow the well layer and the barrier layer at
their optimal growth temperatures.
[0092] In addition, since the thin film layer has a band gap energy
larger than that of the well layer and smaller than that of the
barrier layer, almost the same level of carriers as the case of
forming no thin film layer can be confined within the well
layer.
[0093] Moreover, like the MQW of the first embodiment, in the MQW
of the second embodiment, the formed thin film layer can also relax
strain created between the well layer and the barrier layer as
compared to the conventional MQW.
[0094] When comparison is made between FIG. 13 and FIG. 15, the MQW
of the first embodiment has a flatter top surface. This is probably
because the thin film layer of the MQW in the first embodiment can
be formed at a temperature closer to the optimal growth
temperature.
[0095] In the MQW of the second embodiment,
Al.sub.0.02In.sub.0.02Ga.sub.0.96N that is a quarternary crystal is
used as the material for the well layer, but the material for the
well layer is not limited to this. As long as a mixed crystal
containing In and Ga is used as the material for the well layer,
the optimal growth temperature thereof is lower than that of a
mixed crystal containing Al and Ga. Therefore, the same effects as
the MQW of the second embodiment can be exerted. The barrier layer
may also be made of a material other than AlGaN. For example, in
the case where the thin film layer is made of GaN, even though the
well layer is formed of InGaN (a ternary crystal) and the barrier
layer is made of AlInGaN (a quarternary crystal), the same effects
as the MQW of the second embodiment can be exerted. That is to say,
it is sufficient that the well layer is made of
In.sub.xAl.sub.yGa.sub.1-x-yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, and 0<x+y<1) and the barrier layer is
made of In.sub.wAl.sub.zGa.sub.1-z-wN (0.ltoreq.w<1,
0<z<1, and 0<z+w<1). In the MQW of the second
embodiment, GaN is used as the material for the thin film layer,
but it is sufficient to use the material having an energy band gap
larger than that of the well layer and smaller than that of the
barrier layer.
[0096] Even if the barrier layer is undoped or n-doped, the MQW can
exert the same effects. The example shown in FIG. 4 uses a sapphire
substrate as the substrate 14, but use of another substrate such as
a SiC substrate, a ZnO substrate, a GaN substrate, or a Si
substrate can also provide the same effects as the MQW of the
second embodiment.
Third Embodiment
[0097] FIG. 6 is a sectional view showing a nitride semiconductor
device with a MQW according to a third embodiment of the present
invention. FIG. 7 is a graph showing a profile of the temperature
for growing nitride semiconductor layers constituting the MQW
according to the third embodiment.
[0098] Referring to FIG. 6, the nitride semiconductor device of the
third embodiment includes: a substrate 27 of sapphire; a buffer
layer 28 of GaN provided on the substrate 27; an underlying layer
29 of GaN provided on the buffer layer 28; and a MQW 84 of nitride
semiconductor provided on the underlying layer 29. The MQW 84 has a
structure in which a barrier layer of Al.sub.0.15Ga.sub.0.85N, a
thin film layer of GaN provided on the barrier layer, and a well
layer of Al.sub.0.02In.sub.0.02Ga.sub.0.96N provided on the thin
film layer are repeatedly stacked in this order. In the example
shown in FIG. 6, on the underlying layer 29, a barrier layer 30, a
thin film layer 31, a well layer 32, a barrier layer 33, a thin
film layer 34, a well layer 35, a barrier layer 36, a thin film
layer 37, a well layer 38, and a barrier layer 39 are sequentially
provided from bottom to top. The thin film layers 31, 34, and 37
have a thickness of, for example, 4 nm or smaller, more preferably
2 nm or smaller. The well layers 32, 35, and 38 have a thickness
of, for example, 2 nm. The barrier layers 30, 33, 36, and 39 have a
thickness of, for example, 10 nm.
[0099] In the nitride semiconductor device of the third embodiment,
the magnitudes of the band gap energies of the barrier layer, the
thin film layer, and the well layer satisfy the barrier
layer>the thin film layer>the well layer.
[0100] A characteristic of the MQW of the third embodiment is that
the thin film layer of nitride semiconductor is provided on the
barrier layer containing Al and Ga and under the well layer
containing In and Ga.
[0101] The nitride semiconductor device with the MQW in the third
embodiment is fabricated by the following procedure using an MOCVD
method. Note that the flow rates of gases supplied and the pressure
within a reaction tube applied in the fabrication steps are set at
the same values as those of the first embodiment.
[0102] First, the substrate 27 of sapphire having been adequately
cleaned is inserted into the reaction tube of an MOCVD apparatus,
and then with nitrogen and hydrogen flowed into the reaction tube,
the substrate 27 is heated at about 1100.degree. C. for ten minutes
to clean the surface of the substrate 27.
[0103] Next, the temperature of the substrate 27 is lowered to
about 570.degree. C., and nitrogen, TMG, and ammonia are flowed
into the reaction tube to form the buffer layer 28 of GaN on the
substrate 27.
[0104] While TMG supply is stopped and nitrogen and ammonia are
flowed, the temperature of the substrate 27 is elevated to about
1150.degree. C. Under this temperature, nitrogen, hydrogen, TMG,
and ammonia are flowed into the reaction tube to grow the
underlying layer 29 of GaN on the buffer layer 28.
[0105] Subsequently, TMG supply is stopped, and then the
temperature of the substrate 27 is lowered to about 1100.degree. C.
Under this temperature, nitrogen, hydrogen, TMG, TMA, and ammonia
are flowed into the reaction tube to grow the 10 nm-thick barrier
layer 30 of Al.sub.0.15Ga.sub.0.85N.
[0106] Then, supply of TMA and hydrogen is stopped. While lowering
the temperature of the substrate 27 to 900.degree. C., nitrogen,
TMG, and ammonia are flowed to grow the thin film layer 31 of GaN
on the barrier layer 30.
[0107] When the temperature of the substrate 27 reaches 900.degree.
C., supply of TMG is stopped. With the substrate temperature kept
at 900.degree. C., nitrogen, TMG, TMA, TMI, and ammonia are flowed
to grow the 2 nm-thick well layer 32 of
Al.sub.0.02In.sub.0.02Ga.sub.0.96N.
[0108] Thereafter, supply of TMG, TMA, and TMI is stopped, and the
temperature of the substrate 27 is elevated from 900.degree. C. to
1100.degree. C.
[0109] With the temperature of the substrate 27 kept at
1100.degree. C., nitrogen, hydrogen, TMG, TMA, and ammonia are
flowed to grow the 10 nm-thick barrier layer 33 of
Al.sub.0.15Ga.sub.0.85N.
[0110] Then, supply of TMA and hydrogen is stopped. While lowering
the temperature of the substrate 27 to about 900.degree. C.,
nitrogen, TMG, and ammonia are flowed to grow the thin film layer
34 of GaN. Thereafter, the well layer, the barrier layer, and the
thin film layer are sequentially grown by repeatedly conducting the
same procedure to fabricate the MQW. By adjusting the amount of TMG
supplied during elevation of temperature of the substrate 27, the
thicknesses of the thin film layers 31, 34, and 37 made of GaN are
controlled within the range of more than 0 nm and no more than 4
nm.
[0111] FIG. 16 is a graph showing the relation between the
thickness of the thin film layer and the light emission intensity
(PL intensity) in the MQW of the third embodiment. The number of
nitride semiconductor layers stacked and the measurement condition
are identical to those of the test in FIG. 12.
[0112] As can be seen from FIG. 16, the PL intensity of the MQW of
the third embodiment significantly varied depending on the
thickness of the thin film layer, and particularly the PL intensity
was maximum when the thin film layer had a thickness of 2 nm or
smaller. From this result, it is found that in the MQW of the third
embodiment, it is particularly preferable to have the thin film
layer with a thickness more than 0 nm and no more than 2 nm.
However, for all of the measured thicknesses, the PL intensity of
the MQW of the third embodiment exceeded the PL intensity of the
conventional MQW.
[0113] FIG. 17 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW of the third embodiment. From the result shown in FIG. 17, it
is found that the top surface flatness of the MQW significantly
varied depending on the thickness of the thin film layer, and in
particular, when the thin film layer had a thickness of 2 nm or
smaller, the RMS value of surface flatness of the MQW became
smaller. From this result, it is found that also in regard to the
surface flatness, it is preferable to have the thin film layer with
a thickness of 2 nm or smaller. However, for all of the measured
thicknesses, the top surface of the MQW of the third embodiment was
flatter than that of the conventional MQW.
[0114] From the results described above, it is found that even
though the thin film layers 31, 34, and 37 are formed on the
barrier layers 30, 33, and 36, respectively, like the third
embodiment, the top surface of the MQW can be made flatter than
that of the conventional MQW to provide a more improved light
emission efficiency. This is conceivably because with the MQW of
the third embodiment, removal of nitrogen from the barrier layer
can be prevented to avoid degradation of the quality of the barrier
layer and because the well layer and the barrier layer can be grown
at their optimal growth temperatures. Moreover, with the MQW of the
third embodiment, the change in the composition of the barrier
layer can also be prevented.
[0115] Furthermore, like the MQW of the first and second
embodiments, with the MQW of the third embodiment, the formed thin
film layer can also relax strain created between the well layer and
the barrier layer as compared to the conventional MQW. This also
brings about improvement of the PL intensity. In addition, in the
MQW of the third embodiment, since the thin film layer is formed
during lowering of the temperature of the substrate 27, the period
of time taken for formation of the thin film layer does not have to
be prepared additionally. Therefore, formation of the MQW of the
third embodiment can be done for the same period of time and at the
same cost as the conventional MQW.
[0116] In the MQW of the third embodiment,
Al.sub.0.02In.sub.0.02Ga.sub.0.96N that is a quarternary crystal is
used as the material for the well layer, but the material for the
well layer is not limited to this. As long as a mixed crystal
containing In and Ga is used as the material for the well layer,
the optimal growth temperature thereof is lower than that of a
mixed crystal containing Al and Ga. Therefore, the same effects as
the MQW of the third embodiment can be exerted. The barrier layer
may also be made of a material other than AlGaN. For example, in
the case where the thin film layer is made of GaN, even though the
well layer is formed of InGaN (a ternary crystal) and the barrier
layer is made of AlInGaN (a quarternary crystal), the same effects
as the MQW of the third embodiment can be exerted. That is to say,
it is sufficient that the well layer is made of
In.sub.xAl.sub.yGa.sub.1-x-yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, and 0<x+y<1) and the barrier layer is
made of In.sub.wAl.sub.zGa.sub.1-z-wN (0.ltoreq.w<1,
0<z<1, and 0<z+w<1). In the MQW of the third
embodiment, GaN is used as the material for the thin film layer,
but it is sufficient to use the material having an energy band gap
larger than that of the well layer and smaller than that of the
barrier layer.
[0117] Even if the barrier layer is undoped or n-doped, the MQW can
exert the same effects. The example shown in FIG. 6 uses a sapphire
substrate as the substrate 27, but use of another substrate such as
a SiC substrate, a ZnO substrate, a GaN substrate, or a Si
substrate can also provide the same effects as the MQW of the third
embodiment.
Fourth Embodiment
[0118] FIG. 8 is a sectional view showing a nitride semiconductor
device with a MQW according to a fourth embodiment of the present
invention. FIG. 9 is a graph showing a profile of the temperature
for growing nitride semiconductor layers constituting the MQW
according to the fourth embodiment. The nitride semiconductor
device of the fourth embodiment has the same structure as that of
the nitride semiconductor device according to the third embodiment,
but these embodiments differ in the temperature for formation of
the thin film layer.
[0119] Specifically, the nitride semiconductor device of the fourth
embodiment includes, from bottom to top, a substrate 40, a buffer
layer 41, an underlying layer 42, a barrier layer 43, a thin film
layer 44, a well layer 45, a barrier layer 46, a thin film layer
47, a well layer 48, a barrier layer 49, a thin film layer 50, a
well layer 51, and a barrier layer 52. All the layers lying between
the barrier layer 43 and the barrier layer 52 constitute a MQW
86.
[0120] The nitride semiconductor device with the MQW structure in
the fourth embodiment is fabricated by the following procedure
using an MOCVD method. Note that the flow rates of gases supplied
and the pressure within a reaction tube applied in the fabrication
steps are set at the same values as those of the first
embodiment.
[0121] First, in the same procedure as those of the first to third
embodiments, the buffer layer 41, the underlying layer 42, and the
barrier layer 43 are sequentially formed on the substrate 40 of
sapphire.
[0122] Next, as shown in FIG. 9, supply of TMA and hydrogen into
the reaction tube is stopped. With the temperature of the substrate
40 kept at 1100.degree. C., nitrogen, TMG, and ammonia are flowed
to grow the thin film layer 44 of GaN on the barrier layer 43. As
described above, growing the thin film layer 44 at the growth
temperature of the barrier layer is a characteristic of the
fabrication method according to the fourth embodiment.
[0123] Then, supply of TMG is stopped, and then the temperature of
the substrate 40 is lowered to 900.degree. C. When the temperature
of the substrate 40 reaches 900.degree. C., nitrogen, TMQ TMA, TMI,
and ammonia are flowed into the reaction tube with the substrate
temperature kept at 900.degree. C., thereby growing the 2 nm-thick
well layer 45 of Al.sub.0.02In.sub.0.02Ga.sub.0.96N.
[0124] Thereafter, supply of TMG, TMA, and TMI is stopped, and the
temperature of the substrate 40 is elevated from 900.degree. C. to
1100.degree. C. With the temperature of the substrate 40 kept at
1100.degree. C., nitrogen, hydrogen, TMG, TMA, and ammonia are
flowed into the reaction tube to grow the 10 nm-thick barrier layer
46 of Al.sub.0.15Ga.sub.0.85N.
[0125] Next, supply of TMA is stopped. With the temperature of the
substrate 40 kept at 1100.degree. C., nitrogen, TMG, and ammonia
are flowed into the reaction tube to grow the thin film layer 47 of
GaN.
[0126] The temperature of the substrate 40 is lowered to
900.degree. C. When the temperature of the substrate 40 reaches
900.degree. C., the substrate temperature is kept at that value and
the well layer 48 is formed. Thereafter, the same procedure is
repeated to fabricate the MQW. By adjusting the amount of TMG
supplied during elevation of temperature of the substrate 40, the
thicknesses of the thin film layers 44, 47, and 50 made of GaN are
controlled within the range of more than 0 nm and no more than 4
nm.
[0127] FIG. 18 is a graph showing the relation between the
thickness of the thin film layer and the light emission intensity
(PL intensity) in the MQW of the fourth embodiment. The number of
nitride semiconductor layers stacked and the measurement condition
are identical to those of the test in FIG. 12.
[0128] As can be seen from FIG. 18, the PL intensity of the MQW of
the fourth embodiment significantly varied depending on the
thickness of the thin film layer, and particularly the PL intensity
was maximum when the thin film layer had a thickness of 2 nm or
smaller. From this result, it is found that in the MQW of the
fourth embodiment, it is particularly preferable to have the thin
film layer with a thickness more than 0 nm and no more than 2 nm.
However, for all of the measured thicknesses, the PL intensity of
the MQW of the fourth embodiment greatly exceeded the PL intensity
of the conventional MQW.
[0129] FIG. 19 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW of the fourth embodiment.
[0130] From FIG. 19, it is found that the surface flatness of the
MQW significantly varied depending on the thickness of the thin
film layer, and in particular, when the thin film layer had a
thickness of 2 nm or smaller, the RMS value of surface flatness
(surface roughness) of the MQW became smaller. From this result, it
is found that also in regard to the surface flatness, it is
preferable to have the thin film layer with a thickness of 2 nm or
smaller. However, for all of the measured thicknesses, the top
surface of the MQW of the fourth embodiment was flatter than that
of the conventional MQW. This is conceivably because with the MQW
of the fourth embodiment, removal of nitrogen from the barrier
layer can be prevented to avoid degradation of the quality of the
barrier layer and because the well layer and the barrier layer can
be grown at their optimal growth temperatures. Moreover, with the
MQW of the fourth embodiment, the change in the composition of the
barrier layer can also be prevented.
[0131] Moreover, in the MQW of the fourth embodiment, since the
thin film layer is formed between the well layer and the barrier
layer, strain created between the layers can be relaxed as compared
to the conventional MQW. This also brings about improvement of the
PL intensity.
[0132] In the MQW of the fourth embodiment,
Al.sub.0.02In.sub.0.02Ga.sub.0.96N that is a quarternary crystal is
used as the material for the well layer, but the material for the
well layer is not limited to this. As long as a mixed crystal
containing In and Ga is used as the material for the well layer,
the optimal growth temperature thereof is lower than that of a
mixed crystal containing Al and Ga. Therefore, the same effects as
the MQW of the fourth embodiment can be exerted. The barrier layer
may also be made of a material other than AlGaN. For example, in
the case where the thin film layer is made of GaN, even though the
well layer is formed of InGaN (a ternary crystal) and the barrier
layer is made of AlInGaN (a quarternary crystal), the same effects
as the MQW of the fourth embodiment can be exerted. That is to say,
it is sufficient that the well layer is made of
In.sub.xAl.sub.yGa.sub.1-x-yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, and 0<x+y<1) and the barrier layer is
made of In.sub.wAl.sub.zGa.sub.1-z-wN (0.ltoreq.w<1,
0<z<1, and 0<z+w<1). In the MQW of the fourth
embodiment, GaN is used as the material for the thin film layer,
but it is sufficient to use the material having an energy band gap
larger than that of the well layer and smaller than that of the
barrier layer.
[0133] Even if the barrier layer is undoped or n-doped, the MQW can
exert the same effects. The example shown in FIG. 8 uses a sapphire
substrate as the substrate 40, but use of another substrate such as
a SiC substrate, a ZnO substrate, a GaN substrate, or a Si
substrate can also provide the same effects as the MQW of the
fourth embodiment.
Fifth Embodiment
[0134] FIG. 10 is a sectional view showing a nitride semiconductor
device with a MQW according to a fifth embodiment of the present
invention. FIG. 11 is a graph showing a profile of the temperature
for growing nitride semiconductor layers constituting the MQW
according to the fifth embodiment.
[0135] Referring to FIG. 10, the nitride semiconductor device of
the fifth embodiment includes: a substrate 53 of sapphire; a buffer
layer 54 of GaN provided on the substrate 53; an underlying layer
55 of GaN provided on the buffer layer 54; and a MQW 88 of nitride
semiconductor provided on the underlying layer 55. The MQW 88 has a
structure in which a barrier layer of Al.sub.0.15Ga.sub.0.85N, a
first thin film layer of GaN provided on the barrier layer, a well
layer of Al.sub.0.02In.sub.0.02Ga.sub.0.96N provided on the first
thin film layer, and a second thin film layer of GaN provided on
the well layer are repeatedly stacked in this order. In this
structure, the uppermost layer is the barrier layer.
[0136] In the example shown in FIG. 10, on the underlying layer 55,
a barrier layer 56, a first thin film layer 57, a well layer 58, a
second thin film layer 59, a barrier layer 60, a first thin film
layer 61, a well layer 62, a second thin film layer 63, a barrier
layer 64, a first thin film layer 65, a well layer 66, a second
thin film layer 67, and a barrier layer 68 are sequentially
provided from bottom to top. The first thin film layers 57, 61, and
65 and the second thin film layers 59, 63, and 67 have a thickness
of, for example, 4 nm or smaller, more preferably 2 nm or smaller.
The well layers 58, 62, and 66 have a thickness of, for example, 2
nm. The barrier layers 56, 60, 64, and 68 have a thickness of, for
example, 10 nm.
[0137] In the nitride semiconductor device of the fifth embodiment,
the magnitudes of the band gap energies of the barrier layer, the
thin film layer, and the well layer satisfy the barrier
layer>the first thin film layer=the second thin film
layer>the well layer.
[0138] A characteristic of the MQW of the fifth embodiment is that
the first thin film layer of nitride semiconductor is provided
under the well layer containing In and Ga and that the second thin
film layer of nitride semiconductor is provided on the well
layer.
[0139] The nitride semiconductor device with the MQW in the fifth
embodiment is fabricated by the following procedure using an MOCVD
method. Note that the flow rates of gases supplied and the pressure
within a reaction tube applied in the fabrication steps are set at
the same values as those of the first embodiment.
[0140] First, by the same procedure as those of the first to fourth
embodiments, the buffer layer 54, the underlying layer 55, and the
barrier layer 56 are sequentially formed on the substrate 53 of
sapphire.
[0141] Then, supply of TMA and hydrogen is stopped. While lowering
the temperature of the substrate 53 from about 1100.degree. C. to
about 900.degree. C., nitrogen, TMG, and ammonia are flowed to grow
the first thin film layer 57 of GaN.
[0142] Then, when the temperature of the substrate 53 reaches
900.degree. C., supply of TMG is stopped. With the substrate
temperature kept at 900.degree. C., nitrogen, TMG, TMA, TMI, and
ammonia are flowed to grow the 2 nm-thick well layer 58 of
Al.sub.0.02In.sub.0.02Ga.sub.0.96N.
[0143] Thereafter, supply of TMA and TMI is stopped. While
elevating the temperature of the substrate 53 from 900.degree. C.
to 1100.degree. C., nitrogen, TMG, and ammonia are flowed to grow
the second thin film layer 59 of GaN.
[0144] After the temperature of the substrate 53 reaches
1100.degree. C., supply of TMG is stopped. With the temperature of
the substrate 53 kept at 1100.degree. C., nitrogen, hydrogen, TMG,
TMA, and ammonia are flowed into the reaction tube to grow the 10
nm-thick barrier layer 60 of Al.sub.0.15Ga.sub.0.85N.
[0145] Then, supply of TMG, TMA, and hydrogen is stopped. While
lowering the temperature of the substrate 53 to about 900.degree.
C., nitrogen, TMG, and ammonia are flowed to grow the first thin
film layer 61 of GaN. Hereafter, the same procedure is repeatedly
conducted to fabricate the MQW.
[0146] By adjusting the amount of TMG supplied during elevation of
temperature of the substrate 53, the thicknesses of the first thin
film layers 57, 61, and 65 and the second thin film layers 59, 63,
and 67 made of GaN are controlled within the range of more than 0
nm and no more than 4 nm.
[0147] FIG. 20 is a graph showing the relation between the
thickness of the thin film layer and the light emission intensity
(PL intensity) in the MQW of the fifth embodiment. The number of
nitride semiconductor layers stacked and the measurement condition
are identical to those of the test in FIG. 12.
[0148] As can be seen from FIG. 20, the PL intensity of the MQW of
the fifth embodiment significantly varied depending on the
thickness of the thin film layer, and particularly the PL intensity
was maximum when the thin film layer had a thickness of 2 nm or
smaller. From this result, it is found that in the MQW of the fifth
embodiment, it is particularly preferable to have the thin film
layer with a thickness more than 0 nm and no more than 2 nm, more
preferably, no more than 1 nm. However, for all of the measured
thicknesses, the PL intensity of the MQW of the fifth embodiment
greatly exceeded the PL intensity of the conventional MQW.
[0149] FIG. 21 is a graph showing the relation between the
thickness of the thin film layer and the surface flatness in the
MQW of the fifth embodiment.
[0150] From the result shown in FIG. 21, it is found that the
surface flatness of the MQW significantly varied depending on the
thickness of the thin film layer, and in particular, when the thin
film layer had a thickness of 2 nm or smaller, the RMS value of
surface flatness (surface roughness) of the MQW became smaller.
From this result, it is found that also in regard to the surface
flatness, it is preferable to have the thin film layer with a
thickness of 2 nm or smaller. However, for all of the measured
thicknesses, the top surface of the MQW of the fifth embodiment was
flatter than that of the conventional MQW. Not only that, the top
surface of the MQW of the fifth embodiment was flatter than those
of the first to fourth embodiments. This is conceivably because the
first thin film layer prevents removal of nitrogen and the like
from the barrier layer and concurrently the second thin film layer
prevents removal of nitrogen from the well layer. Thereby, the MQW
of the fifth embodiment prevents degradation of the qualities of
the barrier layer and the well layer to provide a very high light
emission efficiency. Moreover, in the MQW of the fifth embodiment,
by forming the thin film layer, the first and second thin film
layers are provided between the well layer and the respective
barrier layers. Therefore, strain placed to the well layer by both
the above and below layers is reduced. This also brings about
improvement of the PL intensity.
[0151] In the MQW of the fifth embodiment,
Al.sub.0.02In.sub.0.02Ga.sub.0.96N that is a quarternary crystal is
used as the material for the well layer, but the material for the
well layer is not limited to this. As long as a mixed crystal
containing In and Ga is used as the material for the well layer,
the optimal growth temperature thereof is lower than that of a
mixed crystal containing Al and Ga. Therefore, the same effects as
the MQW of the fifth embodiment can be exerted. The barrier layer
may also be made of a material other than AlGaN. For example, in
the case where the first and second thin film layers are made of
GaN, even though the well layer is formed of InGaN (a ternary
crystal) and the barrier layer is made of AlInGaN (a quarternary
crystal), the same effects as the MQW of the fifth embodiment can
be exerted. That is to say, it is sufficient that the well layer is
made of In.sub.xAl.sub.yGa.sub.1-x-yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, and 0<x+y<1) and the barrier layer is
made of In.sub.wAl.sub.zGa.sub.1-z-wN (0.ltoreq.w<1,
0<z<1, and 0<z+w<1). In the MQW of the fifth
embodiment, GaN is used as the materials for the first and second
thin film layers, but it is sufficient to use the material having
an energy band gap larger than that of the well layer and smaller
than that of the barrier layer.
[0152] Even if the barrier layer is undoped or n-doped, the MQW can
exert the same effects. The example shown in FIG. 10 uses a
sapphire substrate as the substrate 53, but use of another
substrate such as a SiC substrate, a ZnO substrate, a GaN
substrate, or a Si substrate can also provide the same effects as
the MQW of the fifth embodiment.
INDUSTRIAL APPLICABILITY
[0153] As described above, the present invention is useful for
producing a high-quality multiple quantum well structure with
little crystal degradation, and thereby can be employed for various
semiconductor devices such as light emitting elements.
* * * * *