U.S. patent application number 16/674824 was filed with the patent office on 2020-05-07 for semiconductor device and method of forming p-type nitride semiconductor layer.
This patent application is currently assigned to Kyoto University. The applicant listed for this patent is Kyoto University Nichia Corporation. Invention is credited to Mitsuru FUNATO, Yoichi KAWAKAMI, Katsuhiro KISHIMOTO, Kunimichi OMAE.
Application Number | 20200144372 16/674824 |
Document ID | / |
Family ID | 70459042 |
Filed Date | 2020-05-07 |
United States Patent
Application |
20200144372 |
Kind Code |
A1 |
KISHIMOTO; Katsuhiro ; et
al. |
May 7, 2020 |
SEMICONDUCTOR DEVICE AND METHOD OF FORMING P-TYPE NITRIDE
SEMICONDUCTOR LAYER
Abstract
A semiconductor device includes a p-type nitride semiconductor
layer, the p-type nitride semiconductor layer including an
Al-containing nitride semiconductor layer and an Al-containing
compound layer containing Al and C as main constituent elements and
provided on the surface of the Al-containing nitride semiconductor
layer.
Inventors: |
KISHIMOTO; Katsuhiro;
(Kyoto-shi, JP) ; FUNATO; Mitsuru; (Kyoto-shi,
JP) ; KAWAKAMI; Yoichi; (Kyoto-shi, JP) ;
OMAE; Kunimichi; (Anan-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kyoto University
Nichia Corporation |
Kyoto-shi
Anan-shi |
|
JP
JP |
|
|
Assignee: |
Kyoto University
Kyoto-shi
JP
Nichia Corporation
Anan-shi
JP
|
Family ID: |
70459042 |
Appl. No.: |
16/674824 |
Filed: |
November 5, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/0254 20130101;
H01L 21/02458 20130101; H01L 21/0262 20130101; H01L 21/02505
20130101; H01L 29/2003 20130101; H01L 29/7786 20130101; H01L 33/007
20130101; H01L 33/0075 20130101; H01L 21/0242 20130101; H01L 33/32
20130101; H01L 21/02389 20130101; H01L 29/78 20130101 |
International
Class: |
H01L 29/20 20060101
H01L029/20; H01L 33/32 20060101 H01L033/32; H01L 33/00 20060101
H01L033/00; H01L 21/02 20060101 H01L021/02; H01L 29/78 20060101
H01L029/78 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 7, 2018 |
JP |
2018-210033 |
Claims
1. A semiconductor device comprising a p-type nitride semiconductor
layer, the p-type nitride semiconductor layer comprising: an
Al-containing nitride semiconductor layer; and an Al-containing
compound layer containing Al and C as main constituent elements and
provided on a surface of the Al-containing nitride semiconductor
layer.
2. The semiconductor device according to claim 1, wherein the
p-type nitride semiconductor layer comprises a hole accumulation
layer in the vicinity of an interface between the Al-containing
nitride semiconductor layer and the Al-containing compound
layer.
3. The semiconductor device according to claim 1, wherein, in the
Al-containing compound layer, a composition ratio of C to Al is in
a range of 0.2 to 3.
4. The semiconductor device according to claim 1, wherein the
Al-containing compound layer contains O and/or N.
5. The semiconductor device according to claim 1, wherein the
Al-containing nitride semiconductor layer is composed of
Al.sub.XGa.sub.1-XN (0<X.ltoreq.1).
6. The semiconductor device according to claim 1 comprising: a
first semiconductor layer having an n-type nitride semiconductor
layer, a second semiconductor layer having the p-type nitride
semiconductor layer an active layer between the first semiconductor
layer and the second semiconductor layer, the active layer
containing a nitride semiconductor.
7. The semiconductor device according to claim 6, wherein the
active layer emits deep ultraviolet light.
8. The semiconductor device according to claim 6 comprising: a
p-electrode provided on a surface of the Al-containing compound
layer.
9. The semiconductor device according to claim 6, wherein a total
thickness of the Al-containing nitride semiconductor layer and the
Al-containing compound layer is 10 nm or less.
10. The semiconductor device according to claim 1 comprising: a
drain electrode and a source electrode both provided on or above
the Al-containing compound layer, and a gate electrode located
between the source electrode and the drain electrode, the gate
electrode being provided above the Al-containing compound layer via
an insulating film.
11. The semiconductor device according to claim 1, wherein a
thickness of the Al-containing compound layer is less than 10
nm.
12. A method of forming a p-type nitride semiconductor layer
comprising: preparing a base; and providing a p-type nitride
semiconductor layer on or above the base by forming an
Al-containing nitride semiconductor layer on or above the base and
then forming an Al-containing compound layer containing Al and C as
main constituent elements on a surface of the Al-containing nitride
semiconductor layer by supplying a source gas including a source
gas of Al and a source gas of C.
13. The method according to claim 12, wherein the providing the
p-type nitride semiconductor layer comprises: forming the
Al-containing nitride semiconductor layer on or above the base by
supplying a source gas containing a source gas of Al and a source
gas of N and, forming the Al-containing compound layer on the
surface of the Al-containing nitride semiconductor layer.
14. The method according to claim 12, wherein the providing the
p-type nitride semiconductor layer comprises: forming the
Al-containing nitride semiconductor layer on or above the base by
supplying a source gas containing a source gas of Al and a source
gas of N and, forming the Al-containing compound layer on the
surface of the Al-containing nitride semiconductor layer by
supplying the source gas containing the source gas of Al and the
source gas of C.
15. The method according to claim 12, wherein, in the preparing the
base, the base is disposed at a merging position between a first
flow path and a second flow path, and wherein, in the forming the
Al-containing compound layer, the Al-containing compound layer is
formed by supplying the source gas of Al from the first flow path
and the source gas of C from the second flow path in the forming
the Al-containing compound layer.
16. The method according to claim 15, wherein an Al metal is
vaporized in the first flow path for the source gas of Al.
17. The method according to claim 12, wherein C.sub.3H.sub.8 is
used as the source gas of C.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Japanese Patent
Application No. 2018-210033, filed on Nov. 7, 2018, the disclosure
of which is hereby incorporated by reference in its entirety.
BACKGROUND
Technical Field
[0002] The present disclosure relates to semiconductor devices, and
more specifically, to a semiconductor device including a nitride
semiconductor and a method for forming a p-type nitride
semiconductor layer.
Description of the Related Art
[0003] In recent years, nitride semiconductor light-emitting
devices, such as light-emitting diodes (LEDs) formed using nitride
semiconductors, have been widely used. A nitride semiconductor
light-emitting device is fabricated, for example, by growing a
plurality of nitride semiconductor layers, which includes an n-type
nitride semiconductor layer, a light-emitting layer, and a p-type
nitride semiconductor layer, on a sapphire substrate. Usually, in
the nitride semiconductor light-emitting device, a nitride
semiconductor doped with Si is used as the n-type nitride
semiconductor layer, while a nitride semiconductor doped with Mg is
used as the p-type nitride semiconductor layer.
[0004] However, the p-type nitride semiconductor doped with Mg has
a problem of not achieving sufficient concentration of holes
depending on the composition of the nitride semiconductor, for
example, in a nitride semiconductor containing Al. In addition,
light-emitting devices with a short emission wavelength have been
increasingly developed recently, but in particular nitride
semiconductors mainly used in this type of light-emitting device
contain a large amount of Al, which poses a problem that the hole
concentration is further lowered. To produce a p-type nitride
semiconductor by doping with Mg, the doped Mg needs to be
activated, for example, by being annealed at a temperature of
several hundred degrees after the doping. However, the activation
may not be sufficiently accomplished by only the annealing,
depending on annealing conditions, particularly in an Al-containing
nitride semiconductor even though the Al content is small. To solve
these problems, JP 2014-179584 A has proposed doping with C as a
p-type impurity in place of Mg.
SUMMARY
[0005] However, also as mentioned in JP 2014-179584 A, the p-type
nitride semiconductor obtained by doping with Mg or C has a
limitation on the acceptable content of Al therein.
[0006] This makes it difficult to provide a semiconductor device
that requires a nitride semiconductor having a large content of Al,
in other words, a large bandgap. Thus, the semiconductor device
including the nitride semiconductor cannot be used enough in
various applications.
[0007] Accordingly, it is an object of the present disclosure to
provide a semiconductor device which includes a nitride
semiconductor and can be used in various applications.
[0008] In addition, it is another object of the present disclosure
to provide a method for forming a p-type nitride semiconductor
layer which enables the manufacture of a semiconductor device
usable in various applications and including a nitride
semiconductor so that the formed p-type nitride semiconductor layer
can exhibit p-type conductivity without being limited to any Al
content.
[0009] To achieve the foregoing object, a semiconductor device
according to the present disclosure is a semiconductor device that
includes a p-type nitride semiconductor layer. The p-type nitride
semiconductor layer includes an Al-containing nitride semiconductor
layer and an Al-containing compound layer containing Al and C as
main constituent elements and provided on a surface of the
Al-containing nitride semiconductor layer.
[0010] A method for forming a p-type nitride semiconductor layer
according to the present disclosure includes the steps of.
[0011] preparing a base; and
[0012] providing a p-type nitride semiconductor layer on or above
the base by forming an Al-containing nitride semiconductor layer on
or above the base and then forming an Al-containing compound layer
containing Al and C as main constituent elements on a surface of
the Al-containing nitride semiconductor layer by supplying a source
gas including a source gas of Al and a source gas of C.
[0013] In the semiconductor device according to the present
disclosure mentioned above, the p-type nitride semiconductor layer
includes the Al-containing nitride semiconductor layer and the
Al-containing compound layer containing Al and C as main
constituent elements and provided on the surface of the
Al-containing nitride semiconductor layer. With this configuration,
the semiconductor device includes the nitride semiconductor layer
and can be used in various applications.
[0014] The method for forming a p-type nitride semiconductor layer
according to the present disclosure includes the step of providing
the p-type nitride semiconductor layer on or above the base by
forming the Al-containing nitride semiconductor layer on or above
the base and then forming the Al-containing compound layer on the
surface of the Al-containing nitride semiconductor layer by
supplying the source gases including the source gas of Al and the
source gas of C, the Al-containing compound layer containing Al and
C as main constituent elements. With this configuration, the method
makes it possible to provide the p-type nitride semiconductor layer
that can exhibit the p-type conductivity without being limited to
any Al content.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a cross-sectional view of a p-type nitride
semiconductor layer in a first embodiment according to the present
disclosure.
[0016] FIG. 2 is a cross-sectional view showing a hole accumulation
layer added to the cross-sectional view of FIG. 1.
[0017] FIG. 3 shows the band diagram of the vicinity of an
interface between an Al-containing nitride semiconductor layer and
an Al-containing compound layer shown in FIG. 2.
[0018] FIG. 4 is a process flow in a method for forming a p-type
nitride semiconductor layer of the first embodiment.
[0019] FIG. 5 is a process flow showing a specific example of a
step S2 of forming a p-type nitride semiconductor layer shown in
FIG. 4.
[0020] FIG. 6 is a cross-sectional view of a growth reactor for
growing an Al-containing compound layer in the first
embodiment.
[0021] FIG. 7 shows a cross-sectional view and a partially enlarged
view of a nitride semiconductor light-emitting device in a second
embodiment according to the present disclosure.
[0022] FIG. 8 is a cross-sectional view of a field effect
transistor in a third embodiment according to the present
disclosure.
[0023] FIG. 9 is a schematic cross-sectional view when measuring a
surface resistance in the vicinity of the surface of the p-type
nitride semiconductor layer in Examples according to the present
disclosure.
[0024] FIG. 10 is a schematic cross-sectional view of the p-type
nitride semiconductor layer according to Examples of the present
disclosure, when measuring the surface resistance in the vicinity
of the surface of the p-type nitride semiconductor layer after the
Al-containing compound layer is removed.
[0025] FIG. 11 is a cross-sectional view of a base prior to forming
the Al-containing compound layer in Examples 4 and 5 according to
the present disclosure.
DETAILED DESCRIPTION OF THE EMBODIMENT
[0026] Embodiments according to the present disclosure will be
described below with reference to the accompanying drawings.
First Embodiment
[0027] A first embodiment according to the present disclosure
relates to a p-type nitride semiconductor layer that can be used in
a semiconductor device including a nitride semiconductor.
[0028] As shown in FIG. 1, a p-type nitride semiconductor layer 11
of the first embodiment includes (a) an Al-containing nitride
semiconductor layer 11a and (b) an Al-containing compound layer 11b
containing Al and C as main constituent elements and provided on
the surface of the Al-containing nitride semiconductor layer. The
p-type nitride semiconductor layer 11 is considered to exhibit
p-type conductivity without depending on an impurity level which is
formed in a bandgap by p-type impurities, such as Mg or C.
[0029] That is, the p-type nitride semiconductor layer 11 of the
first embodiment is considered to exhibit the p-type conductivity
even when each of the Al-containing nitride semiconductor layer 11a
and the Al-containing compound layer 11b does not have any impurity
level associated with the doping of p-type impurities, such as Mg
or C.
[0030] The reason why the p-type nitride semiconductor layer 11 of
the first embodiment exhibits the p-type conductivity is not clear,
but is considered to be that a hole accumulation layer is formed in
the vicinity of the interface between the Al-containing nitride
semiconductor layer 11a and the Al-containing compound layer 11b.
Specifically, it is considered that the band diagram of energy
levels in the p-type nitride semiconductor layer 11 curves upward
(toward the higher energy side) at the interface between the
Al-containing nitride semiconductor layer 11a and the Al-containing
compound layer 11b, so that a hole accumulation layer is formed in
the vicinity of the interface at the curved part of the band
diagram, thereby exhibiting the p-type conductivity.
[0031] For example, as shown in FIG. 3, the band diagram on the
Al-containing nitride semiconductor layer 11a side curves upward
(toward the higher energy side) in the vicinity of the interface
between the Al-containing nitride semiconductor layer 11a and the
Al-containing compound layer 11b due to the composition (for
example, Al content) of the Al-containing nitride semiconductor
layer 11a and the composition (for example, Al content) of the
Al-containing compound layer 11b. Consequently, holes are
accumulated in a curved region on the Al-containing nitride
semiconductor layer 11a side to form the hole accumulation
layer.
[0032] It is noted that in FIG. 2, the hole accumulation layer
formed in the Al-containing nitride semiconductor layer 11a in the
vicinity of the interface is denoted by reference numeral 11a2,
whereas a part of the Al-containing nitride semiconductor layer 11a
excluding the hole accumulation layer 11a2 is denoted by reference
numeral 11a1.
[0033] Since the band diagram itself at the interface between the
Al-containing nitride semiconductor layer 11a and the Al-containing
compound layer 11b realizes the p-type conductivity in this way,
the p-type conductivity is considered not to be due to the impurity
level formed in the bandgap by the p-type impurities, such as Mg or
C. The hole accumulation layer could be formed on the Al-containing
compound layer 11b side in the vicinity of the interface, depending
on the composition (for example, Al content) of the Al-containing
nitride semiconductor layer 11a and the composition (for example,
Al content) of the Al-containing compound layer 11b.
[0034] By considering the results shown in Examples below, it is
understood that the p-type conductivity, which is exhibited in the
Al-containing nitride semiconductor layer 11a and the Al-containing
compound layer 11b, is not necessarily exhibited only in the
Al-containing nitride semiconductor layer 11a having a specific Al
content and the Al-containing compound layer 11b having a specific
Al content.
[0035] That is, the Al-containing nitride semiconductor layer 11a
only needs to contain at least aluminum and may be composed of, for
example, Al.sub.XGa.sub.1-XN (0<X.ltoreq.1). The Al-containing
nitride semiconductor layer 11a may further contain In. Since the
composition of a quaternary compound semiconductor, such as
AlInGaN, is more likely to fluctuate in composition ratio, the
Al-containing nitride semiconductor layer 11a is preferably
composed of a binary compound semiconductor AlN or a ternary
compound semiconductor AlGaN.
[0036] The Al-containing compound layer 11b only needs to contain
at least Al and C. The Al content in the Al-containing compound
layer 11b is set so that the composition ratio of C to Al is, for
example, 0.1 to 10, preferably 0.2 to 3, and more preferably 0.5 to
2. The Al-containing compound layer 11b may further contain other
elements, such as oxygen (O) and nitrogen (N), and inevitable
impurities mixed therein. Examples of the material of the
Al-containing compound layer 11b include AlC.sub.2,
Al.sub.4C.sub.3, Al.sub.5C.sub.3N, Al.sub.2OC, Al.sub.4O.sub.4C,
and the like. When the surfaces of samples produced by the
inventors in an investigation process were analyzed by a
time-of-flight secondary ion mass spectrometry (TOF-SIMS), it was
confirmed that an Al-containing compound layer represented by
AlC.sub.2 with a thickness of less than 10 nm was formed in each
sample.
[0037] The p-type nitride semiconductor layer 11 may be thin. For
example, a total thickness of the Al-containing nitride
semiconductor layer 11a and the Al-containing compound layer 11b
may be 10 nm or less. Since the Al-containing nitride semiconductor
layer 11a has a relatively high resistance, when using the p-type
nitride semiconductor layer 11 as a contact layer of an LED or the
like as mentioned later, the total thickness of the Al-containing
nitride semiconductor layer 11a and the Al-containing compound
layer 11b is preferably thin and is preferably 10 nm or less. The
thickness of the Al-containing compound layer 11b may be less than
10 nm.
[0038] As shown in FIG. 4, the method for forming a p-type nitride
semiconductor layer 11 in the above-mentioned first embodiment
includes, for example, a step S1 of preparing a base and a step S2
of forming a p-type nitride semiconductor layer.
[0039] In the step S2 of forming a p-type nitride semiconductor
layer, the Al-containing nitride semiconductor layer 11a is formed
on or above the base, and then the Al-containing compound layer 11b
is formed on a surface of the Al-containing nitride semiconductor
layer 11a by supplying source gases including a source gas of Al
and a source gas of C so as to contain Al and C as main constituent
elements.
[0040] The formation of the Al-containing compound layer 11b in the
step S2 of forming the p-type nitride semiconductor layer can be
performed using, for example, a growth reactor 100 shown in FIG. 6,
which has a first flow path p1 and a second flow path p2 that has
its one end merging with the first flow path p1. In the growth
reactor 100, the second flow path p2 is provided so that the gas
flowing through the second flow path p2 merges with the gas flowing
through the first flow path p from above.
[0041] In the growth reactor 100 configured as mentioned above, a
base 105 is disposed at a merging position of the first flow path p
with the second flow path p2, and then the source gas of Al is
supplied from the first flow path p, while the source gas of C is
supplied from the second flow path p2. Thus, the Al-containing
compound layer is formed on or above the base 105 disposed at the
merging position between the first flow path and the second flow
path.
[0042] Here, in the growth reactor 100, an Al metal 104 disposed
within a raw material storage portion 103 in the first flow path p1
is heated with a first heater h1 provided in a raw material zone z1
to produce vaporized source gas of Al, which is used as the source
gas of the process.
[0043] For example, C.sub.3H.sub.8, C.sub.2H.sub.2, CBr.sub.4,
CCl.sub.4 or the like can also be used as the source gas of C, and
especially C.sub.3H.sub.8 (propane) is preferably used.
[0044] As shown in FIG. 6, in the growth reactor 100, a second
heater h2 is provided in a growth zone z2. By controlling the
second heater h2, the growth zone z2 can be set at a predetermined
temperature.
[0045] When using the growth reactor 100 configured as mentioned
above, the step S21 of forming the Al-containing nitride
semiconductor layer and the step S22 of forming the Al-containing
compound layer shown in FIG. 5 can be performed in sequence to
thereby form the p-type nitride semiconductor layer.
[0046] For example, the base 105 formed of a sapphire substrate or
the like is disposed at a sample setting portion 110 located at the
merging position of the first flow path p1 with the second flow
path p2. Then, the source gas of Al is supplied from the first flow
path p1, and another source gas as the raw material of the
Al-containing nitride semiconductor layer 11a containing N is also
supplied from the second flow path p2 to the substrate. Thus, the
Al-containing nitride semiconductor layer 11a is grown on or above
the base 105 (step S21).
[0047] After growing the Al-containing nitride semiconductor layer
11a, the Al-containing compound layer 11b is formed thereon by
supplying the source gas of Al from the first flow path p1 and the
source gas of C from the second flow path p2 to the Al-containing
nitride semiconductor layer 11a (step S22).
[0048] In this way, the p-type nitride semiconductor layer 11 can
be formed using the growth reactor 100.
[0049] When forming the Al-containing nitride semiconductor layer
11a, the source gas of C may or may not be supplied. If the source
gas of C is supplied together with the source gas of N, the step of
forming the Al-containing nitride semiconductor layer 11a and the
step of forming the Al-containing compound layer 11b can be
performed in one process. As mentioned in Examples below, it is
confirmed that the Al-containing compound layer 11b is formed only
at the outermost surface of the p-type nitride semiconductor layer
11 even though the source gas of C is continuously supplied when
forming the p-type nitride semiconductor layer 11. In this case, it
is also considered that the only Al-containing nitride
semiconductor layer is formed while supplying the source gases at
the growth temperature maintained, for example, at 1,500.degree.
C., and then the Al-containing compound layer is formed thereon
while decreasing this temperature after the end of growing the
Al-containing nitride semiconductor layer. Thus, even if the source
gas of C is continuously supplied when forming the p-type nitride
semiconductor layer 11, a multilayer structure including the
Al-containing nitride semiconductor layer 11a and the Al-containing
compound layer 11b can be obtained. The temperature of the growth
zone z2 in this case is set at, for example, 1,500.degree. C.
[0050] Alternately, the process in step S21 of forming the
Al-containing nitride semiconductor layer and the process in step
S22 of forming the Al-containing compound layer may be separately
performed. In this case, in step S22, the temperature of the raw
material zone z1 can be set at a temperature at which the Al metal
104 is vaporized (for example, 1,400.degree. C.), while the
temperature of the growth zone z2 can be set lower than the
temperature of the raw material zone z1. Thus, the possibility of
damaging the Al-containing nitride semiconductor layer 11a or a
layer previously formed on the base 105 can be reduced. The
temperature of the growth zone z2 in this case is, for example,
1,000.degree. C. or higher, and can be set at 1,050.degree. C.
[0051] In step S2 of forming the p-type nitride semiconductor
layer, the process in step S21 of forming the Al-containing nitride
semiconductor layer and the process in step S22 of forming the
Al-containing compound layer may be individually performed in
different growth reactors. For example, an Al-containing nitride
semiconductor layer may be formed by metalorganic chemical vapor
deposition (MOCVD) in a growth reactor different from the growth
reactor 100 (S21). Thereafter, an Al-containing compound layer may
be formed by disposing the base, with the Al-containing nitride
semiconductor layer formed thereon, in the growth reactor 100, and
then by supplying a source gas of Al from the first flow path p1
and a source gas of C from the second flow path p2 to the
Al-containing nitride semiconductor layer (S22).
[0052] Accordingly, the first embodiment configured as mentioned
above can provide the p-type nitride semiconductor layer 11 and the
method for forming the same which can exhibit p-type conductivity
without being limited to any Al content.
[0053] Furthermore, the p-type nitride semiconductor layer 11 of
the first embodiment configured as mentioned above can provide a
semiconductor device which includes a nitride semiconductor layer
and is usable in various applications. Hereinafter, a description
will be made on a specific example of the semiconductor device
including the p-type nitride semiconductor layer 11 of the first
embodiment.
Second Embodiment
[0054] A second embodiment according to the present disclosure
relates to a nitride semiconductor light-emitting device 200 which
is an example of the semiconductor device including the p-type
nitride semiconductor layer of the first embodiment. More
specifically, the nitride semiconductor light-emitting device 200
of the second embodiment is an example in which the p-type nitride
semiconductor layer according to the present disclosure is applied
to a nitride semiconductor light-emitting device 200 that emits
ultraviolet light and is formed mainly using a nitride
semiconductor having a large Al content.
[0055] As shown in FIG. 7, the nitride semiconductor light-emitting
device 200 of the second embodiment includes, for example, a
substrate 260 formed of sapphire, an underlayer 250 provided on the
substrate 260, an n-side first semiconductor layer 240 provided on
the underlayer 250, an active layer 230 provided on the first
semiconductor layer 240, an electron blocking layer 220 provided on
the active layer 230, and a p-side second semiconductor layer 210
provided on the electron blocking layer 220. The first
semiconductor layer 240 includes an n-type nitride semiconductor
layer.
[0056] In the nitride semiconductor light-emitting device 200 of
the second embodiment, the second semiconductor layer 210 includes
a p-side contact layer and a p-side cladding layer 212, and the
p-side contact layer is constituted of the p-type nitride
semiconductor layer 11 according to the present disclosure.
[0057] In the nitride semiconductor light-emitting device 200 of
the second embodiment, the p-type nitride semiconductor layer,
which is the p-side contact layer, is the same as the p-type
nitride semiconductor layer 11 of the first embodiment, and thus a
description thereof is omitted below. Parts of the nitride
semiconductor light-emitting device 200 other than the p-type
nitride semiconductor layer 11 will be specifically described
below.
[0058] A substrate on which a nitride semiconductor can be grown,
for example, a substrate formed of sapphire (Al.sub.2O.sub.3), AlN,
or AlGaN can be used as the substrate 260.
[0059] For example, an AlN film may be used as the underlayer
250.
[0060] The n-side first semiconductor layer 240 includes, for
example, a first composition gradient layer 242 and a second
composition gradient layer 241. The first composition gradient
layer 242 is formed of, for example, undoped Al.sub.aGa.sub.1-aN,
in which the Al composition ratio "a" sequentially or gradually
decreases upward. The second composition gradient layer 241 is
formed of, for example, Al.sub.bGa.sub.1-bN doped with n-type
impurities, such as Si, in which the Al composition ratio "b"
sequentially or gradually decreases upward. The n-side first
semiconductor layer may not be a composition gradient layer and may
be a layer that has a substantially single composition, such as an
n-type AlGaN layer.
[0061] The active layer 230 has, for example, a quantum-well
structure that includes an Al.sub.cGa.sub.1-cN well layer and an
Al.sub.dGa.sub.1-dN barrier layer. For example, to emit deep
ultraviolet light with a wavelength of 220 to 350 nm, the active
layer 230 is set at have a composition corresponding to a desired
wavelength at an Al composition ratio "c" of the well layer that is
within the range of 0.4<c.ltoreq.1.0.
[0062] The barrier layer is set at have an Al composition ratio "d"
within the range of, for example, c<d.ltoreq.1.0 so as to make a
bandgap energy of the barrier layer larger than that of the well
layer.
[0063] In the semiconductor light-emitting device that emits deep
ultraviolet light with a peak wavelength of 280 nm, for example,
the well layer is constituted of a nitride semiconductor formed of
an Al.sub.0.45Ga.sub.0.55N in which an Al composition ratio "c" is
0.45, while the barrier layer is constituted of a nitride
semiconductor formed of Al.sub.0.56Ga.sub.0.44N in which an Al
composition ratio "d" is 0.56.
[0064] The second semiconductor layer 210 includes, for example,
the p-side cladding layer 212 and the p-side contact layer. The
p-side contact layer is constituted of the p-type nitride
semiconductor layer 11 of the first embodiment.
[0065] The p-side cladding layer 212 can include a nitride
semiconductor formed of, for example, Al.sub.0.03Ga.sub.0.37N.
[0066] The nitride semiconductor light-emitting device 200 of the
second embodiment includes the electron blocking layer 220, which
has a bandgap energy larger than that of the barrier layer, between
the p-side second semiconductor layer 210 and the active layer
230.
[0067] An n-electrode 207 is disposed on an exposed portion of the
second composition gradient layer 241 created by removing a partial
region of the semiconductor multilayer structure. The n-electrode
207 can be formed of, for example, an alloy of titanium and
aluminum by a sputtering method.
[0068] A p-electrode 208 is provided on a surface of the
Al-containing compound layer 11b. The p-electrode 208 can be formed
of, for example, a nickel layer and an aluminum layer by the
sputtering method. The p-electrode 208 may be constituted of a
laminate structure of a nickel layer and a magnesium layer or a
single rhodium layer. In addition, the p-electrode 208 may be
formed using a material that is transparent to light emitted from
the active layer 230.
[0069] The above-mentioned nitride semiconductor light-emitting
device 200 of the second embodiment is configured to include the
p-type nitride semiconductor layer of the first embodiment as the
p-side contact layer. This configuration makes it possible to
easily form the p-side contact layer with the p-type conductivity
using a nitride semiconductor which has a large Al content. Thus,
absorption of ultraviolet light emitted from the active layer in
the p-side contact layer can be suppressed, thereby enhancing the
light extraction efficiency.
[0070] That is, even in a nitride semiconductor light-emitting
device that emits ultraviolet light and includes a well layer
containing Al, the necessary p-type conductivity is obtained by
forming a p-side contact layer using GaN, which has a bandgap
smaller than that of the well layer, but in this case, the
ultraviolet light emitted in the active layer is absorbed in the
p-side contact layer, resulting in reduced light extraction
efficiency.
[0071] In contrast, however, the above-mentioned nitride
semiconductor light-emitting device 200 of the second embodiment is
configured to include the p-type nitride semiconductor layer of the
first embodiment as the p-side contact layer, which enables the
formation of the p-side contact layer having the p-type
conductivity, using the nitride semiconductor that has a larger Al
content than that in the well layer. Thus, the absorption of
ultraviolet light emitted from the active layer 230 in the p-side
contact layer can be suppressed, thereby enhancing the light
extraction efficiency.
Third Embodiment
[0072] A third embodiment according to the present disclosure shows
another example of the semiconductor device that includes the
p-type nitride semiconductor layer of the first embodiment. The
present third embodiment relates to a field effect transistor 300
that has a metal-insulator-semiconductor (MIS) structure including
the p-type nitride semiconductor layer according to the present
disclosure.
[0073] The field effect transistor 300 of the third embodiment
includes the p-type nitride semiconductor layer 11 of the first
embodiment that is provided on a substrate 360 made of, for
example, sapphire via a nitride semiconductor layer 370 made of,
for example, AlN. A source electrode 381, a gate electrode 385, and
a drain electrode 383 are provided over the p-type nitride
semiconductor layer 11. The gate electrode 385 is located between
the source electrode 381 and the drain electrode 383, each of which
is in ohmic contact with the p-type nitride semiconductor layer 11.
The gate electrode 385 is provided over the p-type nitride
semiconductor layer 11 via an insulating film 387.
[0074] The field effect transistor 300 of the third embodiment
configured as mentioned above includes the p-type nitride
semiconductor layer 11 of the first embodiment which can exhibit
the p-type conductivity even though the Al content therein is
large. This configuration enables the formation of a hole channel
with a wide bandgap, especially, the provision of power device
transistors. It is noted that the nitride semiconductor layer 370
may not be provided.
EXAMPLES
[0075] Examples according to the present disclosure will be
described below.
[0076] Examples 1 to 3 below are examples of fabricating samples
using the growth reactor 100 shown in FIG. 6. Comparative Example 1
is an example of fabricating a sample for comparison with Examples
1 to 3.
Examples 1 to 3
[0077] In Examples 1 to 3, a sapphire substrate 460 was disposed at
a merging position of the first flow path p1 with the second flow
path p2 in the growth reactor 100. Then, the source gas of Al was
supplied along the upper surface of the sapphire substrate 460 via
the first flow path p1 while N.sub.2 gas (source gas of N) and
C.sub.3H.sub.8 gas (source gas of C) were supplied from above to
the upper surface of the sapphire substrate 460 via the second flow
path p2. In this way, an Al-containing nitride semiconductor layer
was formed of the AlN layer, and then an Al-containing compound
layer was formed on the AlN layer. The total thickness of the
Al-containing nitride semiconductor layer and the Al-containing
compound layer was approximately 0.5 .mu.m.
[0078] In Examples 1 to 3, the samples were fabricated on the same
conditions except for the flow rate of C.sub.3H.sub.8 (source gas
of C).
[0079] Specifically, in Example 1, the flow rate of C.sub.3H.sub.8
was set at 0.2 sccm; in Example 2, the flow rate of C.sub.3H.sub.8
was set at 0.4 sccm; and in Example 3, the flow rate of
C.sub.3H.sub.8 was set at 0.8 sccm. In Examples 1 to 3, the flow
rate of N.sub.2 (source gas of N) was set at 3,000 sccm.
[0080] In Examples 1 to 3, the respective flow rates of the source
gases other than C.sub.3H.sub.8 (source gas of C) were set as
follows.
[0081] Source gas of Al: 2.5 sccm
[0082] Source gas of N: 3,000 sccm
[0083] Carrier gas (Ar) for Al source: 600 sccm
[0084] When fabricating the samples of Examples 1 to 3, in the
growth reactor 100, the temperature of the raw material zone z1 was
set at 1,450.degree. C., while the temperature of the growth zone
z2 was set at 1,500.degree. C. After the temperature of the raw
material zone Z1 and the temperature of the growth zone z2 were
maintained at 1,450.degree. C. and 1,500.degree. C., respectively,
for 60 minutes, the temperature of the raw material zone z1 and the
temperature of the growth zone z2 were naturally decreased while
causing the source gases to flow in the growth reactor.
[0085] The sheet carrier density and the conductivity type of the
p-type nitride semiconductor layer formed on the sapphire substrate
were measured for the respective samples in Examples 1 to 3
fabricated in the way mentioned above. The sheet carrier densities
of the samples were as follows.
[0086] Example 1: 4.45.times.10.sup.13/cm.sup.2
[0087] Example 2: 3.85.times.10.sup.13/cm.sup.2
[0088] Example 3: 2.64.times.10.sup.14/cm.sup.2
[0089] Each of the samples exhibited the p-type conductivity.
Comparative Example 1
[0090] In Comparative Example 1, a sample was fabricated in the
same way as that in Examples 1 to 3 except that C.sub.3H.sub.8
(source gas of C) was not supplied. In the sample of Comparative
Example 1, substantially no electricity flowed, and no p-type
conductivity was exhibited.
[0091] In contrast, in Examples 1 to 3, all the samples that were
fabricated on the same conditions except for the flow rate of
C.sub.3H.sub.8 (source gas of C) exhibited the p-type
conductivity.
[0092] As is understood from this, it is not necessarily right that
the p-type conductivity was exhibited only when the Al-containing
compound layer 11b had the specific composition. That is, when
changing the flow rate of C.sub.3H.sub.8 (source gas of C) while
making the flow rate of the source gas of Al constant among
Examples 1 to 3, the composition ratio of C to Al in the
Al-containing compound layer varies because of the different flow
rates of C.sub.3H.sub.8 (source gas of C). Nevertheless, all the
samples in Examples 1 to 3 exhibited the p-type conductivity, which
shows that the specific composition of the Al-containing compound
layer 11b is not essential to the p-type conductivity.
[0093] To evaluate the influence of the Al-containing compound
layer in the p-type nitride semiconductor layer formed on the
sapphire substrate, the sample of Example 1 was examined for a
current value flowing through the vicinity of the surface of the
p-type nitride semiconductor layer 11 by the methods shown in FIGS.
9 and 10.
[0094] Specifically, as shown in FIG. 9, measurement electrodes 491
made of Ni were separately formed on the surface of the
Al-containing compound layer of the sample in Example 1, and then
the surface current flowing between the electrodes was examined. As
a result, the current of approximately 20 .mu.A flowed at a voltage
of 5 V.
[0095] Then, while the measurement electrodes 491 shown in FIG. 9
were maintained, parts of the p-type nitride semiconductor layer 11
with a thickness of 10 nm from its surface were removed by reactive
ion etching so as to leave regions of the semiconductor layer 11
directly under the measurement electrodes 491. In this state, the
surface current flowing between the electrodes was examined as
shown in FIG. 10. Since the thickness of the Al-containing compound
layer in the sample of Example 1 was estimated to be less than 10
nm, the Al-containing compound layer was thought not to exist
between the electrodes in the state shown in FIG. 10.
[0096] As a result, the current flowing at a voltage of 5 V was
approximately 3 to 4 nA, which was lower by three or more orders of
magnitude than that that before the reactive ion etching, which
meant that substantially no current flowed.
[0097] From the above-mentioned results, it can be seen that only
the Al-containing nitride semiconductor layer from which the
Al-containing compound layer is removed in the p-type nitride
semiconductor layer 11 does not exhibit p-type conductivity.
Examples 4 and 5
[0098] In Examples 4 and 5, first, as shown in FIG. 11, by the
metalorganic vapor phase epitaxy, an underlayer 462 made of AlN was
grown on the sapphire substrate 460 so as to have a thickness of
3.5 .mu.m, and then an Al-containing nitride semiconductor layer
11a made of AlGaN (Mg-doped AlGaN) containing Al and Ga doped with
Mg was formed on the underlayer 462 so as to have a thickness of
0.5 .mu.m. In this way, the base was fabricated.
[0099] Then, the thus-obtained base was disposed at a merging
position of the first flow path p1 with the second flow path p2 in
the growth reactor 100, and subsequently the source gas of Al was
supplied along the upper surface of the Al-containing nitride
semiconductor layer 11a via the first flow path p1, while
C.sub.3H.sub.8 gas (source gas of C) was supplied from the vertical
direction to the upper surface of the Al-containing nitride
semiconductor layer 11a via the second flow path p2. In this way,
an Al-containing compound layer was formed on the Al-containing
nitride semiconductor layer 11a. When forming the Al-containing
compound layer, the flow rate of the source gas supplied from each
of the first flow path p1 and the second flow path p2 in the growth
reactor 100 in Examples 4 and 5 was set at the same level as that
in Example 1.
[0100] In the way mentioned above, the samples of Examples 4 and 5,
each including the p-type nitride semiconductor layer which was
composed of the Al-containing nitride semiconductor layer 11a made
of the Mg-doped AlGaN and the Al-containing compound layer 11b,
were fabricated.
[0101] Here, Examples 4 and 5 differ from each other in the
composition ratio between Al and Ga in the Al-containing nitride
semiconductor layer 11a, but are the same in other aspects.
Specifically, the Al-containing nitride semiconductor layer 11a in
Example 4 was made of Al.sub.0.3Ga.sub.0.7N, and the Al-containing
nitride semiconductor layer 11a of Example 5 was made of
Al.sub.0.8Ga.sub.0.2N. In each of Examples 4 and 5, the growth
conditions were set so that the Mg concentration was within the
range of 2.0.times.10.sup.19 to 4.0.times.10'.sup.9/cm.sup.3.
[0102] The sheet carrier density and the conductivity type of the
p-type nitride semiconductor layer (Mg-doped AlGaN
layer+Al-containing compound layer) formed above the sapphire
substrate were measured and examined for the respective samples in
Examples 4 and 5 fabricated in the way mentioned above.
[0103] The sheet carrier densities of the samples were as
follows.
[0104] Example 4: 6.47.times.10'.sup.3/cm.sup.2
[0105] Example 5: 4.89.times.10.sup.13/cm.sup.2
[0106] Each of the samples exhibited the p-type conductivity.
[0107] The current-voltage characteristics at this time exhibited
ohmic characteristics.
Comparative Examples 2 and 3
[0108] In Comparative Example 2, the conductivity type of an
Al-containing nitride semiconductor layer 11a which was composed of
Al.sub.0.3Ga.sub.0.7N without having an Al-containing compound
layer was examined. As a result, the Al-containing nitride
semiconductor layer 11a did not exhibit any p-type conductivity,
and the current value was lower by two orders of magnitude than a
current value at the same voltage in Example 4. The current-voltage
characteristics at this time exhibited Schottky
characteristics.
[0109] In Comparative Example 3, the conductivity type of an
Al-containing nitride semiconductor layer 11a which was composed of
Al.sub.0.8Ga.sub.0.2N without having an Al-containing compound
layer was examined. As a result, the Al-containing nitride
semiconductor layer 11a did not exhibit any p-type conductivity,
and the current value was lower by four or more orders of magnitude
than a current value at the same voltage in Example 5. The
current-voltage characteristics at this time exhibited Schottky
characteristics.
[0110] After removing the Al-containing compound layer from each
sample of Examples 4 and 5 by the method shown in FIG. 10, the
current flowing through the vicinity of the surface of the sample
was measured. As a result, the current value of each of the samples
in Examples 4 and 5 was higher than the current value measured at
the same voltage in Comparative Examples 2 and 3. This is
considered to be because the Al-containing compound layer remaining
directly under the electrode maintains the p-type conductivity
directly under the electrode.
* * * * *