U.S. patent application number 13/373102 was filed with the patent office on 2012-03-01 for method of manufacturing p-type nitride semiconductor and semiconductor device fabricated by the method.
Invention is credited to Shigetoshi Ito, Teruyoshi Takakura, Mototaka Taneya, Yuhzoh Tsuda, Yoshihiro Ueta.
Application Number | 20120049328 13/373102 |
Document ID | / |
Family ID | 37114903 |
Filed Date | 2012-03-01 |
United States Patent
Application |
20120049328 |
Kind Code |
A1 |
Tsuda; Yuhzoh ; et
al. |
March 1, 2012 |
Method of manufacturing p-type nitride semiconductor and
semiconductor device fabricated by the method
Abstract
The present invention includes a first step of forming a nitride
semiconductor layer by metal organic chemical vapor deposition by
using a first carrier gas containing a nitrogen carrier gas and a
hydrogen carrier gas of a flow quantity larger than that of the
nitrogen carrier gas to thereby supply a raw material containing Mg
and a Group V raw material containing N, and a second step of
lowering a temperature by using a second carrier gas to which a
material containing N is added, and hence solves the problems
encountered in the art.
Inventors: |
Tsuda; Yuhzoh; (Sakurai-shi,
JP) ; Ito; Shigetoshi; (Shijonawate-shi, JP) ;
Taneya; Mototaka; (Nara-shi, JP) ; Ueta;
Yoshihiro; (Yamatokoriyama-shi, JP) ; Takakura;
Teruyoshi; (Tenri-shi, JP) |
Family ID: |
37114903 |
Appl. No.: |
13/373102 |
Filed: |
November 4, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11887153 |
Sep 26, 2007 |
8076165 |
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PCT/JP2006/303985 |
Mar 2, 2006 |
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13373102 |
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Current U.S.
Class: |
257/615 ;
257/E21.09; 257/E29.089; 438/478 |
Current CPC
Class: |
H01L 33/007 20130101;
B82Y 20/00 20130101; C23C 16/4481 20130101; H01S 5/04252 20190801;
H01S 5/305 20130101; H01S 2304/04 20130101; H01L 21/02579 20130101;
H01L 21/0262 20130101; H01S 5/3211 20130101; H01S 5/2009 20130101;
H01S 5/3063 20130101; H01L 21/0254 20130101; C23C 16/34 20130101;
H01S 5/34333 20130101; H01S 5/22 20130101 |
Class at
Publication: |
257/615 ;
438/478; 257/E21.09; 257/E29.089 |
International
Class: |
H01L 29/20 20060101
H01L029/20; H01L 21/20 20060101 H01L021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 1, 2005 |
JP |
2005-105827 (P) |
Claims
1. A method of manufacturing a p-type nitride semiconductor,
comprising: a first step of forming a nitride semiconductor layer
by metal organic chemical vapor deposition by using a first carrier
gas containing a nitrogen gas and a hydrogen gas of a flow quantity
larger than a flow quantity of said nitrogen gas to thereby supply
a raw material containing magnesium and a Group V raw material
containing nitrogen (N); and a second step of lowering a
temperature of the first step by using a second carrier gas to
which a material containing nitrogen (N) is added at a
concentration of at least 0.01% and less than 20%.
2. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein ammonia is used at a concentration of
at least 0.01% and at most 5% as said material containing nitrogen
(N).
3. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein in said first carrier gas, a ratio of
the hydrogen gas with respect to a total flow quantity of the
nitrogen gas and the hydrogen gas is more than 70% and at most
95%.
4. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein a total layer thickness of said
nitride semiconductor layer is at least 0.5 .mu.m.
5. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein said nitride semiconductor layer is
made of a plurality of layers, and a total layer thickness of a
layer including a nitride semiconductor with an aluminum
composition ratio of at least 2% is at least 0.3 .mu.m.
6. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein said raw material containing
magnesium is bis (ethyl cyclopentadienyl) magnesium.
7. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein a flow quantity of said Group V raw
material containing nitrogen (N) is smaller than the flow quantity
of the hydrogen gas in the first carrier gas.
8. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein a flow quantity of the second carrier
gas is larger than the flow quantity of the nitrogen gas in the
first carrier gas.
9. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein the second carrier gas is argon or
nitrogen.
10. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein said second step is a step of
lowering the temperature to 400.degree. C. or lower, and a time
required for reaching 400.degree. C. is at most 25 minutes.
11. The method of manufacturing the p-type nitride semiconductor
according to claim 1, wherein dimethyl hydrazine is used at a
concentration of at least 0.01% and at most 2% as said material
containing nitrogen (N).
12. The method of manufacturing the p-type nitride semiconductor
according to claim 1, further comprising: a third step of
subjecting the nitride semiconductor layer to an annealing
treatment after said second step in an inert gas for at least one
minute and at most 15 minutes.
13. The method of manufacturing the p-type nitride semiconductor
according to claim 12, wherein the annealing treatment in the third
step is performed at a temperature of at least 700.degree. C. and
at most 900.degree. C.
14. The method of manufacturing the p-type nitride semiconductor
according to claim 12, wherein in said third step, ammonia is
further added at a concentration of at least 0.01% and at a most 5%
in addition to said inert gas.
15. The method of manufacturing the p-type nitride semiconductor
according to claim 12, wherein in said third step, hydrogen is
further added at a concentration of at least 10 ppm and at most 500
ppm in addition to said inert gas.
16. A semiconductor device including a p-type nitride semiconductor
layer fabricated by the manufacturing method recited in claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This U.S. nonprovisional application is a continuation
application under 35 U.S.C. .sctn.120 of U.S. application Ser. No.
11/887,153, filed Sep. 26, 2007, which is a U.S. national stage
application under 35 U.S.C. .sctn.365 of International Application
No. PCT/JP2006/303985, filed Mar. 2, 2006, which claims priority
under 35 U.S.C. .sctn.119 to Japanese Application No. 2005-105827,
filed on Apr. 1, 2005, the disclosures of each of which are hereby
incorporated herein in their entirety by reference.
TECHNICAL FIELD
[0002] The present invention relates to a method of manufacturing a
p-type nitride semiconductor and a semiconductor device fabricated
by the method.
BACKGROUND
[0003] Conventionally, a nitride semiconductor layer fabricated by
a metal organic chemical vapor deposition method (MOCVD method) and
doped with p-type impurities was electrically neutral and could not
obtain a p-type electric property, and had significantly high
resistance. Accordingly, Japanese Patent Laying-Open No. 05-183189
(Patent Document 1) describes that, after the completion of crystal
growth, the nitride semiconductor layer doped with p-type
impurities and having significantly high resistance is subjected to
an annealing treatment in an atmosphere that contains substantially
no hydrogen and at a temperature of at least 400.degree. C., to
thereby eliminate hydrogen from the nitride semiconductor layer, so
that a nitride semiconductor having p-type conductivity and low
resistance can be obtained from a nitride semiconductor having high
resistance.
[0004] Meanwhile, Japanese Patent Laying-Open No. 09-129929 (Patent
Document 2) and Japanese Patent Laying-Open No. 2004-103930 (Patent
Document 3) describe a manufacturing method for obtaining a high
p-type carrier concentration or low resistivity after the
completion of crystal growth even without an annealing treatment
for obtaining a p-type property.
[0005] According to the manufacturing method disclosed in Patent
Document 2, when a p-type cladding layer and a p-type contact layer
are stacked, for example, they are fabricated under the condition
that an inert gas is excessive with respect to hydrogen and a flow
quantity ratio of hydrogen to the inert gas is at least 0.75%, and
after the completion of crystal growth, a substrate is naturally
cooled with a flow quantity of the inert gas increased. Here, in
the stage where the flow quantity of the inert gas is increased, a
flow quantity ratio between the inert gas and ammonia is set to
2:1. According to this document, such a manufacturing method
enables electrical activation of at least 7% of p-type impurities
elements, and obtainment of a significantly high carrier
concentration of at least 2.4.times.10.sup.18 cm.sup.-3.
[0006] Furthermore, according to the manufacturing method disclosed
in Patent Document 3, a GaN-based semiconductor crystal containing
p-type impurities is grown in a crystal growing atmosphere
containing an inert gas at a ratio of at least 50 vol %, and after
the completion of crystal growth, the atmosphere is replaced with a
cooling atmosphere containing ammonia at a ratio of 0.1%-30 vol %
at the crystal growth temperature, and the semiconductor crystal is
cooled in the cooling atmosphere. According to this document, such
a manufacturing method enables a treatment for activating p-type
impurities to be stably performed without a heat treatment after
growth. [0007] Patent Document 1: Japanese Patent Laying-Open No.
05-183189 [0008] Patent Document 2: Japanese Patent Laying-Open No.
09-129929 [0009] Patent Document 3: Japanese Patent Laying-Open No.
2004-103930
PROBLEMS TO BE SOLVED
[0010] However, the p-type nitride semiconductor layer obtained
through an annealing treatment after growth, as disclosed in Patent
Document 1, did not have sufficiently low resistivity. Reduction in
resistivity directly leads to reduction in power consumption, and
hence is significantly important in view of the application to a
nitride semiconductor-based light-emitting element or an electronic
device. As to a nitride semiconductor, a p-type one generally has
resistivity significantly higher than that of an n-type one, so
that further reduction in resistivity of the p-type one has been
sought. Furthermore in Patent Document 1, there also arose a
problem of thermal damage to, and hence degradation of, a nitride
semiconductor layer containing In (e.g. a light-emitting element
including an InGaN active layer), because of the annealing
treatment after crystal growth. In contrast, the manufacturing
methods disclosed in Patent Documents 2 and 3, which require no
annealing treatment step, or if any, only require short annealing
treatment time, are quite beneficial because thermal degradation of
the element in the manufacturing step can be prevented. However,
when the inventors of the present invention conducted a
supplementary test, there was a case where a p-type electric
property was not exhibited after crystal growth, or if a p type was
obtained, resistivity was significantly high (or a p-type carrier
concentration was significantly low), so that the properties as
disclosed in the inventions could not be obtained.
[0011] A first problem to be solved by the present invention is to
provide a method of manufacturing a p-type nitride semiconductor
having low resistivity and exhibiting p-type conductivity with high
reproducibility, without subjecting a nitride semiconductor layer
doped with p-type impurities to an annealing treatment after the
completion of its growth. This manufacturing method requires no
annealing treatment step, or if any, only requires short annealing
treatment time after the completion of crystal growth, so that it
is possible to prevent thermal damage to, and hence degradation of,
a nitride semiconductor layer containing In (e.g. a light-emitting
element containing an InGaN active layer).
[0012] Next, a second problem to be solved by the present invention
is to provide a manufacturing method of forming the p-type nitride
semiconductor according to the present invention and obtained by
solving the first problem above, into a p-type nitride
semiconductor with much lower resistivity. This makes it possible
to further reduce power consumption in a nitride
semiconductor-based light-emitting element or an electronic
device.
SUMMARY OF MEANS FOR SOLVING THE PROBLEMS
[0013] The present invention includes a first step of forming a
nitride semiconductor layer by metal organic chemical vapor
deposition by using a first carrier gas containing a nitrogen gas
(nitrogen carrier gas) and a hydrogen gas (hydrogen carrier gas) of
a flow quantity larger than a flow quantity of the nitrogen gas to
thereby supply a raw material containing magnesium (Mg) and a Group
V raw material containing nitrogen (N); and a second step of
lowering a temperature by using a second carrier gas to which a
material containing nitrogen (N) is added at a concentration of at
least 0.01% and less than 20%, and hence can solve the first
problem thereof. Here, the concentration of at least 0.01% and less
than 20% as to the material containing N was calculated by (a flow
quantity of the material containing N)/(the flow quantity of the
material containing N+a flow quantity of the second carrier
gas).
[0014] It is preferable that ammonia is used at a concentration of
at least 0.01% and at most 5% as the material containing N of the
present invention. The use of the present manufacturing method
enables obtainment of a p-type nitride semiconductor having low
resistivity with high reproducibility without an annealing
treatment for obtaining a p-type property after the completion of
the second step.
[0015] The present invention is characterized in that in the first
carrier gas, a ratio of the hydrogen carrier gas with respect to
the total flow quantity of the nitrogen carrier gas and the
hydrogen carrier gas is more than 70% and at most 95%. Here, the
ratio of the hydrogen carrier gas is calculated by (a flow quantity
of the hydrogen carrier gas)/(a flow quantity of the nitrogen
carrier gas+the flow quantity of the hydrogen carrier gas).
[0016] In the present invention, it is preferable that the total
layer thickness of the nitride semiconductor layer fabricated by
the first step is at least 0.5 .mu.m.
[0017] In the present invention, it is preferable that the total
layer thickness of a layer including a nitride semiconductor with
an aluminum (Al) composition ratio of at least 2%, out of a
plurality of nitride semiconductor layers fabricated by the first
step, is at least 0.3 .mu.m.
[0018] The present invention is characterized in that the raw
material containing magnesium is bis(ethyl cyclopentadienyl)
magnesium.
[0019] The present invention is characterized in that a flow
quantity of the Group V raw material containing N is smaller than
the flow quantity of the hydrogen carrier gas in the first carrier
gas.
[0020] The present invention is characterized in that a flow
quantity of the second carrier gas is larger than the flow quantity
of the nitrogen carrier gas in the first carrier gas.
[0021] In the present invention, it is preferable that the second
carrier gas is argon or nitrogen.
[0022] The second step of the present invention is characterized in
that it is a step of lowering the temperature to 400.degree. C. or
lower, and that time required for reaching 400.degree. C. is at
most 25 minutes.
[0023] The present invention is characterized in that dimethyl
hydrazine is used at a concentration of at least 0.01% and at most
2% as the material containing N. Here, the concentration of
dimethyl hydrazine is calculated by (a flow quantity of dimethyl
hydrazine).times.(a vapor pressure of dimethyl hydrazine)/(an
atmospheric pressure)/(a flow quantity of the second carrier
gas+the flow quantity of dimethyl hydrazine.times.the vapor
pressure of dimethyl hydrazine/the atmospheric pressure).
[0024] The present invention is characterized in that it includes a
third step of subjecting the nitride semiconductor layer to an
annealing treatment after the second step in an inert gas
atmosphere for at least one minute and at most 15 minutes. The
present invention can thereby solve the second problem thereof.
Here, the nitride semiconductor layer after the second step has
already exhibited p-type conductivity after the completion of
crystal growth, owing to the present invention described above.
[0025] The present invention is characterized in that an annealing
temperature is at least 700.degree. C. and at most 900.degree.
C.
[0026] The present invention is characterized in that in the third
step, ammonia is further added at a concentration of at least 0.01%
and at most 5% in addition to the inert gas.
Here, the concentration of ammonia in the third step is calculated
by (a flow quantity of ammonia)/(the flow quantity of ammonia+a
flow quantity of the inert gas).
[0027] The present invention is characterized in that in the third
step, hydrogen is further added at a concentration of at least 10
ppm and at most 500 ppm in addition to the inert gas. Here, the
concentration of hydrogen is calculated by (a flow quantity of
hydrogen)/(the flow quantity of hydrogen+a flow quantity of the
inert gas).
[0028] The present invention relates to a semiconductor device
including a p-type nitride semiconductor layer fabricated by the
method of manufacturing the p-type nitride semiconductor according
to the present invention.
EFFECTS
[0029] By using the manufacturing method according to the present
invention, a nitride semiconductor layer which contains Mg and a
crystal of which is grown by a metal organic chemical vapor
deposition method can be formed into a p-type nitride semiconductor
layer having low resistivity and exhibiting p-type conductivity
with high reproducibility without a heat treatment after the
completion of its growth. For example, if the present manufacturing
method is applied to a nitride semiconductor light-emitting element
including an active layer made of InGaN, the method requires no
annealing treatment step for allowing a nitride semiconductor layer
containing Mg to obtain a p-type property, or if any, only requires
short annealing treatment time, after the completion of crystal
growth. Accordingly, it is possible to prevent thermal damage to,
and hence degradation of, the InGaN active layer due to the
annealing treatment step. As a result, luminous efficiency is
improved and longer lifetime is obtained. Furthermore, a p-type
nitride semiconductor can be obtained with high reproducibility, so
that yield rate in nondefective products is improved as well.
[0030] Furthermore, by applying the annealing treatment according
to the present invention to the p-type nitride semiconductor layer,
resistivity obtained after the completion of growth can further be
lowered. It is thereby possible to further reduce power consumption
in a nitride semiconductor-based light-emitting element or an
electronic device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 provides experimental data showing a relationship
between annealing time and resistivity.
[0032] FIG. 2 provides an example of experimental data showing a
relationship between annealing time and a hole carrier
concentration.
[0033] FIG. 3 shows an example of a nitride semiconductor laser
element.
DESCRIPTION OF THE REFERENCE SIGNS
[0034] 10: sample A, 20: sample B, 100: n-type GaN substrate, 101:
n-type GaN layer, 102: n-type AlGaN cladding layer, 103: n-type GaN
light guide layer, 104: light-emitting layer, 105: intermediate
layer, 106: carrier block layer made of AlGaN, 107: p-type GaN
light guide layer, 108: p-type AlGaN cladding layer, 109: p-type
GaN contact layer, 110: n electrode, 111: p electrode, 112:
SiO.sub.2 dielectric film.
DETAILED DESCRIPTION
First Embodiment
[0035] A first embodiment can mainly solve the first problem.
[0036] Initially, a substrate is placed in a metal organic chemical
vapor deposition device (MOCVD device). Here, the substrate
collectively refers to a base material such as sapphire, GaN,
AlGaN, SiC, Si, or ZrB.sub.2, or the one including a nitride
semiconductor layer, a crystal of which is grown on the base
material. The substrate is heated to a certain temperature. By
using a first carrier gas containing a nitrogen carrier gas and a
hydrogen carrier gas of a flow quantity larger than that of the
nitrogen carrier gas, a Group III raw material, a raw material
containing Mg, and a Group V raw material containing N are supplied
to form a nitride semiconductor layer. The step described above is
referred to as a first step.
[0037] After the first step, a second step of lowering a
temperature of the substrate by using a second carrier gas to which
a material containing N is added at a concentration of at least
0.01% and less than 20% is completed.
[0038] Through these steps, the nitride semiconductor layer ensures
crystallinity of high quality, and exhibits p-type conductivity
with high reproducibility without an annealing step after the
second step, so that it was possible to obtain a nitride
semiconductor layer having low resistivity (see Example 1 described
below).
[0039] Here, the nitride semiconductor layer is specifically GaN,
AlGaN, InAlGaN, InGaN, or the like, containing Mg.
[0040] If the nitride semiconductor layer is GaN containing Mg or
AlGaN containing Mg, a temperature at which such a semiconductor
layer obtains favorable crystallinity is at least 900.degree. C.
and at most 1100.degree. C., more preferably at least 1000.degree.
C. and at most 1080.degree. C. If the nitride semiconductor layer
is InAlGaN containing Mg, a temperature at which it obtains
favorable crystallinity is at least 700.degree. C. and at most
1000.degree. C. If the nitride semiconductor layer is InGaN
containing Mg, a temperature at which it obtains favorable
crystallinity is at least 700.degree. C. and at most 850.degree.
C.
[0041] As the nitrogen carrier gas used in the first step, it is
preferable to use nitrogen having a moisture concentration
controlled to be at most 1 ppb. Although nitrogen is preferable for
its inexpensiveness, it generally has hygroscopicity and hence
contains much moisture, so that it is important to lower the
moisture concentration thereof so as to obtain a nitride
semiconductor having a p-type electric property. As a method for
stably supplying to the MOCVD device the nitrogen having a moisture
concentration of at most 1 ppb, there is used a nitrogen refining
device utilizing an gettering effect. As to the generally-used,
adsorption and regeneration-type nitrogen refining device, moisture
once adsorbed (or impurities such as oxygen) is/are released again
in a regeneration step in an adsorption column, and hence a purity
of nitrogen may become unstable. In contrast, once the nitrogen
refining device utilizing a gettering effect captures moisture (or
impurities such as oxygen), it never releases it/them again, which
enables stable supply of high-purity nitrogen. However, the
gettering-type device can no longer produce the effect above upon
exhaustion of a target, and hence requires high cost. As a method
of arranging the nitrogen refining device in view of stable supply
of high-purity nitrogen, it is preferable to provide in series one
or more nitrogen refining devices utilizing an gettering effect. If
economical efficiency is considered, it is preferable to provide in
series at least two adsorption and regeneration-type nitrogen
refining devices, or provide in series a plurality of adsorption
and regeneration-type nitrogen refining devices and then provide a
nitrogen refining device utilizing an gettering effect. As a method
of arranging a nitrogen refining device that enables stable supply
of high-purity nitrogen and achieves excellent economic efficiency,
there are used two nitrogen refining devices including an
adsorption and regeneration-type nitrogen refining device provided
on a first stage (on a nitrogen supply side), and a nitrogen
refining device utilizing an gettering effect provided on a
subsequent stage (on the MOCVD side).
[0042] The raw material containing Mg specifically refers to
cyclopentadienyl magnesium or bis(ethyl cyclopentadienyl)
magnesium. Out of these raw materials, the use of bis(ethyl
cyclopentadienyl) magnesium was particularly preferable because it
could improve the effect of the present invention. Depending on the
raw materials containing Mg, the way how Mg serving as p-type
impurities is taken in into the nitride semiconductor layer may
subtly differ. Furthermore, bis(ethyl cyclopentadienyl) magnesium
is a liquid raw material, and thus is highly responsive to
variations in amount of the raw material to be supplied, when
compared with cyclopentadienyl magnesium, which is a solid raw
material.
[0043] Furthermore, the present invention uses in the first step
the hydrogen carrier gas of a flow quantity larger than that of the
nitrogen carrier gas, and thus produces an effect of improving
diffusional efficiency of the raw material containing Mg in the
carrier gases. It is considered that improvement in responsiveness
to variations in amount of the raw material to be supplied, and
improvement in diffusional efficiency of the raw material
containing Mg, aid in uniform addition of Mg serving as p-type
impurities into the nitride semiconductor layer, and contribute to
obtainment of p-type nitride semiconductor layer having low
resistivity and exhibiting p-type conductivity with high
reproducibility without a heat treatment after the completion of
crystal growth.
[0044] For the Group III raw material, it is possible to use
tri-methyl gallium, tri-ethyl gallium, tri-methyl aluminum,
tri-ethyl aluminum, tri-methyl indium, tri-ethyl indium, or the
like.
[0045] An amount of the Group V raw material containing N to be
supplied, which is used in the first step of the present invention,
is preferably at least one liter and at most 10 liters per minute,
and the flow quantity of the Group V raw material containing N is
suitably smaller than that of the hydrogen carrier gas. The raw
material containing Mg is more likely to diffuse in hydrogen than
in the Group V raw material containing N, and hence the
above-described value is suitably used as a flow quantity of the
Group V raw material containing N. Here, as the Group V raw
material containing N, it is possible to use ammonia, dimethyl
hydrazine, monomethyl hydrazine, or the like. Improvement in
diffusion of the raw material containing Mg enables uniform
addition of Mg serving as p-type impurities into the nitride
semiconductor layer, and hence formation of a nitride semiconductor
layer of high quality. It is thereby possible to obtain a p-type
nitride semiconductor layer having low resistivity and exhibiting
p-type conductivity with high reproducibility, without a heat
treatment after the completion of crystal growth.
[0046] The nitrogen carrier gas or the hydrogen carrier gas in the
first step also includes a bubbling gas used for vaporizing or
sublimating the Group III raw material to supply the same to the
substrate. For example, if bubbling is performed with hydrogen to
vaporize tri-methyl gallium serving as the Group III raw material,
the flow quantity thereof is added to the flow quantity of the
hydrogen carrier gas. Similarly, if bubbling is performed with
nitrogen, the flow quantity thereof is added to the flow quantity
of the nitrogen carrier gas.
[0047] In the first step, it is preferable that the flow quantity
of the nitrogen carrier gas is at least two liters and at most 10
liters per minute, while the flow quantity of the hydrogen carrier
gas is at least 10 liters and at most 45 liters per minute. In the
present invention, however, the flow quantity of the hydrogen
carrier gas must be larger than that of the nitrogen carrier gas.
This idea is the inverse of that in the conventional technique. The
details thereof will hereinafter be described.
[0048] In the conventional technique, in order to allow a nitride
semiconductor layer having p-type impurities added thereto to have
a p-type electric property after the completion of crystal growth
without an annealing treatment, it was necessary to form such a
nitride semiconductor layer in an atmosphere containing a hydrogen
carrier gas and a nitrogen carrier gas of an amount larger than
that of the hydrogen carrier gas, as disclosed in, for example,
Patent Document 2 or 3. One of the reasons is that failure to
obtain a p-type nitride semiconductor after the completion of
growth is generally considered to be attributable to Mg, which has
been added into the nitride semiconductor, binding to hydrogen and
failing to act as p-type impurities (Mg being deactivated by
hydrogen).
[0049] However, resistivity of the nitride semiconductor layer
fabricated by such a fabricating method was very high after the
completion of crystal growth, and p-type conductivity was not
exhibited, or even if p-type conductivity was exhibited, uniform
resistivity was not obtained in an arbitrary surface of the
substrate, so that a low resistivity part and a high resistivity
part exist therein in a mixed manner. As such, with the
conventional manufacturing method, it was not possible to obtain
with high reproducibility a nitride semiconductor layer exhibiting
a p-type property after the completion of crystal growth.
Furthermore, when a current was injected into an element fabricated
by the conventional manufacturing method, a voltage increased with
elapse of the time, resulting in breakage of multiple elements.
[0050] Accordingly, to examine the causes thereof, a nitride
semiconductor layer having Mg serving as p-type impurities added
thereto was fabricated with a function of x, where x represents a
ratio of the hydrogen carrier gas with respect to the sum of the
nitrogen carrier gas and the hydrogen carrier gas. As a result, an
X-ray analysis revealed that when the nitride semiconductor layer
was formed with x.ltoreq.50%, a tilt component (a component
contributing to an orientation of the crystal) in a direction of
crystal growth of the nitride semiconductor layer and/or a twist
component (a component contributing to an edge dislocation density
of the crystal) in a crystal plane was/were increased, resulting in
deterioration of crystallinity. This tendency became remarkable as
the ratio x of the hydrogen carrier gas was decreased. Furthermore,
a surface morphology of the nitride semiconductor layer also
deteriorated as the ratio x of the hydrogen carrier gas was
decreased. The surface morphology more remarkably deteriorated as a
layer thickness of the nitride semiconductor layer was increased.
For example, a surface morphology started deteriorating
approximately when the layer thickness exceeded 0.3 .mu.m in the
case of GaN having Mg added thereto, and approximately when the
layer thickness exceeded 0.2 .mu.m in the case of AlGaN having Mg
added thereto. In view of the foregoing, it is considered that the
problems in the conventional technique described above are caused
by deterioration in crystal quality of the nitride semiconductor
layer having Mg serving as p-type impurities added thereto. If
there is no need to consider reproducibility or yields, even the
conventional technique described above may be applicable to a
light-emitting diode or the like including a p-type nitride
semiconductor layer having a relatively small thickness. However,
the conventional technique is not applicable to a laser diode that
requires a p-type nitride semiconductor layer of high quality and
having a large thickness (at least 0.3 .mu.M or more).
[0051] As the reason why crystal quality of the nitride
semiconductor layer having Mg added thereto depended on the ratio x
of the hydrogen carrier, the following three points can be
considered. Firstly, decrease in ratio x of the hydrogen carrier
causes reduction in decomposition efficiency of the Group III raw
material and the raw material containing Mg that constitute the
nitride semiconductor layer, and thus causes deterioration in
crystal quality. Secondly, decrease in ratio x of the hydrogen
carrier causes insufficient diffusion of these raw material gases
in the carrier gases (nitrogen has diffusional efficiency lower
than that of hydrogen), resulting in nonuniform distribution of the
raw materials in the substrate. Thirdly, to grow a nitride
semiconductor layer of high quality (particularly when a nitride
semiconductor layer containing Al is to be formed), it is necessary
to grow a crystal at a flow velocity of at least 50 cm/second.
However, the higher flow velocity makes the above-described second
tendency more remarkable.
[0052] To solve these problems, in the first step of the present
invention, the flow quantity of the hydrogen carrier gas is set to
be larger than that of the nitrogen carrier gas. Crystallinity of
the nitride semiconductor layer fabricated as such was
significantly favorable, hardly depending on its layer thickness.
This seems to be because increase in ratio of the hydrogen carrier
gas with respect to (the nitrogen carrier gas+the hydrogen carrier
gas) in the first carrier gas causes improvement in decomposition
efficiency of the raw material gases that constitute the nitride
semiconductor layer, as well as sufficient diffusion of these raw
material gases. Furthermore, even if the crystal was grown at a
flow velocity of at least 50 cm/second, which is required to grow a
crystal of a nitride semiconductor layer of high quality,
nonuniform distribution did not occur in the substrate. As such,
with the use of the first step according to the present invention,
a nitride semiconductor layer of high quality can be formed even if
the total layer thickness of the nitride semiconductor layer is at
least 0.5 .mu.m, or even if the total layer thickness of a layer
including a nitride semiconductor with an Al composition ratio of
at least 2% is at least 0.3 .mu.m. In other words, this means that
a p-type nitride semiconductor layer necessary for fabricating a
laser diode can be provided.
[0053] Here, the ratio of the hydrogen carrier gas in the first
carrier gas preferably falls within a range of higher than 50% and
lower than 100%. As the ratio of the hydrogen carrier gas
increases, crystallinity of the nitride semiconductor layer becomes
favorable. However, remarkable improvement in crystallinity can no
longer be observed approximately when 70% is exceeded, and
crystallinity becomes approximately constant. If the ratio of the
hydrogen carrier gas exceeds 95%, crystal growth is decelerated in
a direction parallel to (in an a-axis direction of) the substrate
surface of the nitride semiconductor, and if 100% is reached, a
minute pit is often generated at a surface of the nitride
semiconductor layer. This is because the hydrogen carrier gas
serves to promote crystal growth in a direction of a
crystallographic axis (in a c-axis direction) of the nitride
semiconductor, while the nitrogen carrier gas serves to promote
crystal growth in an in-plane direction (in the a-axis direction)
of the nitride semiconductor. Deceleration of the growth in the
a-axis direction is not preferable because this generates a minute
pit at a surface of the nitride semiconductor layer, or causes
reduction in mobility in the in-plane direction in the p-type
electric property. Accordingly, the most preferable range for the
ratio of the hydrogen carrier gas in the first carrier gas is
higher than 70% and at most 95%.
[0054] Next, the nitride semiconductor layer of high quality
obtained by the first step of the present invention can be formed
into a p-type nitride semiconductor having low resistivity and
exhibiting p-type conductivity with high reproducibility by the
second step according to the present invention, without an
annealing treatment after the completion of growth. The second step
according to the present invention is characterized in that a
second carrier gas to which a material containing N is added at a
concentration of at least 0.01% and less than 20% is used to lower
a temperature from the temperature at which the crystal growth is
completed. An optimal material as the material containing N, which
is used at a concentration of at least 0.01% and less than 20%, is
ammonia to be used at a concentration of at least 0.01% and at most
5%.
[0055] Here, the temperature at which the crystal growth is
completed is defined as a temperature immediately after the
completion of crystal growth of the last nitride semiconductor
layer, out of a plurality of nitride semiconductor layers, crystal
of which should be grown in the MOCVD device. If the nitride
semiconductor layer formed in the first step corresponds to the
last nitride semiconductor layer described above, a temperature for
initiating the second step corresponds to the temperature
immediately after the completion of crystal growth of the nitride
semiconductor layer in the first step. If a layer is formed by
another step between the first step and the second step, a
temperature immediately after the completion of crystal growth of
the relevant layer corresponds to the temperature at which the
crystal growth is completed.
[0056] To obtain a nitride semiconductor layer with excellent
crystallinity, it is generally necessary to grow a crystal of the
nitride semiconductor at a significantly high growth temperature of
at least 700.degree. C. and at most 1100.degree. C. Furthermore,
nitrogen to be constitute the nitride semiconductor layer has high
volatility, so that intake of nitrogen is easily inhibited (a
nitride semiconductor is decomposed (etched)) if ammonia, which
serves as a nitrogen raw material of the nitride semiconductor, is
not supplied in a range of the high growth temperature described
above. To prevent this, in the conventional practice, a temperature
is lowered after the completion of crystal growth of the nitride
semiconductor layer, with an excessive amount of ammonia supplied.
When a temperature of the nitride semiconductor layer formed by the
first step of the present invention was lowered with the use of a
second carrier gas containing an excessive amount of ammonia at a
concentration of 30% in accordance with the conventional
manufacturing method (here, nitrogen was used as the second carrier
gas), the nitride semiconductor layer achieved a significantly
favorable surface. However, it had significantly high resistance
after the completion of crystal growth, and exhibited no p-type
electric property. According to the detailed test results by the
inventors of the present invention, it was found that if a
temperature was lowered at an ammonia concentration of at least
0.01% and at most 5%, which concentration is much lower than that
in the conventional practice, the nitride semiconductor layer
fabricated in the first step of the present invention exhibited
favorable p-type electric property after the completion of crystal
growth with high reproducibility (the favorable p-type electric
property specifically refers to exhibition of low resistivity or a
high hole carrier concentration) (for the details thereof, see
Example 1 described below). Furthermore, observation at least under
an optical microscope revealed that a surface of the nitride
semiconductor layer was not roughened. An ammonia concentration of
lower than 0.01% was not preferable because the nitride
semiconductor layer had its surface roughened during the
temperature lowering process and exhibited no p-type electric
property. In contrast, if an ammonia concentration was higher than
5%, the nitride semiconductor layer after the completion of crystal
growth had resistivity increased in accordance with the ammonia
concentration, even though it exhibited p-type conductivity. An
ammonia concentration exceeding 10% caused remarkable increase in
resistivity, and an ammonia concentration of at least 20% caused
high resistance, so that resistivity could not be measured by Hall
measurement. Here, the ammonia concentration is a concentration
calculated by (a flow quantity of ammonia)/(the flow quantity of
ammonia+a flow quantity of a second carrier gas). With this
formula, it is possible to calculate a flow quantity at which an
ammonia concentration of at least 0.01% and at most 5% is
achieved.
[0057] For the second carrier gas used in the second step, it is
possible to use nitrogen or argon. If nitrogen is used as the
second carrier gas, it is preferable to use nitrogen having a
moisture concentration of at most 1 ppb. This is because, although
a nitrogen gas is preferable for its inexpensiveness, it generally
has hygroscopicity and contains much moisture. To obtain a nitride
semiconductor with a p-type electric property, it is important to
lower the moisture concentration thereof. A method for obtaining
nitrogen having a moisture concentration of at most 1 ppb is
similar to that in the case of the nitrogen carrier gas described
above. The use of argon as the second carrier gas is preferable for
obtaining a nitride semiconductor with a p-type electric property
because argon has less hygroscopicity.
[0058] In the second step of the present invention, supply of the
Group III raw material may be stopped, or may be continued.
Continuous supply of the Group III raw material makes it possible
to prevent an outermost surface of the nitride semiconductor layer
from being roughened by reevaporation in the temperature lowering
process. An amount of the Group III raw material to be supplied in
the case of continuous supply is preferably set to be, for example,
at most one-fifth of the amount to be supplied when the nitride
semiconductor layer is formed. An amount to be supplied exceeding
one-fifth is not preferable because a droplet-like product is
formed on the nitride semiconductor layer, or a nitride
semiconductor layer of poor quality is stacked. In the present
embodiment described below, the case where supply of the Group III
raw material is stopped is mainly described for simplicity.
[0059] A flow quantity of the second carrier gas in the second step
is preferably larger than that of the nitrogen carrier gas in the
first step. This is because an ammonia concentration in the second
step according to the present invention is a concentration
significantly lower than that in the conventional practice, and
hence it is necessary to lower the temperature as fast as possible
after the first step in order to prevent a surface of the nitride
semiconductor layer from being roughened. Particularly if argon is
used as the second carrier gas, argon has thermal conductivity
lower than that of nitrogen, so that a larger flow quantity is
required than in the case where nitrogen is used.
[0060] In the step of lowering a temperature in the second step,
time required for lowering the temperature to 400.degree. C. is
preferably at most 25 minutes, and more preferably at most 20
minutes. The time required for cooling down to 400.degree. C.
exceeding 25 minutes is not preferable because there arises a
problem such as a roughened surface of the nitride semiconductor
layer, or failure to obtain an ohmic electrode after the completion
of crystal growth (increase in resistivity). Although the lower
limit value of the cooling time is not particularly set, it is
preferably at least three minutes, and more preferably at least
five minutes, because the nitride semiconductor layer more easily
obtains p-type electric property after the completion of crystal
growth when being exposed to the ammonia atmosphere for a certain
amount of time.
[0061] In the step of lowering a temperature in the second step,
after the temperature is lowered to 600.degree. C., supply of
ammonia may be stopped, or ammonia may be allowed to flow
continuously until a room temperature is reached. If ammonia is
allowed to flow until 600.degree. C. is reached, p-type
conductivity can be exhibited in low resistivity with high
reproducibility after the completion of crystal growth.
Furthermore, supply of ammonia until 400.degree. C. is reached is
preferable because a surface of the nitride semiconductor layer is
hardly roughened even if supply of ammonia is subsequently stopped
and the second carrier gas is exclusively supplied, and even if at
least 30 minutes elapse for reaching an allowable temperature for
substrate transfer (approximately 180.degree. C.) from 400.degree.
C.
[0062] In the second step, it is also possible to use dimethyl
hydrazine or monomethyl hydrazine at a concentration of at least
0.01% and at most 2% as the material containing N. In the case of
dimethyl hydrazine, for example, the concentration is calculated by
(a flow quantity of dimethyl hydrazine).times.(a vapor pressure of
dimethyl hydrazine)/(an atmospheric pressure)/(a flow quantity of
the second carrier gas+the flow quantity of dimethyl
hydrazine.times.the vapor pressure of dimethyl hydrazine/the
atmospheric pressure). Dimethyl hydrazine or monomethyl hydrazine
is a reducing agent more effective than ammonia in reduction, so
that it enables obtainment of a nitride semiconductor layer
exhibiting p-type conductivity after the completion of crystal
growth more effectively when compared with ammonia. If dimethyl
hydrazine or monomethyl hydrazine is used as the material
containing N in the second step, cooling down to 600.degree. C. is
preferably performed within 18 minutes.
[0063] A p electrode suitable for the p-type nitride semiconductor
obtained by the present invention contains at least palladium or
platinum. The use of a p electrode containing such a material
enables further reduction in contact resistance and further
reduction in power consumption in a nitride semiconductor-based
light-emitting element or an electronic device.
[0064] <A Mode for Obtaining with Higher Reproducibility a
P-Type Nitride Semiconductor Having Low Resistivity after the
Second Step>
[0065] To obtain a nitride semiconductor layer exhibiting a p-type
electric property after the completion of growth as in the present
invention, it is preferable to lower a crystal defect density in
the nitride semiconductor layer. This is because the crystal defect
inhibits activation of Mg and adversely affects a p-type electric
property. The experimental results obtained by the inventors of the
present invention revealed that if a nitride semiconductor layer
having p-type impurities added thereto as well as a nitride
semiconductor layer serving as an underlying layer thereof failed
to have favorable crystallinity, it was difficult to obtain much
higher reproducibility and sufficient resistivity in the nitride
semiconductor layer after the completion of growth. If a
light-emitting element is to be fabricated, an underlying layer of
the p-type nitride semiconductor layer is usually an n-type
semiconductor layer having n-type impurities added thereto. Here,
the n-type semiconductor layer refers to an n-type GaN layer, an
n-type AlGaN layer, an n-type InAlGaN layer, an n-type InGaN, or
the like.
[0066] If the nitride semiconductor layer having n-type impurities
added thereto has poor crystallinity, a nitride semiconductor layer
formed thereon is also affected thereby. To prevent this, accurate
control of a concentration of the n-type impurities was found to be
important. More specifically, if Si is used as the n-type
impurities, an Si concentration in the n-type nitride semiconductor
is preferably at most 7.times.10.sup.17 cm.sup.-3, and more
preferably at most 5.times.10.sup.17 cm.sup.-3. Furthermore, it is
preferable that the n-type nitride semiconductor layer having such
an Si concentration accounts for at least 50% of the plurality of
n-type semiconductor layers. This lowers a crystal defect density
in the n-type semiconductor layers, and consequently, also lowers a
crystal defect density in the nitride semiconductor layers, so that
it is possible to obtain with much higher reproducibility a p-type
nitride semiconductor having low resistivity after the completion
of crystal growth.
Second Embodiment
[0067] A second embodiment of the present invention can mainly
solve the second problem.
[0068] After the first and second steps according to the present
invention, the nitride semiconductor layer having a p-type electric
property was subjected to an annealing treatment in an inert gas
for at least one minute and at most 15 minutes. This is the third
step according to the present invention. By adding the third step
to the first and second steps according to the present invention,
resistivity of the nitride semiconductor layer is further
reduced.
[0069] Most important effects obtained in the second embodiment of
the present invention (the effects found in Example 2 described
below) will be described in advance, and then various modes of the
third step will be described, and the effects of the present
invention will be described in detail, illustrating specific
experimental data in Example 2.
[0070] Clearly, resistivity of a nitride semiconductor layer
obtained by subjecting the nitride semiconductor layer, which has
been obtained by the first and second steps according to the
present invention, to the annealing treatment in the third step,
was lower than that of a nitride semiconductor obtained by
subjecting the nitride semiconductor, which had significantly high
resistance and failed to exhibit p-type conductivity after the
completion of crystal growth (which was subjected only to the first
step), to the annealing treatment in the same third step.
Furthermore, however long the latter sample was subjected to the
annealing treatment, it could not obtain resistivity comparable to
that of the former sample. Moreover, when the annealing treatment
in the third step was performed to measure annealing time when
resistivity of each of the samples became approximately constant,
the former sample required shorter time than the latter sample.
This means that the use of the method of manufacturing the p-type
nitride semiconductor according to the present invention only
requires shorter annealing time even if the annealing treatment
(the third step) is performed. This is convenient for a
light-emitting element that essentially requires an active layer
containing In because it is possible to prevent thermal damage to,
and hence degradation of, the active layer containing In due to the
annealing treatment step. Consequently, improvement in luminous
efficiency and increased lifetime of the light-emitting element can
be expected.
[0071] After the second step of the present invention, the
substrate may be removed from the MOCVD device and subjected to the
annealing treatment in the third step, or alternatively, the
substrate may be subjected to the annealing treatment in the third
step still in the MOCVD device. If the substrate is removed from
the MOCVD device and subjected to the annealing treatment in the
third step, an RTA (a Rapid Thermal Annealing) device can be used
as the annealing treatment device. The RTA device is preferably
used because it quickly raises and lowers the temperature, causing
less thermal damage to the nitride semiconductor layer. The RTA
device is preferably used when several nitride semiconductor layers
stacked on the substrate include an active layer made of InGaN or
InAlGaN, for example, because such an active layer is particularly
vulnerable to thermal damage caused by annealing.
[0072] Here, for the inert gas in the third step, nitrogen or argon
can be used. Argon has thermal conductivity lower than that of
nitrogen, and hence heat is less likely to be drawn by the inert
gas existing on the periphery of the substrate, so that improvement
in heat distribution in the substrate makes it possible to achieve
uniform distribution of a p-type electric property in a
surface.
[0073] An amount of argon to be supplied per minute is preferably
at least one liter and at most 10 liters. Argon has low thermal
conductivity, and hence after the completion of the annealing
treatment at a desired annealing temperature, cooling is preferably
performed by allowing argon to flow at a flow quantity larger than
that in the annealing treatment. For example, there is adopted a
flow quantity approximately 1.2 times to two times as large as that
of argon supplied in the annealing treatment. This makes it
possible to suppress thermal damage to the nitride semiconductor
layer (particularly a nitride semiconductor layer containing In) in
the temperature lowering process after the completion of the
annealing treatment. It is more preferable that after the
completion of the annealing treatment, argon is substituted by
nitrogen to perform cooling with nitrogen, which has higher thermal
conductivity than argon. For the argon, argon having a purity of
99.9999% to 99.99% can be used. The use of argon having a purity of
99.99% exhibited slightly greater improvement in p-type electric
property.
[0074] The annealing temperature in the third step according to the
present invention is preferably at least 700.degree. C. and at most
900.degree. C. An annealing temperature lower than 700.degree. C.
and an annealing temperature exceeding 900.degree. C. are not
preferable because the former provides almost no improvement in
electric property by the annealing treatment, while the latter
tends to damage the nitride semiconductor. The more preferable
annealing temperature is at least 800.degree. C. and at most
850.degree. C.
[0075] Annealing time in the third step is preferably at least one
minute and at most 15 minutes, and more preferably at least three
minutes and at most 10 minutes. Annealing time exceeding 15 minutes
is not preferable because the nitride semiconductor layer is
thermally damaged and its p-type electric property tends to
deteriorate. Annealing time less than 1 minute is not preferable
because an annealing treatment cannot sufficiently be performed, so
that the p-type electric property obtained thereby remains almost
the same as that after the second step, and the effects of the
third step cannot be observed. If the RTA device is selected as the
annealing treatment device, the annealing time is set such that the
total time required for reaching a desired annealing temperature,
namely, at least 700.degree. C. and at most 900.degree. C., more
preferably at least 800.degree. C. and at most 850.degree. C., is
at least one minute and at most 15 minutes. For example, if a step
of annealing at a desired annealing temperature only for two
minutes and then performing cooling for three minutes (the cooling
herein refers to lowering of the temperature to 400.degree. C. or
lower) is repeated five times, the total annealing time is two
minutes.times.five times=10 minutes in total. By repeating such a
thermal cycle, it is possible to further prevent thermal damage to
the nitride semiconductor layer (particularly a nitride
semiconductor layer containing In) than in continuous
annealing.
[0076] As a more preferable mode in the third step according to the
present invention, ammonia is further added at a concentration of
at least 0.01% and at most 5% in addition to the inert gas. This
improves a p-type electric property. Other effects are similar to
those described above. At that time, the annealing temperature may
be set to at least 700.degree. C. and at most 920.degree. C., more
preferably at least 800.degree. C. and at most 900.degree. C., and
further preferably at least 810.degree. C. and at most 880.degree.
C. This is because if ammonia is added in addition to the inert
gas, the upper limit value of the temperature at which the nitride
semiconductor is thermally damaged is raised, so that the annealing
temperature can be set higher. However, the annealing treatment at
a temperature of at least 850.degree. C. tends to cause thermal
damage to the nitride semiconductor layer (particularly a nitride
semiconductor layer containing In) even though ammonia is added,
and hence the annealing time is preferably at least one minute and
at most 10 minutes.
[0077] As a more preferable mode in the third step according to the
present invention, hydrogen is further added at a concentration of
at least 10 ppm and at most 500 ppm in addition to the inert gas.
This slightly improves a p-type electric property according to the
present invention. Other effects are similar to those described
above. The annealing temperature at that time is at least
700.degree. C. and at most 850.degree. C., more preferably at least
800.degree. C. and at most 850.degree. C. However, annealing in a
hydrogen-containing atmosphere for long time results in the nitride
semiconductor layer being etched and warped, and hence
deterioration in p-type electric property. Therefore, the annealing
time is required to be controlled accurately. The annealing time in
the case where hydrogen is further added in addition to the inert
gas is preferably at least one minute and at most 10 minutes, and
preferably at least three minutes and at most seven minutes.
Furthermore, the RTA device is desirably used as the annealing
device.
Example 1
[0078] A detailed example of the first embodiment will hereinafter
be described.
[0079] A sample A in the present example was made of a C-plane
sapphire substrate (corresponds to the substrate in the present
application), an undoped GaN layer of approximately 3 .mu.m
provided on the sapphire substrate, and a GaN layer doped with Mg
and of approximately 0.6 .mu.m (which corresponds to the nitride
semiconductor layer formed in the first step) provided on the
undoped GaN layer. A method of manufacturing the Mg-doped GaN layer
by the first step of the present invention included the steps of
growing the Mg-doped GaN layer by supplying a hydrogen carrier gas
and a nitrogen carrier gas at 25 liters per minute and at 6.5
liters per minute, respectively, at a growth temperature of
1030.degree. C., and supplying tri-methyl gallium as a Group III
raw material at 40 cc per minute, bis(ethyl cyclopentadienyl)
magnesium as a raw material containing Mg at 60 cc per minute, and
ammonia as a Group V raw material containing N at a concentration
of at least 0.01% and less than 20% at six liters per minute. As
such, the first step according to the present invention was
completed.
[0080] Successively, in the second step according to the present
invention, supply of the hydrogen carrier gas, the tri-methyl
gallium, and the bis(ethyl cyclopentadienyl) magnesium was stopped
and heating of the substrate was stopped, and nitrogen was supplied
as the second carrier gas at 14.7 liters per minute, while the flow
quantity of ammonia was lowered to 0.3 liter per minute. Here, the
ammonia concentration was 2%. Cooling down from 1030.degree. C. to
400.degree. C. was performed for approximately 16 minutes, and the
second step was completed. Cooling down from 400.degree. C. to
normal temperature was performed by stopping the supply of ammonia
and further increasing the flow rate of the second carrier gas to
15 liters per minute.
[0081] Furthermore, a sample B was fabricated as a comparative
example. Here, sample B was fabricated by the same fabricating
method as that of the sample A, except that an ammonia
concentration in the second step was 30%. More specifically, a
temperature of sample B was lowered after the completion of the
first step of the present invention, while nitrogen and ammonia
were supplied as the second carrier gas at 10.5 liters per minute
and at 4.5 liters per minute, respectively.
[0082] Next, electric properties of sample A and sample B were
compared with each other. Hall measurement was used as a method of
evaluating the electric properties of these samples. The results
showed that sample A exhibited a p-type electric property after the
second step without being subjected to an annealing treatment. For
specific electric property values, resistivity was approximately
3.8 .OMEGA.cm, and a hole carrier concentration was
2.4.times.10.sup.17 cm.sup.3. In contrast, sample B had
significantly high resistance after the second step, and hence its
electric property (resistivity and a hole carrier concentration)
could not be evaluated by Hall measurement. Accordingly,
measurement of resistivity of sample B was attempted by means of a
measurement device utilizing a four-probe method, with which
resistivity of up to 12 k.OMEGA.cm can be measured. However, owing
to its significantly high resistance, resistivity could not be
measured. In view of a measurement limitation of the measurement
device, it was found that sample B had resistivity higher than at
least 12 k.OMEGA.cm.
Example 2
[0083] A detailed example of the second embodiment will hereinafter
be described.
[0084] Samples A and B, which were fabricated by the method of
Example 1, were further subjected to the third step according to
the present invention. Here, nitrogen was used as an inert gas in
the third step, and an annealing temperature of 800.degree. C. was
used.
[0085] The results are shown in FIGS. 1 and 2. An axis of abscissas
in each of FIGS. 1 and 2 represents annealing time, and an axis of
ordinates in FIG. 1 represents resistivity, while an axis of
ordinates in FIG. 2 represents a hole carrier concentration. In
both of FIGS. 1 and 2, sample A of the present application is
denoted as 10, while sample B as a comparative example is denoted
as 20. Here, an electric property at annealing time of 0 minute in
these drawings refers to the one obtained immediately after these
samples were removed from the MOCVD device (after the completion of
crystal growth). The electric property after the completion of
crystal growth is as shown in Example 1.
[0086] FIGS. 1 and 2 show that sample A has resistivity lower than
that of sample B (lower by approximately at least 20% with respect
to sample B as a comparative example), and also has a higher hole
carrier concentration. Here, the following points should be noted.
Firstly, sample B, which had high resistance as an electric
property obtained immediately after it was removed from the MOCVD
device and failed to exhibit a p-type electric property, could not
achieve an electric property comparable to that obtained by sample
A, for whatever long annealing time sample B was subjected to the
subsequent annealing treatment. With reference to FIG. 1, when
annealing time is increased, more specifically, approximately when
10 minutes elapse, gradual increase in resistivity starts. When
annealing time exceeds 15 minutes and is further increased, the
nitride semiconductor layer is thermally damaged, resulting in
further deterioration in resistivity. In view of this, it was
determined that for whatever long annealing time sample B was
subjected to the annealing treatment, it was not comparable to
sample A. Secondly, sample B achieved approximately constant
resistivity at annealing time of approximately seven minutes,
whereas sample A achieved approximately constant resistivity at
annealing time of approximately three minutes. In other words, it
was found that sample A, which was fabricated by the manufacturing
method of the present invention, had resistivity lower (a carrier
concentration higher) than that of sample B as a comparative
example, and only required short time for the annealing treatment.
The latter feature, in particular, is convenient to a
light-emitting element essentially requiring an active layer
containing In such as InGaN, because the active layer containing In
is vulnerable to thermal damage. As such, the use of the method of
manufacturing the p-type nitride semiconductor according to the
present invention only requires short annealing time for obtaining
low resistivity, so that it is possible to reduce thermal damage to
the nitride semiconductor layer.
[0087] It is not known why such features are exhibited. It is
estimated that, since even the same annealing step causes a
significant difference in p-type electric property, sample A (which
exhibited p-type electric property after the completion of crystal
growth) obtains lower resistance in a p type in a mechanism
different from that of sample B (which had significantly high
resistance and failed to exhibit a p-type electric property after
the completion of crystal growth).
Example 3
[0088] A nitride semiconductor laser element in FIG. 3 includes a
(0001)-plane n-type GaN substrate 100, an n-type GaN layer 101, an
n-type AlGaN cladding layer 102, an n-type GaN light guide layer
103, a light-emitting layer 104, an intermediate layer 105, a
carrier block layer 106 made of p-type AlGaN, a p-type GaN light
guide layer 107, a p-type AlGaN cladding layer 108, a p-type GaN
contact layer 109, an n electrode 110, a p electrode 111, and an
SiO.sub.2 dielectric film 112. Here, n-type AlGaN cladding layer
102 is configured with a first n-type AlGaN cladding layer 102a, a
second n-type AlGaN cladding layer 102b, and a third n-type AlGaN
cladding layer 102c.
[0089] Initially, an underlying layer of 1 .mu.m as n-type GaN
layer 101 was formed on n-type GaN substrate 100 at a growth
temperature of 1050.degree. C. by using the MOCVD device and
applying ammonia as the Group V raw material, tri-methyl gallium as
the Group III raw material, and SiH.sub.4. The n-type GaN layer was
stacked in order to improve surface morphology of the n-type GaN
substrate, and relieve stress strain caused by grinding, which
remains in the surface of the GaN substrate, to thereby form an
outermost surface suitable for epitaxial growth.
[0090] Next, tri-methyl aluminum was used as the Group III raw
material to grow n-type AlGaN cladding layer 102 made of first
n-type Al.sub.0.062Ga.sub.0.938N cladding layer 102a having a
thickness of 2.3 .mu.M (an Si impurities concentration of
5.times.10.sup.17/cm.sup.3), second n-type Al.sub.0.10Ga.sub.0.90N
cladding layer 102b having a thickness of 0.15 .mu.m (an Si
impurities concentration of 5.times.10.sup.17/cm.sup.3), and third
n-type Al.sub.0.062Ga.sub.0.938N cladding layer 102c having a
thickness of 0.1 .mu.m (an Si impurities concentration of
5.times.10.sup.17/cm.sup.3). Successively, n-type GaN light guide
layer 103 having a thickness of 0.1 .mu.m (an Si impurities
concentration of 3.times.10.sup.17/cm.sup.3) was grown.
Subsequently, a temperature of the substrate was lowered to
750.degree. C. to form light-emitting layer 104 having a
three-cycle multiple quantum well structure. Here, light-emitting
layer 104 used in the present embodiment was grown by forming an
undoped In.sub.0.003Ga.sub.0.997N barrier layer having a thickness
of 20 nm, an undoped In.sub.0.09Ga.sub.0.91N well layer having a
thickness of 4 nm, an undoped In.sub.0.003Ga.sub.0.997N barrier
layer having a thickness of 8 nm, an undoped
In.sub.0.09Ga.sub.0.91N well layer having a thickness of 4 nm, an
undoped In.sub.0.003Ga.sub.0.997N barrier layer having a thickness
of 8 nm, and an undoped In.sub.0.09Ga.sub.0.91N well layer having a
thickness of 4 nm, in this order.
[0091] Next, intermediate layer 105 (a layer thickness of 70 nm)
was grown. Intermediate layer 105 refers to a layer in which an
undoped In.sub.0.003Ga.sub.0.997N layer having a thickness of 20
nm, an Si-doped GaN layer having a thickness of 10 nm (an Si
impurities concentration of approximately
7.times.10.sup.17/cm.sup.3), and an undoped GaN layer having a
thickness of 40 nm were grown in this order.
[0092] Here, the n-type semiconductor layers doped at an Si
concentration of at most 7.times.10.sup.17 cm.sup.-3 was designed
to account for at least 50% of the n-type semiconductor layers.
[0093] Next, the temperature of the substrate was raised again to
1050.degree. C. to successively grow p-type carrier block layer 106
made of AlGaN and having a thickness of 20 nm, p-type GaN light
guide layer 107 of 20 nm, p-type AlGaN cladding layer 108 of 0.5
.mu.m, and p-type GaN contact layer 109 of 0.1 .mu.m. Here, the
composition ratio of Al in p-type carrier block layer 106 made of
AlGaN was 30%, while the composition ratio of Al in p-type AlGaN
cladding layer 108 was 5.5%. Furthermore, Mg (bis(ethyl
cyclopentadienyl) magnesium) was used as the p-type impurities.
[0094] The following conditions were used for the gas for each
layer having the p-type impurities added thereto. P-type carrier
block layer 106 made of AlGaN required a nitrogen carrier gas at
19.9 liters/minute, a hydrogen carrier gas at 0.6 liter/minute, and
ammonia at 9.5 liters/minute. Each of p-type GaN light guide layer
107, p-type AlGaN cladding layer 108, and p-type GaN contact layer
109 required a nitrogen carrier gas at 8.5 liters/minute, a
hydrogen carrier gas at 35.4 liters/minute, and ammonia at five
liters/minute, at a flow velocity of 108 cm/second. Accordingly,
p-type GaN light guide layer 107, p-type AlGaN cladding layer 108,
and p-type GaN contact layer 109 correspond to the nitride
semiconductor layer formed by the first step of the present
application. Accordingly, the total layer thickness of the nitride
semiconductor layers formed in the first step was 0.62 and the
total layer thickness of a layer including a nitride semiconductor
having at least an Al composition ratio of at least 2%, out of
these nitride semiconductor layers, was 0.5 .mu.m.
[0095] In addition to a function as a carrier block layer, p-type
carrier block layer 106 made of AlGaN also has a function of
preventing In in the light-emitting layer made of InGaN from
evaporating during growth. Accordingly, in order to utilize the
latter function rather than the effects of the present application,
the ratio of the nitrogen carrier gas was made to be higher than
that of the hydrogen carrier gas.
[0096] Successively, after p-type GaN contact layer 109 was grown,
the flow quantities of the nitrogen carrier gas, the hydrogen
carrier gas, and ammonia were switched to 18.9 liters/minute, 0,
and 0.6 liter/minute, respectively, and the temperature was lowered
from 1050.degree. C. to 400.degree. C. for 18 minutes. The epi
substrate was then removed from the MOCVD device and subjected to
an annealing treatment for annealing time of five minutes, at an
annealing temperature of 820.degree. C., in an atmosphere of argon
having a purity of 99.99%.
[0097] N electrode 110 was formed at a back surface of the epi
substrate in an order of Hf/Al. Au was then evaporated onto n
electrode 110 as an n-type electrode pad. Instead of the n
electrode material described above, Ti/Al, Ti/Mo, Hf/Au or the like
may also be used.
[0098] The p electrode portion was etched in a stripe-like manner
to form a ridge stripe portion. The width of the ridge stripe
portion was 1.6 .mu.m. Subsequently, SiO.sub.2 dielectric film 112
was evaporated thereon to achieve a thickness of 200 nm, and p-type
GaN contact layer 109 was exposed, and p electrode 111 was formed
by evaporating Pd (15 nm)/Mo (15 nm)/Au (200 nm) thereon in this
order. Although Pd was used in the present example, Pt may also be
used instead thereof.
[0099] The composition ratio of Al in p-type carrier block layer
106 made of AlGaN as described above was 30%. However, it can be
adjustable within a range of at least 10% and at most 35%.
[0100] Each of the n-type AlGaN cladding layer and the p-type AlGaN
cladding layer described above may adopt another Al composition
ratio. Instead of p-type AlGaN cladding layer 108, it may be
possible to use a superlattice made of Mg-doped GaN/Mg-doped
AlGaN.
Example 4
[0101] The present example is similar to Example 3, except that a
p-type InGaN contact layer was used instead of p-type GaN contact
layer 109 in Example 3.
[0102] The p-type InGaN contact layer was formed on p-type AlGaN
cladding layer 108 to have a layer thickness of 0.05 .mu.m.
Although the present example adopted an In composition ratio of 3%
in the p-type InGaN contact layer, the p-type InGaN contact layer
may also be fabricated at another In composition ratio. The
conditions for gases forming the p-type InGaN contact layer were a
nitrogen carrier gas at 8.5 liters/minute, no hydrogen carrier gas,
and ammonia at 3.5 liters/minute. P-type AlGaN cladding layer 108
was grown at 1050.degree. C., while the p-type InGaN contact layer
was grown at 770.degree. C. Accordingly, p-type GaN light guide
layer 107 and p-type AlGaN cladding layer 108 correspond to the
nitride semiconductor layers formed in the first step of the
present invention. The total layer thickness of the layers
corresponding to the nitride semiconductor layers formed in the
first step of the present invention was 0.52 .mu.m, and the total
layer thickness of a layer including a nitride semiconductor having
at least an Al composition ratio of at least 2%, out of the nitride
semiconductor layers, was 0.5 .mu.m.
[0103] Successively, after the p-type InGaN contact layer was
grown, the flow quantities of the nitrogen carrier gas, the
hydrogen carrier gas, and ammonia were switched to 19
liters/minute, 0, and one liter/minute, respectively, and the
temperature was lowered from 770.degree. C. to 400.degree. C. for 8
minutes. Here, a temperature at which the second step of the
present application is to be initiated was identified as a
temperature immediately before the completion of crystal growth,
namely, 770.degree. C. Next, the epi substrate was removed from the
MOCVD device and subjected to an annealing treatment for annealing
time of 10 minutes, at an annealing temperature of 810.degree. C.,
in a nitrogen atmosphere. The subsequent manufacturing method is
similar to that of Example 3.
Example 5
[0104] There was examined a difference in resistivity of a p-type
nitride semiconductor after the completion of the growth according
to the present invention (after the second step) between the case
where bis(ethyl cyclopentadienyl) magnesium was used, and the case
where biscyclopentadienyl magnesium was used, as the raw material
containing Mg in the first step in the first embodiment according
to the present invention. Here, the sample used for the examination
was GaN having Mg added thereto. As a result, in the case of bis
(ethyl cyclopentadienyl) magnesium, resistivity after the
completion of growth was approximately 3-6 .OMEGA.cm. In contrast,
in the case of biscyclopentadienyl magnesium, resistivity was
approximately 6-8 .OMEGA.cm. In view of these results, bis (ethyl
cyclopentadienyl) magnesium is superior to biscyclopentadienyl
magnesium in resistivity after the completion of growth.
Example 6
[0105] An annealing treatment was performed by further adding
methane to the inert gas in the third step in the second embodiment
of the present invention. A sample used in the present example was
GaN having Mg added thereto. The GaN having Mg added thereto was
fabricated by the method of the first embodiment. Here,
biscyclopentadienyl magnesium was used as the raw material
containing Mg in the first step. In the second step, a temperature
was lowered to 400.degree. C. while ammonia was allowed to flow at
a concentration of 0.5%. Resistivity after the completion of growth
(after the second step) was approximately 6 .OMEGA.cm, and p-type
conductivity was exhibited.
[0106] Next, in the third step, annealing was performed in an
atmospheric gas containing argon and methane at 800.degree. C. for
15 minutes. Here, a methane concentration in argon was 10 ppm.
After the annealing, a treatment with buffered hydrofluoric acid
was performed for one minute, to form a p electrode containing Pd
at a surface of the GaN sample having Mg added thereto. At that
time, resistivity was approximately 2 .OMEGA.cm.
[0107] In the present example, the reason why methane was further
added to the inert gas in the third step to perform annealing was
to remove oxygen attached to the surface of the sample (remove an
oxide film). This reduces abnormal contact resistance and improves
yields. Although a methane concentration in argon was set to be 10
ppm in the present example, a concentration of at least 1 ppm and
at most 100 ppm can produce similar effects. Although methane was
added to argon in the present example, addition of methane to
nitrogen also produces similar effects.
[0108] (As to a Semiconductor Device Utilizing the Manufacturing
Method According to the Present Invention)
[0109] With the use of the method of manufacturing the p-type
nitride semiconductor according to the present invention, it is
possible to fabricate a semiconductor device including a p-type
nitride semiconductor layer. This makes it possible to further
reduce power consumption in the semiconductor device when compared
with the conventional one, and to implement downsizing of the
semiconductor device (provision for mobility) and driving for a
long period of time. For example, it can preferably be used for a
semiconductor device such as a nitride semiconductor laser, a
nitride semiconductor light-emitting diode, a nitride semiconductor
electronic device (nitride semiconductor-based transistor), an
optical pickup device, a magnetooptical reproducing and recording
device, a high-density recording and reproducing device, a laser
printer, a bar code reader, a projector, or a white LED light
source.
[0110] It should be understood that the embodiments disclosed
herein are illustrative and not limitative in all aspects. The
scope of the present invention is shown not by the description
above but by the scope of the claims, and is intended to include
all modifications within the equivalent meaning and scope of the
claims.
* * * * *