U.S. patent application number 15/991317 was filed with the patent office on 2019-11-28 for group 13 element nitride crystal substrate and function element.
The applicant listed for this patent is NGK INSULATORS, LTD.. Invention is credited to Makoto IWAI, Takashi YOSHINO.
Application Number | 20190360119 15/991317 |
Document ID | / |
Family ID | 59012957 |
Filed Date | 2019-11-28 |
![](/patent/app/20190360119/US20190360119A9-20191128-D00000.png)
![](/patent/app/20190360119/US20190360119A9-20191128-D00001.png)
![](/patent/app/20190360119/US20190360119A9-20191128-D00002.png)
![](/patent/app/20190360119/US20190360119A9-20191128-D00003.png)
![](/patent/app/20190360119/US20190360119A9-20191128-D00004.png)
United States Patent
Application |
20190360119 |
Kind Code |
A9 |
IWAI; Makoto ; et
al. |
November 28, 2019 |
GROUP 13 ELEMENT NITRIDE CRYSTAL SUBSTRATE AND FUNCTION ELEMENT
Abstract
A crystal substrate 1 includes an underlying layer 2 and a thick
film 3. The underlying layer 2 is composed of a crystal of a
nitride of a group 13 element and includes a first main face 2a and
a second main face 2b. The thick film 3 is composed of a crystal of
a nitride of a group 13 element and provided over the first main
face of the underlying layer. The underlying layer 2 includes a low
carrier concentration region 5 and a high carrier concentration
region 4 both extending between the first main face 2a and the
second main face 2b. The low carrier concentration region 5 has a
carrier concentration of 10.sup.17/cm.sup.3 or lower and a defect
density of 10.sup.7/cm.sup.2 or lower. The high carrier
concentration region 4 has a carrier concentration of
10.sup.19/cm.sup.3 or higher and a defect density of
10.sup.8/cm.sup.2 or higher. The thick film 3 has a carrier
concentration of 10.sup.18/cm.sup.3 or higher and
10.sup.19/cm.sup.3 or lower and a defect density of
10.sup.7/cm.sup.2 or lower.
Inventors: |
IWAI; Makoto; (Kasugai-city,
JP) ; YOSHINO; Takashi; (Ama-city, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGK INSULATORS, LTD. |
Nagoya-city |
|
JP |
|
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20180274128 A1 |
September 27, 2018 |
|
|
Family ID: |
59012957 |
Appl. No.: |
15/991317 |
Filed: |
May 29, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2016/075175 |
Aug 29, 2016 |
|
|
|
15991317 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/0262 20130101;
H01L 21/0242 20130101; C30B 19/02 20130101; H01L 21/02458 20130101;
H01L 21/208 20130101; C30B 29/38 20130101; C30B 19/12 20130101;
H01L 21/02505 20130101; C30B 25/183 20130101; H01L 33/0075
20130101; H01L 21/02573 20130101; H01L 21/02494 20130101; H01L
21/205 20130101; H01L 21/0254 20130101; H01L 33/32 20130101; C30B
29/406 20130101 |
International
Class: |
C30B 29/38 20060101
C30B029/38; H01L 21/205 20060101 H01L021/205; H01L 21/208 20060101
H01L021/208; H01L 33/32 20060101 H01L033/32; C30B 29/40 20060101
C30B029/40; H01L 21/02 20060101 H01L021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 11, 2015 |
JP |
2015-242068 |
Claims
1. A group 13 nitride crystal substrate comprising an underlying
layer and a thick layer: said underlying layer comprising a crystal
of a nitride of a group 13 element and having a first main face and
a second main face; said thick film comprising a crystal of a
nitride of a group 13 element and provided over said first main
face of said underlying layer; wherein said underlying layer
comprises a low carrier concentration region and a high carrier
concentration region both extending between said first main face
and said second main face; wherein said low carrier concentration
region has a carrier concentration of 1.times.10.sup.16/cm.sup.3 or
higher and 10.sup.17/cm.sup.3 or lower; wherein said low carrier
concentration region has a defect density of
2.times.10.sup.6/cm.sup.2 or higher and 10.sup.7/cm.sup.2 or lower;
wherein said high carrier concentration region has a carrier
concentration of 10.sup.19/cm.sup.3 or higher and
5.times.10.sup.19/cm.sup.3 or lower; wherein said high carrier
concentration region has a defect density of 10.sup.8/cm.sup.2 or
higher and 5.times.10.sup.8/cm.sup.2 or lower; wherein said thick
film has a carrier concentration of 10.sup.18/cm.sup.3 or higher
and 10.sup.19/cm.sup.3 or lower; and wherein said thick film has a
defect density of 10.sup.7/cm.sup.2 or lower.
2. The crystal substrate of claim 1, wherein a defect density at a
surface of said thick film is 10.sup.7/cm.sup.2 or lower.
3. The crystal substrate of claim 1, wherein said thick film has a
thickness of 1 .mu.m or larger and wherein said underlying layer
has a thickness of 50 .mu.m or larger and 200 .mu.m or smaller.
4. The crystal substrate of claim 1, wherein said thick film is
formed by a flux method or a vapor phase method.
5. The crystal substrate of claim 1, wherein a surface of said
thick film comprises a polished surface.
6. The crystal substrate of claim 1, wherein said crystal of said
nitride of said group 13 element forming said underlying layer and
said crystal of said nitride of said group 13 element forming said
thick film comprises gallium nitride.
7. A functional device comprising said crystal substrate of claim 1
and a functional layer formed over said thick film of said crystal
substrate and comprising a nitride of a group 13 element.
8. The functional device of claim 7, wherein said functional layer
has a function of emitting a light.
9. The functional device of claim 7, further comprising a seed
crystal comprising a nitride of a group 13 element, wherein said
crystal substrate is provided over said seed crystal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of International Application No.
PCT/JP2016/075175, filed Aug. 29, 2016, which claims the priority
of Japanese Patent Application No. 2015-242068, filed Dec. 11,
2015, the entire contents of which are incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates to a substrate of a crystal of
a group 13 nitride and a functional device utilizing the same. For
example, the present invention may be used in technical fields
requiring high quality, such as a blue LED with improved color
rendering index and expected as a future substitute of a
luminescent lamp, a blue-violet laser for high-speed and
high-density optical memory, a power device for an inverter for a
hybrid car or the like.
RELATED ART STATEMENT
[0003] As white LED's are widely applied, it has been required
improved performances for the LED chips. The improved performances
mean improved efficiency and luminance. HVPE method is well known
as a method of obtaining self-supporting substrate of gallium
nitride. Among them, as methods of obtaining crystals of high
quality, DEEP method (patent document 1 and non-patent document 1)
and VAS method (patent documents 2 and 3) are disclosed.
[0004] Flux method is a kind of liquid phase methods. In the case
of gallium nitride, by using sodium metal as flux, it is possible
to reduce a temperature and pressure required for crystal growth of
gallium nitride to about 800.degree. C. and several MPa,
respectively. Specifically, nitrogen gas is dissolved into mixed
melt of sodium metal and gallium metal, so that gallium nitride
becomes supersaturation state and grows as a crystal. According to
such liquid phase process, dislocations are reduced than that in
the case of vapor phase process, so that it is possible to obtain
gallium nitride of high quality and having a lower dislocation
density (Patent document 4).
Related Documents
[0005] (Non-patent document 1) SEI Technical Review 2009, July,
Vol. 175, pages 10 to 18, "Development of GaN Substrates" (Patent
document 1) Japanese patent No. 3801125B1 (Patent document 2)
Japanese patent No. 3631724B1 (Patent document 3) Japanese patent
No. 4396816B1 (Patent document 4) Japanese patent publication No.
2009-012986A
SUMMARY OF THE INVENTION
[0006] It is necessary to make the conductivity of a nitride
crystal layer of a group 13 element high, for producing LED's and
laser diodes. For this, the inventors have tried to dope a dopant,
such as Si or oxygen, into the crystal layer, so that the
conductivity of the crystal layer and its output power are
improved.
[0007] However, even if the conductivity of the crystal layer is
actually increased, there were cases that an output power of
emitted light is not improved, in the case of so-called vertical
type light-emitting devices and that its current is raised, for
example.
[0008] An object of the present invention is, in a substrate of a
crystal of a nitride of a group 13 element, to obtain desired
conductivity and to effectively utilize the conductivity of the
crystal of the nitride of the group 13 element to improve its
function.
[0009] The present invention provides a crystal substrate
comprising an underlying layer and a thick layer:
[0010] the underlying layer comprising a crystal of a nitride of a
group 13 element and having a first main face and a second main
face;
[0011] the thick film comprising a crystal of a nitride of a group
13 element and provided over the first main face of the underlying
layer;
[0012] wherein the underlying layer comprises a low carrier
concentration region and a high carrier concentration region both
extending between the first main face and the second main face;
[0013] wherein the low carrier concentration region has a carrier
concentration of 10.sup.17/cm.sup.3 or lower;
[0014] wherein the low carrier concentration region has a defect
density of 10.sup.7/cm.sup.2 or lower;
[0015] wherein the high carrier concentration region has a carrier
concentration of 10.sup.19/cm.sup.3 or higher;
[0016] wherein the high carrier concentration region has a defect
density of 10.sup.8/cm.sup.2 or higher;
[0017] wherein the thick film has a carrier concentration of
10.sup.18/cm.sup.3 or higher and 10.sup.19/cm.sup.3 or lower;
and
[0018] wherein the thick film has a defect density of
10.sup.7/cm.sup.2 or lower.
[0019] The present invention further provides a device
comprising:
[0020] the crystal substrate; and
[0021] a functional layer comprising a nitride of a group 13
element and formed over the crystal substrate.
Effects of the Invention
[0022] For example, in the case that LED's or laser diodes of
vertical type structures are produced, a high conductivity is
required. However, for providing the conductivity by doping a
dopant, such as Si or oxygen, into a crystal of the nitride of a
group 13 element, it is proved that it is difficult to dope the
dopant uniformly over the whole surface of a wafer. It is further
proved that the crystallinity is deteriorated and dislocation
density is increased in regions where the content of the dopant is
higher.
[0023] Then, as the mechanism of the current leakage is considered
in detail, it was found the followings. That is, in a
cross-sectional structure of the crystal substrate of the nitride
of the group 13 element, the crystallinity is deteriorated and a
current easily flows in a region having a high conductivity. This
results in the local concentration of current so that the current
leakage occurs. That is, it was successfully confirmed that the
temperatures of the locations with the current leakage occurred
were raised, by observing the cross section by a microscopic
thermography.
[0024] It was further confirmed the followings. That is, in the
cross-sectional structure of the crystal substrate, the number of
defects is large and the current leakage tends to occur in the
relatively high carrier concentration region. In addition to this,
it was confirmed that such pillar-shaped high carrier concentration
region is elongated between the upper face and bottom face of the
crystal substrate to penetrate through the substrate. At the same
time, it was also confirmed that the low carrier concentration
region was elongated in a pillar shape between the upper and bottom
faces of the crystal substrate to penetrate through the substrate.
It means that the current concentration is likely to occur through
the high carrier concentration regions.
[0025] Further, a reduction of the luminous efficiency in the case
of raising the current density applied onto an LED chip has been
known as efficiency droop phenomenon. Its cause was reported to be
carrier overflow, non-radiative recombination, Auger recombination
or the like. However, such phenomenon is mainly found in horizontal
type LED's.
[0026] The above phenomenon found by the inventors is different
from the efficiency droop phenomenon, has not been reported until
now and newly discovered by the inventors. The local current
concentration is newly discovered this time based on the
observation as described above.
[0027] Based on the discovery, the inventors tried to re-grow
crystal of a nitride of a group 13 element on an underlying layer
of a nitride of a group 13 element to form a thick film and to
facilitate the association of defects in the thick film by
controlling the growth conditions of it. As a result, it is
confirmed that the pillar-shaped high carrier concentration regions
in the crystal substrate are dispersed in the thick film so that
the carriers are dispersed uniformly to realize relatively uniform
luminescence in CL. It is thus possible to successfully prevent the
current leakage between a pair of the main faces of the crystal
substrate by providing the thick film having such cross-sectional
structure on the underlying layer. It is thus possible to prevent
the current leakage in the case that the voltage applied on the
crystal substrate is raised, so that the conductivity of the
crystal substrate can be effectively utilized and the function can
be further improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1(a) is a view schematically showing a crystal
substrate 1 according to an embodiment of the present invention,
and FIG. 1(b) is a view schematically showing a functional device
16 with the substrate 1 and a functional device structure 6 formed
thereon.
[0029] FIG. 2(a) shows the state that a seed crystal film 12 is
provided on a supporting body 11, FIG. 2(b) shows the state that an
underlying film 13 composed of a crystal of a nitride of a group 13
element is provided on the seed crystal film 12, and FIG. 2(c)
shows the state that a thick film 14 is formed on the underlying
film 13.
[0030] FIG. 3(a) is a view schematically showing nuclei 20
generated on the seed crystal 12, and FIG. 3(b) is a view
schematically showing directions of growth from the nuclei 20.
[0031] FIG. 4(a) shows the state that the underlying layer 13 and
thick film 14 are separated from the seed crystal film, and FIG.
4(b) shows a crystal substrate 1 obtained by processing the
underlying layer 13 and thick film 14 of FIG. 4(a).
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0032] The present invention will be described further in detail,
appropriately referring to attached drawings.
[0033] (Crystal Substrate)
[0034] According to a preferred embodiment, as shown in FIG. 1(a),
a thick film 3 composed of a nitride of a group 13 element is
formed on an underlying layer 2 composed of a nitride of a group 13
element. The underlying layer 2 includes a first main face 2a and a
second main face 2b. The thick film 3 is provided on the first main
face 2a of the underlying layer 2.
[0035] The underlying layer 2 includes low carrier concentration
regions 5 and high carrier concentration regions 4 each extending
between the first main face 2a and the second main face 2b. It is
confirmed that each of the region 4 and 5 is generated to extend
through the underlying layer 2 between the first and second main
faces.
[0036] Here, the low carrier concentration region 5 has a carrier
concentration of 10.sup.17/cm.sup.3 or lower, and the low carrier
concentration region 5 has a defect density of 10.sup.7/cm.sup.2 or
lower. Further, the high carrier concentration region 4 has a
carrier concentration of 10.sup.19/cm.sup.3 or higher, and the high
carrier concentration region 4 has a defect density of
10.sup.8/cm.sup.2 or higher. According to the mechanism described
above, the dopants are concentrated in the high carrier
concentration region so that its carrier concentration is raised,
and defects are also concentrated into the same region, which is
elongated to extend between the main faces of the underlying
layer.
[0037] Here, the low carrier concentration region may preferably
have a carrier concentration of 8.times.10.sup.16/cm.sup.3 or
lower. Further, in the underlying layer including the high carrier
concentration region described above, the carrier concentration of
the low carrier concentration region may be
1.times.10.sup.17/cm.sup.3 or lower and 1.times.10.sup.16/cm.sup.3
or higher in many cases. Further, the defect density of the low
carrier concentration region may preferably be
8.times.10.sup.6/cm.sup.2 or lower. Further, the defect density of
the low carrier concentration region may be
2.times.10.sup.6/cm.sup.2 or higher in many cases.
[0038] Further, the carrier concentration of the high carrier
concentration region 4 is 10.sup.19/cm.sup.3 or higher and may
preferably be 2.times.10.sup.19/cm.sup.3 or higher. Further, in the
underlying layer including the low carrier concentration region
described above, the carrier concentration of the high carrier
concentration region may be 5.times.10.sup.19/cm.sup.3 or lower in
many cases. Further, the defect density of the high carrier
concentration region may preferably be 5.times.10.sup.7/cm.sup.2 or
lower.
[0039] According to the present invention, the carrier
concentration of the thick film 3 is 10.sup.18/cm.sup.3 or higher
and 10.sup.19/cm.sup.3 or lower, and the defect density of the
thick film is 10.sup.7/cm.sup.2 or lower. Although the defect
density of the thick film is preferably lower, as the thick film is
provided on the underlying layer of the structure described above,
the defect density of the thick film is 2.times.10.sup.6/cm.sup.2
or higher in many times.
[0040] Here, in the case that the dopant is an n-type dopant (such
as Si, Ge, oxygen or the like), the activation ratio is as high as
98 percent or higher, so that the dopant concentration can be
deemed as the carrier concentration. The carrier concentration will
be described below on the viewpoint of the present invention for
preventing leakage due to the current concentration. However, the
activation ratio can be deemed as 100 percent and the carrier
concentration can be read as the dopant concentration.
[0041] According to the present invention, the high carrier
concentration region and low carrier concentration region are
measured and distinguished as follows.
[0042] It is used a system of measuring cathode luminescence (for
example, MP series supplied by HORIBA, Ltd.) at a magnitude of 50
to 500 folds and in a rectangular region of taking an image of 0.1
mm and 1 mm.
[0043] The carrier concentration and defect density are to be
measured according to the method and conditions described in the
Examples section.
[0044] The group 13 element constituting the underlying layer and
thick film is a group 13 element according to the Periodic Table
determined by IUPAC. The group 13 element is specifically gallium,
aluminum, indium, thallium or the like. The nitride of the group 13
element may preferably be gallium nitride, aluminum nitride or
gallium aluminum nitride. Further, as an additive, it may be listed
carbon, a metal having a low melting point (tin, bismuth, silver,
gold), and a metal having a high melting point (a transition metal
such as iron, manganese, titanium, chromium).
[0045] It is necessary, in the thick film, to associate the defects
so as to prevent the high carrier concentration regions from
penetrating through the thick film to the surface of the thick
film. On the viewpoint, the thickness T3 of the thick film may
preferably be 1 .mu.m or larger, more preferably be 50 .mu.m or
larger and most preferably be 100 .mu.m or larger. Further, as the
thickness T3 of the thick film is too large, the conductivity as a
whole becomes low. The thickness of the thick film may preferably
be 300 .mu.m or smaller and more preferably be 200 .mu.m or
smaller.
[0046] The thickness T2 of the underlying film may preferably be 50
.mu.m or smaller, on the viewpoint of easiness of handling.
Further, the thickness of the underlying film may preferably be 200
.mu.m or smaller, on the viewpoint of conductivity.
[0047] Further a total thickness (T2+T3) of the crystal substrate,
that is, of the underlying layer and thick film may preferably be
250 to 450 .mu.m.
[0048] (Functional Layer and Functional Device)
[0049] A functional layer may be formed on the thick film to obtain
a functional device.
[0050] The functional layer as described above may be composed of a
single layer or a plurality of layers. Further, as the functions,
it may be used as a white LED with high brightness and improved
color rendering index, a blue-violet laser disk for high-speed and
high-density optical memory, a power device for an inverter for a
hybrid car or the like.
[0051] As a semiconductor light emitting diode (LED) is produced on
the crystal substrate by a vapor phase process, preferably by metal
organic vapor phase deposition (MOCVD) method, the dislocation
density inside of the LED can be made comparable with that of the
thick film of the crystal substrate.
[0052] The film-forming temperature of the functional layer may
preferably be 950.degree. C. or higher, and more preferably be
1000.degree. C. or higher, on the viewpoint of inhibiting the
incorporation of impurities such as carbon. Further, on the
viewpoint of preventing defects, the film-forming temperature of
the functional layer may preferably be 1200.degree. C. or lower and
more preferably be 1150.degree. C. or lower.
[0053] The material of the functional layer may preferably be a
nitride of a group 13 element. Group 13 element means group 13
element according to the Periodic Table determined by IUPAC. The
group 13 element is specifically gallium, aluminum, indium,
thallium or the like.
[0054] The light emitting device structure includes, an n-type
semiconductor layer, a light emitting region provided on the n-type
semiconductor layer and a p-type semiconductor layer provided on
the light emitting region, for example. In a light emitting device
16 shown in FIG. 1(b), an n-type contact layer 6a, n-type clad
layer 6b, active layer 6c, p-type clad layer 6d and p-type contact
layer 6e are formed on the thick film 3 of the crystal substrate to
constitute the light emitting structure 6.
[0055] Further, the light emitting structure described above may
preferably further include an electrode for the n-type
semiconductor layer, an electrode for the p-type semiconductor
layer, a conductive adhesive layer, a buffer layer and a conductive
supporting body or the like not shown.
[0056] According to the light emitting structure, as light is
emitted in the light emitting region through re-combination of
holes and electrons injected through the semiconductor layers, the
light is drawn through the side of a translucent electrode on the
p-type semiconductor layer or the film of the nitride single
crystal of the group 13 element. Besides, the translucent electrode
means an electrode capable of transmitting light and made of a
metal thin film or transparent conductive film formed substantially
over the whole of the p-type semiconductor layer.
[0057] The n-type semiconductor layer or p-type semiconductor layer
is composed of a semiconductor of III-V group compound
semiconductor, which includes the followings.
Al.sub.yIn.sub.xGa.sub.1-x-yN(0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1)
[0058] As a dopant for imparting n-type conductivity, silicon,
germanium and oxygen are listed. Further, as a dopant for imparting
p-type conductivity, magnesium and zinc are listed.
[0059] The method of growing each of the semiconductor layers
constituting the light emitting structure includes various kinds of
vapor phase growing methods. For example, metal organic chemical
vapor deposition (MOCVD; MOVPE), molecular beam epitaxy (MBE),
hydride vapor phase epitaxy (HVPE) or the like may be used. Among
them, it is possible to obtain semiconductor layers with good
crystallinity and flatness by MOCVD method. According to MOCVD
method, an alkyl metal compound such as TMG (trimethyl gallium) and
TEG (triethyl gallium) or the like is used as the Ga source in many
cases and a gas such as ammonia and hydrazine are used as the
nitrogen source.
[0060] The light emitting region includes a quantum well active
layer. The material of the quantum well active layer is designed so
that the band gap is made smaller than those of the n-type and
p-type semiconductor layers. The quantum well active layer may be a
single quantum well active layer (SQW) structure or a multi quantum
well active layer (MQW) structure. The material of the quantum well
active layer includes the followings.
[0061] As a preferred example of the quantum well active layer, it
is listed an MQW structure including three to ten periods of
quantum well active layers each made of
Al.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN (x=0.15, y=0.20) series
with a film thickness of 3 nm/8 nm.
[0062] (Production of Crystal Layer)
[0063] According to a preferred embodiment, the underlying layer
and thick film are formed on a seed crystal in this order. The seed
crystal may form a self-supporting substrate (supporting body) or
may be a seed crystal film formed on a separate supporting body.
The seed crystal film may be composed of a single layer or may
include a buffer layer on the side of the supporting body.
[0064] For example, as shown in FIG. 2(a), a seed crystal film 12
is formed on a surface lla of a supporting body 11. The seed
crystal film 12 is composed of a nitride of a group 13 element.
[0065] In the case that the seed crystal film is formed on the
supporting body, although the single crystal forming the supporting
body is not limited, it includes sapphire, AlN template, GaN
template, self-supporting GaN substrate, silicon single crystal,
SiC single crystal, MgO single crystal, spinel (MgAl.sub.2O.sub.4),
LiAlO.sub.2, LiGaO.sub.2, and perovskite composite oxide such as
LaAlO.sub.3, LaGaO.sub.3 or NdGaO.sub.3 and SCAM (ScAlMgO.sub.4). A
cubic perovskite composite oxide represented by the composition
formula [A.sub.1-y(Sr.sub.1-xBa.sub.x).sub.y]
[(Al.sub.1-zGa.sub.z).sub.1-uD.sub.u]O.sub.3 (wherein A is a rare
earth element; D is one or more element selected from the group
consisting of niobium and tantalum; y=0.3 to 0.98; x=0 to 1; z=0 to
1; u=0.15 to 0.49; and x+z=0.1 to 2) may be also used.
[0066] The direction of growth of the nitride crystal layer of the
group 13 element may be a direction normal to c-plane of the
wurtzite structure or a direction normal to each of the a-plane and
m-plane.
[0067] The dislocation density at the surface 12a of the seed
crystal is preferably lower, on the viewpoint of reducing the
dislocation density of the crystal substrate provided on the seed
crystal. On the viewpoint, the dislocation density at the surface
12a of the seed crystal layer may preferably be 7.times.10.sup.8
cm.sup.-2or lower and more preferably be
5.times.10.sup.8cm.sup.-2or lower. Further, as the dislocation
density of the seed crystal may preferably be lower on the
viewpoint of the quality, the lower limit is not particularly
provided, but it may generally be 5.times.10.sup.7cm.sup.-2 or
higher in many cases.
[0068] The method of forming the seed crystal film is not
particularly limited, and vapor phase process such as metal organic
chemical vapor deposition (MOCVD) method, hydride vapor phase
epitaxy (HVPE) method, pulse-excited deposition (PXD) method, MBE
method and sublimation method, and liquid phase process such as
flux method are exemplified.
[0069] Then, as shown in FIG. 2(b), the underlying layer 13 is
formed on the seed crystal film 12. Then, as shown in FIG. 2(c),
the thick film 14 is formed on the surface 13a of the underlying
layer 13.
[0070] According to a preferred embodiment, the underlying layer
and thick film are grown by flux method. In this case, the kind of
the flux is not particularly limited, as far as it is possible to
grow the nitride of the group 13 element. According to a preferred
embodiment, it is used a flux containing at least one of an alkali
metal and alkaline earth metal, and flux containing sodium metal is
particularly preferred.
[0071] A raw material of the nitride of the group 13 element is
mixed to the flux and used. As the raw material, a single metal, an
alloy and a compound are applicable, and the single metal of the
group 13 element is suitably used from the viewpoint of
handling.
[0072] In the case that the underlying layer is produced by flux
method, for promoting the vertical growth, the supersaturation
degree is held at a low value and the flow of the melt is reduced,
so that only the concentration gradient is utilized as the driving
force for growing the crystal. The nucleation is restrained and the
crystal is grown upwardly from nuclei 20 (arrows C) as shown in
FIG. 3(b).
[0073] Specifically, the underlying layer is preferably grown
according to the following method.
[0074] (1A) The average growth temperature of the melt in the
crucible is made high, so that the supersaturation degree is
increased to inhibit the formation of nuclei 20 (Refer to FIG.
3(a)).
[0075] (2A) The temperature at an upper part of the crucible is
made higher than that at a bottom part of the crucible so that flow
in the melt is inhibited.
[0076] (3A) The melt is not agitated, or its agitation rate is made
lower if agitated.
[0077] (4A) The partial pressure of a nitrogen-containing gas is
made lower.
[0078] For example, the following conditions are applicable.
[0079] (1A) The average growth temperature of the melt in the
crucible is made 870 to 885.degree. C.
[0080] (2A) The temperature at the upper part in the crucible is
made higher than that at the bottom part of the crucible by 0.5 to
1.degree. C.
[0081] (3A) The melt is not agitated, or the agitation rate is made
30 rpm or smaller. Further, the direction of the agitation is
fixed.
[0082] (4A) The partial pressure of the nitrogen-containing gas is
made 3.5 to 3.8 MPa.
[0083] Under such conditions, a large number of crystal defects is
present in the high carrier concentration regions. This is because,
according to the structure of crystal grown upwardly and vertically
in a shape of a pillar, the numbers of the carriers and crystal
defects tend to increase in the sites with many dopants.
[0084] The crystal defects referred to herein means threading
dislocations, including three kinds of dislocations: screw
dislocation, edge dislocation and the mixed dislocations. These
kinds of the dislocations can be confirmed by a transmission type
electron microscope (TEM) or cathode luminescence (CL).
[0085] Then, a nitride of a group 13 element is re-grown in the
same melt by flux method so that the thick film as described above
can be formed.
[0086] That is, the growth conditions are changed to increase the
crystal growth rate. As a result, as shown in FIG. 3(b), crystal
growth 21 is mainly constituted with lateral growth (arrows A and
B).
[0087] During the stage of forming the thick film, the following
modifications are preferred.
[0088] (1B) The average growth temperature of the melt in the
crucible is made 850 to 860.degree. C. Further, a difference
between the average growth temperature and that in the initial
stage is made 10 to 25.degree. C.
[0089] (3B) The melt is agitated, and the direction of the
agitation is periodically changed. Further, the rotation of the
crucible is stopped when the direction of the rotation is changed.
In this case, the time period of the stopping the rotation may
preferably be 100 to 6000 seconds and more preferably be 600 to
3600 seconds. Further, the time periods of the rotation before and
after stopping the rotation may preferably be 10 to 600 seconds and
the rotational rate may preferably be 10 to 30 rpm.
[0090] (4B) The partial pressure of the nitrogen-containing gas is
made 4.0 to 4.2 MPa, and the partial pressure is made higher than
the partial pressure of the nitrogen-containing gas in the initial
state by 0.2 to 0.5 MPa.
[0091] Here, preferably, in the stage of forming the thick film,
the production conditions are gradually changed so that the
low-dislocation density and carrier concentration are balanced.
Specifically, the agitation rate of the melt is gradually elevated,
and the retention time at the maximum rotational rate during the
agitation is made gradually longer.
[0092] The crystal defects are thereby elongated from the lower
main face contacting the underlying layer to the surface as the
crystal is grown, while they are associated with each other. The
carrier concentration and defect density are thus uniformly
distributed in the thick film so that the concentration of the leak
current can be prevented.
[0093] According to the flux method, a single crystal is grown in
an atmosphere containing nitrogen atom-containing gas. For this
gas, nitrogen gas may be preferably used, and ammonia may be used.
Any other gas except the nitrogen atom-containing gas in the
atmosphere is not limited; but an inert gas may be preferably used,
and argon, helium, or neon may be particularly preferred.
[0094] It is preferred to hold for one hour or longer, more
preferably two hours or longer, under the conditions (1A) to (4A)
as described above, in the stage of growing the underlying layer.
The retention time in the stage of forming the underlying layer may
preferably be 10 hours or shorter.
[0095] A ratio (molar ratio) of the nitride of the group 13
element/flux (for example, sodium) in the melt may preferably be
made higher, preferably be 18 mol % or higher and more preferably
be made 25 mol % or higher, on the viewpoint of the present
invention. However, as this ratio becomes too high, the crystal
quality tends to be deteriorated, so that the ratio may preferably
be 40 mol % or lower.
[0096] (Vapor Phase Method)
[0097] For example, in the case of HVPE method, in initial stage of
the growth, the mixed ratio of hydrogen is made higher (for example
50 percent or higher) so that the growth rate is made lower (for
example, 10 to 20 microns/hr), so that the crystal is grown
primarily in the direction shown by an arrow C in FIGS. 3(a) and
3(b). Thereafter, the mixed ratio of hydrogen is lowered (for
example 30 percent or lower) and the ratio of V/III is made higher
(for example 2000 or higher) so as to realize the growth primarily
in the directions shown in arrows A and B in FIG. 3(b).
[0098] The nitride of the group 13 element grown by flux method
emits fluorescence (blue fluorescence) having a broad peak in a
wavelength of 440 to 470 nm, in the case that it is irradiated
light of a wavelength of 330 to 385 nm (for example light emitted
from a mercury lamp). On the other hand, the nitride of the group
13 element produced by a vapor phase process emits fluorescence
(yellow fluorescence) having a broad peak in a wavelength of 540 to
580 nm, in the case that it is irradiated light of a wavelength of
330 to 385 nm. It is thereby possible to distinguish the nitride
crystals of the group 13 element obtained by flux method and vapor
phase process, based on the color of the fluorescence emitted by
irradiating the light of a wavelength of 330 to 385 nm.
[0099] (Processing and Shape of Crystal Layer)
[0100] As shown in FIGS. 2(b) and 2(c), the crystal substrate 1A is
formed on the seed crystal film, and then a specific functional
layer may be formed on the surface 14a of the thick film 14.
Alternatively, the crystal substrate 1A may be removed from the
seed crystal 12 by grinding, lift-of or the like to separate the
crystal substrate 1A as shown in FIG. 4(a). In this case, the
specific functional layer is formed on the thick film 14 of the
separated crystal substrate 1A. At this time, the bottom face 13b
of the underlying layer 13 may be subjected to polishing to reduce
the warpage of the crystal substrate.
[0101] Alternatively, the surface 14a of the thick film 14 may be
polished to make the surface 3a of the thick film 3 an polished
surface as shown in FIG. 4(b). Further, at this time, the bottom
face 13b of the underlying layer 13 may be subjected to polishing
to make the bottom face 2a of the underlying layer 2 a polished
surface as shown in FIG. 4(b).
[0102] According to a preferred embodiment, the layer of the
crystal of the nitride of the group 13 element has a shape of a
circular plate, and it may have another shape such as a rectangular
plate. Further, according to a preferred embodiment, the dimension
of the gallium nitride substrate is of a diameter .phi. of 25 mm or
larger. It is thereby possible to provide the crystal layer which
is suitable for the mass production of functional devices and easy
to handle.
[0103] It will be described below the cases of grinding and
polishing the surfaces of the crystal substrate of the nitride of
the group 13 element.
[0104] Grinding is that an object is contacted with fixed
abrasives, obtained by fixing the abrasives by a bond and rotating
at a high rotation rate, to grind a surface of the object. By such
grinding, a roughened surface is formed. In the case that a bottom
face of the gallium nitride substrate is ground, it is preferably
used the fixed abrasives containing the abrasives, composed of SiC.
Al.sub.2O.sub.3, diamond, CBN (cubic boron nitride, same applies
below) or the like having a high hardness and having a grain size
of about 10 .mu.m to 100 .mu.m.
[0105] Further, lapping is that a surface plate and an object are
contacted, while they are rotated with respect to each other,
through free abrasives (it means abrasives which are not fixed,
same applies below), or fixed abrasives and the object are
contacted while they are rotated with respect to each other, to
polish a surface of the object. By such lapping, it is formed a
surface having a surface roughness smaller than that in the case of
the grinding and larger than that in the case of micro lapping
(polishing). It is preferably used abrasives composed of SiC.
Al.sub.2O.sub.3, diamond, CBN or the like having a high hardness
and having a grain size of about 0.5 .mu.m or larger and 15 .mu.m
or smaller.
[0106] Micro lapping (polishing) means that a polishing pad and an
object are contacted with each other through free abrasives while
they are rotated with each other, or fixed abrasives and the object
are contacted with each other while they are rotated with each
other, for subjecting a surface of the object to micro lapping to
flatten it. By such polishing, it is possible to obtain a crystal
growth surface having a surface roughness smaller than that in the
case of the lapping.
EXAMPLES
Example
[0107] The crystal substrate and light emitting device were
produced, according to the procedure described referring to FIGS. 1
to 4.
[0108] (Production of a Seed Crystal Substrate)
[0109] A low-temperature GaN buffer layer was deposited in 20 nm at
530.degree. C. using MOCVD method, on a c-plane sapphire substrate
11 having a diameter of 2 inches and a thickness of 500 .mu.m. A
seed crystal film 12 made of GaN and having a thickness of 2 .mu.m
was deposited thereon at 1050.degree. C. The defect density
observed by TEM (transmission type electron microscope) was proved
to be 1.times.10.sup.9/cm.sup.2. The obtained assembly was
subjected to ultrasonic washing with organic solvent and ultra-pure
water for 10 minutes, respectively, and then dried to obtain a seed
crystal substrate.
[0110] (Growth of Underlying Layer)
[0111] In a glove box filled with an inert gas, Ga metal and Na
metal were weighed in a molar ratio of 20:80 and then placed on a
bottom of an alumina crucible with the seed crystal substrate.
Further, as a dopant, liquid germanium was added thereto in an
amount of 1 mol/cm.sup.3 with respect to Ga. Three crucibles were
stacked, and an alumina lid was mounted on the top of the three
crucibles. These were contained in a holding container (reaction
container) of stainless steel. Four reaction containers, each
containing the stack of the crucibles, were stacked and then
contained in a holding container (inner container) made of
stainless steel.
[0112] Then, heaters provided in a pressure container were heated
to melt raw materials in the crucibles to generate Ga--Na mixed
melts. While the crucibles were heated to maintain the temperature
at 880.degree. C., nitrogen gas was supplied from a nitrogen gas
bombe until a pressure of 4.0 MPa to initiate the crystal growth.
Five hours later, the temperature in the crucible was lowered to
850.degree. C. over 20 hours and the pressure was changed to 4.2
MPa. At this time, the agitation was initiated by inverting the
rotational direction of the rotational table continuously to change
the growth mode of the crystal. Thereafter, the pressure was
changed to 4.0 MPa for 30 minutes and the agitation was continued
by inverting the rotational direction of the rotational table
continuously to grow the crystal.
[0113] As to the conditions of the rotation, the rotational table
was rotated around a central axis at a rate of 20 rpm clockwise and
anti-clockwise at a specific interval. The time period for
acceleration was made 6 seconds, the holding time was made 200
seconds, the rate-decreasing time was made 6 seconds and the
stopping time was made 1 second. Such condition was held for 24
hours. The thickness of the GaN crystal was increased by about 300
microns.
[0114] The surface of the thus obtained gallium nitride crystal
substrate was subjected to polishing to make the thickness 250
microns. The density of dark spots on the surface was measured by
cathode luminescence and proved to be 2.times.10.sup.5/cm.sup.2.
The density of the dark spots was deemed as the defect density. A
sample was cut out from the crystal substrate in dimensions of 6
mm.times.6 mm, and the samples was subjected to hole measurement to
obtain the carrier density, which was proved to be
5.times.10.sup.15/cm.sup.3.
[0115] (Growth of Thick Film)
[0116] Then, n-GaN was further grown in 100 microns using the
liquid phase process. N-type dopants were doped so that the carrier
density was made 1.times.10.sup.18/cm.sup.3.
[0117] As to the growth conditions, the rotational rate as
described above was gradually increased from 20 rpm to 40 rpm over
4 hours, and the retention time at 35 rpm was increased from 200
seconds to 600 second stepwise by 100 seconds over 4 hours.
Thereafter, crystal was cooled to room temperature and collected.
It was grown the crystal of about 100 microns for a total time of 8
hours.
[0118] (Production of a Wafer)
[0119] Thereafter, the substrate was subjected to laser lift-off so
that the supporting substrate and seed crystal film were separated
from the gallium nitride crystal substrate. Both faces of the
crystal substrate were subjected to mechanical processing and dry
etching to perform the flattening to obtain a 2-inch wafer 1 having
a thickness of 290 .mu.m. The thicknesses of the thick film 3 and
underlying layer 2 after the polishing were about 40 microns and
250 microns, respectively.
[0120] The cross-sectional structure of the wafer was observed by
CL. As a result, the cross section was divided into brightly light
emitting regions 5 and dark regions 4 in the underlying layer. Each
of the regions was grown in a pillar shape in the cross section.
Further, dark lines due to concentration of dislocations were
observed in the central part of the brightly light emitting region
4. The brightly light emitting region 4 has a defect density of 2
to 5.times.10.sup.8/cm.sup.2 and a carrier concentration of
1.times.10.sup.18/cm.sup.3. The region 5 of a lower luminous
intensity has a substantially uniform defect density of
3.times.10.sup.6/cm.sup.2 and a carrier concentration of
8.times.10.sup.16/cm.sup.3.
[0121] On the other hand, in the thick film 3, uniform luminescence
was observed and the dislocation-concentrated regions were not
observed. The defect density was approximately uniform and
3.times.10.sup.6/m.sup.3, and the carrier concentration was
1.times.10.sup.18/cm.sup.3.
[0122] The carrier concentration was measured as follows. CL
measurement was carried out for four samples whose carrier
concentrations are measured in advance by eddy current system
(1.times.10.sup.17/cm.sup.3; 1.times.10.sup.18/cm.sup.3;
5.times.10.sup.18/cm.sup.3; 1.times.10.sup.19/cm.sup.3). The
brightness of each of the corresponding images was subjected to
image processing in 8 bits (255 gradations) to provide a
calibration line concerning the carrier density and brightness of
the CL image. The thus obtained calibration line was used to
calculate the carrier concentration.
[0123] Further, light of a wavelength of 330 to 385 nm was
irradiated from a mercury lamp to the thus obtained thick film 3.
It was oscillated fluorescence (blue fluorescence) having a broad
peak at 440 to 470 nm.
[0124] (Production of Light Emitting Device)
[0125] The thus obtained crystal substrate was used to produce an
LED structure by MOCVD method. Electrodes were then patterned and
the back face was polished so that the thickness of the underlying
layer after the back-polishing was made 70 microns. It was then
produced a blue LED having a dimensions of 1 mm.times.1 mm by
dicing and of the horizontal type structure. The internal quantum
efficiency was measured at a driving current of 350 mA and proved
to be as high as about 80 percent. The luminous intensity was
uniform over the plane. Further, the driving current was raised to
1000 mA and the internal quantum efficiency was measured to prove
to be as high as 65 percent. Further, the leak current at an
application voltage of 2V was as low as 0.01 .mu.A.
Comparative Example 1
[0126] It was produced a crystal substrate according to the same
procedure as the Example 1, and the LED structure was fabricated
thereon. However, different from the Example 1, the growth of the
crystal was terminated at the stage of forming the underlying layer
and the thick film was not formed.
[0127] As the cross section of the underlying layer was observed
according to the same procedure as the Example 1, brightly light
emitting regions 4 and dark regions 5 are elongated in pillar shape
in the direction of growth, respectively, and do not intersect each
other. Many black lines, which are considered to be dislocation
lines, are observed in the brightly light emitting regions 4. On
the other hand, the black lines were scarcely observed in the dark
regions 5. Further, the defects densities and carrier
concentrations of the respective regions 4 and 5 were substantially
same as those in the Example 1.
[0128] The internal quantum efficiency at a driving current of 350
mA was measured for the thus obtained LED device, it was obtained a
relatively high value of about 75 percent but the light emitting
intensity was proved to be not uniform in a plane. Further, it was
elevated to 1000 mA and the internal quantum efficiency was
measured. It was proved to be lowered to 55 percent and the
deviation of the light emitting efficiency became further
considerable. Further, the leak current at an application of 2V was
1 mA, which was larger than that in the Example 1.
[0129] According to the experimental results described above, it
was proved that the defect concentrated region in the brightly
light emitting regions 4 in the underlying layer are the cause of
the leak current.
* * * * *