U.S. patent application number 11/684724 was filed with the patent office on 2007-09-20 for crystal preparing device, crystal preparing method, and crystal.
Invention is credited to Akihiro Fuse, Hirokazu Iwata, Seiji Sarayama.
Application Number | 20070215034 11/684724 |
Document ID | / |
Family ID | 38177275 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070215034 |
Kind Code |
A1 |
Iwata; Hirokazu ; et
al. |
September 20, 2007 |
CRYSTAL PREPARING DEVICE, CRYSTAL PREPARING METHOD, AND CRYSTAL
Abstract
In a crystal preparing device, a crucible holds a mixed molten
metal containing alkali metal and group III metal. A container has
a container space contacting the mixed molten metal and holds a
molten alkali metal between the container space and an outside of
the container, the molten alkali metal contacting the container
space. A gas supply device supplies nitrogen gas to the container
space. A heating device heats the crucible to a crystal growth
temperature. The crystal preparing device is provided so that a
vapor pressure of the alkali metal which evaporates from the molten
alkali metal is substantially equal to a vapor pressure of the
alkali metal which evaporates from the mixed molten metal.
Inventors: |
Iwata; Hirokazu; (Miyagi,
JP) ; Sarayama; Seiji; (Miyagi, JP) ; Fuse;
Akihiro; (Miyagi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
38177275 |
Appl. No.: |
11/684724 |
Filed: |
March 12, 2007 |
Current U.S.
Class: |
117/79 ; 117/206;
423/409 |
Current CPC
Class: |
C30B 17/00 20130101;
C30B 29/406 20130101; Y10T 117/1024 20150115; C30B 29/403 20130101;
C30B 9/10 20130101 |
Class at
Publication: |
117/79 ; 423/409;
117/206 |
International
Class: |
C30B 9/00 20060101
C30B009/00; C01B 21/06 20060101 C01B021/06; C30B 11/00 20060101
C30B011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2006 |
JP |
2006-068614 |
Mar 22, 2006 |
JP |
2006-078173 |
Claims
1. A crystal preparing device comprising: a crucible holding a
mixed molten metal containing alkali metal and group III metal; a
reaction container having a container space which contacts the
mixed molten metal and holding a molten alkali metal between the
container space and an outside of the container, the molten alkali
metal contacting the container space; a gas supply device supplying
a nitrogen gas to the container space; and a heating device heating
the crucible to a crystal growth temperature, wherein the crystal
preparing device is provided so that a vapor pressure of the alkali
metal which evaporates from the molten alkali metal is
substantially equal to a vapor pressure of the alkali metal which
evaporates from the mixed molten metal.
2. The crystal preparing device according to claim 1, further
comprising a heater/condenser controlling a temperature of the
molten alkali metal to a specific temperature at which a vapor
pressure of the alkali metal which evaporates from the molten
alkali metal is substantially balanced with a vapor pressure of the
alkali metal which evaporates from the mixed molten metal.
3. The crystal preparing device according to claim 1, wherein a
temperature of the molten alkali metal is lower than a temperature
of the mixed molten metal.
4. The crystal preparing device according to claim 1, further
comprising a piping disposed below the crucible and connected with
the reaction container, wherein the crucible is contained in the
reaction container and the molten alkali metal is held at least in
the piping.
5. The crystal preparing device according to claim 4, wherein the
molten alkali metal is held only in the piping.
6. The crystal preparing device according to claim 1, wherein the
gas supply device supplies nitrogen gas to the container space of
the reaction container through the alkali molten metal.
7. A crystal preparing method which produces a group III nitride
crystal using a crystal preparing device, the crystal preparing
device including a crucible holding a mixed molten metal containing
alkali metal and group III metal, a reaction container having a
container space which contacts the mixed molten metal and holding a
molten alkali metal between the container space and an outside of
the container, the molten alkali metal contacting the container
space, and a piping disposed below the crucible and connected with
the reaction container, the crystal preparing method comprising: a
first step of placing the alkali metal and the group III metal into
the crucible in an inert gas or nitrogen gas atmosphere; a second
step of placing the alkali metal into the piping in the inert gas
or nitrogen gas atmosphere; a third step of filling the container
space of the reaction container with nitrogen gas; a fourth step of
heating the crucible to a crystal growth temperature; a fifth step
of heating the molten alkali metal at a specific temperature at
which a vapor pressure of the alkali metal which evaporates from
the molten alkali metal is substantially equal to a second vapor
pressure of the alkali metal which evaporates from the mixed molten
metal; and a sixth step of holding a temperature of the crucible to
the crystal growth temperature, and holding a temperature of the
molten alkali metal to the specific temperature for a predetermined
time.
8. The crystal preparing method according to claim 7, wherein the
specific temperature is lower than the crystal growth
temperature.
9. The crystal preparing method according to claim 7, wherein the
crystal preparing device further includes a molten metal holding
member provided in the piping to hold the molten alkali metal by a
surface tension of the alkali molten metal, and wherein the second
step is provided to place the alkali metal between the crucible and
the reaction container so that the molten alkali metal containing
the alkali metal is held at least in the piping.
10. The crystal preparing method according to claim 9, wherein the
second step is provided to place the alkali metal between the
crucible and the reaction container so that the molten alkali metal
containing the alkali metal is held only in the piping.
11. The crystal preparing method according to claim 7 further
comprising a seventh step of supplying nitrogen gas to the
container space through the molten alkali metal after the fifth
step is performed.
12. A crystal preparing method which produces a group III nitride
crystal using a crystal preparing device including a reaction
container holding a mixed molten metal containing alkali metal and
group III metal, the crystal preparing method comprising: a first
step of placing the alkali metal and the group III metal in a
reaction container in an inert gas or nitrogen gas atmosphere
containing a predetermined amount of impurities; a second step of
filling a container space of the reaction container with nitrogen
gas; a third step of heating the reaction container to a crystal
growth temperature; a fourth step of holding a temperature of the
reaction container at the crystal growth temperature for a
predetermined time; and a fifth step of supplying nitrogen gas to
the reaction container to hold a pressure in the container space of
the reaction container at a predetermined pressure.
13. The crystal preparing method according to claim 12, wherein the
predetermined amount of impurities contains oxygen gas of 10 ppm or
less.
14. The crystal preparing method according to claim 13, wherein the
predetermined amount of impurities contains oxygen gas of 0.1-10
ppm.
15. The crystal preparing method according to claim 13, wherein the
predetermined amount of impurities contains oxygen gas below 0.1
ppm.
16. The crystal preparing method according to claim 12, wherein the
predetermined amount of impurities contains oxygen gas of 10 ppm or
less and moisture of 10 ppm or less.
17. The crystal preparing method according to claim 16, wherein the
predetermined amount of impurities contains oxygen gas of 0.1-10
ppm and moisture of 0.1-10 ppm.
18. The crystal preparing method according to claim 16, wherein the
predetermined amount of impurities contains oxygen gas below 0.1
ppm and moisture of 0.1-10 ppm.
19. The crystal preparing method according to claim 16, wherein the
predetermined amount of impurities contains oxygen gas of 0.1-10
ppm and moisture below 0.1 ppm.
20. The crystal preparing methods according to claim 12, wherein
the predetermined amount of impurities contains moisture of 10 ppm
or less.
21. The crystal preparing method according to claim 20, wherein the
predetermined amount of impurities contains moisture of 0.1-10
ppm.
22. The crystal preparing method according to claim 20, wherein the
predetermined amount of impurities contains moisture below 0.1
ppm.
23. The crystal preparing method according to claim 12, further
comprising a sixth step of heating the reaction container holding
an intermetallic compound of the alkali metal and the group III
metal under a predetermined reduced pressure, wherein the
intermetallic compound is placed, in the first step, in the
reaction container in the inert gas or nitrogen gas atmosphere, and
the second to fifth steps are performed after the sixth step is
performed.
24. A group III nitride crystal containing group III metal and
nitrogen, wherein the group III nitride crystal has a dislocation
density of 10.sup.3cm.sup.-2 or less and contains oxygen in a
concentration of 10.sup.20cm.sup.-3 or less.
25. The group III nitride crystal according to claim 24, wherein
the concentration of oxygen is in a range of
10.sup.18-10.sup.20cm.sup.-3.
26. The group III nitride crystal according to claim 24, wherein
the concentration of oxygen is lower than 10.sup.18cm.sup.-3.
27. The group III nitride crystal according to claim 24, wherein
the group III nitride crystal further contains hydrogen in a
concentration of 10.sup.20cm.sup.-3 or less.
28. The group III nitride crystal according to claim 27, wherein
the concentration of hydrogen is in a range of
10.sup.19-10.sup.20cm.sup.-3.
29. The group III nitride crystal according to claim 27, wherein
the concentration of hydrogen is lower than 10.sup.19cm.sup.-3.
30. The group III nitride crystal according to claim 24, wherein
the group III nitride crystal further contains alkali metal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a crystal preparing device
which carries out crystal growth of a group III nitride crystal,
and a crystal preparing method which carries out crystal growth of
a group III nitride crystal.
[0003] 2. Description of the Related Art
[0004] Most of InGaAlN devices (or group III nitride semiconductor
devices) which are currently used as ultraviolet, purple, blue, or
green light sources are produced by carrying out crystal growth of
a group III nitride crystal on a sapphire or silicone carbide (SiC)
substrate by using the MOCVD method (organic metal chemical vapor
deposition), the MBE method (molecular beam crystal growth method),
etc.
[0005] When sapphire and silicone carbide are used as the substrate
in this case, the substrate and the group III nitride semiconductor
greatly differ in the coefficient of thermal expansion and the
grating constant, and many crystal defects will be included in the
group III nitride semiconductor. Such crystal defects relate
directly to the drawbacks which cause deterioration of the device
characteristics, for example, short operating life and heavy
operation power in a luminescence device.
[0006] A sapphire substrate is an insulator, and taking out
electrodes from such a substrate as in the conventional
luminescence devices is impossible. Thus, it is necessary to take
out electrodes from the group III nitride semiconductor. As a
result, the area of the semiconductor device becomes large and
there is a problem that the semiconductor device must be produced
with high cost. Moreover, if the area of the semiconductor device
becomes large, another problem of a curvature of the substrate may
arise due to the use of the combination of different materials: the
substrate and the group III nitride semiconductor.
[0007] Moreover, in the case of the group III nitride semiconductor
device produced on the sapphire substrate, performing the chip
separation by cleavage is difficult and preparing the resonator end
surface needed for a laser diode (LD) is difficult.
[0008] For this reason, currently, the sapphire substrate is ground
or dry etched to a thickness of 100 micrometers or less, and then
the chip separation is performed in a manner analogous to that of
the cleavage separation so that the resonator end surface is
formed. Therefore, it is difficult to perform the formation of the
resonator end surface and the chip separation at a single process
as in the conventional laser diode (LD), which will increase the
manufacturing cost due to complication of the processes.
[0009] In order to obviate the problems, there is proposed an
improved method in which a group III nitride semiconductor is
selectively grew up in a transverse direction on the sapphire
substrate, in order to reduce the amount of crystal defects
included in the group III nitride semiconductor. Although it is
possible to reduce the amount of crystal defects by using the
improved method, the above problems related to the dielectric
feature of the sapphire substrate and the difficulty of the
cleavage separation still remain.
[0010] One method to solve the above problems is to use a gallium
nitride (GaN) substrate which is of the same material as the
crystal which is subjected to crystal growth on the substrate. For
this reason, there are proposed various methods of carrying out
crystal growth of a bulk GaN through vapor growth, molten metal
growth, etc. However, a GaN substrate which has good quality and
practical size is not yet realized.
[0011] As one method of realizing a GaN substrate, there is
proposed a GaN crystal growth method which uses sodium (Na) as
flux. See U.S. Pat. No. 5,868,837. In this method, sodium azide
(NaN3) and metal Ga are used as raw materials, NaN3 and metal Ga
are placed into a reaction container made of stainless steel (the
inside dimensions of the container: inside diameter=7.5 mm,
length=100 mm) in a nitrogen atmosphere, and the reaction container
is held at the temperature of 600-800 degrees C. for 24 to 100
hours, so that a GaN crystal grows.
[0012] According to the above-mentioned method, the crystal growth
in a range of comparatively low temperatures: 600-800 degrees C. is
attained, and the internal pressure of the container is about 100
kg/cm.sup.2 which is comparatively low. Thus, the above-mentioned
method provides practical crystal growth conditions.
[0013] Moreover, there is recently proposed a crystal preparing
method in which a mixed molten metal containing alkali metal and
group III metal is made to react with a group V raw material
including nitrogen, so that a quality group III nitride crystal is
prepared. See Japanese Laid-Open Patent Application No.
2001-058900.
[0014] Moreover, there is also proposed a crystal preparing method
in which a mixed molten metal containing metal Na and metal Ga is
put into a crucible, the crucible is placed in a reaction
container, and molten metal Na is held outside the crucible and
within the reaction container. By raising the vapor pressure of the
molten metal Na, crystal growth of a GaN crystal is carried out.
See Japanese Laid-Open Patent Application No. 2005-263571.
[0015] However, the conventional crystal preparing devices which
carry out crystal growth of a GaN crystal have a problem that
holding the mixing ratio of metal Na and metal Ga in the mixed
molten metal at a controlled ratio during the crystal growth of a
GaN crystal is difficult. There may be the case in which the vapor
pressure of metal Na is too high so that the vapor is spread from
the mixed molten metal to the exterior. Conversely, there may be
also the case in which the vapor pressure of metal Na is too low so
that the vapor is diffused from the exterior to the mixed molten
metal.
[0016] Moreover, metal Ga is consumed out of the mixed molten metal
with the progress of preparation of crystal growth. As a result, it
is difficult to maintain the mixing ratio of metal Na and metal Ga
in the mixed molten metal at a controlled ratio stably.
SUMMARY OF THE INVENTION
[0017] According to one aspect of the invention, there is provided
an improved crystal preparing device and method in which the
above-described problems are eliminated.
[0018] According to one aspect of the invention there is provided a
crystal preparing device which stably holds the mixing ratio of an
alkali metal and a group III metal at a controlled ratio and
carries out crystal growth of a group III nitride crystal.
[0019] According to one aspect of the invention there is provided a
crystal preparing method which stably holds the mixing ratio of an
alkali metal and a group III metal at a controlled ratio to produce
a group III nitride crystal.
[0020] In an embodiment of the invention which solves or reduces
one or more of the above-mentioned problems, there is provided a
crystal preparing device comprising: a crucible holding a mixed
molten metal containing alkali metal and group III metal; a
reaction container having a container space which contacts the
mixed molten metal and holding a molten alkali metal between the
container space and an outside of the container, the molten alkali
metal contacting the container space; a gas supply device supplying
a nitrogen gas to the container space; and a heating device heating
the crucible to a crystal growth temperature, wherein the crystal
preparing device is provided so that a vapor pressure of the alkali
metal which evaporates from the molten alkali metal is
substantially equal to a vapor pressure of the alkali metal which
evaporates from the mixed molten metal.
[0021] In an embodiment of the invention which solves or reduces
one or more of the above-mentioned problems, there is provided a
crystal preparing method which produces a group III nitride crystal
using a crystal preparing device, the crystal preparing device
including a crucible holding a mixed molten metal containing alkali
metal and group III metal, a reaction container having a container
space which contacts the mixed molten metal and holding a molten
alkali metal between the container space and an outside of the
container, the molten alkali metal contacting the container space,
and a piping disposed below the crucible and connected with the
reaction container, the crystal preparing method comprising: a
first step of placing the alkali metal and the group III metal into
the crucible in an inert gas or nitrogen gas atmosphere; a second
step of placing the alkali metal into the piping in the inert gas
or nitrogen gas atmosphere; a third step of filling the container
space of the reaction container with nitrogen gas; a fourth step of
heating the crucible to a crystal growth temperature; a fifth step
of heating the molten alkali metal at a specific temperature at
which a vapor pressure of the alkali metal which evaporates from
the molten alkali metal is substantially equal to a second vapor
pressure of the alkali metal which evaporates from the mixed molten
metal; and a sixth step of holding a temperature of the crucible to
the crystal growth temperature, and holding a temperature of the
molten alkali metal to the specific temperature for a predetermined
time.
[0022] According to the embodiments of the crystal preparing device
and method of the invention, a vapor pressure of alkali metal which
evaporates from a mixed molten metal containing alkali metal and
group III metal and a vapor pressure of alkali metal which
evaporates from a molten alkali metal are substantially equal to
each other, and a group III nitride crystal is produced through
crystal growth. In this case, movement of alkali metal from the
mixed molten metal to the molten alkali metal and movement of
alkali metal from the molten alkali metal to the mixed molten metal
are balanced during the crystal growth of the group III nitride
crystal. Therefore, according to the embodiments of the invention,
the mixing ratio of the alkali metal and the group III metal in the
mixed molten metal can be held at a controlled ratio.
[0023] In an embodiment of the invention which solves or reduces
one or more of the above-mentioned problems, there is provided a
crystal preparing method which produces a group III nitride crystal
using a crystal preparing device including a reaction container
holding a mixed molten metal containing alkali metal and group III
metal, the crystal preparing method comprising: a first step of
placing the alkali metal and the group III metal in a reaction
container in an inert gas or nitrogen gas atmosphere containing a
predetermined amount of impurities; a second step of filling a
container space of the reaction container with nitrogen gas; a
third step of heating the reaction container to a crystal growth
temperature; a fourth step of holding a temperature of the reaction
container at the crystal growth temperature for a predetermined
time; and a fifth step of supplying nitrogen gas to the reaction
container to hold a pressure in the container space of the reaction
container at a predetermined pressure.
[0024] In an embodiment of the invention which solves or reduces
one or more of the above-mentioned problems, there is provided a
group III nitride crystal containing group III metal and nitrogen,
wherein the group III nitride crystal has a dislocation density of
10.sup.3cm.sup.-2 or less and contains oxygen in a concentration of
10.sup.20cm.sup.-3 or less.
[0025] According to the embodiments of the invention, a group III
nitride crystal is produced by placing alkali metal and group III
metal in an inert gas or nitrogen gas atmosphere containing oxygen
and/or moisture in a concentration of 10 ppm or less. As a result,
the group III nitride crystal having a dislocation density of
10.sup.3cm.sup.-2 or less and containing oxygen of
10.sup.20cm.sup.-3 or less, or the group III nitride crystal having
a dislocation density of 10.sup.3cm.sup.-2 or less and containing
oxygen of 10.sup.20cm.sup.-3 or less and hydrogen of
10.sup.20cm.sup.-3 or less is produced. Therefore, according to the
embodiments of the invention, the group III nitride crystal
suitable for use in optical devices or electronic devices can be
produced.
[0026] Moreover, according to the embodiments of the invention, the
group III nitride crystal has a dislocation density of
10.sup.3cm.sup.-2 or less and contains oxygen in a concentration of
10.sup.20cm.sup.-3 or less. Moreover, the group III nitride crystal
has a dislocation density of 10.sup.3cm.sup.-2 or less and contains
oxygen of 10.sup.20cm.sup.-3 or less and hydrogen of
10.sup.20cm.sup.-3 or less. As a result, it is possible to produce
any of an insulator containing a group III nitride crystal, a group
III nitride semiconductor with a low resistance, and a group III
nitride semiconductor with a high resistance. Therefore, according
to the embodiments of the invention, the resulting group III
nitride crystal can be used for optical devices or electron
devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Other objects, features and advantages of the present
invention will be apparent from the following detailed description
when reading in conjunction with the accompanying drawings.
[0028] FIG. 1 is a diagram showing the composition of a crystal
preparing device in an embodiment of the invention.
[0029] FIG. 2 is a perspective view of the molten metal holding
member shown in FIG. 1.
[0030] FIG. 3 is a plan view showing the mounting state of the
molten metal holding member to piping.
[0031] FIG. 4A and FIG. 4B are enlarged views of the supporting
device, the piping, and the thermocouple shown in FIG. 1.
[0032] FIG. 5 is a diagram showing the composition of an
up-and-down movement device shown in FIG. 1.
[0033] FIG. 6 is a timing chart of a vibration detection
signal.
[0034] FIG. 7 is a timing chart of the temperature of a crucible
and a reaction container.
[0035] FIG. 8A and FIG. 8B are diagrams showing the change of state
in the crucible and the reaction container before and after timing
t1 shown in FIG. 7.
[0036] FIG. 9 is a diagram showing the state in the crucible and
the reaction container between timing t1 and timing t2 shown in
FIG. 7.
[0037] FIG. 10 is a diagram showing the relation between the
temperature of seed crystal and the flow rate of nitrogen gas.
[0038] FIG. 11 is a diagram showing the relation between the
nitrogen gas pressure and the crystal growth temperature in the
case of growing up a GaN crystal.
[0039] FIG. 12 is a flowchart for explaining the crystal preparing
method of a GaN crystal.
[0040] FIG. 13A and FIG. 13B are diagrams for explaining the amount
of metal Na.
[0041] FIG. 14 is a diagram showing the composition of a crystal
preparing device in an embodiment of the invention.
[0042] FIG. 15 is a flowchart for explaining the crystal preparing
method of a GaN crystal.
[0043] FIG. 16 is a diagram showing an example of the molten metal
holding member.
[0044] FIG. 17 is a diagram for explaining the fixing method of the
molten metal holding member shown in FIG. 16.
[0045] FIG. 18A and FIG. 18B are diagrams showing another example
of the molten metal holding member.
[0046] FIG. 19 is a diagram showing the composition of a crystal
preparing device in an embodiment of the invention.
[0047] FIG. 20 is a timing chart of an vibration detection
signal.
[0048] FIG. 21 is a diagram showing the state in the reaction
container and the external reaction container between timing t1 and
timing t2 shown in FIG. 7.
[0049] FIG. 22 is a diagram showing the relation between the
nitrogen gas pressure and the crystal growth temperature in the
case of growing up a GaN crystal.
[0050] FIG. 23 is a diagram for explaining the method of setting
metal Na and metal Ga to a reaction container and an external
reaction container.
[0051] FIG. 24 is a flowchart for explaining the crystal preparing
method of a GaN crystal.
[0052] FIG. 25 is a diagram showing the state in the reaction
container and the external reaction container in step S9 shown in
FIG. 24.
[0053] FIG. 26 is a diagram showing the state in the reaction
container and the external reaction container in step S10 shown in
FIG. 24.
[0054] FIG. 27 is a diagram showing the state of the reaction
container and the external reaction container at the end of
preparation of a GaN crystal according to the flowchart shown in
FIG. 24.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0055] A description will be given of embodiments of the invention
with reference to the accompanying drawings.
[0056] FIG. 1 shows the composition of a crystal preparing device
in an embodiment of the invention. As shown in FIG. 1, the crystal
preparing device 100 of this embodiment includes a crucible 10, a
reaction container 20, piping 30, piping 200, a bellows 40, a
supporting device 50, a molten metal holding member 60, heating
devices 70, 80, temperature sensors 71, 81, gas supply lines 90,
110, 250, valves 120, 121, 160, a pressure regulator 130, gas
cylinders 140, 270, an exhaust pipe 150, a vacuum pump 170,
pressure sensor 180, molten metal 190, a thermocouple 210, an
up-and-down movement device 220, a vibration applying device 230, a
vibration detecting device 240, a flowmeter 260, and a temperature
control device 280.
[0057] The crucible 10 has a generally cylindrical shape and is
made of boron nitride (BN) or stainless steel (SUS316L). The
reaction container 20 is arranged around the crucible 10 with a
given interval separate from the crucible 10. Namely, the reaction
container 20 accommodates the crucible 10 inside the reaction
container 20. The reaction container 20 includes a body part 21 and
a lid part 22. Each of the body part 21 and the lid part 22 is made
of stainless steel (SUS316L). The reaction container 20 is sealed
by a metal O-ring (not shown) disposed between the body part 21 and
the lid part 22.
[0058] The piping 30 extends from the bottom of the reaction
container 20 in gravity direction DR1, and one end of the piping 30
is connected with the inside of the reaction container 20 on the
bottom of the crucible 10 and the other end thereof is connected
with the gas supply line 110.
[0059] The bellows 40 extends from the top of the reaction
container 20 in gravity direction DR1 and is connected with the
reaction container 20 on the top of the crucible 10. The supporting
device 50 is in the form of a hollow cylinder, and the lower half
of the supporting device 50 is inserted into space 23 of the
reaction container 20 through the bellows 40.
[0060] The molten metal holding member 60 is made of, for example,
a metal, ceramics, etc., and disposed in the piping 30 at a
location below the connection part of the reaction container 20 and
the piping 30.
[0061] The heating devices 70 are arranged so that the outer
peripheral surface 20A of the reaction container 20 is surrounded
by the heating devices 70. The heating devices 80 are arranged so
that they confront the bottom 20B of the reaction container 20. The
temperature sensors 71 and 81 are arranged in the vicinity of the
heating devices 70 and 80, respectively.
[0062] The gas supply line 90 is arranged so that one end of the
gas supply line 90 is connected with the reaction container 20
through the valve 120 and the other end thereof is connected with
the gas cylinder 140 through the pressure regulator 130.
[0063] The gas supply line 110 is arranged so that one end of the
gas supply line 110 is connected with the piping 30 through the
valve 121 and the other end thereof is connected with the gas
supply line 90.
[0064] The valve 120 is disposed to the gas supply line 90 near the
reaction container 20. The pressure regulator 130 is disposed to
the gas supply line 90 near the gas cylinder 140. The gas cylinder
140 is connected with the gas supply line 90.
[0065] The exhaust pipe 150 is arranged so that one end of the
exhaust pipe 150 is connected with the reaction container 20
through the valve 160, and the other end thereof is connected with
the vacuum pump 170. The valve 160 is disposed to the exhaust pipe
150 with near the reaction container 20. The vacuum pump 170 is
connected with the exhaust pipe 150.
[0066] The pressure sensor 180 is attached to the reaction
container 20. The molten metal 190 includes molten metal sodium
(Na) and is held in the piping 30 by the molten metal holding
member 60.
[0067] The piping 200 and the thermocouple 210 are inserted in the
inside of the supporting device 50. The up-and-down mechanism 220
is attached to the supporting device 50 above the bellows 40. The
gas supply line 250 is arranged so that one end of the gas supply
line 250 is connected with the piping 200 and the other end thereof
is connected with the gas cylinder 270 through the flowmeter 260.
The flowmeter 260 is disposed to the gas supply line 250 near the
gas cylinder 270. The gas cylinder 270 is connected with the gas
supply line 250.
[0068] The crucible 10 holds a mixed molten metal 290 containing
metal Na and metal Ga. The reaction container 20 covers the
circumference of the crucible 10. The piping 30 holds molten metal
190. The bellows 40 holds the supporting device 50 and intercepts
the inside and the outside of the reaction container 20. The
bellows 40 is expanded or contracted in gravity direction DR1 in
accordance with movement of the supporting device 50 in gravity
direction DR1. The supporting device 50 supports a seed crystal
(GaN crystal) 5 at its bottom end which is inserted into the
reaction container 20.
[0069] The molten metal holding member 60 has a concavo-convex
structure on the outer peripheral surface so that a number of small
holes on the order of tens of micrometers are formed between the
holding member 60 and the inner wall of the piping 30. The nitrogen
gas supplied to the space 31 in the piping 30 passes through the
small holes of the holding member 60 in the direction to the molten
metal 190, so that the nitrogen gas is supplied to the space 23 of
the reaction container 20 through the molten metal 190. The molten
metal holding member 60 holds the molten metal 190 in the piping 30
by the surface tension of the molten metal 190.
[0070] Each of the heating devices 70 includes a heater and a
current source. According to a control signal CTL1 from the
temperature control device 280, the current source of each heating
device 70 supplies electric current through the heater, so that the
heating devices 70 heat the crucible 10 and the reaction container
20 from the outer peripheral surface 20A of the reaction container
20 to a crystal growth temperature.
[0071] The temperature sensor 71 detects a temperature T1 of the
heater of each heating device 70, and outputs the detected
temperature T1 to the temperature control device 280.
[0072] Each of the heating devices 80 also includes a heater and a
current source. According to a control signal CTL2 from the
temperature control device 280, the current source of each heating
device 80 supplies electric current through the heater, so that the
heating devices 80 heat the crucible 10 and the reaction container
20 from the bottom 20B of the reaction container 20 to a crystal
growth temperature.
[0073] The temperature sensor 81 detects a temperature T2 of the
heater of each heating device 80, and outputs the detected
temperature T2 to the temperature control device 280.
[0074] The gas supply line 90 supplies the nitrogen gas which is
fed through the pressure regulator 130 from the gas cylinder 140,
to the reaction container 20 through the valve 120. The gas supply
line 110 supplies the nitrogen gas which is fed through the
pressure regulator 130 from the gas cylinder 140, to the piping 30
through the valve 121.
[0075] The valve 120 supplies the nitrogen gas in the gas supply
line 90 to the reaction container 20, or suspends the supply of the
nitrogen gas to the reaction container 20. The pressure regulator
130 regulates the nitrogen gas from the gas cylinder 140 at a
predetermined pressure, and supplies the nitrogen gas under the
regulated pressure to the gas supply lines 90, 110.
[0076] The gas cylinder 140 holds nitrogen gas. The exhaust pipe
150 passes the gas in the reaction container 20 to the vacuum pump
170. The valve 160 connects the exhaust pipe 150 and the reaction
container 20 spatially, or intercepts the exhaust pipe 150 and the
reaction container 20 spatially. The vacuum pump 170 performs
vacuum suction in the reaction container 20 through the exhaust
pipe 150 and the valve 160.
[0077] The pressure sensor 180 detects the pressure in the reaction
container 20. The molten metal 190 serves to supply the nitrogen
gas entered through the molten metal holding member 60, to the
space 23 of the reaction container 20.
[0078] The piping 200 supplies the nitrogen gas fed from the gas
supply line 250 to one end of the piping 200, to the supporting
device 50, in order to cool the seed crystal 5. The thermocouple
210 detects a temperature T3 of the seed crystal 5, and outputs the
detected temperature T3 to the temperature control device 280.
[0079] According to a vibration detection signal BDS from the
vibration detecting device 240, the up-and-down movement device 220
moves up or down the supporting device 50 so that the seed crystal
5 may touch the gas-liquid interface 3 of the space 23 and the
mixed molten metal 290.
[0080] The vibration applying device 230 includes, for example, a
piezoelectric element and applies vibrations having a predetermined
frequency to the supporting device 50. The vibration detecting
device 240 includes, for example, an acceleration pickup and
detects the vibrations of the supporting device 50. The vibration
detecting device 240 outputs the vibration detection signal BDS,
indicating the vibrations of the supporting device 50, to the
up-and-down movement device 220.
[0081] The gas supply line 250 supplies the nitrogen gas fed
through the flowmeter 260 from the gas cylinder 270, to the piping
200. The flowmeter 260 adjusts the flow rate of the nitrogen gas
supplied from the gas cylinder 270, according to a control signal
CTL3 from the temperature control device 280, and supplies the
nitrogen gas to the gas supply line 250 at the controlled flow
rate. The gas cylinder 270 holds nitrogen gas.
[0082] The temperature control device 280 receives the detected
temperatures T1, T2, and T3 from the temperature sensors 71 and 81
and the thermocouple 210, respectively, and generates the control
signals CTL1-CTL3 based on the received temperatures T1, T2, and
T3.
[0083] The temperature control device 280 outputs the generated
control signals CTL1 and CTL2 to the heating devices 70 and 80,
respectively, and outputs the generated control signal CTL3 to the
flowmeter 260.
[0084] FIG. 2 is a perspective view of the molten metal holding
member 60 in FIG. 1. As shown in FIG. 2, the molten metal holding
member 60 includes a plug 61 and a number of projections 62. The
plug 61 has a generally cylindrical shape. Each of the projections
62 has a cross-section in the form of a semi-circle, and is formed
on the outer peripheral surface of the plug 61 along the
longitudinal direction DR2 of the plug 61.
[0085] FIG. 3 is a plan view showing the mounting state of the
molten metal holding member 60 to the piping 30. As shown in FIG.
3, the plurality of projections 62 are arranged in the
circumferential direction of the plug 61 at intervals "d" on the
order of tens of micrometers. Each of the projections 62 has a
height H from the outer peripheral surface of the plug 61 on the
order of tens of micrometers. The projections 62 of the molten
metal holding member 60 touch the inner wall 30A of the piping 30.
Thereby, the molten metal holding member 60 is fitted into the
inner wall 30A of the piping 30.
[0086] Since the projections 62 have the height H on the order of
tens of micrometers and are arranged at the intervals d on the
order of tens of micrometers on the outer peripheral surface of the
plug 61, a number of voids 63 each having a diameter on the order
of tens of micrometers are formed between the molten metal holding
member 60 and the inner walls 30A of the piping 30 when the molten
metal holding member 60 is fitted into the inner wall 30A of the
piping 30.
[0087] This void 63 serves to pass nitrogen gas through it in the
longitudinal direction DR2 of the plug 61, and serves to hold the
molten metal 190 by the surface tension of the molten metal 190,
thereby preventing the passing of the molten metal 190 in the
longitudinal direction DR2 of the plug 61.
[0088] FIG. 4A and FIG. 4B are enlarged views of the supporting
device 50, the piping 200, and the thermocouple 210 shown in FIG.
1. As shown in FIG. 4A and FIG. 4B, the supporting device 50
includes a cylindrical member 51 and holding members 52 and 53.
[0089] The cylindrical member 51 has a generally circular
cross-section. The holding member 52 has a generally L-shaped
cross-section, and is fixed at one end 511 of the cylindrical
member 51 to the outer peripheral surface 51A and the bottom 51B of
the cylindrical member 51. The holding member 53 has a generally
L-shaped cross section, and is fixed at one end 511 of the
cylindrical member 51 to the outer peripheral surface 51A and the
bottom 51B of the cylindrical member 51. The holding members 52 and
53 are arranged symmetrically about the centerline of the
cylindrical member 51. As a result, a space 54 is formed in the
area that is surrounded by the cylindrical member 51 and the
holding members 52 and 53.
[0090] The piping 200 has a generally circular cross-section and is
arranged inside the cylindrical member 51. In this case, the bottom
200A of the piping 200 is arranged so that it confronts the bottom
51B of the cylindrical member 51. A plurality of holes 201 are
formed in the bottom 200A of the piping 200. The nitrogen gas
supplied to the piping 200 is sprayed through the holes 201 to the
bottom 51B of the cylindrical member 51.
[0091] The thermocouple 210 is arranged inside the cylindrical
member 51 so that one end 210A of the thermocouple 210 touches the
bottom 51B of the cylindrical member 51 (see FIG. 4A).
[0092] The seed crystal 5 is configured so that it is fitted into
the space 54 and supported by the supporting device 50 through the
fitting of the seed crystal 5 to the space 54. In this case, the
seed crystal 5 touches the bottom 51B of the cylindrical member 51
(see FIG. 4B).
[0093] Therefore, the heat conductivity between the seed crystal 5
and the cylindrical member 51 is raised. As a result, the
temperature of the seed crystal 5 can be detected by the
thermocouple 210, and the seed crystal 5 can be easily cooled with
the nitrogen gas sprayed from the piping 200 to the bottom 51B of
the cylindrical member 51.
[0094] In this embodiment, the seed crystal itself is configured so
that it is fitted into the supporting device 50 like the seed
crystal 5. Alternatively, a hexagon-shaped seed crystal may be held
through an adapter having the form of the seed crystal 5, if it is
configured to touch the supporting device 50.
[0095] FIG. 5 shows the composition of the up-and-down movement
device 220 shown in FIG. 1. As shown in FIG. 5, the up-and-down
movement device 220 includes a rack member 221, a gear 222, a shaft
member 223, a motor 224, and a control unit 225.
[0096] The rack member 221 has a generally triangular cross-section
and is fixed to the outer peripheral surface 51A of the cylindrical
member 51. The gear 222 is fixed to one end of the shaft member 223
and is engaged with the rack member 221. The shaft member 223 is
connected at its one end with the gear 222, and the other end of
the shaft member 223 is connected with the shaft (not shown) of the
motor 224.
[0097] The motor 224 rotates the gear 222 in the direction
indicated by the arrow 226 or 227 in accordance with a control
signal from the control unit 225. The control unit 225 controls the
motor 224 based on the vibration detection signal BDS from the
vibration detecting device 240 to rotate the gear 222 in the
direction indicated by the arrow 226 or 227.
[0098] The supporting device 50 is moved upward in the gravity
direction DR1 when the gear 222 rotates in the direction indicated
by the arrow 226 in FIG. 5, and when the gear 222 rotates in the
direction indicated by the arrow 227 in FIG. 5, the supporting
device 50 is moved downward in the gravity direction DR1.
Therefore, the clockwise or counter-clockwise rotation of the gear
222 in the direction indicated by the arrow 226 or 227 is
equivalent to the upward or downward movement of the supporting
device 50 in the gravity direction DR1. The length of the rack
member 221 in the gravity direction DR1 is equivalent to the
distance by which the seed crystal 5 is raised or lowered by the
supporting device 50.
[0099] FIG. 6 is a timing chart of the vibration detection signal
BDS. As shown in FIG. 6, the vibration detection signal BDS
detected by the vibration detecting device 240 includes a signal
component SS1 when the seed crystal 5 is not in contact with the
mixed molten metal 290. On the other hand, when the seed crystal 5
is in contact with the mixed molten metal 290, the vibration
detection signal BDS includes a signal component SS2.
[0100] The seed crystal 5 vibrates greatly by the vibrations
applied by the vibration applying device 230 when the seed crystal
5 is not in contact with the mixed molten metal 290, and the
vibration detection signal BDS in this case includes the signal
component SS1 having a comparatively large amplitude.
[0101] On the other hand, the seed crystal 5 does not vibrate
greatly due to the viscosity of the mixed molten metal 290 even if
vibrations are applied thereto by the vibration applying device 230
when the seed crystal 5 is in contact with the mixed molten metal
290, and the signal detection signal BDS in this case includes the
signal component SS2 having a comparatively small amplitude.
[0102] Referring back to FIG. 5, when the vibration detection
signal BDS is received from the vibration detecting device 240, the
control unit 225 detects the signal component of the received
vibration detection signal BDS. When the detected signal component
includes the signal component SS1, the control unit 225 controls
rotation of the motor 224 so that the supporting device 50 is
lowered in the gravity direction DR1, until the detected signal
component of the vibration detection signal BDS is changed to the
signal component SS2.
[0103] Specifically, the control unit 225 controls the motor 224 to
rotate the gear 222 in the direction indicated by the arrow 227,
and the motor 224 rotates the gear 222 in the direction of arrow
227 through the shaft member 223 according to the control from the
control unit 225. In this manner, the supporting device 50 is moved
downward in the gravity direction DR1.
[0104] If the signal component of the vibration detection signal
BDS received from the vibration detecting device 240 is changed
from the signal component SS1 to the signal component SS2, the
control unit 225 controls the motor 224 to suspend rotation of the
gear 222, and the motor 224 stops rotation of the gear 222
according to the control from the control unit 225. In this manner,
the supporting device 50 suspends movement and holds the seed
crystal 5 to the gas-liquid interface 3.
[0105] On the other hand, when the vibration detection signal BDS
including the signal component SS2 is received from the vibration
detecting device 240, the control unit 225 controls the motor 224
to suspend movement of the supporting device 50. Thus, based on the
vibration detection signal BDS detected by the vibration detecting
device 240, the up-and-down movement device 220 moves up or down
the supporting device 50 in the gravity direction DR1 so that the
seed crystal 5 may touch the mixed molten metal 290.
[0106] FIG. 7 is a timing chart of the temperature of the crucible
10 and the reaction container 20. FIG. 8A and FIG. 8B are diagrams
showing the change of state in the crucible 10 and the reaction
container 20 before and after timing t1 shown in FIG. 7. FIG. 9 is
a diagram showing the state in the crucible 10 and the reaction
container 20 between timing t1 and timing t2 shown in FIG. 7. FIG.
10 is a diagram showing the relation between the temperature of
seed crystal 5 and the flow rate of nitrogen gas.
[0107] In FIG. 7, the straight line k1 indicates the temperature of
the crucible 10 and the reaction container 20, and the curve k2 and
the straight line k3 indicate the temperature of the seed crystal
5.
[0108] As shown in FIG. 7, the heating devices 70 and 80 heat the
crucible 10 and the reaction container 20 so that the temperature
rises according to the straight line k1 and is held at 800 degrees
C.
[0109] When the heating devices 70 and 80 start heating the
crucible 10 and the reaction container 20, metal Na 6 exists in the
piping 30 and metal Na 6 and metal Ga 7 exist in the crucible 10
(see FIG. 8A).
[0110] When the temperature of the crucible 10 and the reaction
container 20 rises to 98 degrees C. in timing t1, metal Na in the
crucible 10 is melted and mixed with metal Ga which is already been
melted at about 30 degrees C. As a result, the mixed molten metal
290 is produced in the crucible 10.
[0111] Then, the temperature of the crucible 10 and reaction
container 20 rises to 800 degrees C. in timing t2. In the process
in which the temperature of the crucible 10 and the reaction
container 20 rises to 800 degrees C., the temperature of the piping
30 above the molten metal holding member 60 rises to 98 degrees C.
or higher. At such temperature, metal Na 6 held in the piping 30 is
melted and the molten metal 190 is produced in the piping 30. And
the molten metal holding member 60 holds the molten metal 190 by
the surface tension of the molten metal 190.
[0112] In this case, the heating devices 80 raise the temperature
of the portion of the piping 30 which touches the molten metal 190
to a specific temperature at which the vapor pressure PNa of metal
Na which evaporates from the molten metal 190 is substantially
balanced with the vapor pressure PNa--Ga of metal Na which
evaporates from the mixed molten metal 290. Namely, the temperature
of the portion of the piping 30 which touches the molten metal 190
is raised to the specific temperature at which evaporation of metal
Na from the molten metal 190 and evaporation of metal Na from the
mixed molten metal 290 become equilibrium by the heating device 80
(see FIG. 8B).
[0113] Accordingly, movement of metal Na from the molten metal 190
to the mixed molten metal 290 and movement of metal Na from the
mixed molten metal 290 to the molten metal 190 become equilibrium,
and movement of metal Na is seemingly suspended between the molten
metal 190 and the mixed molten metal 290. As a result, it is
possible to prevent changing of the mixing ratio of metal Na and
metal Ga in the mixed molten metal 290 due to the evaporation of
metal Na from the molten metal 190 and the mixed molten metal 290.
If the temperature of the molten metal holding member 60 at this
time is above the melting point of metal Na and substantial
evaporation of Na does not occur a this temperature, diffusion of
metal Na to the space 31 of the piping 30 can be disregarded, and
it is possible to prevent changing of the mixing ratio of metal Na
and metal Ga in the mixed molten metal 290. The temperature at
which substantial evaporation of Na does not occur is, for example,
a temperature in a range of 200-300 degrees C. The vapor pressure
of Na at 200 degrees C. is about 1.8.times.10.sup.-2 Pa, and the
vapor pressure of Na at 300 degrees C. is about 1.8 Pa. When the
specific temperature is beyond the above-mentioned range, there is
diffusion by some evaporation but it is possible to prevent
changing of the mixing ratio of metal Na and metal Ga in the mixed
molten metal 290.
[0114] When the temperature of the crucible 10 and the reaction
container 20 rises to 98 degrees C. in timing t1 and rises to 800
degrees C. to timing t2, the nitrogen gas 4 in the space 23 cannot
be spread into the space 31 of the piping 30 through the molten
metal 190 (=molten metal Na) and the molten metal holding member
60, but it is enclosed in the space 23 (see FIG. 9).
[0115] In timing t2 when the temperature of the crucible 10 and the
reaction container 20 rises to 800 degrees C., the up-and-down
movement device 220 moves up or down the supporting device 50 based
on the vibration detection signal BDS from the vibration detecting
device 240, so that the seed crystal 5 is brought in contact with
the mixed molten metal 290.
[0116] In the high temperature state in which the temperature of
the crucible 10 and the reaction container 20 is about 800 degrees
C., the nitrogen gas 4 in the space 23 is captured into the mixed
molten metal 290 through the metal Na. In this case, the
concentration of nitrogen or the concentration of Ga.sub.xN.sub.y
(where x and y are real numbers) in the mixed molten metal 290 near
the gas-liquid interface 3 of the space 23 and the mixed molten
metal 290 is the highest, and crystal growth begins from the seed
crystal 5 where the GaN crystal touches the gas-liquid interface
3.
[0117] In this invention, Ga.sub.xN.sub.y is called "group III
nitride" and concentration of Ga.sub.xN.sub.y is called "group III
nitride concentration".
[0118] When nitrogen gas is not supplied to the piping 200, the
temperature T3 of seed crystal 5 is the same as the temperature
(800 degrees C.) of the mixed molten metal 290. However, in this
invention, in order to raise the degree of supersaturation of
nitrogen or a group III nitride in the mixed molten metal 290 in
the neighborhood of the seed crystal 5, nitrogen gas is supplied to
the piping 200 so that the seed crystal 5 is cooled, and the
temperature T3 of the seed crystal 5 is made lower than the
temperature of the mixed molten metal 290.
[0119] More specifically, after timing t2, the temperature T3 of
seed crystal 5 is set to the temperature Ts1 (which is lower than
800 degrees C.) according to the curve k2. This temperature Ts1 is
equal to 790 degrees C., for example.
[0120] The method which sets the temperature T3 of the seed crystal
5 to the temperature Ts1 will be explained. When the temperatures
T1 and T2 received from temperature sensors 71 and 81 reach
(800+alpha) .degree. C. (=temperature of the heaters contained in
the heating devices 70 and 80 when setting the crucible 10 and the
reaction container 20 to 800 degrees C.), the temperature control
device 280 generates a control signal CTL3 for pouring the nitrogen
gas at a flow rate which sets the temperature T3 of the seed
crystal 5 to the temperature Ts1 and outputs the control signal
CTL3 to the flowmeter 260.
[0121] In response to the control signal CTL3, the flowmeter 260
pours the nitrogen gas from the gas cylinder 270 into the piping
200 through the gas supply line 250 at the flow rate which sets the
temperature T3 to the temperature Ts1.
[0122] The temperature of the seed crystal 5 falls from 800 degrees
C. proportionally in accordance with the flow rate of the nitrogen
gas. When the flow rate of the nitrogen gas is set to the flow rate
fr1 (sccm), the temperature T3 of the seed crystal 5 is set to the
temperature Ts1 (see FIG. 10). Therefore, the flowmeter 260 pours
the nitrogen gas into the piping 200 at the flow rate fr1.
[0123] The nitrogen gas supplied to the piping 200 is sprayed to
the bottom 51B of the cylindrical member 51 through the small holes
201 of the piping 200. The seed crystal 5 is cooled through the
bottom 51B of the cylindrical member 51, and the temperature T3 of
the seed crystal 5 falls to the temperature Ts1 in timing t3, and
it is held at this temperature Ts1 up to timing t4.
[0124] There is a predetermined difference of temperature alpha
between the temperatures T1 and T2 of the heaters of the heating
devices 70 and 80 and the temperature of the mixed molten metal
290. And the temperature control device 280 controls, when the
temperature T3 of the seed crystal 5 begins to fall from 800
degrees C., the heating devices 70 and 80 by outputting the control
signals CTL1 and CTL2, so that the temperatures T1 and T2 received
from the temperature sensors 71 and 81 are set to (800+alpha)
.degree. C.
[0125] In this invention, the temperature T3 of the seed crystal 5
is preferably controlled to fall according to the straight line k3
after timing t2. That is, the temperature T3 of the seed crystal 5
falls from 800 degrees C. to the temperature Ts2 (<Ts1) during
the period between timing t2 and timing t4. In this case, the
flowmeter 260 increases the flow rate of the nitrogen gas poured
into the piping 200 from 0 to the flow rate fr2 (>fr1) according
to the straight line k4 based on the control signal CTL3 from the
temperature control device 280.
[0126] If the flow rate of nitrogen gas is set to the flow rate
fr2, the temperature T3 of the seed crystal 5 is set to the
temperature Ts2 (which is lower than the temperature Ts1). This
temperature Ts2 is 750 degrees C., for example.
[0127] In this manner, by increasing gradually the difference
between the temperature (=800 degrees C.) of the mixed molten metal
290 and the temperature T3 of the seed crystal 5, the degree of
supersaturation of nitrogen or a group III nitride in the mixed
molten metal 290 in the neighborhood of the seed crystal 5 becomes
large gradually, and the crystal growth of a GaN crystal can be
continued.
[0128] In performing crystal growth of a GaN crystal using the
crystal preparing device 100, the seed crystal 5 is replaced by a
GaN crystal which grows up in the crystal preparing device 100
without using the seed crystal 5.
[0129] FIG. 11 is a diagram showing the relation between the
nitrogen gas pressure and the crystal growth temperature in the
case of growing up a GaN crystal.
[0130] In FIG. 11, the horizontal axis expresses the crystal growth
temperature and the vertical axis expresses the nitrogen gas
pressure. In FIG. 11, the range REG2 is a range in which many
nuclei occur in the bottom and the side of the crucible 10 which
touch the mixed molten metal 290 and a GaN crystal in the
pillar-like form growing up in the direction of c axis
(<0001>) is produced.
[0131] When producing seed crystal 5, a GaN crystal is grown up
using the nitrogen gas pressure and the crystal growth temperature
which are located in the range REG2. In this case, many nuclei
occur on the bottom and the side wall in the crucible 10, and a
pillar-shaped GaN crystal which is grown up in the direction of c
axis is produced.
[0132] One GaN crystal is selected from among many GaN crystals
produced through crystal growth, and the selected GaN crystal is
cut into the shape shown in FIG. 4A so that the seed crystal 5 is
produced. Therefore, the projection part 5A (see FIG. 4B) of the
seed crystal 5 includes the GaN crystal which is grown up in the
direction of c axis (<0001>).
[0133] By the above-mentioned method, the produced seed crystal 5
is fitted into the space 54 of the supporting device 50, and it is
fixed to the supporting device 50.
[0134] FIG. 12 is a flowchart for explaining the crystal preparing
method of a GaN crystal.
[0135] Upon start of the crystal preparing method of FIG. 12, the
crucible 10, the reaction container 20, and the piping 30 are put
in into the glove box which is filled up with Ar gas.
[0136] Metal Na is put into the piping 30 in the Ar gas atmosphere
(step S1). The moisture content of the Ar gas in this case is 1 ppm
or less, and the concentration of oxygen in the AR gas is 1 ppm or
less.
[0137] Then, metal Na and metal Ga are put into the crucible 10 in
the Ar gas atmosphere (step S2). In this case, metal Na and metal
Ga are put into the crucible 10 at the mixing ratio of 5:5. The
crucible 10 containing metal Na and metal Ga is placed in the
reaction container 20.
[0138] Then, the seed crystal 5 is placed on top of the metal Na
and metal Ga in the crucible 10 in the Ar gas atmosphere (step S3).
More specifically, the seed crystal 5 is placed on top of the metal
Na and metal Ga in the crucible 10, by fitting the seed crystal 5
into the space 54 formed at the end 511 of the supporting device 50
(see FIG. 4B).
[0139] Then, the crucible 10 and the reaction container 20 are
taken out from the glove box 300, and the crucible 10 and the
reaction container 20 are set to the crystal preparing device 100
in the state where the inside of the crucible 10 and the reaction
container 20 is filled up with the Ar gas.
[0140] The valve 160 is opened, and the Ar gas which is filled in
the crucible 10 and the reaction container 20 is exhausted using
the vacuum pump 170.
[0141] After vacuum suction of the inside of the crucible 10 and
the reaction container 20 to a predetermined pressure (0.133 Pa or
less) is performed using the vacuum pump 170, the valve 160 is
closed and the valves 120, 121 are opened so that the inside of the
crucible 10 and the reaction container 20 is filled up with the
nitrogen gas fed from the gas cylinder 140 through the gas supply
lines 90, 110.
[0142] In this case, the nitrogen gas is supplied to the crucible
10 and the reaction container 20 so that the pressure in the
crucible 10 and the reaction container 20 is set to about 0.1 MPa
using the pressure regulator 130.
[0143] When the pressure in the reaction container 20 detected by
the pressure sensor 180 is set to about 0.1 MPa, the valves 120,
121 are closed and the valve 160 is opened so that the nitrogen gas
which is filled in the crucible 10 and the reaction container 20 is
exhausted using the vacuum pump 170. Also in this case, the vacuum
suction of the inside of the crucible 10 and the reaction container
20 to the predetermined pressure (0.133 Pa or less) is performed
using the vacuum pump 170.
[0144] The supply of nitrogen gas to the crucible 10 and the
reaction container 20 and the vacuum suction of the reaction
container 20 and the crucible 10 are repeatedly performed several
times.
[0145] Then, after the out vacuum suction of the inside of the
crucible 10 and the reaction container 20 to the predetermined
pressure is performed using the vacuum pump 170, the valve 160 is
closed the valves 120, 121 are opened so that the nitrogen gas is
supplied to the crucible 10 and the reaction container 20 and the
pressure in the crucible 10 and the reaction container 20 is set to
the range of 1.01-5.05 MPa using the pressure regulator 130 (step
S4).
[0146] In this case, the metal Na in the piping 30 is solid, the
nitrogen gas is supplied from the space 31 of the piping 30 to the
space 23 of the reaction container 20 through the molten metal
holding member 60. Then, the valve 120 is closed when the pressure
in the space 23 detected by the pressure sensor 180 is set to the
range of 1.01-5.05 MPa.
[0147] Then, the crucible 10 and the reaction container 20 are
heated to 800 degrees C. using the heating devices 70 and 80 (step
S5). The molten metal 190 is heated to the specific temperature at
which the vapor pressure PNa of metal Na which evaporates from the
molten metal 190 is substantially balanced with the vapor pressure
PNa--Ga of metal Na which evaporates from the mixed molten metal
290 (step S6).
[0148] In this case, since the melting point of metal Na held in
the piping 30 is about 98 degrees C., the metal Na is melt in the
process in which the crucible 10 and the reaction container 20 are
heated to 800 degrees C., and it becomes the molten metal 190. The
two gas-liquid interfaces 1 and 2 occur (see FIG. 1). The
gas-liquid interface 1 is located at the interface of the molten
metal 190 and the space 23 in the reaction container 20, and the
gas-liquid interface 2 is located at the interface of the molten
metal 190 and the molten metal holding member 60.
[0149] In the process in which the crucible 10 and the reaction
container 20 are heated to 800 degrees C., the metal Na and metal
Ga in the crucible 10 are also liquefied, and the mixed molten
metal 290 of metal Na and metal Ga is generated in the crucible 10.
The vapor pressure PNa is substantially equal to the vapor pressure
PNa--Ga.
[0150] Then, the up-and-down movement device 220 moves up or down
the seed crystal 5 so that the seed crystal 5 contacts the mixed
molten metal 290 by the above-mentioned method (step S7).
[0151] In the high temperature state (the temperature of the
crucible 10 and the reaction container 20 is about 800 degrees C.),
the nitrogen gas in the space 23 is captured into the mixed molten
metal 290 through the metal Na, a GaN crystal begins to grow up
from the seed crystal 5.
[0152] The temperature of the crucible 10 and the reaction
container 20 and the nitrogen gas pressure in the reaction
container 20 at this time are located in the range REG3 shown in
FIG. 11. This range REG3 is a range in which crystal growth of a
GaN crystal tends to occur preferentially over the seed
crystal.
[0153] Then, the temperature of the crucible 10 and the reaction
container 20 is held at 800 degrees C. for a predetermined time (in
a range of tens of hours to hundreds of hours) (step S8). And the
temperature T3 of the seed crystal 5 is set to the temperature Ts1
or Ts2 which is lower than the temperature (=800 degrees C.) of the
mixed molten metal 290 by the above-mentioned method (step S9).
[0154] With advance of the GaN crystal growth, the nitrogen gas in
the space 23 is consumed and the nitrogen gas in the space 23 is
decreased. Then, the pressure P1 in the space 23 becomes lower than
the pressure P2 in the space 31 (P1<P2), a differential pressure
occurs between the space 23 and the space 31, and the nitrogen gas
in the space 31 is supplied gradually to the space 23 through the
molten metal holding member 60 and the molten metal 190 (=molten
metal Na).
[0155] That is, nitrogen gas is supplied to the space 23 of the
crucible 10 and the reaction container 20 (step S10). Then, the
seed crystal 5 is moved up or down by the above-mentioned method so
that the seed crystal 5 contacts the mixed molten metal 290 (step
S11). A GaN crystal having a large size will be produced.
[0156] After the predetermined time passes, the temperature of the
crucible 10 and the reaction container 20 is lowered (step S12).
The preparation of a GaN crystal is completed.
[0157] As mentioned above, in this embodiment, the vapor pressure
PNa--Ga of metal Na which evaporates from the mixed molten metal
290 is substantially balanced with the vapor pressure PNa of metal
Na which evaporates from the molten metal 190, and the GaN crystal
is produced through crystal growth.
[0158] Therefore, movement of metal Na between the molten metal 190
and the mixed molten metal 290 does not occur, seemingly, and
change of the mixing ratio of metal Na and metal Ga in the mixed
molten metal 290 can be prevented.
[0159] As a result, the mixing ratio of metal Na and metal Ga can
be held at a controlled ratio, and the GaN crystal is produced
through crystal growth.
[0160] Although the bottom 20B of the reaction container 20 is
heated by the heating device 80, in step S1 of the flowchart of
FIG. 12, the amount of metal Na which is put into the piping 30 is
set to an amount in which the molten metal 190 does not enter into
the reaction container 20 when the metal Na in the piping 30
becomes the molten metal 190.
[0161] When the temperature of the crucible 10 and the reaction
container 20 is increased to 800 degrees C., the temperature of the
molten metal 190 is lower than that of the mixed molten metal 290,
the vapor pressure PNa can be easily balanced with the vapor
pressure PNa--Ga. It is possible to stabilize the mixing ratio of
metal Na and metal Ga in the mixed molten metal 290 from the start
time of crystal growth, and the GaN crystal can be produced.
[0162] FIG. 13A and FIG. 13B are diagrams for explaining the amount
of metal Na. The case where molten metal 190 teaches metal Na of
the amount which exists also in reaction container 20 into piping
30 will be explained.
[0163] Suppose that the gas-liquid interface 1A of the molten metal
190 and the space 23 exists in the almost same position as the
gas-liquid interface 3 of the mixed molten metal 290 and space 23
(see FIG. 13A).
[0164] The molten metal 190 is heated by the heating devices 70 and
80 through the reaction container 20, and the temperature of the
molten metal 190 is almost equal to the temperature of the mixed
molten metal 290. As a result, the vapor pressure PNa of metal Na
which evaporates from the molten metal 190 becomes higher than the
vapor pressure PNa--Ga of metal Na which evaporates from the mixed
molten metal 290.
[0165] If the vapor pressure PNa is higher than the vapor pressure
PNa--Ga, the metal Na moves from the molten metal 190 to the mixed
molten metal 290. When the vapor pressure PNa is substantially
equal to the vapor pressure PNa--Ga, movement of metal Na from the
molten metal 190 to the mixed molten metal 290 is suspended. That
is, when the gas-liquid interface 1A of the molten metal 190 and
the space 23 falls to the gas-liquid interface 1B in the piping 30,
movement of metal Na from the molten metal 190 to the mixed molten
metal 290 is suspended (see FIG. 13B). Movement of metal Na in this
case means gaseous phase transportation.
[0166] Thus, even when metal Na in the amount to make the molten
metal 190 enter the reaction container 20 is set to the piping 30,
the metal Na moves from the molten metal 190 to the mixed molten
metal 290 so that the vapor pressure PNa of metal Na which
evaporates from the molten metal 190 is substantially balanced with
the vapor pressure PNa--Ga of metal Na which evaporates from the
mixed molten metal 290.
[0167] Therefore, in this invention, the step S1 of the flowchart
shown in FIG. 12 may be provided so that the metal Na in the amount
existing between the crucible 10 and the reaction container 20 and
in the piping 30 is supplied to the piping 30. Generally, metal Na
in the amount in which the molten metal 190 exists at least in the
piping 30 is set to the piping 30.
[0168] In this case, crystal growth of a GaN crystal is performed
at the mixing ratio of metal Na and metal Ga in the mixed molten
metal 290 when the vapor pressure PNa of metal Na which evaporates
from the molten metal 190 is substantially equal to the vapor
pressure PNa--Ga of metal Na which evaporates from the mixed molten
metal 290.
[0169] As mentioned above, it is preferred that, when the
temperature of the crucible 10 and the reaction container 20 is
increased to 800 degrees C., the amount of metal Na when the vapor
pressure PNa of metal Na which evaporates from the molten metal 190
is balanced with the vapor pressure PNa--Ga of metal Na which
evaporates from the mixed molten metal 290 is entered into the
piping 30.
[0170] Even when the temperature of the crucible 10 and the
reaction container 20 is increased to 800 degrees C. and the
gas-liquid interface 1A of the molten metal 190 and the space 23 is
in the position that is the same as that of the gas-liquid
interface 3 of the mixed molten metal 290 and the space 23 (see
FIG. 13A), movement of metal Na from the molten metal 190 to the
mixed molten metal 290 occurs. And, finally, the vapor pressure PNa
of metal Na which evaporates from the molten metal 190 is
substantially balanced with the vapor pressure PNa--Ga of metal Na
which evaporates from the mixed molten metal 290.
[0171] Conversely, when PNa<PNa--Ga (i.e., when a small mount of
metal Na remains), movement of Na vapor from the mixed molten metal
290 to the molten metal 190 arises. And, finally, the vapor
pressures PNa and PNa--Ga becomes substantially in agreement.
[0172] The crystal preparing method of the invention may be
provided so that the metal Na is set in the amount in which the
molten metal 190 can exist in the reaction container 20 when the
temperature of the crucible 10 and the reaction container 20 is
increased to 800 degrees C., and crystal growth of a GaN crystal is
performed. Moreover, the crystal preparing method of the invention
may be provided so that the metal Na is set in the amount in which
the molten metal 190 can exist between the space 23 and the molten
metal holding member 60, and crystal growth of a GaN crystal is
performed.
[0173] This is because the molten metal 190 existing between the
space 23 and the molten metal holding member 60 is equivalent to
balancing the vapor pressure PNa of metal Na which evaporates from
the molten metal 190 with the vapor pressure PNa--Ga of metal Na
which evaporates from the mixed molten metal 290.
[0174] Therefore, in the crystal preparing device which supplies
nitrogen gas through the molten metal 190 to the space 23 which
contacts the mixed molten metal 290, and performs crystal growth of
a GaN crystal, if the metal Na (solid) exists in the passage for
supplying the nitrogen gas to the space 23 after the crystal growth
of the GaN crystal is completed, it means performing crystal growth
of a GaN crystal using the crystal preparing device of the
invention.
[0175] If Ga in the mixed molten metal 290 is consumed and the
amount of Na contained in the mixed molten metal 290 is increased
during the crystal growth, the vapor pressure PNa--Ga is increased.
As a result, movement of Na to the molten metal 190 takes place,
and the Na vapor pressure is balanced between the mixed molten
metal 290 and the molten metal 190 and held at a controlled ratio.
Thus, it is possible to perform stable crystal growth of the GaN
crystal, and improvement of quality of the GaN crystal and increase
of crystal size can be realized.
[0176] FIG. 14 shows the composition of a crystal preparing device
100A in an embodiment of the invention. As shown in FIG. 14, in the
crystal preparing device 100A, the temperature control device 280
of the crystal preparing device 100 shown in FIG. 1 is replaced
with a temperature control device 280A, and a heater/condenser 300
and a temperature sensor 301 are added. Other composition of this
crystal preparing device 100A is the same as that of the crystal
preparing device 100.
[0177] The heater/condenser 300 is arranged around the portion of
the piping 30 where the molten metal holding member 60 is arranged,
and it is located above the molten metal holding member 60.
[0178] The temperature sensor 301 is arranged near the
heater/condenser 300. The temperature control device 280A receives
the temperature T4 from the temperature sensor 301.
[0179] The temperature control device 280A generates a control
signal CTL4 for heating the molten metal 190 to the specific
temperature at which the vapor pressure PNa of metal Na which
evaporates from the molten metal 190 is substantially equal to the
vapor pressure PNa--Ga of metal Na which evaporates from the mixed
molten metal 290, based on the received temperature T4. The
generated control signal CTL4 is outputted to the heater/condenser
300. In addition, the temperature control device 280A performs the
same function as the temperature control device 280.
[0180] In response to the control signal CTL4 from the temperature
control device 280A, the heater/condenser 300 heats or cools the
molten metal 190 to the specific temperature.
[0181] Namely, when the temperature of the molten metal 190 is
lower than the specific temperature, the heater/condenser 300 heats
the molten metal 190 so that the temperature of the molten metal
190 is increased to the specific temperature. On the other hand,
when the temperature of the molten metal 190 is higher than the
specific temperature, the heater/condenser 300 cools the molten
metal 190 so that the temperature of the molten metal 190 is
decreased to the specific temperature.
[0182] The temperature sensor 301 detects the temperature T4 of the
heater/condenser 300, and outputs the detected temperature T4 to
the temperature control device 280A.
[0183] In the crystal preparing device 100A, the amount of metal Na
set in the piping 30 is controlled so that the position of the
gas-liquid interface 1 between the molten metal 190 and the space
23 is set in a position lower than the heating device 80.
[0184] In the crystal preparing device 100A, a GaN crystal is
produced according to the flowchart shown in FIG. 15. FIG. 15 is a
flowchart for explaining the crystal preparing method of a GaN
crystal in an embodiment of the invention.
[0185] In the flowchart shown in FIG. 15, step S6 in the flowchart
shown in FIG. 12 is replaced with step S6A, and other steps in the
flowchart shown in FIG. 15 are the same as in the flowchart shown
in FIG. 12.
[0186] In the above-mentioned step S5, when the crucible 10 and the
reaction container 20 are heated to 800 degrees C. using the
heating devices 70 and 80, the temperature of the molten metal 190
is independently controlled by the heater/condenser 300 to the
specific temperature such that the vapor pressure PNa of metal Na
which evaporates from the molten metal 190 is substantially
balanced with the vapor pressure PNa--Ga of metal Na which
evaporates from the mixed molten metal 290 (step S6A).
[0187] Then, the procedure of steps S7-S12 mentioned above is
sequentially performed, and the crystal preparing method is
completed.
[0188] By controlling the temperature of the molten metal 190
independently of the temperature (=the temperature of the crucible
10 and the reaction container 20) of the mixed molten metal 290 as
mentioned above, it is possible to easily make the vapor pressure
PNa--Ga of metal Na which evaporates from the mixed molten metal
290 equal to the vapor pressure PNa of metal Na which evaporates
from the molten metal 190.
[0189] The gas cylinder 140, the pressure regulator 130, the gas
supply lines 90, 110, the valve 121, the piping 30, the molten
metal holding member 60, and the molten metal 190 constitute a gas
supply device.
[0190] FIG. 16 is a diagram showing an example of the molten metal
holding member 60. FIG. 17 is a diagram for explaining the fixing
method of the molten metal holding member 400 shown in FIG. 16. As
shown in FIG. 16, the molten metal holding member 400 includes a
plug 401 and a number of projections 402.
[0191] The plug 401 has a generally cylindrical shape, and the
diameter of the plug 401 changes in the longitudinal direction DR3.
Each of the projections 402 has a semi-spherical cross-section, and
the diameter thereof is on the order of tens of micrometers. The
projections 402 are formed on the outer peripheral surface 401A of
the plug 401 at random.
[0192] However, the interval between two adjoining projections 402
is set to the order of tens of micrometers.
[0193] As shown in FIG. 17, the molten metal holding member 400 is
fixed by the supporting members 403, 404 in the piping 30.
[0194] More specifically, the molten metal holding member 400 is
fixed, when the supporting member 403 fixed to the inner wall of
piping 30 and the supporting member 404 by which the end is fixed
to the inner wall of piping 30 on the other hand face across an end
on the other hand.
[0195] In this case, the projections 402 of the molten metal
holding member 400 may be in contact with the piping 30, and does
not need to touch. When the molten metal holding member 400 is
fixed so that the projections 402 may not touch the piping 30, the
interval of the projections 402 and the piping 30 is set to the
interval which can hold the molten metal 190 by the surface tension
of the molten metal 190, and the molten metal holding member 400 is
fixed by the supporting members 403, 404.
[0196] Since metal Na into which it is put in piping 30 is a solid
before heating of the crucible 10 and the reaction container 20 is
started, the nitrogen gas supplied from gas cylinder 140 can
diffuse between space 23 in reaction container 20, and piping 30
through the molten metal holding member 400.
[0197] When heating of the crucible 10 and the reaction container
20 is started and the temperature of the piping 30 is increased to
98 degrees C. or more, the metal Na in the piping 30 is melt, and
it becomes the molten metal 190, which is confined in the nitrogen
gas in the space 23.
[0198] The molten metal 190 flows into space 31, and bends, and the
molten metal holding member 400 holds molten metal 190 with the
surface tension of molten metal 190 like.
[0199] The molten metal 190 and the molten metal holding member 400
will confine in space 23 nitrogen gas and the metal Na vapor which
evaporated from molten metal 190 and the mixed molten metal 290, if
GaN crystal growth advances.
[0200] As a result, the further evaporation of metal Na from the
mixed molten metal 290 can be controlled, and change of the
quantitative ratio of metal Na and metal Ga by evaporation of the
metal Na in the mixed molten metal 290 can be controlled.
[0201] If GaN crystal growth follows on going on and the nitrogen
gas in space 23 decreases in number, pressure P1 in space 23
becomes lower than pressure P2 of space 31, and the molten metal
holding member 400 will pass the nitrogen gas of space 31 in the
direction of reaction container 20, and it will supply it to space
23 through the molten metal 190.
[0202] Thus, the molten metal holding member 400 acts like the
molten metal holding member 60 mentioned above. Therefore, the
molten metal holding member 400 is replaced with the molten metal
holding member 60, and is used for crystal preparing device
100.
[0203] In the above-mentioned embodiment, the molten metal holding
member 400 includes the projections 402, the molten metal holding
member 400 does not need to have the projections 402. In this case,
the molten metal holding member 400 is fixed to the inner wall of
the piping 30 by the supporting members 403, 404 so that the
interval between the plug 401 and the reaction container 20 and the
piping 30 may be set to tens of micrometers.
[0204] The molten metal holding member 400 (the case where it does
not have a case where it has the projections 402, and the
projections 402 are included), the interval of being the same and
piping 30 may be made to be determined according to the temperature
of the molten metal holding member 400.
[0205] In this case, when the temperature of the molten metal
holding member 400 is relatively high, the interval of the molten
metal holding member 400 and the piping 30 is set up small
relatively.
[0206] When the temperature of the molten metal holding member 400
is relatively low, the interval of the molten metal holding member
400 and piping 30 is set up greatly relatively.
[0207] The interval of the molten metal holding member 400 and
piping 30 which can hold molten metal 190 changes with the
temperature of the molten metal holding member 400 with surface
tension. Therefore, the interval of the molten metal holding member
400 and piping 30 is changed according to the temperature of the
molten metal holding member 400, and it enables it to hold molten
metal 190 certainly by the surface tension.
[0208] Temperature control of the molten metal holding member 400
is performed by the heating device 80. That is, in carrying out
temperature rising of the temperature of the molten metal holding
member 400 to a temperature higher than 150 degrees C., it heats
the molten metal holding member 400 with the heating device 80.
[0209] When the molten metal holding member 400 is used, the gas
cylinder 140, the pressure regulator 130, the gas supply lines 90,
110, the valve 121, the piping 30, the molten metal holding member
400, and the molten metal 190 constitute a gas supply device.
[0210] FIG. 18A and FIG. 18B are diagrams showing another example
of the molten metal holding member. As shown in FIG. 18A and FIG.
18B, the molten metal holding member 410 includes a plug 411 and a
number of penetration holes 412 formed therein. The penetration
holes 412 are formed along with the longitudinal direction DR2 of
the plug 411. Each of the penetration holes 412 has a diameter of
tens of micrometers (see FIG. 18A). In the molten metal holding
member 410, at least one penetration hole 412 is formed.
[0211] The molten metal holding member 420 includes a plug 421 and
a number of penetration holes 422 formed therein. The penetration
holes 422 are formed along with the longitudinal direction DR2 of
the plug 421. Each of the penetration holes 422 has diameters r1,
r2, and r3 which are changed in the longitudinal direction DR2 in
two or more steps. Each of the diameters r1, r2, and r3 is
determined in the molten metal 190 in the range which can be held
by the surface tension. For example, each diameter is determined to
be in a range of several micrometers to tens of micrometers (see
FIG. 18B).
[0212] In the molten metal holding member 420, at least one
penetration hole 422 is formed. The diameter of the penetration
hole 422 is changed to at least two pieces. The diameter of
penetration hole 422 may be changed continuously in the
longitudinal direction DR2.
[0213] The molten metal holding member 410 or 420 is replaced with
and used for the molten metal holding member 60 of crystal
preparing device 100.
[0214] When the molten metal holding member 420 is replaced with
and used for the molten metal holding member 60, it is set to the
crystal preparing device 100.
[0215] With either of the diameters changed into two or more steps
and the molten metal 190 can be held by the surface tension of
molten metal 190 even if it does not perform temperature control of
the molten metal holding member 420 precisely. Even if it does not
perform temperature control of the molten metal holding member 420
precisely, the GaN crystal which has large size can be
produced.
[0216] When the molten metal holding member 410 or 420 is used, the
gas cylinder 140, the pressure regulator 130, the gas supply lines
90, 110, the valve 121, the piping 30, the molten metal holding
member 410 or 420, and the molten metal 190 constitute a gas supply
device.
[0217] In this invention, instead of the molten metal holding
member 60, a porous plug or a check valve may be used. The porous
plug is made of a sintered metal in which stainless powders are
sintered, and has the structure where many small holes of tens of
micrometers are formed. Therefore, the porous plug can hold the
molten metal 190 by the surface tension of the molten metal 190
similar to the molten metal holding member 60 mentioned above. The
check valve include both the check valve of spring type used for a
low temperature part, and the check valve of piston type used for a
high temperature part. In the check valve of piston type, when the
pressure P2 in the space 31 is higher than the pressure P1 in the
space 23, the piston moves upward along with one pair of guides by
the difference of pressure P2 and pressure P1, and the nitrogen gas
of the space 31 is supplied to the space 23 through the molten
metal 190. And when P1>=P2, the piston moves to close the
connection part of the reaction container 20 and the piping 30 by
gravity. Therefore, the check valve of piston type can be used in a
high temperature part.
[0218] In the case of the above embodiment, the crystal growth
temperature is 800 degrees C. This invention is not limited to the
above embodiment. Alternatively, the crystal growth temperature may
be 600 degrees C. or higher. And the nitrogen gas pressure may be a
pressure which allows crystal growth using the crystal growth
method in the pressurization state of 0.4 MPa or higher. That is,
the upper limit of the nitrogen gas pressure may not be limited to
5.05 MPa in the above embodiment. Alternatively, the upper limit of
the nitrogen gas pressure may be a pressure of 5.05 MPa or
higher.
[0219] In the crystal preparing device of this invention, the
up-and-down movement device 220, the vibration applying device 230,
and the vibration detecting device 240 may be omitted from the
crystal preparing device 100. In this case, although the seed
crystal 5 is not moved in the up-and-down direction, the seed
crystal 5 is supported by the supporting device 50 so that metal Na
and metal Ga which is put in the crucible 10 and set in the molten
state may contact the mixed molten metal 290. Therefore, a GaN
crystal may be produced from the seed crystal 5. As a result, a GaN
crystal having a large size can be produced.
[0220] In the crystal preparing device of this invention, the
thermocouple 210, the piping 200, the gas supply line 250, the
flowmeter 260, and the gas cylinder 270 may be omitted from the
crystal preparing device 100. In this case, although temperature T3
of seed crystal 5 is not controlled to a temperature lower than the
temperature of the mixed molten metal 290 during the crystal growth
of a GaN crystal, the seed crystal 5 is contacted by the mixed
molten metal 290 using the supporting device 50, a GaN crystal is
produced from the seed crystal 5. As a result, a GaN crystal having
a large size can be produced.
[0221] In the crystal preparing device of this invention, the
thermocouple 210, the piping 200, the up-and-down movement device
220, the vibration applying device 230, the vibration detecting
device 240, the gas supply line 250, the flowmeter 260, and the gas
cylinder 270 may be omitted from the crystal preparing device 100.
In this case, although the seed crystal 5 is not moved in the
up-and-down direction and the temperature T3 of the seed crystal 5
is not controlled to a temperature lower than the temperature of
the mixed molten metal 290, the seed crystal 5 is supported by the
supporting device 50 so that metal Na and metal Ga which is put in
the crucible 10 and set in the molten state may contact the mixed
molten metal 290. Therefore, a GaN crystal is produced from the
seed crystal 5. As a result, a GaN crystal having a large size can
be produced.
[0222] In this invention, by using a crystal preparing device in
which the thermocouple 210, the piping 200, the up-and-down
movement device 220, the vibration applying device 230, the
vibration detecting device 240, the gas supply line 250, flowmeter
260, and the gas cylinder 270 are omitted from the crystal
preparing device 100, crystal growth of a GaN crystal may be
performed without using the seed crystal 5. In this case, although
a GaN crystal grows from the bottom and the side of the crucible
10, the vapor pressure PNa of metal Na which evaporates from the
molten metal 190 is substantially balanced with the vapor pressure
PNa--Ga of metal Na which evaporates from the mixed molten metal
290, the mixing ratio of metal Na and metal Ga in the mixed molten
metal 290 is held at a controlled ratio, and the crystal growth of
a GaN crystal can be performed.
[0223] In the above embodiment, metal Na and metal Ga are put into
the crucible 10 in the Ar gas atmosphere and metal Na is put into
the piping 30 in the Ar gas atmosphere. Alternatively, metal Na and
metal Ga may be put into the crucible 10 and metal Na may be put
into the piping 30 in a nitrogen gas atmosphere or an inert-gas
atmosphere of any other inert gas, different than Ar gas, such as
He, Ne, or Kr gas. In this case, the moisture concentration of the
inert gas or nitrogen gas is 1 ppm or less, and the concentration
of oxygen of the inert gas or nitrogen gas is 1 ppm or less.
[0224] In the above embodiment, the metal which is mixed with metal
Ga is sodium Na. Alternatively, alkali metals, such as lithium (Li)
and potassium (K), or alkaline earth metals, such as magnesium
(Mg), calcium (Ca) and strontium (Sr) may be mixed with metal Ga so
that the mixed molten metal 290 is produced. In this case, the
mixed molten metal in which the alkali metal is melted constitutes
the molten alkali metal, and the mixed molten metal in which the
alkaline earth metal is melted constitutes the molten alkaline
earth metal.
[0225] Moreover, instead of nitrogen gas, any compound which
contains nitrogen, such as sodium azide and ammonia, may be used.
The compound constitutes the nitrogen material gas. Instead of Ga,
any of group III metals, such as boron (B), aluminum (Al), and
indium (In), may be used.
[0226] Generally, the crystal preparing device and method of this
invention may be provided to produce a group III nitride crystal
using the mixed molten metal of an alkali metal or an alkaline
earth metal, and a group III metal (boron is included). The group
III nitride crystal produced using the crystal preparing device or
method of this invention is used for production of group III
nitride semiconductor devices, such as a light emitting diode, a
semiconductor laser, photodiode, and a transistor.
[0227] The present invention is applied to the crystal preparing
device which carries out crystal growth of a group III nitride
crystal with a controlled mixing ratio of an alkali metal and a
group III metal. Moreover, the present invention is applied to the
crystal preparing method which produces a group III nitride crystal
with a controlled mixing ratio of an alkali metal and a group III
metal.
[0228] Next, FIG. 19 shows the composition of a crystal preparing
device in an embodiment of the invention. As shown in FIG. 19, the
crystal preparing device 100 in this embodiment includes a reaction
container 10, an external reaction container 20, a piping 30, a
piping 200, a bellows 40, a supporting device 50, a
control/introduction plug 60, heating devices 70, heating devices
80, temperature sensors 71, 81, gas supply lines 90, 110, 250, a
valve 120, a valve 121, a valve 160, a pressure regulator 130, gas
cylinders 140, 270, an exhaust pipe 150, a vacuum pump 170, a
pressure sensor 180, a molten metal 190, a thermocouple 210, an
up-and-down movement device 220, a vibration applying device 230, a
vibration detecting device 240, a flowmeter 260, and a temperature
control device 280.
[0229] The reaction container 10 has a pillar-shaped configuration
and includes boron nitride (BN). The external reaction container 20
separates a gap the reaction container 10 and predetermined, and is
arranged around the reaction container 10. And the external
reaction container 20 includes a body part 21 and a lid part
22.
[0230] Each of the body part 21 and the lid part 22 includes
SUS316L, and the portion between the body part 21 and the lid part
22 is sealed with a metal O-ring. Therefore, metal Na is not leaked
to the outside.
[0231] The piping 30 is connected with the external reaction
container 20 with the reaction container 10 down side in gravity
direction DR1. The bellows 40 is connected with the external
reaction container 20 with the reaction container 10 up side in the
gravity direction DR1.
[0232] The supporting device 50 is in the form of a hollow
cylinder, and it is partially inserted into the space 23 of the
external reaction container 20 through the bellows 40.
[0233] The control/introduction plug 60 is made of metal, ceramics,
etc., and held in the piping 30 below the connection part of the
external reaction container 20 and the piping 30.
[0234] The heating device 70 is arranged so that the outer
peripheral surface 20A of the external reaction container 20 may be
surrounded.
[0235] The heating devices 80 are arranged so that they confront
the bottom 20B of the external reaction container 20. The
temperature sensors 71 and 81 approach the heating devices 70 and
80, respectively, and are arranged.
[0236] One end of the gas supply line 90 is connected with the
external reaction container 20 through the valve 120, and the other
end thereof is connected with the gas cylinder 140 through the
pressure regulator 130.
[0237] One end of the gas supply line 110 is connected with the
piping 30 through the valve 121, and the other end thereof is
connected with the gas supply line 90.
[0238] The gas supply line 90 includes the valve 120 near the
external reaction container 20. The gas supply line 110 includes
the valve 121 near the piping 30. The gas supply line 90 includes
the pressure regulator 130 near the gas cylinder 140. The gas
cylinder 140 is connected with the gas supply line 90.
[0239] One end of the exhaust pipe 150 is connected with the
external reaction container 20 through the valve 160, and the other
end thereof is connected with the vacuum pump 170. The exhaust pipe
150 includes the valve 160 near the external reaction container 20.
The vacuum pump 170 is connected with the exhaust pipe 150.
[0240] The pressure sensor 180 is attached to the external reaction
container 20. The molten metal 190 includes molten metal sodium
(Na) and is held in the piping 30 and between the reaction
container 10 and the external reaction container 20.
[0241] The piping 200 and the thermocouple 210 are inserted in the
inside of the supporting device 50. The up-and-down movement device
220 is attached to the supporting device 50 at the location above
the bellows 40. One end of the gas supply line 250 is connected to
the piping 200, and the other end thereof is connected to the gas
cylinder 270 through the flowmeter 260. The gas supply line 250
includes the flowmeter 260 near the gas cylinder 270. The gas
cylinder 270 is connected with the gas supply line 250.
[0242] The reaction container 10 holds the mixed molten metal 290
containing metal Na and metal Ga (metal gallium). The external
reaction container 20 is a wrap about the circumference of the
reaction container 10.
[0243] The piping 30 leads the nitrogen gas (N2 gas) supplied from
the gas cylinder 140 through the gas supply lines 90, 110 to the
control/introduction plug 60.
[0244] The bellows 40 intercepts the inside and the outside of the
external reaction container 20 while holding the supporting device
50. Moreover, the bellows 40 is expanded or contracted in the
gravity direction DR1 in accordance with movement of the supporting
device 50 in the gravity direction DR1. The supporting device 50
supports the seed crystal 5 (which includes a GaN crystal) at the
end thereof which is inserted into the external reaction container
20.
[0245] The control/introduction plug 60 has a concavo-convex
structure on its circumferential surface, and holes on the order of
tens of micrometers are formed between the plug 60 and the wall of
the piping 30, so that they passes the nitrogen gas through the
piping 30 in the direction to the molten metal 190, and supplies
the nitrogen gas into the space 23 through the molten metal
190.
[0246] Moreover, the control/introduction plug 60 holds the molten
metal 190 by using the surface tension of the holes of tens of
micrometers between the reaction container 10 and the external
reaction container 20 and in the piping 30.
[0247] The heating device 70 includes a heater and a current
source. And according to the control signal CTL1 from the
temperature control device 280, the heating device 70 sends the
current from the current source through the heater, and heats the
reaction container 10 and the external reaction container 20 from
the outer peripheral surface 20A of the external reaction container
20 to a crystal growth temperature.
[0248] The temperature sensor 71 detects the temperature T1 of the
heater of the heating device 70, and outputs the detected
temperature T1 to the temperature control device 280.
[0249] The heating device 80 also includes a heater and a current
source. And according to control signal CTL2 from the temperature
control device 280, the heating device 80 sends current from the
current source through the heater, and heats the reaction container
10 and the external reaction container 20 from the bottom 20B of
the external reaction container 20 to a crystal growth temperature.
The temperature sensor 81 detects the temperature T2 of the heater
of the heating device 80, and outputs the detected temperature T2
to the temperature control device 280.
[0250] The gas supply line 90 supplies the nitrogen gas fed from
the gas cylinder 140 through the pressure regulator 130, to the
external reaction container 20 through the valve 120. The gas
supply line 110 supplies the nitrogen gas fed from the gas cylinder
140 through the pressure regulator 130, to the piping 30 through
the valve 121.
[0251] The valve 120 supplies the nitrogen gas in the gas supply
line 90 into the external reaction container 20, or stops supply
into the external reaction container 20 of nitrogen gas. A valve
121 supplies the nitrogen gas in the gas supply line 110 to piping
30, or stops supply for the piping 30 of nitrogen gas. The pressure
regulator 130 makes predetermined pressure the nitrogen gas from a
gas cylinder 140, and supplies it to the gas supply line 90,
110.
[0252] The gas cylinder 140 holds nitrogen gas. The exhaust pipe
150 passes the gas in the external reaction container 20 to the
vacuum pump 170. The valve 160 connects the exhaust pipe 150
spatially in the external reaction container 20, or intercepts the
exhaust pipe 150 spatially in the external reaction container 20.
The vacuum pump 170 performs vacuum suction in the external
reaction container 20 through the exhaust pipe 150 and the valve
160.
[0253] The pressure sensor 180 detects the pressure in the external
reaction container 20. Nitrogen gas is introduced through the
molten metal 190 and the control/introduction plug 60 into the
space 23.
[0254] The piping 200 supplies the nitrogen gas supplied from the
gas supply line 250 to the supporting device 50, and cools the seed
crystal 5. The thermocouple 210 detects the temperature T3 of the
seed crystal 5, and outputs the detected temperature T3 to the
temperature control device 280.
[0255] According to the vibration detection signal BDS from the
vibration detecting device 240, the up-and-down movement device 220
moves up and down the supporting device 50 so that the seed crystal
5 may touch the gas-liquid interface 3 of space 23 and mixed molten
metal 290.
[0256] The vibration applying device 230 includes, for example, a
piezoelectric device, and applies vibrations having a predetermined
frequency to the supporting device 50.
[0257] The vibration detecting device 240 includes, for example, an
acceleration pickup and detects vibrations of the supporting device
50, and outputs the vibration detection signal BDS indicating
vibrations of the supporting device 50, to the up-and-down movement
device 220.
[0258] The gas supply line 250 supplies the nitrogen gas fed from
the gas cylinder 270 through the flowmeter 260, to the piping 200.
The flowmeter 260 adjusts the flow rate of the nitrogen gas
supplied from the gas cylinder 270 according to the control signal
CTL3 from the temperature control device 280, and supplies the
nitrogen gas with the adjusted flow rate to the gas supply line
250. The gas cylinder 270 holds nitrogen gas.
[0259] The composition of the control/introduction plug 60 in FIG.
19 is as shown in FIG. 2. As shown in FIG. 2, the
control/introduction plug 60 contains the plug 61 and the convex
parts 62. The plug 61 has the cross-section in a generally circular
shape.
[0260] Each of the convex parts 62 has the cross-section in the
shape of a semi-circle, and is formed on the outer peripheral
surface of the plug 61 along the longitudinal direction DR2 of the
plug 61.
[0261] The mounting state of the control/introduction plug 60 to
the piping 30 is as shown in FIG. 3. As shown in FIG. 3, the
plurality of convex parts 62 are formed in the circumferential
direction of the plug 61, and the convex parts 62 are formed at
intervals d on the order of tens of micrometers. Moreover, each of
the convex parts 62 has the height H on the order of tens of
micrometers.
[0262] The convex parts 62 in the control/introduction plug 60 are
in contact with the wall 30A of the piping 30. Thereby, the
control/introduction plug 60 is fitted into the wall 30A of the
piping 30.
[0263] In the state where the control/introduction plug 60 is
fitted into the wall 30A of the piping 30 a plurality of air gaps
63 each having a diameter on the order of 10 micrometers are formed
in the outer peripheral surface of the plug 61 between the
control/introduction plug 60 and the wall 30A of the piping 30.
[0264] The air gaps 63 hold the molten metal 190 by the surface
tension of the molten metal 190, and prevent the passing of the
molten metal 190 in the longitudinal direction DR2 of the plug 61.
The air gaps 63 pass nitrogen gas in the longitudinal direction DR2
of the plug 61.
[0265] The composition of the supporting device 50, the piping 200,
and the thermocouple 210 in FIG. 19 is as shown in FIG. 4A and FIG.
4B. As shown in FIG. 4A and FIG. 4B, the supporting device 50
contains the cylindrical member 51 and the holding members 52 and
53.
[0266] The cylindrical member 51 has the cross-section in a
generally circular form. The holding member 52 has the
cross-section in a generally L-shaped form, and is fixed at the end
511 of the cylindrical member 51 to the outer peripheral surface
51A and the bottom 51B of the cylindrical member 51.
[0267] Moreover, the holding member 53 has the cross-section in a
generally L-shaped form, and is fixed to the outer peripheral
surface 51A and the bottom 51B of the cylindrical member 51 so that
it is symmetrically arranged with the holding member 52 at the end
511 of the cylindrical member 51. As a result, the space 54 is
formed in the location surrounded by the cylindrical member 51 and
the holding members 52 and 53.
[0268] The piping 200 has the cross-section in a generally circular
shape, and is arranged inside the cylindrical member 51. In this
case, the bottom 200A of the piping 200 is arranged so as to
confront the bottom 51B of the cylindrical member 51. And a number
of small holes 201 are formed in the bottom 200A of the piping 200.
The nitrogen gas supplied into piping 200 is sprayed on the bottom
51B of the cylindrical member 51 through the small holes 201.
[0269] The thermocouple 210 is arranged inside the cylindrical
member 51 so that the end 210A of the thermocouple 210 touches the
bottom 51B of the cylindrical member 51 (see FIG. 4A).
[0270] And the seed crystal 5 is configured to have a shape which
fits into the space 54, and it is supported by the supporting
device 50 by fitting the same into the space 54. In this case, the
seed crystal 5 touches the bottom 51B of the cylindrical member 51
(see FIG. 4B). The seed crystal in this embodiment has the shape of
the seed crystal 5 shown in FIG. 4B. Alternatively, if the seed
crystal is in the shape which touches the supporting device 50, a
hexagonal shape seed crystal may be held using an adapter which has
the shape of the seed crystal 5 shown in FIG. 4B.
[0271] Therefore, the thermal conductivity between the seed crystal
5 and the cylindrical member 51 becomes high. As a result, the
temperature of the seed crystal 5 can be detected using the
thermocouple 210, and the seed crystal 5 can be easily cooled with
the nitrogen gas sent to the bottom 51B of the cylindrical member
51 from the piping 200.
[0272] The composition of the up-and-down movement device 220 in
FIG. 19 is as shown in FIG. 5. As shown in FIG. 5, the up-and-down
movement device 220 contains the rack member 221, the gear 222, the
shaft member 223, the motor 224, and the control unit 225.
[0273] The rack member 221 has the cross-section in a generally
triangular shape, and is fixed to the outer peripheral surface 51A
of the cylindrical member 51. The gear 222 is fixed to the end of
the shaft member 223, and the gear 222 is engaged with the rack
member 221. One end of the shaft member 223 is connected with the
gear 222, and the other end thereof is connected with the shaft
(not shown) of the motor 224.
[0274] The motor 224 rotates the gear 222 in the direction
indicated by the arrow 226 or 227 in accordance with the control
signal from the control unit 225. The control unit 225 controls the
motor 224 based on the vibration detection signal BDS from the
vibration detecting device 240 so that the gear 222 is rotated in
the direction indicated by the arrow 226 or 227.
[0275] When the gear 222 is rotated in the direction indicated by
the arrow 226, the supporting device 50 is moved upward in the
gravity direction DR1. When the gear 222 is rotated in the
direction indicated by the arrow 227, the supporting device 50 is
moved downward in the gravity direction DR1. Therefore, rotating
the gear 222 in the direction indicated by the arrow 226 or 227 is
equivalent to moving the supporting device 50 up and down in the
gravity direction DR1.
[0276] FIG. 20 is a timing chart of the vibration detection signal
BDS. As shown in FIG. 20, the vibration detection signal BDS
detected by the vibration detecting device 240 includes the signal
component SS1 when the seed crystal 5 is not in contact with the
mixed molten metal 290. The vibration detection signal BDS includes
the signal component SS2 when the seed crystal 5 is in contact with
the mixed molten metal 290. The vibration detection signal BDS
includes the signal component SS3 when the seed crystal 5 is
immersed in the mixed molten metal 290.
[0277] The seed crystal 5 vibrates greatly by the vibrations
applied by the vibration applying device 230 when the seed crystal
5 is not in contact with the mixed molten metal 290, and the
vibration detection signal BDS at this time includes the signal
component SS1 having a relatively large amplitude.
[0278] On the other hand, when the seed crystal 5 is in contact
with the mixed molten metal 290, the seed crystal 5 does not
vibrate greatly according to the viscosity of the mixed molten
metal 290 even if the vibrations are applied by the vibration
applying device 230, and the vibration detection signal BDS at this
time includes the signal component SS2 having a relatively small
amplitude.
[0279] Moreover, when the seed crystal 5 is immersed in the mixed
molten metal 290, the seed crystal 5 becomes further difficult to
vibrate according to the viscosity of the mixed molten metal 290,
and the vibration detection signal BDS at this time includes the
signal component SS3 having an amplitude smaller than that of the
signal component SS2.
[0280] Referring back to FIG. 5, when the vibration detection
signal BDS is received from the vibration detecting device 240, the
control unit 225 detects the signal component in the vibration
detection signal BDS. And when the detected signal component
includes the signal component SS1, the control unit 225 controls
the motor 224 so that the supporting device 50 is moved down in the
gravity direction DR1 until the detected signal component in the
vibration detection signal BDS is changed to the signal component
SS2.
[0281] Specifically, the control unit 225 controls the motor 224 to
rotate the gear 222 in the direction indicated by the arrow 227,
and the motor 224 rotates the gear 222 in the direction indicated
the arrow 227 through the shaft member 223 according to the control
signal from the control unit 225. Thereby, the supporting device 50
is moved downward in the gravity direction DR1.
[0282] And when the signal component in the vibration detection
signal BDS received from the vibration detecting device 240 is
changed from the signal component SS1 to the signal component SS2,
the control unit 225 controls the motor 224 to suspend rotation of
the gear 222, the motor 224 stops rotation of the gear 222
according to the control signal from the control unit 225. Thereby,
the supporting device 50 suspends movement and holds the seed
crystal 5 in contact with the gas-liquid interface 3.
[0283] On the other hand, when the vibration detection signal BDS
including the signal component SS2 is received from the vibration
detecting device 240, the control unit 225 controls the motor 224
to suspend movement of the supporting device 50. In this case, the
seed crystal 5 is already in contact with the mixed molten metal
290.
[0284] In this manner, based on the vibration detection signal BDS
detected by the vibration detecting device 240, the up-and-down
movement device 220 moves the supporting device 50 up or down in
the gravity direction DR1 so that the seed crystal 5 may touch the
mixed molten metal 290.
[0285] The timing chart of the temperature of the reaction
container 10 and the external reaction container 20 is as shown in
FIG. 7. FIG. 21 shows the state in the reaction container 10 and
the external reaction container 20 between timing t1 and timing t2
shown in FIG. 7. And the relation between the temperature of the
seed crystal 5 and the flow rate of nitrogen gas is as shown in
FIG. 10.
[0286] In FIG. 7, the straight line k1 shows the temperature of the
reaction container 10 and the external reaction container 20, and
the curve k2 and the straight line k3 show the temperature of the
seed crystal 5.
[0287] As shown in FIG. 7, the heating devices 70 and 80 heat the
reaction container 10 and the external reaction container 20 so
that the temperature is increased to 800 degrees C. according to
the straight line k1 and the temperature is held at 800 degrees
C.
[0288] In this case, the reaction container 10 and the external
reaction container 20 are filled up with the nitrogen gas of
1.01-5.05 MPa. If the heating devices 70 and 80 begin to heat the
reaction container 10 and the external reaction container 20, the
temperature of the reaction container 10 and the external reaction
container 20 begins to rise. In timing t1, the temperature exceeds
98 degrees C., and in timing t2 the temperature is increased to 800
degrees C.
[0289] At this time, metal Na held between the reaction container
10 and the external reaction container 20 is melt, and it becomes
the molten metal 190 (=molten metal Na). Moreover, metal Na and
metal Ga which are held in the reaction container 10 are also melt,
and they become the mixed molten metal 290.
[0290] And the nitrogen gas 4 in the space 23 cannot be diffused to
the space 31 in the piping 30 through the molten metal 190 (=molten
metal Na) and the control/introduction plug 60, but it is enclosed
in the space 23.
[0291] Moreover, the quantity of Na which evaporates from the
molten metal 190 and the mixed molten metal 290 to the space 23
increases as the temperature of the reaction container 10 and the
external reaction container 20 approaches 800 degrees C. And the Na
vapor is confined in the space 23 as the metal Na vapor 7.
Therefore, the nitrogen gas 4 and the metal Na vapor 7 coexist in
the space 23 after timing t1 (see FIG. 21).
[0292] Moreover, in the timing t2 in which the temperature of the
reaction container 10 and the external reaction container 20 rises
to 800 degrees C., the up-and-down movement device 220 moves up or
down the supporting device 50 based on the vibration detection
signal BDS from the vibration detecting device 240 by the
above-mentioned method, so that the seed crystal 5 contacts the
mixed molten metal 290.
[0293] And in the state in which the temperature of the reaction
container 10 and the external reaction container 20 is high (about
800 degrees C.), the nitrogen gas 4 in the space 23 is captured
into the mixed molten metal 290 through the metal Na. In this case,
the nitrogen concentration in the mixed molten metal 290 or the
concentration of Ga.sub.xN.sub.y (where x and y are real numbers)
is the highest in the vicinity of the gas-liquid interface 3 of the
space 23 and the mixed molten metal 290, a GaN crystal begins to
grow from the seed crystal 5 which is in contact with the
gas-liquid interface 3.
[0294] When nitrogen gas is not into the piping 200, the
temperature T3 of the seed crystal 5 is the same (800 degrees C.)
as the temperature of the mixed molten metal 290. In this
embodiment, in order to raise the degree of supersaturation of
nitrogen or group III nitride in the mixed molten metal 290 of the
seed crystal 5 neighborhood, nitrogen gas is supplied into the
piping 200, so that the seed crystal 5 is cooled and the
temperature T3 of the seed crystal 5 is made lower than the
temperature of the mixed molten metal 290.
[0295] More specifically, after timing t2, the temperature T3 of
the seed crystal 5 is set to the temperature Ts1 which is lower
than 800 degrees C. according to the curve k2. This temperature Ts1
is 790 degrees C., for example.
[0296] The method of setting the temperature T3 of the seed crystal
5 to the temperature Ts1 is essentially the same as that in the
previously described embodiment of FIG. 7.
[0297] There are two reasons the difference between the temperature
(=800 degrees C.) of the mixed molten metal 290 and the temperature
T3 of the seed crystal 5 is gradually increased in this embodiment.
The first reason is that a GaN crystal adheres to the seed crystal
5 with advance of the crystal growth of the GaN crystal. If the
temperature of the seed crystal 5 is not decreased gradually, it
becomes difficult to set the temperature of the GaN crystal
produced through crystal growth from the seed crystal 5 to a
temperature lower than the temperature of the mixed molten metal
290. The second reason is that Ga in the mixed molten metal 290 is
consumed with advance of the crystal growth of a GaN crystal, the
ratio .gamma.=Na/(Na+Ga) becomes large, and the degree of
supersaturation of the nitrogen concentration in the mixed molten
metal 290 or the group III nitride concentration becomes small. If
the temperature of the seed crystal 5 is not decreased gradually,
it becomes difficult to hold the nitrogen concentration in the
mixed molten metal 290 or the group III nitride concentration to
supersaturation.
[0298] Moreover, by decreasing the temperature of the seed crystal
5 gradually with advance of the crystal growth of a GaN crystal,
the degree of supersaturation of the nitrogen in the mixed molten
metal 290 of the seed crystal 5 neighborhood or the group III
nitride becomes large gradually, and the growth rate of a GaN
crystal becomes quick with advance of crystal growth. As a result,
it is possible to increase the size of a GaN crystal more
quickly.
[0299] When performing crystal growth of the GaN crystal in the
crystal preparing device 100, the seed crystal 5 is a GaN crystal
which is produced through crystal growth without using the seed
crystal 5 in the crystal preparing device 100. FIG. 22 is a diagram
showing the relation between the nitrogen gas pressure and the
crystal growth temperature in the case of growing up a GaN
crystal.
[0300] In FIG. 22, the horizontal axis indicates the crystal growth
temperature and the vertical axis indicates the nitrogen gas
pressure. In FIG. 22, the region REG is a region in which a
pillar-shaped GaN crystal grows up in the direction of c axis
(<0001>) in the bottom and the side which touch the mixed
molten metal 290 of the reaction container 10.
[0301] Therefore, when producing the seed crystal 5, a GaN crystal
is grown up using the nitrogen gas pressure and the crystal growth
temperature which are located in the region REG. In this case, many
cores occur on the bottom and the side wall in the reaction
container 10, and a pillar-shaped GaN crystal which is grown up in
the direction of c axis is produced. And a GaN crystal is selected
from among many GaN crystals produced through crystal growth, and
cut into the shape shown in FIG. 4A, so that the seed crystal 5 is
produced. Therefore, the projection part 5A (see FIG. 4B) of the
seed crystal 5 includes the GaN crystal grown up in the direction
of c axis (<0001>). The thus produced seed crystal 5 is
fitted in the space 54 of the supporting device 50 and it is fixed
to the supporting device 50.
[0302] FIG. 23 is a diagram for explaining the method of setting
metal Na and metal Ga to the reaction container 10 and the external
reaction container 20. As shown in FIG. 23, the glove box 300
contains a heater 310 inside. And the interior space 301 of the
glove box 300 is filled up with Ar gas.
[0303] The gas purification device 320 is connected with the glove
box 300 with the pipes 330 and 340. Moreover, the gas purification
device 320 is connected with the gas cylinder (not shown) which is
filled up with Ar gas.
[0304] The gas purification device 320 receives Ar gas from the gas
cylinder and purifies the received Ar gas. And the gas purification
device 320 supplies the purified Ar gas to the interior space 301
of the glove box 300 through the pipe 330.
[0305] Moreover, the gas purification device 320 attracts Ar gas
from the interior space 301 of the glove box 300 through the pipe
340, purifies the attracted Ar gas, and supplies the same to the
interior space 301 of the glove box 300. That is, the gas
purification device 320 performs circulation of Ar gas between the
device 320 and the interior space 301 of the glove box 300 as
indicated by the arrows 302 and 303.
[0306] The gas purification device 320 purifies Ar gas by reducing
the oxygen concentration and the moisture concentration in the Ar
gas to 10 ppm or less. Preferably, the gas purification device 320
purifies Ar gas by reducing the oxygen concentration and the
moisture concentration in the Ar gas in a range of 0.1-10 ppm.
Still more preferably, the gas purification device 320 purifies Ar
gas by reducing the oxygen concentration and the moisture
concentration in Ar gas to less than 0.1 ppm.
[0307] Moreover, the gas purification device 320 purifies Ar gas by
reducing the oxygen concentration in the Ar gas to a 10 ppb level,
and reducing the moisture concentration to 10 ppm or less.
Preferably, the gas purification device 320 purifies Ar gas by
reducing the oxygen concentration in the Ar gas to a 10 ppb level
and reducing the moisture concentration in the range of 0.1-10 ppm.
Still more preferably, the gas purification device 320 purifies Ar
gas by reducing the oxygen concentration in the Ar gas to a 10 ppb
level and reducing the moisture concentration to less than 0.1
ppm.
[0308] Furthermore, the gas purification device 320 purifies Ar gas
by reducing the moisture concentration in Ar gas to a 10 ppb level
and reducing the oxygen concentration to 10 ppm or less.
Preferably, the gas purification device 320 purifies Ar gas by
reducing the moisture concentration in the Ar gas to a 10 ppb level
and reducing the oxygen concentration in the range of 0.1-10 ppm.
Still more preferably, the gas purification device 320 purifies Ar
gas by reducing the moisture concentration in the Ar gas to a 10
ppb level and reducing the oxygen concentration to less than 0.1
ppm.
[0309] Since the oxygen concentration or moisture concentration of
a 10 ppb level is lower than the moisture concentration or oxygen
concentration of a 10 ppm level by triple figures, it can be
disregarded when compared with the moisture concentration or oxygen
concentration of a 10 ppm level.
[0310] Therefore, the gas purification device 320 performs the Ar
gas purification in any of the following eight patterns: [0311] (A)
reduce oxygen concentration and moisture concentration in the range
of 0.1-10 ppm; [0312] (B) reduce oxygen concentration to less than
0.1 ppm and reduce moisture concentration in the range of 0.1-10
ppm; [0313] (C) reduce oxygen concentration in the range of 0.1-10
ppm and reduce moisture concentration to less than 0.1 ppm; [0314]
(D) reduce moisture concentration in the range of 0.1-10 ppm;
[0315] (E) reduce oxygen concentration and moisture concentration
to less than 0.1 ppm; [0316] (F) reduce moisture concentration to
less than 0.1 ppm; [0317] (G) reduce oxygen concentration in the
range of 0.1-10 ppm; and [0318] (H) reduce oxygen concentration to
less than 0.1 ppm.
[0319] Since the purification patterns (D) and (F) mean reducing
the oxygen concentration to a 10 ppb level, it is changed to
reducing only the moisture concentration to a predetermined
concentration. Similarly, since the purification patterns (G) and
(H) mean reducing the moisture concentration to a 10 ppb level, it
is changed to reducing only the oxygen concentration to a
predetermined concentration.
[0320] One purification pattern is selected from among the
above-mentioned 8 purification patterns (A)-(H), the gas
purification device 320 purifies Ar gas using the se and supplies
the purified Ar gas to the interior space 301 of the glove box
300.
[0321] In order to use the purification patterns (A)-(H) for Ar gas
purification, the gas purification device 320 is provided with
corresponding filters F1-F8, respectively. The filter F1 is a
filter which reduces both the oxygen concentration and the moisture
concentration in Ar gas to the range of 0.1-10 ppm. The filter F2
is a filter which reduces the oxygen concentration to less than 0.1
ppm, and reduces the moisture concentration to the range of 0.1-10
ppm.
[0322] Moreover, the filter F3 is a filter which reduces the oxygen
concentration to the range of 0.1-10 ppm, and reduces the moisture
concentration to less than 0.1 ppm. The filter F4 is a filter which
reduces the oxygen concentration to a 10 ppb level, and reduces the
moisture concentration to the range of 0.1-10 ppm.
[0323] Furthermore, the filter F5 is a filter which reduces both
the oxygen concentration and the moisture concentration to less
than 0.1 ppm. The filter F6 is a filter which reduces the oxygen
concentration to a 10 ppb level, and reduces the moisture
concentration to less than 0.1 ppm.
[0324] Furthermore, the filter F7 is a filter which reduces the
moisture concentration to a 10 ppb level, and reduces the oxygen
concentration to the range of 0.1-10 ppm. The filter F8 is a filter
which reduces the moisture concentration to a 10 ppb level, and
reduces the oxygen concentration to less than 0.1 ppm.
[0325] Therefore, any of the filters F1-F8 mentioned above is
chosen and the chosen filter is installed in to the gas
purification device 320. And with the installed filter, the gas
purification device 320 reduces the oxygen concentration and the
moisture concentration in Ar gas to the predetermined range, and
supplies the purified Ar gas to the interior space 301 of the glove
box 300.
[0326] After the crystal growth of the GaN crystal in the crystal
preparing device 100 is completed, the external reaction container
20 is removed from the crystal preparing device 100, and is
installed on the heater 310 of the glove box 300.
[0327] Then, the heater 310 is turned on and the reaction container
10A and the external reaction container 20 are heated at 100
degrees C. or higher. And metal Na solidified between the reaction
container 10A and the external reaction container 20 is melt, and
it becomes the molten metal 350. When the molten metal 350 is
produced between the reaction container 10A and the external
reaction container 20, the reaction container 10A is in a removable
state. The reaction container 10A is changed with a new reaction
container 10B.
[0328] Then, melting the metal Na, it is put into the new reaction
container 10B, and the heater 310 is turned off. The molten metal
350 and the metal Na put in the reaction container 10B will be
solidified. Subsequently, metal Ga is put into the reaction
container 10B. If metal Ga is put into the reaction container 10B
after metal Na solidifies, the reaction of metal Na and metal Ga
can be prevented and metal Na and metal Ga can be put into the
reaction container 10B.
[0329] After metal Na and metal Ga are put into the reaction
container 10B, metal Na is filled up between the reaction container
10B and the external reaction container 20 if needed. And the seed
crystal 5 is placed in the supporting device 50, the lid part 22
(not shown) is attached to close the body part 21 of the external
reaction container 20, and the portion between the body part 21 and
the lid parts 22 is sealed with the metal seal.
[0330] Then, the external reaction container 20 is taken out from
the glove box 300, and the external reaction container 20 is set to
the crystal preparing device 100.
[0331] As described above, metal Na and metal Ga are set to the
reaction container 10 and the external reaction container 20 in an
Ar gas atmosphere where the quantity of oxygen and/or moisture is
reduced to 10 ppm or less, and the external reaction container 20
is set to the crystal preparing device 100.
[0332] FIG. 24 is a flowchart for explaining the crystal preparing
method of a GaN crystal.
[0333] Upon start of the crystal preparing method of FIG. 24, the
reaction container 10 and the external reaction container 20 are
put into the glove box 300 which is filled up with Ar gas and
contains a predetermined amount of impurities. In addition, the
predetermined amount of impurities in this embodiment means oxygen
in a concentration of 10 ppm or less, and/or moisture in a
concentration of 10 ppm or less.
[0334] And metal Na and metal Ga are put into the reaction
container 10 in the Ar gas atmosphere containing the predetermined
amount of impurities (step S1). In this case, metal Na and metal Ga
are put into the reaction container 10 in the mole ratio of
5:5.
[0335] Then, metal Na is placed between the reaction container 10
and the external reaction container 20 in the Ar gas atmosphere
containing the predetermined amount of impurities (step S2). And
the seed crystal 5 is placed on top of the metal Na and metal Ga in
the reaction container 10 in the Ar gas atmosphere containing the
predetermined amount of impurities (step S3).
[0336] Specifically, the seed crystal 5 is placed on top of the
metal Na and metal Ga in the reaction container 10 by fitting the
seed crystal 5 into the space 54 which is formed at the end 511 of
the supporting device 50 (see FIG. 4B).
[0337] Then, the reaction container 10 and the external reaction
container 20 are taken out from the glove box 300, and the reaction
container 10 and the external reaction container 20 are set as the
crystal preparing device 100 in the state where it is filled up
with Ar gas in the reaction container 10 and the external reaction
container 20.
[0338] And the valve 160 is opened and Ar gas with which it filled
up in the reaction container 10 and the external reaction container
20 with the vacuum pump 170 is exhausted.
[0339] After carrying out vacuum suction of the inside of the
reaction container 10 and the external reaction container 20 to the
predetermined pressure (0.133 Pa or less) using the vacuum pump
170, the valve 160 is closed and the valves 120, 121 are opened so
that the nitrogen gas is supplied from the gas cylinder 140 to the
reaction container 10 and the external reaction container 20
through the gas supply lines 90, 110.
[0340] In this case, nitrogen gas is supplied into the reaction
container 10 and the external reaction container 20 so that the
pressure in the reaction container 10 and the external reaction
container 20 may be set to about 0.1 MPa(s) using the pressure
regulator 130.
[0341] And when the pressure in the external reaction container 20
detected by the pressure sensor 180 is set to about 0.1 MPa, the
valve 160 is opened and the valves 120, 121 are closed so that the
nitrogen gas which is filled in the reaction container 10 and the
external reaction container 20 is exhausted using the vacuum pump
170.
[0342] Also in this case, vacuum suction of the inside of the
reaction container 10 and the external reaction container 20 to the
predetermined pressure (0.133 Pa or less) is performed using the
vacuum pump 170.
[0343] And the supply of the nitrogen gas to the reaction container
10 and the external reaction container 20 and the vacuum suction of
the inside of the reaction container 10 and the external reaction
container 20 is repeatedly performed several times.
[0344] Then, after carrying out vacuum suction of the inside of the
reaction container 10 and the external reaction container 20 to the
predetermined pressure by the vacuum pump 170, the valves 120, 121
are opened and the valve 160 is closed so that the nitrogen gas is
supplied into the reaction container 10 and the external reaction
container 20 to make the pressure in the reaction container 10 and
the external reaction container 20 is in the range of 1.01-5.05 MPa
by the pressure regulator 130 (step S4)
[0345] In this case, metal Na between the reaction container 10 and
the external reaction container 20 is solid, and nitrogen gas is
supplied from the space 31 of the piping 30 to the space 23 in the
external reaction container 20 through the control/introduction
plug 60.
[0346] And the valve 120 is closed when the pressure in the space
23 detected by the pressure sensor 180 is set to 1.01-5.05 MPa.
[0347] Then, the reaction container 10 and the external reaction
container 20 are heated to 800 degrees C. by the heating devices 70
and 80 (step S5). In this case, the melting point of the metal Na
held between the reaction container 10 and the external reaction
container 20 is about 98 degrees C., and it is melt in the process
in which the reaction container 10 and the external reaction
container 20 are heated to 800 degrees C., and it becomes the
molten metal 190. And the gas-liquid interfaces 1 and 2 occur (see
FIG. 19). The gas-liquid interface 1 is located in the interface of
the molten metal 190 and the space 23 in the external reaction
container 20, and the gas-liquid interface 2 is located in the
interface of the molten metal 190 and the control/introduction plug
60.
[0348] Moreover, when the temperature of the reaction container 10
and the external reaction container 20 is increased to 800 degrees
C., the temperature of the control/introduction plug 60 is 150
degrees C. Therefore, the vapor pressure of the molten metal 190
(=molten metal Na) in the gas-liquid interface 2 is
7.6.times.10.sup.-4 Pa, and the molten metal 190 (=molten metal Na)
hardly evaporates through the air gap 63 of the
control/introduction plug 60. As a result, the molten metal 190
(=metal Na melt) hardly decreases.
[0349] Even if the temperature of the control/introduction plug 60
is increased to 300 degrees C. or 400 degrees C., the vapor
pressure of molten metal 190 (=metal Na melt) is 1.8 Pa or 47.5 Pa,
and the decrease of the molten metal 190 (=metal Na melt) with the
vapor pressure of this level can be disregard.
[0350] Thus, in the crystal preparing device 100, the temperature
of the control/introduction plug 60 is set to the temperature at
which the molten metal 190 (=metal Na melt) does not decrease
substantially by evaporation.
[0351] Furthermore, in the process in which the reaction container
10 and the external reaction container 20 are heated to 800 degrees
C., metal Na and metal Ga in the reaction container 10 become
fluid, and the mixed molten metal 290 of metal Na and metal Ga is
produced in the reaction container 10. And the up-and-down movement
device 220 contacts the seed crystal 5 to the mixed molten metal
290 by the method mentioned above (step S6).
[0352] Furthermore, if the temperature of the reaction container 10
and the external reaction container 20 is increased to 800 degrees
C., the nitrogen gas in the space 23 is captured into the mixed
molten metal 290 through metal Na, and a GaN crystal begins to grow
from the seed crystal 5.
[0353] Then, the temperature of the reaction container 10 and the
external reaction container 20 is held for a predetermined time
(tens of hours-hundreds of hours) at 800 degrees C. (step S7). And
the temperature T3 of the seed crystal 5 is set to a temperature
Ts1 (or temperature Ts2) which is lower than the temperature (=800
degrees C.) of the mixed molten metal 290 by the above-mentioned
method (step S8).
[0354] And when the growth of a GaN crystal advances, the nitrogen
gas in the space 23 will be consumed and the nitrogen gas in the
space 23 is decreased. Then, the pressure P1 in the space 23 is
lower than the pressure P2 of the space 31 in the piping 30
(P1<P2), and a differential pressure occurs between the inside
of the space 23 and the inside of the space 31, and the nitrogen
gas of the space 31 is supplied gradually into the space 23 through
the control/introduction plug 60 and the molten metal 190 (=metal
Na melt) (step S9).
[0355] Then, the seed crystal 5 is dropped by the method mentioned
above so that the seed crystal 5 contacts the mixed molten metal
290 (step S10). Thereby, a GaN crystal having a large size
grows.
[0356] After a predetermined time passes, the temperature of the
reaction container 10 and the external reaction container 20 is
lowered (step S11), and the preparation of a GaN crystal is
completed.
[0357] FIG. 25 shows the state in the reaction container 10 and the
external reaction container 20 in the step S9 shown in FIG. 24. As
shown in FIG. 25, the temperature of the reaction container 10 and
the external reaction container 20 is held at 800 degrees C. from
timing t2 to timing t4, and the growth of a GaN crystal in the
mixed molten metal 290 advances. At this time, metal Na evaporates
from the molten metal 190 and the mixed molten metal 290, and the
nitrogen gas 4 and the metal Na vapor 7 coexist in the space
23.
[0358] And the pressure P1 in the space 23 is lower than the
pressure P2 of the space 31 in the piping 30 by the consumption of
the nitrogen gas 4. Then, the molten metal 190 moves to the space
23 and the gas-liquid interfaces 1 and 2 of the molten metal 190 go
up.
[0359] In such a state, nitrogen gas is supplied from the space 31
of the piping 30 to the molten metal 190 through the
control/introduction plug 60, and a bubble 191 is moved in the
inside of molten metal 190, and it is supplied to the space 23 from
the gas-liquid interface 1. And when the pressure P1 in the space
23 becomes almost the same as the pressure P2 in the space 31, the
molten metal 190 is lowered to the original position, and the
supply of nitrogen gas from the space 31 of the piping 30 to the
reaction container 10 and the external reaction container 20
through the control/introduction plug 60 and the molten metal 190
is stopped.
[0360] Thus, the control/introduction plug 60 supplies nitrogen gas
into the reaction container 10 and the external reaction container
20 from the space 31 in the piping 30 while the plug 60 holds the
molten metal 190 (=metal Na melt) between the reaction container 10
and the external reaction container 20 by the surface tension of
the molten metal 190. Therefore, the control/introduction plug 60
has the structure which prevents the passage of molten metal
190.
[0361] FIG. 26 shows the state in the reaction container 10 and the
external reaction container 20 in step S10 shown in FIG. 24. If the
crystal growth of a GaN crystal advances and the mixed molten metal
290 decreases, the gas-liquid interface 3 is lowered and the GaN
crystal 6 produced from the seed crystal 5 is separated from the
mixed molten metal 290.
[0362] The vibration detection signal BDS at this time includes the
signal component SS1 (shown in FIG. 20), and the up-and-down
movement device 220 lowers the supporting device 50 through the
above-mentioned method based on the vibration detection signal BDS
so that the GaN crystal 6 contacts the mixed molten metal 290.
Thereby, the GaN crystal 6 contacts the mixed molten metal 290
again, and the GaN crystal 6 grows preferentially.
[0363] Thus, the seed crystal 5 or the GaN crystal 6 which is
produced through crystal growth from the seed crystal 5 is always
contacted to the mixed molten metal 290 during the crystal growth
of the GaN crystal. Therefore, a GaN crystal having a large size
can be produced.
[0364] FIG. 27 shows the state of the reaction container 10 and the
external reaction container 20 at the end of preparation of the GaN
crystal according to the flowchart shown in FIG. 24. As shown in
FIG. 27, when the crystal growth of the GaN crystal 6 advances and
all the metal Ga in the mixed molten metal 290 is consumed, the
mixed molten metal 290 contains only metal Na and the gas-liquid
interface 3 does not fall further. At this time, the size of the
GaN crystal 6 which is produced through crystal growth from the
seed crystal 5 becomes large so that the GaN crystal 61 is
prepared.
[0365] The relation between the amount of impurities contained in
the GaN crystal 61 produced according to the flowchart shown in
FIG. 24 and the Ar-gas purification patterns (A)-(H) is shown in
Table 1 below.
TABLE-US-00001 TABLE 1 Concentration of O.sub.2 Amount of O & H
in & H.sub.2O in Ar Gas [ppm]: GaN Crystal [cm.sup.-3]: Spl.1:
O.sub.2 = 0.1 10, O = 10.sup.18 10.sup.20, H.sub.2O = 0.1 10 H =
10.sup.19 10.sup.20 Spl.2: O.sub.2 = below 0.1, O = 10.sup.18
10.sup.20, H.sub.2O = 0.1 10 H = 10.sup.19 10.sup.20 Spl.3: O.sub.2
= 0.1 10, O = 10.sup.18 10.sup.20, H.sub.2O = below 0.1 H = below
10.sup.19 Spl.4: -- O = 10.sup.18 10.sup.19, H.sub.2O = 0.1 10 H =
10.sup.19 10.sup.20 Spl.5: O.sub.2 = below 0.1, O = below
10.sup.18, H.sub.2O = below 0.1 H = below 10.sup.19 Spl.6: -- O =
below 10.sup.18, H.sub.2O = below 0.1 H = below 10.sup.19 Spl.7:
O.sub.2 = 0.1 10, O = 10.sup.18 10.sup.20 -- H: below limit Spl.8:
O.sub.2 = below 0.1, O = below 10.sup.18 -- H: below limit
[0366] In the above Table 1, the amount of oxygen (O) and the
amount of hydrogen (H) have been measured by using the SIMS
(Secondary Ion Mass Spectroscopy).
[0367] In the above Table 1, the amounts of oxygen and the amounts
of hydrogen for the samples Spl.1-Spl.8 of the GaN crystal 61
produced when purifying the Ar gas by using the purification
patterns (A)-(H) are indicated respectively.
[0368] In this manner, the oxygen concentration and the hydrogen
concentration in the GaN crystal 61 which is produced through
crystal growth using the crystal preparing device 100 are
controllable by controlling the oxygen concentration and the
moisture concentration in the Ar gas contained in the interior
space 301 of the glove box 300.
[0369] The samples Spl.1 and Spl.2 are the GaN crystals containing
the amount of oxygen of 10.sup.18-10.sup.20cm.sup.-3, and the
amount of hydrogen of 10.sup.19-10.sup.20cm.sup.-3. The sample
Spl.4 is a GaN crystal containing the amount of oxygen of
10.sup.18-10.sup.19cm.sup.-3, and the amount of hydrogen of
10.sup.19-10.sup.20cm.sup.-3.
[0370] Thus, the samples Spl.1, Spl.2, and Spl.4 are the GaN
crystals containing the amount of hydrogen of
10.sup.19-10.sup.20cm.sup.-3. And the GaN crystal containing the
amount of hydrogen of 10.sup.19-10.sup.20cm.sup.-3 is analogous to
an insulator. Therefore, the purification patterns (A), (B), and
(D) are suitable when preparing a GaN crystal analogous to an
insulator.
[0371] Moreover, the sample Spl.3 is a GaN crystal containing the
amount of oxygen of 10.sup.18-10.sup.20cm.sup.-3 and the amount of
hydrogen below 10.sup.19cm.sup.-3. The sample Spl.7 is a GaN
crystal containing the amount of oxygen of
10.sup.18-10.sup.20cm.sup.-3 and the amount of hydrogen below the
detectable limit.
[0372] Thus, the samples Spl.3 and Spl.7 contain the amount of
oxygen of 10.sup.18-10.sup.20cm.sup.-3. Since oxygen of
10.sup.18-10.sup.20cm.sup.-3 may act as the n type dopant in a GaN
crystal, each of the samples Spl.3 and Spl.7 is a low resistance
GaN crystal. Therefore, the GaN crystal of each of the samples
Spl.3 and Spl.7 is suitable for the substrate when producing an
optical device, such as a semiconductor laser, thereon.
[0373] Thus, since the GaN crystal used as the substrate for the
optical device can be produced, when a semiconductor laser is
produced on the GaN crystal, the electrode for pouring the current
for laser oscillation into the active layer of the semiconductor
laser can be formed in the thickness direction. The current can be
poured into the active layer more efficiently than in the case
where the electrode for current pouring is formed in the transverse
direction, and the semiconductor laser in which the threshold
current for laser oscillation is reduced can be produced.
[0374] In addition, hydrogen functions to inactivate dopant in a
GaN crystal, and the GaN crystal of the sample Spl.7 has a specific
resistance which is lower than that of the GaN crystal of the
sample Spl.3. In this manner, the amount of hydrogen below
10.sup.19cm.sup.-3 can function to control the specific resistance
of a GaN crystal.
[0375] Furthermore, the samples Spl.5 and Spl.6 are the GaN
crystals containing the amount of oxygen below 10.sup.18cm.sup.-3
and the amount of hydrogen below 10.sup.19cm.sup.-3, and the sample
Spl.8 is a GaN crystal containing the amount of oxygen below
10.sup.18cm.sup.-3 and the amount of hydrogen below the detectable
limit.
[0376] Thus, the samples Spl.5, Spl.6, and Spl.8 contain the amount
of oxygen below 10.sup.18cm.sup.-3. And the amount of oxygen below
10.sup.18cm.sup.-3 does not act as n type dopant in a GaN crystal.
Therefore, the samples Spl.5, Spl.6, and Spl.8 include a high
resistance GaN crystal, and they are suitable for producing an
electronic device, such as a field-effect transistor.
[0377] The GaN crystals of all the samples Spl.1-Spl.8 mentioned
above have a dislocation density of below 10.sup.3cm.sup.-2. Some
of them may contain metal Na (alkali metal), others may not contain
metal Na (alkali metal).
[0378] Thus, by controlling the oxygen concentration and the
moisture concentration of Ar gas contained in the glove box 300, it
is possible to produce a GaN crystal having a dislocation density
of 10.sup.3cm.sup.-2 or less and containing oxygen in a
concentration of 10.sup.20cm.sup.-3 or less (the samples Spl.7 and
Spl.8), or it is possible to produce a GaN crystal having a
dislocation density of 10.sup.3cm.sup.-2 or less and containing
oxygen of 10.sup.20cm.sup.-3 or less and hydrogen of
10.sup.20cm.sup.-3 or less (the samples Spl.1-Spl.6).
[0379] Therefore, according to this embodiment, the oxygen
concentration and the hydrogen concentration of Ar gas in the glove
box 300 are controlled, and any of an insulator, a low resistance
GaN semiconductor, and a high resistance GaN semiconductor can be
produced and a GaN crystal appropriate for use in optical devices
or electronic devices can be produced.
[0380] Although the mutual reaction is prevented, metal Ga and
metal Na are put in to the reaction container 10 and it explained
that a GaN crystal is produced, the intermetallic compound of not
only this but the metal Ga and metal Na is put into the reaction
container 10, and it is possible to produce a GaN crystal according
to the above embodiment of the invention.
[0381] The metal Ga and the metal Na are placed into the reaction
container 10 within the glove box, and an intermetallic compound is
produced by heating temperature at 100 degrees C. or higher. At
this time, the intermetallic compound of Na.sub.5Ga.sub.8 is
produced by setting the ratio of Na and Ga to 5:8. This
intermetallic compound is in the form of powders.
[0382] The produced powdered intermetallic compound may be put into
the new reaction container 10 in the glove box 300, the reaction
container 10 and the external reaction container 20 may be set in
the crystal preparing device 100, so that a GaN crystal may be
produced.
[0383] In addition, in the flowchart shown in FIG. 24, when the
reaction container 10 and the external reaction container 20 are
heated to 800 degrees C., the seed crystal 5 is contacted to the
mixed molten metal 290 containing metal Na and metal Ga (steps S5
and S6 in FIG. 24). Alternatively, when the reaction container 10
and the external reaction container 20 are heated to 800 degrees C.
(step S5), the seed crystal 5 may be held in the mixed molten metal
290 containing metal Na and metal Ga in step S6. That is, when the
reaction container 10 and the external reaction container 20 are
heated to 800 degrees C., the seed crystal 5 is immersed in the
mixed molten metal 290, so that crystal growth of a GaN crystal
from the seed crystal 5 may be performed.
[0384] And the operation which contacts the seed crystal 5 to the
mixed molten metal 290 includes step A which detects a vibration
detection signal BDS indicating vibrations of the supporting device
50, by applying vibrations to the supporting device 50 using the
vibration applying device 230, as well as step B which moves the
supporting device 50 using the up-and-down movement device 220 so
that the detected vibration detection signal BDS may be an
vibration detection signal (the signal component SS2 of the
vibration detection signal BDS) when the seed crystal 5 touches the
mixed molten metal 290.
[0385] Moreover, the operation which holds the seed crystal 5 in
the mixed molten metal 290 includes step A which detects a
vibration detection signal BDS indicating the vibrations of the
supporting device 50, by applying vibrations to the supporting
device 50 using the vibration applying device 230, as well as step
C which moves the supporting device 50 using the up-and-down
movement device 220 so that the detected vibration detection signal
BDS may be an vibration detection signal (the signal component SS3
of the vibration detection signal BDS) when the seed crystal 5 is
immersed in the mixed molten metal 290.
[0386] The supporting device 50 is moved using the up-and-down
movement device 220 in the above step B and the above step C.
According to the relation between the capacity of the reaction
container 10 and the total amount of metal Ga and metal Na
contained in the reaction container 10, the position of the liquid
surface (=the interface 3) of the mixed molten metal 290 within the
reaction container 10 is changed and when the mixed molten metal
290 is produced within the reaction container 10, the seed crystal
5 may be held in the space 23 or the seed crystal 5 may be immersed
in the mixed molten metal 290. In order to immerse the seed crystal
5 in the mixed molten metal 290 or to contact the seed crystal 5
with the mixed molten metal 290, it is necessary to move the seed
crystal 5 up or down in the gravity direction DR1.
[0387] Moreover, in step S10 of the flowchart in FIG. 24, the seed
crystal 5 is dropped so that the seed crystal 5 contacts the mixed
molten metal 290. Generally, the step S10 of the flowchart in FIG.
24 includes step D which moves the supporting device 50 using the
up-and-down movement device 220 so that a GaN crystal which is
produced through crystal growth from the seed crystal 5 contacts
the mixed molten metal 290 during the crystal growth of the GaN
crystal.
[0388] Although Ga in the mixed molten metal 290 is consumed and
the liquid surface (=interface 3) of the mixed molten metal 290 is
lowered with the crystal growth of the GaN crystal. The GaN crystal
which is produced through crystal growth from the seed crystal 5
may be moved downward or the GaN crystal which is produced through
crystal growth from the seed crystal 5 may be moved upward in
accordance with the relation between the speed at which the liquid
surface (=interface 3) falls and the crystal growth speed of the
GaN crystal.
[0389] That is, when the falling speed of the liquid surface
(=interface 3) is higher than the crystal growth speed of the GaN
crystal, the GaN crystal which is produced through crystal growth
from the seed crystal 5 is moved downward, so that the GaN crystal
is contacted with the liquid surface (=interface 3) of the mixed
molten metal 290. On the other hand, when the falling speed of the
liquid surface (=interface 3) is slower than the crystal growth
speed of the GaN crystal, the GaN crystal which is produced through
crystal growth from the seed crystal 5 is moved upward, so that the
GaN crystal is contacted with the liquid surface (=interface 3) of
the mixed molten metal 290.
[0390] Thus, since it is necessary to move up or down the GaN
crystal which is produced through crystal growth from the seed
crystal 5 in the gravity direction DR1 in accordance with the
relation between the falling speed of the liquid surface
(=interface 3) and the crystal growth speed of the GaN crystal, the
supporting device 50 is moved using the up-and-down movement device
220'' in the above step D.
[0391] And in the above step D, the operation which contacts the
GaN crystal which is produced through crystal growth from the seed
crystal 5 to the mixed molten metal 290 includes the above step A
and the above step B.
[0392] As mentioned above, a GaN crystal with a controlled amount
of oxygen and/or a controlled amount of hydrogen can be produced by
controlling the oxygen concentration and/or the moisture
concentration of Ar gas in the glove box 300 when placing metal Na
and metal Ga into the reaction container 10. According to this
feature, a GaN crystal appropriate for use in optical devices or
electronic devices can be produced.
[0393] Moreover, the crystal preparing device 100 of this
embodiment is characterized by growing up a GaN crystal where metal
Na vapor 7 is enclosed in the space 23. According to this feature,
evaporation of metal Na from the mixed molten metal 290 can be
controlled, the rate of a mole ratio of metal Na and metal Ga in
the mixed molten metal 290 can be held almost uniformly, and a
quality GaN crystal having a large size can be produced.
[0394] Furthermore, the crystal preparing device 100 of this
embodiment is characterized by setting the temperature T3 of the
seed crystal 5 to a temperature lower than the temperature (or the
temperature of the heaters of the heating devices 70 and 80) of the
reaction container 10 and the external reaction container 20, so
that a GaN crystal is produced.
[0395] According to this feature, it is controllable so that the
degree of supersaturation of the nitrogen in the mixed molten metal
290 of the seed crystal 5 neighborhood or group III nitride is made
high while a GaN crystal is produced from the seed crystal 5. That
is, it is controllable to prevent occurrence of a core other than
the seed crystal 5, and a GaN crystal is produced only from the
seed crystal 5. As a result, the GaN crystal having a large size
can be produced. And the seed crystal 5 is lowered using the
up-and-down movement device 220 so that the seed crystal 5 may
contact the mixed molten metal 290 during crystal growth of a GaN
crystal. It is possible to hold the state in which a GaN crystal is
produced preferentially from the seed crystal 5. As a result, the
GaN crystal having a large size can be produced.
[0396] Furthermore, the temperature T4 in the crystal preparing
device 100 of the gas-liquid interface 1 of the space 23 in the
external reaction container 20, and the molten metal 190 or the
gas-liquid interface 1 neighborhood is substantially equal to the
temperature T5 in the gas-liquid interface of the space 23 and the
mixed molten metal 290 or the gas-liquid interface 3 neighborhood,
the heating device 70 heats the reaction container 10 and the
external reaction container 20.
[0397] Thus, by making the temperature T5 in the gas-liquid
interface 3 or the gas-liquid interface 3 neighborhood
substantially equal to the temperature T4 in the gas-liquid
interface 1 or the gas-liquid interface 1 neighborhood, the metal
Na vapor which evaporated from molten metal 190, and the metal Na
vapor which evaporated from the mixed molten metal 290 will be in
equilibrium in space 23, and it can control that metal Na vapor of
the gas-liquid interface 3 neighborhood diffuses in the direction
of the gas-liquid interface 1.
[0398] As a result, evaporation of metal Na from the mixed molten
metal 290 is controlled certainly, the rate of a mole ratio of
metal Na and metal Ga in the mixed molten metal 290 can be
stabilized, it is stabilized and the GaN crystal which has large
size can be produced.
[0399] Furthermore, in the crystal preparing device 100, the
reaction container 10 and the external reaction container 20 may be
heated so that the temperature T4 is higher than the temperature
T5. In this case, a heating device is further installed between the
reaction container 10 and the external reaction container 20, the
reaction container 10 is heated with that installed heating device,
the gas-liquid interface 3 or the gas-liquid interface 3
neighborhood is heated to temperature T5, and the gas-liquid
interface 1 or the gas-liquid interface 1 neighborhood is heated to
temperature T4 with the heating device 70.
[0400] Thus, by setting the temperature T4 to a temperature higher
than the temperature T5, the vapor pressure of metal Na in the
gas-liquid interface 1 becomes higher than the vapor pressure of
metal Na in the gas-liquid interface 3, and metal Na vapor diffuses
from the gas-liquid interface 1 neighborhood to the gas-liquid
interface 3 neighborhood. In this case, in the gas-liquid interface
3 neighborhood, the concentration of metal Na vapor becomes high
and can control further evaporation of metal Na from the mixed
molten metal 290. As a result, the rate of a molar ratio of metal
Na and metal Ga in the mixed molten metal 290 can be stabilized
certainly, it is stabilized and the GaN crystal having a large size
can be produced.
[0401] Therefore, in the crystal preparing device 100, the
temperature T4 is set to be higher than the temperature T5, and
preparation of a GaN crystal is performed.
[0402] In the above embodiment, vibrations are applied to the
supporting device 50 and vibrations of the supporting device 50 are
detected so that the seed crystal 5 or the GaN crystal 6 contacts
the mixed molten metal 290. Alternatively, the position of the
gas-liquid interface 3 may be detected, and the seed crystal 5 or
the GaN crystal 6 may be contacted with the mixed molten metal 290.
In this case, the current is sent through the lead wire in the
state of lead wire where connected the end to the external reaction
container 20 from the outside, and the end is made immersed into
the mixed molten metal 290, and the length of the lead wire into
which it is put in the external reaction container 20 in case the
current switches from OFF to ON detects the position of the
gas-liquid interface 3.
[0403] If the other end of lead wire is immersed in the mixed
molten metal 290, the current flows into lead wire through the
mixed molten metal 290, the reaction container 10, molten metal
190, and the external reaction container 20 and the other end of
lead wire is not immersed in the mixed molten metal 290, current
does not flow into the lead wire.
[0404] Therefore, the position of the gas-liquid interface 3 is
detectable with the length of the lead wire put in the external
reaction container 20 when application of the current is switched
from OFF to ON. And if the position of the gas-liquid interface 3
is detected, the seed crystal 5 or the GaN crystal 6 is dropped to
the position of the detected the gas-liquid interface 3 using the
up-and-down movement device 220.
[0405] Alternatively, a sound wave may be generated and applied to
the gas-liquid interface 3, and the time for the sound wave to go
and come back between the gas-liquid interfaces 3 is measured, so
that the position of the gas-liquid interface 3 is detected.
[0406] Furthermore, it is possible to detect the position of the
gas-liquid interface 3 from the length of the thermocouple which is
inserted into the reaction container 10 from the external reaction
container 20 when the temperature detected by the thermocouple is
changed.
[0407] Moreover, the temperature of the seed crystal 5 is lower
than the temperature of the mixed molten metal 290 by cooling the
seed crystal 5 in the above embodiment. Alternatively, a heater may
be installed in the piping 200 to control the temperature of the
seed crystal 5 by this heater. When the reaction container 10 and
the external reaction container 20 are heated with the heating
devices 70 and 80, the temperature of the seed crystal 5 may not
rise like the temperature of the mixed molten metal 290. In such a
case, the seed crystal 5 is heated by using the heater installed in
the piping 200, so that it is controlled so that the temperature of
the seed crystal 5 changes according to the curve k2 or the
straight line k3 shown in FIG. 7.
[0408] Therefore, the heating devices 70 and 80 and the heater in
the piping 200 may be controlled so that the difference between the
temperature of the mixed molten metal 290 and the temperature of
the seed crystal 5 matches with a difference in temperature between
the straight line k1 and the curve k2 shown in FIG. 7, or a
difference in temperature between the straight line k1 and the
straight line k3 shown in FIG. 7.
[0409] Furthermore, in the above embodiment, the height H of the
convex part 62 of the control/introduction plug 60 and the gap d of
the convex parts 62 is on the order of tens of micrometers.
Alternatively, the height H of the convex part 62 and the gap d of
the convex parts 62 may be determined according to the temperature
of the control/introduction plug 60. In this case, when the
temperature of the control/introduction plug 60 is relatively high,
the height H of the convex part 62 is relatively made high, and the
gap d of the convex parts 62 is made relatively small. Moreover,
when the temperature of the control/introduction plug 60 is
relatively low, the height H of the convex part 62 is relatively
made low and the gap d of the convex parts 62 is made relatively
large.
[0410] That is, when the temperature of the control/introduction
plug 60 is relatively high, the size of the air gap 63 between the
control/introduction plug 60 and the piping 30 is made relatively
small, and when the temperature of the control/introduction plug 60
is relatively low, the size of the air gap 63 between the
control/introduction plug 60 and the piping 30 is made relatively
large.
[0411] The size of the air gap 63 is determined according to the
height H of the convex part 62 and the gap d of the convex parts
62, and the size of the air gap 63 which can hold the molten metal
190 using the surface tension changes in accordance with the
temperature of the control/introduction plug 60. Therefore, the
height H of the convex part 62 and the gap d of the convex parts 62
are changed according to the temperature of the
control/introduction plug 60, and it is possible to hold the molten
metal 190 certainly by using the surface tension.
[0412] And the control of the temperature of the
control/introduction plug 60 is performed by the heating device 80.
That is, in increasing the temperature of the control/introduction
plug 60 to a temperature higher than 150 degrees C., the
control/introduction plug 60 is heated by using the heating device
80.
[0413] Moreover, in the above embodiment, metal Na and metal Ga are
placed into the reaction container 10 in the Ar gas atmosphere
containing a predetermined amount of impurities, and metal Na is
placed between the reaction container 10 and the external reaction
container 20 in the Ar gas atmosphere containing the predetermined
amount of impurities. Alternatively, metal Na and metal Ga are
placed into the reaction container 10 in a nitrogen gas atmosphere
which contains gas other than Ar gas, such as He, Ne, Kr, etc. and
contains a predetermined amount of impurities, and metal Na may be
placed between the reaction container 10 and the external reaction
container 20. Generally, it is adequate to place metal Na and metal
Ga into the reaction container 10 in the nitrogen gas atmosphere
containing a predetermined amount of impurities or the inert gas
containing a predetermined amount of impurities, and to place metal
Na between the reaction container 10 and the external reaction
container 20.
[0414] The present invention is not limited to the above-described
embodiments, and variations and modifications may be made without
departing from the scope of the present invention.
[0415] Further, the present application is based on and claims the
benefit of priority of Japanese patent application No. 2006-068614,
filed on Mar. 14, 2006, and Japanese patent application No.
2006-078173, filed on Mar. 22, 2006, the entire contents of which
are hereby incorporated by reference.
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