U.S. patent application number 10/134381 was filed with the patent office on 2002-11-07 for high purity gallium for producing compound semiconductor, refining process and apparatus for the same.
This patent application is currently assigned to DOWA MINING CO., LTD.. Invention is credited to Kato, Hidekazu, Ohgami, Takashi, Okuda, Kanichi, Tayama, Kishio, Yamamura, Takeharu.
Application Number | 20020162419 10/134381 |
Document ID | / |
Family ID | 17989446 |
Filed Date | 2002-11-07 |
United States Patent
Application |
20020162419 |
Kind Code |
A1 |
Yamamura, Takeharu ; et
al. |
November 7, 2002 |
High purity gallium for producing compound semiconductor, refining
process and apparatus for the same
Abstract
In a process for separating impurities from a raw gallium
material containing impurities, a process for refining gallium
comprising progressively solidifying a raw gallium material
provided in a liquid state inside a vessel while applying stirring,
such that the diameter of the tubular solidification boundary
gradually advances from the inner wall plane of the vessel towards
the center of the vessel to reduce the diameter of the tubular
solidification boundary, and separating the liquid phase remaining
in the central portion of the vessel from the solidified phase
before the entire raw material inside the vessel is solidified. The
process above is repeated as required by using, as the raw gallium
material, the solidified phase from which the liquid phase is
separated. A metallic gallium favorably used for the preparation of
a compound semiconductor can be obtained by analyzing the impurity
concentration of the impurity-concentrated Ga separated from the
solidified layer.
Inventors: |
Yamamura, Takeharu;
(Hachiouji-shi, JP) ; Kato, Hidekazu;
(Funabashi-shi, JP) ; Ohgami, Takashi;
(Edogawa-ku, JP) ; Tayama, Kishio; (Akita-ken,
JP) ; Okuda, Kanichi; (Tokyo, JP) |
Correspondence
Address: |
McDermott, Will & Emery
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
DOWA MINING CO., LTD.
Tokyo
JP
|
Family ID: |
17989446 |
Appl. No.: |
10/134381 |
Filed: |
April 30, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10134381 |
Apr 30, 2002 |
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09581840 |
Jun 19, 2000 |
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09581840 |
Jun 19, 2000 |
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PCT/JP99/05943 |
Oct 27, 1999 |
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Current U.S.
Class: |
75/688 ; 266/205;
420/555 |
Current CPC
Class: |
C22B 9/02 20130101; Y02P
10/20 20151101; C22B 9/14 20130101; C22B 58/00 20130101 |
Class at
Publication: |
75/688 ; 266/205;
420/555 |
International
Class: |
C22B 007/04; C22C
028/00; C22B 058/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 29, 1998 |
JP |
10-309144 |
Claims
1. In a process for separating impurities from a raw gallium
material containing impurities, a process for refining gallium
comprising progressively solidifying a raw gallium material
provided in a liquid state inside a vessel while applying stirring,
such that the diameter of the tubular solidification boundary
gradually advances from the inner wall plane of the vessel towards
the center of the vessel to reduce the diameter of the tubular
solidification boundary, and separating the liquid phase remaining
in the central portion of the vessel from the solidified phase
before the entire raw material inside the vessel is solidified.
2. A process for refining gallium as claimed in claim 1, wherein
stirring is applied by a magnetic field.
3. A process for refining gallium as claimed in claim 1 or 2,
wherein stirring is applied by a magnetic field in such a manner
that a circular flow is generated in the liquid phase in the
circumferential direction.
4. In a process for separating impurities from a raw gallium
material containing impurities, a process for refining gallium
comprising progressively solidifying a raw gallium material
provided in a liquid state inside a vessel while applying stirring,
such that the diameter of the tubular solidification boundary
gradually advances from the inner wall plane of the vessel towards
the center of the vessel to reduce the diameter of the tubular
solidification boundary, separating the liquid phase remaining in
the central portion of the vessel from the solidified phase before
the entire raw material inside the vessel is solidified, and after
melting the solidified phase inside the vessel, repeating the same
process steps above.
5. A process for refining gallium as claimed in claim 4, wherein a
solid phase is reserved as a seed crystal on the inner wall plane
of the vessel on melting the solidified phase.
6. An apparatus for refining gallium comprising a vessel having a
cylindrical inner wall, a cooling zone attached to the outer
peripheral plane of the vessel, a heating zone provided on the
inner side of the inner wall of the vessel, a suction pipe
installed at the central portion of the vessel, and a magnetic
rotator placed on the lower side of the vessel.
7. An apparatus for refining gallium comprising a vessel having a
cylindrical inner wall, a cooling and heating zone attached to the
outer peripheral plane of the vessel, a suction pipe installed at
the central portion of the vessel, and a magnetic rotator placed on
the lower side of the vessel.
8. An apparatus for refining gallium as claimed in claim 7, wherein
the cooling and heating zone is operated by switching cold water
and hot water to pass therethrough.
9. An apparatus for refining gallium as claimed in claim 6, 7, or
8, wherein a heating zone is provided at the bottom portion of the
vessel and to the outer periphery of the suction pipe.
10. An apparatus for refining gallium as claimed in claim 6, 7, 8,
or 9, wherein a means for reserving a seed crystal is provided to
the inner wall of the vessel or in the vicinity of said inner
wall.
11. In a high purity raw Ga material for use in the preparation of
a compound semiconductor, a raw Ga material used for preparing a
compound semiconductor which has a difference
.DELTA.C=.vertline..SIGMA.An-.SIGMA.- Bn.vertline. of 5 ppm by
atomic or lower when subjected to a "test method for
impurity-concentrated Ga" as defined below, where .SIGMA.An
represents the total quantity of the components contained in the
sample of an impurity-concentrated Ga, which is at least one
element of group A components selected from the group consisting of
B, Na, Mg, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Au, Hg, Pb, and
Bi; and .SIGMA.Bn represents the total quantity of the components
contained in the sample of impurity-concentrated Ga, which is at
least one element of group B components selected from the group
consisting of F, Si, S, Cl, Ge, Se, Sn, and Te. "Test method for
impurity-concentrated Ga" is defined as a test method comprising:
using an apparatus for refining gallium comprising a vessel, having
a cylindrical inner wall made of a 3 mm thick SUS304 steel sheet
provided with a 0.3 mm thick fluororesin coated inner wall plane,
said vessel having an inner radius of 60 mm and a height of 40 mm,
a cooling zone attached to the outer peripheral plane of said
vessel, a suction pipe installed at the central portion of said
vessel, and a magnetic rotator provided to the lower portion of
said vessel; filling said vessel with a raw Ga material in liquid
state at a quantity as such that it amounts to 30 mm in height
inside the vessel while purging the space inside the vessel with an
inert gas; and obtaining a sample of impurity-concentrated Ga as
follows: while applying a circular flow of 100.+-.10 rpm to the
liquid raw Ga material by using the rotator, maintaining the liquid
raw Ga material at a temperature of 29.6.+-.0.5.degree. C., and
passing a cooling water at a temperature of 5.degree. C. through
the cooling zone, thereby allowing progressive solidification of
the liquid to proceed from the inner wall of the vessel towards the
central portion of the vessel at a solidification rate as such that
the entire liquid may solidify in 60.+-.5 minutes, then sampling
the liquid phase through the suction pipe when the radius of the
remaining liquid phase becomes 20 mm.
12. A raw Ga material used for preparing a compound semiconductor
as claimed in claim 11, wherein said compound semiconductor is a
single crystal of GaAs.
13. A raw Ga material used for preparing a compound semiconductor
as claimed in claim 11, wherein said compound semiconductor is a
crystal of GaP.
14. A raw Ga material used for preparing a compound semiconductor
as claimed in claim 11, 12, or 13, wherein .SIGMA.An is 1 ppm by
atomic or lower.
15. A raw Ga material used for preparing a compound semiconductor
as claimed in claim 11, 12, or 13, wherein .SIGMA.Bn is 1 ppm by
atomic or lower.
16. A raw Ga material used f or preparing a compound semiconductor,
having a refined purity of 6N as refined by the process of refining
gallium as claimed in claim 1.
17. A raw Ga material used for preparing a compound semiconductor,
having a refined purity of 7N or higher as refined by the process
of refining gallium as claimed in claim 1.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a refining process and an
apparatus for metallic gallium (Ga), and it further refers to a
high purity Ga suitable for obtaining a compound semiconductor such
as a GaAs single crystal.
BACKGROUND OF THE INVENTION
[0002] Among the compound semiconductors, the Group III-V
compounds, particularly GaAs single crystals, are widely used as
the substrates of electronic devices and optical devices such as
high speed ICs and photoelectronic integrated circuits because they
not only have superior high electron mobility which is about five
times as high as that of an elemental semiconductor such as
silicon, but also exhibit excellency in, for example, high
frequency characteristics, magnetic conversion functions,
photoreceptor functions and light emitting functions.
[0003] Wafers of GaAs single crystal are manufactured through
various processes. The basic process thereof must comprise a step
of growing GaAs crystals from a melt of Ga--As and a step of
slicing them into wafers. A wafer (semi-insulating GaAs substrate)
thus prepared is then subjected to selective ion injection or
various types of epitaxial growth processes to finally obtain the
desired semiconductor device element.
[0004] In using a GaAs single crystal (GaAs ingot) as a
semi-insulating substrate, it is an indispensable requirement that
the single crystal stably maintains a specific resistance (referred
to hereinafter as resistivity) of 1.times.10.sup.7 Q.multidot.cm or
higher. Although it is most desirable to obtain an intrinsic GaAs
single crystal completely free from impurities and lattice defects,
it is practically difficult to produce such an intrinsic GaAs
single crystal of high purity because of the unavoidable crystal
defects and residual impurities. As a reason for causing such
difficulties, there can be mentioned the presence of impurities
that accompany the raw material for Ga (gallium) used in the step
of growing a GaAs crystal from the Ga--As melt.
[0005] In growing a GaAs crystal from a Ga--As melt, generally used
is the LEC (Liquid Encapsulated Czocralski) process. This process
comprises covering the surface of a Ga--As melt placed inside a
crucible with B.sub.2O.sub.3, and pulling up a seed crystal of GaAs
through the B.sub.2O.sub.3 layer while rotating the melt and
applying pressure in an inert gas atmosphere. In carrying out this
process, various improvements are devised to reduce the
incorporation of impurities into the GaAs single crystal as much as
possible, such as using a crucible made of PBN (Pyrolytic Boron
Nitride) or controlling the gaseous atmosphere.
[0006] In spite of the improvements made on the constitution of the
apparatus and on the process conditions, the probability of
incorporating the impurities into the GaAs single crystal increases
if the concentration of impurities incorporated in the starting
melt from which the GaAs crystal is grown remains high. That is, it
is still difficult to obtain a high quality GaAs single crystal if
the purity of the raw materials for Ga and As remains low. Among
the impurities which accompany the raw materials for Ga and As,
there certainly are impurity elements having a low segregation
index that are less incorporated into the growing crystal and
reside in the melt; however, from the viewpoint of improving the
yield in producing GaAs single crystals, it is still undesirable to
result in a melt containing impurity elements at high
concentration. Accordingly, the concentration of impurities in the
raw materials for Ga and As is preferably as low as possible and it
is further desirable to previously recognize the type and content
of each impurity present in the raw material.
[0007] Concerning the raw materials for Ga and As for use in
producing GaAs single crystals, it is relatively easy to find a
commercially available high purity As (arsenic) having a purity of
7N (seven nines; stands for a 99.99999% purity, and is sometimes
used hereinafter to express the purity). However, the case for raw
Ga materials is not so simple. Any raw Ga material contains, to
some extent, a variety of impurities in various forms depending on
its origin, and, the quantity of the impurities fluctuates in
general. It is therefore difficult to stably obtain a raw Ga
material free from impurities which are inconvenient for the
production of GaAs single crystals. Furthermore, with the present
day analytical technology (glow discharge mass spectrometer) for
analyzing the content of the impurity elements present in metallic
Ga, it is difficult to obtain reliable results for each of the
components incorporated at a level of 0.01 ppm or lower. It can be
understood therefrom that it is even difficult to know the exact
concentration of each of the impurity elements contained in trace
quantities in the raw Ga material to be used for the production of
GaAs single crystals.
[0008] In addition to the aforementioned GaAs single crystals,
compound semiconductors using Ga include GaP, GaN, etc. Because a
GaP single crystal has excellent photoreceptor and light emitting
functions, it is used as a substrate for optical devices such as
light emitting devices. A GaP single crystal wafer is produced by
first synthesizing a polycrystalline GaP, pulling up the
polycrystalline GaP as a GaP single crystal and by means of a
process similar to, for example, the aforementioned LEC process,
and slicing the resulting GaP single crystal ingot. A light
emitting device can be finally obtained by performing liquid layer
expitaxy. To obtain a light emitting device of high luminance in
this case, the incorporation of impurities in the GaP single
crystal substrate must be suppressed to the lowest limit.
Particularly harmful are the impurities which increase the
concentration of the carriers on synthesizing the polycrystalline
GaP and lowers the resistivity. Similar to the case of GaAs,
furthermore, the incorporation of such harmful impurities is
believed to be originated from the raw Ga material in many
cases.
[0009] As a process for refining metallic gallium to remove
impurities from the raw materials, conventionally known processes
include acid processing, electrolytic smelting, zone melting,
pulling up crystals, recrystallization by melting and
solidification, etc. Among these processes, the recrystallization
process comprising melting and solidification is advantageous in
that it enables refining using a relatively simple installation and
operation. In solidifying a liquid of a raw gallium material
containing an impurity, there is known a phenomenon as such that
the impurity concentration of the crystal becomes lower than that
of the residual liquid. The principle of this process is based on
this phenomenon.
[0010] For the process of refining gallium by utilizing the
phenomenon above, proposals for improving the process conditions
and operations can be found in, for example, JP-A-Sho62-270494 (the
term "JP-A-" as referred herein signifies "an unexamined published
Japanese patent application"), JP-A-Sho63-242996, JP-A-Hei2-50926,
JP-A-Hei2-50927, JP-B-Hei2-53500 (the term "JP-B-" as referred
herein signifies "an examined published Japanese patent
application"), JP-A-Hei6-136467, etc.
OBJECT OF THE INVENTION
[0011] At present, in producing compound semiconductors such as
GaAs and GaP, it is practically impossible to obtain a highly pure
metallic gallium having a purity of 6N or 7N or even higher and
also provided with reliable analytical data for each of the
impurity contents. Since this caused a problem in producing high
quality compound semiconductors such as GaAs and GaP, a first
object of the present invention is to overcome this problem.
[0012] Among the prior art technologies for producing high purity
gallium, the recrystallization process using melting and
solidification comprises separating the crystalline gallium (solid
phase) containing impurities at a low concentration level from the
residual liquid (liquid phase) containing impurities at a higher
concentration, and is based on the concept of separating the solid
phase from the liquid phase differing in impurity concentration. In
order to separate the high purity solid phase from the liquid
phase, it is necessary to separate the solid phase when the
residual liquid is still present at a relatively large quantity.
This inevitably results in a low yield of high purity gallium.
[0013] For instance, in JP-A-Hei6-136467 is disclosed a process
which comprises inserting a cooled tube into the central portion of
the molten raw gallium material placed inside a vessel, thereby
allowing solid gallium to precipitate on the surface of the tube,
and then pulling up the resulting tube having thereon the
precipitated gallium. The disclosure teaches, however, that it is
preferred to complete the operation of precipitation at a stage
that the solidification ratio (the ratio of the precipitate)
accounts for 30 to 40%, and that even under the most controlled
conditions, the solidification ratio attainable is in the range of
60 to 70%. Accordingly, this results in a large quantity of gallium
remaining in the liquid phase, and this limits the yield of the
refined product.
[0014] The recrystallization process using melting and
solidification is rarely adopted as a mass production technology to
produce a high purity gallium having a high purity in the level of
6N or 7N in an industrial scale due to its poor controllability and
productivity.
[0015] Accordingly, a second object of the present invention is to
establish a process for producing high purity gallium at high yield
and with superior controllability.
[0016] Furthermore, a third object of the present invention is to
provide, for a high purity metallic gallium for use in the
preparation of single crystals of compound semiconductors such as
GaAs and GaP, a means to recognize the concentration of each of the
impurities that are unable to reliably quantify their concentration
by the existing analytical method using glow discharge mass
spectrometer; at the same time, it is also the object of the
present invention to provide a high purity gallium containing such
impurities in a trace level, but with given approximate
concentration.
DISCLOSURE OF THE INVENTION
[0017] According to an aspect of the present invention, there is
provided in a process for separating impurities from a raw gallium
material containing impurities, a process for refining gallium
comprising progressively solidifying a raw gallium material
provided in a liquid state inside a vessel while applying stirring,
such that the diameter of the tubular solidification boundary
gradually advances towards the center of the vessel to reduce the
diameter of the tubular solidification boundary, and separating the
liquid phase remaining in the central portion of the vessel from
the solidified phase before the entire raw material inside the
vessel is solidified.
[0018] In the process above, the stirring can be applied by a
magnetic field, and particularly, the stirring is preferably
applied by a magnetic field in such a manner that a circular flow
is generated in the liquid phase in the circumferential
direction.
[0019] In the aforementioned refining process, the solidified phase
inside the vessel can be molten again after separating the liquid
phase remaining in the central portion of the vessel from the
solidified phase, and by repeating the process above, the purity of
the solidified phase can be progressively increased. In this case,
the solid phase is preferably reserved as a seed crystal on the
inner wall plane of the vessel on melting the solidified phase.
[0020] As an apparatus for performing the refining process above,
there is provided an apparatus for refining gallium comprising a
vessel having a cylindrical inner wall, a cooling zone attached to
the outer peripheral plane of the vessel, a heating zone provided
on the inner side of the inner wall of the vessel, a suction pipe
installed at the central portion of the vessel, and a magnetic
rotator placed on the lower side of the vessel. Furthermore, as
another apparatus for performing the refining process above, there
is provided an apparatus for refining gallium comprising a vessel
having a cylindrical inner wall, a cooling and heating zone
attached to the outer peripheral plane of the vessel, a suction
pipe installed at the central portion of the vessel, and a magnetic
rotator placed on the lower side of the vessel. In these
apparatuses, a heating zone can also be provided at the bottom
portion of the vessel and to the outer periphery of the suction
pipe.
[0021] In addition to the aforementioned processes and apparatuses
for refining gallium, an aspect of the present invention provides a
high purity raw Ga material for use in the preparation of a
compound semiconductor, characterized by a raw Ga material used for
preparing a compound semiconductor which yields a difference
.DELTA.C=.vertline..SIGM- A.An-.SIGMA.Bn.vertline. of 5 ppm by
atomic or lower when subjected to a "test method for
impurity-concentrated Ga" as defined below, where .SIGMA.An
represents the total quantity of the components contained in the
sample of an impurity-concentrated Ga, which is at least one
element of group A components selected from the group consisting of
B, Na, Mg, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Au, Hg, Pb, and
Bi; and .SIGMA.Bn represents the total quantity of the components
contained in the sample of impurity-concentrated Ga, which is at
least one element of group B components selected from the group
consisting of F, Si, S, Cl, Ge, Se, Sn, and Te. "Test method for
impurity-concentrated Ga" is defined as a test method
comprising:
[0022] using an apparatus for refining gallium comprising a vessel,
having a cylindrical inner wall made of a 3 mm thick SUS304 steel
sheet provided with a 0.3 mm thick fluororesin coated inner wall
plane, the vessel having an inner radius of 60 mm and a height of
40 mm, a cooling zone attached to the outer peripheral plane of the
vessel, a suction pipe installed to the central portion of the
vessel, and a magnetic rotator provided to the lower portion of the
vessel;
[0023] filling the vessel with a raw Ga material in liquid state at
a quantity as such that it amounts to 30 mm in height inside the
vessel while purging the space inside the vessel with an inert gas;
and
[0024] obtaining a sample of impurity-concentrated Ga as
follows:
[0025] while applying a circular flow of 100.+-.10 rpm to the
liquid raw Ga material by using the rotator, maintaining the liquid
raw Ga material at a temperature of 29.6.+-.0.5.degree. C., and
passing a cooling water at a temperature of 5.degree. C. through
the cooling zone, thereby allowing progressive solidification of
the liquid to proceed from the inner wall of the vessel towards the
central portion of the vessel at a solidification rate as such that
the entire liquid may solidify in 60.+-.5 minutes, then sampling
the liquid phase through the suction pipe described above when the
radius of the remaining liquid phase becomes 20 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a system diagram showing an equipment allocation
in an installation for performing a process according to the
present invention;
[0027] FIG. 2 is a schematically drawn cross section view for the
refining vessel portion shown in FIG. 1;
[0028] FIG. 3 is a cross section viewed from the direction shown by
an arrow in FIG. 2;
[0029] FIG. 4 is an explanatory diagram showing the operation
sequence in performing a process according to the present invention
using the installation shown in FIG. 1;
[0030] FIG. 5 is a system diagram showing another equipment
allocation in an installation for performing a process according to
the present invention;
[0031] FIG. 6 is an explanatory diagram showing the operation
sequence in performing a process according to the present invention
using the installation shown in FIG. 5;
[0032] FIG. 7 is a schematically drawn cross section view showing
the structure of the wall portion of the vessel provided as a means
for reserving a seed crystal on initiating solidification; and
[0033] FIG. 8 is a diagram showing the relation between the
resistivity of a single crystal GaAs and the difference
N.sub.A-N.sub.B between the total quantity N.sub.A of Group A
impurity elements (acceptors) and the total quantity N.sub.B of
Group B impurity elements (donors).
BEST MODE FOR CARRING OUT THE INVENTION
[0034] Metallic gallium obtained from various types of Ga recovery
processes or products of processed Ga-containing scraps contain
trace quantities of Sn, In, Cu, Pb, Zn, Au, and other impurity
elements, and, although it is possible to obtain a metallic gallium
having a purity of about four nines by reducing the concentration
of such impurities, the concentration is still high as a raw Ga
material for use in the production of a GaAs single crystal.
Accordingly, it is required to increase the purity of such raw Ga
materials to a higher purity of, for instance, 6N or 7N, or to a
even higher purity. The definition of the method for specifying the
purity of Ga to 6N or 7N concentration is as follows.
[0035] By performing mass analysis on the elements to be analyzed
specified in paragraph (1) below in accordance with paragraph (2),
and for the impurities present at a quantitatively analyzed
concentration exceeding the quantitative lower limit defined in
paragraph (3), the total of the quantitatively obtained
concentration values is subtracted from 100% by weight; the
numerical value thus obtained in terms of the number of "9" is then
defined as the purity (N) of a high purity gallium.
[0036] (1) Elements to be analyzed include 18 elements of Al, Si,
P, Cl, K, Ca, Cr, Fe, Ni, Cu, Zn, Ge, As, In, Sn, Au, Hg, and
Pb.
[0037] (2) Method of analysis: The refined gallium is heated and
dissolved while thoroughly stirring, thereby obtaining a
homogeneous sample. About 50 g portion per one refined unit is
sampled to be subjected to the analysis. Quantitative analysis is
then performed on the thus prepared sample by using a GDMS (glow
discharge mass spectrometer) for each of the elements specified in
paragraph (1) above.
[0038] (3) Lower limits of quantitative analysis: The lower limits
of quantitative detection in accordance with the mass analysis
specified in paragraph (2) above are as follows.
[0039] 0.01 ppm (by weight): Al, Si, P, Cl, K, Ca, Cr, Fe, Ni, Cu,
Zn, As, In, Sn, Hg, and Pb
[0040] 0.1 ppm (by weight): Ge
[0041] 0.2 ppm (by weight): Au
[0042] The purity of a high purity Ga is specified as above
because, with the present analytical technique using GDMS, reliable
quantitative analysis on Ga can be performed only to a
concentration of 0.01 ppm or higher for the elements in Ga (except
for Ge and Au, which are 0.1 ppm or higher and 0.2 ppm or higher,
respectively).
[0043] The present invention relates to an industrially
advantageous process and apparatus for refining metallic gallium
containing various types of impurity elements to produce metallic
gallium having a higher purity. Although the process and the
apparatus according to the present invention are applicable to any
type of raw gallium materials so long as they contain gallium as
the principal component, they are more advantageously applied to
raw gallium materials having a purity in the range of from about 1
to 4 N.
[0044] The present inventors have found a fact that the refining of
crystals can be performed at an extremely efficient manner by
placing the raw gallium materials above in a vessel having a
cylindrical inner wall and allowing the solidification to proceed
from the inner wall side towards the center of the vessel while
applying stirring. That is, in an ideal case of uniformly allowing
the solidification to proceed from the inner wall plane of a
cylinder in the direction headed to the center, the tubular
boundary of solidification advances in such a manner to gradually
decrease the diameter of the tubular boundary of solidification;
thus, the boundary area of solidification is gradually reduced. The
behavior of this change in area is completely reverse to the case
disclosed in the aforementioned JP-A-Hei6-136467. In the case of
the disclosure, the diameter of the tubular solidification boundary
extends from the surface of the tube immersed in the liquid gallium
phase; that is, the solidification boundary in the prior art
disclosure increases its area.
[0045] Furthermore, in the process according to the present
invention, the liquid phase becomes cylindrical and the diameter of
the cylindrical liquid phase decreases with the progress of
solidification. However, the center of the liquid cylinder remains
the same. Thus, the liquid phase containing impurities at a higher
concentration accumulates on the central portion, and this
facilitates the separation of the liquid phase with higher impurity
concentration. For instance, the residual liquid at the central
portion can be sucked out to easily perform the solid-liquid
separation. This also allows the repetition of the refining
operation, and results in a solidified phase with higher purity
with increasing repetition times of the operation. Furthermore,
because the liquid phase is obtained in a cylindrical form having a
center, stirring of the liquid phase can be continuously applied in
a uniform manner during solidification. This signifies that uniform
stirring is performed during the progress of solidification in such
a manner to prevent difference from occurring in the peripheral and
the vertical direction of the solidification boundary. The fact
that the stirring of the liquid phase is performed in a favorable
manner, particularly, that the stirring is favorably performed at
the solidification boundary, greatly contributes to the results
obtained by the refining process for crystals according to the
present invention. The liquid phase can be stirred by applying a
magnetic field.
[0046] Thus, by allowing the solidification of the liquid raw
gallium material to proceed from the cylindrical inner wall side
towards the center while applying stirring, crystal refining can be
achieved in an extremely efficient manner. Furthermore, even if the
solidification is proceeded to such a degree that the liquid phase
is accumulated at the central portion accounting for 10% by volume
or less, or, in some cases, for about 5%, it is still possible to
obtain a solidified phase with high purity.
[0047] The present invention is described below in further detail
by way of preferred embodiments with reference to the attached
drawings.
[0048] FIG. 1 shows an equipment allocation in a representative
installation for performing a process according to the present
invention. Referring to the figure, the apparatus for refining
gallium comprises a vessel 2 having a cylindrical inner wall 1, a
cooling zone 3 through which cold water is passed and attached to
the outer peripheral plane of the vessel 2, a heating zone 4
comprising a coil through which hot water is passed through and
provided on the inner side of the inner wall 1 of the vessel, a
suction pipe 5 installed at the central portion of the vessel, and
a rotator 6 made of a permanent magnet and placed at the lower
portion of the vessel.
[0049] The vessel 2 shown in the figure is formed into a shape
having a round bottom and a cylindrical wall by using a stainless
steel plate, and it preferably has a lid 9. Preferably, the plane
of the stainless steel sheet (for example, a SUS304 stainless steel
sheet) that is brought into contact with liquid gallium is coated
with a resin coating. In this manner, the components contained in
the stainless steel can be prevented from migrating into gallium.
Most preferred resin for use in the resin coating is fluororesin.
As fluororesins, usable is PFA (Ethylene
tetrafluoride-perfluoroalkoxyethylene copolymer). Similarly, the
surface of the suction pipe 5 that is brought into contact with
liquid gallium is preferably subjected to resin coating. In this
manner, the members that are brought into contact with liquid
gallium inside the vessel are preferably coated with a resin, and
in some cases, the members may be formed with a resin. As the resin
for the outermost surface layer, polypropylene and polyethylene
resins can be used other than fluororesin.
[0050] Ideally, the cylindrical inner wall 1 has a cross section of
true circle, but also acceptable are cases in which the cross
section is formed as polygons having edge portions or ellipsoids.
Thus, the term "cylindrical inner wall" refers to not only the
cylindrical inner walls having a cross section of true circle, but
also those having polygonal or ellipsoidal cross sections.
[0051] Referring to the figure, the cylindrical inner wall 1 is
formed to have the same diameter along the entire length from the
lower end to the upper end. However, in order to establish a heat
conduction as such to form a cylindrical solidification boundary,
it is also possible to form the cylinder having a circumferential
plane in such a manner that the radius in the lower end may differ
from that in the upper end. As such cylindrical forms, there can be
mentioned a form having a smaller radius at the middle, or on the
contrary, having an enlarged radius at the middle. Also, the
cylinder may have a reduced diameter at the top or the bottom
portion.
[0052] A cooling zone is provided to the outer peripheral plane of
the vessel 2, and in the case shown in the figure, the cooling zone
3 is constructed by a water-cooling jacket 3 through which cold
water is flown. The water-cooling jacket 3 is attached in such a
manner that it surrounds the vessel 2. A water-cooling coil may be
used in place of the water-cooling jacket, or various types of
coolants can be used instead of cold water. At any rate, in an
ideal case, the cooling zone is provided as such that it cools the
raw gallium material 10 inside the vessel 2 uniformly in the
circumferential direction and the vertical direction from the inner
wall 1 of the vessel; i.e., the heat is uniformly emitted radially
from the upper and the lower sides of the raw gallium material.
[0053] A heating zone 4 comprising a coil through which hot water
is flown (referred to hereinafter as a hot water coil 4) is
provided in the vessel on the inner side of the inner wall 1. The
hot water coil 4 is provided slightly distant from the inner wall 1
of the vessel, and the pitch of the coil is taken sufficiently
large so that the solidified phase may move between the coils. By
flowing hot water through the hot water coil 4, the raw gallium
material solidified inside the vessel can be molten, but by
providing the hot water coil 4 at a distant from the inner wall 1,
the portion which is brought into contact with the inner wall plane
can be left without being molten (at this instance, cold water is
flown through the cooling zone 3), and this portion being left in
the form of solid can be reserved as a seed crystal for use in the
next solidification.
[0054] A suction pipe 5 is located at the central portion of the
vessel. The suction pipe is provided to discharge the residual
liquid in the central portion out of the vessel. A heating zone 7
through which hot water is flown (referred to hereinafter as a hot
water pipe 7) is provided to the outer peripheral plane of the
suction pipe 5. By using the hot water pipe 7, heat can be applied
to the residual liquid to maintain the liquid phase and to
establish a smooth sucking of the liquid. Furthermore, the raw
gallium material provided as a liquid to be processed may be fed
into the vessel 2 by using this suction pipe 5. In such a case, a
supply pipe equipped with a valve can be connected from the vessel
filled with the liquid raw gallium material (not shown) to the
suction piping 11 shown in the figure. The suction pipe 5 can be
provided in such a manner that it can be slid vertically along the
center axis of the vessel, and in some cases, it can be moved to
the outside of the vessel.
[0055] Preferably, a heating zone 8 (referred to as a "hot water
spiral pipe"), through which hot water is flown, is provided at the
bottom of the vessel, so as to accelerate the melting of metallic
gallium solidified inside the vessel, or to maintain the
temperature of the metallic gallium inside the vessel to a
predetermined temperature range. The hot water spiral pipe can be
provided by placing a spiral pipe at the bottom of the vessel, so
that hot water can be flown therethrough. Alternatively, a hot
water jacket can be used in place of the spiral pipe.
[0056] A rotator 6 made of a permanent magnet is placed at the
lower portion of the vessel. A magnetized rare earth magnet is used
for the rotator 6, and stirring is performed by aligning the ro
tating axis 12 of the magnet with the center of the vessel, and by
rotating it in the plane parallel to the bottom of the vessel
around the center of the vessel using a motor 13. In this manner,
stirring is applied to the liquid gallium by means of a magnetic
force. Thus, the stirring of the liquid phase is applied by a
circular flow produced by the rotation of the rotator 6.
[0057] On the other hand, the suction pipe 5 is connected to a
vacuum vessel 15 via a suction piping 11 equipped with a valve 14.
A ladle 16 is set inside the vacuum vessel 15, and the inside of
the vacuum vessel 15 is maintained under reduced pressure by
operating a vacuum pump 17. In FIG. 1, a cold water pump 18 is
incorporated in the piping through which cold water is circulated
between the cooling zone 3 and the cold water source, while a hot
water pump 19 is provided in a circuit for circulating hot water
between the heating zone 8 and a hot water source.
[0058] FIG. 3 is the schematically shown cross section of the
refining vessel viewed from the direction shown by an arrow in FIG.
2. Referring to FIG. 3, the water cooling jacket 3 is installed in
contact with the outer periphery of the vessel 2, and the hot water
coil 4 is installed concentrically at a distance from the inner
plane of the vessel 2. The suction pipe 5 is provided at the center
of the vessel.
[0059] The operation of refining gallium using the apparatus for
refining gallium having the constitution above is described below.
The refining operation is preferably carried out inside a clean
room while maintaining the chamber at a temperature not lower than
the melting point of gallium (e.g., at a temperature not lower than
30.degree. C.). Further preferred is to perform the refining
operation under a pressure slightly higher than the atmospheric
pressure, using a refining vessel 2 provided with an airtight lid
9, and purging the atmosphere inside, the vessel with an inert gas
such as gaseous nitrogen or gaseous argon.
[0060] FIG. 4 is a diagram showing the operation steps for refining
gallium using the apparatus shown in FIG. 1. In FIG. 4 (A) is shown
the state of vessel 2 into which a liquid raw gallium material 10
is placed. In this state, the liquid raw gallium material 10 is
maintained at a temperature not lower than the melting point by
supplying hot water to the hot water coil 4 and the hot water pipe
7, while simultaneously supplying hot water to the heating zone
(hot water spiral pipe) 8 at the bottom portion of the vessel. By
flowing cold water to the water cooling jacket 3 at the same time,
the solidified layer obtained in the previous stage can be reserved
as seed crystals on the inner wall of the vessel.
[0061] FIG. 4(B) shows the state at the midway of solidification
after initiating the solidification from the state shown in FIG.
4(A). In this state, the solidification proceeds in such a manner
that the diameter of the tubular solidification boundary gradually
advances towards the center of the vessel to reduce the diameter of
the tubular solidification boundary by the operations as
follows:
[0062] Driving the motor 13 of the magnetic rotator 6;
[0063] Supplying cold water to the water cooling jacket 3;
[0064] Stopping hot water supply to the hot water coil 4;
[0065] Stopping hot water supply to the hot water spiral pipe 8;
and
[0066] Supplying hot water to the hot water pipe 7.
[0067] Accordingly, while being stirred, the liquid phase gradually
reduces its diameter into a cylinder having a smaller diameter to
be remained at the central portion. Referring to FIG. 4(B), more
specifically, the solidification boundary B between the solidified
phase S shown as the hatched portion and the liquid phase L shown
by the non-hatched portion gradually reduces its diameter toward
the center of the vessel.
[0068] In this instance, the liquid phase L at the central portion
is prevented from being solidified because hot water is flown
through the hot water pipe 7 of the suction pipe, and stirring
using the magnetic force can be favorably maintained. In
particular, if the solidification boundary B is provided in the
form of a cylinder with a center located at the center of the
vessel, a circular flow can be imparted to the liquid phase L by
the rotator 6 being rotated around the center axis. Thus, a
favorable flow generates uniformly in the liquid phase in the
vicinity of the solidification boundary B. In this manner,
favorable stirring is performed on the entire boundary in the
liquid side where solidification is in progress, and the
segregation of impurity elements in this portion can be suppressed.
Furthermore, from the viewpoint on the solidified phase S, the area
for growing the crystal is gradually reduced because the
solidification area gradually decreases due to the gradual
reduction in diameter of the solidification boundary B. Thus, by
the favorably performed stirring and the reduction in diameter of
the solidification boundary, the impurity elements are prevented
from being entrained in the solidified phase, and as a result,
gallium can be refined at a high yield. Ideally, the solidification
boundary B has little difference in radius along the vertical
direction and is cylindrical with a cross section near to true
circle, but in the practical operation, the influence of having
some difference in radius or of not having a cross section of true
circle is not so serious.
[0069] FIG. 4(C) shows a state in which the solidification is
further proceeded from the state shown in FIG. 4(B), and the
residual liquid RL remaining in the central portion of the vessel
is separated from the solidified phase S just before the entire raw
material in the vessel is solidified. The operation of the
apparatus is continued in a manner similar to the operation
performed in the state shown in FIG. 4(B), except that the valve 14
provided to the piping 11 connected to the vacuum vessel 15 is
opened to draw out the residual liquid RL through the suction pipe
5 from the central portion of the vessel. Because the residual
liquid RL contains the impurities present in the raw gallium liquid
10, the impurities can be removed from the raw gallium liquid 10 by
drawing out the residual liquid RL. The concentration of the
impurities in the residual liquid increases with decreasing the
amount of residual liquid. In the process according to the present
invention, the incorporation of impurities into the solidified
phase can be suppressed even if the solidification ratio (i.e., the
ratio of the volume of the solidified phase to that of the raw
liquid material) is increased to 90% or higher, or in some cases,
to 95% or higher. Accordingly, the solidification can be proceeded
to a state in which the amount of residual liquid is small. This
also implies that the repetition times of remelting and
solidification can be increased, and thereby high purity gallium
can be collected at a high yield.
[0070] FIG. 4(D) shows a state in which the solidified phase
remaining in the vessel after the residual liquid RL is drawn out
in the previous state shown in FIG. 4(C) is remolten. Remelting is
performed by the same operation as that employed in the step shown
in FIG. 4(A) above. By supplying cold water to the water-cooling
jacket 3 while flowing hot water through the hot water coil 4 in
this case, remelting can be performed while maintaining the
solidified phase present in the vicinity of the inner wall of the
vessel at a temperature not higher than the melting point. Thus, a
part of the solidified phase remaining in the vicinity of the inner
wall of the vessel can be reserved as seed crystals S.sub.1. The
seed crystals S.sub.1 can be left in a state directly attached to
the inner wall of the vessel, but in order to assure the presence
of the crystal nuclei, it is preferred to leave many seed crystals
in spots. For this purpose, it is effective to provide
irregularities on the inner wall of the vessel; or, in some cases,
members such as mesh members and porous plates may be provided to
the inner wall of the vessel to facilitate the generation of seed
crystals in a large quantity.
[0071] By repeating the sequence of steps A to D above, gallium
with a higher purity can be obtained inside the vessel 2 every time
the sequence is repeated, while gallium with a lower purity is
collected inside the ladle 16 provided in the vacuum vessel 15. The
process according to the present invention which repeats the
sequence above comprises, in other words, increasing the purity of
the raw material inside the vessel by drawing out the concentrated
impurity portions from the vessel in small quantities. Thus, this
is an extremely efficient refining operation. Since the control
operation for each of the process steps is simple, the entire
process excels in operability, and automatic control can be easily
introduced thereto.
[0072] FIG. 5 is a diagram showing an equipment allocation in
another installation for performing a process according to the
present invention. Referring to the figure, the apparatus for
refining gallium comprises a vessel 2 having a cylindrical inner
wall 1, a cooling and heating zone 20 attached to the outer
peripheral plane of the vessel 2, a suction pipe 5 installed at the
central portion of the vessel, and a rotator 6 made of a permanent
magnet and placed at the lower portion of the vessel. Furthermore,
a heating zone 7 through which a hot water is flown is provided to
the outer periphery of the suction pipe 5, and another heating zone
8 through which a hot water is flown is provided at the bottom
portion of the vessel. Cold water or hot water is provided from a
cold water source 21 or a hot water source 22 to the cooling and
heating zone 20 by switching from one to the other.
[0073] The structure of the apparatus shown in FIG. 5 is
substantially the same as that shown in FIG. 1, except for removing
the hot water coil 4 from the apparatus shown in FIG. 1 and for
replacing the water cooling jacket 3 by a cooling and heating zone
20 to which cold water or hot water is switchably supplied. The
cooling and heating zone 20 is constructed similarly to the jacket
shown in FIG. 1 in a manner that it surrounds the outer peripheral
plane of the vessel 2, and cold or hot water is supplied to the
jacket. More specifically, the cold water source 21 and the hot
water source 22 are communicated with the jacket by the switching
operation of three way valves 23 and 24, and cold water or hot
water is supplied to the jacket by driving the pump 25. The
switching operation for the hot water and the cold water is carried
out in such a manner that a process similar to that explained with
reference to FIG. 4 can be performed on the apparatus shown in FIG.
1. An example of the operation steps is shown in FIG. 6.
[0074] FIG. 6(A) shows the state of the vessel 2 into which a
liquid raw gallium material 10 is placed. In this state, the liquid
raw gallium material 10 is maintained at a temperature not lower
than the melting point by supplying hot water to the jacket 20 and
the heating zone (jacket) 8 provided at the bottom portion of the
vessel. Hot water is continuously supplied to the heating zone 7 of
the suction pipe throughout the entire process.
[0075] FIG. 6(B) shows the state at the midway of solidification
after initiating the solidification from the stage shown in FIG.
6(A). In this state, cold water is supplied to the jacket 20 while
continuing the rotation of the magnetic rotator 6 and stopping the
supply of hot water to the heating zone 8. At this stage, the
solidification proceeds in a manner similar to the case explained
with reference to the operation shown in FIG. 4(B). That is, the
solidification boundary B gradually advances towards the center of
the vessel while maintaining the tubular shape and reducing the
diameter.
[0076] Similar to FIG. 4(C), FIG. 6(C) shows the operation of
drawing out the residual liquid RL remaining in the central portion
of the vessel through the suction pipe 5. The state of supplying
hot water or cold water is substantially the same as that shown in
FIG. 6(B), but hot water may be supplied to the heating zone 8
provided at the bottom portion of the vessel, so that the residual
liquid RL may not be solidified at the bottom portion of the
vessel.
[0077] FIG. 6(D) shows a state in which the solidified phase
remaining in the vessel is remolten after the residual liquid RL is
drawn out in the previous state. The remelting is performed by the
same operation shown in FIG. 6(A), and the similar process steps
can be repeated thereafter.
[0078] It is preferred that the seed crystals of gallium are
present on the inner wall of the vessel on shifting from the stage
shown in FIG. 6(A) or 6(D) to the stage shown in FIG. 6(B). If no
seed crystal exists on supplying cold water to the jacket 20 to
initiate the solidification, the liquid phase would be supercooled.
If the solidification is allowed to proceed from such a supercooled
state, the efficiency of refining would be lowered.
[0079] FIG. 7 shows a means for reserving the seed crystals.
Referring to FIG. 7, a concave portion 26 is provided to a part of
the inner wall 1 of the vessel 2, and a cold water box 27 is
provided independent to the jacket 20 in such a manner that the
cold water box 27 may surround the concave portion 26. By using
such a constitution, cold water is supplied to the cold water box
27 even while hot water is flown through the jacket 20. In this
manner, solidified phase of gallium can be reserved inside the
concave portion 26, so that it may function as a seed crystal at
the initiation of solidification and prevent the occurrence of
supercooling on the liquid phase.
[0080] Instead of the means shown in FIG. 7, a pipe supplied with
cold water may be inserted from the upper portion of the vessel 2
to the vicinity of the inner wall 1 of the vessel. Thus, a
solidified phase which functions as a seed crystal can be formed on
the surface of the pipe. In addition, supercooling can be prevented
by feeding granules of solid gallium as seed crystals in the
vicinity of the inner wall of the vessel.
[0081] In accordance with the process for refining gallium using
the apparatus above, metallic gallium improved in purity can be
obtained every time the refining process is repeated. Thus, a
metallic gallium having a purity of not only 6N, but also 7N, or
even higher in some cases, can be obtained at a never achieved high
yield. The fact that a metallic gallium having such a high purity
is available by an industrially advantageous process greatly
contributes to the preparation of a GaAs single crystal;
particularly, by considering the process for refining gallium
according to the present invention from a different viewpoint, this
method can be regarded as a process for concentrating impurities in
gallium. That is, the impurity concentration in metallic gallium
can be specified by applying this process; hence, the process
according to the present invention provides a raw metallic gallium
material highly advantageous in preparing a GaAs single crystal. In
other words, the impurities contained in trace quantities in
gallium, their quantitative analysis has not been possible by glow
discharge mass spectrometer (GDMS), can be recognized by the
process according to the present invention as described below. The
fact that the concentration of each of the impurity element can be
recognized means that the contents of elements which function as
acceptors and donors when incorporated into a GaAs single crystal
can be presumed, and this is greatly advantageous in obtaining a
GaAs single crystal of higher quality.
[0082] For instance, let us suppose a 7N metallic gallium is
obtained industrially by using the refining apparatus according to
the present invention. The fact that a metallic gallium having a
purity as high as 7N is industrially available is itself surprising
from the viewpoint of the conventional technological level;
however, reliable quantitative analysis obtainable on such a
metallic gallium by using a conventional GDMS is, as described
hereinbefore, in the level of, for instance, 0.1 ppm or higher for
Ge, 0.2 ppm or higher for Au, 0.01 ppm or higher for most of the
other elements, and, for the impurities present at a concentration
lower than these levels, the error level becomes too large (still,
however, the metallic gallium can be defined as having a purity of
7N) according to the above-described definition. This signifies
that the analysis performed by using GDMS fails to quantitatively
identify the content of acceptor type elements or donor type
elements. The process according to the present invention enables
the quantitative identification of acceptor type and donor type
elements.
[0083] More specifically, when the 7N metallic gallium above is
subjected to the apparatus and the process for refining gallium
according to the present invention, the residual liquid (i.e., Ga
containing impurities at a high concentration; referred to
hereinafter as "impurity-concentrated Ga") which remains in small
quantity at the central portion of the vessel can be sampled and
analyzed by using GDMS. Since the concentration of each of the
impurity elements are higher for this sample, quantitative analysis
is now possible by using GDMS. Then, the group of elements
quantitatively detected by GDMS can be classified into Group A
elements, i.e., the acceptor type elements, and Group B elements,
i.e., the donor type elements. By taking the difference between the
total concentration of Group A elements and that of Group B
elements, it has been found that the 7N metallic gallium yielding
the difference at a value not higher than a predetermined level is
particularly favorable for the starting Ga material for preparing a
GaAs single crystal having a high resistivity. Similarly, it has
been found that such a metallic gallium is particularly preferable
for preparing a GaP single crystal.
[0084] In further detail, when subjecting metallic gallium to a
test defined as "Test method for impurity-concentrated Ga" herein,
the impurity-concentrated Ga is obtained as a sample to be analyzed
by GDMS, and from the analytical results, a difference
.DELTA.C=.vertline..SIGMA.A- n-.SIGMA.Bn.vertline., where .SIGMA.An
represents the total quantity of the group A components (B, Na, Mg,
K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Au, Hg, Pb, and Bi), and
.SIGMA.Bn represents the total quantity of the group B components
(F, Si, S, Cl, Ge, Se, Sn, and Te) can be obtained; then, it has
been found that, as is shown in the Examples hereinafter, if the
difference AC is 5 ppm by atomic or lower, the metallic Ga enables
stable preparation of GaAs single crystal having a higher
resistivity than any one in the prior art.
[0085] The conditions utilized in the apparatus and the process for
refining Ga according to the present invention are specified in
"Test method for impurity-concentrated Ga" defined herein to
establish a testing method. As the testing apparatus, firstly,
there is used an apparatus for refining gallium comprising a vessel
having a cylindrical inner wall made of a 3 mm thick SUS304 steel
sheet provided with a 0.3 mm thick fluororesin coated inner wall
plane, the vessel having an inner radius of 60 mm and a height of
40 mm, a cooling zone attached to the outer peripheral plane of the
vessel, a suction pipe installed to the central portion of the
vessel, and a magnetic rotator provided to the lower portion of the
vessel. A lid is provided air-tight to the vessel so that the
inside of the vessel can be purged with an inert gas. The inner
wall side of the SUS304 steel sheet is provided with a fluororesin
coating in order to prevent the contamination from SUS304. As the
fluororesin, preferred is PFA
(tetrafluoroethylene-perfluoroalkoxyethylen- e copolymer), but is
not particularly limited thereto, and any type is acceptable so
long as it prevents contamination from SUS304. The SUS304 is a
stainless steel which contains, all percentages expressed by
weight, 0.08% or less C, 1.00% or less Si, 2.00% or less Mn, 0.045%
or less P, 0.030% 36 or less S, 8.00 to 10.50% Ni, 18.00 to 20.00%
Cr, and balance Fe with unavoidable impurities.
[0086] The cooling zone is a annular jacket 3 which can
continuously flow cold water as described in FIG. 1, and the height
thereof is the same as that of the vessel, i.e., 40 mm. The annular
jacket is a tube having an inner radius of 5 mm and comprises a
double tube consisting of an inner tube using the wall of the
vessel itself and an outer tube made of a 3 mm thick stainless
steel sheet (SUS304 steel sheet) similar to that used for the
vessel. The upper portion and the bottom portion of the jacket are
both produced by using a 3 mm thick SUS304 steel sheet. The suction
pipe is installed movable in the upward and downward directions
through the lid of the vessel, and a member corresponding to the
heating zone 7 through which hot water is flown, as described in
FIG. 1, is provided to the outer peripheral plane thereof.
Preferably, a member corresponding to the heating zone 8 shown in
FIG. 1 is provided at the bottom of the vessel, but an electric
heater may be used in the place thereof.
[0087] The test is performed by using the testing apparatus above,
and the vessel of the apparatus is filled with a raw Ga material in
liquid state at a quantity as such that it amounts to 30 mm in
height inside the vessel while purging the space inside the vessel
with an inert gas to maintain the pressure inside the vessel at a
value slightly higher than the atmospheric pressure. Then, while
applying a circular flow of 100.+-.10 rpm to the liquid raw Ga
material by using a magnetic rotator, the liquid raw Ga material is
maintained at a temperature of 29.6.+-.0.5.degree. C., and a
cooling water is passed through the cooling zone at a temperature
of 5.degree. C., thereby proceeding solidification of the liquid at
a solidification rate as such that the entire liquid may solidify
in 60.+-.5 minutes from the inner wall of the vessel towards the
central portion of the vessel. The solidification rate above can be
set easily by trial tests comprising repetition of solidification
and melting while changing the flow rate of the cooling water and
the supply rate of heat for melting the solidified product. In the
practical test for sampling the impurity-concentrated Ga, the
solidification is allowed to proceed under the conditions set
above, and sampling of the liquid phase through the suction pipe is
performed when the radius of the liquid phase remaining at the
central portion of the vessel becomes 20 mm. The liquid phase thus
sampled is denoted as the impurity-concentrated Ga.
[0088] The thus sampled impurity-concentrated Ga is then subjected
to analysis using GDMS to count the content of each of the impurity
elements. If the thus obtained
.DELTA.C=.vertline..SIGMA.An-.SIGMA.Bn.ver- tline. is 5 ppm by
atomic or lower, the metallic gallium subjected to the test is an
approved product suitable for use as the raw Ga material for
preparing a compound semiconductor, for instance, a product
particularly preferred for use in the preparation of a GaAs single
crystal.
[0089] As described above, the present invention not only enables
the production of high purity metallic gallium having a purity of
6N or 7N, but also provides a metallic gallium suitable for use in
the preparation of high purity GaAs single crystal provided with
the prediction on the distribution of impurity elements functioning
as acceptors and of impurity elements functioning as donors.
[0090] As explained below, the aforementioned fact is extremely
advantageous in preparing a semi-insulating GaAs single
crystal.
[0091] A GaAs single crystal requires a resistivity of
1.times.10.sup.7 Q.multidot.cm or higher, but the resistivity
changes depending on the number of impurities. In such a case, the
behavior is different depending on whether the impurity functions
as an acceptor or a donor, but the difference between the number of
acceptors and donors is related with the resistivity.
[0092] For instance, FIG. 8 shows the relation between the
resistivity of a single crystal GaAs and the difference
N.sub.A-N.sub.B between the total quantity N.sub.A of Group A
impurity elements (acceptors) and the total quantity N.sub.B of
Group B impurity elements (donors), wherein N.sub.A-N.sub.B is
taken as the abscissa and the resistivity of the GaAs single
crystal is taken as the ordinate. The resistivity increases from
1.times.10.sup.7 .OMEGA..multidot.cm for a difference
N.sub.A-N.sub.B of about 3.times.10.sup.14/cm.sup.3, but it drops
abruptly with the difference N.sub.A-N.sub.B exceeding a value of
about 1.times.10.sup.16/cm.sup.3 to indicate that the GaAs single
crystal becomes electrically conductive. This phenomenon has been
utilized heretofore; for instance, elements for increasing N.sub.B
were added in the case the impurity elements include high N.sub.A,
and on the contrary, in the case N.sub.B was high, impurities for
increasing N.sub.A were added to control the difference
N.sub.A-N.sub.B. This operation is known as dope control of
impurities.
[0093] In accordance with the present invention, a semi-insulating
GaAs single crystal having a resistivity of 1.times.10.sup.7
.OMEGA..multidot.cm or higher can be obtained without performing
the dope control of the impurities. More specifically, the present
invention provides an advantageous effect of producing a
semi-insulating GaAs single crystal without doping any impurities.
It can be understood according to the present invention that, if
the value .DELTA.C=.vertline..SIGMA.An-.SIGMA.Bn.vertline. obtained
by analyzing an impurity-concentrated Ga using GDMS is 5 ppm by
atomic or lower, a GaAs single crystal prepared by using this
metallic gallium results in a difference N.sub.A-N.sub.B falling in
the range of from 3.times.10.sup.14 to
1.times.10.sup.16/cm.sup.3.
[0094] In this case, it is particularly preferred that the
impurity-concentrated Ga yields a GDMS analytical value .SIGMA.An
of, for instance, 1 ppm by atomic or lower, and furthermore, it is
still more preferred that .SIGMA.Bn is, for instance, 1 ppm by
atomic or lower. In accordance with the present invention and as
shown in Examples 3 and 4 below, a high purity metallic Ga for
preparing GaAs single crystal satisfying such requirements is
available. As a matter of course, the present invention provides
high purity metallic gallium suitable not only for GaAs single
crystals, but also for preparing Ga based compound semiconductors
inclusive of GaP, GaN, etc.
EXAMPLES
Example 1
[0095] A raw gallium material containing impurities at
concentrations shown in Table 1 was refined by using an apparatus
shown in FIG. 1. For the refining operation, cold water at a
temperature of 5.degree. C. was used for the water to be flown
through the water-cooling jacket 3, and hot water at a temperature
of 70.degree. C. was used for the water to be flown through the hot
water coil 4, hot water spiral pipe 8, and the hot water pipe 7.
The temperature of the liquid phase L during solidification was
proceeded was controlled to fall in the range of
29.6.+-.0.5.degree. C. A rare earth magnet was used for the magnet
of the rotator 6, and the rotation speed of the liquid phase on the
initiation of solidification was controlled to be 100 rpm by
setting the revolution of the magnet to a constant rate of 500 rpm.
Gaseous nitrogen was introduced into the vessel 2 provided with a
lid, and the entire apparatus was operated inside a clean room of
Class 100 level (set at a temperature of 35.degree. C.)
[0096] A 150-kg portion of raw gallium material was charged inside
the stainless steel vessel 2 having an inner radius of 200 mm and a
height of 300 mm, made of a SUS304, and the process steps A to D
shown in FIG. 4 was repeated 7 times. The refining took 33 hours to
complete the operation. As shown in Table 1, 110 Kg of refined
gallium and 40 Kg of separated gallium were obtained as a result.
The separated gallium was collected inside the ladle 16 set inside
the vacuum vessel 15. The impurities were analyzed by using
GDMS.
1 TABLE 1 Weight Impurity (ppm) kg Al Si P Cl K Ca Cr Fe Ni Cu Raw
gallium material 150 0.02 0.25 <0.01 0.15 0.03 0.05 0.05 0.06
0.04 1.8 Refined gallium 110 <0.01 0.02 <0.01 0.02 <0.01
<0.01 <0.01 <0.01 <0.01 <0.01 Weight Impurity (ppm)
kg Zn Ge As In Sn Au Hg Pb Total Raw gallium material 150 4.8
<0.1 <0.01 1.4 18 <0.2 1.9 3.9 Refined gallium 110
<0.01 <0.1 <0.01 <0.01 <0.01 <0.2 <0.01
<0.01 0.04 Note) Impurities were analyzed using GDMS (glow
discharge mass spectrometer).
[0097] As shown in Table 1, a 7-nine high purity gallium was
prepared from a 4-nine raw gallium material at a yield of %.
Example 2
[0098] Refining operation was performed in the same manner as
described in Example 1, except for using a raw gallium material
differing in origin. The refining operation was repeated 7 times
and it took a duration of 24 hours to complete the operation. The
results are shown in Table 2. The impurities were analyzed by using
GDMS.
2 TABLE 2 Weight Impurity (ppm) kg Al Si P Cl K Ca Cr Fe Ni Cu Raw
gallium material 150 0.02 0.13 <0.01 0.12 0.05 0.08 0.05 0.12
0.06 2.5 Refined gallium 122 <0.01 0.01 <0.01 0.02 <0.01
<0.01 <0.01 <0.01 <0.01 <0.01 Weight Impurity (ppm)
kg Zn Ge As In Sn Au Hg Pb Total Raw gallium material 150 0.52
<0.1 <0.01 1.4 25 0.25 2.4 8.1 40.80 Refined gallium 122
<0.01 <0.1 <0.01 <0.01 <0.01 <0.2 <0.01
<0.01 0.03 Note) Impurities were analyzed using GDMS (glow
discharge mass spectrometer).
[0099] In this example again, as shown in Table 2, a 7-nine high
purity gallium was prepared from a 4-nine raw gallium material at a
yield of 81%.
Example 3
[0100] Refining operation was performed in the same manner as
described in Example 1, except for using a raw gallium material
(4N) having a chemical analysis data obtained by GDMS as shown in
Table 3. The refining operation was repeated 7 times. The GDMS
analytical results for the thus obtained refined gallium (7N) are
shown in the column of refined gallium of Table 3.
[0101] The refined gallium (7N) thus obtained was subjected to the
test defined in the present specification as "Test method for
impurity-concentrated Ga". More specifically, the test was
performed by using an apparatus for refining gallium comprising a
vessel having a cylindrical inner wall made of a 3 mm thick SUS304
steel sheet and provided with an inner wall plane coated with a
fluororesin (PFA: tetrafluoroethylene-perfluoroalkoxyethylene
copolymer) 0.3 mm in thickness, the vessel having an inner radius
of 60 mm and a height of 40 mm, a cooling zone attached to the
outer peripheral plane of the vessel, a suction pipe installed at
the central portion of the vessel, and a magnetic rotator provided
to the lower portion of the vessel. Then, the vessel of the
apparatus was filled with the refined gallium (7N) in liquid state
at a quantity as such that it amounts to 30 mm in height inside the
vessel while purging the space inside the vessel with gaseous
nitrogen. Subsequently, while applying a circular flow of 100.+-.10
rpm to the liquid Ga material by using a magnetic rotator, the
liquid Ga was maintained at a temperature of 29.6.+-.0.5.degree.
C., and a cooling water was passed through the cooling zone at a
temperature of 5.degree. C., thereby proceeding solidification of
the liquid at a solidification rate as such that the entire liquid
may solidify in 60.+-.5 minutes from the inner wall of the vessel
towards the central portion of the vessel. Thus, sampling of the
liquid phase (impurity-concentrated Ga) through the suction pipe
was performed when the radius of the liquid phase remaining at the
central portion of the vessel became 20 mm.
[0102] The thus sampled impurity-concentrated Ga was then subjected
to analysis using GDMS to obtain the content of each of the
impurity elements. As a result, the values given in the column of
impurity-concentrated Ga (analytical values obtained by GDMS) of
Table 3 were obtained. The DMS values (ppm by weight; referred to
hereinafter as "wt.ppm") for each of the impurities in the
impurity-concentrated Ga were reduced to values expressed in terms
of ppm by atomic (referred to hereinafter as "at.ppm") in
accordance with the conversion equation below, and the reduced
values are given in the column of impurity-concentrated Ga (reduced
values) of Table 3.
Concentration (at.ppm) of element n=Concentration (wt.ppm) of
element n.times.atomic weight of Ga (69.72)/atomic weight of
element n
[0103] As a result, the following values were obtained:
.SIGMA.An=0.628 at.ppm and .SIGMA.Bn=0.694 at.ppm. Thus,
.DELTA.C=.vertline.An-.SIGMA.Bn.- vertline.=0.065 at.ppm was
obtained.
3TABLE 3 (Example 3) Raw gallium Refined gallium Impurity- Impurity
material (7 times) concentrated Ga concentrated Ga Impurity GDMS
value GDMS value GDMS value Reduced value element (wt. ppm) (wt.
ppm) (wt. ppm) (at. ppm) B 0.02 <0.01 0.02 0.129 Group A Na 0.68
<0.01 0.03 0.091 .SIGMA.An = 0.628 Mg 0.9 <0.01 0.03 0.086 K
0.47 <0.01 0.03 0.053 Ca 0.35 <0.01 0.02 0.035 Cr 1.1
<0.01 0.01 0.013 Mn 1.2 <0.01 0.01 0.013 Fe 1.2 <0.01 0.01
0.012 Co 0.5 <0.01 0.01 0.012 Ni 1.2 <0.01 0.01 0.012 Cu 1.2
<0.01 0.01 0.011 Zn 3.5 <0.01 0.01 0.011 Mo <0.01 <0.01
0.01 0.007 Cd 1.2 <0.1 0.10 0.062 Au <0.2 <0.2 0.20 0.071
Hg 1.3 <0.01 0.01 0.003 Pb 2.5 <0.01 0.01 0.003 Bi 0.8
<0.01 0.01 0.003 F 0.01 <0.01 0.03 0.110 Group B Si 0.5
<0.01 0.05 0.124 .SIGMA.Bn = 0.694 S 0.2 <0.01 0.04 0.087 Cl
0.15 0.03 0.12 0.236 Ge <0.1 <0.1 0.10 0.096 Se 0.1 <0.01
0.02 0.018 Sn 20.0 <0.01 0.03 0.018 Te 0.05 <0.01 0.01 0.005
.DELTA.C = .vertline. .SIGMA.An - .SIGMA.Bn .vertline. = 0.065 at.
ppm
Example 4
[0104] By using the same raw gallium material as that used in
Example 3, a process similar to that described in Example 3 was
performed, except for repeating the refining operation for 5 times.
Thus was obtained a refined gallium with GDMS analytical results
given in the column of refined gallium in Table 4. Similar to the
refined gallium obtained in Example 3, the product thus obtained
had a purity of 7N in terms of GDMS value. Similarly, the refined
gallium (7N) was subjected to "Test method for
impurity-concentrated Ga". The thus sampled impurity-concentrated
Ga was then subjected to analysis using GDMS to obtain the content
of each of the impurity elements. As a result, the values given in
the column of impurity-concentrated Ga (GDMS value) of Table 4 were
obtained. The DMS values (wt.ppm) for each of the impurities in the
impurity-concentrated Ga were reduced to values (at.ppm) in a
manner similar to that of Example 3, and the reduced values are
given in the column of impurity-concentrated Ga (reduced values) of
Table 4.
[0105] As a result, the following values were obtained: .SIGMA.An
1.172 at.ppm and .SIGMA.Bn=0.672 at.ppm. Thus,
.DELTA.C=.vertline..SIGMA.An-.SI- GMA.Bn.vertline.=0.500 at.ppm was
obtained.
[0106] On comparing the results with those of Example 3, it can be
understood that, although refined gallium samples having the same
GDMS analytical values of 7N are used, the product of Example 4
obtained by repeating the refining operation for 5 times yields
higher values for .SIGMA.An and .SIGMA.Bn, and also for .DELTA.C,
as compared with those of the product of Example 3 obtained by
repeating the refining operation for 7 times.
4TABLE 4 (Example 4) Raw gallium Refined gallium Impurity-
Impurity- material (5 times) concentrated Ga concentrated Ga
Impurity GDMS value GDMS value GDMS value Reduced value element
(wt. ppm) (wt. ppm) (wt. ppm) (at. ppm) B 0.02 <0.01 0.02 0.129
Group A Na 0.68 <0.01 0.08 0.243 .SIGMA.An = 1.172 Mg 0.9
<0.01 0.10 0.287 K 0.47 <0.01 0.07 0.125 Ca 0.35 <0.01
0.04 0.070 Cr 1.1 <0.01 0.02 0.027 Mn 1.2 <0.01 0.02 0.025 Fe
1.2 <0.01 0.03 0.037 Co 0.5 <0.01 0.02 0.024 Ni 1.2 <0.01
0.02 0.024 Cu 1.2 <0.01 0.01 0.011 Zn 3.5 <0.01 0.02 0.021 Mo
<0.01 <0.01 0.01 0.007 Cd 1.2 <0.1 0.10 0.062 Au <0.2
<0.2 0.20 0.071 Hg 1.3 <0.01 0.01 0.003 Pb 2.5 <0.01 0.01
0.003 Bi 0.8 <0.01 0.01 0.003 F 0.01 <0.01 0.03 0.110 Group B
Si 0.5 <0.01 0.05 0.124 .SIGMA.Bn = 0.672 S 0.2 <0.01 0.04
0.087 Cl 0.15 0.03 0.10 0.197 Ge <0.1 <0.1 0.10 0.096 Se 0.1
<0.01 0.02 0.018 Sn 20.0 <0.01 0.06 0.035 Te 0.05 <0.01
0.01 0.005 .DELTA.C = .vertline. .SIGMA.An - .SIGMA.Bn .vertline. =
0.065 at. ppm
Comparative Example 1
[0107] A commercially available metallic gallium with a nominal
purity of 6N was analyzed by GDMS to obtain the analytical values
given in the column of commercially available gallium in Table 5.
The commercially available gallium (6N) was subjected to "Test
method for impurity-concentrated Ga" in a manner similar to that of
Example 3. The thus sampled impurity-concentrated Ga was then
subjected to analysis using GDMS to obtain the content of each of
the impurity elements. As a result, the values given in the column
of impurity-concentrated Ga (GDMS value) of Table 5 were obtained.
The DMS values (wt.ppm) for each of the impurities in the
impurity-concentrated Ga were reduced to values (at.ppm) in a
manner similar to that of Example 3, and the reduced values are
given in the column of impurity-concentrated Ga (reduced values) of
Table 5.
[0108] As a result, the following values were obtained:
.SIGMA.An=0.600 at.ppm and VBn=5.798 at.ppm. Thus,
.DELTA.C=.vertline..SIGMA.An-.SIGMA.Bn- .vertline.=5.198 at.ppm was
obtained.
Comparative Example 2
[0109] The same test as that of Comparative Example 1 was
performed, except for using a different commercially available
metallic gallium having a nominal purity of 6N. The results are
given in Table 6 in a manner similar to those of Comparative
Example 1, and the following values were obtained: .SIGMA.An=5.997
at.ppm, .SIGMA.Bn=0.630 at.ppm, and
.DELTA.C=.vertline..SIGMA.An-.SIGMA.Bn.vertline.=5.367 at.ppm.
5TABLE 5 (Comparative Example 1) Commercially available gallium
Impurity- Impurity- 6N concentrated Ga concentrated Ga Impurity
GDMS value GDMS value Reduced value element (wt.ppm) (wt.ppm)
(at.ppm) Group A .SIGMA.An = 0.600 B <0.01 0.02 0.129 Na
<0.01 0.03 0.091 Mg <0.01 0.02 0.577 K <0.01 0.03 0.053 Ca
<0.01 0.02 0.035 Cr <0.01 0.01 0.013 Mn <0.01 0.01 0.013
Fe <0.01 0.01 0.012 Co <0.01 0.01 0.012 Ni <0.01 0.01
0.012 Cu <0.01 0.01 0.011 Zn <0.01 0.01 0.011 Mo <0.01
0.01 0.007 Cd <0.1 0.10 0.062 Au <0.2 0.20 0.071 Hg <0.01
0.01 0.003 Pb <0.01 0.01 0.003 Bi <0.01 0.01 0.003 Group B
.SIGMA.Bn = 5.798 F <0.01 0.03 0.110 Si 0.2 0.8 1.986 S 0.2 0.8
1.739 Cl 0.15 0.60 1.180 Ge <0.1 0.10 0.096 Se <0.01 0.02
0.018 Sn 0.1 1.0 0.587 Te 0.02 0.15 0.082 .DELTA.C = .vertline.
.SIGMA.An-.SIGMA.Bn .vertline. = 5.198 at.ppm
[0110]
6TABLE 6 (Comparative Example 2) Commercially available gallium
Impurity- Impurity- 6N concentrated Ga concentrated Ga Impurity
GDMS value GDMS value Reduced value element (wt.ppm) (wt.ppm)
(at.ppm) Group A .SIGMA.An = 5.998 B <0.01 0.02 0.129 Na 0.1
0.31 0.940 Mg 0.2 0.60 1.721 K 0.1 0.27 0.481 Ca 0.1 0.42 0.731 Cr
0.1 0.45 0.603 Mn <0.01 0.01 0.013 Fe 0.05 0.51 0.637 Co
<0.01 0.01 0.012 Ni 0.02 0.15 0.178 Cu 0.01 0.07 0.077 Zn 0.03
0.20 0.213 Mo <0.01 0.01 0.007 Cd <0.1 0.10 0.062 Au <0.2
0.20 0.071 Hg 0.02 0.10 0.035 Pb 0.02 0.18 0.061 Bi 0.01 0.08 0.027
Group B .SIGMA.Bn = 0.630 F <0.01 0.02 0.073 Si <0.01 0.05
0.124 S <0.01 0.04 0.087 Cl 0.03 0.10 0.197 Ge <0.1 0.10
0.096 Se <0.01 0.02 0.018 Sn <0.01 0.05 0.029 Te <0.01
0.01 0.005 .DELTA.C = .vertline. .SIGMA.An-.SIGMA.Bn .vertline. =
5.367 at.ppm
Example 5
[0111] Single crystals of GaAs were prepared in accordance with LEC
(Liquid Encapsulated Czocralski) method described in Example 1 of
Japanese patent application Heill-098528 filed by the present
applicants (assignors), by using each of the refined gallium
obtained in Examples 3 and 4 and the commercially available
metallic gallium shown in Comparative Examples 1 and 2 for the raw
Ga material for preparing GaAs single crystal, and using
commercially available high purity arsenic having a purity of 7N
for the raw As material. The commercially available high purity
arsenic (7N) was sampled and analyzed by using GDMS to find that
the concentrations of all of the elements belonging to Group A and
Group B were not higher than 0.01 ppm, the limit of quantitative
analysis using GDMS.
[0112] The process for preparing GaAs single crystal by LEC method
in accordance with the disclosure of Japanese patent application
Hei11-098528 is briefly described below. In a PBN (Pyrolytic Boron
Nitride) crucible were placed 5,000 g of a raw Ga material and
5,500 g of a raw As material, and after charging further thereon a
liquid sealant (B203) with water content of 200 ppm by weight at a
sufficient amount, the entire crucible was set inside a pressure
vessel. Then, after introducing a gas of pure Ar pressurized to 37
kgf/cm.sup.2 (.congruent.3.6 MPa) into the pressure vessel, the PBN
crucible was heated by using a heater provided inside the pressure
vessel. In this manner, the raw Ga material was allowed to react
with the raw As material inside the crucible to synthetically
obtain polycrystalline GaAs.
[0113] The polycrystalline GaAs thus obtained inside the crucible
was further heated to obtain a melt of GaAs. A pressure of 65
kgf/cm.sup.2 (.congruent.6.4 MPa) was attained inside the pressure
vessel at this instance. The pressure was then reduced to 4
kgf/cm.sup.2 (.congruent.0.4 MPa), and the crucible was allowed to
stand for 1 hour to degas the B.sub.20.sub.3 layer. Then, pure
gaseous nitrogen was introduced into the vessel to increase the
pressure to 24 kgf/cm.sup.2 (.congruent.2.4 MPa), until an
Ar/N.sub.2 mixing ratio of 1/6 was attained, and was allowed to
stand for 30 minutes. The pressure of the mixed gas was then
reduced to 4 kgf/cm.sup.2 (.apprxeq.0.4 MPa), and was further left
for 30 minutes. Pure gaseous nitrogen was introduced again into the
vessel to increase the pressure to 24 kgf/cm.sup.2 (.apprxeq.2.4
MPa), until an Ar/N.sub.2 mixing ratio of 1/36 was attained, and
was allowed to stand for 30 minutes. Presumably, by thus
controlling the gas pressure and the ratio of mixing gases, boron
mixed into the GaAs melt can be reacted with pure gaseous nitrogen
through B.sub.2O.sub.3, and is discharged out of the system in the
form of boron nitride.
[0114] At this stage, a seed crystal was introduced downward into
the crucible while rotating it at a rate of about 5 rpm, and was
brought into contact with the surface of the GaAs melt. In this
case, the crucible was also rotated at a rate of about 25 rpm, and
upon completion of seeding, the seed crystal was pulled up to form
a cone portion. Subsequently, a straight cylindrical body portion
having a uniform crystal diameter was formed at a rate of 8 mm/hr
while rotating it at a constant rate of 5 rpm, and a tail portion
was formed to grow a GaAs single crystal. After being pulled up,
the product was cooled to obtain a 4-inch diameter ingot of GaAs
single crystal.
[0115] By employing the process above, GaAs single crystals were
prepared while maintaining the same conditions, except for changing
the raw Ga materials. Then, the resulting GaAs single crystals were
subjected to the measurement of resistivity, and the results as
follows were obtained depending on the difference in raw Ga
materials.
7 Resistivity of GaAs single crystal Refined Ga of Example 3 5
.times. 10.sup.7 .OMEGA. .multidot. cm Refined Ga of Example 4 2
.times. 10.sup.7 .OMEGA. .multidot. cm Commercially available Ga of
Comparative Example 1 5 .times. 10.sup.4 .OMEGA. .multidot. cm
Commercially available Ga of Comparative Example 2 1 .times.
10.sup.5 .OMEGA. .multidot. cm
[0116] From the results obtained above, it can be understood that
although refined gallium samples having the same purity 7N were
used, the product of Example 3 having a value of 0.065 at.ppm for
.DELTA.C=.vertline..SIGMA- .An-.SIGMA.Bn.vertline. as defined in
"Test method for impurity-concentrated Ga" according to the present
specification enabled a GaAs single crystal having a higher
resistivity as compared with that having a .DELTA.C value of 0.500
at.ppm obtained in Example 4. Furthermore, even though a purity of
6N was achieved, the commercially available metallic gallium (used
in Comparative Examples 1 and 2) having a .DELTA.C value exceeding
5 at.ppm failed to produce GaAs single crystals having a
resistivity of 1.times.10.sup.7 .OMEGA..multidot.cm or higher.
Example 6
[0117] GaP crystals were prepared in accordance with a known SSD
(Synthesis Solute Diffusion) process by using each of the refined
gallium obtained in Examples 3 and 4 and the commercially available
metallic gallium shown in Comparative Examples 1 and 2 for the raw
Ga material for preparing GaP crystal, and using commercially
available high purity phosphorus having a purity of 7N for the raw
P material. The commercially available high purity phosphorus (7N)
was sampled and analyzed by using GDMS to find that the
concentrations for all of the elements belonging to Group A and
Group B were not higher than 0.01 ppm, the limit of quantitative
analysis using GDMS.
[0118] In performing the SSD process, 140 g of raw Ga material was
charged into a 30-mm diameter quartz crucible provided with a
support rod, and the crucible charged with the raw Ga material was
vacuum sealed inside a quartz ampoule together with 70 g of a raw
phosphorus material placed at the bottom of the same ampoule. Then,
while heating the raw phosphorus material provided at the bottom of
the ampoule to a temperature of 430.degree. C., the inside of the
crucible was heated to 900.degree. C. and the inner pressure of the
ampoule was maintained at 1 atm. In this manner, GaP crystals were
allowed to precipitate on the bottom portion of the raw Ga material
(melt) provided inside the crucible. Thus, sampling was made on the
GaP crystal grown by this method at the stages in which the GaP
crystal grew to a length of 10 mm and 50 mm (30 days after
initiation of the synthesis), and measurements were made thereon to
obtain the carrier density and the resistivity. As a result, the
following values were obtained.
8 (10-mm length) (50-mm length) Carrier density of GaP crystals
(/cm.sup.3) Refined Ga of Example 3 1.2 .times. 10.sup.15 3.4
.times. 10.sup.15 Refined Ga of Example 4 1.8 .times. 10.sup.15 4.8
.times. 10.sup.15 Commercially available Ga of 1.3 .times.
10.sup.16 3.1 .times. 10.sup.16 Comparative Example 1 Commercially
available Ga of 1.1 .times. 10.sup.16 2.8 .times. 10.sup.16
Comparative Example 2 Resistivity of GaP crystals (.OMEGA.
.multidot. cm) Refined Ga of Example 3 40 12 Refined Ga of Example
4 20 8 Commercially available Ga of 2 1.3 Comparative Example 1
Commercially available Ga of 4 1.5 Comparative Example 2
[0119] From the results above, it can be understood that although
refined gallium samples having the same purity of 7N were used, the
product of Example 3 enabled a GaP crystal having a lower carrier
density and a higher resistivity as compared with that obtained in
Example 4. Furthermore, even though a purity of 6N was achieved,
the commercially available metallic gallium (used in Comparative
Examples 1 and 2) having a value of
.DELTA.C=.vertline..SIGMA.An-.SIGMA.Bn.vertline. as defined in
"Test method for impurity-concentrated Ga" according to the present
specification exceeding 5 at.ppm failed to produce GaP crystals
having a carrier density of 1.times.10.sup.16/cm.sup.3 or lower and
a resistivity of 5 .OMEGA..multidot.cm or higher.
[0120] While the invention has been described in detail by making
reference to specific examples, it should be understood that
various changes and modifications can be made without departing
from the scope and the spirit of the present invention.
* * * * *