Method of and apparatus for preparing single crystal

A method and an apparatus for pulling a compound single crystal from a raw material molten solution is constructed to cause the solution to flow into a second crucible provided in a first crucible containing the raw material molten solution which is continuously synthesized from a plurality of raw materials, through a communicating hole formed in the bottom portion of the second crucible. The single crystal is pulled while the raw material molten solution is continuously synthesized from the plurality of raw materials, whereby it is possible to pull a long single compound crystal through a single pulling step from the raw material molten solution which is contained in the second crucible. An excellent state of a solid-liquid interface is maintained to obtain a quality single crystal.

FIELD OF THE INVENTION 
The present invention relates to a method of and an apparatus for preparing 
a single crystal of a group III and V compound semiconductor such as GaAs, 
InP or the like, a group II and VI compound semiconductor such as CdTe, 
ZnSe or the like, an oxide containing a volatile constituent, or the like. 
BACKGROUND OF THE INVENTION 
A compound semiconductor single crystal is generally prepared by a 
horizontal Bridgeman method (HB method) or a liquid encapsulated 
Czochralski method (LEC method). 
When a single crystal is prepared according to the LEC method, such a 
single crystal is pulled by a rotary pulling shaft from a raw material 
molten solution having a surface covered with a liquid sealant. An 
apparatus for such pulling comprises an airtight housing, a rotatable, 
vertically movable upper shaft having a lower end on which a seed crystal 
is mounted, a crucible for containing a raw material, a lower shaft for 
supporting the crucible, an evacuator, an inert gas introducing system, a 
heater, and the like. In such apparatus, the upper and lower shafts are 
rotated for providing uniform temperature conditions in the direction of 
rotation. In addition, the raw material molten solution is covered with a 
liquid sealant and supplied with a high pressure inert gas, in order to 
suppress decomposition and evaporation of a raw material element having a 
high vapor pressure near its melting point. 
When a single crystal of a compound semiconductor or the like is prepared 
by a pulling method, generally employed is a method of filling up a 
crucible with a raw material polycrystal or a raw material single element 
or impurity only once at the start and supplying no raw material in an 
intermediate stage. In such a method, the size of the pulled single 
crystal is inevitably restricted by the amount of the raw material which 
is filled into the crucible at the start. In an apparatus for carrying out 
such a method, the raw material molten solution contained in the crucible 
is reduced as the pulling proceeds. When the quantity of the molten 
solution is less than a prescribed value, it is no longer possible to pull 
a single crystal. If a long single crystal can be grown through single 
pulling, it is possible to reduce loss at each end of the crystal, while 
it is also possible to reduce time loss in a preparation step for the 
crystal growth, an extraction step after the growth and the like. Thus, 
the cost can be remarkably reduced as compared with a case of growing 
short crystals repeatedly. 
In order to pull a longer single crystal through a single growth step, the 
crucible must be continuously supplied with the raw material. As to a 
method of pulling a silicon single crystal, methods enabling a continuous 
supply of raw materials have been proposed in British Patent No. 755,422 
(Aug. 22, 1956), Japanese Patent Laying-Open No. 59-79000 (May 8, 1984), 
U.S. Pat. No. 4,659,421 (Apr. 21, 1987), U.S. Pat. No. 2,977,258 (Mar. 28, 
1961) and U.S. Pat. No. 4,650,540 (Mar. 17, 1987), for example. 
Each of these methods utilizes a wide crucible to pull a single crystal 
from a certain pulling region of the crucible while dissolving a solid raw 
material in another region of the crucible for supplying the same into the 
pulling region. When the supply quantity of the solid raw material is 
substantially equal to the pulling quantity, the crucible is regularly 
provided therein with a constant quantity of the raw material molten 
solution. Thus, it is possible to pull a silicon single crystal until the 
solid raw material is used up or the pulling reaches the geometrical limit 
of the upper shaft in the apparatus. Since it is possible to grow a 
sufficiently long ingot of a silicon single crystal, the cost for 
preparing the single crystal can be reduced. 
Each of these apparatuses is adapted to pull a single crystal from and 
supply a solid raw material into the same crucible. The crucible is formed 
to have a wide surface area and a thin bottom. While the crucible is 
rotatable about its central axis, the center of the pulling shaft is not 
aligned with that of the lower shaft supporting the crucible. However, the 
temperature of the raw material molten solution is uniformalized by 
stirring since the crucible is being rotated. In the case of a silicon 
single crystal, it is possible to relatively easily grow a long single 
crystal by supplying a raw material simultaneously with pulling, as 
described above. In practice, a silicon single crystal for a wafer of 152 
mmU or 203 mmU having a length of at least 1m has been pulled by such a 
method. 
In the case of a group III and V compound semiconductor single crystal, 
however, it is difficult to apply the aforementioned method In general, 
the surface of a raw material molten solution is covered with a liquid 
sealant in order to suppress dissociation of a group V element having a 
high vapor pressure In order to supply a solid raw material as described 
above, it is necessary to prevent evaporation of the group V element from 
this solid. Therefore, the solid raw material to be supplied must be 
covered with a liquid sealant, for example. 
*1* Japanese Patent Laying-Open No. 61-158897 (application filed on Dec. 
29, 1984) and *2* Japanese Patent Laying-Open No. 60-137891 (application 
filed on Dec. 24, 1983) by the applicants discloses methods or apparatuses 
for preparing single crystals of groups III and V compound semiconductors. 
FIG. 4 schematically illustrates a concrete example of apparatuses 
proposed in the above specifications A rotatable crucible 60 contains a 
GaAs molten solution 61 and B.sub.2 O.sub.3 62 serving as a liquid 
sealant. This crucible 60 is rotatably supported by a lower shaft. A GaAs 
single crystal 63 is pulled from the GaAs molten solution 61. A cylinder 
64 is provided in a peripheral edge portion of the crucible 60. This 
cylinder 64 is dipped into the molten solution 61, while a polycrystalline 
rod 66 of GaAs is inserted therein. The polycrystalline rod 66 is entirely 
covered with a liquid sealant 65 which is filled into the cylinder 64. 
Further, an auxiliary heater 68 is provided around the cylinder 64, in 
order to prevent solidification of the liquid sealant 65. In this 
apparatus, supply of the raw material molten solution is carried out by 
gradually moving down the polycrystalline rod 66 and dissolving the same 
into the molten solution 61. The aforementioned specification *1* shows 
that a long non-doped GaAs single crystal is pulled while a continuous 
supply of a raw material takes place. The specification *2* shows that an 
increase in the impurity concentration was prevented by the supply of a 
non-doped polycrystal in order to uniformly dope an impurity having a 
segregation coefficient of not more than 1. 
Further, *3* Japanese Patent Laying-Open No. 61-158896 (application filed 
on Dec. 29, 1984) discloses a method and an apparatus in which Ga and As 
are added to a raw material molten solution from different paths in place 
of a solid raw material. FIG. 5 is a typical diagram showing a concrete 
example of an apparatus described in this specification. A container 71 
containing As is mounted under a crucible 70. A small hole 72 is formed in 
the bottom portion of the crucible 70, in order to introduce As vapor from 
the container 71. As is supplied into a raw material molten solution 73 
through the small hole 72. A Ga container 74 is provided above the 
crucible 70. In the crucible 70, the supplied As and Ga react with each 
other to provide a GaAs molten solution. A single crystal is directly 
pulled from the raw material molten solution synthesized in such a manner. 
In each of the methods and apparatuses shown in the above specifications 
*1* and *2*, it is necessary to perform a prior synthesizing of a 
polycrystal serving as a raw material to be supplied into the melt. Since 
the raw material to be supplied is solid, the following disadvantages take 
place: 
(1) The cost for synthesizing a group III and V compound semiconductor raw 
material is increased. 
(2) During the step of synthesizing the supplied polycrystalline raw 
material may entrap impurities Thus, the raw material molten solution may 
be contaminated with such impurities contained in the raw material being 
supplied. 
(3) In the apparatus shown in FIG. 4, the raw material solid to be supplied 
must be sealed in the cylinder with the liquid sealant. Due to geometrical 
limitations of such a sealing mechanism, the solid to be supplied may not 
be much increased in size. Since the supply quantity of the raw material 
is thus limited, the length of the pulled single crystal is also limited. 
(4) In the pulling apparatus, a supply mechanism, an auxiliary heater and 
the like must be arranged in a narrow space above the crucible, and hence 
the apparatus is complicated in structure. 
In the method shown in specification *3*, no expense is required for a 
prior synthesizing of a polycrystalline raw material since the supply raw 
material is directly synthesized in the apparatus. However, the method has 
the following problems: 
(1) Heat generated in the synthesis of the raw material molten solution in 
the crucible disturbs the temperature environment in the molten solution. 
This exerts a bad influence on the state of the solid-liquid interface. 
(2) It is difficult to form a small hole which allows passage of As vapor 
with no leakage of the raw material molten solution from the bottom 
portion of the crucible. 
(3) While the synthesis quantity of the raw material molten solution is 
controlled by the supply quantity of Ga, it is difficult to 
stoichiometrically control As. 
OBJECTS OF THE INVENTION 
An object of the present invention is to solve the aforementioned problems 
and to provide a method and an apparatus which can prepare a single 
crystal that is longer than a conventionally produced single crystal and 
the production shall take place in a single step at a cost lower than that 
for the conventionally produced single crystal. 
Another object of the present invention is to provide a method and an 
apparatus which can pull a long compound semiconductor single crystal 
while preventing the aforementioned contamination by impurities. 
Still another object of the present invention is to provide a method which 
can pull a particularly long compound semiconductor single crystal with an 
apparatus having a simpler structure. 
A further object of the present invention is to provide a method and an 
apparatus which can pull a particularly long compound semiconductor single 
crystal from a raw material solution that is being maintained in a 
stoichiometric composition with an excellent state of a solid-liquid 
interface. 
SUMMARY OF THE INVENTION 
A method of preparing a single crystal according to the present invention 
is characterized by preparing a single crystal from a raw material molten 
solution, which is synthesized from a plurality of raw materials, by a 
pulling method wherein the single crystal is pulled from the raw material 
molten solution flowing into a second crucible provided in a first 
crucible containing the raw material molten solution that is being 
continuously synthesized from the plurality of raw materials through a 
communicating hole formed in the second crucible. 
According to this method, the raw material molten solution is continuously 
synthesized from a plurality of raw materials. The synthesized raw 
material molten solution is contained in the first crucible. The raw 
material molten solution contained in the first crucible flows into the 
second crucible through the communicating hole formed in the second 
crucible. A single crystal is pulled from the raw material molten solution 
contained in the second crucible. The quantity of the raw material molten 
solution contained in the first crucible is reduced while pulling the 
single crystal and while a newly synthesized raw material molten solution 
is supplied into the first crucible. Therefore, the quantities of the raw 
material molten solutions contained in the first and second crucibles are 
maintained in constant ranges. Since the raw material is continuously 
supplied in such a manner, it is possible to pull a long single crystal 
through a single pulling step. Thus, the cost for preparation of the 
single crystal is reduced. In addition, it is not necessary to synthesize 
a polycrystalline raw material to be supplied, dissimilarly to the 
conventional case, whereby the costs for preparing such a supply raw 
material is reduced. Further, there is no possibility that the raw 
material molten solution is contaminated by impurities contained in the 
supplied raw material, dissimilarly to the conventional case. 
Disadvantages such as vibration of the molten solution, a non-uniform 
temperature distribution of the molten solution and the like of the raw 
material molten solution contained in the first crucible are avoided by 
and in the second crucible. Further, the state of the solid-liquid 
interface is excellent when the single crystal is pulled from the raw 
material molten solution contained in the second crucible, as compared 
with the first crucible. The crystal is thus pulled from the second 
crucible, whereby a bad influence such as generation of a twin or a 
dislocation on the crystal growth is suppressed. In addition, it is 
possible to rotate the second crucible in order to improve the state of 
the raw material molten solution contained in the second crucible. As 
described above, it is possible to pull a long single crystal having a 
high purity and a low dislocation density. 
The method according to the present invention is particularly suitable for 
pulling a compound single crystal containing a high dissociation pressure 
component, such as a group III and V compound seimconductor or the like. 
When the raw material contains a high dissociation pressure component, the 
single crystal is preferably pulled in a closed space. Further, liquid 
sealants may be provided on the raw material molten solutions which are 
contained in the first and second crucibles. 
The raw material molten solution may be synthesized in a third crucible, by 
supplying a plurality of raw materials into the third crucible 
respectively. In this case, it is preferable to continuously supply the 
raw material molten solution from the third crucible into the first 
crucible Particularly when the raw materials contain high dissociation 
pressure components, synthesis of the raw material molten solution in the 
third crucible is preferably carried out in a closed space which is not in 
contact with an atmosphere, for pulling a single crystal. When the raw 
materials contain high dissociation pressure components, a liquid sealant 
should be provided on the liquid which is contained in the third crucible. 
On the other hand, the raw material molten solution may alternatively be 
synthesized in the first crucible by supplying a plurality of raw 
materials into the first crucible respectively. In this case, it is 
preferable to rotate the first crucible. Further, synthesis of the raw 
material molten solution in the first crucible is preferably carried out 
in a closed space which is in common with an atmosphere for pulling a 
single crystal. 
A single crystal preparation apparatus according to the present invention 
for practicing the present method, comprises a plurality of raw material 
containers for containing a plurality of raw materials respectively, a 
mixing crucible for mixing the raw materials therein for synthesizing a 
raw material molten solution, a piping system for feeding the raw 
materials from the raw material containers to the mixing crucible, a 
storage crucible for storing the synthesized raw material molten solution 
while pulling a single crystal, an inner crucible provided in the storage 
crucible and having a communicating hole in its bottom portion for 
introducing the raw material molten solution into the inner crucible, a 
heater provided around the storage crucible controlling the temperature of 
the raw material molten solution contained in the storage crucible, and a 
rotatable and/or vertically movable crystal pulling shaft for pulling a 
single crystal from the raw material molten solution flowing into the 
inner crucible. 
The apparatus according to the present invention may comprise an airtight 
housing for pulling the single crystal under an atmosphere closed off from 
the atmospheric air. 
The apparatus according to the present invention may further comprise 
heaters for heating the raw materials in the respective raw material 
containers. 
The apparatus according to the present invention may further comprise 
rotation means for rotating the aforementioned inner crucible about a 
rotary shaft whose center is substantially in conformity to that of the 
crystal pulling shaft. 
The piping system according to the present invention may further comprise a 
flow rate control mechanism for controlling the supply quantity of the raw 
materials. 
The storage crucible according to the present invention may be identical to 
the mixing crucible. In other words, the storage crucible can also serve 
as the mixing crucible in the apparatus according to the present 
invention. In this case, the storage crucible or mixing crucible is 
preferably rotatably supported by a lower shaft. 
On the other hand, the storage crucible may be independent of the mixing 
crucible. In this case, the storage crucible and the mixing crucible may 
be connected with each other by a pipe. A heater may be provided around 
the mixing crucible. In addition, the mixing crucible may be covered with 
an airtight housing, in order to carry out synthesis of the raw material 
molten solution in the mixing crucible in a closed space which is closed 
off against atmospheric air for pulling a single crystal. A heater may be 
provided around this airtight housing.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE 
OF THE INVENTION 
A first embodiment according to the present invention is now described. 
Referring to FIG. 1, a pulling furnace 1 comprises a housing from which 
vapor contained in its interior can be withdrawn by a vacuum pump. The 
housing may be filled with a gas at a high pressure. A crucible 2 for 
storing a raw material molten solution, a closed housing 3, a container 19 
for containing a group V raw material, a pulling shaft 5 and the like are 
provided in the pulling furnace 1. The crucible 2 contains a group III and 
V raw material molten solution 6. The molten solution 6 is heated by a 
main heater 7 positioned around the periphery of the crucible 2. The 
surface of the raw material molten solution 6 is covered with a liquid 
sealant 8. The pulling shaft 5 has a double structure including an outer 
shaft 9 and an inner shaft 10 having a common axis of rotation. Both 
shafts are independently rotatable and vertically movable. A seed crystal 
1 is mounted on the lower end of the inner shaft 10. A single crystal 12 
is pulled by the inner shaft 10. 
Similarly to a general LEC method, an inert gas 13 under a high pressure is 
filled into the interior of the pulling furnace 1. The high pressure of 
the inert gas 13 is applied to the liquid sealant 8 whereby dissociation 
of the group V element from the surface of the raw material molten 
solution 6 is inhibited. On the other hand, the crucible 2, which must be 
rotated in a conventional pulling method, is fixed according to the 
invention. Since the temperature distribution from the center toward the 
outer periphery in the raw material molten solution 6 is ununiformalized 
in this state, an inner crucible 14 is dipped into the raw material molten 
solution 6 in the crucible 2. The inner hole 15 in its center. This inner 
crucible 14 is coupled to the outer shaft 9 by a suspending jig 4, whereby 
it is possible to rotate the inner crucible 14 by rotating the outer shaft 
9. Since the raw material molten solution 6 is rotated by the rotation of 
the inner crucible 14, it is possible to stably attain a substantially 
rotation-symmetrical temperature distribution in the raw material molten 
solution. 
A mixing crucible 16 is provided in the closed housing 3. The raw materials 
are mixed with each other in the crucible 16, for synthesizing the raw 
material molten solution. A heater 17 is positioned around the outer 
periphery of the closed housing 3. Another heater 17' is positioned around 
the crucible 16 in the closed housing 3. A container 18 holding a group 
III raw material (Ga, In or the like) is positioned outside the pulling 
furnace 1. Another container 19 holding a group V raw material (P, As, Sb 
or the like) is provided inside the pulling furnace 1. The group III raw 
material contained in liquid form in the container 18 is supplied into the 
crucible 16 through a pipe 20. The group V raw material contained as a gas 
or liquid in the container 19 is supplied into the crucible 16 through 
another pipe 21. Sealing members 22 and 23 containing liquid sealants in 
dish-type containers having holes are provided in portions around the 
pipes 20 and 21 passing through the upper wall of the closed housing 3. 
The sealing members 22 and 23 maintain sealed states while allowing 
passage of the pipes 20 and 21 respectively. Heaters 24 for heating the 
liquid sealants are provided in the vicinity of the sealing members 22 and 
23. 
A heater 25 for heating the raw material as needed is provided in the 
vicinity of the container 18 for the group III raw material. Further, a 
flow control mechanism for example a flow control valve 26 is provided in 
the pipe 20 close to an exhaust port of the container 18, so that it is 
possible to freely adjust the supply quantity. 
A heater 27 for heating the group V raw material is provided in the 
vicinity of the container 19. The group V raw material (P or As, for 
example) which is contained in the container 19 in a solid state is 
sublimated by heating, whereby the resulting group V gas raw material 
enters the crucible 16 through the pipe 21. Since the vapor pressure of 
the group V raw material is determined by the heating temperature, it is 
possible in this case to adjust the supply quantity of the raw material by 
the heater. 
Although the container 19 for the group V raw material is provided in the 
interior of the pulling furnace 1 in the apparatus shown in FIG. 1, the 
container 19 may alternatively be positioned outside of the furnace 1. On 
the other hand, the container for the group III raw material, which is 
shown outside of the furnace 1, may alternatively be positioned in the 
container. The gas or liquid of the group V raw material and the liquid of 
the group III raw material which are mixed in the crucible 16, react with 
each other at an appropriate temperature, to form a group III and V 
compound. This compound is held in the crucible 16 in a state of a molten 
solution 28. The molten solution 28 enters the crucible 2 at the bottom 
through a communicating tube 29 which connects the crucible 2 with the 
crucible 16. Thus, the newly synthesized raw material is supplied to the 
crucible 2 through the communicating tube 29. A heater 30 is also provided 
around the communicating tube 29. 
The relationship between the vertical positions of the raw material molten 
solutions which are contained in the crucibles 2 and 16 respectively, is 
determined on the basis of pressure balance conditions. It is possible to 
supply the crucible 2 with a constant quantity of the raw material molten 
solution by supplying the crucible 16 with proper quantities of the group 
III raw material and the group V raw material. When the single crystal 12 
is pulled, it is possible to maintain the vertical position of the liquid 
surface in the crucible 2 constant by equalizing the supply quantity of 
the raw material molten solution to the consumption for pulling. In this 
case, the following equality holds assuming that dS/dt represents single 
crystal pulling weight in a unit time and dQ/dt represents the weight of 
the supplied raw material: 
##EQU1## 
At this time, the vertical elevations of the liquid surfaces in the 
crucibles 2 and 16 remain unchanged. 
The relationship between the vertical elevations of the liquid surfaces is 
provided as follows: Suppose that H represents the vertical elevation of 
the surface of the raw material molten solution 6 contained in the 
crucible 2 and K represents the vertical elevation of the surface of the 
molten solution 28 in the crucible 16 relative to the bottom of the 
crucible 2. Suppose that d represents the thickness of the liquid sealant 
8 in the crucible 2. Suppose that .rho..sub.0 represents the density of 
the raw material molten solution, and .rho..sub.1 represents the partial 
pressure of the inert gas (N.sub.2, Ar, Ne or the like) contained in the 
internal space of the pulling furnace 1, and Q.sub.0 represents the gas 
partial pressure of the group V element. P.sub.0 is further greater than 
Q.sub.0. Suppose that P.sub.1 represents the partial pressure of the inert 
gas contained in the internal space of the closed housing 3, and Q.sub.1 
represents the gas partial pressure of the group V element. Suppose that 
.DELTA. represents the difference between the internal and external 
pressures in the sealing members 22 and 23. Suppose that .DELTA.&gt;0 when 
the internal pressure of the closed housing 3 is high. Under the 
aforementioned conditions, a pressure balance is provided by the following 
equation: 
EQU P.sub.0 +Q.sub.0 +.rho..sub.1 dg+.rho..sub.0 Hg=P.sub.1 +Q.sub.1 
+.rho..sub.0 Kg-.DELTA. (2) 
In this equation, g represents gravitational acceleration. .DELTA., which 
is restricted by the thickness of the liquid sealant and the diameters of 
the holes of the sealing portions etc., is not necessarily zero. If the 
pressures of the gases are identical and .DELTA.=0, the following equation 
holds: 
EQU .rho..sub.1 d=.rho..sub.0 (K-H) (3) 
The thickness d of the liquid sealant is constant since the same is 
determined by the quantity introduced in the crucible 2 at the start. If 
the vertical elevation K of the liquid surface of the raw material molten 
solution 28 contained in the crucible 16 is constant, the vertical 
elevation H of the liquid surface in the crucible 2 is constant. The 
condition that K and H are constant is the optimum mode for carrying out 
the present invention. 
On the other hand, a mode of changing K is also possible. K and H may be 
constant when a non-doped single crystal is pulled, while it is better to 
reduce K at a constant rate when an impurity is doped. When a single 
crystal which is doped with an impurity having a segregation coefficient k 
of not more than 1 is pulled, the relation between the supply quantity and 
the pulling quantity may be set in accordance with the following equation 
in order to make the impurity concentration constant: 
##EQU2## 
The liquid surface is lowered little by little since the supply quantity 
is less than the pulling quantity. However, this change is slight when k 
is small. In this case it is also possible to pull a long single crystal. 
Pulling of a non-doped GaAs single crystal using the aforementioned 
apparatus will now be described. The crucible 2 was formed by a PBN 
crucible having a diameter of 152 mm. The closed housing 3 was formed by a 
housing of carbon having inner and outer surfaces were coated with PBN. 
The crucible 2 was connected with the crucible 16 by a communicating tube 
which was made of PBN. 
The container 19 for containing an As raw material was made of carbon and 
its inner surface was coated with PBN. The container 18 for containing a 
Ga raw material, which was provided outside of the pulling furnace 1, was 
made of carbon, and its inner surface was coated with PBN. The sealing 
members 22 and 23 contained proper quantities of liquid sealants (B.sub.2 
O.sub.3) respectively. 
The inner crucible 14 was made of carbon and its surface was coated with 
PBN. 
An operation for pulling a crystal will now be described. 5 kg of a GaAs 
polycrystal and 700 g of B.sub.2 O.sub.3 were contained in the crucible 2. 
At this time, the polycrystal was so positioned that part of the GaAs 
polycrystal blocked the communicating tube 29. 
The container 18 contained 5 kg of Ga. The container 19 contained 5.4 kg of 
As. The mole ratio of Ga to As, which were raw materials to be supplied, 
was 1:1.005. This ratio was in anticipation of loss of As resulting from 
dissociation. 
A seed crystal 11 was mounted on the lower end of the inner shaft 10 of the 
pulling shaft 5. The inner crucible 14 was suspended from the outer shaft 
9 through the suspending jig 4. 
The gas contained in the pulling furnace 1 was discharged by a vacuum pump 
(not shown), and gaseous nitrogen was filled into the furnace at a 
prescribed pressure level. The closed housing 3 was entirely heated by the 
heater 17 and so controlled that a lowest temperature portion including 
the interior of the housing 3 was at 617.degree. C. This is such a 
temperature that the vapor pressure of As is about 1 atm. The lowest 
temperature was set at 617.degree. C., since the vapor pressure of As is 
determined by the lowest temperature in a closed space. 
Then B.sub.2 O.sub.3 and GaAs contained in the crucible 2 were melted by 
the main heater 7. The raw material molten solution of GaAs was covered 
with B.sub.2 O.sub.3. A part of the raw material molten solution flowed 
into the crucible 16 through the communicating hole 29. The liquid level 
in the crucible 16 was raised and the lower ends of the pipes 20 and 21 
were submerged in the molten solution. 
The heaters 25 and 27 were energized to heat the Ga and As raw materials. 
The temperature of the container 19 was set at 617.degree. C., and the 
temperature of the pipe 21 connected thereto was maintained at a 
temperature above 617.degree. C. The flow rate control mechanism 26 was so 
adjusted that a proper quantity of the Ga raw material gradually flowed 
from the container 18. 
Then, the vertical position of the outer shaft 9 was controlled to locate 
the inner crucible 14 in the raw material molten solution. The inner shaft 
10 and the outer shaft 9 were rotated at speeds of 3 r.p.m. and 10 r.p.m. 
respectively in directions opposite to dip the seed crystal 11 into the 
raw material molten solution 6. After seeding, crystal growth was started. 
While the single crystal 12 of GaAs was grown under the seed crystal, the 
vertical position of the liquid surface of the raw material molten 
solution contained in the crucible 2 was maintained constant. At this 
time, a constant quantity of the Ga raw material was continuously 
supplied, while the As raw material was supplied as a gas of about 1 atm. 
As hereinabove described, the synthesis quantity of GaAs per unit time was 
balanced with the pulling quantity of the single crystal. The synthesis 
quantity was controlled by controlling the supply quantity of GaAs. So far 
as the As partial pressure was controlled to about 1 atm., the composition 
of the GaAs molten solution was stoichiometrically maintained. Through the 
aforementioned operation, a GaAs single crystal of 76 mm in diameter and 
533 mm in length was grown. This was a long single crystal, which had been 
impossible to attain by the conventional method. This single crystal was 
entirely semiinsulative, and had uniform characteristics. 
A second embodiment according to the present invention will to pull a 
single crystal in a pulling furnace 1 with an inner atmosphere of an inert 
gas 13. The pulling furnace 1 is provided with a crucible 2 for containing 
a raw material molten solution and another crucible 16 for synthesizing 
the molten solution respectively. The lower portion of the crucible 2 is 
connected with the lower portion of the crucible 16 by a communicating 
tube 29. Heaters 7 and 37 are arranged around these crucibles 
respectively. The crucible 2 contains a raw material molten solution 6, 
and is provided with an inner crucible 14. A communicating hole 15 is 
provided in the bottom portion of the inner crucible 14, so that the raw 
material molten solution 6 flows into the interior of crucible 14. On the 
other hand, a rotatable and/or vertically movable inner shaft 10 is 
suspended above the center of the crucible 2, and a seed crystal 11 is 
mounted on its lower end. A cylindrical outer shaft 9 is rotatably mounted 
to surround the inner shaft 10. The outer shaft 9 is connected with the 
inner crucible 14 by a suspending jig 4, so that the inner crucible 14 is 
rotated following rotation of the outer shaft 9. Both the inner shaft 10 
and the outer shaft 9 pass through the upper portion of the pulling 
furnace 1. In the pulling furnace 1, an ampoule 39 for containing a raw 
material is provided above the crucible 16. The ampoule 39 is connected 
with a pipe 21, which extends into the crucible 16. A heater 27 is 
provided around the ampoule 39. Another ampoule 38 is positioned outside 
of the pulling furnace 1. This ampoule 38 is connected with a pipe 20, 
which extends into the crucible 16 through the pulling furnace 1. The 
portion around the pipe 20, which passes through the pulling furnace 1, is 
sealed. 
A GaAs single crystal was grown using the apparatus having the 
aforementioned structure. First, 5 kg of a GaAs polycrystal and 700 g of a 
sealant B.sub.2 O.sub.3 were contained in the crucible 2 having a diameter 
of 152 mm which was made of PBN. At the same time, 500 g of a sealant 
B.sub.2 O.sub.3 was contained in the crucible 16. The ampoule 39 was made 
of carbon, and its inner surface was coated with PBN. 5.4 kg of As raw 
material was contained in the ampoule 39. The ampoule 38 was formed of 
carbon, and its inner surface was coated with PBN. 5 kg of Ga raw material 
was contained in the ampoule 38. Then, N.sub.2 gas was filled into the 
pulling furnace 1 at a prescribed pressure, and the previously charged 
GaAs polycrystal and B.sub.2 O.sub.3 were melted by the heaters 7 and 37. 
The molten solution contained in the crucible 2 flowed into the crucible 
16 through the communicating tube 29. 
The forward end portions of the pipes 20 and 21 were submerged in this 
molten solution. Then, the temperature of the ampoule 39 was set at 
617.degree. C. by the heater 27, and the temperature of the overall area 
of the As supply system was set at 617.degree. C. also with a heater (not 
shown) provided around the pipe 21. In the aforementioned state, the 
positions of the inner shaft 10 and the outer shaft 9 were adjusted to dip 
the inner crucible 14 into the raw material molten solution 6. The inner 
shaft 10 and the outer shaft 9 were rotated at 3 r.p.m. and 10 r.p.m. 
respectively in directions opposite to each other, and the inner shaft 10 
was moved down to dip the seed crystal 11 into the molten solution. The 
molten solution was controlled at a growth temperature by the respective 
heaters, and the inner shaft 10 was rotated and moved up to pull the 
single crystal. When the single crystal was pulled, Ga was continuously 
supplied from the ampoule 38 so that the level of the molten solution 
surface in the crucible 16 was not changed and As was supplied from the 
ampoule 39 as a gas at about 1 atm., as shown in the first embodiment. 
Thus, the stoichiometric composition of the raw material molten solution 
synthesized in the crucible 16 was continuously maintained constant. 
A GaAs single crystal of 76 mm in diameter and 533 mm in length was grown 
according to the aforementioned step, whereby the as-obtained crystal was 
semi-insulative along its overall area and it had uniform characteristics. 
As shown in the first embodiment, the ampoule for supplying As may 
alternatively be positioned outside of the high-pressure housing also in 
the apparatus of the second embodiment. A third embodiment according to 
the present invention will now be described. Referring to FIG. 3, this 
apparatus is adapted to pull a single crystal in an airtight housing 31. 
The airtight housing 31 is formed by a cylindrical body 32 and a cover 33. 
The cover 33 engages into an annular groove portion 32a which is formed in 
the upper end portion of the body 32 and is sealed with a liquid sealant 
8, to define a closed space. A rotatable lower shaft 35 passes through the 
bottom center of the body 32. The portion passed by the lower shaft 35 is 
sealed with a liquid sealant 8. The lower shaft 35 extends into the 
interior of the body 32 and a support portion 34 for supporting a crucible 
42 is formed on the upper end of shaft 35. The crucible 42 on the support 
portion 34 contains a raw material molten solution 6 and is provided with 
an inner crucible 54. A communicating hole 55 is provided in the bottom 
portion of the inner crucible 54, so that the molten solution 6 flows into 
the interior of crucible 54. A rotatable and/or vertically movable upper 
shaft 45 is provided above the center of the crucible 42 and a seed 
crystal 11 is mounted on the forward or lower end of the upper shaft 45. 
The upper shaft 45 passes through the upper portion of the cover 33, and 
the passed portion is sealed with a liquid sealant 8. A high dissociation 
pressure element 80 in the raw material molten solution is stored in the 
bottom portion of the body 32. Heaters 57, 47 and 67 are provided around 
the airtight housing 31 for controlling the temperatures of the upper, 
intermediate and lower portions of the housing respectively. 
In the apparatus of FIG. 2 having the aforementioned structure, ampoules 48 
and 49 for containing raw materials for the molten solution are positioned 
outside and above the airtight housing 31. The ampoules 48 and 49 are 
connected with raw material supply tubes 41 and 40 respectively. The raw 
material supply tubes 41 and 40 pass through the cover 33 and extend into 
the crucible 42. Openings of the supply tubes are dipping into the molten 
solution 6 contained in the crucible 42 respectively. Heaters 77 and 87 
are provided around the ampoules 48 and 49 respectively. The portions of 
the raw material supply tubes 40 and 41 passing through the cover portion 
33 are sealed. 
A GaAs single crystal was grown using the apparatus of FIG. 3 having the 
aforementioned structure. The crucible 42 made of BN had a diameter of 152 
mm and was charged with 4 kg of a GaAs polycrystal serving as a raw 
material. The temperatures of the liquid sealants 8 in the respective 
sealed portions were raised to at least 500.degree. C. to close the 
airtight housing 31, and thereafter the temperature around the crucible 42 
was raised to 1300.degree. C. by the heater 47. The raw material 
polycrystal was melted by such heating. The temperatures were set by the 
respective heaters so that the lowest temperature in the airtight housing 
31 was not less than 617.degree. C. On the other hand, As 50 was contained 
in the ampoule 48. Further, Ga 51 was contained in the ampoule 49. Then, 
the upper shaft 45 was moved down to dip the seed crystal 11 in the molten 
solution 6. After seeding, a single crystal was pulled. When the raw 
materials contained in the ampoules are liquids such as Ga, the raw 
materials may be supplied by application of pressure into the ampoules 
from the exterior by injecting an inert gas. These raw materials were 
mixed and synthesized in the crucible 42. Further, the As partial pressure 
in the airtight housing 31 was adjusted, thereby controlling the 
stoichiometric composition of the raw material molten solution. 
A single crystal was pulled in the aforementioned manner, whereby a single 
crystal of 75 mm in diameter and 300 to 400 mm in length was obtained. It 
was possible to pull the crystal with a small temperature gradient, since 
the vapor pressure in the space for growing the crystal was controlled in 
this method. The dislocation density of the as-obtained single crystal was 
at an extremely low average value of not more than 300 cm.sup.-2. Thus, a 
crystal having excellent characteristics was obtained. Further, a crystal 
having a homogeneous composition was obtained since the stoichiometric 
composition was controlled as to the raw material molten solution, as 
described above. In the so-obtained crystal, deposition of As was small 
while the E12 density was 1.times.10.sup.16 cm.sup.-3, which is not at all 
inferior to a high-quality crystal formed by a conventional LEC method. 
When the raw materials are supplied from the ampoules in the form of gases 
in the aforementioned apparatus, the pressure of the gas in the airtight 
housing 31 is increased if the supplied raw material gases are evaporated 
from the molten solution surface. Such increase of the gas pressure 
facilitates the synthesis reaction on the surface of the molten solution. 
Therefore, it is possible to pull a single crystal of a desired 
composition by satisfactorily balancing the pressures of the supplied raw 
materials and the vapor pressure in the high-pressure housing. 
Further, it is also possible to cover the molten solution surface with a 
liquid sealant B203 in the aforementioned apparatus. In this case, the gas 
evaporated from the raw material molten solution only unilaterally 
increases the pressure in the high-pressure housing, and cannot return 
into the molten solution. When the pressure in the housing is increased, 
therefore, the supply quantity of the raw material gas is reduced and 
hence the speed of reaction for synthesizing the raw material is reduced. 
Although the inner crucible is so constructed that it floats on the molten 
solution in the aforementioned apparatus, such an inner crucible may 
alternatively be fixed to an outer crucible or the like. 
According to the present invention, as hereinabove described, it is 
possible to pull a single crystal while supplying raw materials, whereby a 
long single crystal can be pulled without increasing the size of a 
crucible containing a raw material molten solution. Thus, the cost for 
manufacturing the single crystal is reduced. Further, there is no 
possibility of contamination caused by impurities resulting from a raw 
material crystal for supply, dissimilarly to the conventional case. Thus, 
a high-purity single crystal can be obtained. Further, the aforementioned 
raw material supply mechanism has a simple structure. In addition, it is 
possible to control the stoichiometric composition of the as-pulled single 
crystal by controlling the supply quantities of the raw materials. 
In view of the aforementioned advantages, the present invention is 
particularly useful for growing a long single crystal of a compound 
semiconductor containing a high dissociation component or an oxide.