Method of preparing group II-VI or III-V compound single crystal

Abstract: Provided is a method which can industrially prepare a group II-VI or III-V compound single crystal having a small number of crystal defects by effectively preventing polycrystallization. The method is adapted to prepare a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound, and comprises the steps of coating an inner surface of a crucible with a film consisting of powdered solids and a vitreous substance, placing the polycrystalline compound in the coated crucible, placing the crucible which is provided therein with the polycrystalline compound in heating means, heating the crucible which is arranged in the heating means with the heating means for melting the polycrystalline compound in the crucible, and cooling the crucible and the melted compound for growing a single-crystalline compound.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of preparing a group II-VI or III-V compound single crystal, and more particularly, it relates to a method of preparing a group II-VI or III-V compound single crystal which is employed for optoelectronic devices, such as light emitting diodes (LED), laser diodes and the like, and microwave devices. Description of the Background Art Group III-V and II-VI compound semiconductor single crystals include those of gallium arsenide (GaAs), gallium phosphide (GaP), indium phosphide (InP) and cadmium telluride (CdTe), for example. It is known that such compound semiconductor single crystals are generally prepared by various industrial methods such as horizontal Bridgeman method (HB method), liquid encapsulated Czochralski method (LEC method), vertical Bridgeman method (VB), and vertical gradient freeze method (VGF method). Among these methods, the VB method, for example, is adapted to prepare a compound semiconductor single crystal in the following manner: In general, a vertical furnace having at least one high temperature zone and at least one low temperature zone is employed in the VB method. These zones are designed to provide a temperature profile consisting of high and low temperature zones having relatively homogeneous temperatures, which are divided by a critical zone having a temperature gradient of about 5 to 20°C/cm. First, a vertically arranged crucible (made of pBN in general) which is suitable for containing a group II-VI or III-V compound is arranged in a sealed ampoule. Growth of a single crystal is progressed by slowly moving up the furnace while immovably holding the crucible-ampoule assembly. The VB method of preparing a compound semiconductor comprises the steps of placing a seed single crystal on a bottom portion of the crucible, introducing a polycrystalline substance into the crucible, placing the crucible in the ampoule and thereafter sealing the ampoule, placing this crucible-ampoule assembly on a crucible support which is provided inside the aforementioned vertically arranged furnace, heating the polycrystalline substance and the upper portion of the seed single crystal to a temperature exceeding the melting point thereof, and upwardly moving the furnace by the length of the melt obtained by melting of the polycrystalline substance thereby forming a solid single-crystalline substance. The ingot of the group II-VI or III-V compound single crystal prepared in the aforementioned manner is then taken out from the crucible, and sliced into wafers for various optoelectronic and microwave devices. The VB method having the aforementioned manner is expected as a promising method which can prepare a high-quality crystal, having a lower defect density at a lower cost as compared with the remaining methods. However, a problem of occurrence of crystal defects cannot still be completely solved by this VB method. Namely, thermal stress is caused in a contact part due to difference between thermal expansion coefficients of a container such as a crucible and the crystal to result in generation of dislocations, and resulting polycrystallization. In order to prevent such polycrystallization, it is effective to reduce the coefficient of friction at the contact surface between the container and the crystal. In more concrete terms, it may be effective to make the container and a raw material melt hardly wettable, or to employ a lubricant. In order to improve the wettability of the container and the raw material melt, employment of quartz as the material for the container is generally studied, for example. According to this method, no contamination is caused by boron. However, this method is unsuitable for industrial preparation since it is difficult to increase the dimensions of the container consisting of quartz. On the other hand, a method employing liquefied boron oxide (B 2 O 3 ) is studied as the method of employing a lubricant, for example. Such employment of B 2 O 3 is also adapted to prevent a melted or crystallized compound such as GaAs from decomposition and volatilization of a volatile component such as arsenic (As). Under the room temperature, however, B 2 O 3 is solidified and hence the crystal and the crucible are fixed to each other. In case of a crucible of pyrolytic boron nitride (pBN), therefore, pBN is disadvantageously delaminated to reduce the life of the crucible. In growth of a silicon (Si) doped GaAs single crystal, for example, replacement reaction takes place between the dopant of Si and boron (B) contained in B 2 O 3 at the interface between the GaAs melt and B 2 O 3 . Thus, decrease of the Si concentration and increase of the boron concentration are disadvantageously caused. U.S. Patent No. 4,999,082 (1991: hereinafter referred to as "prior art") discloses a method of improving such a conventional method to prevent polycrystallization. The method disclosed in this prior art is characterized in that an inner surface of a crucible is coated with powdered solids in place of B 2 O 3 . In more concrete terms, solid boron nitride (BN) powder is employed. According to this prior art, no crystal is stuck with the crucible dissimilarly to the case of employing liquefied B 2 O 3 , whereby the life of the crucible can be effectively increased. However, adhesion of the BN powder to the crucible is insufficient under conditions disclosed in the prior art. Thus, the BN powder cannot homogeneously adhere to the inner surface of the crucible. According to this prior art, the BN powder is applied by a method of sprinkling the powder on the inner surface of the crucible in a dry state, or a method of applying a slurry of the solid powder and a carrier liquid consisting of a mixture of water and alcohol or the like, for example, to the inner surface of the crucible and thereafter evaporating the carrier liquid. Thus, the adhesion of the BN powder to the crucible is so small that the applied BN powder disadvantageously falls from the inner surface of the crucible. Fig. 14 typically illustrates the state of an interface between a crucible and a GaAs melt in preparation of a GaAs single crystal by the prior art. Referring to Fig. 14, BN particles 3 are applied to a surface of a pBN crucible 1 according to the prior art, while the BN particles 3 partially fall at a part A. In such a particle fall part A, the crucible 1 and the GaAs melt 2 are wetted, which causes frictional force, resulting in the occurrence of polycrystallization. Also when the BN particles 3 have large particle sizes, the GaAs melt 2 and the BN particles 3 are wetted to cause crystal defects. On the other hand, the VB method of preparing a single crystal also has the following problem: When the seed crystal is arranged on the bottom portion of the crucible, a gap is disadvantageously formed between the crucible and the seed crystal. When a raw material melt such as a GaAs melt infiltrates into this gap, solidification of the melt is started not from the seed crystal side but from the crucible wall surface side, to disadvantageously cause polycrystallization. SUMMARY OF THE INVENTION An object of the present invention is to provide a method which can industrially prepare a group II-VI or III-V compound single crystal having a low defect density by solving the aforementioned problems and effectively preventing polycrystallization. According to an aspect of the present invention, a method of preparing a group II-VI or III-V compound single crystal is provided. This method is adapted to prepare a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound, and comprises the steps of coating an inner surface of a crucible with a film consisting of powdered solids having a melting point which is higher than that of the polycrystalline compound and a vitreous substance, placing the polycrystalline compound in the coated crucible, placing the coated crucible which is provided therein with the polycrystalline compound in heating means, heating the crucible which is arranged in the heating means with the heating means for melting the polycrystalline compound in the crucible, and cooling the crucible and the melted compound for growing a single-crystalline compound. According to another aspect of the present invention, a method of preparing a group II-VI or III-V compound single crystal is provided. This method is adapted to prepare a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound in a vertically arranged crucible, and comprises the steps of coating an inner surface of the crucible with a film consisting of powdered solids having a melting point which is higher than that of the polycrystalline compound and a vitreous substance, placing a seed crystal consisting of a single-crystalline compound on a bottom portion of the crucible, placing the polycrystalline compound in the remaining portion of the crucible, placing the crucible which is provided therein with the seed crystal and the polycrystalline compound in a vertically arranged furnace which is capable of forming an upper high temperature zone and a lower low temperature zone, adjusting the position of the furnace and the temperature of the upper high temperature zone thereby melting the polycrystalline compound while maintaining a part of the seed crystal in a solid state in the crucible, setting the lower low temperature zone at a temperature which is less than the melting point of the single-crystalline compound while setting the upper high temperature zone at a temperature which is higher than the melting point thereby forming a solid-liquid interface, and upwardly vertically moving the furnace and the solid-liquid interface while substantially retaining the temperature setting in the lower low temperature zone and the upper high temperature zone thereby growing a single-crystalline compound. In the present invention, the step of coating the inner surface of the crucible with the film consisting of the powdered solids and the vitreous substance preferably includes the steps of applying a mixed solution containing the powdered solids and a silanol compound to the inner surface of the crucible, and vacuum-heating the crucible which is supplied with the mixed solution thereby forming the film containing the powdered solids and the vitreous substance consisting of silicon oxide on the inner surface of the crucible. The mixed solution containing the powdered solids and the silanol compound may be applied to the inner surface of the crucible in a liquid state, or the former may be sprayed onto the latter. The mixed solution is preferably applied to the inner surface of the crucible by recoating of at least twice. When a thick film is formed by single coating, the film may be cracked or readily separated from the crucible. In such recoating, the first layer must be temporarily baked for forming a next layer. The vacuum heat treatment following the application of the mixed solution to the crucible is preferably performed at a temperature of at least 500°C for at least 1 hour, more preferably at a temperature of about 1000°C for 2 or 3 hours. Due to the vacuum heat treatment, silanol contained in the applied mixed solution is converted to silicon oxide (SiO 2 ) through heating/polymerization reaction. According to the present invention, the vitreous substance preferably has a softening temperature which is lower than the melting point of the polycrystalline compound. The vitreous substance is prepared from diboron trioxide (B 2 O 3 ) or a mixture of diboron trioxide (B 2 O 3 ) and silicon dioxide (SiO 2 ), for example. According to the present invention, the powdered solids are prepared from boron nitride, for example. According to the present invention, further, the powdered solids can be prepared from a substance having lubricity such as graphite, in place of boron nitride. When boron nitride is employed in the present invention, its mean particle size is preferably at least 0.05 µm and not more than 10 µm. The powdered solids are easy to wet to cause crystal defects, if the mean particle size of boron nitride is in excess of 10 µm. Throughout the specification, the particle sizes of boron nitride are defined as those of primary particles 10 also when the primary particles 10 of boron nitride (BN) aggregate to form a secondary particle 20 as shown in Fig. 16, for example. In the present invention, the inner surface of the crucible preferably has prescribed surface roughness in an uncoated state. As to the prescribed surface roughness, the maximum roughness R max is preferably 10 to 150 µm, and the center line average roughness R a is preferably 3 to 15 µm. The term "maximum roughness R max " indicates a value obtained by measuring a space between two straight lines, which are parallel to an average line, holding a profile line of a portion of an evaluation length Lm extracted from the profile line in the direction of its average line in the direction of an axial magnification. The term "profile line" indicates a contour appearing on a section when a measured surface is cut along a plane which is perpendicular to a mean surface of the measured surface. The term "average line" indicates a straight line having the geometry of the measured surface in the extracted portion of the profile line, which is set to minimize the sum of squares of deviation from the line to the profile line. The term "center line average roughness R a " indicates a value obtained by the following equation when a portion of the evaluation length Lm is extracted from a roughness curve in the direction of its center line, the center line of the extracted portion and the direction of axial magnification are set on X- and Y-axes respectively, and the roughness curve is expressed as Y(x): The term "roughness curve" indicates a curve obtained by a measuring method having a characteristic of removing a low frequency component from the profile line. The term "center line" indicates a straight line, which is parallel to the average line of the roughness curve, making areas enclosed with the straight line and the roughness curve equal on both sides of the straight line. All of the above indices of the surface roughness are in accordance with JIS B0601-1976. Adhesion of the film to the crucible is improved due to such in-crucible surface roughness. According to still another aspect of the present invention, a method of preparing a group II-VI or III-V compound single crystal is provided. This method is adapted to prepare a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound in a vertically arranged crucible, and comprises the steps of coating an outer surface of a seed crystal consisting of a single-crystalline compound with a film consisting of powdered solids having a melting point which is higher than that of the polycrystalline compound and a vitreous substance, placing the coated seed crystal on a bottom portion of the crucible, placing the polycrystalline compound in the remaining portion of the crucible, placing the crucible which is provided therein with the seed crystal and the polycrystalline compound in a vertically arranged furnace which is capable of forming an upper high temperature zone and a lower low temperature zone, adjusting the position of the furnace and the temperature of the upper high temperature zone thereby melting the polycrystalline compound while maintaining a part of the seed crystal in a solid state in the crucible, setting the lower low temperature zone at a temperature which is less than the melting point of the single-crystalline compound while setting the upper high temperature zone at a temperature which is higher than the melting point thereby forming a solid-liquid interface, and upwardly vertically moving the furnace and the solid-liquid interface while substantially retaining the temperature setting in the lower low temperature zone and the upper high temperature zone thereby growing a single-crystalline compound. In the present invention, the step of coating the outer surface of the seed crystal with the film consisting of the powdered solids and the vitreous substance preferably includes the steps of applying a mixed solution containing the powdered solids and a silanol compound to the outer surface of the seed crystal, and vacuum-heating the seed crystal which is supplied with the mixed solution thereby forming the film containing the powdered solids and the vitreous substance consisting of silicon oxide on the outer surface of the seed crystal. After the application of the mixed solution to the seed crystal, the vacuum heat treatment is preferably performed at a temperature of about 500 to 600°C for at least 30 minutes, more preferably at a temperature of 500 to 600°C for 2 to 3 hours, in order to prevent the seed crystal from decomposition/deterioration. Due to the vacuum heat treatment, silanol contained in the applied mixed solution is converted to silicon oxide (SiO 2 ) through heating/polymerization reaction. In the present invention, the step of applying the mixed solution to the outer surface of the seed crystal is carried out by dipping the seed crystal in the mixed solution, or spraying the mixed solution onto the outer surface of the seed crystal. When the mixed solution is sprayed, acetone is preferably mixed into the mixed solution. Spray applicability is improved and internal strain of the film is reduced due to such mixing of acetone. According to the present invention, the vitreous substance for coating the outer surface of the seed crystal preferably has a softening temperature which is lower than the melting point of the polycrystalline compound, similarly to the case of coating the inner surface of the crucible. The vitreous substance is prepared from diboron trioxide (B 2 O 3 ) or a mixture of diboron trioxide (B 2 O 3 ) and silicon dioxide (SiO 2 ), for example. According to the present invention, the powdered solids are prepared from boron nitride, for example. When boron nitride is employed, its mean particle size is preferably at least 0.05 µm and not more than 10 µm. The powdered solids are easy to wet to cause crystal defects, if the mean particle size of boron nitride is in excess of 10 µm. According to a further aspect of the present invention, a method of preparing a group II-VI or III-V compound single crystal is provided. This method is adapted to prepare a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound in a vertically arranged crucible, and comprises the steps of coating an inner surface of the crucible with a film consisting of powdered solids having a melting point which is higher than that of the polycrystalline compound and a vitreous substance, coating an outer surface of a seed crystal consisting of a single-crystalline compound with a film consisting of powdered solids having a melting point which is higher than that of the polycrystalline compound and a vitreous substance, placing the coated seed crystal on a bottom portion of the crucible, placing the polycrystalline compound in the remaining portion of the crucible, placing the crucible which is provided therein with the seed crystal and the polycrystalline compound in a vertically arranged furnace which is capable of forming an upper high temperature zone and a lower low temperature zone, adjusting the position of the furnace and the temperature of the upper high temperature zone thereby melting the polycrystalline compound while maintaining a part of the seed crystal in a solid state in the crucible, setting the lower low temperature zone at a temperature which is less than the melting point of the single-crystalline compound while setting the upper high temperature zone at a temperature which is higher than the melting point thereby forming a solid-liquid interface, and upwardly vertically moving the furnace and the solid-liquid interface while substantially retaining the temperature setting in the lower low temperature zone and the upper high temperature zone thereby growing a single-crystalline compound. This method preferably further comprises a step of coating a gap between the crucible and the seed crystal with a film consisting of powdered solids and a vitreous substance after placing the seed crystal on the bottom portion of the crucible. In this case, a mixed solution containing the powdered solids and a silanol compound is preferably applied to the gap between the crucible and the seed crystal to be thereafter vacuum-heated, similarly to the case of forming the film on the inner surface of the crucible or the outer surface of the seed crystal. The vacuum heating is preferably performed at a temperature of about 500 to 600°C, in order to prevent the seed crystal from decomposition/deterioration. When the seed crystal is arranged on the bottom portion of the crucible, the gap between the crucible and the seed crystal is completely filled by such formation of the film, thereby preventing solidification and resulting polycrystallization of a raw material melt infiltrating into the gap. In the method of preparing a group II-VI or III-V compound single crystal according to the present invention, the group II-VI or III-V compound is prepared from gallium arsenide, for example. Furthermore, the compound employed in the present invention may be prepared from a group III-V compound such as InP or GaP, or a group II-VI compound such as ZeSe or CdTe, for example. According to a further aspect of the present invention, a method of preparing a group II-VI or III-V compound single crystal is provided. This method is adapted to prepare a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound, and comprises the steps of coating an inner surface of a crucible with powdered solids having a melting point which is higher than that of the polycrystalline compound through a binder consisting of a vitreous substance, placing the polycrystalline compound in the coated crucible, placing the crucible which is provided therein with the polycrystalline compound in heating means, heating the crucible which is arranged in the heating means with the heating means thereby melting the polycrystalline compound in the crucible, and cooling the crucible and the melted compound thereby growing a single-crystalline compound. According to the present invention, the inner surface of the crucible is coated with a film consisting of the powdered solids and the vitreous substance. Fig. 13 typically illustrates the state of an interface between a crucible 1, having an inner surface which is coated with a film consisting of powdered solids 3 and a vitreous substance 13, and a raw material melt 2. Referring to Fig. 13, vitreous substance parts 13a which are positioned between the respective powdered solids 3 and vitreous substance parts 13b which are positioned between the powdered solids 3 and the crucible 1 act as a binder, whereby high adhesion of the film to the crucible 1 is attained. Thus, the film will not be separated from the crucible 1, dissimilarly to the prior art. Consequently, the powdered solids 3 can sufficiently provoke an effect for serving as a lubricant. According to the present invention, the surfaces of the powdered solids 3 are coated with vitreous substance parts 13c. The vitreous substance 13 such as SiO 2 is hardly wetted with the raw material melt 2 such as a GaAs melt, as compared with those of BN or the like. Thus, it is possible to effectively prevent occurrence of crystal defects caused by wetting. According to a further aspect of the present invention, a method of preparing a group II-VI or III-V compound single crystal is provided. This method is adapted to prepare a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound in a vertically arranged crucible, and comprises the steps of coating an inner surface of the crucible with powdered solids having a melting point which is higher than that of the polycrystalline compound through a binder consisting of a vitreous substance, placing a seed crystal consisting of a single-crystalline compound on a bottom portion of the crucible, placing the polycrystalline compound in the remaining portion of the coated crucible, placing the crucible which is provided therein with the seed crystal and the polycrystalline compound in a vertically arranged furnace which is capable of forming an upper high temperature zone and a lower low temperature zone, adjusting the position of the furnace and the temperature of the upper high temperature zone thereby melting the polycrystalline compound while maintaining a part of the seed crystal in a solid state in the crucible, setting the lower low temperature zone at a temperature which is less than the melting point of the single-crystalline compound while setting the upper high temperature zone at a temperature which is higher than the melting point thereby forming a solid-liquid interface, and upwardly vertically moving the furnace and the solid-liquid interface while substantially retaining the temperature setting in the lower low temperature zone and the upper high temperature zone thereby growing a single-crystalline compound. According to a further aspect of the present invention, a method of preparing a group II-VI or III-V compound single crystal is provided. This method is adapted to prepare a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound, and comprises the steps of coating an outer surface of a seed crystal consisting of a single-crystalline compound with powdered solids having a melting point which is higher than that of the polycrystalline compound through a binder consisting of a vitreous substance, placing the coated seed crystal on a bottom portion of a crucible, placing the polycrystalline compound in the remaining portion of the coated crucible, placing the crucible which is provided therein with the seed crystal and the polycrystalline compound in a vertically arranged furnace which is capable of forming an upper high temperature zone and a lower low temperature zone, adjusting the position of the furnace and the temperature of the upper high temperature zone thereby melting the polycrystalline compound while maintaining a part of the seed crystal in a solid state in the crucible, setting the lower low temperature zone at a temperature which is less than the melting point of the single-crystalline compound while setting the upper high temperature zone at a temperature which is higher than the melting point thereby forming a solid-liquid interface, and upwardly vertically moving the furnace and the solid-liquid interface while substantially retaining the temperature setting in the lower low temperature zone and the upper high temperature zone thereby growing a single-crystalline compound. According to the present invention, the outer surface of the seed crystal is coated with a film consisting of the powdered solids and the vitreous substance. Therefore, no gap is formed between the crucible and the seed crystal when the seed crystal is arranged on the bottom portion of the crucible, whereby solidification of a raw material melt from a wall surface of the crucible and resulting polycrystallization are prevented. According to the present invention, further, the film consists of the powdered solids and the vitreous substance. Thus, it is possible to form a homogeneous film having high adhesion to the seed crystal. Fig. 17 typically illustrates the state of an outer surface of a seed crystal 7 which is coated with a film consisting of powdered solids 3 and a vitreous substance 13 according to the present invention. Referring to Fig. 17, vitreous substance parts 13a which are positioned between the respective powdered solids 3 serving as a lubricant and vitreous substance parts 13b which are positioned between the powdered solids 3 serving as a lubricant and the seed crystal 7 act as a binder, whereby a homogeneous film having strong adhesion to the seed crystal 7 is formed as described above. According to a further aspect of the present invention, a method of preparing a group II-VI or III-V compound single crystal is provided. This method is adapted to prepare a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound in a vertically arranged crucible, and comprises the steps of coating an inner surface of the crucible with powdered solids having a melting point which is higher than that of the polycrystalline compound through a binder consisting of a vitreous substance, coating an outer surface of a seed crystal consisting of a single-crystalline compound with powdered solids having a melting point which is higher than that of the polycrystalline compound through a binder consisting of a vitreous substance, placing the coated seed crystal on a bottom portion of the crucible, placing the polycrystalline compound in the remaining portion of the coated crucible, placing the crucible which is provided therein with the seed crystal and the polycrystalline compound in a vertically arranged furnace which is capable of forming an upper high temperature zone and a lower low temperature zone, adjusting the position of the furnace and the temperature of the upper high temperature zone thereby melting the polycrystalline compound while maintaining a part of the seed crystal in a solid state in the crucible, setting the lower low temperature zone at a temperature which is less than the melting point of the single-crystalline compound while setting the upper high temperature zone at a temperature which is higher than the melting point thereby forming a solid-liquid interface, and upwardly vertically moving the furnace and the solid-liquid interface while substantially retaining the temperature setting in the lower low temperature zone and the upper high temperature zone thereby growing a single-crystalline compound. According to the present invention, as hereinabove described, polycrystallization is effectively prevented due to improvement of the adhesion and wettability of the film, whereby a group II-VI or III-V compound single crystal having a small number of crystal defects can be prepared. Also in growth of an Si-doped GaAs single crystal, replacement reaction hardly takes place between the dopant of Si and BN or B 2 O 3 contained in BN, whereby decrease of the Si concentration is suppressed while the contamination by B is reduced. According to the present invention, further, the film which is formed on the inner surface of the crucible contains the vitreous substance so that the film is not separated from the crucible, whereby a homogeneous film can be formed. Thus, it is possible to cope with size increase of the crystal, whereby the present invention is particularly effective for preparation in an industrial scale. According to the present invention, in addition, the inner surface of the crucible is not damaged by formation of the film, whereby the life of the crucible is not reduced. Consequently, it is possible to prepare the single crystal at a low cost. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates exemplary steps of preparing a group II-VI or III-V compound single crystal; Fig. 2 illustrates an exemplary crucible pretreatment step according to the present invention; Fig. 3 illustrates a state of applying a mixed solution to a crucible by spraying according to the present invention; Fig. 4 illustrates a state of vacuum-heating the crucible supplied with the mixed solution according to the present invention; Fig. 5 is a sectional view showing an exemplary crucible employed in the present invention; Fig. 6 illustrates a state of applying a mixed solution to a seed by dipping the latter in the former according to the present invention; Fig. 7 illustrates a state of vacuum-heating the seed supplied with the mixed solution according to the present invention; Fig. 8 illustrates a state of filling a gap between the crucible and the seed according to the present invention; Fig. 9 is an enlarged sectional view showing a part IX in Fig. 8; Fig. 10 is a sectional view showing a state of quartz ampoule sealing; Fig. 11 illustrates the dimensions of a crucible employed in Example of the present invention; Fig. 12 illustrates the dimensions of a quartz ampoule employed in Example of the present invention; Fig. 13 typically illustrates the state of an interface between a crucible having an inner surface which is coated with a film consisting of powdered solids and a vitreous substance according to the present invention and a raw material melt; Fig. 14 typically illustrates the state of an interface between a crucible and a GaAs melt in preparation of a GaAs single crystal in accordance with the prior art; Fig. 15 is a sectional view showing a crystal grown according to Example of the present invention; Fig. 16 is adapted to illustrate particle sizes of BN particles; and Fig. 17 typically illustrates the state of an outer surface of a seed crystal coated with a film consisting of powdered solids and a vitreous substance according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 illustrates exemplary steps of preparing a group II-VI or III-V compound single crystal. The respective steps shown in Fig. 1 are now described. (1) Raw Material Pretreatment Step First, a raw material consisting of a GaAs polycrystal which was synthesized by a boat method was chamfered, in order to prevent a film formed on an inner surface of a crucible from being separated by an edge of the raw material. On the other hand, a seed crystal (hereinafter referred to as a seed) consisting of a GaAs single crystal was also chamfered, similarly to the polycrystalline raw material, in order to prevent the film formed on the inner surface of the crucible from being separated upon insertion of the seed in a bottom portion of the crucible. Then, etching was performed on the polycrystalline raw material and the seed treated in the aforementioned manner, for removing impurities from surfaces thereof. The etching was performed under the following ordinary etching conditions: 1) Exemplary Etching Conditions 1 (in case of employing sulfuric acid etching solution) etching solution composition: sulfuric acid:hydrogen peroxide:water = 3:1:1 temperature of etching solution: 60°C etching time: about 20 minutes for the polycrystalline raw material and 1 to 2 minutes for the seed 2) Exemplary Etching Conditions 2 (in case of employing aqua regia) etching solution composition: nitric acid:hydrochloric acid = 1:3 temperature of etching solution: room temperature etching time: about 20 minutes for the polycrystalline material and about 8 minutes for the seed (2) Crucible Pretreatment Step (Example 1) Fig. 2 illustrates an exemplary crucible pretreatment step. The crucible pretreatment step is now described with reference to Fig. 2. (a) Preparation of Mixed Solution BN powder of not more than 10 µm in mean particle size was employed. This powder had oxygen concentration of not more than 3 wt%. The oxygen concentration of the powder is preferably reduced to hardly react with a dopant. Silanol compounds were prepared from OCD by Tokyo Ohka Kogyo Co., Ltd., which is a liquid consisting of a silanol compound and a solvent. In this experiment, two types of compounds including an inorganic system Type 2 (containing 5.9 wt.% of Si(OH) 4 with a solvent of ethyl acetate) and an organic system Type 7 (containing 12 wt.% of RnSi(OH) 4-n , where R represents an alkyl group, with a solvent of methanol). Both of the Types 2 and 7 were employable up to concentrations of 20 wt.%. With such BN particles and silanol compounds, mixed solutions were prepared as follows: 1) In case of employing (applying) the mixed solution in a liquid state: BN Powder OCD Aceton Mixed Solution Mixing Example 1 Crystal 10µm 33g inorganic system 5.9wt% 87-88g not used In Table 1, Crystal means the particle size of the BN powder. 2) In case of employing (spraying) the mixed solution: BN Powder OCD Aceton Mixed Solution Mixing Example 2 Crystal 0.5 µm 16-24g inorganic 5.9wt% 87-88g 87-88g Mixed Solution Mixing Example 3 Crystal 0.5 µm 16-24g inorganic 5.9wt% 174-176g no used Mixed Solution Mixing Example 4 Crystal 10 µm 33g inorganic 5.9wt% 87-88g 1 to 3 times of OCD Mixed Solution Mixing Example 5 Crystal 10 µm 33g inorganic 5.9wt% 174-176g not used In Table 2, Crystal means the particle size of the BN powder. As shown in Table 2, the mixed solution is preferably mixed with acetone, when the same is applied by spraying. The first reason for mixing acetone resides in improvement of spray applicability. Ethyl acetate or methanol forming the solvent of OCD is liquefied upon continuous application due to its low volatility, and readily cracked or mottled after drying. Further, spray particles of OCD having high viscosity are so hardly atomized that it is difficult to form a homogeneous film. When such OCD is mixed with acetone, its volatility is so improved that the same is hardly liquefied. Further, a homogeneous film is effectively obtained due to atomization. The second reason for mixing acetone resides in reduction of internal strain of the film. When OCD is at a high ratio, the film is so hardened that its internal strain is increased, although the same is hardly wetted with a GaAs melt. A film having high internal strain is readily separated due to cracking. When acetone is mixed, the OCD concentration is reduced to reduce internal strain of the film, thereby suppressing cracking or separation. Tables 3 and 4 show results of study which was made on particle sizes of BN powder and acetone mixing ratios as to the mixed solutions. Referring to Tables 3 and 4, the term "defects" indicates generation of dislocations or polycrystals. BN Particle Size 0.5µm (not more than 1µm) 10µm Lubricity excellent few defects inferior many defects Wetting few few twins few defects strong (many) many twins many defects Surface Smoothness excellent few twins inferior many twins Aceton Mixing Ratio Low (High OCD Concentration) High (Law OCD Concentration) Film Quality hard easy to crack soft hard to crack Film Internal Stress high easy to separate low hard to separate Wetting hard to wet hard to generate defect easy to wet easy to generate defect (b) Application of Mixed Solution to Crucible The mixed solution can be applied to a crucible by applying the mixed solution to an inner surface of the crucible in a liquid state (liquid application), or spraying the mixed solution. Fig. 3 illustrates a state of applying a mixed solution to a crucible by spraying. Referring to Fig. 3, a pBN crucible 1 is preferably heated by a band heater 4 between 50 and 150°C, so that a mixed solution 5 is sprayed thereto by an atomizer 6. In this case, a solvent contained in the sprayed mixed solution 5 can be quickly dried for preventing the mixed solution 5 from dripping. When the mixed solution 5 is sprayed through an opening 1a in an upper part of the crucible 1, the mixed solution 5 hardly adheres to a seed receiving portion 1c and a tapered portion 1b provided in a lower part of the crucible 1 since the same is sprang back by the wall of the crucible 1. Therefore, the mixed solution 5 is sucked from the side of the seed receiving portion 1c, so that its adhesion to the tapered portion 1b and the seed receiving portion 1c is improved. Fig. 5 is a sectional view showing an exemplary crucible 1 employed in the present invention. Referring to Fig. 5, an inner surface of this crucible 1 is finely irregularized. When such a crucible 1 is employed, a film which is formed on its inner surface is hardly separated. As to the surface irregularity, the maximum roughness R max and the center line average roughness R a are preferably 10 to 150 µm and 3 to 15 µm respectively, for example. When the mixed solution is thickly applied to the inner surface of the crucible at once, the film may be cracked or readily separated. Therefore, the mixed solution is preferably applied to the crucible at least twice. A preferable thickness of a layer which is formed by single application is 50 to 500 µm in case of liquid application, and 50 to 1000 µm in case of spraying. When the mixed solution is applied twice, therefore, four types of application methods shown in Table 5 are conceivable, for example. First Layer Second Layer Example 1 liquid application spraying Example 2 spraying spraying Example 3 liquid application liquid application Example 4 spraying liquid application When the mixed solution is applied by recoating of at least twice, a layer which is formed by first application is preferably temporarily heat treated at about 1000°C in advance of second application. Due to this heat treatment, the strength of the layer formed by the first application is so improved that the film is not cracked upon recoating. (c) Application of Mixed Solution to Seed The mixed solution can be applied to a seed by dipping the seed in the mixed solution (submersion), or spraying the mixed solution to an outer surface of the seed. The mixed solution mixing example 1 shown in Table 1 is preferable for the submersion, for example, while the mixed solution mixing example 2 shown in Table 2 is preferable for the spraying, for example. A film is thus formed on the seed for the following reason: When a GaAs melt infiltrates into a gap between the seed and the crucible, the melt may be solidified not from the seed side but from the crucible wall surface side, to be polycrystallized. When the film is formed on the seed, it is possible to prevent the seed and the crucible from making a gap therebetween, thereby preventing infiltration of the GaAs melt. Fig. 6 illustrates a state of applying a mixed solution 5 to a seed 7 by dipping the latter in the former. Referring to Fig. 6, a portion of the seed 7 excluding a forward end portion 7a is dipped in the mixed solution 5, so that the seed 7 is inserted in a crucible from the forward end portion 7a. (d) Vacuum Heating of Crucible and Seed Inorganic system OCD which is heated in a vacuum is converted to SiO 2 (quartz glass), due to removal of water from Si(OH) 4 by heating/polymerization reaction. When organic system OCD is heated in a vacuum, on the other hand, a small amount of graphite remains in addition to SiO 2 . This graphite is so wettable with the GaAs melt that crystal defects (dislocations) may be caused by wetting. Table 6 shows preferable vacuum heating conditions for the crucible and the seed. Degree of Vacuum Temperature Retention Time Crucible 1,33-1,33·10 -6 Pa (10 -2 - 10 -8 Torr) (preferably 1,33 10 -4 -1,33·10 -6 Pa (10 - 6 - 10 -8 Torr) around 1000°C at least 1h. (preferably 2 to 3h.) Seed 1,33-1,33·10 -6 Pa (10 -2 - 10 -8 Torr) (preferably 1,3310 -4 -1,33·10 -6 Pa (10 - 6 - 10 -8 Torr) 500 - 600°C at least 30 min. (preferably 2 to 3h.) When the heating temperature is in excess of 500°C, no problem arises in formation of SiO2 . However, it is conceivable that heating at 500 to 600°C is suitable for a GaAs seed in order to prevent decomposition/deterioration of GaAs, although a pBN crucible can be heated at a high temperature of about 1500°C, for example. The degree of vacuum does not exert strong influence on formation of SiO2 in particular. A gas flow of nitrogen (N2) or argon (Ar) may be employed, for example, so far as water can be removed. Fig. 4 illustrates a state of vacuum-heating the crucible 1 which is supplied with the mixed solution 5. Referring to Fig. 4, the crucible 1 supplied with the mixed solution 5 is arranged in a quartz vessel 8, which in turn is evacuated by a vacuum pump and heated by a heater 14 from the exterior. On the other hand, Fig. 7 illustrates a state of vacuum-heating the seed 7 which is supplied with the mixed solution 5. Referring to Fig. 7, the seed 7 supplied with the mixed solution 5 is heat-treated in a vacuum by a heater 24. Due to such a vacuum heat treatment, films consisting of BN powder and SiO 2 are formed on the inner surface of the crucible and the outer surface of the seed 7. (e) Seed Attachment The seed which is formed with the film on its outer surface in the aforementioned manner is inserted in the crucible which is provided with the film on its inner surface, and arranged on its bottom portion. If the forward end portion of the seed has an edge, the film formed on the inner surface of the crucible is disadvantageously separated. Therefore, the seed is preferably inserted in the crucible from a chamfered side. After the seed is attached, a cap or a plug which is made of BN or pBN is preferably attached to the bottom portion of the crucible, so that the seed will not fall from the crucible. (f) Filling of Gap between Crucible and Seed When the seed is attached to the bottom portion of the crucible in the aforementioned manner, a gap may be formed between the crucible and the seed. When a GaAs melt infiltrates into this gap, solidification may be started from this portion to cause polycrystallization. Therefore, the gap between the crucible and the seed is preferably filled as follows: First, the mixed solution is applied to the gap between the crucible and the seed. This application of the mixed solution can be performed by dropping the mixed solution into the gap by a dropping pipette, or spraying the former to the latter. When the mixed solution is applied by spraying, it is necessary to wipe out the mixed solution adhering to an upper end surface of the seed after application. Alternatively, the upper end surface of the seed may be previously masked before spraying, in order to prevent adhesion of the mixed solution. The mixed solution mixing example 1 shown in Table 1 is preferable for the dropping, for example, while the mixed solution mixing example 2 shown in Table 2 is preferable for the spraying, for example. Fig. 8 is a sectional view showing a state of filling a gap between a crucible 1 and a seed 7, and Fig. 9 is a partially enlarged sectional view of a portion IX in Fig. 8. Referring to Figs. 8 and 9, a gap 9 between the crucible 1 which is formed with a film 15 on its inner surface and the seed 7 which is formed with another film 75 on its outer surface is filled with a mixed solution 5. (g) Vacuum Heating Then, a vacuum heat treatment is performed in order to heat/polymerize the mixed solution filling the gap. For example, the heat treatment is preferably performed at a temperature of 500 to 600°C causing no damage on the seed for 2 to 3 hours under an atmosphere having a degree of vacuum of 1,33·10 -4 -1,33·10 -6 Pa (10 -6 to 10 -8 Torr). Due to such a vacuum heat treatment, a film consisting of BN powder and SiO2 is formed in the gap between the crucible and the seed.. (Example 2) While the above Example 1 has been described with reference to the films coating the inner surface of the crucible and the outer surface of the seed consisting of only BN serving as powdered solids and Si02 serving as vitreous substances, no softening is caused at the melting point of GaAs when only SiO2 is employed as the vitreous substance. However, when (1) B203 or (2) a mixture of B203 and SiO2 is employed as the vitreous substance, for example, softening is caused at the melting point of GaAs. Consequently, the vitreous substance serving as a binder will not inhibit movement of BN particles serving as a lubricant, whereby the effect of the BN particles serving as a lubricant in the present invention can be further improved. Methods of forming films and effects thereof are now described with reference to (1) B 2 O 3 and (2) a mixture of B 2 O 3 and SiO 2 employed as vitreous substances. 1) In Case of Employing B 2 O 3 as Vitreous Substance (a) Methods of Film Formation (Method 1) First, BN powder is applied to an inner surface of a crucible. Then, the crucible supplied with the BN powder is heated under an atmosphere of oxygen (O 2 ) gas or O 2 mixed gas at a temperature of 900 to 1200°C. Thus, the surface of the BN powder is oxidized along the following reaction (1): 4BN + 3O 2 → 2B 2 O 3 + 2N 2 Consequently, a film consisting of the BN powder and B 2 O 3 is formed on the inner surface of the crucible. (Method 2) First, BN powder is mixed with boric acid (H 3 BO 3 ) powder, and this mixture is further mixed with a solvent such as water or alcohol, to prepare a mixed solution. The prepared mixed solution is then applied to an inner surface of a crucible or an outer surface of a seed. Then, the crucible supplied with the mixed solution is heat treated under an atmosphere of nitrogen (N 2 ) gas, argon (Ar) gas or O 2 gas at a temperature of 300 to 1200°C. On the other hand, the seed supplied with the mixed solution is heat treated under an atmosphere of N 2 gas or Ar gas at a temperature of 300 to 600°C. Consequently, a film consisting of BN particles and B 2 O 3 is formed on the inner surface of the crucible or the outer surface of the seed. (Method 3) First, BN powder is mixed with boron oxide (B 2 O 3 ) powder, and this mixture is further mixed with a solvent such as water or alcohol, to prepare a mixed solution. In such preparation of the mixed solution, the B 2 O 3 powder may be previously dissolved in the solvent such as water or alcohol, to be then mixed with the BN powder. The prepared mixed solution is then applied to an inner surface of a crucible or an outer surface of a seed. Then, the crucible supplied with the mixed solution is heat treated under an atmosphere of N 2 gas, Ar gas or O 2 gas at a temperature of 300 to 1200°C. On the other hand, the seed supplied with the mixed solution is heat treated under an atmosphere of N 2 gas or Ar gas at a temperature of 300 to 600°C. Consequently, a film consisting of BN particles and B 2 O 3 is formed on the inner surface of the crucible or the outer surface of the seed. (b) Effect This method is effective particularly in preparation of a semi-insulating crystal since there is no such apprehension that Si is mixed into the raw material melt, as compared with the case of employing SiO 2 as the vitreous substance. 2) In Case of Employing Mixture of B 2 O 3 and SiO 2 as Vitreous Substance (a) Methods of Film Formation (Method 1) First, BN powder is mixed with a silanol compound, to prepare a mixed solution. The silanol compound can be prepared from the aforementioned OCD, for example. Then, the prepared mixed solution is applied to an inner surface of a crucible. Then, the crucible which is supplied with the mixed solution is oxidized. Due to such oxidation, the BN powder contained in the mixed solution is oxidized to be partially converted to B 2 O 3 , while silanol is converted to silicon oxide (SiO 2 ). Consequently, a film consisting of BN particles and a mixture of B 2 O 3 and SiO 2 is formed on the inner surface of the crucible. (Method 2) First, BN powder, B 2 O 3 powder and a silanol compound are mixed with each other, to prepare a mixed solution. The silanol compound can be prepared from the aforementioned OCD, for example. On the other hand, the B 2 O 3 powder may be replaced with boric acid (H 3 BO 3 ) powder. Then, the prepared mixed solution is applied to an inner surface of a crucible or an outer surface of a seed. Then, the crucible which is supplied with the mixed solution is heat treated under an atmosphere of nitrogen (N 2 ) gas, argon (Ar) gas or oxygen (O 2 ) gas at a temperature of 300 to 1200°C. On the other hand, the seed supplied with the mixed solution is heat treated under an atmosphere of N 2 gas or Ar gas at a temperature of 300 to 600°C. Consequently, a film consisting of BN particles and a mixture of B 2 O 3 and SiO 2 is formed on the inner surface of the crucible or the outer surface of the seed. (Method 3) First, BN powder, SiO 2 powder and B 2 O 3 powder are mixed with each other, and this mixture is further mixed with a solvent such as water or alcohol, to prepare a mixed solution. The B 2 O 3 powder may be replaced with boric acid (H 3 BO 3 ) powder. Then, the prepared mixed solution is applied to an inner surface of a crucible or an outer surface of a seed. Then, the crucible which is supplied with the mixed solution is heat treated under an atmosphere of N 2 gas, Ar gas or O 2 gas at a temperature of 300 to 1200°C. On the other hand, the seed supplied with the mixed solution is heat treated under an atmosphere of N 2 gas or Ar gas at a temperature of 300 to 600°C. Consequently, a film consisting of BN particles and a mixture of B 2 O 3 and SiO 2 is formed on the inner surface of the crucible or the outer surface of the seed. (Method 4) First, BN powder and SiO 2 powder are mixed with each other, and this mixture is further mixed with a solvent such as water or alcohol, to prepare a mixed solution. Then, the prepared mixed solution is applied to an inner surface of a crucible. Then, the crucible which is supplied with the mixed solution is oxidized. Consequently, a film consisting of BN particles and a mixture of B 2 O 3 and SiO 2 is formed on the inner surface of the crucible. (b) Effect While a sufficient binder effect is attained with only B 2 O 3 employed as the vitreous substance contained in the film, B 2 O 3 is readily influenced by moisture absorption, as follows: Namely, B 2 O 3 is converted by moisture absorption to boric acid (H 3 BO 3 ), which has no adhesion and is scattered by heating. When B 2 O 3 is converted to H 3 BO 3 , therefore, adhesion of the film is reduced and the vitreous substance contained in the film for serving as a binder runs short due to scattering caused by heating for melting the raw material, to separate the film. This may consequently result in crystal defects such as generation of twins or microtwins or polycrystallization. This apprehension can be eliminated by employing a mixture prepared by adding SiO 2 to B 2 O 3 as the vitreous substance. In preparation of a GaAs single crystal, for example, the concentration of added SiO 2 is preferably 0.1 to 80 mol.%. The softening temperatures of B 2 O 3 and SiO 2 are 300 to 350°C and around 1500°C respectively, and hence the softening temperature of the mixture thereof exceeds that of GaAs if the SiO 2 concentration exceeds 80 mol.%. If the SiO 2 concentration is smaller than 0.1 mol.%, on the other hand, B 2 O 3 is influenced by moisture absorption. Further, the concentration of added SiO 2 is preferably 1 to 70 mol.%, and more preferably 5 to 60 mol.%. In preparation of a GaP single crystal having a higher melting point than GaAs, on the other hand, the concentration of added SiO 2 is 0.1 to 90 mol.%, preferably 1 to 80 mol.%, and more preferably 5 to 70 mol.%. In preparation of an InP single crystal having a lower melting point than GaAs, the concentration of added SiO 2 is 0.1 to 65 mol.%, preferably 1 to 55 mol.%, and more preferably 5 to 45 mol.%. Advantages of the mixture of B 2 O 3 and SiO 2 for serving as the vitreous substance are now described with reference to high and low SiO 2 concentrations. When the SiO 2 concentration is low, i.e., when the B 2 O 3 concentration is high in the mixture of B 2 O 3 and SiO 2 , the raw material is hardly contaminated with Si, and hence this mixture is preferably applied to preparation of a semi-insulating crystal. When the SiO 2 concentration is high, i.e., when the B 2 O 3 concentration is low in the mixture of B 2 O 3 and SiO 2 , on the other hand, replacement reaction between the dopant of Si and boron (B) forming B 2 O 3 is so hardly caused that the concentration of the dopant of Si is less reduced and the vitreous substance is less contaminated with boron (B). Thus, this mixture is preferably applied to preparation of an Si-doped crystal. When the SiO 2 concentration is increased, further, conversion of B 2 O 3 to boric acid (H 3 BO 3 ) resulting from moisture absorption is further effectively prevented, whereby the effect of the binder is sufficiently provided to improve adhesion of the film, while no H 3 BO 3 is scattered in heating and hence no shortage of the vitreous substance for serving as a binder in the film is caused. (3) Raw Material Charging A crucible of pBN which was subjected to the aforementioned pretreatment was charged with 5 kg of GaAs polycrystalline raw material synthesized by a boat method (HB method). A dopant was prepared from 110 mg of a high purity silicon (Si) wafer. In order to keep the interior of a quartz ampoule at an equilibrium vapor pressure (1 atm.), it is preferable to add 1 g of high-purity arsenic (As) in the crucible along with the GaAs polycrystalline raw material. (4) Vacuum Sealing Fig. 10 is a sectional view showing a state of quartz ampoule sealing. A crucible 1 which was charged with polycrystalline raw material 12 in the aforementioned manner was arranged in a quartz ampoule 50, which was then covered with a quartz cap 51, evacuated and vacuum-sealed through a burner 52. The crucible 1 was charged with the GaAs polycrystalline raw material 12 and an Si wafer 22 serving as a dopant, which was stored in a depression formed by the polycrystalline raw material 12. (5) Crystal Growth GaAs crystal growth was made under ordinary conditions. The VB method is adapted to reduce the temperature from a seed crystal side by upwardly moving a heater or downwardly moving an ampoule, thereby growing a single crystal. On the other hand, the VGF method is adapted to change the temperature profile of a heater while fixing the positional relation between the heater and an ampoule, thereby reducing the temperature from a seed crystal side and growing a single crystal. Exemplary growth conditions for the VB method are now described. Figs. 11 and 12 show the dimensions of a crucible and a quartz ampoule as employed respectively. First, the heater temperature was increased or the ampoule was moved toward a higher temperature side at a speed of 3 to 5°C/min., to melt the polycrystalline raw material. Then, the heater temperature was increased at a speed of 5 to 10°C/h., for partially melting the seed crystal thereby performing seeding. Then, VB growth was made under the following conditions: temperature of high temperature zone: 1245 to 1265°C temperature of low temperature zone: 1000 to 1150°C temperature gradient: 5 to 10°C/cm heater moving speed: 2 to 7 mm/h. heater moving distance: around 300 mm After the VB growth, the heater movement was stopped and cooling was made at 5 to 180°C/h. The cooling can be made from the temperature upon completion of the growth to the room temperature at a speed of 180°C/h., for example. In order to suppress generation of crystal defects, the cooling can alternatively be made from the temperature upon completion of the growth to 1000°C at a speed of 10°C/h., from 1000°C to 800°C at a speed of 30°C/h., and from 600°C to the room temperature at a speed of 100°C/h. Table 7 shows the characteristics of the crystal grown in the aforementioned manner. EPD (Etch Pit Density) (cm -2 ) Si Concentration (=Carrier Concentration) (cm -3 ) Boron Concentration (cm -3 ) Shoulder Portion 3,000-4,000 2 × 10 17 5 × 10 15 - 5 × 10 16 Middle Portion 2,000-3,000 4 - 5 × 10 17 5 × 10 15 - 5 × 10 16 Tail Portion 500 - 1,000 1.5 - 2 × 10 18 1 × 10 16 - 1 × 10 17 As to the shoulder, middle and tail portions shown in Table 7, refer to Fig. 15. (6) Ampoule Breaking After the crystal growth, the ampoule was broken to take out the GaAs single crystal. Table 8 shows the effects of the aforementioned Example of the present invention in comparison with those of the prior art. Referring to Table 8, "conventional example" indicates the case of employing liquid boron oxide (B 2 O 3 ), and "prior art" indicates the case of employing solid boron nitride (BN). Film Adhesion Crystallinity Deviation in Si Concentration (Si - Doped GaAs) B Concentration (In Case of Si 2 × 10 17 cm -3 ) Life of Crucible Conventional Example strong excellent large 20-30% high at least 1 × 10 18 cm -3 short (5 times) Prior Art weak easy to cause crystal defects small within 5 % low not more than 5 × 10 16 cm -3 long (20 - 30 times) Example of This Invention Strong excellent small within 5% low not more than 5 × 10 16 cm -3 long (20 - 30 times)

Claims: A method of preparing a group II-VI or III-V compound single crystal for preparing a single-crystalline group II-VI or III-V compound from a polycrystalline group II-VI or III-V compound, comprising the steps of: coating an inner surface of a crucible (1) with a film (15) consisting of powdered solids (3) having a melting point being higher than that of said polycrystalline compound and a vitreous substance (13); placing said polycrystalline compound (12) in said coated crucible (1); placing said coated crucible (1) being provided therein with said polycrystalline compound (12) in heating means; heating said crucible (1) being arranged in said heating means with said heating means thereby melting said polycrystalline compound (12) in said crucible (1); and cooling said crucible (1) and said melted compound (12) for growing a single-crystalline compound. A method of preparing a group II-VI or III-V compound single crystal according to claim 1, further comprising the steps of: placing a seed crystal (7) consisting of said single-crystalline compound on a bottom portion of said coated crucible (1); placing said polycrystalline compound (12) in the remaining portion of said crucible (1); placing said crucible (1) being provided therein with said seed crystal (7) and said polycrystalline compound (12) in a furnace, said furnace being vertically arranged and being capable of forming an upper high temperature zone and a lower low temperature zone; adjusting the position of said furnace and the temperature of said upper high temperature zone thereby melting said polycrystalline compound (12) while maintaining a part of said seed crystal (7) in a solid state in said crucible (1); setting said lower low temperature zone at a temperature being lower than the melting point of said single-crystalline compound while setting said upper high temperature zone at a temperature being higher than said melting point thereby forming a solid-liquid interface; and vertically moving said solid-liquid interface from said seed crystal by upwardly moving said furnace or downwardly moving said crucible while substantially retaining said temperature setting in said lower low temperature zone and said upper high temperature zone thereby growing said single-crystalline compound. A method of preparing a group II-VI or III-V compound single crystal according to claim 1, further comprising the steps of: coating an outer surface of a seed crystal (7) consisting of said single-crystalline compound with a film (75) consisting of powdered solids (3) having a melting point being higher than that of said polycrystalline compound and a vitreous substance (13); placing said coated seed crystal (7) in a portion of said coated crucible (1); placing said polycrystalline compound (12) in the remaining portion of said crucible (1); placing said crucible (1) being provided therein with said seed crystal (7) and said polycrystalline compound (12) in said heating means; adjusting the position of said heating means and the temperature thereby melting said polycrystalline compound (12) while maintaining a part of said seed crystal (7) in a solid state in said crucible (1); cooling said crucible (1) and said melted compound (12) for growing said single-crystalline compound. A method of preparing a group II-VI or III-V compound single crystal according to claim 2 or 3; further comprising the steps of: coating an outer surface of a seed crystal (7) consisting of said single-crystalline compound with a film (75) consisting of powdered solids (3) having a melting point being higher than that of said polycrystalline compound and a vitreous substance (13); placing said coated seed crystal (7) on a bottom portion of said coated crucible (1); placing said polycrystalline compound (12) in the remaining portion of said crucible (1); placing said crucible (1) being provided therein with said seed crystal (7) and said polycrystalline compound (12) in a vertically arranged furnace being capable of forming an upper high temperature zone and a lower low temperature zone; adjusting the position of said furnace and the temperature of said upper high temperature zone thereby melting said polycrystalline compound (12) while maintaining a part of said seed crystal (7) in a solid state in said crucible (1); setting said lower low temperature zone at a temperature being lower than the melting point of said single-crystalline compound while setting said upper high temperature zone at a temperature being higher than said melting point thereby forming a solid-liquid interface; and vertically moving said solid-liquid interface from said seed crystal by upwardly moving said furnace or downwardly moving said crucible while substantially retaining said temperature setting in said lower low temperature zone and said upper high temperature zone thereby growing said single-crystalline compound. The method of preparing a group II-VI or III-V compound single crystal in accordance with claim 4, further comprising a step of coating a gap between said crucible (1) and said seed crystal (7) with a film consisting of powdered solids (3) and a vitreous substance (13) after placing said seed crystal on said bottom portion of said crucible. The method of preparing a group II-VI or III-V compound single crystal in accordance with one of the claims 1 to 5, wherein said vitreous substance (13) contains silicon oxide. The method of preparing a group II-VI or III-V compound single crystal in accordance with claim 1 or 6, wherein said step of coating said inner surface of said crucible (1) or said outer surface of said seed crystal (7) with said film (75) consisting of said powdered solids (3) and said vitreous substance (13) comprises the steps of: applying a mixed solution (5) containing said powdered solids (3) and a silanol compound to said inner surface of said crucible (1) or said outer surface of said seed crystal (7), and heating said crucible (1) or said seed crystal (7) being supplied with said mixed solution (5) thereby forming said film (15, 25) containing said powdered solids (3) and said vitreous substance (13) consisting of silicon oxide on said inner surface of said crucible (1) or said outer surface of said seed crystal (7). The method of preparing a group II-VI or III-V compound single crystal in accordance with any of claims 1 to 5, wherein said vitreous substance (13) has a softening temperature being lower than the melting point of said polycrystalline compound (12). The method of preparing a group II-VI or III-V compound single crystal in accordance with claim 8, wherein said vitreous substance (13) contains B 2 O 3 . The method of preparing a group II-VI or III-V compound in accordance with claim 8, wherein said vitreous substance (13) contains a mixture of B 2 O 3 and SiO 2 . The method of preparing a group II-VI or III-V compound single crystal in accordance with any of claims 1 to 10, wherein said powdered solids (3) contain boron nitride. The method of preparing a group II-VI or III-V compound single crystal in accordance with claim 11, wherein said boron nitride is at least 0.05 µm and not more than 10 µm in mean particle size. The method of preparing a group II-VI or III-V compound single crystal in accordance with any of claims 1 to 12, wherein said inner surface of said crucible has a prescribed surface roughness in an uncoated state, said prescribed surface roughness being 10 to 150 µm in maximum height R y , and 3 to 15 µm in arithmetical mean roughness R a . The method of preparing a group II-VI or III-V compound single crystal in accordance with any of claims 1 to 13, wherein said coumpound includes gallium arsenide.

Inventors: KAWASE TOMOHIRO, TATSUMI MASAMI, WAKAYAMA YOSHIHIRO, KAWASE, TOMOHIRO, TATSUMI, MASAMI, WAKAYAMA, YOSHIHIRO, Kawase, Tomohiro, Sumitomo Elec. Ind., Ltd., Tatsumi, Masami, Sumitomo Elec. Ind., Ltd., Wakayama, Yoshihiro, Sumitomo Elec. Ind., Ltd.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1996
Main classification: C30B11
IPC codes: C30B11