METHOD OF MANUFACTURING CUBIC BORON NITRIDE P-N JUNCTION BODY

Abstract: In a method of manufacturing a cubic boron nitride p-n junction body, cBN seed crystals (1) of a first conductivity type are made coexist with original BN (3) and a solvent (2) containing a doping material of a second conductivity type, to grow cBN of the second conductivity type from the seed crystals (1). The solvent (2) contains 0.1 to 5 percent by weight of the doping material. The seed crystals (1) are disposed one by one in a plurality of blocks of the solvent (2), which blocks are separated from each other, to be in contact with nothing but the solvent (2). Due to such disposal, no spontaneous nucleation is caused in portions other than the seed crystals (1). A large-grained cBN p-n junction body is obtained as the result.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of manufacturing a p-n junction body of cubic boron nitride (hereinafter referred to as cBN), which is effectively applied to a diode, for example. Description of the Background Art cBN, which is synthesized under superhigh pressure, has a wide band gap of 6.6 eV, and is stable under a temperature up to 900°C in the atmosphere. Further, a p-type or n-type semiconductor material can be obtained by doping cBN with an appropriate substance. Therefore, a cBN p-n junction body prepared by joining p-type cBN with n-type cBN has great expertations as a semiconductor device which can stably operate under a high temperature. United States Patent No. 3,078,232 (corresponding to the Japanese Patent Publication Gazette No. 7309/1963) discloses a p-type cBN in which Be is doped. United States Patent No. 3,141,802 (corresponding to the Japanese Patent Publication Gazette No. 24856/1963) discloses an n-type cBN in which Si and/or Ge is doped and a p-n junction cBN grain. United States Patent No. 3,216,942 (corresponding to the Japanese Patent Publication Gazette No. 24857/1963) discloses an n-type cBN in which S and/or Se is doped and a p-n junction cBN grain. A conventional method of manufacturing a cBN p-n junction body is carried out as follows: (1) Be is added to a mixture of boron nitride (hereinafter referred to as BN) source and a solvent material. Then the mixture is exposed to superhigh pressure/temperature enabling formation of cBN, thereby to synthesize cBN having a p-type semiconductor property, the crystal lattice of which is partially replaced by Be. (2) At least one of Si, Ge, S and Se is added to a mixture of a solvent material and BN. The p-type cBN obtained in the item (1) is so disposed and exposed to superhigh pressure/temperature as to grow cBN having an n-type semiconductor property, the crystal lattice of which is partially replaced by Si, for example, on the p-type CBN. Thus, a cBN p-n junction body is obtained. A new method of manufacturing a larger cBN junction body is proposed on page 184-185 of "Program and Abstracts" of The 28th High Pressure Conference of Japan held at Kobe on Nov. 4-6, 1987. In this method, the so-called temperature gradient method is employed to grow a larger cBN crystal on the seed crystal by providing the seed crystal at the interface between BN source and a solvent material with a temperature relatively higher than that of a seed crystal part. In this case, seed crystals are prepared from p-type cBN and Si is added to the solvent, thereby to manufacture a cBN p-n junction body. The cBN p-n junction body obtained in this method is larger in size and higher in quality than that obtained in the aforementioned method, since spontaneous nucleation and the growth rate of cBN can be controlled by adjusting the temperature gradient. In the aforementioned method disclosed in United States Patent Nos. 3,141,802 and 3,216,942, a large amount of cBN p-n junction bodies can be obtained at a low cost. However, each cBN p-n junction body thus obtained has extremely small size of about 300 µm, to cause difficulty in handling in later steps. The size of the junction body is thus limited to about 300 µm, since formation/growth of cBN is unavoidably caused by spontaneous nucleation from portions other than the seed crystals, to restrict spaces allowing growth of cBN from the seed crystals. On the other hand, a cBN p-n junction body having size of 2 or 3 millimeters can be obtained in the method employing the temperature gradient method. However, this method requires the extreme long time of tens of hours, whereby the cost is inevitably increased. SUMMARY OF THE INVENTION An object of the present invention is to solve the aforementioned problems and provide a method of manufacturing a large amount of large-grained cBN p-n junction bodies at a low cost. According to the inventive method of manufacturing a cBN p-n junction body, cBN seed crystals having a certain conductivity type semiconductor property are made to coexist with BN source and a solvent material containing a doping material causing a reverse conductivity type and capable of dissolving BN, to grow cBN having a reverse conductivity type semiconductor property on the seed crystals under a thermodynamically stable condition of cBN at a temperature exceeding the melting point of the solvent material. According to the present invention, the solvent material contains 0.1 to 5 percent by weight of the doping material. The seed crystals are disposed one by one in a plurality of blocks of the solvent, which blocks are separated from each other, to be in contact with nothing but the solvent material. Thus, no spontaneous nucleation is caused in portions other than the seed crystals. As the result, a large-grained cBN p-n junction body can be obtained. According to the present invention, further, the cBN p-n junction body can be manufactured in an extremely short time as compared with the conventional method employing the temperature gradient method. These 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 is a longitudinal sectional view showing exemplary structure of the present invention; Fig. 2 is a sectional view taken along the line II - II in Fig. 1. Fig. 3 is a longitudinal sectional view showing specific structure of the present invention; and Fig. 4 is a sectional view taken along the line IV - IV in Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors have studied the cause for spontaneous nucleation and have come to the conclusion that solvent parts unnecessary for growth of cBN on the seed crystals cause spontaneous nucleation in the conventional method. In order to prevent such spontaneous nucleation, therefore, the solvent parts unnecessary for growth on the seed crystals may be removed. According to the present invention, seed crystals are disposed one by one in a Plurality of blocks of the solvent, which blocks are separated from each other, to be in contact with nothing but the solvent material. Figs. 1 and 2 show exemplary structure of the present invention, which is based on the aforementioned conclusion. Referring to Figs. 1 and 2, each seed crystal 1 of cBN having a certain conductivity type semiconductor property is enclosed by a block 2 of a solvent, which contains a doping material causing a reverse conductivity type and is capable of dissolving BN source. A plurality of such solvent blocks 2 are distributed/disposed in a BN source member 3. The solvent contains 0.1 to 5 percent by weight of the doping material, which causes the conductivity type reverse to that of the seed crystals 1. If the seed crystals 1 have a p-type semiconductor property, the doping material is prepared from at least one element selected from a group of Si, Ge, S and Se, in order to grow n-type cBN. If the seed crystals 1 are of n-type, the doping material is prepared from Be, for example, in order to grow p-type cBN. The BN source member employed in the present invention, such as that of hexagonal BN (hereinafter referred to as hBN) or amorphous BN, is not particularly restrictive to the crystal structure. However, a hot-pressed hBN sintered body is preferable since it is easy to handle. The solvent material is preferably prepared from boronitride rf an alkaline metal or an alkaline earth metal such as Li₃BN₂ or Ca₃B₂N₄, due to its high solvent-ability. When the diameter of solvent material is 1 to 2 mm, a more preferable result is obtained. The p-type or n-type cBN seed crystals employed in the present invention can be easily synthesized by " exposing a mixture of a solvent material containing doping material and BN source to superhigh pressure/temperature. The detail of such a method is disclosed in United States Patent Nos. 3,078,232, 3,141,802 and 3,216,942, for example. It is possible to manufacture a p-n-p type or n-p-n type cBN junction by further repeating the joining method for the cBN p-n junction body according to the present invention. Examples of the present invention are now described in detail. Example 1 Be was added to a mixture of BN source and a solvent material, and the mixture was exposed to superhigh pressure/temperature enabling formation of cBN. The BN source was prepared from hBN powder, while the solvent material was prepared from Li₃BN₂ and the doping material was prepared from Be(NO₃)₂ powder. The ratio of hBN to the solvent material was 200 percent by weight, and the ratio of Be(NO₃)₂ to the solvent material was 10 percent by weight (0.68 percent by weight in Be conversion). The mixture was held under pressure of 50 kbar (kb) at a temperature of 1600°C for one hour, to obtain a large amount of p-type cBN crystals of 300 to 500 µm. A cBN p-n junction body was prepared from the p-type cBN seed crystals obtained in the aforementioned manner, through sample chamber structure shown in Figs. 3 and 4. In order to obtain the sample chamber structure shown in Figs. 3 and 4, a second BN plate 10b provided with many holes 11 is placed on a first BN plate 10a without any hole. Half part 12a of a solvent material, which contains a doping material capable of causing a certain conductivity type, is then introduced into the holes 11. Then, cBN seed crystal 13, which is reverse in conductivity type to the doping material, and remaining half part 12b of the solvent material is introduced in this order. Thereafter another first BN plate 10a is placed thereon. Thus, the sample chamber structure shown in Figs. 3 and 4 is obtained. BN source was prepared by deoxidizing a hot-pressed hBN product in N₂ at 2050°C. The solvent material containing a doping material was prepared by a pressed body of 1.5 mm in diameter and 2 mm in thickness, which was formed of Li₃BN₂ powder containing 3 percent by weight of Si powder. The structure was held under pressure of 50 kb at a temperature of 1650°C for three hours, whereby most of the seed crystals caused growth of cBN, 40 percent of which reached #20 US mesh size (840 µm). The cBN p-n junction body thus obtained was subjected to voltage of 5 V under the room temperature, to present a rectifying property causing a flow of current of at least five times that in the reverse direction. Example 2 Seed crystals were prepared from n-type cBN, which was synthesized in a similar manner to Example 1 except for that the solvent material contained 3 percent by weight of S powder as a doping material. A large amount of n-type cBN crystals of 350 to 500 µm were obtained by holding under pressure of 50 kb at a temperature of 1600°C for one hour. The n-type cBN crystals thus obtained were employed as seed crystals, to prepare a cBN p-n junction body through the sample chamber structure shown in Figs. 3 and 4. BN source was prepared by deoxidizing a hot-pressed hBN product, similarly to Example 1. The solvent material containing a doping material was prepared by a pressed body of 2 mm in diameter and 3 mm in thickness, which was formed of LiSrBN₂ powder containing 1 percent by weight of Be powder. The structure was held under pressure of 52 kbar (kb) at a temperature of 1650°C for six hours, whereby seed crystals of 30 percent were grown to #16 US mesh size (1190 µm). The cBN p-n junction body thus obtained was subjected to voltage of 5 V under the room temperature, to present a rectifying property causing a flow of current of at least five times that in the reverse direction. According to the present method of manufacturing a cBN p-n junction body, the seed crystals are disposed one by one in a plurality of solvent blocks, which are separated from each other, to be in contact with nothing but the solvent material. Thus, a large amount of large-grained cBN p-n junction bodies can be prepared at a low cost.

Claims: 1. A method of manufacturing a cubic boron nitride p-n junction body, comprising the steps of: providing a boron nitride source member (3), providing cubic boron nitride seed crystals (1) having a certain conductivity type semiconductor property, providing a plurality of blocks of a solvent material (2) being capable of dissolving boron nitride and containing 0.1 to 0.5 percent by weight doping material of a reverse conductivity type to said seed crystals (1), enclosing each of said seed crystals (1) by a block of solvent material (2) so that each crystal is in contact with said solvent material (2) only, disposing said blocks containing said seed crystals (1) into said boron nitride source member (3) so that said blocks are separated from each other by said source member (3), and growing cubic boron nitride having a reverse conductivity type semiconductor property on said seed crystals (1) under a thermodynamically stable condition for cubic boron nitride at a temperature exceeding the melting point of said solvent material (2). 2. A method of manufacturing a cubic boron nitride p-n junction body in accordance with claim 1, wherein said seed crystals (1) have a p-type semiconductor property, and said doping material is prepared from at least one or two substances selected from a group including Si, Ge, S and Se. 3. A method of manufacturing a cubic boron nitride p-n junction body in accordance with claim 1, wherein said seed crystals (1) have an n-type semiconductor property, and said doping material is prepared from Be. 4. A method of manufacturing a cubic boron nitride p-n junction body in accordance with claim 1, wherein said boron nitride source (3) is prepared by a hexagonal boron nitride sintered body. 5. A method of manufacturing a cubic boron nitride p-n junction body in accordance with claim 1, wherein said solvent material (2) is boronitride of an alkaline metal or that of an alkaline earth metal. 6. A method of manufacturing a cubic boron nitride p-n junction body in accordance with claim 5, wherein said solvent (2) is Li₃NB₂ or Ca₃B₂N₄. 7. A method of manufacturing a cubic boron nitride p-n junction body in accordance with claim 1, wherein each block of said solvent (2) is within a range of 1 to 2 mm in diameter

Inventors: DEGAWA, JUNJI, TSUJI, KAZUWO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1989
Main classification: H01L29
IPC codes: C30B29, B01J3, H01L29