Abstract:
A method is provided for preparing, with high reproducibility, a carbon-doped group III-V compound semiconductor crystal having favorable electrical characteristics and having impurities removed therefrom, and in which the amount of doped carbon can be adjusted easily during crystal growth. This method includes the steps of: filling a crucible with compound raw material, solid carbon, and boron oxide; sealing the filled crucible gas impermeable material; heating and melting the compound raw material under the sealed state in the airtight vessel; and solidifying the melted compound raw material to grow a carbon-doped compound semiconductor crystal.

Description:
Notice: More than one reissue application has been filed for the reissue of U.S. Pat No.  6 , 007 , 622 . The reissue applications include application Ser. Nos.  11 / 187 , 036  ( the present application )  and  09 / 824 , 965 . The present application is a continuation of application Ser. No.  09 / 824 , 965  filed Apr.  3 ,  2001   ( now U.S. Pat. No. RE 39 , 778   ).  
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of preparing a group III-V compound semiconductor crystal. Particularly, the present invention relates to a method of preparing a group III-V compound semiconductor crystal in which carbon is doped. 
     2. Description of the Background Art 
     Conventionally, there are various prior arts as set forth in the following regarding the method of preparing a group III-V compound semiconductor crystal in which carbon is doped. 
     In Japanese Patent Laying-Open No. 64-79087 (referred to as “prior art 1” hereinafter), a method of preparing a carbon-doped GaAs single crystal according to the gradient freeze method or horizontal Bridgman method (HB method) is disclosed. 
       FIG. 6  is a diagram for describing a method of preparing a carbon-doped GaAs single crystal according to prior art 1. 
     Referring to  FIG. 6 , a graphite boat  51  as a carbon source is arranged at one side in a quartz ampoule  55 . Raw material which is gallium (Ga)  52  is provided in graphite boat  51 . Arsenic (As)  57  is provided at the other side in quartz ampoule  55 . Quartz ampoule  55  is sealed in vacuum and then installed in an electric furnace to be heated. After the GaAs raw material is synthesized, the temperature is reduced maintaining a constant temperature gradient, whereby a GaAs single crystal is grown. 
     The carbon of graphite boat  51  reacts with oxygen supplied from As 2 O 3 , Ga 2 O and the like remaining in quartz ampoule  55  to result in gas of CO, CO 2  and the like to be doped into the growing GaAs crystal. 
     It is described that the doping amount of carbon can be controlled according to the total amount of oxygen in the sealed quartz ampoule  55 , the synthesization reaction condition, or single crystal growth condition, and the like. 
     In Journal of the Japanese Association of Crystal Growth, 1991, Vol. 18, No. 4, pp. 88-95 (referred to as “prior art 2” hereinafter), a method of preparing a carbon-doped GaAs single crystal by the vertical gradient freeze method (VGF method) is disclosed. 
       FIG. 7  is a diagram for describing a method of preparing a carbon-doped GaAs single crystal according to prior art 2. 
     Referring to  FIG. 7 , raw material  62  having carbon doped in advance, directly synthesized by the LEC method and boron oxide (B 2 O 3 )  64  are provided in a crucible  61  and sealed in vacuum in a quartz ampoult  65 . This is installed in a vertical furance and heated to melt the raw material and boron oxide. By reducing the temperature in the furnace while maintaining a constant temperature gradient, a GaAs single crystal is grown. 
     Here, boron oxide containing water of 200 ppm spreads around only the periphery of the upper surface of GaAs melt  62 . The center area of the upper surface of GaAs melt  62  is exposed to the ambient. According to the method of prior art 2, the upper surface of the melt must be exposed to the ambient to control the stoichiometry of the GaAs melt. The vapor pressure in quartz ampoule  65  is controlled by arsenic  67 . 
     According to this method, the carbon concentration of the crystal depends on the carbon concentration of the raw material. 
     In U.S. Pat. No. 4,999,082 (referred to as “prior art 3” hereinafter), a method of preparing carbon-doped GaAs single crystal by the vertical Bridgman method (VB method) is disclosed. 
       FIG. 8  is a diagram for describing a method of preparing carbon-doped GaAs single crystal according to prior art 3. 
     Referring to  FIG. 8 , a crucible  71  is filled with GaAs raw material  72 . After carbon source  73  is arranged outside of crucible  71 , a quartz ampoule  75  is sealed. Quartz ampoule  75  is placed in a vertical furnace and heated to melt the raw material. The furnace is moved upwards while substantially maintaining the set temperature profile. By solidifying the raw material from a seed crystal  77 , a GaAs single crystal is grown. 
     According to this method, carbon source  73  is in fluid communication with compound raw material  72  to allow gas transfer. 
     Japanese Patent Laying-Open No. 3-252399 (referred to as “prior art 4” hereinafter) discloses a method of preparing a semi-insulating GaAs substrate. 
     Prior art 4 is characterized in that the impurity which becomes the acceptor is doped so as to result in 1˜3×10 15  atoms/cm 3  after subtracting the concentration of the impurity which becomes the donor in a GaAs crystal. 
     Japanese Patent Laying-Open No. 2-74597 (referred to as “prior art 5” hereinafter) discloses a chromium-doped semi-insulating GaAs single crystal and a method of preparing thereof. This prior art 5 is characterized in that carbon is contained having a concentration n c  that satisfies both the relations of:
 
1×10 15 cm −3 ≦n c &lt;n si  and n si −n c ≦4.4×10 15 cm −3  
 
for the residual Si concentration of n si  remaining in the single crystal, with the resistivity of at least 10 6 Ω·cm.
 
     The above-described prior art have various disadvantages. In prior art 1, boron oxide is not used. Therefore, impurity contamination can be expected. Furthermore, since the amount of the carbon source cannot be controlled in this method, it is difficult to control the carbon concentration. 
     In prior art 2, carbon cannot be doped during the crystal growth since carbon source is not used. There is a problem that the carbon concentration cannot be adjusted during crystal preparation. Furthermore, a part of the carbon in the GaAs melt reacts with oxygen, that is generated as a result of the water in the boron oxide decomposing, to be lost as CO gas. There was a problem that the carbon concentration in the GaAs crystal is lowered. 
     In prior art 3, it is difficult to control the carbon concentration since the carbon source is located outside the crucible. Furthermore, impurity contamination can be expected since boron oxide is not used. 
     In prior art 4, carbon is recited as the impurity serving as the acceptor. However, only the doping of zinc and copper is disclosed as the example. There is no description of carbon doping. 
     Prior art 5 describes a chromium-doped semi-insulating GaAs single crystal containing carbon. However, this prior art 5 is silent about the method of doping carbon. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of the present invention is to provide a method of preparing in high reproducibility a group III-V compound semiconductor crystal of favorable electrical characteristics having impurities removed, and in which the amount of doped carbon can easily be adjusted during crystal growth. 
     According to an aspect of the present invention, a method of preparing a group III-V compound semiconductor crystal is provided. This method of preparing a group III-V compound semiconductor crystal having carbon doped includes the steps of: filling a crucible or boat with compound raw material, solid carbon, and boron oxide; sealing the crucible or boat filled with compound raw material, solid carbon, and boron oxide in an airtight vessel formed of a gas impermeable material; heating and melting the compound material in a sealed state in the airtight vessel; and solidifying the melted compound material to grow a carbon-doped compound semiconductor crystal. 
     Since the crucible or boat is filled with compound raw material, solid carbon, and boron oxide according to the present invention, the boron oxide softened by heating is brought into contact with at least a portion of the solid carbon is the state where the compound raw material is melted. 
     According to the present invention, the carbon concentration in the raw material does not have to be adjusted since carbon can be doped during crystal growth. Good controllability of the carbon concentration is obtained. In other words, the target carbon concentration can be obtained in high reproducibility. By using boron oxide which has an impurity removal effect, the contamination of impurities in the crystal can be suppressed to obtain a crystal of favorable electrical characteristics. 
     Quartz or pBN (pyrolytic boron nitride) and the like can be enumerated as the gas impermeable material. 
     Preferably, boron oxide contains water. 
     This is because the water in boron oxide is essential to remove impurities. Furthermore, it is considered that the water in the boron oxide effects the incorporation of carbon into the crystal. 
     Boron oxide preferably contains water of 10-500 wt ppm. 
     In the present invention, the amount of solid carbon to be filled is preferably larger than the amount of carbon doped into the compound semiconductor crystal. 
     This is to promote reaction using an excessive amount of carbon since the reaction rate of solid carbon is extremely low. Furthermore, consumption of the part of the solid carbon at the gas generation of the carbon compound must be supplied. Thus, by using solid carbon of an amount larger than the total amount of carbon doped into the crystal, the advantage of the present invention works effectively. 
     Specifically, the amount of solid carbon must be at least ten times, preferably at least 100 times larger than the weight of the carbon doped into the crystal. 
     In the present invention, it is preferred that the solid carbon is subjected to a heat treatment under reduced pressure before being filled in the crucible or boat. 
     By this process, the impurity element remaining in carbon is removed to result in a crystal of higher purity. 
     The pressure in applying a heat treatment on carbon is preferably from 1 Torr to 1×10 −8 Torr. The appropriate temperature of the heat treatment is 500° C.-2000° C. The above-described effect can be obtained by carrying out the heat treatment for at least one hour. It was found that a greater effect can be obtained as the time for the heat treatment becomes longer. However, there is very little change in the effect when the time for the heat treatment exceeds 12 hours. Considering that the cost for production is increased as the time for the heat treatment becomes longer, the time period for the heat treatment of not more than 12 hours is appropriate. 
     In the present invention, it is preferable to keep the compound raw material in its melted state for a certain time period before it is solidified for crystal growth. 
     By this purpose, the impurities of Si and the like in the GaAs polycrystalline raw material can be removed by gettering with boron oxide. Although Si of approximately 1×10 16 cm −3  is included as impurities in the raw material synthesized by the HB method, the amount of Si in the GaAs subjected to the above-described process is less than 1×10 15 cm −3 , which is below the detection limit of an analyzer. Si of an amount over 1×10 15 cm −3  was detected from those not subjected to the above-described process. 
     Thus, carbon can be sufficiently melted in the GaAs melt from the solid carbon by the above-described process. This process also provides the advantage that the temperature of the GaAs melt is stabilized, and the carbon concentration and impurity concentration in the melt can be made uniform. 
     The above-described effect can be obtained when the holding time period in the melted state of raw material is at least 3 hours. Further favorable characteristics can be obtained stably when the holding time is at least 6 hours. Although a greater effect can be obtained as the holding time becomes longer, the degree of change in the effect gradually becomes smaller when the holding time period exceeds 36 hours. There is very little change in the effect when the holding time exceeds 72 hours. Considering that the cost for production becomes higher as the holding time is increased, the holding time is preferably not more than 72 hours, further preferably not more than 36 hours. 
     In the present invention, powder carbon can be used as the solid carbon. 
     Powder carbon is advantageous in promoting the reaction due to its greater specific surface area. Increase in the reaction speed allows carbon to be doped efficiently in the crystal. 
     Also, the amount of carbon to be doped into the crystal can easily be adjusted according to the grain size, the weight, and the like of the used powder. For example, powder of a smaller grain size has a greater specific surface area to increase the reaction speed, whereby the amount of doped carbon is increased. 
     Therefore, the grain size of the powder carbon is preferably smaller. More specifically, the average grain size is preferably not more than 100 μm, more preferably not more than 50 μm. When powder carbon is used, the powder carbon spreads in the boron oxide softened by heating the state where the compound raw material is melted. 
     In the present invention, fiber carbon, as well as powder carbon, can be used as the solid carbon. 
     Fiber carbon is advantageous in that the diameter of the fiber is small and a greater surface area can be obtained to result in a faster reaction speed. It is therefore possible to dope carbon into the crystal efficiency. Also, the amount of carbon doped into the crystal can easily be adjusted according to the diameter or weight of the fiber that is used. Uniform distribution of the carbon concentration can be obtained from the shoulder to the tail of the prepared crystal when fiber carbon is used. 
     The diameter of the fiber carbon is preferably smaller. Specifically, the average diameter is preferably not more than 50 μm, more preferably not more than 10 μm. 
     Usage of fiber carbon allows carbon to spread in boron oxide that is softened by heating in the state where the compound raw material is melted. Also, the carbon can float above boron oxide to be exposed to the ambient. 
     In the present invention, bulk carbon can be used as solid carbon, in addition to powder carbon and fiber carbon. 
     Bulk carbon is advantageous in that the amount of carbon to be doped in the crystal can easily be adjusted by the weight and configuration of the carbon used. Uniform distribution of carbon concentration can be obtained from the shoulder to the tail of the prepared crystal when bulk carbon is used. 
     Bulk carbon is preferably used in a disk shape that is smaller than the inner diameter of the crucible. The amount of doped carbon can easily be controlled by the diameter of the disk. 
     The bulk solid carbon is preferably a sintered compact of carbon powder. The reaction speed is particularly high for the sintered compact of powder having high porosity. Sintered carbon powder is advantageous in distributing carbon uniformly in the crystal. 
     When bulk solid carbon is used, a state can be obtained in which at least a portion of the bulk solid carbon is immersed in the softened boron oxide. 
     In the present invention, the crucible or boat is preferably formed on pBN (pyrolytic boron nitride). 
     Depending upon the constituent element of the crucible or boat, there is a possibility that boron oxide or carbon reacts with the crucible to induce contamination of the raw material melt. pBN is most appropriate as the material of the crucible or boat to suppress reaction with boron oxide or carbon. 
     The present invention is particularly effective as a method of doping carbon into a GaAs crystal. 
     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  is a diagram for describing an example of a method of preparing a group III-V compound semiconductor crystal according to the present invention. 
         FIG. 2  is a diagram showing the state of carrying out crystal growth using a vertical furnace. 
         FIG. 3  is a diagram for describing another example of a method of preparing a group III-V compound semiconductor crystal according to the present invention. 
         FIG. 4  is a diagram for describing a further example of a method of preparing a group III-V compound semiconductor crystal according to the present invention. 
         FIG. 5  is a diagram for describing each portion of a crystal. 
         FIG. 6  is a diagram for describing a method of preparing a carbon-doped group III-V compound semiconductor crystal single crystal according to an example of prior art. 
         FIG. 7  is a diagram for describing a method of preparing a carbon-doped group III-V compound semiconductor crystal single crystal according to another example of prior art. 
         FIG. 8  is a diagram for describing a method of preparing a carbon-doped group III-V compound semiconductor crystal single crystal according to a further example of prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
       FIG. 1  is a diagram for describing an example of preparing a group III-V compound semiconductor crystal according to the present invention. 
     Referring to  FIG. 1 , GaAs polycrystalline raw material  2 , carbon powder  13  subjected to heat treatment under reduced pressure in advance, boron oxide (B 2 O 3 )  4 , and a seed crystal  7  were placed in a pBN crucible  1 . The seed crystal was placed at the bottom portion of the crucible  1 . In crucible  1 , arrangement was provided so that carbon powder  13  and boron oxide  4  were brought into contact with each other, and also boron oxide  4  and raw material  2  were brought into contact with each other when the raw material was melted. 
     Crucible  1  was inserted in a quartz ampoule  5  together with solid arsenic. Ampoule  5  was sealed under reduced pressure with a quart cap  6 . 
     Respective conditions of Example 1 are shown in the following Table 1. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 GaAs 
                   
               
               
                   
                 polycrystal 
               
               
                   
                 (raw material) 
                 3 kg used 
               
               
                   
                   
               
             
             
               
                   
                 Carbon powder 
                 350 mesh (grain size 45 μm and below), 
               
               
                   
                   
                 100 mg used 
               
               
                   
                   
                 Heat treatment at 1000° C. for 6 hours at 
               
               
                   
                   
                 the pressure of 10 −2  Torr 
               
               
                   
                 B 2 O 3   
                 Water concentration 50 wt ppm, 50 g used 
               
               
                   
                 pBN crucible 
                 Inner diameter 80 mm, entire length 250 min 
               
               
                   
                 Solid arsenic 
                 1 g used 
               
               
                   
                   
               
             
          
         
       
     
     Referring to  FIG. 2 , the above-described quartz ampoule  5  was heated at the rate of approximately 200° C./hour by a heater  8  using a vertical furnace  50 . 
     During this process of heating, boron oxide  4  was softened and melted. Also, GaAs polycrystalline raw material  2  was melted. 
     At this point, boron oxide  4  was present as a film  4 a having a thickness of less than 1 mm between pBN crucible  1  and GaAs raw material melt  2 . The remainder of boron oxide  4  covered the upper surface of GaAs melt  2 . The thickness of the boron oxide layer  4 b covering the upper surface of GaAs melt  2  was approximately 5 mm. Carbon powder  13  was dispersed in this boron oxide layer  4 b. 
     The condition mentioned above was kept for approximately 36 hours. 
     Then, heater  8  was moved upwards at the rate of 4 mm/hour, whereby solidification started from the portion of seed crystal  7 . Thus, a single crystal was grown. The characteristics of the obtained single crystal is shown in the following Table 2. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 Crystal diameter 
                 80 mm 
                   
                   
               
               
                   
                 Length of φ80 mm portion 
                 100 mm 
               
               
                   
                 Carbon concentration 
                 Shoulder 
                 1.4 × 10 15   
                 cm −3   
               
               
                   
                   
                 Tail 
                 0.8 × 10 15   
                 cm −3   
               
               
                   
                 Resistivity 
                 Shoulder 
                 2.9 × 10 7   
                 Ωcm 
               
               
                   
                   
                 Tail 
                 1.5 × 10 7   
                 Ωcm 
               
               
                   
                 Dislocation density 
                 Shoulder 
                 900 
                 cm −2   
               
               
                   
                   
                 Tail 
                 1200 
                 cm −2   
               
               
                   
                   
               
             
          
         
       
     
     In the present specification, the “shoulder” and “tail” of a crystal corresponds to the relevant portions shown in FIG.  5 . 
     The role of solid arsenic (As) sealed under reduced pressure in the quartz ampoule in the present example is set forth in the following. 
     The dissociation pressure at the melting point of GaAs is approximately 1 atm. When GaAs is melted, the airtight vessel is filled with As vapor of approximately 1 atm at the temperature of the melting point. This As vapor is generated as a result of the GaAs melt being decomposed. Therefore, the composition of the GaAs melt is shifted from the original composition of Ga:As=1:1 to Ga rich composition. By sealing solid arsenic in the quartz ampoule in addition to GaAs, the shift from the composition of Ga:As=1:1 caused by decomposition of the GaAs melt can be suppressed. 
     EXAMPLE 2 
       FIG. 3  is a diagram for describing another example of a method of preparing a group III-V compound semiconductor crystal of the present invention. 
     Referring to  FIG. 3 , GaAs polycrystalline raw material  2 , carbon fiber  23  subjected to heat treatment under reduced pressure in advance, boron oxide  4 , and a seed crystal  7  were placed in a pBN crucible  1 . Seed crystal  7  was placed at the bottom portion of the crucible  1 . In crucible  1 , arrangement was provided so that carbon fiber  23  and boron oxide  4  were brought into contact with each other and also boron oxide  4  and raw material  2  were brought into contact with each other when the raw material was melted. 
     Crucible  1  was inserted in a quartz ampoule  5  together with solid arsenic. Quartz ampoule  5  was sealed under reduced pressure with a quartz cap  6 . 
     Respective conditions of Example 2 are shown in the following Table 3. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 GaAs 
                   
               
               
                   
                 polycrystal 
               
               
                   
                 (raw material) 
                 10 kg used 
               
               
                   
                   
               
             
             
               
                   
                 Carbon fiber 
                 Average diameter 5-8 μm, 40 mg used, 
               
               
                   
                   
                 Heat treatment at 800° C. for 3 hours at 
               
               
                   
                   
                 the pressure of 10 −7  Torr 
               
               
                   
                 B 2 O 3   
                 Water concentration 70 wt ppm, 100 g used 
               
               
                   
                 pBN crucible 
                 Inner diameter 105 mm, entire length 400 mm 
               
               
                   
                 Solid arsenic 
                 1.5 g used 
               
               
                   
                   
               
             
          
         
       
     
     Quartz ampoule  5  was heated at the rate of approximately 120° C./hour by a heater  8  using a vertical furnace  50 , as shown in FIG.  2 . 
     During the process of heating, boron oxide  4  was softened and melted. Also, GaAs polycrystalline raw material  2  was melted. 
     At this time point, boron oxide  4  was present as a film  4 a having a thickness of not more than 1 mm between pBN crucible  1  and GaAs melt  2 . The remainder of boron oxide  4  covered the upper surface of the GaAs melt. This boron oxide layer  4 b covering the upper surface of GaAs melt  2  was approximately 5 mm. The carbon fiber  23  was partially dispersed in boron oxide layer  4 b on GaAs melt  2 , and partially floated. Furthermore, a portion of carbon fiber  23  was present also at the proximity of the interface between GaAs melt  2  and boron oxide layer  4 b. 
     Then, the condition mentioned above was kept for approximately 12 hours. 
     Then, heater  8  was moved upwards at the rate of 3 mm/hour, whereby solidification started from the portion of seed crystal  7 . Thus, a single crystal was grown. The characteristics of the obtained single crystal are shown in the following Table 4. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 Crystal diameter 
                 105 mm 
                   
                   
               
               
                   
                 Length of φ105 mm portion 
                 200 mm 
               
               
                   
                 Carbon concentration 
                 Shoulder 
                 6.5 × 10 15   
                 cm −3   
               
               
                   
                   
                 Tail 
                 7.0 × 10 15   
                 cm −3   
               
               
                   
                 Resistivity 
                 Shoulder 
                 4.1 × 10 8   
                 Ωcm 
               
               
                   
                   
                 Tail 
                 5.0 × 10 8   
                 Ωcm 
               
               
                   
                 Dislocation density 
                 Shoulder 
                 800 
                 cm −2   
               
               
                   
                   
                 Tail 
                 1500 
                 cm −2   
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 3 
     A carbon-doped GaAs single crystal was grown using 20 mg of carbon fiber similar to that of Example 2. 
     The other conditions of the experiment was identical to those of Example 2, and their description will not be repeated. 
     The characteristics of the obtained single crystal are shown in the following Table 5. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
             
             
               
                   
                 Crystal diameter 
                 105 mm 
                   
                   
               
               
                   
                 Length of φ105 mm portion 
                 200 mm 
               
               
                   
                 Carbon concentration 
                 Shoulder 
                 2.3 × 10 15   
                 cm −3   
               
               
                   
                   
                 Tail 
                 2.2 × 10 15   
                 cm −3   
               
               
                   
                 Resistivity 
                 Shoulder 
                 8.8 × 10 7   
                 Ωcm 
               
               
                   
                   
                 Tail 
                 8.4 × 10 7   
                 Ωcm 
               
               
                   
                 Dislocation density 
                 Shoulder 
                 1000 
                 cm −2   
               
               
                   
                   
                 Tail 
                 1800 
                 cm −2   
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 4 
     A carbon-doped GaAs single crystal was grown using 7.5 mg of carbon fiber similar to those of Examples 2 and 3. 
     The other conditions are identical to those of Examples 2 and 3, and their description will not be repeated. 
     The characteristics of the obtained single crystal are shown in the following Table 6. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
             
             
               
                   
                 Crystal diameter 
                 105 mm 
                   
                   
               
               
                   
                 Length of φ105 mm portion 
                 200 mm 
               
               
                   
                 Carbon concentration 
                 Shoulder 
                 1.3 × 10 15   
                 cm −3   
               
               
                   
                   
                 Tail 
                 1.2 × 10 15   
                 cm −3   
               
               
                   
                 Resistivity 
                 Shoulder 
                 2.5 × 10 8   
                 Ωcm 
               
               
                   
                   
                 Tail 
                 2.3 × 10 8   
                 Ωcm 
               
               
                   
                 Dislocation density 
                 Shoulder 
                 1500 
                 cm −2   
               
               
                   
                   
                 Tail 
                 2000 
                 cm −2   
               
               
                   
                   
               
             
          
         
       
     
     It is appreciated from Examples 2, 3 and 4 that the carbon concentration in the crystal can easily be adjusted by just adjusting the amount of solid carbon to be doped according to the present invention. 
     EXAMPLE 5 
       FIG. 4  is a diagram for describing another example of a method of preparing a group III-V compound semiconductor crystal according to the present invention. 
     Referring to  FIG. 4 , GaAs polycrystalline raw material  2 , a disk  43  made of sintered carbon powder subjected in advance to a heat treatment under reduced pressure, boron oxide  4 , and a seed crystal  7  were placed in a pBN crucible  1 . Seed crystal  7  was placed at the bottom portion of the crucible  1 . In crucible  1 , arrangement was provided so that carbon disk  43  and boron oxide  4  were brought into contact with each other, and also boron oxide  4  and raw material  2  were brought into contact with each other when the raw material was melted. 
     This crucible  1  was inserted in a quartz ampoule  5  together with solid arsenic. Quartz ampoule  5  was sealed under reduced pressure using quartz cap  6 . 
     Respective conditions of example 4 are indicated in the following Table 7. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 GaAs 
                   
               
               
                   
                 polycrystalline 
               
               
                   
                 raw material 
                 3 kg used 
               
               
                   
                   
               
             
             
               
                   
                 Carbon disk 
                 Diameter 30 mm, thickness 10 mm used 
               
               
                   
                   
                 Heat treatment at 1500° C. for 12 hours at 
               
               
                   
                   
                 the pressure of 1 Torr 
               
               
                   
                 B 2 O 3   
                 Water concentration 300 wt ppm, 50 g used 
               
               
                   
                 pBN crucible 
                 Inner diameter 80 mm, entire length 250 mm 
               
               
                   
                 Solid arsenic 
                 1 g used 
               
               
                   
                   
               
             
          
         
       
     
     The above-described quartz ampoule  5  was heated at the rate of approximately 200° C./hour by heater  8  using vertical furnace  50 . 
     During the process of heating, boron oxide  4  was softened and melted. Also, GaAs polycrystalline raw material  2  was melted. 
     At this time point, boron oxide  4  was present as a film  4 a having a thickness of less than 1 mm between pBN crucible  1  and GaAs melt  2 . The remainder of boron oxide  4  covered the upper surface of GaAs melt  2 . The thickness of the boron oxide layer  4 b covering the upper surface of GaAs melt  2  was approximately 6 mm. Carbon disk  43  had its bottom surface in contact with raw material melt  2 , and its top surface exposed to the ambient. The side surface thereof was surrounded by boron oxide layer  4 b. 
     The condition mentioned above was kept for approximately 6 hours. 
     Then, heater  8  was moved upwards at the rate of 4 mm/hour, whereby solidification started from the portion of seed crystal  7 . Thus, a single crystal was grown. The characteristics of the obtained single crystal are shown in the following Table 8. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 8 
               
               
                   
                   
               
             
             
               
                   
                 Crystal diameter 
                 80 mm 
                   
                   
               
               
                   
                 Length of φ80 mm portion 
                 100 mm 
               
               
                   
                 Carbon concentration 
                 Shoulder 
                 6.8 × 10 15   
                 cm −3   
               
               
                   
                   
                 Tail 
                 7.1 × 10 15   
                 cm −3   
               
               
                   
                 Resistivity 
                 Shoulder 
                 4.5 × 10 8   
                 Ωcm 
               
               
                   
                   
                 Tail 
                 5.2 × 10 8   
                 Ωcm 
               
               
                   
                 Dislocation density 
                 Shoulder 
                 1200 
                 cm −2   
               
               
                   
                   
                 Tail 
                 1500 
                 cm −2   
               
               
                   
                   
               
             
          
         
       
     
     In a semi-insulating GaAs crystal, the resistivity is one of the most important characteristics. It is preferable that variation in resistivity is smaller. Furthermore, since this resistivity value depends on the carbon concentration in the GaAs crystal, variation in the carbon concentration in the crystal should be as small as possible. 
     In the above-described examples where carbon fiber or bulk carbon was used as the solid carbon, the carbon was doped substantially uniformly from the shoulder to the tail of the crystal. It is appreciated that carbon fiber and bulk carbon are preferable as solid carbon sources. The shape of bulk carbon is not limited to the disk shape shown in Example 5, and any shape can be used. Also, bulk carbon is preferably a sintered compact of carbon powder. 
     Comparison of the effect of the present invention depending upon difference in the type of solid carbon is shown in the following Table 9. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 Difference in effect among powder, fiber, and bulk carbon 
               
             
          
           
               
                 Type of solid carbon 
                 Carbon distribution in a crystal from shoulder to tail 
               
               
                   
               
               
                 Carbon powder 
                 Gradual decrease of carbon from shoulder to tail 
               
               
                 Carbon fiber 
                 Uniform distribution of carbon from shoulder to tail 
               
               
                 Bulk carbon 
                 Uniform distribution of carbon from shoulder to tail 
               
               
                   
               
             
          
         
       
     
     Comparison of the carbon concentration in a GaAs crystal between the present invention and the prior art is shown in the following Table 10. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 10 
               
               
                   
               
               
                 Comparison of carbon concentration in GaAs crystal 
               
               
                  Carbon concentration 
               
               
                  (cm −3 ) 
               
               
                  Shoulder 
                 Tail 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Present invention 
                 Carbon powder 
                   
                 1.4 × 10 15   
                 0.8 × 10 15   
               
               
                   
                 Carbon fiber 
                 Example 2 
                 6.5 × 10 15   
                 7.0 × 10 15   
               
               
                   
                   
                 Example 3 
                 2.3 × 10 15   
                 2.2 × 10 15   
               
               
                   
                   
                 Example 4 
                 1.3 × 10 15   
                 1.2 × 10 15   
               
               
                   
                 Carbon disk 
                   
                 6.8 × 10 15   
                 7.1 × 10 15   
               
               
                 Prior art 
                 Prior art 2 
                   
                 0.5 × 10 15   
                 0.4 × 10 15   
               
               
                   
                 Prior art 3 
                   
                 2.2 × 10 15   
                 1.4 × 10 15   
               
               
                   
               
             
          
         
       
     
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.