Method of preparing a compound semiconductor crystal

Abstract

A method of preparing a compound semiconductor crystal is able to dope the crystal with carbon with high reproducibility. The method includes the steps of sealing a carbon oxide gas of a predetermined partial pressure and a compound semiconductor material in a gas-impermeable airtight vessel, increasing the temperature of the vessel to melt the compound semiconductor material sealed in the vessel, and then decreasing the temperature of the vessel to solidify the melted compound semiconductor material to grow a compound semiconductor crystal containing a predetermined amount of carbon. With this method, a compound semiconductor crystal with a carbon concentration of 0.1×10 15 cm −3 to 20×10 15 cm −3 is prepared with high reproducibility.

1. A method of preparing a compound semiconductor crystal containing carbon, comprising the following steps:
 a) selecting a partial pressure of a carbon oxide gas;  
 b) sealing said carbon oxide gas having said partial pressure, and a semiconductor raw material, in a gas-impermeable airtight vessel;  
 c) after said step b), increasing a temperature of said airtight vessel to melt said raw material in said airtight vessel and thereby form a melted material from said raw material; and  
 d) after said step c), decreasing said temperature of said airtight vessel to solidify said melted material and thereby grow said compound semiconductor crystal containing a resultant content of said carbon, wherein said resultant content of said carbon is dependent on said partial pressure of said carbon oxide gas selected in said step a).  
 
     
     
       2. The method according to  claim 1 , wherein said carbon oxide gas includes at least one of CO gas and CO 2  gas. 
     
     
       3. The method according to  claim 1 , wherein said steps a) and b) are carried out so that said partial pressure of said carbon oxide gas is in a range from 0.1 to 100 Torr at 25° C. 
     
     
       4. The method according to  claim 1 , further comprising a preliminary step of selecting a carbon content value, wherein said step a) comprises selecting said partial pressure of said carbon oxide gas dependent on said carbon content value, and wherein said resultant content of said carbon in said compound semiconductor crystal is equal to said carbon content value. 
     
     
       5. The method according to  claim 1 ,
 wherein said steps b) and c) are carried out so that said melted material is located on a first side of a plane extending along a melt interface of said melted material in said airtight vessel, and a gas-filled space containing at least a portion of said carbon oxide gas is provided within said airtight vessel on a second side of said plane opposite said first side, and  
 further comprising, during said step d), controlling said temperature of said airtight vessel, on said second side of said plane, so that a hottest portion of said gas-filled space and a coolest portion of said gas-filled space have a temperature difference of no more than 300° C. therebetween.  
 
     
     
       6. The method according to  claim 1 , wherein said resultant content of said carbon in said compound semiconductor crystal is a resultant local content of carbon at a location in said compound semiconductor crystal defined by a fraction solidified (g) of 0.1. 
     
     
       7. The method of  claim 1 , further comprising a preliminary step of selecting a carbon content value, and wherein said step a) comprises selecting said partial pressure dependent on said carbon content value. 
     
     
       8. The method according to  claim 1 , wherein said step a) comprises selecting said partial pressure according to a formula:
   C CARBON =α×P 0.5 ,  
 
       wherein C CARBON  represents a value of said resultant content of said carbon per cm 3  of said compound semiconductor crystal to be achieved in said step d). 
     
     
       9. The method of preparing a compound semiconductor crystal according to  claim 1 , wherein said carbon oxide gas is sealed in said airtight vessel according to an expression:
   C CARBON =α×P 0.5 ,  
 
       wherein C CARBON  represents a carbon concentration in said compound semiconductor crystal, P (Torr) represents a partial pressure of said carbon oxide gas represented in, and α represents any coefficient. 
     
     
       10. The method according to  claim 1 , further comprising a step of subjecting said airtight vessel to a vacuum heat treatment before said step of sealing said carbon oxide gas in said airtight vessel. 
     
     
       11. The method according to  claim 10 , wherein said vacuum heat treatment is carried out at an elevated temperature no more than 350° C. 
     
     
       12. The method according to  claim 1 , further comprising providing said airtight vessel to have at least an internal vessel wall formed of a material excluding carbon, and further ensuring that no additional carbon is introduced into said carbon oxide gas during at least one of said steps c) and d). 
     
     
       13. The method according to  claim 12 , wherein said material excluding carbon includes at least one material selected from the group consisting of quartz, silicon nitride, boron nitride, pyrolytic boron nitride and alumina. 
     
     
       14. The method according to  claim 1 , further comprising a preliminary step of providing said semiconductor raw material so that said compound semiconductor crystal contains GaAs. 
     
     
       15. The method according to  claim 14 , wherein said steps are carried out so that said resultant content of said carbon in said compound semiconductor crystal is in a range from 0.1×10 15 cm −3  to 20×10 15 cm −3 . 
     
     
       16. The method according to  claim 1 , wherein in said step c), said melted material occupies a melt volume that is bounded along one plane by a melt interface of said melted material in said airtight vessel, said melt volume is located on a first side of said plane extending along said melt interface in said airtight vessel, said carbon oxide gas occupies a first gas-filled volume within said airtight vessel on said first side of said plane and a second gas-filled volume within said airtight vessel on a second side of said plane opposite said first side, and said second gas-filled volume is larger than said first gas-filled volume. 
     
     
       17. The method according to  claim 16 , wherein said second gas-filled volume is at least twice as large as said first gas-filled volume. 
     
     
       18. The method according to  claim 1 , further comprising a preliminary step of selecting a carbon content value, and wherein said step a) comprises selecting said partial pressure according to a formula:
   C CARBON =α×P 0.5 ,  
 
       wherein C CARBON  represents said carbon content value in atoms of carbon per cm 3  of said compound semiconductor crystal, P represents said partial pressure in Torr, and a represents a dependence coefficient. 
     
     
       19. The method according to  claim 18 , wherein said dependence coefficient α is in a range from 0.25×10 15 cm −3 /Torr 0.5  to 4×10 15 cm −3 /Torr 0.5 . 
     
     
       20. The method according to  claim 18 , wherein said dependence coefficient α is in a range from 0.5×10 15 cm −3 /Torr 0.5  to 2×10 15 cm −3 /Torr 0.5 . 
     
     
       21. The method according to  claim 1 , further comprising providing said airtight vessel to have at least a portion thereof formed of quartz. 
     
     
       22. The method according to  claim 21 , wherein said airtight vessel is provided to have a wall thickness of said portion formed of quartz being at least 1.5 mm. 
     
     
       23. The method according to  claim 21 , further comprising controlling said temperature during said steps c) and d) so that said portion formed of quartz never exceeds 1270° C. 
     
     
       24. The method according to  claim 1 , wherein said step b) further comprises sealing a boron oxide based substance together with said carbon oxide gas and said raw material in said airtight vessel, said step c) further comprises melting said boron oxide based substance to form a melted boron oxide based substance, and said melted material is at least partially in contact with said melted boron oxide based substance during said step d). 
     
     
       25. The method according to  claim 24 , wherein said melted boron oxide based substance forms a continuous layer entirely along an interface of said melted material on a plane between said melted material and said melted boron oxide based substance. 
     
     
       26. The method according to  claim 24 , further comprising selecting said boron oxide based substance to comprise boron oxide and a water content of not more than 300 ppm of water. 
     
     
       27. The method according to  claim 24 , further comprising selecting said boron oxide based substance to comprise boron oxide and a water content of not more than 100 ppm of water. 
     
     
       28. The method according to  claim 24 , further comprising selecting said boron oxide based substance to comprise boron oxide and a water content of water, wherein said water content has a variation in a range from −20% to +20% relative to an average of said water content. 
     
     
       29. The method according to  claim 1 , further comprising providing said airtight vessel to have at least an internal vessel wall thereof formed of a first material excluding carbon, and further providing that at least one content of said airtight vessel has at least an outer surface formed of a second material excluding carbon. 
     
     
       30. The method according to  claim 29 , wherein said step b) further comprises placing said raw material in a crucible and sealing said crucible in said airtight vessel, wherein said step b) additionally comprises sealing a boron oxide based substance in said airtight vessel, and wherein said content includes said raw material, said crucible, and said boron oxide based substance. 
     
     
       31. The method according to  claim 29 , wherein each one of said first material and said second material respectively comprises at least one respective material selected from the group consisting of quartz, silicon nitride, boron nitride, pyrolytic boron nitride and alumina. 
     
     
       32. The method according to  claim 29 , wherein said first material and said second material both consist of the same material. 
     
     
       33. The method according to  claim 29 , wherein said first material and said second material consist respectively of two different materials. 
     
     
       34. The method according  claim 1 , further comprising:
 a first preliminary step of providing a correlation between partial pressure values for said carbon oxide gas and carbon content values for said compound semiconductor crystal; and  
 a second preliminary step of selecting a carbon content value;  
 wherein said step a) comprises selecting said partial pressure to have a partial pressure value correlated to said carbon content value by said correlation.  
 
     
     
       35. The method according to  claim 34 , wherein said resultant content of said carbon in said compound semiconductor crystal is equal to said carbon content value that has been selected in said second preliminary step. 
     
     
       36. The method according to  claim 34 , wherein said correlation is defined by a formula:
   C CARBON =α×P 0.5 ,  
 
       wherein C CARBON  represents said carbon content value, P represents said partial pressure value, and a represents a dependence coefficient. 
     
     
       37. The method according to  claim 36 , wherein said dependence coefficient α is in a range from 0.25×10 15 cm −3 /Torr 0.5  to 4×10 15 cm −3 /Torr 0.5  for C CARBON  being in terms of atoms of carbon per cm 3  and P being in terms of Torr. 
     
     
       38. The method according to  claim 1 , wherein said steps are carried out so that said resultant content of said carbon in said compound semiconductor crystal is in a range from 0.1×10 15 cm −3  to 20×10 15 cm −3 . 
     
     
       39. The method according to  claim 38 , wherein said resultant content of said carbon in said compound semiconductor crystal is a resultant local content of carbon at a location in said compound semiconductor crystal defined by a fraction solidified (g) of 0.1. 
     
     
       40. The method according to  claim 38 , wherein said resultant content of said carbon in said compound semiconductor crystal is a resultant local content of carbon at a location in said compound semiconductor crystal. 
     
     
       41. The method according to  claim 40 , wherein said location is at a shoulder of said compound semiconductor crystal. 
     
     
       42. The method according to  claim 40 , wherein said location is a location defined by a fraction solidified (g) of 0.1 in said compound semiconductor crystal. 
     
     
       43. A method of preparing a compound semiconductor crystal containing carbon, comprising: sealing a semiconductor raw material and a boron oxide based substance in a gas- impermeable airtight vessel and increasing a temperature of said airtight vessel to melt said raw material and said boron oxide based substance in said airtight vessel and thereby form a melted material from said raw material; and decreasing said temperature of said airtight vessel to solidify said melted material and thereby grow said compound semiconductor crystal containing a resultant content of said carbon, characterized by selecting a partial pressure of a carbon oxide gas, and making said carbon oxide gas having said selected partial pressure coexistent with said semiconductor raw material and boron oxide based substance prior to sealing said vessel, so that said resultant content of said carbon is dependent on said selected partial pressure of said carbon oxide gas.   
     
     
       44. The method according to  claim 43 , wherein said partial pressure selecting step is carried out so that said resultant content of said carbon in said compound semiconductor crystal is in a range from  0 . 1 × 10   15   cm   −3    to  20 × 10     15   cm   −3 . 
     
     
       45. A method of preparing a compound semiconductor crystal containing carbon, comprising
   making a selected partial pressure of a carbon oxide gas coexistent with a melted material of a compound semiconductor raw material and a melted boron oxide base substance in a gas - impermeable airtight vessel, and then        decreasing a temperature of said airtight vessel to solidify said melted material and thereby grow said compound semiconductor crystal containing a controlled amount of carbon.     
     
     
       46. The method according to  claim 45 , wherein said steps are carried out so that said controlled amount of carbon in said compound semiconductor crystal is in a range from  0 . 1 × 10   15   cm   −3    to  20 × 10     15   cm   −3 . 
     
     
       47. A method of preparing a compound semiconductor crystal containing carbon, comprising:
   sealing a semiconductor raw material  (   2   )  and a boron oxide based substance  (   4   )  in a gas - impermeable airtight vessel, heating said raw material and said boron oxide based substance in said airtight vessel to melt them and thereby forming a melted material from said raw material; and        decreasing said temperature of said airtight vessel to solidify said melted material and thereby grow said compound semiconductor crystal containing a resultant content of said carbon,        characterized by      selecting a partial pressure of a carbon oxide gas  (   7   )  and making said carbon oxide gas coexistent with said semiconductor raw material and boron oxide based substance prior to sealing said vessel, so that said resultant content of said carbon is dependent on said selected partial pressure of said carbon oxide gas.       
     
     
       48. The method according to  claim 47 , wherein said carbon oxide gas (   7   )  includes at least one type of gas selected from the group consisting CO gas and CO   2    gas.   
     
     
       49. The method of preparing a compound semiconductor crystal according to  claim 47 , wherein said carbon oxide gas (   7   )  is sealed in said airtight vessel  (   8   )  having a partial pressure of  13 . 3  to  13333  Pa  (   0 . 1  to  100  Torr )  at  25 ° C.   
     
     
       50. The method of preparing a compound semiconductor crystal according to  claim 47 , wherein growing said compound semiconductor crystal, a space behind a raw- material melt  (   2   )  formed from said melted semiconductor material has a most heated portion and a least heated portion with a temperature difference of no more than  300 ° C. therebetween.   
     
     
       51. The method of preparing a compound semiconductor crystal according to  claim 47 , wherein said semiconductor raw material (   2   )  comprises a compound semiconductor material.   
     
     
       52. The method of preparing a compound semiconductor crystal according to  claim 47 , wherein said semiconductor raw material (   2   )  comprises GaAs.   
     
     
       53. The method according to  claim 47 , wherein said melted semiconductor material (   2   )  is at least partially kept in contact with said boron oxide based substance  (   4   )  in growing said compound semiconductor crystal.   
     
     
       54. The method according to  claim 53 , wherein said melted semiconductor material (   2   )  has an upper surface entirely covered with said boron oxide based substance  (   4   )  in growing said compound semiconductor crystal.   
     
     
       55. The method according to  claim 53 , further comprising selecting said boron oxide based substance (   4   )  to comprise boron oxide and a water content of water, wherein said water content has a variation in a range from − 20   %  to + 20   %  relative to an average of said water content.   
     
     
       56. The method according to  claim 53 , wherein said boron oxide based substance (   4   )  has a water content of no more than a first upper limit value of  300  ppm.   
     
     
       57. The method according to  claim 56 , wherein said boron oxide based substance (   4   )  has a water content of no more than a second upper limit value of  100  ppm.   
     
     
       58. The method of preparing a compound semiconductor crystal according to  claim 47 , wherein said carbon oxide gas (   7   )  is sealed in said airtight vessel  (   8   )  according to an expression:        C   carbon   =a×P   0.5      
         wherein C   carbon  ( cm   −3 )  represents a carbon concentration in said compound semiconductor crystal, P  ( Pa )  represents a partial pressure of said carbon oxide gas  (   7   ),  and “a” represents any coefficient.   
     
     
       59. The method of preparing a compound semiconductor crystal according to  claim 58 , wherein said coefficient a ranges from  0 . 217 × 10   14    to  3 . 46 × 10     14    cm   −3   /Pa   0.5  (   0 . 25 × 10     15    to  4 × 10     15    cm   −3   /Torr   0.5 ). 
     
     
       60. The method of preparing a compound semiconductor crystal according to  claim 59 , wherein said coefficient a ranges from  0 . 433 × 10   14    to  1 . 73 × 10     14    cm   −3   /Pa   0.5  (   0 . 5 × 10     15    to  2 × 10     15    cm   −3   /Torr   0.5 ). 
     
     
       61. The method of preparing a compound semiconductor crystal according to  claim 47 , wherein a step of subjecting said airtight vessel (   8   )  to a vacuum heat treatment is provided before the step of sealing said airtight vessel  (   8   ). 
     
     
       62. The method of preparing a compound semiconductor crystal according to  claim 61 , wherein said vacuum heat treatment is performed at no more than  350 ° C. 
     
     
       63. The method of preparing a compound semiconductor crystal according to  claim 47 , wherein at least an internal wall of said airtight vessel (   8   )  and at least an outer surface of the content of said airtight vessel  (   8   )  other than said compound semiconductor material and said boron oxide based substance  (   4   )  are formed from a material excluding carbon.   
     
     
       64. The method of preparing a compound semiconductor crystal according to  claim 63 , wherein said material excluding carbon includes at least one material selected from the group consisting of quartz, silicon nitride, boron nitride, pyrolytic boron nitride and alumina. 
     
     
       65. The method of preparing a compound semiconductor crystal according to  claim 47 , wherein said gas- impermeable airtight vessel  (   8   )  at least has a portion formed from quartz.   
     
     
       66. The method of preparing a compound semiconductor crystal according to  claim 65 , wherein said portion formed from quartz has a thickness of no less than  1 . 5  mm. 
     
     
       67. The method of preparing a compound semiconductor crystal according to  claim 65 , wherein said portion formed from quartz is controlled to have a temperature of at most  1270 ° C. in growing said compound crystal. 
     
     
       68. The method of preparing a compound semiconductor crystal according to  claim 47 , wherein a space behind a raw- material melt  (   2   )  formed from said melted semiconductor material is larger in volume than a space on the side of said raw - material melt.   
     
     
       69. The method of preparing a compound semiconductor crystal according to  claim 68 , wherein a space behind said raw- material melt  (   2   )  is no less than twice larger in volume than said space on the side of said raw - material melt.   
     
     
       70. The method of  claim 47 , wherein said compound semiconductor crystal has a carbon concentration of  0 . 1 × 10   15   cm   −3    to  20 × 10     15   cm   −3 . 
     
     
       71. The method of  claim 70 , wherein said compound semiconductor contains GaAs. 
     
     
       72. A method of preparing a compound semiconductor crystal containing carbon, comprising
   making a selected partial pressure of a carbon oxide gas coexistent with a melted material of a compound semiconductor raw material and a melted boron oxide base substance in a crucible in a gas - impermeable airtight vessel, and then        decreasing a temperature of said airtight vessel to solidify said melted material in the crucible and thereby grow said compound semiconductor crystal containing a controlled amount of carbon.