Process for producing high quality large size silicon carbide crystals
View Patent ↗The invention is an improvement in the method of producing a high quality bulk single crystal of silicon carbide in a seeded sublimation system. In a first embodiment, the improvement comprises reducing the number of macrosteps in a growing crystal by incorporating a high concentration of nitrogen atoms in the initial one (1) millimeter of crystal growth.
1. In a method of producing a high quality bulk single crystal of silicon carbide in a seeded sublimation system, the improvement comprising:
reducing excess Si at a growing surface by initiating the sublimation growth from a source powder on a seed crystal at sublimation temperatures in the presence of a partial pressure of nitrogen sufficient to produce a concentration of nitrogen of between about 10 18 cm −3 and about 10 19 cm −3 in the crystal for about the first one (1) millimeter of sublimation growth; and
continuing sublimation growth to a thickness of at least 5 millimeters in the presence of a partial pressure of nitrogen that reduces the concentration of nitrogen in the growing crystal to less than about 10 18 cm −3 for the remainder of the sublimation growth of the bulk crystal.
2. A method according to claim 1 wherein the step of initiating sublimation growth in the presence of nitrogen includes introducing nitrogen to the sublimation system at about 400 torr Ar pressure.
3. A method according to claim 1 further comprising reducing the partial pressure of nitrogen in the seeded sublimation system after the initial growth stage of the single crystal silicon carbide.
4. A method according to claim 1 comprising initiating growth with a seed crystal of silicon carbide substrate that has a polytype selected from the group consisting of the 3C, 4H, 6H, and 15R polytypes.
5. A method according to claim 4 comprising growing the bulk single crystal at substantially the same diameter as the seed crystal.
6. A method of producing a high quality bulk single crystal of silicon carbide in a seeded sublimation system, the method comprising:
placing a SiC seed on a seed holder in a crucible;
evacuating the crucible to remove ambient air and other impurities;
placing the crucible under inert gas pressure;
heating the system to SiC sublimation temperatures;
reducing the inert gas pressure to initiate sublimation SiC growth from a source powder to the seed crystal;
introducing dopant gases at approximately 400 torr or below inert gas pressure to produce a dopant concentration of between about 10 18 cm −3 and about 10 −3 cm in the first one (1) millimeter of sublimation growth to thereby reduce free Si on a growing surface of the seed crystal; and
continuing the sublimation growth while reducing the partial pressure of dopant gases to the appropriate levels to achieve dopant concentrations at or below about 10 18 cm −3 to a thickness of at least 5 millimeters throughout the remainder of the growth process.
7. A method according to claim 6 wherein the step of placing a SiC seed on a seed holder comprises placing the seed on a graphite seed holder.
8. A method according to claim 6 further comprising stopping growth by raising the inert gas pressure in the crucible to above about 400 torr and lowering the temperature to below about 1900° C. to stop crystal growth.
9. A method according to claim 8 wherein the step of stopping growth occurs after between about 5 mm and about 75 mm of crystal growth.
10. A method according to claim 6 wherein the step of placing the crucible under inert gas pressure involves introducing an inert gas selected from the group consisting of noble gases and mixtures thereof.
11. A method according to claim 6 wherein the step of heating the system to SiC growth temperatures involves heating to temperatures between about 1900° and 2500° C.
12. A method according to claim 6 wherein the step of introducing dopant gases involves introducing dopant gases selected from the group consisting of N, P, As, Sb, Bi, and mixtures thereof.
13. A method according to claim 6 wherein the step of introducing dopant gases involves introducing nitrogen to the growth system.
14. A method according to claim 6 wherein the step of reducing the inert gas pressure to initiate SiC growth comprises reducing the inert gas pressure to below 400 torr.
15. A method according to claim 6 further comprising annealing the crystal after the completion of the crystal growth process.
16. A method of producing a high quality bulk single crystal of silicon carbide in a seeded sublimation system, the method comprising:
reducing the presence of macrosteps at a crystal growth surface by first initiating the seeded sublimation growth from a source powder at sublimation temperatures in the presence of a partial pressure of nitrogen sufficient to produce a concentration of between about 10 18 cm −3 and about 10 19 cm −3 in the crystal for about the first one (1) millimeter of sublimation growth; and
continuing sublimation growth to a thickness of at least 5 millimeters in the presence of a partial pressure of nitrogen that reduces the concentration of nitrogen in the crystal to about 10 18 cm −3 or less for the remainder of the sublimation growth.
17. A method according to claim 16 wherein said step of initiating crystal growth occurs at or below about 400 torr Ar pressure.
18. A method according to claim 16 wherein the step of reducing the partial pressure of nitrogen for the remainder of the crystal growth process involves additional crystal growth of between about 5 mm and 75 mm.
19. A method according to claim 17 wherein the step of initiating crystal growth occurs at temperatures between about 2100 and 2500° C.
20. A method for reducing step bunching in the production of high quality bulk single crystal silicon carbide during seeded sublimation growth, the method comprising:
increasing the partial pressure of dopant gases during the first millimeter of sublimation crystal growth from a source powder at sublimation temperatures to produce a high concentration of dopant atoms in the vapor phase of the sublimation system during the first one millimeter of crystal growth; and
continuing sublimation growth to a thickness of at least 5 millimeters while removing liquid silicon present on a growing surface of the bulk single crystal of silicon carbide by reaction with the dopant atoms in the vapor phase.
21. A method according to claim 20 wherein said dopant gases are selected from the group consisting of n-type dopant atoms.
22. A method according to claim 21 wherein said n-type dopant atoms are selected from the group consisting of N, P, As, Sb, Bi, and mixtures thereof.
23. A method according to claim 20 further comprising initiating growth with a seed crystal of silicon carbide substrate that has a polytype selected from the group consisting of the 3C, 4H, 6H, and 15R polytypes.