Methods for controllable doping of aluminum nitride bulk crystals

• US 20070243653A1
• Filed: 03/30/2007
• Published: 10/18/2007
• Est. Priority Date: 03/30/2006
• Status: Active Grant

• Current Assignee: Crystal IS, Inc. (Asahi Kasei Corporation)
• Original Assignee: Crystal IS, Inc. (Asahi Kasei Corporation)
• Inventors: Schowalter, Leo, Morgan, Kenneth, Slack, Glen
• Granted Patent: US 8,012,257 B2
• Time in Patent Office: Days
• US Class Current: 438/46
• CPC Class Codes: C01B 21/0722 Preparation by direct nitridation of aluminium
• CPC Class Codes: C04B 2235/3852 Nitrides, e.g. oxynitrides, carbonitrides, oxycarbonitrides, lithium nitride, magnesium nitride
• CPC Class Codes: C04B 2235/3856 Carbonitrides, e.g. titanium carbonitride, zirconium carbonitride
• CPC Class Codes: C04B 2235/3873 Silicon nitrides, e.g. silicon carbonitride, silicon oxynitride
• CPC Class Codes: C04B 2235/402 Aluminium
• CPC Class Codes: C04B 2235/428 Silicon
• CPC Class Codes: C04B 2235/46 Gases other than oxygen used as reactant, e.g. nitrogen used to make a nitride phase
• CPC Class Codes: C04B 2235/656 characterised by specific heating conditions during heat treatment
• CPC Class Codes: C04B 2235/72 Products characterised by the absence or the low content of specific components, e.g. alkali metal free alumina ceramics
• CPC Class Codes: C04B 2235/723 Oxygen content
• CPC Class Codes: C04B 2235/79 Non-stoichiometric products, e.g. perovskites (ABO3) with an A/B-ratio other than 1
• CPC Class Codes: C04B 35/581 based on aluminium nitride
• CPC Class Codes: C04B 35/65 Reaction sintering of free metal- or free silicon-containing compositions C04B35/573, C04B35/591 take precedence
• CPC Class Codes: C30B 23/00 Single-crystal growth by condensing evaporated or sublimed materials
• CPC Class Codes: C30B 29/403 AIII-nitrides
• CPC Class Codes: C30B 33/02 Heat treatment C30B33/04, C30B33/06 take precedence

Abstract

Fabrication of doped and undoped stoichiometric polycrystalline AlN ceramics with high purity is accomplished by, for example, reacting Al pellets with nitrogen gas. Such polycrystalline AlN ceramics may be utilized in the fabrication of high purity AlN single crystals, which may be annealed to enhance a conductivity thereof.

Claims

1. A method of forming polycrystalline AlN, the method comprising the steps of:

providing within a crucible a pellet comprising Al; and

reacting the pellet at a reaction temperature and a reaction pressure with nitrogen gas to form a polycrystalline AlN ceramic,

wherein the polycrystalline AlN ceramic is approximately stoichiometric.

2. The method of claim 1, wherein the pellet consists essentially of Al.

3. The method of claim 2, wherein the reaction temperature is within the range of approximately 1900°

C. to approximately 2200°

C.

4. The method of claim 1, wherein the pellet comprises a first concentration of a dopant species.

5. The method of claim 4, wherein the dopant species comprises Si.

6. The method of claim 4, wherein the first concentration is less than approximately 12% by weight.

7. The method of claim 4, wherein the polycrystalline AlN ceramic comprises a second concentration of the dopant species.

8. The method of claim 7, wherein the second concentration is less than approximately 12% by weight.

9. The method of claim 4, wherein the reaction temperature is within the range of approximately 1600°

C. to approximately 2200°

C., and the reaction pressure is within the range of approximately 1 bar to approximately 60 bars.

10. The method of claim 1, wherein an oxygen concentration of the polycrystalline AlN ceramic is less than approximately 400 ppm.

11. The method of claim 10, wherein the oxygen concentration of the polycrystalline AlN ceramic is less than approximately 100 ppm.

12. The method of claim 1, wherein substantially all of the pellet is reacted.

13. The method of claim 1, further comprising:

providing within the crucible at least one additional pellet comprising Al; and

reacting the at least one additional pellet at the reaction temperature and the reaction pressure with nitrogen gas, thereby increasing a volume of the polycrystalline AlN ceramic.

14. The method of claim 1, further comprising:

providing the polycrystalline AlN ceramic at a first end of and within a crystal growth enclosure;

subliming the polycrystalline AlN ceramic at a formation temperature; and

forming an AlN single crystal at a second end of the crystal growth enclosure.

15. The method of claim 14, wherein the formation temperature is within the range of approximately 2000°

C. to approximately 2750°

C.

16. The method of claim 14, wherein an oxygen concentration of the AlN single crystal is less than approximately 400 ppm.

17. The method of claim 16, wherein the oxygen concentration of the AlN single crystal is less than approximately 100 ppm.

18. The method of claim 14, wherein an oxygen concentration of the AlN single crystal is less than approximately 4.5×

10¹⁹/cm³.

19. The method of claim 14, wherein a thermal conductivity of the AlN single crystal is greater than approximately 250 W/m·

K.

20. The method of claim 14, wherein the polycrystalline AlN ceramic comprises a dopant species, and the AlN single crystal comprises the dopant species at a first concentration.

21. The method of claim 20, wherein the dopant species comprises Si.

22. The method of claim 20, wherein the first concentration is greater than approximately 10¹⁶/cm³.

23. The method of claim 14, wherein the AlN single crystal has a conductivity greater than approximately 10⁻

4 Ω

<sup>−

1</sup>cm<sup>−

1</sup> at room temperature.

24. The method of claim 14, wherein at least a portion of the crucible is disposed proximate and in contact with the polycrystalline AlN ceramic.

25. A method of forming a doped AlN wafer, the method comprising the steps of:

providing a single-crystal wafer comprising AlN and a dopant species; and

annealing the single-crystal wafer to electrically activate the dopant species.

26. The method of claim 25, wherein the dopant species comprises Si.

27. The method of claim 25, wherein the step of annealing decreases a concentration of aluminum vacancies in the single crystal wafer.

28. The method of claim 25, wherein the step of annealing increases a concentration of nitrogen vacancies in the single-crystal wafer.

29. The method of claim 25, wherein the step of annealing is performed at an annealing temperature and an annealing nitrogen pressure, and the annealing nitrogen pressure is greater than approximately a nitrogen pressure required to form AlN from Al at the first temperature.

30. The method of claim 29, wherein the annealing nitrogen pressure is less than approximately twice the nitrogen pressure required to form AlN from Al at the first temperature.

31. The method of claim 30, wherein the annealing nitrogen pressure is within the range of approximately 0.1 mbar to approximately 5 bars.

32. The method of claim 29, wherein the annealing temperature is greater than approximately 1900°

C.

33. The method of claim 25, wherein after annealing the single-crystal wafer has a conductivity greater than approximately 10⁻

4 Ω

<sup>−

1</sup>cm<sup>−

1</sup> at room temperature.

34. The method of claim 33, wherein after annealing the single-crystal wafer has a conductivity greater than approximately 10⁻

2 Ω

<sup>−

1</sup>cm<sup>−

1</sup> at room temperature.

35. A method of forming an AlN wafer, the method comprising the steps of:

providing a single-crystal wafer comprising undoped AlN and having a first conductivity; and

annealing the single-crystal wafer in an ambient at an annealing temperature and an annealing pressure,

wherein after annealing the single-crystal wafer has a second conductivity greater than the first conductivity.

36. The method of claim 35, wherein the annealing temperature is within the range of approximately 1700°

C. to approximately 2200°

C.

37. The method of claim 35, wherein the annealing pressure is within the range of approximately 2 bars to approximately 30 bars.

38. The method of claim 35, wherein the ambient consists essentially of nitrogen and at least one inert gas.

39. The method of claim 38, wherein the at least one inert gas comprises argon.

40. The method of claim 38, wherein a pressure of the nitrogen in the ambient is greater than a nitrogen pressure required to form AlN from Al at the annealing temperature.

41. The method of claim 35, wherein the second conductivity is greater than approximately 10⁻

2 Ω

<sup>−

1</sup>cm<sup>−

1</sup> at room temperature.

42. An AlN single crystal having a thickness greater than approximately 100 μ

m, a cross-sectional area greater than approximately 1 cm², and a conductivity greater than approximately 10<sup>−

4</sup> Ω

<sup>−

1</sup>cm<sup>−

1</sup> at room temperature.

43. The AlN single crystal of claim 42, further comprising a substitutional dopant species and exhibiting n-type conductivity.

44. The AlN single crystal of claim 43, wherein the substitutional dopant species comprises a group IV element.

45. The AlN single crystal of claim 44, wherein the substitutional dopant species comprises Si.

46. The AlN single crystal of claim 42 consisting essentially of AlN and nitrogen vacancies.

47. The AlN single crystal of claim 42, wherein the thickness is greater than approximately 200 μ

m.

48. The AlN single crystal of claim 47, wherein the thickness is greater than approximately 2 mm.

49. The AlN single crystal of claim 42, wherein the conductivity is greater than approximately 10⁻

2 Ω

<sup>−

1</sup>cm<sup>−

1</sup> at room temperature.

50. The AlN single crystal of claim 42, wherein a thermal conductivity thereof is greater than approximately 250 W/m·

K.