Aluminum nitride bulk crystals having high transparency to untraviolet light and methods of forming them

• US 9,447,519 B2
• Filed: 04/16/2015
• Issued: 09/20/2016
• 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: Schujman, Sandra B., Rao, Shailaja P., Bondokov, Robert T., Morgan, Kenneth E., Slack, Glen A., Schowalter, Leo J.
• Primary Examiner(s): SONG, MATTHEW J
• Application Number:
  US14/687,993
• Publication Number:
  US 20150218729A1
• Time in Patent Office: 523 Days
• Field of Search: 117/84, 117/88, 117/952
• US Class Current: 1/1
• CPC Class Codes: C30B 23/00 Single-crystal growth by condensing evaporated or sublimed materials
• CPC Class Codes: C30B 23/002 Controlling or regulating
• CPC Class Codes: C30B 23/02 Epitaxial-layer growth
• CPC Class Codes: C30B 23/066 Heating of the material to be evaporated
• CPC Class Codes: C30B 29/403 AIII-nitrides
• CPC Class Codes: C30B 33/02 Heat treatment C30B33/04, C30B33/06 take precedence
• CPC Class Codes: C30B 35/00 Apparatus not otherwise provided for, specially adapted for the growth, production or after-treatment of single crystals or of a homogeneous polycrystalline material with defined structure
• CPC Class Codes: H01L 21/02389 Nitrides
• CPC Class Codes: Y10T 428/2982 Particulate matter [e.g., sphere, flake, etc.]

Abstract

In various embodiments, methods of forming single-crystal AlN include providing a substantially undoped polycrystalline AlN ceramic having an oxygen concentration less than approximately 100 ppm, forming a single-crystal bulk AlN crystal by a sublimation-recondensation process at a temperature greater than approximately 2000° C., and cooling the bulk AlN crystal to a first temperature between approximately 1500° C. and approximately 1800° C. at a first rate less than approximately 250° C./hour.

Claims

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

providing a pellet comprising Al within a crucible;

reacting the pellet at a reaction temperature and a reaction pressure with nitrogen to form a bulk polycrystalline AlN ceramic, wherein, when the polycrystalline AlN ceramic reaches approximately room temperature after the reaction, the polycrystalline AlN ceramic has (i) less than approximately 1% excess Al and (ii) an oxygen concentration of less than approximately 100 ppm;

after forming the polycrystalline AlN ceramic, and with no sublimation-recondensation treatment of the polycrystalline AlN ceramic therebetween, providing the polycrystalline AlN ceramic and at least a portion of the crucible within a crystal growth enclosure; and

subliming the polycrystalline AlN ceramic at a formation temperature, the at least a portion of the crucible being in contact with the polycrystalline AlN ceramic thereduring, whereby an AlN single crystal is formed within the crystal growth enclosure.

2. The method of claim 1, wherein the reaction temperature is within the range of approximately 1900°

C. to approximately 2200°

C.

3. The method of claim 1, wherein the pellet comprises a first concentration of a non-oxygen dopant species.

4. The method of claim 3, wherein the dopant species comprises Si.

5. The method of claim 3, wherein the first concentration is less than approximately 12% by weight.

6. The method of claim 3, 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.

7. 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 with nitrogen gas, thereby increasing a volume of the polycrystalline AlN ceramic.

8. The method of claim 1, wherein the formation temperature is within the range of approximately 2000°

C. to approximately 2750°

C.

9. The method of claim 1, wherein the pellet is reacted in an atmosphere (i) comprising nitrogen gas and (ii) substantially free of hydrogen.

10. The method of claim 1, further comprising cleaning the pellet prior to providing the pellet within the crucible.

11. The method of claim 10, wherein cleaning the pellet comprises exposing the pellet to at least one of hydrofluoric acid or nitric acid.

12. The method of claim 1, wherein the polycrystalline AlN ceramic is stoichiometric.

13. The method of claim 1, wherein the polycrystalline ceramic has less than approximately 0.1% excess Al.

14. The method of claim 1, wherein an oxygen concentration of the AlN single crystal is less than approximately 400 ppm.

15. The method of claim 1, wherein the oxygen concentration of the AlN single crystal is less than approximately 100 ppm.

16. The method of claim 1, wherein an oxygen concentration of the AlN single crystal is less than approximately 4.5×

10¹⁹/cm³.

17. The method of claim 1, wherein a thermal conductivity of the AlN single crystal is greater than approximately 250 W/m·

K.

18. The method of claim 1, wherein the polycrystalline AlN ceramic comprises a non-oxygen dopant species, and the AlN single crystal comprises the dopant species at a first concentration.

19. The method of claim 18, wherein the dopant species comprises Si.

20. The method of claim 18, wherein the first concentration is greater than approximately 10¹⁶/cm³.

21. The method of claim 1, wherein the AlN single crystal has a conductivity greater than approximately 10⁻

4Ω

<sup>−

1</sup> cm<sup>−

1</sup> at room temperature.

22. The method of claim 1, further comprising annealing the AlN single crystal.

23. The method of claim 22, wherein after annealing the AlN single crystal has a conductivity greater than approximately 10⁻

2Ω

<sup>−

1</sup> cm<sup>−

1</sup> at room temperature.

24. The method of claim 1, wherein the AlN single crystal has an optical absorption coefficient less than 100/cm in a wavelength range of approximately 210 nm to approximately 480 nm.

25. The method of claim 24, wherein the optical absorption coefficient of the AlN single crystal is less than 100/cm at a wavelength of 280 nm.

26. The method of claim 1, wherein the crucible comprises tungsten.

27. The method of claim 1, further comprising cooling the AlN single crystal to a first temperature between approximately 1500°

C. and approximately 1800°

C. at a first rate less than approximately 250°

C./hour.

28. The method of claim 27, further comprising cooling the AlN single crystal from the first temperature to a second temperature lower than the first temperature at a second rate faster than the first rate.

29. The method of claim 1, further comprising cooling the AlN single crystal from the growth temperature to a first temperature between approximately 1500°

C. and approximately 1800°

C. at a first rate ranging from 70°

C./hour to 150°

C./hour, the first rate being slowed via controlled application of heat from a furnace, and (ii) thereafter, cooling the AlN single crystal from the first temperature to a second temperature lower than the first temperature at a second rate greater than approximately 250°

C./hour.

30. The method of claim 1, wherein an absorption coefficient of the AlN single crystal is less than approximately 20 cm⁻

1 in the entire wavelength range between about 4500 nm and approximately 215 nm.

31. The method of claim 1, wherein an absorption coefficient of the AlN single crystal is less than approximately 10 cm⁻

1 for the entire wavelength range between approximately 400 nm and approximately 250 nm.

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

providing a pellet comprising Al within a crucible;

reacting the pellet at a reaction temperature and a reaction pressure with nitrogen to form a bulk polycrystalline AlN ceramic, wherein, when the polycrystalline AlN ceramic reaches approximately room temperature after the reaction, the polycrystalline AlN ceramic has (i) less than approximately 1% excess Al and (ii) an oxygen concentration of less than approximately 100 ppm;

after forming the polycrystalline AlN ceramic, and with no sublimation-recondensation treatment of the polycrystalline AlN ceramic therebetween, providing the polycrystalline AlN ceramic and at least a portion of the crucible within a crystal growth enclosure; and

subliming the polycrystalline AlN ceramic at a formation temperature, the at least a portion of the crucible being in contact with the polycrystalline AlN ceramic thereduring, whereby an AlN single crystal is formed within the crystal growth enclosure,

wherein the crucible comprises a bottom plug and a foil wrap.

33. The method of claim 32, wherein the bottom plug comprises tungsten, the foil wrap comprises tungsten, and the polycrystalline AlN ceramic forms in direct contact with the bottom plug and the foil wrap.

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

providing a pellet comprising Al within a crucible;

reacting the pellet at a reaction temperature and a reaction pressure with nitrogen to form a bulk polycrystalline AlN ceramic, wherein, when the polycrystalline AlN ceramic reaches approximately room temperature after the reaction, the polycrystalline AlN ceramic has (i) less than approximately 1% excess Al and (ii) an oxygen concentration of less than approximately 100 ppm;

after forming the polycrystalline AlN ceramic, and with no sublimation-recondensation treatment of the polycrystalline AlN ceramic therebetween, providing the polycrystalline AlN ceramic and at least a portion of the crucible within a crystal growth enclosure; and

subliming the polycrystalline AlN ceramic at a formation temperature, the at least a portion of the crucible being in contact with the polycrystalline AlN ceramic thereduring, whereby an AlN single crystal is formed within the crystal growth enclosure,

wherein the crucible and the crystal growth enclosure both comprise a first material inert to AlN.