Defect reduction in seeded aluminum nitride crystal growth
Bulk single crystal of aluminum nitride (AlN) having an areal planar defect density≦100 cm −2 . Methods for growing single crystal aluminum nitride include melting an aluminum foil to uniformly wet a foundation with a layer of aluminum, the foundation forming a portion of an AlN seed holder, for an AlN seed to be used for the AlN growth. The holder may consist essentially of a substantially impervious backing plate.
1. A method for growing single-crystal aluminum nitride (AlN), the method comprising:
mounting an AlN seed on a seed holder, thereby forming a seed-seed holder assembly;
disposing the seed-seed holder assembly within a crystal-growth crucible;
heating the crystal-growth crucible to apply thereto (i) a radial thermal gradient of less than 50° C./cm and (ii) a vertical thermal gradient greater than 1° C./cm and less than 50° C./cm; and
depositing aluminum and nitrogen onto the AlN seed under conditions suitable for growing single-crystal AlN originating at the AlN seed.
2. The method of claim 1 , further comprising disposing AlN source material within the crystal-growth crucible, the deposited aluminum and nitrogen evolving from the AlN source material during heating of the crystal-growth crucible.
3. The method of claim 2 , wherein the AlN source material is polycrystalline.
4. The method of claim 1 , wherein the seed-seed holder assembly is affixed to a lid of the crystal-growth crucible.
5. The method of claim 1 , wherein mounting the AlN seed on the seed holder comprises disposing a foil between the AlN seed and the seed holder.
6. The method of claim 5 , wherein the foil is substantially impervious to aluminum transport.
7. The method of claim 6 , wherein the foil is substantially impervious to nitrogen.
8. The method of claim 5 , wherein the foil is substantially impervious to nitrogen.
9. The method of claim 5 , wherein the foil comprises tungsten.
10. The method of claim 5 , wherein the foil is single-crystalline tungsten.
11. The method of claim 5 , wherein the foil comprises aluminum.
12. The method of claim 1 , wherein the seed holder is substantially impervious to aluminum transport.
13. The method of claim 1 , further comprising disposing a barrier layer over at least a portion of a surface of the AlN seed.
14. The method of claim 13 , wherein the barrier layer comprises at least one of tungsten, Hf, HfN, HfC, W—Re, W—Mo, BN, Ta, TaC, TaN, Ta 2 N, or carbon.
15. The method of claim 13 , wherein the barrier layer consists essentially of tungsten.
16. The method of claim 1 , wherein the AlN seed is a wafer having a diameter of at least 20 mm.
17. The method of claim 1 , wherein the grown single-crystal AlN has a diameter greater than 20 mm, a thickness greater than 0.1 mm, and an areal planar defect density≦100 cm −2 .
18. The method of claim 17 , wherein the areal planar defect density is ≦1 cm −2 .
19. The method of claim 1 , further comprising minimizing or substantially eliminating any gap between the AlN seed and the seed holder by positioning a weight on the seed-seed holder assembly.
20. The method of claim 19 , wherein the weight is positioned on the AlN seed.
21. The method of claim 19 , wherein the weight comprises tungsten.
22. The method of claim 19 , further comprising removing the weight from the seed-seed holder assembly prior to depositing aluminum and nitrogen onto the AlN seed.
23. The method of claim 1 , wherein a ratio of the vertical thermal gradient to the radial thermal gradient is less than 10.
24. The method of claim 1 , wherein a ratio of the vertical thermal gradient to the radial thermal gradient is less than 5.5.
25. The method of claim 1 , wherein a ratio of the vertical thermal gradient to the radial thermal gradient is less than 3.
26. The method of claim 1 , wherein a ratio of the vertical thermal gradient to the radial thermal gradient is greater than 1.2.
27. The method of claim 1 , wherein the radial thermal gradient is larger than 4° C./cm.
28. The method of claim 1 , wherein the vertical thermal gradient is larger than 5° C./cm.
29. The method of claim 1 , wherein applying the radial thermal gradient comprises arranging a plurality of thermal shields outside the crystal-growth crucible.
30. The method of claim 29 , wherein each of the thermal shields comprises a refractory material.
31. The method of claim 29 , wherein each of the thermal shields comprises tungsten.
32. The method of claim 29 , wherein each thermal shield defines an opening therethrough.
33. The method of claim 32 , wherein the openings of the thermal shields are substantially equal in size to each other.
34. The method of claim 32 , wherein the opening of each thermal shield ranges from approximately 10 mm to approximately 2 mm less than a dimension of the growth chamber substantially perpendicular to a growth direction along which the single-crystal AlN grows.
35. The method of claim 32 , wherein the openings of at least two of the thermal shields are different in size.
36. The method of claim 32 , wherein a first thermal shield having a first opening is disposed between the crucible and a second thermal shield, the second thermal shield having a second opening larger than the first opening.
37. The method of claim 29 , wherein at least two of the thermal shields have different thicknesses.
38. The method of claim 29 , wherein a thickness of each of the thermal shields ranges from approximately 0.125 mm to approximately 0.5 mm.
39. The method of claim 29 , wherein the thermal shields are arranged with substantially equal spacings therebetween.
40. The method of claim 29 , wherein spacings between at least two pairs of the thermal shields are different.