The in-situ growth temperature was measured by the same optical pyrometer focused on the top lid of the retort and controlled by the output power. Substrate Preparations 6H-SiC single crystals of various orientations and surface preparation were used, to determine their influence on the AlN growth mode. Morphology Characterization Techniques Macrostuctrual features in the AlN crystals were characterized by Nomarski differential interference contrast microscopy. Results and Discussion 2.1. The AlN grains were 1mm to 2mm in size after several ten hours of growth.
Screw dislocations provide the steps for the growth of crystal.
This shows that the diffusion length of Al or N atoms on the surface is too small to enhance a 2-dimensional nucleation and growth. The large thermal stress between substrates and deposited AlN results in high screw dislocation density. This merging produced the grain boundaries seen in Figure 2 . The large stress introduced by the cooling from sublimation temperature to the room temperature, cracked the AlN into small pieces several mm in size. Occasionally the SiC substrate peeled off from thicker bulk AlN crystal, as shown in Figure 5 .