RELAXATION MECHANISM OF THERMAL-STRESSES IN THE HETEROSTRUCTURE OF GAN GROWN ON SAPPHIRE BY VAPOR-PHASE EPITAXY

Thermal strains and stresses due to the thermal expansion coefficient difference in GaN(0001)/alpha-Al2O3(0001) layered structures are studied by varying the film thickness of GaN from 0.6 to 1200 mum. The strain in GaN is greater in films of less than a few microns thickness. It is decreased in films of thickness from several to about a hundred microns, and is almost completely relaxed in those thicker than 100 mum. The stresses and strains in the heterostructure are calculated using a model in which relaxation due to cracking in the sapphire is considered. Three relaxation mechanisms of the thermal strain are found for different film thicknesses as follows: (a) only lattice deformation (<4 mum), (b) enhancement of interface defects such as ''microcracks'' and/or dislocations (4-20 mum), and (c) generation of ''macrocracks'' in sapphire (> 20 mum).