Relaxation Mechanism Of Thermal Stresses In The Heterostructure Of Gan Grown On Sapphire By Vapor Phase Epitaxy

Abstract

Thermal strains and stresses due to the thermal expansion coefficient difference in GaN(000l)/a-Al203(0001) layered structures are studied by varying the film thickness of GaN from 0.6 to 1200 xm. 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 fxm. 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 μm), (b) enhancement of interface defects such as “microcracks” and/or dislocations (4-20 μm), and (c) generation of “macrocracks” in sapphire (>20 xm). © 1993 The Japan Society of Applied Physics.