Relaxation Mechanism of Thermal Stresses in the Heterostructure of GaN Grown on Sapphire by Vapor Phase Epitaxy

TLDR

The study investigates how thermal strains and stresses in GaN/α‑Al₂O₃ heterostructures vary with GaN film thickness (0.6–1200 µm). A model incorporating sapphire cracking relaxation is used to calculate stresses and strains. Strain is highest in films thinner than a few microns, decreases with thickness up to ~100 µm, and is nearly fully relaxed above 100 µm, with three relaxation mechanisms identified: lattice deformation (<4 µm), interface defect enhancement (4–20 µm), and sapphire macrocracking (>20 µm).

Abstract

Thermal strains and stresses due to the thermal expansion coefficient difference in GaN(0001)/α-Al 2 O 3 (0001) layered structures are studied by varying the film thickness of GaN from 0.6 to 1200 µm. 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 µm. 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 (&lt;4 µm), (b) enhancement of interface defects such as “microcracks” and/or dislocations (4-20 µm), and (c) generation of “macrocracks” in sapphire (&gt;20 µm).