Residual stress relaxation in GaN/sapphire circular pillars measured by Raman scattering spectroscopy

Circular pillar structures of various diameters have been prepared by a focused ion beam (FIB) through a 4-μm-thick epitaxial gallium nitride (GaN) film grown on a sapphire substrate. Micro-Raman scattering is used to measure residual stresses based on the shift of an E 2 phonon in the GaN film. Measurements of residual stress profiles are compared to Winkler's elastic formalism for a shear-supported film with proper boundary conditions. The model, optimized at a cleave edge, is compared to the experimental shape of stress variations inside and outside the pillar structures.