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 E2 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.