Epitaxial strain energy measurements of GaN on sapphire by Raman spectroscopy

This study reports a non-destructive method of measuring the residual strain in the GaN epilayer grown on sapphire substrate by micro-Raman spectroscopy. Operating in confocal mode this method allows a depth-dependent measurement of residual strain in the epitaxial layer without prior treatment of the sample. This approach to measurement of residual strain is demonstrated on GaN epitaxial films grown by both MOCVD and H-MOVPE. In the case of MOCVD grown films, the biaxial strain energy was found to vary from 0 (GaN surface) to 5.0 kJ/mole (GaN/sapphire interface), but in the case of H-MOVPE grown samples the strain energy varied from 6.5 kJ/mole hydrostatic strain (GaN surface) to 25.0 kJ/mole – biaxial strain (GaN/sapphire interface), indicating that the surface layer of the N-terminated HMOVPE material is not free from strain. Estimates are given for the curvature of substrate, lattice parameter of epitaxial layer, and the interface shear modulus.