Residual strain in HVPE GaN free-standing and re-grown homoepitaxial layers

The lattice parameters of as-grown hydride vapor phase epitaxy GaN layers on sapphire, free-standing layers after the substrate lift-off, and homoepitaxial layers grown on the free-standing layers are measured. The in-plane and out-of-plane strains are calculated. It is found that the substrate removal leads to strain relaxation in the crack-free GaN free-standing layers to a certain extent. A small increase of the strain in the GaN homoepitaxial layers compared to the free-standing layers is observed. Cathodoluminescence (CL) spectroscopy and imaging, photoluminescence (PL) and Raman measurements are used as complementary tools in the residual strain analysis.