Editor's Choice Analysis of polarization-dependent photoreflectance studies for c-plane GaN films grown on a-plane sapphire

GaN and related compounds are usu-ally grown on (0001)-oriented (c-)sapphire or SiC sub-strates as well as on (001) silicon resulting in c-plane orientation of the films. The mismatch of the thermal ex-pansion coefficients between substrateand film causes an in-plane strain, which is isotropic in this case. It shifts the free excitonic transitions FX