Optical properties of wurtzite GaN epilayers grown on A -plane sapphire

The exciton binding energies are measured in GaN grown on A-plane sapphire. A value of 25±1meV is found to be consistent with reflectance and photoreflectance experiments. This 25 meV value matches very well the results obtained on GaN epilayers grown on C-plane sapphire with strong biaxial compression, and seems to indicate that exciton energies in GaN epilayers are predominantly influenced by strain fields. From the in-plane anisotropy of optical response, we deduce a value of -2.4eV for the C₅ deformation potential in GaN.