Optical properties of wurtzite GaN epilayers grown onA-plane sapphire

The exciton binding energies are measured in GaN grown on $A$-plane sapphire. A value of $25\pm 1\mathrm{meV}$ 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 $-2.4\mathrm{eV}$ for the $C_5$ deformation potential in GaN.