Optical anisotropy in GaN grown onto A–plane sapphire

GaN epilayers grown on A–plane sapphire experience an orthorhombic strain field giving an in-plane anisotropy of the optical response. By varying the polarisation conditions of reflectivity measurements, we measure the effects of the in-plane anisotropy of the strain field. Also, from a very careful lineshape fitting of the reflectivity spectra, we obtain the splittings between Γ2 and Γ4 excitons and report the first determination of the electron–hole exchange energy in wurtzite GaN, 0.6±0.1 meV. This value is compared to the data obtained for other III–I and II–VI semiconductors, taking into account the length of the chemical bonds.