Résumé
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 Γ
2 and Γ
4 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.