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 Gamma(2) and Gamma(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. (C) 1999 Elsevier Science S.A. All rights reserved.