GaN epilayers grown on A-plane sapphire experience an orthorhombic strain f
ield giving an in-plane anisotropy of the optical response. By varying the
polarisation conditions of reflectivity measurements, we measure the effect
s of the in-plane anisotropy of the strain field. Also, from a very careful
lineshape fitting of the reflectivity spectra, we obtain the splittings be
tween 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 va
lue is compared to the data obtained for other III-I and II-VI semiconducto
rs, taking into account the length of the chemical bonds. (C) 1999 Elsevier
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