Abstract
We calculate the excitonic exchange interaction in alpha-GaN, and find it to be about 2 meV. A theoretical modelling of excitons is performed, and the oscillator strengths of radiative levels are calculated as a function of strain for (0001)-grown epilayers. In particular, we find that the strength of optical transitions are extremely sensitive to the residual strain field in view of the small value of the spin-orbit interaction. Our calculation shows agreement with low-temperature reflectance investigations in GaN epilayers grown on sapphire substrates.