Résumé
We address the strain state of GaN epilayers grown on C plane sapphire, C plane of 6H-SiC, (111) plane of silicon, and on A plane sapphire. We can propose the values of the six deformation potentials in GaN. We next examine the problem of exciton binding energies which are found to be near 25 meV. The A exciton has a binding energy which displays vanishingly small variations with strain. A tentative explanation of the scattering of the data in the literature is proposed in terms of the exciton polariton picture, ansi in terms of confined states in the photon-like branch.