Effect of strain on GaN exciton spectra

The large majority of GaN epitaxial films have been grown on substrates which show both lattice and thermal expansion mismatch and, as a result, are significantly strained. The effect is most readily monitored by measuring exciton energies in photoluminescence spectra, and this paper compares published data on free exciton energies in a homoepitaxial film with a range of heteroepitaxial samples grown on sapphire substrates, all the films having the hexagonal (wurtzite) structure. We observe that strain results in a marked increase in separation of the B and C excitons while leaving the A, B separation relatively unaffected, and interpret this in terms of the quasi-cubic band structure model developed by Hopfield for the very similar case of CdS and other II-VI compounds. Within the limits of this model, there is an ambiguity in the interpretation of exciton energies which depends on the fact that the spin-orbit and crystal field splitting parameters are interchangeable. Consideration of strain effects removes this ambiguity and allows us to derive values of Delta(SO) = 10.4 meV and Delta(CR) = 19.2 meV for the unstrained homoepitaxial sample, consistent with the earlier estimates of Dingle et al based on relative intensity measurements.