EXCITON FINE-STRUCTURE IN UNDOPED GAN EPITAXIAL-FILMS

We report on photoluminescence experiments on hexagonal GaN epitaxial films grown by hydride and organometallic vapor phase epitaxy on sapph ire and 6H-SiC. At low temperatures we observe free and bound exciton recombinations, which allow us to establish the free-exciton binding e nergy and the localization energies of the excitons bound to neutral d onors in undoped films. We demonstrate that the energetic positions of the excitonic recombination lines depend on the layer thickness and t he substrate materials on which the layer was deposited. The influence of strain on the valence-band splittings can be quantified when obser ving the free-exciton transitions onto the different valence bands. Th e experimental results are compared to a theoretical calculation using a first-principle total-energy pseudopotential method within the loca l-density formalism. We present evidence for the existence of two shal low donors in GaN. One of them most likely stems from an intrinsic def ect.