Exciton fine structure in undoped GaN epitaxial films

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Abstract
We report on photoluminescence experiments on hexagonal GaN epitaxial films grown by hydride and organometallic vapor phase epitaxy on sapphire and 6H-SiC. At low temperatures we observe free and bound exciton recombinations, which allow us to establish the free-exciton binding energy and the localization energies of the excitons bound to neutral donors in undoped films. We demonstrate that the energetic positions of the excitonic recombination lines depend on the layer thickness and the substrate materials on which the layer was deposited. The influence of strain on the valence-band splittings can be quantified when observing the free-exciton transitions onto the different valence bands. The experimental results are compared to a theoretical calculation using a first-principle total-energy pseudopotential method within the local-density formalism. We present evidence for the existence of two shallow donors in GaN. One of them most likely stems from an intrinsic defect. © 1996 The American Physical Society.