Electronic and optical properties of nonpolar a-plane GaN quantum wells

In this paper we present a detailed study of the electronic band structure of a series of nonpolar a-plane diffraction, polarization-dependent photoluminescence excitation spectroscopy, and k-p theory. When excited with unpolarized light, excitonic transitions involving different electron subbands are resolved in the excitation spectra. For linearly polarized (E⊥c, E||c) excitation, these are shown to consist of overlapping transitions involving different hole subbands. These results are then analyzed in detail using strain data determined by the x-ray diffraction measurements in combination with the kp theory to calculate the bulk band structure and the relative oscillator strength of an a-plane GaN film under strain. The results are compared with those of an unstrained oplane film. This analysis reveals that the experimentally observed polarization anisotropy can be attributed to anisotropic strain in the C plane. Based on the k-p Hamiltonian, we apply an effective mass approximation, taking into account strain and nonparabolicity effects, to calculate the single-particle states and energies for the different quantum wells. The possible influence of the weak spin-orbit coupling on the results is studied in detail. Starting from the single-particle energies and including excitonic binding energies, the band edge optical transitions are calculated and successfully compared to the experimental data. Our analysis gives an estimate for the conduction-to valence-band offset ratio of 45:55 for nonpolar GaN/AlGaN QW structures. Additionally, our study also allows us to investigate the magnitude of the crystal-field splitting and spin-orbit coupling in GaN systems.