Electronic and optical properties of nonpolar a-plane GaN quantum wells
Schulz, S and Badcock, T J and Moram, M A and Dawson, P and Kappers, M J and Humphreys, C J and O’Reilly, E P
In this paper we present a detailed study of the electronic band structure of a series of nonpolar a-plane GaN/AlGaN multiple quantum wells (QWs) of varying well width using complementary results from x-ray 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 k⋅p 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 c-plane 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.
Department of Materials
Imperial College London
Royal School of Mines
London, SW7 2AZ - UK