Optical and electronic-structure study of cubic and hexagonal GaN thin films

Abstract

The optical properties of cubic and hexagonal GaN films in the region of the fundamental gap are studied with spectroscopic ellipsometry at temperatures between 110 and 630 K. It is verified that the gap of hexagonal GaN is higher than that of the cubic polytype. The parameters of the gaps are determined against temperature and the temperature shifts are found to be lower than and close to those of GaAs and GaP in the cases of cubic and hexagonal GaN, respectively. Additional theoretical calculations of the electronic structure of both polytypes using the full-potential linear-muffin-tin-orbital method reveal a significant contribution to the ${\mathit{E}}_0$ gap from the 8→10 transitions. The resulting gap energies are compared with the literature and the difference between the two GaN polytypes is discussed. The dielectric function ${\mathrm{\varepsilon }}_2$ (ω) is directly calculated from the band structure and its features at energies up to 9.5 eV are discussed and compared to experiment.