OPTICAL AND ELECTRONIC-STRUCTURE STUDY OF CUBIC AND HEXAGONAL GAN THIN-FILMS

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 hex agonal GaN is higher than that of the cubic polytype. The parameters o f the gaps are determined against temperature and the temperature shif ts are found to be lower than and close to those of GaAs and GaP in th e cases of cubic and hexagonal GaN, respectively. Additional theoretic al calculations of the electronic structure of both polytypes using th e full-potential linear-muffin-tin-orbital method reveal a significant contribution to the E(0) gap from the 8 --> 10 transitions. The resul ting gap energies are compared with the Literature and the difference between the two GaN polytypes is discussed. The dielectric function ep silon(2) (omega) is directly calculated from the band structure and it s features at energies up to 9.5 eV are discussed and compared to expe riment.