The optical properties and electronic transitions of cubi and hexagonal GaN films between 1.5 and 10 eV

The optical properties and electronic transitions of cubic (β) and hexagonal (α) GaN films grown by electron-cyclotron resonance molecular beam epitaxy were investigated in the energy region from 1.5 to 10 eV with conventional and synchrotron radiation ellipsometry. This is the first direct observation of the dielectric function of both polytypes in this energy region. The fundamental absorption of β-GaN is located at a lower energy than that of α-GaN. The observed structures are different for both phases and are analysed in the temperature range 80–650 K. With reference to electronic band structure calculations, the transitions are assigned to specific regions in the respective Brillouin zones. Finally, the reflectivities of α-GaN are β-GaN are compared with contradictory results in the literature.