Critical points of the band structure and valence band ordering at the Γ point of wurtzite InN

We compare the ordinary dielectric functions for wurtzite InN which have been determined from ellipsometric studies of an (112 0) a-plane and a low carrier density (0001) c-plane film. Excellent agreement is found over the whole investigated energy range from 0.72 up to 9.5eV. The transition energies in the range of the critical points of the band structure are estimated. A characteristic optical anisotropy is observed below 1 eV which is attributed to the polarization-dependent transition probabilities from the three valence bands at the Γ point of the Brillouin zone into the conduction band. Comparing the experimental absorption edge shift of 25meV for the two polarization directions with calculations of the level splitting and taking into account the relative oscillator strength of transitions, we determine values for the crystal field energy between 19 and 24 meV if spin-orbit energies of 13 and 5 meV, respectively, are assumed. The data emphasis a band gap value of wurtzite InN below 0.7 eV.