We present a detailed analysis of the ordinary and extraordinary dielectric tensor components (DF) of InN corresponding to electric field polarization perpendicular and parallel to the c-axis, respectively. The data were determined by spectroscopic ellipsometry and cover the energy range from 0.72 up to 30 eV. Results for both c-plane and a-plane InN films grown by MBE are compared.
The data indicate unambiguously two absorption edges below 0.9 eV depending on the light polarization direction. Such a behavior is typical for wurtzite material around the band gap. It arises from the different optical selection for transitions from the three valence bands at the center of the Brillouin zone into the conduction band. The extraordinary absorption edge is found 25 meV higher than the ordinary one, which corresponds to the splitting between the uppermost and the lowest-lying valence bands. Assuming a spin-orbit energy of 5 meV we estimate an experimental value for the crystal-field parameter of 24 meV. The extrapolation of the ordinary tensor component emphasizes a band gap of ~0.65 eV at room temperature for wurtzite InN, if carrier induced band gap renormalization, conduction band occupation as well as its non-parabolicity are taken into account.
The shape of the experimental DF’s and the polarization dependence in the energy range from 4.5 up to 9.5 eV are in excellent agreement with theoretical calculations. Low-temperature studies as well as a third-derivate based line shape analysis allow us to determine the transition energies for a large number of critical points of the band structure with highest precision. Finally, we discuss the polarization dependence of the semi-core-level excitations above 16 eV. The data analysis yields a value of 830 meV for the spin-orbit splitting of the InN 4d-bands.

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