The InN semiconductor is a challenge for the theoreticians. There are several pecularities in the electronic band structure and, hence, in the resulting electrical and optical properties: (i) Extremely shallow In4d electrons which influence the valence bands, e.g. by the pd repulsion. (ii) One of the largest electron affinities which is responsible for the small fundamental gap, the small electron effective mass, and the nonparabolicity of the conduction band. (iii) A charge neutrality level in the conduction band being responsible for the n-accumulation layer at the surface. (iv) Strong excitonic effects because of the small electron mass and dielectric constant.

All effects are included in the calculation of the electronic band structure and the many-body effects accompanying electronic excitations. The consequences are demonstrated for the ordinary and extraordinary dielectric functions. After inclusion of the pd repulsion, the quasiparticle band shifts, the attractive electron-hole interaction as well as the electron-hole exchange, we find excellent agreement with recent ellipsometry measurements concerning peak positions and lineshape. There are however deviations concerning the peak intensities above photon energies of 5eV.

The imaginary parts of the dielectric functions show a characteristic lineshape. After a steep onset of the absorption around 1eV, a plateau follows until about 4.5eV. Several pronounced peaks occur in the high-frequency region. We demonstrate that the absorption onset and the plateau-like region are dominated by the nonparabolicity of the conduction band. The interpretation of the high-energy absorption peaks only in terms of optical transitions at critical points is insufficient. The Coulomb interaction leads to an intermixing of transitions from different k-space regions. As a consequence oscillator/spectral strength is redistributed from higher to lower photon energies.

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