The first part of the talk provides an overview how to determine a reliable complex dielectric function (DF) of a material from spectroscopic ellipsometry studies covering the whole spectral range between infrared and vacuum-ultraviolet. Then, the peculiarities of the spectral dependence of the DF for nitride semiconductors with zinc-blende and wurtzite crystal structure will be discussed in detail. It is demonstrated that the inclusion of electron-hole interaction becomes very important for correlating the experimental results with one-electron band-structure calculations, i.e. for determining transition energies for critical points of the band structure. The valence-band ordering at the center of the Brillouin zone has a strong impact on the optical anisotropy of hexagonal compounds around the band gap. Recent results, obtained from measurements on non-polar surface orientations, demonstrate the main aspects. Many-particle effects such as band-gap renormalization and band-filling lead to an energy shift of the absorption edge for high carrier concentrations. Examples for the DF analysis under those conditions will be presented in the final part.

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2. Optical properties of InN grown on Si(111) substrate
3. Impact of stress on the optical properties of AlN layers
4. Optical properties of high-quality cubic AlN, GaN, and AlN/GaN MQWs grown on 3C-SiC
5. Influence of exciton-phonon interaction on the absorption properties of ZnMgO alloys
6. Optical anisotropy of InN from near-IR to deep-UV