Band structures and optical spectra of InN polymorphs: Influence of quasiparticle and excitonic effects

We present ab initio calculations of the electronic structure and the optical properties of InN crystallizing in wurtzite, zinc-blende, and rocksalt structures. They are based on well converged atomic geometries including the effect of the In $4d$ electrons. The quasiparticle aspect is described in the framework of a $GW$ approximation. The opposite influence of quasiparticle effects and the $pd$ repulsion is demonstrated for the band structures. The pecularities of the band structures such as the small gap of the $2H$ and $3C$ polymorphs and their large electron affinity are discussed. The frequency dependence of the dielectric functions is explained in terms of nonparabolic bands and high-energy optical transitions at critical points. Excitonic effects drastically influence the optical absorption by an overall redshift of the entire spectrum and a redistribution of oscillator strengths. The results are critically discussed in the light of recent experiments.