Electronic states in valence and conduction bands of group-III nitrides: Experiment and theory

A comprehensive study of the electronic structure of group-III nitrides (AlN, GaN, InN, and BN) crystallizing in the wurtzite, zinc-blende, and graphitelike hexagonal (BN) structures is presented. A large set of the x-ray emission and absorption spectra was collected at the several synchrotron radiation facilities at installations offering the highest possible energy resolution. By taking advantage of the linear polarization of the synchrotron radiation and making careful crystallographic orientation of the samples, the bonds along c axis (pi) and in plane'' (sigma) in the wurtzite structure could be separately examined. Particularly for AlN we found pronounced anisotropy of the studied bonds. The experimental spectra are compared directly with ab initio calculations of the partial density of states projected on the cation and anion atomic sites. For the GaN, AlN, and InN the agreement between structures observed in the calculated density of states (DOS) and structures observed in the experimental spectra is very good. In the case of hexagonal BN we have found an important influence of insufficient core screening in the x-ray spectra that influences the DOS distribution. The ionicity of the considered nitrides is also discussed.