Theoretical prediction of ELNES/XANES and chemical bondings of AlN polytypes.

The first principles calculations of ELNES/XANES of AlN polytypes were carried out by the first-principles OLCAO method using large supercells composed of more than 100 atoms. It can quantitatively reproduce the experimental spectra from wurtzite AlN using a 108-atoms supercell. ELNES from rock-salt and zinc-blend AlN were predicted by using 128 atoms supercells. The spectral features of rock-salt phase are different from other phases, whereas that of zinc-blend phase have numerous similarities with that from wurtzite AlN. Characteristic differences between the wurtzite and zinc-blend phases are predicted to appear at the first peak of Al L(2,3) and K edges. The first peak of zinc-blend AlN is broader than that of wurtzite AlN. The same tendency was found in the case of SiC. In order to elucidate the cause of the broadness at the first peak, partial density of states and chemical bondings were investigated. The theoretical analysis revealed that the broadness of the first peak is related to the covalency of the compounds. This result suggests that the spectral features at the first peak of L(2,3) and K edges contain information about the covalency at the illuminated area.