Investigation of electronic structure of amorphous niobium oxide based on the density functional theory calculation of crystalline niobium pentoxide polymorphs

Electronic structure of amorphous niobium oxide prepared by a sputtering method was investigated based on optical absorption and photoelectron spectroscopies. In the valence band photoelectron spectra, broad peaks without any characteristic components were observed. Then, theoretical calculations based on a density functional theory were performed to interpret the experimental spectra by using three Nb₂O₅ polymorphs. Among the polymorphs, M-phase with tetragonal structure showed better reproducibility than the other B- and R-phases with monoclinic structure. It was finally concluded that the amorphous niobium oxide had a similar electronic structure to M-Nb₂O₅, and it was supposed that the broad feature in the photoelectron spectra was due to the broad distribution of Nb-O bonds in NbO₆ polyhedra, which was characteristic in M-Nb₂O₅.