Optoelectronic properties of valence-state-controlled amorphous niobium oxide

In order to understand the optoelectronic properties of amorphous niobium oxide (a-NbO _x ), we have investigated the valence states, local structures, electrical resistivity, and optical absorption of a-NbO _x thin films with various oxygen contents. It was found that the valence states of Nb ion in a-NbO _x films can be controlled from 5+  to 4+  by reducing oxygen pressure during film deposition at room temperature, together with changing the oxide-ion arrangement around Nb ion from Nb₂O₅-like to NbO₂-like local structure. As a result, a four orders of magnitude reduction in the electrical resistivity of a-NbO _x films was observed with decreasing oxygen content, due to the carrier generation caused by the appearance and increase of an oxygen-vacancy-related subgap state working as an electron donor. The tunable optoelectronic properties of a-NbO _x films by valence-state-control with oxygen-vacancy formation will be useful for potential flexible optoelectronic device applications.