A state in the band-gap, induced by an O vacancy, was considered to lead to a leakage current and was investigated theoretically. The calculated state which originated from the O vacancy was deeper than the experimentally obtained value. It was suggested that the strongly distorted local structure around the O vacancy could give rise to a shallow energy level, because the energy level of the vacancy state was shallower, before structural optimization, than after it.

Electronic Structure of Oxygen Vacancy in Ta2O5. H.Sawada, K.Kawakami: Journal of Applied Physics, 1999, 86[2], 956-9