Spectroscopic investigation of the hole states in Ni-deficient NiO films

The influence of Ni vacancies on the chemistry and electronic structure of NiO thin films was investigated using X-ray absorption spectroscopy and extended X-ray absorption fine structure analysis. Changes in the electronic structures upon partial oxidation are mainly addressed. It is strongly suggested that the hole carriers are mostly delocalized on oxygen sites while localized holes coexist at both Ni and O sites. Such delocalized carriers are found to be depleted by capping with a thin n-type TiO2 layer. This suggests that the defect states can be healed effectively by the TiO2 capping and its density can be tuned for functionality as a base p-type oxide material. The relationship with threshold resistive switching behavior is also discussed.