Oxygen vacancy mediated conductivity and charge transport properties of epitaxial Ba0.6La0.4TiO3− δ thin films

Abstract

We report on the effects of the oxygen vacancy (VO) regarding the microstructure, conductivity, and charge transport mechanisms of epitaxial Ba0.6La0.4TiO3−δ (BLTO) films. The VO concentration can be largely regulated from 21.5% to 37.8% by varying the oxygen pressure (PO2) during film deposition. Resistivity-temperature and Hall effect measurements demonstrate that the BLTO films can be tuned remarkably from an insulator to a semiconductor, and even to a metallic conductor by regulating the VO concentration. The role of VO concentration in the charge transport mechanism is clarified. For films with low VO concentration, the charge transport is dominated by variable range hopping (VRH) at low temperatures, and it shows small polaron (SP) hopping at high temperatures. For films with high VO concentration, the carrier transport remains VRH at low temperatures, while it changes to SP hopping at moderate temperatures, and is dominated by thermal phonon scattering at high temperatures. Furthermore, the lower starting temperature of SP hopping for films with higher VO concentrations indicates that VO favors electron-phonon coupling. Different charge transport mechanisms are assumed to be due to different VO-induced defect energy levels in the BLTO films, which has been verified by their soft x-ray absorption spectroscopy results.