Leakage current and charge carriers in (Na0.5Bi0.5)TiO3 thin film

A sodium bismuth titanate (Na0.5Bi0.5)TiO3 (NBT) thin film of perovskite structure, synthesized by radio-frequency magnetron sputtering, exhibited a remanent polarization of 11.9 μC cm-2 and a coercive field of 37.9 kV cm-1 at room temperature. It however showed a rather high leakage current density of ∼6 x 10-5 A cm-2 at an applied electric field of 100 kV cm-1. There occurs a change in the controlling mechanism of the electrical behaviours of the NBT thin film from grain interiors to grain boundaries with increasing temperature. The ac conductivity obeys the Jonscher relation. The activation energies for dc conductivity and hopping frequency of the charge carriers are calculated to be 0.92 eV and 1.00 eV, respectively, suggesting oxygen vacancies are the most likely charge carriers at high temperatures. Hopping of oxygen vacancies trapped at the grain boundaries and excitation of polarons in the grain interior are responsible for the relatively high dielectric loss and high dc conductivity. The contribution of hopping charge carriers to the dielectric response is demonstrated by the frequency dispersion observed for the relative permittivity in the low frequency region