Charge transport in epitaxial barium titanate films

The electrical conductivity of epitaxial $\mathrm{BaTi}{\mathrm{O}}_{3}$ films was studied by small-signal impedance spectroscopy at temperatures of 10--720 K using Pt-$\mathrm{BaTi}{\mathrm{O}}_{3}\ensuremath{-}\mathrm{SrRu}{\mathrm{O}}_{3}$ capacitors. The $\ensuremath{\sim}150$-nm-thick $\mathrm{BaTi}{\mathrm{O}}_{3}$ films possessed different lattice strains and degrees of oxygen deficiency. A crossover between the low-temperature hopping of small polarons and the high-temperature semiconductor- to metal-type behavior was demonstrated in all films. It was suggested that the small electron polarons originate from self-trapping at Ti in the stoichiometric tensile-strained film and from trapping at Ti next to the oxygen vacancy in the oxygen-deficient films. The conduction-band transport was ascribed to the thermally activated release of the trapped electrons. It was pointed out that the electronic release can mimic the motion of oxygen vacancies, which are actually immobile.