Space-charge dominated epitaxial BaTiO3 heterostructures

Epitaxial BaTiO3 thin films (∼300 nm thick) with various conductive oxide bottom electrodes including (La0.5Sr0.5)CoO3, SrRuO3 and LaNiO3 were prepared on single-crystalline SrTiO3 substrates using a radiofrequency magnetron sputtering technique in an oxygen-rich atmosphere and at a deposition temperature of 700 °C. The charge transport phenomena of these films were systematically investigated. Comparison of leakage current levels with respect to different BaTiO3-bottom electrode interfaces gave a direct proof of the p-type semiconducting behavior in the BaTiO3 films, whose I-V characteristics were well described by a modified Schottky contact model up to an applied field of ∼60 MV m−1. By adapting this model, we obtained interface potential barriers, space-charge and free charge carrier densities, as well as depletion layer width and interface layer thickness for the epitaxial BaTiO3 films. The dominance of space-charges (∼1020 cm−3) over intrinsic polarizations in these films well explained the apparent reduction in remnant polarization and tilting of the P-V hysteresis loops, as well as an enhanced dielectric strength. Further increase of the applied electric field will result in complete depletion of the BTO films. Consequently, charge transport in these films will become bulk-controlled. This prediction was validated by experimental J-V curves acquired for a broad range of applied electric field, where ionic conduction and Pool-Frenkel emission currents consecutively dominated in the high field (60-220 MV m−1) and ultrahigh field (220-300 MV m−1) regions. Finally, the potential of these lead-free heterostructures as high-energy density dielectric capacitors was demonstrated.