High dielectric constant and tunability of epitaxial SrTiO3 thin film capacitors

Abstract

The dielectric and insulating properties of epitaxial SrTiO3(STO) thin film capacitors were studied. The films were grown by inverted cylindrical magnetron sputtering in the radio frequency mode on (100) STO substrates which were covered with a (001)-oriented YBa2Cu3O7−x (YBCO) layer as a ground electrode. As a top electrode we used YBCO or Au thin films. A high dielectric constant, ε, of up to 5000 was observed at T=80 K. The capacitors revealed a large tunability, i.e., a nonlinear ε(E) dependence, with respect to voltage biasing. By applying 3 V, ε decreased to 1000 which was 20% of its maximum value. The frequency dependence of ε, the temperature dependence of the dielectric loss factor, tan δ, and the direct currency conductivity reflected that variable range hopping via localized states was present and dominated the conduction process in the STO films at low temperatures. The field strength for the electrical breakdown amounted to 300 kV/cm even for rather thin films with a thickness of about 40 nm. Below T=90 K, the STO films were ferroelectric with a high polarization of up to 30 μC/cm2 at T=4.2 K. The ferroelectric phase transition was found to be of second order and of the displacive type.