High dielectric constant and tunability of epitaxial SrTiO3 thin film capacitors

The dielectric and insulating properties of epitaxial SrTiO3(STO) thin film capacitors were studied. The films were grown by inverted cylindrical magn etron sputtering in the radio frequency mode on (100) STO substrates which were covered with a (001)-oriented YBa2Cu3O7-x (YBCO) layer as a ground ele ctrode. As a top electrode we used YBCO or Au thin films. A high dielectric constant, epsilon, of up to 5000 was observed at T=80 K. The capacitors re vealed a large tunability, i.e., a nonlinear epsilon( E) dependence, with r espect to voltage biasing. By applying 3 V, epsilon decreased to 1000 which was 20% of its maximum value. The frequency dependence of epsilon, the tem perature dependence of the dielectric loss factor, tan delta, and the direc t currency conductivity reflected that variable range hopping via localized states was present and dominated the conduction process in the STO films a t low temperatures. The field strength for the electrical breakdown amounte d 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 mu C/cm(2) at T=4.2 K. The ferroelectric phase transition was foun d to be of second order and of the displacive type. (C) 1999 American Insti tute of Physics. [S0021-8979(99)07310-7].