Dielectric characterization of strontium titanate thin films using Josephson-junction-based on-chip resonators

Abstract

Josephson junction oscillators provide a means to determine the dielectric constant (εR) and its frequency, temperature and electric field dependence (εR(ω, T, E)) in thin-film dielectrics in the frequency range 100–900 GHz. Here, we have applied this technique to a 150 nm thick SrTiO3 (STO) film and found a variation in εR at 4.2 K from 363 ± 5 at one end of the sample to 150 ± 6 at the other end and in the absence of external bias voltage V across the STO. On application of such a bias, the εR of the film at 4.2 K was found to change by 30%. The maximum in εR(V ) was found to be at −1 V instead of the expected 0 V, indicating the presence of an internal electric field in the STO layer, which we attribute to the difference in work functions of the gold and YBa2Cu3O7 electrodes in our structure. εR(T ) was markedly different to that of bulk STO. In our thin film, εR(T ) has a broad shoulder in which the temperature dependence is small between about 35 and 50 K, above which it rises again.