Microwave properties of YBa2Cu3O7-x/SrTiO3 planar capacitors

Abstract

The microwave properties of SrTiO3 (STO)/CeO2 varactors deposited on (1102) oriented Al2O3 substrates, using either YBCO or Ag/YBCO electrodes, was investigated. Measurements were conducted using a microstrip resonator at a frequency of 3 GHz, over temperatures ranging from 78–300 K, and bias voltages up to 260 V across a 20 μm capacitance gap. The temperature and bias voltage dependencies of capacitance and loss of the varactors were analyzed. Below the superconducting transition temperature, the microwave frequency dielectric properties of the STO film (tunability and tanδ) were dominant in establishing the varactor properties for both the YBCO and Ag/YBCO electrode structures. At T = 78 K, the capacitance ratio (= C (0 V)/C (260 V)) was 1.6 for both electrode structures. Tanδ was also identical for both structures, with tanδ = 0.04 – 0.05 under zero bias, and reduction of tanδ by a factor of 1.3 under high dc bias conditions observed.