MICROWAVE PROPERTIES OF YBA2CU3O7-X SRTIO3 PLANAR CAPACITORS/

The microwave properties of SrTiO3 (STO)/CeO2 varactors deposited on ( <1(1)under bar 02>) 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 rang ing from 78 - 300 K, and bias voltages up to 260 V across a 20 mu m ca pacitance gap. The temperature and bias voltage dependencies of capaci tance and loss of the varactors were analyzed. Below the superconducti ng transition temperature, the microwave frequency dielectric properti es of the STO film (tunability and tan delta) were dominant in establi shing 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)) w as 1.6 for both electrode structures. Tan delta was also identical for both structures, with tan delta = 0.04 - 0.05 under zero bias, and re duction of tan delta by a factor of 1.3 under high de bias conditions observed.