Electrical tuning of passive HTS microwave devices using single crystal strontium titanate

Over the last few years several groups have fabricated High Temperature Sup erconducting (BTS) thin film microwave devices which have included a ferroe lectric thin film layer in their design. The electric field dependence of t he ferroelectric promises desirable in-situ tuning of the frequency respons e. Here ne present the results of our experiments on the tuning of a coplan ar HTS resonator using single crystal strontium titanate (STO). We have pat terned a simple coplanar resonator device onto Y-Ba-Cu-O thin films deposit ed on (100) magnesium oxide substrates. Careful consideration of the size, position and biasing of these high permittivity STO crystals with respect t o the planar device allo,vs us to minimise the perturbance to the plain; re sonators response,whilst at the same time maximising the degree to which we are able to tune the device. We have also performed a series of capacitanc e measurements on our STO crystals to obtain reliable data for the dependen ce of the permittivity on temperature, applied bias and crystallographic or ientation.