Tuning of Y-Ba-Cu-O ring resonators by Sr-Ti-O thin films

Tunable YBa/sub 2/Cu/sub 3/O/sub 7-(delta)/ (YBCO) ring resonators were produced to study the nonlinear dielectric properties of SrTiO/sub 3/ (STO) thin films. Epitaxial STO/YBCO double layers were deposited by sputtering on LaAlO/sub 3/ substrates and patterned to coplanar ring resonators. By applying an electric field of 100kV/cm the resonant frequency could be shifted by 5% from 5.13 to 5.38 GHz and the unloaded quality factor, Q/sub 0/, of the resonator increased from 195 to 394. The dielectric properties sensitively depended on the deposition temperature of the STO films which was varied from 710 to 810(degree)C. With increasing deposition temperature the relative permittivity, (epsilon)/sub r/, at 77 K raised from 234 to 424. Annealing the film deposited at 810(degree)C in 1 bar oxygen for 2 hours at 1200(degree)C resulted in an additional increase to (epsilon)/sub r/=613. The highest tunability with a 70% reduction of the permittivity at 120 kV/cm was also reached in the annealed film. Although the lowest loss tangent, tan(delta), of 0.006 at 77 K was found for the film deposited at 710(degree)C, the annealed film had the best tunability to loss ratio. The measured temperature and electric field dependence of the permittivity and loss tangent could be described by a phenomenological model which elucidates the loss mechanisms in the STO films.