Epitaxial Ba_xSr_{1 - x}TiO₃/YBa₂Cu₃O₇ bilayers for tunable microwave applications

The feasibility of epitaxial Ba_xSr_{1 - x}TiO₃/YBa₂Cu₃O₇ bilayers was studied for tunable microwave antennae applications. Ferroelectric Ba_xSr_{1 - x}TiO₃ (BST) was epitaxially coupled to YBa₂Cu₃O₇ (YBCO) thin films via the pulsed laser deposition technique. Congruent processing parameters were used for the growth of both layers and it was found that the superconducting phase of the YBCO could still be maintained with sufficient post-deposition anneal in oxygen. X-ray diffraction and Rutherford backscattering spectroscopy results indicate that a mosaic structure was formed in the BST layer. Electrical measurements demonstrate the viability of Ba_0.5Sr_0.5TiO₃ thin films as tuning media for YBCO microwave antennae as the relative dielectric constant was measured to be 360 at 77 K, and 50% modulations were achieved with applied fields as small as 4.5 V.