Epitaxial YBa2Cu3O7−δ /BaxSr1−xTiO3 heterostructures on silicon-on-sapphire for tunable microwave components

Abstract

Epitaxial trilayer heterostructures of YBa₂Cu₃O_7−δ/Ba_xSr_1−xTiO₃/YBa₂Cu₃O_7−δ were grown on silicon‐on‐sapphire buffered by a double layer of CeO₂/Y–ZrO₂. Such structures may be considered for tunable microwave filters. The top and bottom YBa₂Cu₃O_7−δ films were well c‐axis oriented, free from microcracks and had superconducting transitions T_c’s in the range 86–90 K. A thin antidiffusion layer of SrTiO₃ (d≊70 Å) between YBa₂Cu₃O_7−δ and Ba_xSr_1−xTiO₃ (x=0.25–0.9) promoted better crystallinity and higher T_c of the top superconducting film. An Ag/Ba_xSr_1−xTiO₃/YBa₂Cu₃O_7−δ capacitor structure was used to determine the dielectric permittivity and the high frequency loss tan δ of the Ba_xSr_1−xTiO₃ layer. Maximum values of the permittivity of the Ba_xSr_1−xTiO₃ layers were observed around the Curie temperatures of corresponding bulk monocrystals. The dielectric permittivity of the Ba_xSr_1−xTiO₃ (x=0.25–0.75) layers depended strongly (≊20%) on an applied voltage (±2.5 V) at temperatures around 77 K. The tan δ was much higher in films than in bulk crystals.