EPITAXIAL YBA2CU3O7-DELTA/BAXSR1-XTIO3 HETEROSTRUCTURES ON SILICON-ON-SAPPHIRE FOR TUNABLE MICROWAVE COMPONENTS

Epitaxial trilayer heterostructures of YBa2Cu3O7-delta/BaxSr1-xTiO3/YBa2Cu3O7-delta 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 YBa2Cu3O7-delta 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 approximate to 70 Angstrom) between YBa2Cu3O7-delta and BaxSr1-xTiO3 (x = 0.25-0.9) promoted better crystallinity and higher T-c of the top superconducting film. An Ag/BaxSr1-xTiO3/YBa2Cu3O7-delta capacitor structure was used to determine the dielectric permittivity and the high frequency loss tan delta of the BaxSr1-xTiO3 layer. Maximum values of the permittivity of the BaxSr1-xTiO3 layers were observed around the Curie temperatures of corresponding bulk monocrystals. The dielectric permittivity of the BaxSr1-xTiO3 (x = 0.25-0.75) layers depended strongly (approximate to 20%) on an applied voltage (+/- 2.5 V) at temperatures around 77 K. The tan delta was much higher in films than in bulk crystals. (C) 1995 American Institute of Physics.