Impact of microstructure on the tunability of the permittivity and the conductance of the Ba0.25Sr0.75TiO3layer in superconductor/ferroelectric epitaxial heterostructures

Epitaxial heterostructures of YBa 2 Cu 3 O 7- /Ba 0.25 Sr 0.75 TiO 3 /YBa 2 Cu 3 O 7- and YBa 2 Cu 3 O 7- /(7 nm) SrTiO 3 /Ba 0.25 Sr 0.75 TiO 3 /(7 nm) SrTiO 3 /YBa 2 Cu 3 O 7- were grown by pulsed laser deposition on (20 nm) (100) CeO 2 || (1 02) Al 2 O 3 . The epitaxial SrTiO 3 inserted in between the superconducting electrodes and the Ba 0.25 Sr 0.75 TiO 3 layer promotes a better crystallinity of the ferroelectric layer. The improvement in structure resulted in about a 40% increase of the dielectric permittivity and more than one order of magnitude decrease of the conductance of the dielectric ( T 100 K, f = 100 kHz). The of the 400 nm thick dielectric layer of the buffered heterostructure was suppressed 2.7-fold when a ±2.5 V bias voltage was applied between the YBa 2 Cu 3 O 7- electrodes ( T = 100-130 K). The conductance of the Ba 0.25 Sr 0.75 TiO 3 layer increased exponentially with temperature in the range 200-300 K and its electric field dependency followed the relation ln G ~ E 1/2 .