Impact of microstructure on the tunability of the permittivity and the conductance of the Ba0.25Sr0.75TiO3 layer in superconductor/ferroelectric epitaxial heterostructures

Epitaxial heterostructures of YBa2Cu3O7-delta/Ba0.25Sr0.75TiO3/YBa2Cu3O7-delta and YBa2Cu3O7-delta/(7 nm) SrTiO3/Ba0.25Sr0.75TiO3/(7 nm) SrTiO3/YBa2Cu3O7-delta were grown by pulsed laser deposition on (20 nm) (100) CeO(2)parallel to(1102) Al2O3. The epitaxial SrTiO3 inserted in between the superconducting electrodes and the Ba0.25Sr0.75TiO3 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 approximate to 100 K, f = 100 kHz). The epsilon 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 YBa2Cu3O7-delta electrodes (T = 100-130 K). The conductance of the Ba0.25Sr0.75TiO3 layer increased exponentially with temperature in the range 200-300 K and its electric field dependency followed the relation In G similar to E-1/2.