Effect of YBa₂Cu₃O_7-δ film thickness on the dielectric properties of Ba_0.1Sr_0.9TiO₃ in Ag/Ba_0.1Sr_0.9TiO₃/YBa₂Cu₃O_7-δ/LaAlO₃ multilayer structures

Ferroelectric and superconductor bilayers of Ba_0.1Sr_0.9TiO₃ (BSTO)/YBa₂Cu₃O_7-δ (YBCO), with different YBCO film thicknesses, have been fabricated in situ by pulsed-laser deposition on 1.2° vicinal LaAlO₃ substrates. The dielectric properties of BSTO thin films were measured with a parallel-plate capacitor configuration in the temperature range of 77–300 K. We observed a strong dependence of the dielectric properties of BSTO thin films on the thickness of the YBCO layer. As the YBCO-film thickness increases, the temperature of the dielectric permittivity maximum of BSTO thin films shifts to higher values, and the leakage current and dielectric loss increase drastically, while the dielectric constant and dielectric tunability decrease remarkably. The results are explained in terms of the transformation in the growth mode of the YBCO layer from two-dimensional step flow to three-dimensional island that leads to significant deterioration in the dielectric properties of BSTO thin films. We propose that improved dielectric properties could be obtained by reasonably manipulating the growth mode of the YBCO layer in the multilayer structures.