Heteroepitaxial growth of YBCO/STO thin films for voltage tunable varistors

SrTiO3 (STO) ferroelectric (FE) thin films associated with high-temperature superconductors (HTSC) is a good compromise for tunable microwave components. The great challenge encountered for frequently used coplanar structured devices based on FE/HTSC (A/B means A is on B) thin films is that it is difficulty to simultaneously keep good superconductivity of HTSC thin films and optimized non-linear dielectric properties of ferroelectrics. Stripline structured devices based on HTSC/FE thin films are prepared in this paper. In the HTSC/FE structured devices, STO thin films with smooth surface were heteroepitaxial grown on LaAlO3 (1 0 0) (LAO) substrate. The STO thin film showed not only good out-of-plane orientation, but also identical in-plane orientation with substrate. The crystallographic orientation relationship between LAO substrate and STO thin film in a–b plane is [1 0 0] STO∥[1 0 0] LAO. Highly out-of-plane textured YBCO thin films with good superconducting properties were grown onto the STO thin films, and interdigital capacitors with relative capacitance change of 20% and Q-factor of 286 were made from the YBCO/STO/LAO thin films. The experiment results showed that the HTSC/FE multilayers is a good candidate for high-Q varistors.