HTS/Ferroelectric Thin Films for Tunable Microwave Components

We report on the electrical characterization of Ba0.5Sr0.5TiO3/YBa2Cu3O7-δ/LaAlO3multilayer structure. This structure was fabricated using a pulsed laser deposition technique yielding film thicknesses of 300 nm and 800 nm for the YBa2Cu3O7-δ(YBCO) and the Ba0.5Sr0.5TiO3(BST) films, respectively. A transition temperature Te=91.5 K was measured for the YBCO film in this structure after deposition of the BST layer. The structure was patterned into parallel plate capacitors with 400x400 µm gold contacts and YBCO electrodes on top and underneath the BST, respectively. A relative dielectric constant (εr)~425 and a loss tangent (tanδ)=0.040 were measured at 1.0 MHz at 298 K and zero dc voltage (Vdc=0 volts). At 77 K, the dielectric data showed 320≤εr≤260 and tanδ=0.036 at Vdc=0 volts. For 5.0 ≥Vdc≥-3.0 volts εrcould be varied from 180 to 370. For -5.0 ≤Vdc≤-3.0 volts, εrdecreased rapidly with little change in tanδ. The εrversus Vdc data suggest that changes in εrwere affected by electrode space charge layers.