Processing and properties of Ba_0.5Sr_0.5TiO ₃/Bi_1.5Zn_1.0Nb_1.5O ₇/Ba_0.5Sr_0.5TiO₃ sandwich thin films for tunable microwave devices

Ba_0.5Sr_0.5TiO₃/Bi_1.5Zn _1.0Nb_1.5O₇/Ba_0.5Sr _0.5TiO₃ (BST/BZN/BST) sandwich films have been prepared by a sol-gel method. The sandwich thin films were deposited alternatively by spin coating technique and crystallized by rapid thermal annealing. The structures and morphologies of BST/BZN/BST sandwich thin films were analyzed by XRD and FESEM. The XRD results show that the cubic pyrochlore BZN phase and the perovskite BST phase can be observed in the sandwich thin films annealed at 750°C. The diffraction pattern confirms that no measurable reaction occurred between the BST and BZN phases during the annealing process. The surface of sandwich thin films is compact and crack-free. The BST/BZN/BST sandwich thin films annealed at 750°C exhibit a moderate dielectric constant around 91, a low loss tangent of 0.011, and a relative tunability of 27% at a bias field of 600 kV/cm at 10 kHz. Meanwhile, the FOM (defined as the ratio of tunability and loss tangent at room temperature) of BST/BZN/BST sandwich thin films is 24.5. The relative large dielectric constant, low loss tangent, and high FOM suggest that BST/BZN/BST sandwich thin films have potential application for tunable microwave device applications.