Pyrochlore bismuth zinc niobate thin films for tunable microwave circuit applications

Materials that have a non-linear, electric field tunable permittivity are of great interest for tunable microwave devices and circuits applications, such as phase shifters, filters, and tunable matching networks. Achieving low dielectric losses with ferroelectric thin film devices remains a significant challenge and has prevented the widespread application of tunable dielectrics in microwave circuits. In this thesis, a novel material, Bi1.5Zn 1.0Nb1.5O7 (BZN), which has the cubic pyrochlore structure and is not ferroelectric, was studied for low loss tunable microwave applications.; BZN thin films were prepared by rf magnetron sputtering and showed a relatively high permittivity of ∼180 and an extremely low dielectric loss of ∼ 5x10-4 at 1 MHz. This dielectric loss was unusually low for a complex oxide thin film and was of the same magnitude as BZN ceramics. BZN thin films exhibited a tunability of 55% at an applied electric field of 2.4 MV/cm. A total device quality factor (Q), which is the inverse of the dielectric loss, of more than 100 was obtained with BZN thin film capacitors at 10 GHz. This was comparable to that of the best reported BST capacitors in the literature. After accounting for parasitics due to the probe pads, the device Q was over 300 at 10 GHz. This Q still included loss from other extrinsic contributions but was nevertheless much higher than the Q of BZN ceramics of nominally the same composition in this frequency range. The influence of a dielectric relaxation that is observed for BZN ceramics, and causes a roll-off in the permittivity and an increase in loss at microwave frequencies, could not be observed in BZN thin films. The combination of high tunability and low dielectric loss made BZN thin films promising for microwave circuit applications.; To improve the understanding of the differences in the dielectric relaxation behavior between BZN ceramics and thin films, the influence of film strain was studied. We demonstrated that the low-temperature dielectric relaxation of BZN thin films was shifted to a lower temperature when a large tensile strain was imposed by the substrate. BZN had a weakly temperature-dependent tunability, which allowed for temperature-stable tunable devices. A simple model was developed by Dr. A. K. Tagantsev to qualitatively describe the dielectric tunability of bismuth pyrochlore thin films.