Barium zirconate titanate thin films for tunable microwave applications

Abstract

There has been great interest in studying tunable dielectrics for microwave applications such as tunable filters, ring resonators and phase shifters. Perovskites, in particular the family of barium titanate, have attracted much attention due to their moderate permittivity and high dielectric tunability. In the present thesis, highly epitaxial thin films of Ba(ZrxTi1-x)O3were deposited on (LaAlO3)O3 (Sr2AlTaO6) 0.35 [LSAT] (001) single crystal substrates using pulsed laser deposition. The structure of the resulting films was studied with regard to the compositional dependence and effects of the oxygen partial pressure using X-ray diffraction measurements. It was found that both the Zr concentration and oxygen partial pressure had significant effects on the lattice deformation of the thin films. The surface morphology of Ba(ZrxTi1-x)O3 thin films was studied using atomic force microscopy. The grain size and surface roughness, which were related to the dielectric loss, depended strongly on the Zr concentration and oxygen partial pressure. The second part of this thesis was devoted to the study of: (1) the phase transition behaviour of Ba(ZrxTi1-x)O3 (x = 0.20, 0.25, 0.30, 0.35) thin films and ceramics; (2) the dielectric and hysteresis properties of these films over a broad frequency range (1 kHz - 500 MHz) and finally, (3) their dielectric tunability. Depending upon the Zr concentration, transition from normal ferroelectric to paraelectric behaviour was observed by temperature dependent dielectric measurements, which was strongly correlated to the results obtained for ceramics. The dielectric activity of the film was significantly modified by the oxygen partial pressure. For Ba(Zr0.3Ti0.7)O3 thin film deposited under an oxygen pressure of 200 mTorr, the best dielectric tunabilities K = 73 % (at 1 kHz) and K = 50% (at 500 MHz) were obtained, showing its potential for use in tunable microwave devices. A Ba(Zr0.3Ti0.7)O3 thin film-based microwave phase shifter was fabricated and tested. The BZT-based phase shifter showed a phase shift of 62o for an applied bias of 100 V at 14.89 GHz. The simulations for the phase shifter with different thicknesses of BZT film using Sonnet em(R) indicated that a larger shift could be obtained by increasing the thickness of the BZT film. Before testing the phase shifter, a Ba(Zr0.3Ti0.7)O3 thin film-based IDC was used to obtain the quality factor of the optimized Ba(Zr0.3Ti0.7)O3 film at microwave frequencies. Also, a Ba(Zr0.3Ti0.7)O3 thin film-based ring resonator was investigated and the true permittivity of the films at microwave frequencies is about 300, which was determined by fitting the simulated S-parameter spectra using Sonnet em(R) simulation tools.