Study Of Ba_1-xSr_xTiO₃-based Ferroelectric Film Materials And Simulation For Film Phase Shifter In Structure And Performance

A phenomenological thermodynamic description of the Devonshire theory is introduced, the applications of which to the first-order, second-order and diffused phase transition (DPT) of ferroelctrics are analyzed. Meanwhile, the methods on how to compute the spontaneous polarization and permittivity are given. Furthermore, the mechanism of diffused phase transition for ferroelectrics is explained by adopting composition fluctuation hypothesis of Smolenskii based on the microscopic thermodynamic theory. To explain the origin of ferroelectricity in the ABO3 perovskite at the atomic-level, the influence of Ti atomic location on energies and density of state in BaTiO3-based ceramics are studied by first- principle calculation.Based on the above discussion on phase transition of ferroelectric, the fabrication and properties of BST-based ceramics are investigated. The X-ray diffraction (XRD) shows that the lattice constant decreased for BST ceramics with the increasation of Sr content, the lattice distortion of Ba0.6-xPbxSr0.4TiO3 (BPST) ceramics increases with the increasation of Pb content. Dielectric-temperature (ε-T) properties reaveals that Cruie point for BST-based ceramics can be changed by A-site Sr or Pb ion substituation. For example, Cruie temperature (Tc) increases with the increasation for Pb content in BPST (e.g. ranging from -25℃to 120℃), and vice versa for Sr content in BST. This indicates the binding energy of Pb-O or Sr-O bond exerts different influences on TiO6 octahedron compared with Ba-O bond, e.g. Pb substitution on Ba ion enhances the stablity of TiO6 octahedron and Sr substitution Ba ion weakens the stablity of TiO6 octahedron. Furthermore, Raman spectrum shows that characteristic mode peaks of the tetragonal phase at 303 and 710 cm-1 appears when the macroscopic transition to the ferroelectric state of BST ceramics takes place at the vicinity of Tc. In addition, Raman spectra show that the Raman peak corresponding to the stretching mode of TiO6 octahedron at～550 cm-1 is shifted upward. This indicates that the A-site Pb substitution enhance the stablity of TiO6 octahedron. The fitting to the s-T property (measured at the frequency of 10 KHz) by the theory of Smolenskii indicates that the value of diffused exponentγ, determining the degree of diffuse transitions, of BST ceramics with Pb doping obviously is larger compared with BST ceramics without Pb doping. The phenomenon demonstrates that A-site Pb substitution enhances the degree of DPT. Furthermore, theε-T properties ceramic measured at different frequencies show that there has no shift of Tc compared with ferroelectric relaxtors, which implies that DPT of BST-based ceramic is induced by compositional inhomogeneity of Pb, Ba and Sr ions instead of ion structure from inside.Ba0.6Sr0.4TiO3 target is used for depositing BST film on Pt/Ti/SiO2/Si substrate by RF sputtering method, because its Curie temperature is near the room temperature which induces better dielectric and ferroelectric poperties. The influences of RF sputtering parameters,such as substate temperaute,sputtering pressure, substrate-target distance, O2 partial pressure etc., on microstructure and preffered orientation of film are discussed based on the theory of nucleation. It could be concluded that the nucleation of BST film deposited by RF sputtering is co-effected by interface energy, surface energy and bulk nucleation. Then, RF-sputtering parameters were optimized to obtain a developed perovskite (110)-oriented BST film. The P-V hysteresis loops of the film show that remnant polarization (2Pr) and electric coercive field (Ec) of its Pt/BST/Pt capacitor is about 6.2μc/m2 and 33 kV/cm, respectively, under the applied voltage of 5 V. And the film exhibits little fatigue degradation upon about 1010 switching cycles under voltage of 5 V. In addition, the film shows leakage current of 10-9-10-7A/cm2 within -5～5 V voltage, relative permittivity of 161-255 and tunability of about 37% within sweeping voltage of 7 V. In conlusion, BST-based films have many advantages such as high permittivity, low leakeage current and high tunability which make them have widen applications to phase shifter.Finally, the application to phase shifter device of BST film is considered. By the quasi-static field method and the full-wave analysis method, the characteristics impendence solution and equivalent permittivity of copular waveguide phase shifter (CPW) are derived separately. Based on the methods, the parameters of geometry structure of CPW are considered. Meanwhile, the effect of geometry structure on performance (e. g. characteristics impendence, device loss and so on) of CPW are simulated and analyized by High Frequency Structure Simulator (HFSS). The simulation will provide a reference model for the realization of CPW device.