Study of barium strontium zirconate titanate thin films and their microwave device applications

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Study of barium strontium zirconate titanate thin films and their microwave device applications

 

Author: Chan, Ngai-yui
Title: Study of barium strontium zirconate titanate thin films and their microwave device applications
Degree: M.Phil.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Thin films -- Electric properties
Barium compounds -- Electric properties
Department: Dept. of Applied Physics
Pages: xx, 21-187 leaves : ill. ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2425036
URI: http://theses.lib.polyu.edu.hk/handle/200/6042
Abstract: Perovskites, in particular the family of barium titanate, have attracted much attention due to their moderate permittivity and high dielectric tunability. (Ba, Sr) TiO₃ (BST) and Ba (Zr, Ti)O₃ (BZT) materials are perovskite based ferroelectrics which have found extensive applications. For ABO₃ perovskite, substituting A and/or B sites by other ions can modify the dielectric behaviour of the material. (Ba,Sr)(Zr,Ti)O₃ (BSZT) is a solid solution of BST and BZT. It is expected that the addition of Zr and Sr into BaTiO₃ will result in a modified BSZT ceramics. (Ba₁₋x,Srx)(Zr₀.₁,Ti₀.₉)O₃ (x = 0, 0.05, 0.15, 0.25, 0.35, 0.45) ceramics were prepared by a conventional solid state reaction method. The structure, surface morphology, dielectric and ferroelectric properties were investigated. With the increase in Sr content, the Curie temperature decreases linearly. From X-ray diffraction (XRD) analysis, the lattice parameters were found to reduce with the increase in Sr content. Scanning electron microscopy (SEM) was used to reveal the microstructure and it was observed that the grain size decreases with the increase in Sr content. The polarization-electric field loop (P-E loop) shows a decrease in remnant polarization with the increase in Sr content. We have successfully grown BSZT thin films on different substrates [e.g. LaAlO₃(LAO), (La,Sr)(Al,Ta)O₃(LSAT), SrTiO₃ and MgO] with different compositions (x = 0, 0.05, 0.15, 0.25, 0.35, 0.45). The structure, dielectric and ferroelectric properties are studied. The properties are obtained using coplanar configurations on BSZT thin films, as coplanar electrode is much preferable for microwave applications. Using conformal mapping techniques, dielectric constant values ranging from (180 to 800) are obtained. The in-plane dielectric tunabilites obtained are higher than 40%. Structural characterization shows that the lattice parameter reduces with increasing Sr content. Among all compositions, the Curie temperature of (Ba₀.₅₅Sr₀.₄₅)(Zr₀.₁Ti₀.₉)O₃ thin film is below room temperature (10℃), and smallest dielectric dissipation was obtained, giving high Q values. (Ba₀.₅₅Sr₀.₄₅)(Zr₀.₁Ti₀.₉)O₃ thin film was chosen as the base composition for microwave devices (ring resonators and phase shifters) fabrication. Ring resonators are widely used microwave devices and also are tools for material characterization at high frequency (1GHz 20GHz). Ring resonators have been used to characterize the dielectric properties of (Ba₀.₅₅Sr₀.₄₅)(Zr₀.₁Ti₀.₉)O₃ thin film on different substrates in the microwave region. Phase shifters produce various phase shifts in output signal. Ring resonator and coupled microstrip phase shifter (CMPS) have been fabricated on LAO, LSAT and MgO substrates using standard photolithography etching technique. It has been found that, under 12.5 V/μm electric field, the phase shifter provides 60° of phase shift in the GHz region. This is a preliminary demonstration of the performance of the BSZT phase shifter. It is envisaged that by tuning some processing parameters, better results could be obtained.

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