Dielectric properties of barium strontium titanate (BST) thin films and phase shifters based on BST thin films

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Dielectric properties of barium strontium titanate (BST) thin films and phase shifters based on BST thin films

 

Author: Zhou, Xiaoyuan
Title: Dielectric properties of barium strontium titanate (BST) thin films and phase shifters based on BST thin films
Degree: Ph.D.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Thin films.
Dielectrics.
Ferroelectricity.
Department: Dept. of Applied Physics
Pages: xxii, 167 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2189825
URI: http://theses.lib.polyu.edu.hk/handle/200/2569
Abstract: In recent years, barium strontium titanate (abbreviated as BST) thin films have attracted extensive interest for both basic and applied research. Having a perovskite-type crystal structure, BST is known to exhibit a largely diversifiable ferroelectric/dielectric behavior that is influenced by a number of structural factors, such as lattice tetragonality, defects, ratio of barium to strontium, grain size and etc. Mainly due to the structural complexity, many questions on the structure-property relationship in BST thin films remain unanswered and there is also large room for further improving the material's properties. In this work several key issues regarding BST thin films have been studied. First, a clear correlation between lattice tetragonality and ferroelectric and dielectric behaviors has been established in BST thin films. The studied material was composed of Ba0.7Sr0.3TiO3. Grown on single crystal substrates like (LaAlO3)0.3(Sr2AlTaO6)0.7, LaAlO3, MgAl2O4 the thin films were found to have distorted lattices (the cubic lattice of bulk BST with the same composition was regarded as "normal" and used as a reference). This distorted lattice structure leads to the enhanced in-plane dielectric and ferroelectric properties. With increasing film thickness from 20 to 300 nm, the in-plane lattice parameter (a) of BST grown on LaAlO3 increased from 0.395 to 0.402 nm while the out-of-plane lattice parameter (c) remained almost unchanged, which led to an increased a/c ratio (tetragonality) changing from 0.998 to 1.012 and consequently resulted in a shift of Curie temperature from 306 to 360 K associated with an increase of the in-plane remnant polarization and dielectric constant of the films. Similar effects were observed in BST grown on other substrates. The formation mechanism of the lattice distortion was studied in a model system consisting of a SrTiO3 (film)/SiTiO3 (single crystal substrate) homostructure. Among several processing parameters, deposition pressure and the kind of atmosphere were identified to be the critical factors that determined the lattice parameters of the perovskite oxides. Secondly, a phenomenological model was developed to interpret the strain effect on the in-plane dielectric properties of BST thin films with different thickness. The theoretical modeling involved the grouping of strain into biaxial (2-dimensional) and hydrostatic (3-dimensional) components (which were obtained by analyzing the XRD results), the use of the Landau-Ginsburg-Devonshire formalism and mathematical calculations related to the elastic Gibbs free energy. The calculations confirmed that the ferroelectric transition temperature (Curie temperature) and dielectric constant decrease with the decreasing of film thickness. Thirdly, a physical vapor deposition (PVD) process was developed for integrating perovskite-structured thin films on Si substrates. The heterostructure, SrTiO3/Si, was first prepared by laser molecular-beam epitaxy using an ultra-thin Sr layer as the buffer layer. X-ray diffraction measurements indicated that SrTiO3 was well crystallized and epitaxially aligned with Si. Cross-sectional observations in a transmission electron microscope together with X-ray reflectivity measurements revealed that the SrTiO3/Si interface was sharp, smooth and fully crystallized. The thickness of the Sr-template was found to be a critical factor that influenced the quality of the SrTiO3 layer and the interfacial structure. BST thin films were then deposited on the Si/SiO2 substrates with the similar technology. Fourthly, we have fabricated tunable microwave phase shifters on BST/LSAT substrates. High reliability, simple configuration and large phase shift per unit area are the major requirements in the design and fabrication. By carefully optimizing the processing, prototype devices with following specifications were obtained: work frequency = 9 GHz, phase shift up to 70o, figure-of-merit = 60o/dB, insertion loss (S21) = -1.3 dB and return loss (S11) = -14.3 dB.

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