Defense Date


Document Type


Degree Name

Doctor of Philosophy



First Advisor

Hadis Morkoc


Owing to the large electric-field-dependent permittivity, ferroelectric thin films have attracted a great deal of attention on applications in miniature tunable microwave components with high performance and cost reduction, such as phase shifters, tunable oscillators, delay lines, and antennas. These tunable devices require large change in the dielectric constant with applied field and a low loss at microwave frequencies. As one of the promising ferroelectric materials, barium strontium titanate thin film, especially Ba0.5Sr0.5TiO3 (BST) films, have raises great research interests due to its high dielectric constant, which is tunable in an external electric field, combined with relative low loss at microwave frequencies. Tunable microwave components, such as phase shifter, based on the BST films have been widely investigated. Since the polarization, the significant characterization of ferroelectrics, is very sensitive to distortion in crystal structure of ferroelectrics, strain can be effectively utilized to tailor the dielectric properties of BST films. Due to the lattice-mismatch from the substrate and various deposition conditions, epitaxial BST thin film usually contains residual strain generated during film growth. Strain control by improved deposition technique and implementing thermal treatment as well as choosing suitable substrate has attracted intensive attentions in ferroelectric film fabrication. Theory predicts that high dielectric properties can be achieved when free strain or slightly tensile strain left in the BST thin film at room temperature. Microwave application, such as phase shifter, also expects the enhanced tunability by an applied electric field. In this dissertation, single crystalline BST thin films deposited by radio frequency magnetron sputtering on SrTiO3 and DyScO3 substrates were studied. The crystal structure characteristics, including lattice parameters and film strain, were determined using X-ray diffraction. A new growth technique, three-step technique, was introduced and implemented into BST thin film deposition. The application of this new technique in deposition dramatically reduced the compressive strain in the films. We use microwave measurements on coplanar waveguides to evidence the improvement on dielectric properties achieved by tailoring the film strain. Additionally, we studied the BST film deposited by pulsed laser deposition (PLD) with introducing a sputtered seed layer of BST thin film. Compared with the BST film directly deposited on the substrate by PLD deposition, the films with a seed layer showed a large enhancement on the dielectric constant and tunability. The discussion on the change in film strain and dielectric performance of the PLD deposited films further proved the influence of film strain on dielectric properties. We discussed the design, fabrication, and measurement of coplanar waveguide transmission lines as phase shifters fabricated BST films. The thin BST films (~700 nm) on DyScO3 substrates deposited by sputtering demonstrated that the three-step deposition technique improved differential phase shift and microwave figure of merit to a great extent. The introduction of the sputtered seed layer into the PLD deposition of a thicker BST film (~2.15 μm) showed a dramatically enhancement on differential phase shift and microwave figure of merit. The enhanced performance on different series of BST films in microwave frequencies is consistent with the improvement on crystal structure, especially with the change in film strain.


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Available for download on Wednesday, December 14, 2022

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