Processing effects of the evolution of the microstructure and preferred orientation of hydrothermal perovskite thin films
Abstract
Ferroelectric ceramics such as BaTiO3, PbTiO3 and Pb(Zr, Ti)O3 have been widely studied due to their variety of potential application, including ultrasonic sensors, infrared detectors, and ferroelectric random access memories. Ferroelectric films have been synthesized by metal-organic chemical-vapor deposition (MOCVD), laser ablation, sputtering, and sol-gel techniques but all require high temperature treatments to obtain high quality films. However, high temperature processing degrades device performance and thus, a low-temperature route is desirable. The hydrothermal processing method enables the synthesis of ferroelectric thin films at low temperatures (usually below 200°C) which greatly suppress the interdiffusion between substrates, electrodes and thin films. This thesis describes research on the effects of substrate preparation and hydrothermal processing conditions as well as laser post heat treatment on the microstructure and preferred orientation of barium titanate and lead titanate thin films. Post laser treatment is more desirable compared to conventional furnace heating because of its high ramping rate and the ability to heat the sample locally, thereby avoiding potential interdiffusion problems. In previous literature, laser treatment has been applied to amorphous thin films to successfully enhanced the crystallinity, and improve the electrical and ferroelectric properties. In this study, a KrF laser treatment was applied to crystalline hydrothermal barium titanate thin films to investigate the evolution of microstructures, electrical and ferroelectric properties as well as film orientation. The second part of the thesis emphasizes texture analysis on films with the spontaneous polarization direction preferentially-oriented normal to the substrate surface as is necessary to attain a high remanent polarization. PbTiO3 thin films usually undergo phase transformation from a cubic paraelectric structure to a tetragonal ferroelectric structure upon cooling from the growth temperature. The strain that occurs this phase transition can be relaxed by formation domain structures greatly dependent on film thickness, film synthesis techniques and experimental parameters. PbTiO3 films on (001) cubic SrTiO3 substrates fabricated hydrothermally at a temperature lower than the Curie temperature show no phase transition. The effects of lattice and thermal mismatch between the film and substrates and the processing parameters on the preferred orientation of hydrothermal lead titanate were investigated.
Degree
Ph.D.
Advisors
Bowman, Purdue University.
Subject Area
Materials science
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