Georgia Institute of Technology Materials Science and Engineering

Committee Members:

Prof. Nazanin Bassiri-Gharb, Advisor, ME/MSE

Prof. Rosario Gerhardt, MSE

Prof. Hamid Garmestani, MSE

Prof. Asif Khan, ECE

Prof. Eric Vogel, MSE

"Electric-Field-Induced Phase Transitions in Antiferroelectric PbZrO3 Thin Films"

Abstract:

The development of modern microelectronic devices has resulted in increasing demand for miniaturization and multifunctionality, and driven a search for enhanced functional response and lower power consumption. Such demand has necessitated high-performance materials in order to fulfill multiple functional roles. Antiferroelectric thin films, with large dielectric permittivity tunability, electric field induced polarization, as well as high electromechanical response, have gained an increasing interest in a wide range of microelectronic applications from actuators and energy storage devices to high tunability filters.

The high dielectric tunability, large polarization, and superior electromechanical response in antiferroelectric materials are enabled by electric field-induced phase transitions. However, due to the relatively low dielectric breakdown strength, the studies on the phase transition in the prototypical antiferroelectric material, PbZrO3, are limited. To improve the breakdown strength of PbZrO3 thin films, previous work has concentrated on complex doping approaches with up to four cations for the B-site in the perovskite cell. Despite the enhanced dielectric properties, doping increases the complexity in processing and any deviation from the targeted doping concentration may result in a dramatic decrease in functional response. Moreover, without fully understanding the phase transitions in unmodified PbZrO3, the phase transitions in the PbZrO3-based materials systems cannot be effectively tuned for different applications.

The work in this thesis explored the processing-structure-property relationship in PbZrO3 thin films, with a focus on electric field-induced phase transitions. Chemical co-doping with Nb and Mg was found to stabilize the perovskite structure and ferroelectric phase, reducing the electric field necessary to induce the polar phase. However, secondary phases and porosity persisted. In order to obtain high density films with minimal amount of secondary phase(s), the effects of processing conditions, including through control of Pb over-stoichiometry within the precursor solution and use of PbO films (seed layers and over-coats) on the microstructure were investigated. Crystallographic orientation of the films was controlled via PbO seed layers and heat treatment parameters. These investigations uncovered the anisotropic nature of the electric field induced phase transitions in PbZrO3 and provided guidance for “tuning” the phase transitions. The knowledge gained in this prototypical antiferroelectric material (PbZrO3) paves the way for potential applications of antiferroelectric materials in precise high strain actuators, energy storage devices, and tunable dielectrics.