Polarization and polarization fatigue in ferroelectrics.

Abstract

This thesis addresses some fundamental issues in ferroelectricity and its applications through a computational and experimental effort. It focuses on a variety of perovskite-type ferroelectric oxides and investigates the physical basis for spontaneous polarization, domain wall dynamics, and texture development in thin film applications. The dipole-dipole interactions between ionic species in perovskite-type materials have been calculated to determine the local field and the lattice instability. Different ferroelectric and anti-ferroelectric polarization transitions can be realized by taking into account the structure distortion of the parent perovskites. We find the local field is enhanced by short range disorder and its nature varies from disorder to disorder, causing polarization transitions in non-(100) directions. The molecular field theory has also been extended to layered perovskites, which favors in-plane polarization over c-polarization. These theoretical predictions are in agreement with the experimental observations of various perovskites and layered perovskites in both single crystal and thin film forms. Domain switching in PZT has been studied by probing the frequency dependency of polarization hysteresis. A picture of thermally activated domain wall movement is established from the frequency spectra of coercive field. The field dependence of domain wall bulging and the nature of the binding between pinning obstacles and the walls are inferred from such a study. Consistent with this picture, polarization fatigue can be defined as a process of increasing the resistance from pinning defects to domain wall motion. The chemical species that act as pinning defects have been identified through model experiments that control carrier injection, electrode interfaces, and film compositions. Based on these observations, a methodology is proposed to evaluate and predict the fatigue damage of both PZT and layered perovskite thin films. Processing of layered perovskite thin films by metal-organic decomposition (MOD) method has been carried out for $\rm Bi\sb4Ti\sb3O\sb{12},\ PbBi\sb22Nb\sb2O\sb9$ and $\rm SrBi\sb2Nb\sb2O\sb9.$ Either random or c-orientations of the films can be obtained by practicing nucleation control via different thermal treatments prior to the final crystallization of the films. A comparison of ferroelectric properties of different orientations and different materials reveals the effects of layer thickness and A-site and B-site cations in the perovskite units. Physical models are proposed to explain these observations.