Title

Interlayer Coupling in Ferroelectric Multilayers: Domain Structures and Effect of Space Charges

Date of Completion

January 2010

Keywords

Physics, Condensed Matter|Engineering, Materials Science

Degree

Ph.D.

Abstract

A theoretical model is developed for n-layered ferroelectric (FE) heterostructures that employs a non-linear Landau-Devonshire formalism coupled with a detailed analysis of the depolarizing fields arising from the polarization mismatch across interlayer interfaces and the electrical fields of localized space charges at such interfaces. It was shown that in the presence of space charges either horizontal (monolithic FE) or diagonal (graded FE) displacement of the hysteresis loops are thermodynamically stable and phase transition temperature is suppressed. Depending on the spatial distribution of space charges phase transition was found to smear out, lift the degeneracy between positive/negative polarization states and result in lower dielectric constant values. Space charge induced built-in polarization, switchable polarization and total polarization of the heterostructures were also investigated. In the vicinity of the phase transition a recovery in switchable polarization was predicted. Dominance of the space charge induced electric fields over the depolarizing electric field was shown to result in electrostatic decoupling of layers. ^ Developing the theory of wedge domain structure in graded ferroelectrics, a quantitative model of domain evolution and dielectric response of compositionally graded ferroelectric multilayers were presented. It was shown that the domain structure adapts itself to the applied electric field via domain modification. As the applied field rises, the domains are swept away layer-by-layer; resulting in a strong non-linear field dependence of the dielectric constant and tunability of the multilayers. It was also shown that by controlling the relative thicknesses of the layers that make up the graded heterostructure, the dielectric response and tunability could be further enhanced. ^