Computer simulations of ferroelectric domain structure and switching using phase-field approach.

摘要

Ferroelectrics are important functional materials with wide applications in various microelectronic and electrooptical devices such as memories, sensors, and actuators. For the application to information storage devices, the switchability of domains in a ferroelectric by an applied electric field is utilized. The conventional thermodynamic approach to describe switching behavior typically assumes a material to be a perfect crystal while a real ferroelectric material is generally inhomogeneous and contains domains and domain walls, as well as defects such as surfaces, grain boundaries, dislocations and dipolar defects. As a result, prior thermodynamic calculations predicted coercive fields, the minimum applied field to switch a domain, are at least one or more orders of magnitude too high compared to those measured experimentally.;In this work, I developed a three-dimensional (3-D) phase-field model for predicting the domain structures and ferroelectric properties in the presence of structural inhomogeneities in both bulk crystals and thin films. The model takes into account realistic polycrystalline grain structures as well as various energetic contributions including elastic energy, electrostatic energy, and domain wall energy. It is shown that the defects such as existing domain walls, and grain boundaries play a critical role in domain switching and in determining the magnitude of coercive field. It will be demonstrated that the phase-field approach is able to predict the coercive fields and remanent polarizations that are in excellent agreement with experimental measurements. The effect of substrate constraint on phase stability and ferroelectric properties is also discussed.;Further, the phase-field model developed has been extended to study the local tip-induced polarization switching in the presence of twin defects. Epitaxial lead zirconate titanate (PbZr(1-x)TixO3) thin film were studied as a model system. It was observed that the electric potential required to nucleate new domains during polarization switching in ferroelectric thin films varies spatially within the domain structure. The lowest electric field for nucleation is observed near the twin domain boundaries. The spatial distribution of the nucleation voltage obtained from the phase-field approach shows an excellent agreement with experimental measurements using the switching spectroscopy piezoresponse force microscopy(sspfm) technique.