Model of ferroelectric fatigue due to defect/domain interactions

Abstract

A simple model for ferroelectric fatigue is developed from Landau theory. This model proposes that the general fatigue mechanism is due to the formation of mesoscopic structures of interacting charged lattice defects and opposing domains. It shows that the ionic defects may induce a self-trapping potential by polarizing the surrounding ferroelectric lattice. These ionic defects produce additional electrical and mechanical stress on adjacent lattice sites during ferroelectric switching and thereby encourage the formation of neighboring defects. The proposed consequence is that defect planes form during ferroelectric switching that are oriented perpendicular to the polarization axis. Each defect plane separates two opposing domains: as a system, the defect plane and the opposing domains form an energetically self-stabilizing structure. The opposing domains result in a degraded macroscopic electrical response that appears weakly ferroelectric, paraelectric, or linear with increasing charge accumulation at the defect planes. The interaction between the charged defects and polar domains result in an activation energy for the defects that increases with charge accumulation in the defect planes. This leads to the familiar logarithmic decay of the ferroelectric polarization as a function of switching cycles. It is believed that this defect phenomena plays an important role in the fatigue and aging response of ferroelectrics in both thin-film and bulk forms. The model helps explain the apparent spatial orientation of the fatigue mechanism reported in bulk samples, as well as the temperature dependence of the fatigue rate and the electrical response of the fatigued sample.