Microstructural Evolution of Charged Defects in the Fatigue Process of Polycrystalline BiFeO3 Thin Films

TL;DR

This study investigates the microscopic evolution of charged defects, specifically oxygen vacancies, in BiFeO3 thin films during electrical fatigue, revealing their role in fatigue failure through experimental and theoretical analysis.

Contribution

It provides direct experimental evidence and theoretical insights into how charged defect evolution contributes to ferroelectric fatigue in BiFeO3 thin films.

Findings

Oxygen vacancy order forms along grain boundaries during fatigue.

Migration of oxygen vacancies facilitates formation of defect-rich electrode layers.

Ordered vacancy clusters play a key role in fatigue failure.

Abstract

Fatigue failure in ferroelectrics has been intensively investigated in the past few decades. Most of the mechanisms discussed for ferroelectric fatigue have been built on the "hypothesis of variation in charged defects", which however are rarely evidenced by experimental observation. Here, using a combination of complex impedance spectra techniques, piezoresponse force microscopy and first-principles theory, we examine the microscopic evolution and redistribution of charged defects during the electrical cycling in BiFeO3 thin films. The dynamic formation and melting behaviors of oxygen vacancy (VO) order are identified during the fatigue process. It reveals that the isolated VO tend to self-order along grain boundaries to form a planar-aligned structure, which blocks the domain reversals. Upon further electrical cycling, migration of VO within vacancy clusters is accommodated with a lower energy barrier (~0.2 eV) and facilitates the formation of nearby-electrode layer incorporated with highly concentrated VO. The interplay between the macroscopic fatigue and microscopic evolution of charged defects clearly demonstrates the role of ordered VO cluster in the fatigue failure of BiFeO3 thin films.