Oxygen vacancy effects on polarization switching of ferroelectric Bi2FeCrO6 thin films

The controlled switching of spontaneous polarization in ferroelectrics by applying an external electric field is essential for many device operations. Oxygen vacancy defects commonly found in oxide ferroelectrics offer a tempting means for polarization switching regulation at an atomic scale. This study presents a method involving a series of annealing cycles under various environments to modulate the amount of oxygen vacancies in ferroelectric Bi2FeCrO6 thin films. By performing a piezoresponse force microscopy study after each annealing cycle, it is shown that a high content of oxygen vacancies induces specific fingerprints on polarization hysteresis loops. Reducing oxygen vacancy density by annealing in oxygen-rich environments results in hysteresis loops being wider and displaying less imprint, while increasing oxygen vacancy density by annealing in oxygen-poor environments results in thinner and more imprinted hysteresis loops. The process is largely reversible, constituting an alternative method for polarization switching control at specific electric fields. Ferroelectric phase-field calculations and energy level diagrams obtained through ellipsometry, Kelvin probe, and photoemission spectroscopy indicate a combined switching mechanism based on a doping effect due to oxygen vacancies and a built-in electric field originating from the interface. Our study is helpful for designing ferroelectric films with a precise imprint or, more generally, for understanding the impact of charged defects on the polarization states in ferroelectric films.