Polarization switching and relaxation dynamics of bismuth layered ferroelectric thin films: Role of oxygen defect sites and crystallinity

Influence of oxygen vacancies and crystallinity on polarization switching and relaxation dynamics of Bi_3.15Nd_0.85Ti ₃O₁₂ (BNT) thin films on Pt electrodes have been studied by piezoresponse force microscopy (PFM). Heat treatment of the study induced oxygen vacancies in Bi₂O₂⁺2 layers not in TiO₆^-8 octahedra and changes in crystallinity of the films. Oxygen atoms have two different locations unique in the bismuth layered ferroelectrics and, in particular, the oxygen vacancies at the Bi ₂O₂⁺2 layers turned out to be critical for retention of polarization states. Perfect crystallinity in the arrangement of atoms through c-axis is surprisingly found to sustain polarization switching. Electrodynamics of the bound charges in the films with oxygen deficiency and imperfectness of crystallinity are presented in a dynamic model with random-walk motion of point defects and a diffusive migration of mobile and bound charges and their clusters. For the stretched exponential decay of the bound charges in the films, the scaling exponents are 0.086 and 0.289 for well crystalline-thin films.