Asymmetric hysteresis loops and smearing of the dielectric anomaly at the transition temperature due to space charges in ferroelectric thin films

Abstract

Ferroelectric thin films often exhibit a displacement of the polarization versus the electric field hysteresis loops, particularly along the electric field axis. This shift is typically attributed to structural and electronic asymmetry of the film-electrode interfaces, asymmetric surface fields, as well as space charge regions. In this study, we analyze the effect of a spatial, continuous distribution of space charge on the hysteresis response and phase transition characteristics of epitaxial (001) PbZr0.3Ti0.7O3 thin films sandwiched between metallic electrodes on (001) SrTiO3 substrate. Using a nonlinear thermodynamic model, we compute numerically the internal electrical fields and polarizations for several different space charge distributions both in the presence of a triangular external electric field and as a function of temperature at zero applied field. We show that space charge accumulated near the metal-ferroelectric interfaces can dramatically displace the hysteresis along the electric field axis such that the otherwise symmetric coercive fields EC− and EC+ in a bulk ferroelectric related to each other through |EC−|=EC+ may shift depending on the space charge concentration in such a manner that both EC−,EC+>0 or EC−,EC+<0. This gives rise to a very strong imprint. Our findings reveal that the presence of space charges in ferroelectric thin films results in significant changes in the phase transition characteristics, including a reduction in the phase transition temperature, smearing of the transition over a temperature range instead of a sharp dielectric anomaly at the bulk Curie temperature, and a reduction in the dielectric response compared to defect-free ferroelectrics of the same composition.