Asymmetric hysteresis loops and smearing of the dielectric anomaly at the transition temperature due to space charges in ferroelectric thin films
Mısırlıoğlu, Burç and Okatan, M. B. and Alpay, S. P. (2010) Asymmetric hysteresis loops and smearing of the dielectric anomaly at the transition temperature due to space charges in ferroelectric thin films. Journal of Applied Physics, 108 (3). ISSN 0021-8979
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Official URL: http://dx.doi.org/10.1063/1.3457348
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 E-C- and EC+ in a bulk ferroelectric related to each other through vertical bar E-C-vertical bar=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.
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