A review on ferroelectric thin films used for nonvolatile random access memories is given. Particular attention is paid to fundamental limitations on the materials. Optimization of ferroelectric films by impurity doping and grain-size control is first discussed; then size effects are considered (both thickness and lateral dimensions) from the point of view of both depolarization field instabilities and electrical breakdown mechanisms. Finally, dynamic characteristics such as polarization switching and retention are discussed, in which a theory on polarization reversal is presented and three characteristic fields, the breakdown field, the coercive field, and the activation field, are compared.
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