Surface-stimulated phenomena in the polarization response of ferroelectrics

The integration of ferroelectrics in electronic devices requires that they be used in the form of thin films, which implies that for such systems finite-size effects related to the presence of a ferroelectric-electrode interface become important. In this thesis, a number of theoretical studies are presented on the properties of metal-ferroelectric-metal structures, focusing on the impact of the metal-ferroelectric interface on the polarization response of the system. First, a model for reverse domain nucleation in ferroelectrics is introduced, which takes into account the ferroelectric-electrode coupling in both the homogeneous and random cases. The model provides a solution to the coercivity paradox—i.e., the large discrepancy between the observed and predicted coercive fields. The possibility of non-thermally activated nucleation of reverse domains is demonstrated. It is found that small inhomogeneities in the ferroelectric-electrode interface may lead to an exponentially wide spectrum of waiting times for switching. The model predicts that switching is facilitated near morphotropic phase boundaries in perovskite-type ferroelectrics. In order to quantitatively analyze the size-effect problem in metal-ferroelectric-metal systems, an approach is developed which combines first-principles calculations and phenomenological theory. The parameters of the model can be extracted from calculations on ultrathin films, while experimentally verifiable predictions can be made on thick films. Using the developed approach, it is demonstrated how the size effect can be separated into two distinct contributions: a long-range electrostatic and a short-range "chemical" one. By considering symmetric SrRuO3/BaTiO3/SrRuO3 heterostructures with different types of termination (TiO2–TiO2 or RuO2–RuO2), it is shown that the balance between the long-range and the short-range contribution to the size effect can be essentially affected by the type of termination of the ferroelectric and by the polarization hardness of the electrode. The leading role of the long-range contribution to the size effect in SrRuO3/BaTiO3/SrRuO3 heterostructures is demonstrated. Application of the approach to the case of SrRuO3/BaTiO3/SrRuO3 heterostructures with asymmetric interfaces enables to provide a quantitative description of a number of manifestations of such asymmetry in films of technologically meaningful thickness. In particular, it is found that the asymmetry exerts a poling effect on the films, leading to a smearing of the phase transition, to an induced piezoelectric response above the transition temperature, and to the reversal of the polarization asymmetry by application of biaxial strain. Another important result of such calculations is the observation that the ionic relaxations in the metal-oxide electrode play a crucial role in stabilizing the ferroelectric phase of the films. Comparison with frozen-phonon calculations shows that the degree of softness of the SrRuO3 lattice has an essential impact on the screening of ferroelectric polarization, reducing the critical thickness for ferroelectricity of the system. These results provide a possible explanation for the observed beneficial impact of oxide electrodes on the switching and dielectric properties of ferroelectric capacitors.

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