Impact of Anisotropic Biaxial Compressive and Tensile Strain on the Properties of Epitaxial Ferroelectric Films

Abstract

The impact of anisotropic biaxial strain on the ferroelectric properties of thin oxide films (20-100nm) are examined using the example of epitaxial NaNbO3 and SrTiO3 films that are grown on single-crystalline oxide substrates with different lattice mismatch, leading to compressive and tensile in-plane strain, respectively. Generally, tensile in-plane strain leads to an increase of the ferroelectric in-plane transition temperature whereas compressive strain tents to decrease the transition temperature. Shifts of the transition temperature by several 100K can easily be obtained via this method leading to room-temperature permittivity of several 1000. Our investigations have shown that the phase transition itself and the ferroelectric states of the anisotropically strained films turn out to be highly complex. First, the transition temperature depends on the direction of the applied electric field which contradicts the concept of a uniform phase transition for a given system. Second, all systems, that we examined, showed relaxor properties which are usually expected for systems consisting of a mixture of phases. Third, most ferroelectric properties strongly depend on the applied electric field. This can partially be explained by Rayleigh law, however especially for the tensile strained SrTiO3 terms of higher order in the field dependence of the permittivity indicate the strong impact of pinning of domain walls and polar regions (e.g. polar nano regions). Finally at elevated temperature an anisotropic conductivity is observed. The latter might attributed to domain wall conductance. The different observations are discussed in terms of existing models, potential application of the different properties will be sketched.Keywords: anisotropic strain, thin films, ferroelectrics, high-k materia