Impact of Compressional and Tensile Biaxially-Anisotropic Strain on the Ferroelectric Properties of Epitaxial NaNbO3 and SrTiO3 Films

Abstract

Impact of Compressional and Tensile Biaxially-Anisotropic Strain on the Ferroelectric Properties of Epitaxial NaNbO3 and SrTiO3 Films R. Wördenweber1, J. Schwarzkopf2, Biya Cai1, Yang Dai1, D. Braun2, J. Schubert1, E. Hollmann11Peter Grünberg Institute (PGI) and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich, D-52425 Jülich, Germany2Leibniz-Institute for Crystal Growth, Max-Born-Str. 2, D-12489 Berlin, GermanyThe impact of anisotropic biaxial strain on the ferroelectric properties of thin oxide films (20-100nm) are examined for the example of epitaxial NaNbO3 and SrTiO3 films that are grown on different single-crystalline oxide substrates with varying lattice mismatch. 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. 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, ferro- to antiferroelectric transitions are observed. These transitions seem to be connected to the presence of polar nanoregions. Finally, all systems show a distinct frequency dependence of the complex permittivity at low frequencies (typically <1kHz) and intermediate temperatures around room temperature. Again this behavior seems to be connected to the presence of polar nanoregions and points to an additional Maxwell-Wagner like mechanism that seems to be present in the temperature regime where the polar nanoregions are present and mobile