Polar thin film is not ferroelectric

Epitaxial thin film of strontium titanate on a silicon substrate is now demonstrated non-ferroelectric, despite its spontaneous polarization [PRL 105, 217601 (2010)]. This polarization is not due to spontaneous symmetry breaking, rather it is a property of the interface ground state. The symmetry is absent from the outset in the presence of the substrate, which strains the film. Therefore, the polarization is not switchable. This finding is made by DFT computations and STEM techniques.

We use SrTiO3=Si as a model system to elucidate the effect of the interface on ferroelectric behavior in epitaxial oxide films on silicon. Using both first-principles computations and synchrotron x-ray diffraction measurements, we show that structurally imposed boundary conditions at the interface stabilize a fixed
(pinned) polarization in the film but inhibit ferroelectric switching. We demonstrate that the interface chemistry responsible for these phenomena is general to epitaxial silicon-oxide interfaces, impacting on the design of silicon-based functional oxide devices.