Role of interface on structure and properties of epitaxial ferroelectric thin films

It has been recently reported that epitaxial strain can elevate ferroelectric transition temperature (Tc) by hundreds of degrees in both BaTiO3 and SrTiO3 thin films and produce room-temperature ferroelectricity in SrTiO3, a material that normally is not ferroelectric at any temperature. HRTEM analysis of strained BaTiO3 predicts the enhancement of spontaneous polarization by more than 100%, which is consistent with the direct measurements of strained BaTiO3 thin films using epitaxial SrRuO3 electrodes. Furthermore, ultraviolet Raman spectroscopy studies showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 K) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. The ferroelectric transition temperature was tuned by ∼500 °C by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity. This talk will focus on the structure-property relationships of interfaces in such epitaxial ferroelectric thin films and superlattices. The effect of lattice mismatch (epitaxial strain), crystal defects, and chemistry at the film/substrate interfaces on the microstructure and physical properties of different systems will be addressed.