Interesting Evidence for Template-Induced Ferroelectric Behavior in Ultra-Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates

The first evidence for room-temperature ferroelectric behavior in anatasephase titanium dioxide (a-TiO2) is reported. Behavior strongly indicative of ferroelectric characteristics is induced in ultra-thin (20 nm to 80 nm) biaxially-strained epitaxial films of a-TiO2 deposited by liquid injection chemical vapor deposition onto (110) neodymium gallium oxide (NGO) substrates. The films exhibit significant orthorhombic strain, as analyzed by X-ray diffraction and high-resolution transmission electron microscopy. The films on NGO show a switchable dielectric spontaneous polarization when probed by piezoresponse force microscopy (PFM), the ability to retain polarization information written into the film using the PFM tip for extended periods (several hours) and at elevated temperatures (up to 100 degrees C) without significant loss, and the disappearance of the polarization at a temperature between 180 and 200 degrees C, indicative of a Curie temperature within this range. This combination of effects constitutes strong experimental evidence for ferroelectric behavior, which has not hitherto been reported in a-TiO2 and opens up the possibility for a range of new applications. A model is presented for the effects of large in-plane strains on the crystal structure of anatase which provides a possible explanation for the experimental observations.