Evidence for stabilization of ferroelectricity in WO3 thin films under anisotropic epitaxial strain

Interest in ferroelectric materials for processing, memory, and sensing devices has been re-energized in recent years by the discovery of nanoscale ferroelectricity in insulating binary oxides based on hafnia and zirconia deposited in complementary metal-oxide-semiconductor-compatible processes. However, their large bandgap, very limited doping range, and challenges of high coercive voltage leave the search open for an industry-friendly, low-coercive-voltage, semiconducting ferroelectric for photosensitive and resistive-switching applications. Here, we report the deposition of epitaxial WO3 films at 350°C using a chemical atmospheric process. In these films, anisotropic epitaxial strain imposed by the substrate promotes the stabilization of a polar phase at room temperatures with out-of-plane polarization, evidenced by x-ray diffraction, scanning transmission electron microscopy, piezoresponse force measurements, and Raman spectroscopy. Exploring ferroelectricity in ultrathin epitaxial WO3 films could provide a platform for polarization-controlled electronic and optical applications.