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 CMOS-compatible processes. However, their large band gap, very limited doping range and challenges of high coercive voltage leave the search open for an industry-friendly, low-coercive-voltage, semiconducting ferro electric for photosensitive and resistive-switching applications. Here, we report the deposi tion 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.