Enhancement of Tc in single-crystalline 10%La-HfO₂films at two-dimensional limit

Comprehending why ferroelectric HfO₂ films defy the common trend of reduced ferroelectric ordering at the nanoscale presents significant challenges to our traditional knowledge of ferroelectricity based on perovskite-structured proper ferroelectrics. In this work, based on single-crystalline 10%La-HfO₂ (LHO) films epitaxially grown on YSZ(111), we identify that the Curie temperature (Tc) for paraelectric-to-ferroelectric transition increases with the decrease of thickness and reaches up to 850 ± 50 °C at the two-dimensional limit, which is the highest Tc reported in sub-nanometer-thick ferroelectrics. The emergence of ferroelectric-related orthorhombic distortion from initial Hf monolayer as well as the thermodynamic stability rely on the compressive strain imposed by the substrate on the hafnia lattice stretched by La doping. Ferroelectric proximity from LHO further induces orthorhombic distortion on top monolayers of YSZ substrate. These results open the way for atomic engineering of single-phase and single-crystal hafnia oxide as high-temperature ferroelectrics at two-dimensional limit.