Ferroelectricity in Hafnia: The Origin of Nanoscale Stabilization

Abstract: The discovery of ferroelectricity in hafnia-based materials have boosted the potential of incorporating ferroelectrics in advanced electronics, thanks to their compatibility with silicon technology. However, comprehending why these materials defy the common trend of reduced ferroelectric ordering at the nanoscale, and the mechanism that stabilizes the ferroelectric phase (absent in hafnia phase diagram) presents significant challenges to traditional knowledge of ferroelectricity. In this work, we show that the formation of the orthorhombic ferroelectric phase (o-FE, space group Pca21) of the single-crystalline epitaxial films of 10% La-doped HfO2 (LHO) on (111)-oriented yttria stabilized zirconia (YSZ) relies on the stability of the high-pressure orthorhombic antiferroelectric phase (o-AFE, space group Pbca). Our detailed structural characterizations demonstrate that as-grown LHO films represent largely the o-AFE phase being thermodynamically stabilized by the compressive strain. Our Kelvin probe force microscopy studies show, under mechanical poling, the o-AFE phase is converted to the o-FE phase which remains stable under ambient conditions. We find that the orthorhombic phase stability is enhanced in thinner films down to one-unit-cell thickness, a trend that is unknown in any other ferroelectric films. This is due to the vanishing depolarization field of the o-AFE phase and the isomorphic LHO/YSZ interface, supporting strain-enhanced ferroelectricity in the ultrathin films. This results in an unprecedented increase of the Curie temperature up to 850 {\deg}C, the highest reported for sub-nanometer-thick ferroelectrics. Overall, our findings opens the way for advanced engineering of hafnia-based materials for ferroelectric applications and heralding a new frontier of high-temperature ferroelectrics at the two-dimensional limit.