Ferroelectricity in Hafnia: The Origin of Nanoscale Stabilization

The discovery of ferroelectric properties in hafnia-based materials have boosted the potential of incorporating ferroelectricity 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—which is absent in hafnia’s phase diagram—presents significant challenges to our traditional knowledge of ferroelectricity. In this work, we show that the formation of the orthorhombic ferroelectric phase ( o-FE , space group Pca2 ₁ ) of the single-crystalline epitaxial films of 10% La-doped HfO ₂ (LHO) on (111)-oriented yttria stabilized zirconia (YSZ) relies on the stability of the orthorhombic antiferroelectric phase ( o-AFE , space group Pbca) that is present in the high-pressure region of the phase diagram of hafnia. Our detailed x-ray diffraction studies, electron microscopy, and neutron diffraction measurements demonstrate that as-grown LHO films structurally represent an orthorhombic phase that is largely composed of the o-AFE phase being thermodynamically stabilized by the compressive strain imposed by the substrate on the lattice of hafnia stretched by La doping. As follows from our Kelvin probe force microscopy studies, 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 with decreasing film thickness down to one unit cell¾a trend that is unknown in any other ultrathin 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 down to the single-unit-cell thickness, as evident from our electron microscopy and reflection high energy electron diffraction studies. This results in an unprecedented increase of the Curie temperature up to 850 °C—the highest reported for sub-nanometer-thick ferroelectrics. Overall, our findings unveil two long-standing mysteries of ferroelectric hafnia—the stability of the o-FE phase and its enhancement at the nanoscale, opening 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.