  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.
