  Zirconia and hafnia based thin films have attracted tremendous attention in
the last decade due to their unexpected ferroelectric behavior at the
nanoscale, which facilitates the downscaling of ferroelectric devices. The
present work reports a novel ferroelectric rhombohedral phase of ZrO2 that can
be achieved in thin films grown on (111)- Nb:SrTiO3 substrates by ion-beam
sputtering. Structural and ferroelectric characterizations reveal that the
(111)-oriented ZrO2 films are under epitaxial compressive strain and display a
switchable ferroelectric polarization of about 20.2 {\mu}C/cm2 with a coercive
field of 1.5 MV/cm. Moreover, the time dependent polarization reversal
characteristics of Nb:STO/ZrO2/Au film capacitors exhibit bell-shape curves, a
typical feature of ferroelectric films associated with domains reversal. The
estimated activation field is comparable to the coercive field obtained from
polarization-electric field hysteresis loops. Interestingly, the studied films
show ferroelectric behavior per se, i.e., there is no need to apply the wake-up
cycle that is essential to induce ferroelectricity in the conventional
(orthorhombic) ferroelectric phase of ZrO2. Therefore, the present films have a
technologically advantage over the previously studied ferroelectric ZrO2 films,
and may be attractive for nanoscale ferroelectric devices.
