Wake-up Free Ferroelectric Rhombohedral Phase in Epitaxially Strained ZrO₂ Thin Films

Zirconia- and hafnia-based thin films have attracted tremendous attention in the past decade because of their unexpected ferroelectric behavior at the nanoscale, which enables the downscaling of ferroelectric devices. The present work reports an unprecedented ferroelectric rhombohedral phase of ZrO₂ that can be achieved in thin films grown directly on (111)-Nb:SrTiO₃ substrates by ion-beam sputtering. Structural and ferroelectric characterizations reveal (111)-oriented ZrO₂ films under epitaxial compressive strain exhibiting switchable ferroelectric polarization of about 20.2 μC/cm² with a coercive field of 1.5 MV/cm. Moreover, the time-dependent polarization reversal characteristics of Nb:SrTiO₃/ZrO₂/Au film capacitors exhibit typical bell-shaped curve features associated with the ferroelectric domain reversal and agree well with the nucleation limited switching (NLS) model. The polarization-electric field hysteresis loops point to an activation field comparable to the coercive field. Interestingly, the studied films show ferroelectric behavior per se, without the need to apply the wake-up cycle found in the orthorhombic phase of ZrO₂. Overall, the rhombohedral ferroelectric ZrO₂ films present technological advantages over the previously studied zirconia- and hafnia-based thin films and may be attractive for nanoscale ferroelectric devices.