Unravelling the nanoscale mechanism of polarization reversal in a Hf _0.5 Zr _0.5 O ₂ -based ferroelectric capacitor by vector piezoresponse force microscopy

Ferroelectricity is in demand in many device concepts in electronics, energy and microsystem engineering. The performance of ferroelectrics-based devices is determined by either out-of-plane or in-plane polarization, or out-of-plane or in-plane piezoelectric strain. Real prospects for the practical implementation of innovative devices opened up after the discovery of ferroelectricity in ultrathin hafnium oxide films, due to their perfect compatibility with silicon technology. Ferroelectric properties of this material have been assigned to an orthorhombic structural phase with a single polar axis, but the spatial orientation of the polarization vector and the tensorial piezoelectric behaviour, which are inextricably coupled, still remain unknown. Herein, the rotation of the polarization vector in a Hf_0.5Zr_0.5O₂ (10 nm) capacitor during polarization switching and the spatial distribution of longitudinal and shear piezoelectric coefficients are elucidated at the nanoscale using operando vector piezoresponse force microscopy. In most of the capacitor, a 180°-flipping of the polarization vector is observed, which is consistent with the orthorhombic phase structure. However, a rather large fraction of the capacitor is also occupied by nanoregions of ferroelastic (non-180°) switching, which is explained by the effect of the local mechanical stress. To quantify the three-dimensional piezoresponse, a novel approach exploiting the Poisson effect in artificially created non-ferroelectric regions is proposed and it shows that the shear piezoelectric coefficient is twice the longitudinal coefficient. The experimental insights entail an important step in fundamental understanding of the ferroelectric and piezoelectric properties of hafnium oxide and have great potential to trigger new versions of ferroelectric-based devices.