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One of the general features of ferroelectric systems is a complex nature of polarization reversal, which involves domain nucleation and motion of domain walls. Here, time‐resolved nanoscale domain imaging is applied in conjunction with the integral switching current measurements to investigate the mechanism of polarization reversal in yttrium‐doped HfO₂(Y:HfO₂)—currently one of the most actively studied ferroelectric systems. More specifically, the effect of film microstructure on the nucleation process is investigated by performing a comparative study of the polarization switching behavior in the epitaxial and polycrystalline Y:HfO₂thin film capacitors. It is found that although the epitaxial Y:HfO₂capacitors tend to switch slower than their polycrystalline counterparts, they exhibit a significantly higher nucleation density and rate, suggesting that this is a rate‐limiting mechanism. In addition, it is observed that under the external fields approaching the activation field value, the switching kinetics can be described equally well by the nucleation limited switching and the Kolmogorov‐Avrami‐Ishibashi models for both types of capacitors. This signifies convergence of two different mechanisms implying that the polarization reversal proceeds via a homogeneous nucleation process unaffected by the film microstructure, which can be considered as approaching the intrinsic switching limit.

 

The unique nonlinear dielectric properties of antiferroelectric (AFE) oxides are promising for advancements in solid state supercapacitor, actuator, and memory technologies. AFE behavior in high‐k ZrO₂is of particular technological interest, but the origin of antiferroelectricity in ZrO₂remains questionable. The theory of reversible electric field‐induced phase transitions between the nonpolar P4₂/nmc tetragonal phase and the polarPca2₁orthorhombic phase is experimentally tested with local structural and electromechanical characterization of AFE ZrO₂thin films. Piezoresponse force microscopy identifies signature evidence of a field‐induced phase transition. A significant size effect in AFE ZrO₂is experimentally observed as film thickness is scaled down from 14.7 to 4.3 nm. The size effect is explained by modifications to the phase transition energy barrier heights ranging from 0.6 to 7.6 meV f.u⁻¹depending on crystallite size and in‐plane compressive strain with decreasing ZrO₂film thickness. Using the size effect, it is possible to double the energy storage density in ZrO₂from 20 J cm⁻³to greater than 40 J cm⁻³, thus highlighting a feasible route for superior performance in AFE fluorite supercapacitors.

 

Abstract Because of its compatibility with semiconductor-based technologies, hafnia (HfO 2 ) is today’s most promising ferroelectric material for applications in electronics. Yet, knowledge on the ferroic and electromechanical response properties of this all-important compound is still lacking. Interestingly, HfO 2 has recently been predicted to display a negative longitudinal piezoelectric effect, which sets it apart from classic ferroelectrics (e.g., perovskite oxides like PbTiO 3 ) and is reminiscent of the behavior of some organic compounds. The present work corroborates this behavior, by first-principles calculations and an experimental investigation of HfO 2 thin films using piezoresponse force microscopy. Further, the simulations show how the chemical coordination of the active oxygen atoms is responsible for the negative longitudinal piezoelectric effect. Building on these insights, it is predicted that, by controlling the environment of such active oxygens (e.g., by means of an epitaxial strain), it is possible to change the sign of the piezoelectric response of the material.