Effects of Doping, Stress, and Thickness on the Piezoelectric Response and Its Relation with Polarization in Ferroelectric HfO₂

The piezoelectric response in hafnia is of interest because it is a CMOS-compatible material. It is known that polarization depends on the stabilization of the orthorhombic phase, which depends on several parameters, such as doping, stress, and thickness. Here, the piezoelectric response in epitaxial doped HfO2 films characterized by piezoelectric force microscopy, is investigated considering this multifactorial dependence. The impact of different levels of La and Zr doping, variations of stress effects controlled by substrate selection, and thickness effects on piezoresponse is analyzed. It is found that the piezoelectric response is mainly correlated with the polarization, i.e., it is primarily determined by the amount of the orthorhombic phase. Interestingly, Zr doping of hafnia is observed to result in a larger piezoresponse than La doping for films showing similar polarization. The piezoresponse is found to be the greatest for the pure ZrO2 film; however, the actual ferroelectric response is not observed by piezoelectric force microscopy, indicating an exception to the general tendency. It is concluded that in the case of samples showing larger amounts of the orthorhombic phase, extrinsic effects due to the tip radii effect can influence the evaluation of the amplitude of the piezoresponse.