Toward Highly Pure Ferroelectric Hf_1-xZr_xO₂ Thin Films by Tailoring the Strain in an Unstable Thermodynamic System

By utilizing the combined analyses of X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS), a systematic identification on the crystal phases of the polymorphic Hf_1-<i>x</i>Zr<i>_x</i>O₂(HZO) thin films with various series of preparation conditions was presented. The results of the rigorous quantitative analysis show that the monoclinic phase (<i>m</i>-phase) and the tetragonal phase (<i>t</i>-phase) are mutually exclusive and a high fraction of orthorhombic phase (<i>o</i>-phase) up to 97% can be achieved under a specific process condition. Based on the observed correlation between the phase fraction and the in-plane strain, the strain-induced activation energy barrier was proposed to clarify the mechanism of kinetic phase transition process. The endurance test of the device, exhibiting that the wake-up degree (remnant polarization variation Δ<i>P</i>_r/<i>P</i>_max) increases exponentially with increasing the content of tetragonal phase in the pristine state, indicating the transition from <i>t</i><i>-</i> to <i>o</i><i>-</i>phases as a possible mechanism of the wake-up effect. Our study presented herein points the way to efficiently optimize the strain towards highly pure orthorhombic HZO thin film through precisely controlling the process parameters, which is beneficial for the future development of ferroelectric devices.