Growth Window of Ferroelectric Epitaxial Hf_0.5Zr_0.5O₂ Thin Films

The metastable orthorhombic phase of hafnia is generally obtained in polycrystalline films, whereas in epitaxial films, its formation has been much less investigated. We have grown Hf_0.5Zr_0.5O₂ films by pulsed laser deposition, and the growth window (temperature and oxygen pressure during deposition and film thickness) for epitaxial stabilization of the ferroelectric phase is mapped. The remnant ferroelectric polarization, up to ∼24 μC/cm², depends on the amount of orthorhombic phase and interplanar spacing and increases with temperature and pressure for a fixed film thickness. The leakage current decreases with an increase in thickness or temperature, or when decreasing oxygen pressure. The coercive electric field (<i>E</i>_C) depends on thickness (<i>t</i>) according to the <i>E</i>_C – <i>t</i>^–2/3 scaling, which is observed for the first time in ferroelectric hafnia, and the scaling extends to thicknesses down to around 5 nm. The proven ability to tailor the functional properties of high-quality epitaxial ferroelectric Hf_0.5Zr_0.5O₂ films paves the way toward understanding their ferroelectric properties and prototyping devices.