Ferroelastic Domain Switching and Time-Resolved Negative Capacitance in Polar-Axis-Oriented Hf_0.5Zr_0.5O₂ Grown by Atomic Layer Epitaxy

Ferroelectric properties of Hf_0.5Zr_0.5O₂ are strongly correlated with its crystallographic orientation, with the [001] direction serving as the polar axis. However, the epitaxial growth of highly polar-axis-oriented Hf_0.5Zr_0.5O₂ layers with pronounced ferroelectricity is rarely reported. Here epitaxial (001)-oriented Hf_0.5Zr_0.5O₂ thin films grown by atomic layer epitaxy (ALE) is demonstrated, which achieve a state-of-the-art ferroelectric polarization up to 78.9 µC cm^-2. The epitaxial Hf_0.5Zr_0.5O₂ layer experiences a lattice reorientation from (010) to (001) during the wake-up process, as evidenced by plane-view precession electron diffraction. Accordingly, a two-step, 90° ferroelastic domain switching model is proposed to elucidate multiple polarization switching. Furthermore, the observed polarization switching dynamics closely match with the time-resolved negative capacitance, which is quantified as an equivalent high dielectric constant of -170. This study highlights the capability of ALE to precisely control the crystallographic orientation of Hf_0.5Zr_0.5O₂ thin films, providing deep insights into fundamental ferroelectric mechanisms.