Robust Ferroelectricity and Phase Switching in Freestanding Hafnia-Based Membranes

Hafnia-based compounds show considerable potential for nanoelectronics due to their complementary metal-oxide-semiconductor compatibility and robust ferroelectricity at nanoscale sizes. However, this unconventional ferroelectricity remains elusive due to the polymorphic nature of hafnia as well as the absence of suitable methods to characterize the mixed/complex phases of hafnia thin films. Stabilizing O phase instead of nonpolar ground-state monoclinic (M) phase in thin films remains a significant challenge, and understanding the mechanisms that govern phase transitions and FE switching is crucial for rational design of fluorite oxide devices.
Here, we report the centimeter-scale crack-free polycrystalline Hf_0.5Zr_0.5O₂ (HZO) nanomembranes that are well suited for investigating the local crystallographic phases, orientations, and grain boundaries at both the microscopic and mesoscopic scales. [1] Atomic-level imaging of the plan-view crystallographic pattern shows that more than 80% of the grains are ferroelectric orthorhombic phase and that the mean equivalent diameter of these grains is about 12.1 nm, with values ranging from 4-50 nm. These substrate-free HZO membranes have a stable ferroelectric orthorhombic phase, indicating that strain from substrate is not responsible for maintaining this polar phase. Moreover, we directly capture the oxygen shifting during polarization switching and correlated polar-nonpolar phase transitions among multiple metastable phases in freestanding ZrO₂ films by low-dose integrated differential phase-contrast scanning transmission electron microscopy. [2] We investigate the reversibility of O-M phase transition in ZrO₂ freestanding nanocrystals by in-situ visualization of the martensitic transformation at atomic scale, and reveal that the reversible shear deformation pathway from O phase to M state is protected by 90° ferroelectric-ferroelastic switching. [3] In addition, an interface-type dynamic transistor gated by an HZO film was introduced to perform reservoir computing. [4] These findings provide insights into the transition pathways between metastable polymorphs and unravel the evolution of polarization orders in (anti)ferroelectric fluorite oxides.

References:
1. H. Zhong, et al., Adv. Mater. 34, 2109889 (2022).
2. X. Li, et al., Adv. Mater. 35, 2207736 (2023).
3. X. Li, et al., Nat. Mater. 23, 1077 (2024).
4. Z. Liu, et al., Nat. Commun. 14, 7176 (2023).