Twist-Induced Out-of-Plane Ferroelectricity in Bilayer Hafnia

Ferroelectric HfO2 is a promising candidate for next-generation memory devices due to its CMOS compatibility and ability to retain polarization at nanometer scales. However, the polar orthorhombic phase responsible for ferroelectricity is metastable and requires extrinsic stabilization, complicating integration with silicon. Here, we predict that twist-induced symmetry breaking in bilayer 1T-HfO2 enables robust, switchable out-of-plane (OOP) polarization. First-principles density functional theory shows that monolayer 1T-HfO2, cleaved from the (111) surface of cubic HfO2, is dynamically stable. Twisting two aligned monolayers forms a moiré superlattice that breaks interlayer symmetry and yields bistable OOP polarization. At a twist angle of 7.34°, the bilayer exhibits ∼16 μC/cm2 polarization, primarily from polar displacements within AB stacking domains. The polarization can be switched via interlayer sliding with a low energy barrier (∼8 meV per formula unit), enabling electric-field tunability in atomically thin ferroelectric devices.