Phase Synergy Enables Low‐Power Ferroelectric Switching in HfO ₂ Epitaxial Films

ABSTRACT HfO 2 ‐based ferroelectric materials have emerged as leading candidates for next‐generation non‐volatile memory technologies, owing to their nanoscale robust ferroelectricity and complementary metal–oxide–semiconductor (CMOS) compatibility. However, challenges and debates persist in advancing and comprehensively understanding their ferroelectric behavior. In particular, conventional approaches typically regard non‐ferroelectric phases as detrimental and primarily focus on suppressing their formation, yet overlooking their potentially synergistic contributions—particularly those of the tetragonal ( T ) phase. Here, we unambiguously clarify the beneficial role of the T ‐phase and introduce a phase‐boundary engineering strategy that deliberately harnesses it to enhance ferroelectricity in HfO 2 films. By stabilizing optimal coherent boundaries between ferroelectric orthorhombic ( O ) and T phases in epitaxial La‐doped HfO 2 films, we achieve significant improvements in ferroelectric properties—doubling the remanent polarization ( P r ∼ 30 µC/cm 2 ) and substantially reducing the coercive field ( E c ∼ 3 MV/cm) by 30% compared to low‐La doped samples without such boundaries. Atomic‐scale electron microscopy reveals the structural nature of the atomically sharp, coherent O – T boundaries. Combined with deep‐learning enhanced molecular dynamics simulations, our results unravel that these boundaries facilitate intermediate polarization states that lower the switching energy barrier. Consequently, phase coexistence shifts from an inherent drawback to a tunable design element, offering a broadly applicable route to ultra‐low‐power HfO 2 ‐based nanoelectronics.