Samsung Advanced Institute of Technology

HfO₂-based ferroelectrics are highly expected to lead the new paradigm of nano-electronic devices owing to their unexpected ability to enhance ferroelectricity in the ultimate thickness scaling limit (？2 nm). However, understanding of its physical origin remains uncertain, because their direct microstructural and chemical characterization in such a thickness regime is extremely challenging. Herein, we solve the mystery for the continuous retention of high ferroelectricity in an ultrathin hafnium zirconium oxide (HZO) film (~2 nm) by unveiling the evolution of microstructures and crystallographic orientations using a combination of state-of-the-art structural analysis techniques beyond analytical limits and theoretical approaches. We demonstrate that the enhancement of ferroelectricity in ultrathin HZO films originates from textured grains with a preferred orientation along an unusual out-of-plane direction of (112). In principle, (112)-oriented grains can exhibit 62％ greater net polarization than that of the randomly-oriented grains observed in thicker samples (>4 nm). Our first-principles calculations prove that the hydroxyl adsorption during deposition process can significantly reduce the surface energy of (112)-oriented films, thereby stabilizing the high-index facet of (112). This work provides new insights into the ultimate scaling of HfO₂-based ferroelectrics, which may facilitate the design of future extremely small-scale logic and memory devices