Unveiling the Origin of Robust Ferroelectricity in Sub-2 nm Hafnium Zirconium Oxide Films
Hyangsook Lee, Duk‐Hyun Choe, Sanghyun Jo, Jung-Hwa Kim, Hyun Hwi Lee, Hyun‐Joon Shin, Yeehyun Park, Seunghun Kang, Yeonchoo Cho, Seontae Park, Taehwan Moon, Deokjoon Eom, Mirine Leem, Yunseok Kim, Jinseong Heo, Eunha Lee, Hyoungsub Kim
Abstract
HfO2-based ferroelectrics are highly expected to lead the new paradigm of nanoelectronic devices owing to their unexpected ability to enhance ferroelectricity in the ultimate thickness scaling limit (≤2 nm). However, an understanding of its physical origin remains uncertain because its 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 the randomly oriented grains observed in thicker samples (>4 nm). Our first-principles calculations prove that the hydroxyl adsorption during the 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.