Thickness-dependent shift of the ferroelectric composition window in ultrathin Hf1-xZrxO2 thin films

초록

Ferroelectricity in Hf1-xZrxO2 (HZO) films is strongly governed by phase stability, which becomes increasingly sensitive to both film thickness and composition as scaling proceeds into the few-nanometer regime. While near-equimolar Hf:Zr compositions are widely reported to yield robust ferroelectricity in ∼10 nm-thick HZO films, the validity of this conventional composition strategy at ultrathin thicknesses remains unclear. Here, we investigate the coupled effects of thickness and composition on ferroelectric phase stability and switching behavior in atomic-layer-deposited HZO films with thicknesses of 3–5 nm. By independently controlling the film thickness and the Hf:Zr ratio, we reveal a clear thickness-dependent shift of the ferroelectric composition window toward the Hf-rich side. At a thickness of 5 nm, an intermediate composition exhibits the strongest diffraction contribution associated with the orthorhombic phase and remanent polarization, whereas at 3 nm, Hf-rich compositions exhibit the most robust ferroelectric switching, with representative endurance confirmed for the optimized composition. This composition shift is qualitatively consistent with a surface-energy-based Helmholtz free energy framework, in which surface contributions increasingly dominate phase stability at reduced thicknesses. These results establish a thickness–composition design guideline for stabilizing ferroelectricity in ultrathin HZO films and provide insights into phase stability and materials design for nanoscale ferroelectric applications. © 2026

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