Guiding Orthorhombic Phase Evolution in Ultrathin (<4 nm) Hf0.5Zr0.5O2 on Germanium

The integration of scalable, high-performance Hf0.5Zr0.5O2 (HZO) ferroelectric materials with high-mobility channels such as germanium is critical for next-generation logic and memory devices. However, maintaining robust ferroelectricity in films thinner than 5 nm remains a major challenge. In this study, we systematically investigate 3.7-nm HZO films incorporated into a titanium nitride/HZO/Ge capacitor structure. We demonstrate that a conventional single-step post-metallization annealing (PMA) process is insufficient for inducing strong ferroelectricity in ultrathin HZO films. To address this limitation, we formulate a synergistic strategy combining interface engineering with ZrO2-termination and a two-step annealing sequence comprising post-deposition annealing (PDA) followed by PMA. This approach stabilizes the ferroelectric orthorhombic o(020)/o(002) phase and produces robust remanent polarization and improved polarization-voltage loop (P-V loop) squareness, as reflected by an enhanced polarization ratio and (dP/dV)max. Consequently, polarization switching becomes more complete and the P-V loop becomes more rectangular. Comprehensive structural, spectroscopic, theoretical, and electrical analyses further confirm the effectiveness of this strategy. This study provides mechanistic insights into the integration of ultrathin ferroelectric HZO with Ge-based electronic platforms.