Record-High $\mathbf{Pr}\left(2 \mathbf{Pr}>\mathbf{40} \mu \mathbf{C} / \mathbf{cm}^{2}\right)$ in 3 nm (Physical) Ferroelectric HZO Annealed at $\mathbf{450}^{\circ}\mathbf{C}$: High-T (85 °C) Electrical Cycling and Oxygen Vacancy Engineering

A remarkable improvement in remnant polarization (2Pr), from below <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5 \mu \mathrm{C} / \text{cm}^{2}$</tex> to over <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$40 \mu \mathrm{C} / \text{cm}^{2}$</tex>, has been achieved in an ultra-thin 3 nm (physical) ferroelectric (FE) HZO layer annealed at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$450^{\circ} \mathrm{C}$</tex>, facilitated by increased oxygen vacancies (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Vo} / \text{Vo}^{2+}$</tex>) and high-temperature electrical cycling (HTEC) at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$85{ }^{\circ} \mathrm{C}$</tex>. Notably, this enhancement persists upon returning to room temperature (RT). Systematic characterization attributes the record-high 2 Pr in the 3 nm HZO layer to a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Vo} / \text{Vo}^{2+}$</tex>-assisted phase transition from initially dominant tetragonal (t)-phase to FE orthorhombic (o)-phase. First-principles calculations reveal that the increased <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Vo}^{2+}$</tex> during HTEC is crucial for driving the t-to-o phase transition, shifting the free energies of the t- and o-phases, and ultimately stabilizing the o-phase over the t-phase. Our work unlocks new opportunities for optimizing ultra-thin FE layers with a low thermal budget, laying the foundation for the next-generation of FE memory devices.