Epitaxial Orientation-Controlled\nHigh Crystallinity\nand Ferroelectric Properties in Hf<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2</sub> Films

Hafnium-based binary oxides are essential for fabricating\nnanoscale\nhigh-density ferroelectric memory devices. However, effective strategies\nto control and improve their thin-film single crystallinity and metastable\nferroelectricity remain elusive, hindering potential applications.\nHere, using NdGaO<sub>3</sub> (NGO) substrates with four crystalline\norientations, we report a systematic study of the structural characterizations\nand ferroelectric properties of epitaxial Hf<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2</sub> (HZO) films, demonstrating orientation-controlled high\ncrystallinity and enhanced ferroelectric properties. HZO films grown\non NGO(001) and NGO(110) substrates exhibit relatively low crystallinity\nand a significant presence of the monoclinic phase. In contrast, HZO\nfilms grown on NGO(100) and NGO(010) possess high single crystallinity\nand a dominant ferroelectric phase. These differences are attributed\nto the surface symmetry of the NGO substrate, which favors the formation\nof 4- or 2-fold domain configurations. Moreover, the optimized HZO\nfilms exhibit a large polarization (2<i>P</i><sub>r</sub>) of ∼50 μC/cm<sup>2</sup>, enhanced fatigue behavior\nup to 10<sup>11</sup> cycles, improved retention of 2<i>P</i><sub>r</sub> ∼ 40 μC/cm<sup>2</sup> after 10 years,\nand characteristic polarization switching speeds in the submicrosecond\nrange. Our results highlight the importance of modulating the single\ncrystallinity and ferroelectric phase fraction of HfO<sub>2</sub>-based\nfilms to enhance ferroelectric properties, further revealing the potential\nof epitaxial symmetry engineering.