A stable monoclinic variant and resultant robust ferroelectricity in single-crystalline hafnia-based films

The ferroelectricity in nanoscale HfO₂-based films enables their applications more promising than that of the perovskite oxides, taking into account the easy compatibility with the modern silicon-based semiconductor technology. However, the well-known polar orthorhombic phase is thermodynamically metastable, making the applications of HfO₂-based ferroelectrics challenging in terms of uncontrollability and consequently instability of the physical performance in electronic devices. Here we report the robust ferroelectricity in stable monoclinic Hf_0.5Zr_0.5O₂ single-crystalline films, which was known as non-polar before. The as-prepared films display high endurance performance of wake-up free and non-fatigue behavior up to 10¹² cycles. Multimode imaging under aberration-corrected scanning transmission electron microscopy reveals that such an unexpected ferroelectric behavior is resultant from an antiphase boundaries-derived monoclinic polar variant (space group, Pc) intergrown with the nonpolar monoclinic phase (P2₁/c). The switching barrier for the stable polar variant is only 20~50% of that for the metastable orthorhombic phase according to the calculation by the nudged elastic band method. These findings provide a practical approach for designing robust ferroelectricity in hafnia-based materials and would be helpful for the development of lower energy-cost and long-life memory devices compatible with integrated circuit technology.