Unveiling the polarization switching pathway through tetragonal phase as a metastable intermediate state in ferroelectric Hf_xZr_1-xO₂ thin film

The polarization switching pathway in Hf_xZr_1-xO₂-based ferroelectric thin film is still not well clarified and agreed, limiting the fundamental physical understanding and performance engineering. The key question lies in clarifying the transient intermediate state during the polarization switching of orthorhombic phase. In this work, by designing the ferroelectric and dielectric stacks, we theoretically and experimentally demonstrate a polarization switching pathway through an orthorhombic-tetragonal-orthorhombic phase transition in ferroelectric Hf_xZr_1-xO₂ where the non-polar tetragonal phase is metastable. Meanwhile, the phase transition pathway under electric field is experimentally demonstrated by in-situ grazing incidence X-ray diffraction measurement. Furthermore, by engineering the energy barrier of reversible orthorhombic-tetragonal phase transition through controlling the defects and interface properties, a low coercive field ~0.6 MV/cm and a low operation voltage <0.65 V is achieved in an 8 nm Hf_0.5Zr_0.5O₂ film. Our results provide insights into the fundamental physics and performance engineering of ferroelectric Hf_xZr_1-xO₂ materials.