Interface-controlled uniaxial in-plane ferroelectricity in Hf_0.5Zr_0.5O₂(100) epitaxial thin films

Hafnium oxide-based ferroelectric thin films are widely recognized as a CMOS-compatible and highly scalable material platform for next-generation non-volatile memory and logic devices. While out-of-plane ferroelectricity in hafnium oxide films has been intensively investigated and utilized in devices, purely in-plane ferroelectricity of hafnium oxides remains unexplored. In this work, we demonstrate a reversible structural modulation of the orthorhombic phase Hf_0.5Zr_0.5O₂ films between (111)-oriented [HZO(111)_O] multi-domain and (100)-oriented [HZO(100)_O] single-domain configurations by altering perovskite oxide buffer layers. Unlike conventional out-of-plane polarized HZO(111)_O films, the HZO(100)_O films exhibit uniaxial in-plane ferroelectric polarization, sustained even at a thickness of 1.0 nm. Furthermore, the in-plane ferroelectric switching achieves an ultralow coercivity of ~0.5 MV/cm. The HZO(100)_O phase is stabilized by a staggered interfacial reconstruction, driven by the delicate interplays between symmetry mismatch and surface energy. These findings pave the way for innovative device designs and strategies for modulating the functionalities of hafnium oxide-based ferroelectrics.