Hidden structural phase transition assisted ferroelectric domain orientation engineering in Hf_0.5Zr_0.5O₂ films

The polarization of HfO₂-based ferroelectrics originates from the metastable orthorhombic phase formed during the tetragonal to monoclinic phase transition and is typically controlled by tuning the phase content. However, another way to control polarization via modulating ferroelectric domain orientations remains underexplored. This work uncovers a hidden tetragonal-orthorhombic phase transition pathway to engineer domain orientations and further polarization in polycrystalline Hf_0.5Zr_0.5O₂ using single-crystalline TiN substrates. Specifically, (001)_O and/or (010)_O domains, which fully contribute to remanent polarization under an electric field, are controllable in Hf_0.5Zr_0.5O₂ on TiN (001) and (111), enhancing remanent polarization compared to that on TiN (110). The key is the hidden transition from the tetragonal phase's longest c-axis to the orthorhombic phase's shorter b_O/c_O-axis, alongside the reported one to the longest a_O-axis, assisted by periodic dislocations at the TiN/Hf_0.5Zr_0.5O₂ interface. These findings shed light on governing the polarization of Hf_0.5Zr_0.5O₂ films by controlling the interface dislocations and further domain orientations.