Polar Orthorhombic Phase Stabilization of Hf_0.5Zr_0.5O₂ by inserting Seed Layer of HfO₂ and ZrO₂

Ferroelectric materials exhibit electrically switchable polarization states that can be utilized in non-volatile memory components such as ferroelectric random access memory (FeRAM), ferroelectric FET (FeFET), and ferroelectric tunnel junctions (FTJs). Among various ferroelectric materials, Hf_0.5Zr_0.5O₂ (HZO) has attracted significant interest owing to its potential for thickness scalability and compatibility with CMOS technology.<br/>However, hafnia-based ferroelectric materials still have some difficulties needing to be overcome. For example, hafnia-based materials have many polymorphs, including monoclinic phase, tetragonal phase, and orthorhombic phase. Among all the phases, only the orthorhombic phase with space group (Pca2₁) exhibits the ferroelectricity, yet it is a metastable phase for HZO. Therefore, how to stabilize the orthorhombic phase formation at a limited thickness regime is the central issue for a large-scale application of HZO. Another issue is that hafnia-based materials often suffer from the wake-up effect, which may result from the defect accumulation or the transformation from non-polar phase into polar phase. This issue is also related to the orthorhombic phase stabilization and the quality of thin films.<br/>To address these drawbacks and improve the ferroelectric performance of HZO thin films this study proposes another approach by inserting an ultra-thin layer of HfO₂ or ZrO₂ serving as the polar phase stabilizer. This reduces the lattice mismatch between the bottom electrode (La_0.7Sr_0.3MnO₃) and the HZO layer. We found that by optimization of the insertion layer the pure polar phase of HZO can be realized in a larger growth window. We conducted the ferroelectric measurements by piezo force microscopy and ferroelectric tester to analyze the micro/marco ferroelectric properties. The wake-up and imprint effect can be measured by pulse switching measurement and first order reversal curves (FORCs) respectively. Also, by using a brand-new X-ray nano diffraction (XND) technique, it can differentiate the t-phase and o-phase with high energy resolution, which can help to prove the phase transformation from t-phase into o-phase after wake-up cycle.<br/>Our results offer new insights into the stabilization of ferroelectric HZO and the development of wake-up-free hafnia-based devices, with the hope of paving the way for next-generation non-volatile memory applications.