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.
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.
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.
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.