Rhombohedral R3 Phase of Mn-Doped Hf_0.5Zr_0.5O₂ Epitaxial Films with Robust Ferroelectricity

HfO₂ based thin films are becoming key functional materials for next-generation information storage devices due to their excellent sub-nanometer ferroelectric properties and compatibility with silicon-based semiconductor processes. Extensive theoretical and experimental studies have focused on the orthorhombic ferroelectric phase (Pca2₁) to enhance and optimize its ferroelectric stability and durability. However, research on the rhombohedral phase of HfO₂ has been relatively limited due to its higher energy, making it challenging to stabilize [ Nat. Mater. 2018, 17, 1095, Science 2023, 381, 558.].
In this talk, we will present our successful synthesis of a distinct ferroelectric rhombohedral phase is reported, i.e., the R3 phase, in Mn-doped Hf_0.5Zr_0.5O₂ (HZM) epitaxial thin films. By analyzing the bond length distortion within the rhombohedral HfO₂ double-tetrahedral crystal field and incorporating first-principles calculations, we propose that under appropriate out-of-plane tensile stress, surface energy conditions, and co-doping with Zr (4d²) and Mn (3d⁵) elements, the rhombohedral R3 ferroelectric phase can replace the rhombohedral R3m and orthorhombic Pca2₁ ferroelectric phases to become the ground state. This R3 phase HZM film exhibits a remnant polarization of up to 47 µC cm⁻² at room temperature, along with an exceptional retention capability projected to exceed a decade and a good endurance property. Moreover, it is demonstrated that by modulating the concentration of Mn dopant and the film's thickness, it is possible to selectively control the phase transition between the R3, R3m, and Pca2₁ polar phases. Additionally, two evolutionary pathways from the high-symmetry to low-symmetry phases in HfO₂ based ferroelectrics have been summarized, establishing the symmetry inheritance and phase transition relationships among the three ferroelectric phases. This research not only sheds new light on the ferroelectricity of the HfO₂ system but also paves the way for innovative strategies to manipulate ferroelectric properties for enhanced device performance. [Adv. Mater. 2024, 2406038]