Ferroelectric memory has emerged as a highly promising candidate for next-generation non-volatile memory, offering ultra-low power consumption, ultrafast read/write speeds, high reliability, and significant potential for high-density integration. Over the past fifteen years in particular, the discovery of ferroelectricity in HfO₂-based thin films has attracted widespread attention and stimulated substantial progress in this field. Previous review articles have provided comprehensive summaries of representative advances in the field, covering topics including the origin of ferroelectricity in HfO₂, deposition methods, the performances of HfO₂-based ferroelectric thin films and memory devices, as well as progress in device physics, integration strategies, and emerging applications. However, more recent breakthroughs in the multi-level devices have not yet been systematically reviewed. These developments are becoming increasingly critical for mass data storage and emerging paradigms such as in-memory computing. In this review, we summarize recent progress in HfO₂-based ferroelectric multi-level memories, with a focus on material studies, device designs, and emerging applications. By highlighting these advances, we aim to delineate both the persistent challenges and opportunities associated with this technology, with the goal of inspiring further innovation in the field.

High-scalability HfO₂-based ferroelectric thin films are promising for application in fast, energy-efficient, and high-density non-volatile memories. This ferroelectricity is believed to originate from the metastable orthorhombic phase, which is difficult to obtain. Post-metallization annealing with a top electrode capping layer is a useful method for stabilizing the ferroelectric orthorhombic phase. However, direct physical evidence of the top electrode role is lacking. In this study, we visualized the dynamic process of the phase transition in Hf_0.5Zr_0.5O₂ (HZO) thin films with TiN and Pt top electrodes during the heating and cooling processes through in-situ scanning transmission electron microscopy (STEM). The TiN top electrode stabilized the orthorhombic phase, whereas the Pt top electrode induced a phase transition to the monoclinic phase. Subsequently, we elucidated the phase transition mechanism in HZO thin films using the kinetic effect and revealed that it was related to the concentration of oxygen vacancies induced by the top electrode. This study provides valuable insights into the stabilization of the orthorhombic phase in HfO₂-based ferroelectric thin films and contributes to the elucidation of the phase transition mechanism of HfO₂-based ferroelectric thin films.

Ferroelectric thin films based on HfO₂ have garnered increasing attention worldwide, primarily due to their remarkable compatibility with silicon and scalability, in contrast to traditional perovskite-structured ferroelectric materials. Nonetheless, significant challenges remain in their widespread commercial utilization, particularly concerning their notable wake-up effect and limited endurance. To address these challenges, we propose a novel strategy involving the inhomogeneous distribution of Hf/Zr elements within thin films and explore its effects on the ferroelectricity and endurance of Hf_0.5Zr_0.5O₂ thin films. Through techniques such as grazing incidence X-ray diffraction, transmission electron microscopy, and piezoresponse force microscopy, we investigated the structural characteristics and domain switching behaviors of these materials. The experimental results indicate that the inhomogeneous distribution of Hf/Zr contributes to improving the frequency stability and endurance while maintaining a large remnant polarization in Hf_0.5Zr_0.5O₂ ferroelectric thin films. By adjusting the distribution of Zr/Hf within the Hf_0.5Zr_0.5O₂ thin films, significant enhancements in the remnant polarization (2P_r > 35 μC/cm²) and endurance (> 10⁹) along with a reduced coercive voltage can be achieved. Additionally, the fabricated ferroelectric thin films also exhibit high dielectric tunability (≥ 26%) under a low operating voltage of 2.5 V, whether in the wake-up state or not. This study offers a promising approach to optimize both the ferroelectricity and endurance of HfO₂-based thin films.