Mechanism of resistive switching in HfOx-based memristors

We conducted an in-depth study of the resistance-switching mechanism in classical HfO_x-based devices. Through atomic-level characterisation of the dynamic evolution of conductive filaments, we confirmed that the filament system in this type of device has a quasi-core-shell structure, consisting of a metallic Hf₆O core and an insulating HfO₂ crystalline shell. constitute. The study pinpointed multiple factors affecting the crystal structure of the HfO₂ shell, and found that the HfO₂ shell plays a key role in hindering the spontaneous oxidation of the low-resistance core, which is beneficial to the uniformity of parameters during the resistive switching process and the retention performance under electrical stress. This theory fills the gap in the research on the resistance transition mechanism of HfO_x-based devices, and provides theoretical support for the application of such devices in key technology nodes, especially in the sub-5nm regime.

(Abstract) The resistive switching effect in memristors typically stems from the formation and rupture of localized conductive filament paths, and HfO₂ has been accepted as one of the most promising resistive switching materials. However, the dynamic changes in the resistive switching process, including the composition and structure of conductive filaments, and especially the evolution of conductive filament surroundings, remain controversial in HfO₂-based memristors. Here, the conductive filament system in the amorphous HfO₂-based memristors with various top electrodes is revealed to be with a quasi-core-shell structure consisting of metallic hexagonal-Hf₆O and its crystalline surroundings (monoclinic or tetragonal HfO_x). The phase of the HfO_x shell varies with the oxygen reservation capability of the top electrode. According to extensive high-resolution transmission electron microscopy observations and ab initio calculations, the phase transition of the conductive filament shell between monoclinic and tetragonal HfO₂ is proposed to depend on the comprehensive effects of Joule heat from the conductive filament current and the concentration of oxygen vacancies. The quasi-core-shell conductive filament system with an intrinsic barrier, which prohibits conductive filament oxidation, ensures the extreme scalability of resistive switching memristors. This study renovates the understanding of the conductive filament evolution in HfO₂-based memristors and provides potential inspirations to improve oxide memristors for nonvolatile storage-class memory applications.