Mechanism of resistive switching in HfOx-based memristors

We conducted an in-depth study of the resistance-switching mechanism in classical HfOx-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 HfOx-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 HfOx). The phase of the HfOx 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.

Reference
 Evolution of the conductive filament system in HfO2-based memristors observed by direct atomic-scale imaging
Y. Zhang, G.-Q. Mao, X. Zhao*, Y. Li, M. Zhang, Z. Wu, W. Wu, H. Sun, Y. Guo, L. Wang, X. Zhang, Q. Liu, H. Lv, K.-H. Xue*, G. Xu, X. Miao, S. Long*, and M. Liu*
Nature Communications 12, 7232 (2021).
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