Evolution of the conductive filament system in HfO2-based memristors observed by direct atomic-scale imaging

Ying Zhang,Guangwei Xu,Xumeng Zhang,Hangbing Lv,Yu Li,Kan-Hao Xue,Shibing Long,Xiaolong Zhao,Huajun Sun,Ming Liu,Xiangshui Miao,Ge-Qi Mao,Zuheng Wu,Meiyun Zhang,Wei Wu,Qi Liu,Lihua Wang,Yizhong Guo

doi:10.1038/s41467-021-27575-z

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.

Highlights

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
Resistive switching (RS) behavior stemming from the repeatable formation/rupture of conductive filaments (CFs) under an external electric field lays the foundation of oxide memristors[11,12]
Similar to the situation in memristors based on other oxides, CFs in the HfO2-based valence change mechanism (VCM) or thermochemical mechanism (TCM) memristors have been demonstrated as a low-oxygen content region[34,35]

Summary

Introduction

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. The abovementioned studies indicate that the formation/rupture of electric field-induced oxygen-deficient CFs with relatively high conductance contributes to the RS behavior, and this scenario dominates the physical understanding of HfO2-based memristors.

Results

Conclusion