The Interplay between Electronic and Ionic Transport in the Resistive Switching Process of Random Access Memory Devices
Authors: Magyari-Kope Blanka , Zhao Liang , Kamiya Katsumasa , Yang Moon Young , Niwa Masaaki , Shiraishi Kenji , Nishi Yoshio 
Published: February 2020

In resistive random access memory devices (ReRAM), in order to achieve high endurance and good retention characteristics repetitive control over the resistance change between low resistance (LRS) and high resistance (HRS) states is desired. The binary transition metal oxides TiOx, NiOx, HfOx, AlOx, TaOx have been specifically promising to achieve this goal and are the preferential candidates for the next generation of resistance change based devices. To address the critical issues of ReRAM operation, however, atomistic modeling approaches based on quantum mechanical principles are needed to achieve the optimization of material properties and structures of ReRAM cells, as well as of selection devices and memory array configurations.
To explain the process of resistance change observed experimentally, various switching mechanisms were proposed recently and discussed in the technical literature. Generally, in transition metal oxide system such as TiO2, HfO2, NiO, Al2O3, Ta2O5, the diffusion of oxygen vacancies to cluster and create filaments [1-8] and the diffusion of oxygen atoms away from the oxide region to form a thin interfacial reduced oxide, are considered the key ionic processes taking place during forming and switching.
Filamentary models for transition metal oxides had been proposed theoretically and the formation energy implications of a conductive filament channel formation corresponding to the “ON” state or LRS [4-9] was investigated for TiO2, HfO2 and Al2O3 (Fig. 1). It was found that during the clustering process of oxygen vacancies, conductive channels are formed by electron delocalization trends induced by the vacancies. The accompanying electronic effects of charge trapping under applied electrical field during the switching process were also recently addressed [7-9]. Hole injection into a reduced transition metal oxide containing a formed filament were found to favor the dissolution, since isolated vacancies prefer the 2+ charge state configuration. On the other hand electron injection induces filament formation and a transition of vacancy charge state to 1+or neutral configuration. The energy barriers of oxygen vacancy out-diffusion from a filament in various charge states are shown in Fig. 2 as a function of chemical potential and Fermi level control. In addition, preferential impurity doping [10] in these types of systems can favorably affect the forming of the conductive filaments and the transition process between the “ON” and “OFF” states (Fig. 3).
In conclusion, several competing processes may undergo simultaneously at the microscopic level during the switching of ReRAM devices. Atomistic simulations based on density functional theory can provide an accurate description of the role of each component and describe the mechanism of ReRAM switching.
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