Formation Mechanism of Conducting Path in Resistive Random Access Memory by First Principles Calculation Using Practical
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Formation Mechanism of Conducting Path in Resistive Random Access Memory by First Principles Calculation Using Practical Model Based on Experimental Results Takumi Moriyama1, 2, Takahiro Yamasaki 3, Takahisa Ohno3, 4, Satoru Kishida1, 2 and Kentaro Kinoshita1, 2 1 Department of Information and Electronics, Graduate School of Engineering, Tottori University, 4-101 Koyama-Minami, Tottori 680-8552, Japan. 2 Tottori Integrated Frontier Research Center, 4-101 Koyama-Minami, Tottori 680-8552, Japan. 3 Natonal Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan. 4 Institute of Industrial Science, the University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. ABSTRACT For practical use of Resistive Random Access Memory (ReRAM), understanding resistive switching mechanism in transition metal oxides (TMO) is important. Some papers predict its mechanism by using first principles calculation; for example, TMO become conductive by introducing oxygen vacancy in bulk single crystalline TMO. However, most of ReRAM samples have polycrystalline structures. In this paper, we introduced a periodic slab model to depict grain boundary and calculated the surface energy and density of states for surfaces of NiO with various orientations using first-principles calculation to consider the effect of grain boundaries for resistive switching mechanisms of ReRAM. As a results, vacancies can be formed on the side surface of grain more easily than in grain. Moreover, we showed that surface conductivity depends on surface orientation of NiO and the orientation of side surface of grain can change easily by introduction of vacancies, which is the switching mechanism of NiOReRAM INTRODUCTION Flash memory is facing the physical limit of miniaturization. Resistive Random Access Memory (ReRAM) is expected as a substitutional memory for Flash. Compared to Flash, ReRAM has two remarkable advantages over Flash: high applicability to the miniaturization of a cell size and short data transferred time. The former is brought about by the very simple structure of ReRAM, in which binary-transition-metal-oxides such as NiO [1] or TiO2 [2] is sandwiched by top and bottom electrodes. The latter is about by the high write/erase speed switching: 300 ps pulse switching in Pt/HfOx/Pt structures was reported [3]. The memory effect of ReRAM is generated after a breakdown like process called forming. After forming, ReRAM can store data by resistance values of a high resistance states (HRS) and a low resistance states (LRS). Resistive switching from HRS to LRS and vice versa can be caused by applying appropriate voltages. For practical use of ReRAM, understanding resistive switching mechanism in transition metal oxides (TMO) is important. Some papers reported that resistive change of ReRAM were caused in a local conductive path inside TMO [1]. The conductive path were reported to consist of a chain of oxygen vacancies (Vo) and resistive switching was caused by redox reaction of them with migration of oxygen ion [4]. However, it is difficu
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