Experimental and Theoretical Studies of Resistive Switching in Grain Boundaries of Polycrystalline Transition Metal Oxid
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Experimental and Theoretical Studies of Resistive Switching in Grain Boundaries of Polycrystalline Transition Metal Oxide Film Takumi Moriyama1, Sohta Hida1, Takahiro Yamasaki 2, Takahisa Ohno2, Satoru Kishida1 and Kentaro Kinoshita1 1 Department of Information and Electronics, Graduate School of Engineering, Tottori University, 4-101 Koyama-Minami, Tottori 680-8552, Japan. 2 National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan. ABSTRACT Practical use of Resistive Random Access Memory (ReRAM) depends on thorough understanding of the resistive switching (RS) mechanism in polycrystalline metal oxide films. Based on experimental and theoretical results of NiO based ReRAM, we have proposed a grain surface tiling model, in which grain surfaces (i.e. grain boundaries) are composed by insulating and conductive micro surface structures. This paper reports the adequacy of our model to the NiO based ReRAM and universality of surface electronic properties in metal oxides of NiO, CoO and MgO. Experimental results of RS operating modes suggest that the resistance changes in the grain boundaries, supporting our model. First-principles calculation results suggest that our model can be adopted to other metal oxide materials and the RS from a low resistance to a high resistance can be caused at 1000 K, which agrees with previous experimental reports. INTRODUCTION Flash memory is facing the physical limit of miniaturization, and Resistive Random Access Memory (ReRAM) is a likely replacement. ReRAM takes a simple structure that transitionmetal-oxide (TMO) such as NiO [1], TiO2 [2], or TaOx [3] is sandwiched between electrodes. Therefore, ReRAM have high applicability to cell size miniaturization. The memory effect of ReRAM is generated after a breakdown like process called forming. After forming, resistive switching (RS) from a high resistance (HR) state to a low resistance (LR) state and vice versa can be caused applying appropriate bias voltages. The former and the latter process is called set and reset, respectively. For practical use of ReRAM, understanding of the RS mechanism in transition metal oxides (TMO) is important. Some papers predict its mechanism by using firstprinciples calculation; for example, TMO become conductive by introducing oxygen vacancies (Vos) in bulk single crystalline TMO [4] . However, most of ReRAM samples have polycrystalline structures [1] . We have proposed a grain surface tiling model; a grain surface of a polycrystalline NiO film is tiled by insulating and conductive micro surfaces and the conductive micro surface can become the insulating micro surface by the small displacement of the surface atoms, making the RS in the grain boundaries [5]. In this study, we checked the adequacy of the tiling model to NiO based ReRAM by experiments and universality of surface electronic properties in metal oxides of NiO, CoO and MgO by calculating the stabilities and electronic states for various surfaces. Focusing on the estimated temperatures of the conductive paths in reset (430
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