Memory Retention Characteristics of Data Storage Area Written in Transition Metal Oxide Films by Using Atomic Force Micr
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Memory Retention Characteristics of Data Storage Area Written in Transition Metal Oxide Films by Using Atomic Force Microscope. K. Kinoshita1,2, T. Yoda1, and S. Kishida1,2 1
Department of Information and Electronics, Graduate School of Engineering, Tottori University, 4-101 Koyama-Minami, Tottori 680-8552, Japan. 2 Tottori University Electronic Display Research Center, 522-2 Koyama-Kita, Tottori 680-0941, Japan. ABSTRACT Conductive atomic-force microscopy (C-AFM) writing is attracting attention as a technique for clarifying the switching mechanism of resistive random-access memory (ReRAM) by providing a wide area filled with filaments, which can be regarded as one filament with large radius. We observed a C-AFM writing area of NiO films using SEM, and revealed a correlation between the contrast in a secondary electron image (SEI) and the resistance written by C-AFM. In addition, the dependence of the SEI contrast on the beam accelerating voltage (Vaccel) suggests that the resistance-change effect occurs near the surface of the NiO film. As for the effect of electron irradiation on the C-AFM writing area, it was shown that the resistance change effect was caused by exchanging oxygen with the atmosphere at the surface of the NiO film. This result suggests that the low resistance and high resistance areas are, respectively, p-type Ni1+δO (δ < 0) and insulating (stoichiometric) or n-type Ni1+δO (δ 0). INTRODUCTION Reduction/oxidation of conductive filamentary paths consisting of cationic [1] or anionic [2] vacancies are thought to be the most likely origin of a resistance change effect of ReRAM, especially of binary-transition-metal-oxide ReRAM. However, the conductive filamentary paths, termed “filaments”, are sandwiched between a top and bottom electrode. Moreover, it was reported that the diameter of the filaments is very small; some groups even reported the diameter to be less than 50 nm [3-5]. Physical and chemical analyses of the filament by conventional analytical methods have therefore been hindered for a long time. Against this background, overcoming the above-mentioned difficulties by using conductive atomic force microscope (CAFM) was reported to be possible [3,5-7]. That is, by directly contacting transition metal oxide films with an AFM-tip and scanning the tip under application of dc bias voltage, a large area with an arbitrary resistance can be formed without a top electrode being deposited. The AFM-tip plays a role as a top electrode, and the filament is formed between the tip and the bottom electrode. By scanning the AFM-tip, filaments are then generated one after another, and, as a result, the scanned area is filled with the filaments. If occupation density of the filaments in the scanned area is high enough, the scanned area can be regarded as one filament with a large diameter. This feature is a great advantage in analyses on the ReRAM filament. The scanned area will be referred to as a “C-AFM writing area”, hereafter. A C-AFM writing area is visible in a secondary electron images (SEI) of a scan
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