Insight into Distribution and Switching of ReRAM Filaments Based on Variation Analysis of Memory Characteristics
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Insight into Distribution and Switching of ReRAM Filaments Based on Variation Analysis of Memory Characteristics Kentaro Kinoshita1,2, Hayato Tanaka1, Masataka Yoshihara1, and Satoru 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 A hypothesis that probability giving Δ(1/Vset) [= 1/Vset(n) - 1/Vset(n+1)] > 0, P[Δ(1/Vset) > 0], increases with increasing the number of filaments contained in one memory cell, Nfila, and decreases with increasing switching cycle, n, was made to validate a multi-filament model (MFM) as a mechanism causing the cycle to cycle dispersion of Vset in ReRAM. Here, Δ(1/Vset) is the difference between the inverse of set voltages after n-th and (n+1)-th reset processes. This in turn means that Vset will decrease with increasing Nfila and will increase with increasing n. In addition, another hypothesis that probability giving Δ(1/R) [= 1/Rn - 1/Rn+1] > 0, P[Δ(1/R) > 0], agrees with P[Δ(1/Vset) > 0] was made by incorporating the assumption that vset depends on d with the MFM. Here, Rn, vset, and d represent resistance in high resistance state after the n-th reset process, the set voltage of each filament, and the thickness of a gap between the electrode and the edge of the filament. The validity of these two hypotheses were confirmed by measuring the dependence of P[Δ(1/Vset) > 0], P[Δ(1/R) > 0], and the mean value of Vset, , on both the length of the perimeter, L, and n of Pt/NiO/Pt structures to which filaments were introduced by etching the NiO layer. INTRODUCTION Large dispersion of switching characteristics has hindered ReRAM from being put into practical use. The dispersion of memory characteristics such as cycle to cycle dispersion of Vset have been reported to be related to the plurality of filaments [1-4]. However, the relationship between switching characteristics and the number of filaments which are contained in one memory cell, Nfila, is still an open issue due to the difficulty in controlling Nfila. Recently, Yoda et al. [5] reported that filaments, which are generally received as the gathering of oxygen vacancies [6-8], can be introduced on the etched surface of a NiO film due to the reduction effect of the bombardment by the etching gas, Ar+ [9, 10]. Tanaka et al. [11] reported Nfila-dependence of Vset-distribution by measuring the size-dependence of Vset-distribution for etched Pt/NiO/Pt structures, in which Nfila is proportional to the length of the perimeter of the cell, 4L. A multifilament model (MFM), which detail will be described below, was proposed to reproduce the Nfila-dependence of Vset-distribution by the Monte Carlo simulation based on the model [11]. In this paper, we formed a hypothesis, which is led if the MFM is valid as the mechanism causing Vset-dispersion. This hypothesis, probability that Δ(1/Vset) (= 1/Vset(n) - 1/Vset(n+1)) gives positive value inc
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