Gradual Formation of Conductive Filaments in Resistive Switching Cells

Resistive random-access memory is one of the next-generation nonvolatile memories. A resistive switching (RS) phenomenon offers promising applications for not only high-performance nonvolatile memories but also synaptic devices for neuromorphic computing. The RS phenomenon in a RS cell composed of a transition-metal-oxide is often regarded as nonvolatile transitions in the cell resistance between high- and low- resistance states by formation and dissolution of localized conductive filaments. The trigger required for the RS phenomenon in RS cells using a binary oxide such as a nickel oxide (NiO) and titanium dioxide (TiO₂) is an abrupt reduction in resistance called forming. We reported correlation between a crystalline structure of the oxide layer and forming characteristics in the oxide-based RS cells [1,2]. In this study, dependence of the forming characteristics on ambient temperature was investigated under constant voltage stresses to NiO-based RS cells.<br/>A 60-nm-thick platinum (Pt) bottom electrode was deposited by sputtering on a silicon-dioxide/p-type silicon substrate. Next, a polycrystalline 60-nm-thick NiO with a columnar structure was deposited by radio-frequency reactive sputtering under oxygen gas flow rate precisely regulated. Pt top electrodes with thickness of 25 nm were subsequently deposited on the NiO layer through a metal mask. The Pt/NiO/Pt cell area is defined as the top electrode area in the range of 100-300 μm. In all electrical measurements, the Pt bottom electrode was grounded, and the constant voltage was applied to the Pt top electrode. Time to forming in the initial state was measured under constant voltages using an oscilloscope and external resistor of 50 Ω.<br/>Weibull slopes of time to forming were similar with each other in the cells with different cell areas at 290 K. In addition, these Weibull distributions normalized according to the area scaling law fell in the same line, which agrees with our previous reports [1,2]. The results indicate that the formation of conductive filaments at the forming follows a weakest link theory, and that weakest spots are randomly distributed in the NiO film according to the Poisson statistics. Moreover, time dependences of cell resistance with a diameter of 150 μm were investigated at several ambient temperature. Although cell resistance remained unchanged before the forming at 290 K under constant voltage stress of 4.5 V, gradual reduction phenomena in resistance were observed before the forming at a high temperature of 400 K. We also found the voltage dependence of this gradual reduction, which means gradual formation of conductive filaments. The slope of the resistance reduction decreased, and the gradual reduction became suppressed as the voltage decreased. From this result, a percolation model is proposed for breaking the insulating oxide layer under stress due to not only higher temperature but also larger voltage. The formation of defects, that is, the formation of conductive filaments occurs continuously rather than abruptly.<br/>The formation mechanism of the filaments is a key to uncover an origin of the RS phenomenon. The restriction of positive feedback to create the filament by elevating temperature is quite interesting. We also observed multi-step formation of the filament by a thermal effect, which will be shown in our presentation.<br/>[1] Y. Nishi <i>et al</i>., J. Appl. Phys. <b>120</b>, 115308 (2016).<br/>[2] M. Arahata <i>et al</i>., AIP Adv. <b>8</b>, 125010 (2018).