Improved performance of a conducting-bridge random access memory using ionic liquids

The conducting-bridge random access memory (CB-RAM) is a promising candidate for the next-generation memory. In this study, the addition effect of 18 types of ionic liquids (ILs) on the performances of the Cu/HfO 2 /Pt type CB-RAM has been investigated; the switching voltage was lowered by supplying a trace amount of the IL to the HfO 2 layer of the Cu/HfO 2 /Pt cell. It was revealed that the ionic conductivity ( σ i ) of the IL and the coordinating ability ( β ) of the anion in the IL towards the copper (Cu) ion are key competing factors that determine the voltage required for the formation of the Cu filament ( V set ); the increase in σ i and the decrease in β are effective in enhancing the ionization and diffusion of the Cu atoms, resulting in the reduction of V set . Therefore, among the tested ILs, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([bmim][Tf 2 N]), 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)amide ([emim][Tf 2 N]) and 1-butyl-3-methylimidazolium bis(fluorosulfonyl)amide ([bmim][FSA]), which have high σ i and low β values were found to be the best additives for the Cu/HfO 2 /Pt cell. On the other hand, reset was revealed to occur due to the rupture of the Cu filament by the thermal diffusion of the Cu atoms by Joule heat. Reducing V set and making a thin filament are effective in reducing the reset voltages. We further traced the origin of the improved dispersion of the switching voltages by the addition of the Cu(Tf 2 N) 2 doped [bmim][Tf 2 N] to the HfO 2 film, which we recently reported. A complex-impedance measurement suggested that the addition of Cu doped IL involves two competing factors: the decline of the Cu ion diffusion and enhancement of the Cu ionization. The balance of these two factors determines the important parameters such as V set and its dispersion. The performance of the CB-RAM can be optimized by the adjustment of the IL, considering the balance of the competing factors revealed by the present study. The conducting-bridge random access memory (CB-RAM) is a promising candidate for the next-generation memory.