Reversible transition of resistive switching induced by oxygen-vacancy and metal filaments in HfO2

In contrast to the irreversible transition of resistive switching induced by oxygen-vacancy filaments (VF) and metal filaments (MF) reported in the literature, this study reports coexistence and completely reversible transition of VF-and MF-induced resistive switching in a Ni/HfO 2 /SiO x /p +-Si device with three distinct and stable resistance states. In a dual filament model proposed, VF and MF may coexist at the same percolation path, and the formation and rupture proceed in a two-step fashion by choosing appropriate SET/RESET conditions. Exploiting the dependence of different filament compositions on resistive switching may enable new design space for future multi-level-cell resistive-switching memory. Bias-dependent variable resistance in resistive-switching random access memory (RRAM) has been explained by the connection and rupture of conductive filaments in metal oxides [1–3]. Depending on material combination of a large variety of metal oxides and electrodes, filament compositions are generally classified into two categories: metal filament (MF) comprising metal precipitation from electrochemically active electrodes, such as Ni, Cu, and Ag [1,2], and oxygen-vacancy filament (VF) generated by redox of metal oxides [1,3]. However, coexistence of these two filaments in the same RRAM cell was less discussed. Jeong et al. [4] first reported coexistence of bipolar and unipolar resistive switching (RS) in a TiO 2 RRAM cell by controlling current compliance (I comp) at SET. The transition from bipolar RS with low I comp to unipolar RS with high I comp was irreversible. Similar results were also reported in a ZnO RRAM cell [5]. More recently, Raghavan et al. [6] showed that VF and MF can coexist in a NiSi/HfO 2 /Si stack, but the transition from VF-induced bipolar RS mode to MF-induced unipolar RS mode was also irreversible. Our previous study also revealed that the filament composition and RS mode in HfO 2 RRAM was determined by top electrode (TE) material and voltage polarity at electrical forming [7]. This study reports coexistence and completely reversible transition of RS induced by VF and MF in a Ni/HfO 2 /SiO x /p +-Si RRAM cell. This not only led to a better understanding on the interplay of two different filaments during RS, but also enabled new design space for multi-level-cell (MLC) implementation, which is extremely critical for ultra high-density storage. In contrast to the reported MLC RRAM in the literature utilizing only a single RS mechanism [8,9], exploiting the dependence of different filament compositions on RS properties, namely VF …