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

• Reversible transition between bipolar and unipolar resistive switching in HfO2 RRAM.
• Bipolar resistive switching using low compliance current is induced by oxygen-vacancy filaments.
• Unipolar resistive switching using high compliance current is induced by metal filaments.
• Multi-level-cell resistive switching is realized using two different mechanisms.

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/HfO2/SiOx/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.