To address this issue, researchers from the National Institute for Materials Science (NIMS), Japan, have fabricated cross-point structured Cu/Ta2O5/Pt cells and closely examined their current-voltage (I-V) and current-time characteristics. The cells can be initially SET from the off state to the on state only when a positive bias is applied to the Cu electrode.

This first SET operation corresponds to the first formation of a metal filament between the electrodes by inhomogeneous nucleation and subsequent growth of Cu on the Pt electrode, based on the migration of Cu ions into the Ta2O5 film from the Cu electrode. After the formation of the metal filament, the cells exhibit bipolar switching behaviour with the repetition of SET and RESET (from the on state to the off state) operations, as shown in figure (a).

The SET process corresponds to the reformation of the metal filament on a remaining portion by inhomogeneous nucleation as shown in figures (b) to (e), while the reset process is attributed to the thermal dissolution of the metal filament due to the Joule-heating-assisted oxidation followed by the diffusion of Cu ions under the concentration gradient and the applied electric field as illustrated in figures (f) to (i).

Because the reset process is mainly associated with the thermal effect, the reset operation is also possible at positive bias. However, the reset operation at negative bias was found to be essential for a number of repeated switching cycles. This switching model is applicable to other cation-migration-based resistive memories.

Gapless-type atomic switch

Several years ago, the NIMS team proposed a conceptually novel switching device called an "atomic switch", in which resistive switching across a nanometer-scale gap between ionic conductor and counter metal electrodes is based on a similar solid-state electrochemical reaction. From the similarity of the switching mechanisms, the MIM-structured cation-migration-based resistive memory can be considered a "gapless-type atomic switch". The team is conducting further investigations into the microscopic picture of cation conduction and nucleation behaviour in oxide-based atomic switches.

Full details can be found in the journal Nanotechnology.