The switching mechanisms of atomic switches based on poly(ethylene oxide) (PEO) are systematically investigated. By using self-assembled PEO and Ag-PEO thin films, stack-structured devices exhibit stable bipolar switching behavior over 10(3) cycles. Direct observation of filament growth behavior in planar-structured devices reveals the effects of the polymer thin-film morphology, and the presence of electrochemically active electrodes, on the switching characteristics.

Resistance switching is studied in HfO2 as a function of the anode metal (Au, Cu, and Ag) in view of its application to resistive memories (resistive random access memories, RRAM). Current-voltage (I-V) and current-time (I-t) characteristics are presented. For Au anodes, resistance transition is controlled by oxygen vacancies (oxygen-based resistive random access memory, OxRRAM). For Ag anodes, resistance switching is governed by cation injection (Conducting Bridge random access memory, CBRAM). Cu anodes lead to an intermediate case. I-t experiments are shown to be a valuable tool to distinguish between OxRRAM and CBRAM behaviors. A model is proposed to explain the high-to-low resistance transition in CBRAMs. The model is based on the theory of low-temperature oxidation of metals (Cabrera-Mott theory). Upon electron injection, oxygen vacancies and oxygen ions are generated in the oxide. Oxygen ions are drifted to the anode, and an interfacial oxide is formed at the HfO2/anode interface. If oxygen ion mobility is low in the interfacial oxide, a negative space charge builds-up at the HfO2/oxide interface. This negative space charge is the source of a strong electric field across the interfacial oxide thickness, which pulls out cations from the anode (CBRAM case). Inversely, if oxygen ions migration through the interfacial oxide is important (or if the anode does not oxidize such as Au), bulk oxygen vacancies govern resistance transition (OxRRAM case).

Redox reactions at the Cu/Ta2O5 interface and subsequent Cu ion transport in a Ta2O5 film have been investigated by means of cyclic voltammetry (CV) measurements. Under positive bias to the Cu electrode, Cu is preferentially oxidized to Cu2+ and then to Cu+. Subsequent negative bias causes a reduction of the oxidized Cu ions at the interface. It was found that CV curves change drastically with varied relative humidity levels from 5 to 85%. At higher humidity levels, the ion concentrations and diffusion coefficients, estimated from the CV curves, suggest increased redox reaction rates and a significant contribution of proton conduction to the ionic transport. The results indicate that the redox reactions of moisture are rate-limiting and highlight the importance of water uptake by the matrix oxide film in understanding and controlling the resistive switching behavior of oxide-based atomic switches.

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