Thermochemical resistive switching: materials, mechanisms, and scaling projections

In this article, resistive switching based on the thermochemical mechanism (TCM) is reviewed. This mechanism is observed when thermochemical redox processes dominate over electrochemical processes. As the switching is based on thermal effects, it is inherently unipolar, i.e., the transitions between the resistive states can be induced by the same bias voltage polarity. NiO has emerged as a "model material" for resistive switching based on the TCM effect and the discussion of the resistance states and the switching processes are focused on this material with the appropriate electrodes, mainly Pt. Unipolar switching is unambiguously filamentary. Conductive filaments are formed during the electroforming process needed prior to memory switching. The SET operation is interpreted as a sequence of threshold switching and subsequent Joule heating which triggers local redox reactions in which oxygen deficient NiO and, if the amount of released oxygen exceeds a certain amount, also metallic Ni will form. The RESET transition can be described as a thermally activated solid-state process resulting in a local decrease of the metallic Ni species. In terms of operation and reliability, a trade-off between RESET current reduction and retention was experimentally found. This is due to the decreasing long-term stability of the filaments with decreasing size. In addition, the scaling projection of a TCM-based memory technology with NiO is directly related to RESET currents and the availability of appropriate select devices.