Metal-oxide interface reactions and their effect on integrated resistive/threshold switching in NbO _x

Abstract

Reactive metal electrodes (Nb, Ti, Cr, Ta, and Hf) are shown to play an important role in controlling the volatile switching characteristics of metal/Nb2O5/Pt devices. In particular, devices are shown to exhibit stable threshold switching under negative bias but to have a response under positive bias that depends on the choice of metal. Three distinct responses are highlighted: Devices with Nb and Ti top electrodes are shown to exhibit stable threshold switching with symmetric characteristics for both positive and negative polarities; devices with Cr top electrodes are shown to exhibit stable threshold switching but with asymmetric hysteresis windows under positive and negative polarities; and devices with Ta and Hf electrodes are shown to exhibit an integrated thresholdmemory (1S1M) response. Based on thermodynamic data and lumped element modelling these effects are attributed to the formation of a metal-oxide interlayer and its response to field-induced oxygen exchange. These results provide important insight into the physical origin of the switching response and pathways for engineering devices with reliable switching characteristics.

Engineering the Threshold Switching Response of Nb2O5-Based Memristors by Ti Doping

“…In this context, it is interesting to note that the threshold switching response of metal/Nb 2 O 5 /Pt devices is sensitive to the nature of the interlayer formed between the reactive metal electrode and the functional Nb 2 O 5 layer. The device reliability therefore also depends on the choice of metal and its relative thickness to the oxide layer, as discussed in an earlier study …”

“…The data in Figure c also show that there is a critical activation energy (∼0.15 eV) below which the device no longer exhibits CC-NDR. We note that the addition of a series resistance ( R series ) in the model affects the details of the simulations, , but does not affect the general conclusions. (Further information about these dependencies is given in the Supporting Information).…”

“…Significantly, these studies also demonstrated that a critical activation energy is required to observe NDR. Note that the threshold switching reliability of Nb 2 O 5 -based devices also depends on the nature of the interface reaction that take place at the electrode/oxide interface and can be further controlled by varying the thickness of the reactive metal layer (e.g., Ti, TiN, or Nb). , Nevertheless, combining the Ti-doping approach with a proper design of device structure (e.g., choice of electrode) has the potential to offer effective pathways to achieve reliable threshold switching and controllable switching parameters. These results provide an improved understanding of the physical mechanisms responsible for threshold switching and provide new insights for engineering devices for specific applications.…”

“…While memristive switching has been observed in a range of materials and device structures, two terminal metal–oxide–metal (MOM) devices based on niobium oxides have attracted particular attention because of their simple structure, reliability, and diverse switching characteristics. − The latter include non-volatile resistive switching, volatile threshold switching, and a combination of resistive and threshold switching, with the specific response depending on the film stoichiometry, crystallinity, electrode metals, device geometry, and operating conditions (e.g., maximum currents, bias polarity, etc.,). ,,− For example, devices based on NbO 2 and non-stoichiometric NbO x typically exhibit volatile threshold switching, − while those based on Nb 2 O 5 exhibit unipolar (or non-polar) switching when inert (e.g., Au and Pt) top and bottom electrodes are used , and threshold switching when one of the inert electrodes is replaced by a reactive metal. , The dependence on electrode metal highlights the fact that switching is sensitive to interface reactions and field-induced oxygen exchange between the electrode and oxide. ,, Understanding such dependencies is critical for engineering devices with well-defined switching characteristics and remains an active area of research.…”

Schottky-Barrier-Induced Asymmetry in the Negative-Differential-Resistance Response of Nb/NbOx / Pt Cross-Point Devices

“…This can cause a significant increase in temperature due to local Joule heating [25] and thereby modify the oxide stoichiometry and interfacial Schottky barriers. Changes in the filament due to the generation, drift, and diffusion of oxygen vacancies can also result in competition between resistive and threshold switching, in which case it is necessary to limit the operating current [26] or bias polarity in order to achieve pure threshold switching [27]. Threshold switching then occurs in a small volume between the residual filament and electrode due to the localized current flow in this region [26].…”