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

The negative-differential-resistance (NDR) response of $\mathrm{Nb}$/${\mathrm{Nb}\mathrm{O}}_{x}$/$\mathrm{Pt}$ cross-point devices is shown to have a polarity dependence due to the effect of the metal-oxide Schottky barriers on the contact resistance. Three distinct responses are observed under opposite polarity testing: bipolar S-type NDR, bipolar snapback NDR, and combined S-type and snapback NDR, depending on the stoichiometry of the oxide film and device area. In situ thermoreflectance imaging is used to show that these NDR responses are associated with strong current localization, thereby justifying the use of a previously developed two-zone, core-shell thermal model of the device. The observed polarity-dependent NDR responses, and their dependence on stoichiometry and area, are then explained by extending this model to include the effect of the polarity-dependent contact resistance. This study provides an improved understanding of the NDR response of metal-oxide-metal structures and informs the engineering of devices for neuromorphic computing and nonvolatile memory applications.