  The negative differential resistance (NDR) response of Nb/NbOx/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 snap-back NDR, and combined S-type and snap-back 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 localisation, 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 non-volatile memory applications.
