Two-terminal metal/oxide/metal (MOM) structures are known to exhibit characteristic resistance changes when subjected to electrical stress, e.g. voltage or current stimuli. The resistance changes of interest include both non-volatile memory and volatile threshold switching behavior, as well as combinations of these responses which are of interest as active elements in non-volatile memory arrays and neuromorphic computing. While volatile threshold switching has been observed in a range of materials and device structures, two terminal MOM devices based on niobium oxides (NbO_x) and vanadium oxides (VO_x) have attracted particular attention due to their simple structure and reliability. 

The resistance changes in MOM structures are often initiated by a one-step electroforming process that  introduces filamentary conduction, either in the form of a semi-permanent filament created by compositional changes in the oxide or as a transient filament created by inhomogeneous current or field distributions (e.g. current bifurcation). Knowledge about the structure, composition and spatial distribution of filaments is essential for a full understanding of filamentary resistive-switching and for effective modelling and optimisation of associated devices. Additionally, MOM devices based on NbO_x exhibit a wide range of resistive and threshold switching responses that critically depend on operating condition, composition and device geometry. Thus, a proper understanding of these factors is important for achieving reliable switching with desired characteristics for different applications. My PhD project was focussed on understanding the electroforming process and subsequent threshold switching responses in NbO_x, including material and device dependencies.

This talk will briefly introduce the concept of resistive/threshold switching in oxides, then discuss the electroforming and switching processes in NbO_x with particular emphasis on understanding the physical origin of switching, including the origin of a discontinuous ‘snapback’ mode- a distinct threshold switching mode observed under current-controlled testing. Significantly, our results resolve a long-standing controversy about the origin of the snapback response and provide a basis for engineering functional devices with specific characteristics.