Understanding modes of negative differential resistance in amorphous and polycrystalline vanadium oxides

Metal–oxide–metal devices based on amorphous VO_x are shown to exhibit one of two distinct negative differential resistance (NDR) characteristics depending on the maximum current employed for electroforming. For low compliance currents they exhibit a smooth S-type characteristic and have a temperature-dependent device resistance characterized by an activation energy of 0.25 eV, consistent with conduction in polycrystalline VO₂, while for high compliance currents they exhibit an abrupt snap-back characteristic and a resistance characterized by an activation energy of 0.025 eV, consistent with conduction in oxygen deficient VO_x. In both cases, the temperature dependence of the switching voltage implies that the conductivity change is due to the insulator–metal transition in VO₂. From this analysis, it is concluded that electroforming at low currents creates a conductive filament comprised largely of polycrystalline VO₂, while electroforming at high currents creates a composite structure comprised of VO₂ and a conductive halo of oxygen deficient VO_x. The effect of electroforming on the NDR mode is then explained with reference to a lumped element model of filamentary conduction that includes the effect of a parallel resistance created by the halo. These results provide new insight into the NDR response of vanadium-oxide-based devices and a basis for designing devices with specific characteristics.