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

TL;DR

This study investigates the different negative differential resistance behaviors in amorphous VOx devices, linking them to distinct conduction mechanisms and electroforming conditions, providing insights for device design.

Contribution

It reveals how electroforming current influences NDR modes in VOx devices and introduces a model explaining the underlying conduction mechanisms.

Findings

Low compliance current results in a smooth S-type NDR with polycrystalline VO2 conduction.

High compliance current leads to an abrupt NDR with oxygen deficient VOx conduction.

The insulator-metal transition in VO2 underpins the switching voltage behavior.

Abstract

Metal-oxide-metal devices based on amorphous VOx 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 characterised 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 characterised by an activation energy of 0.025 eV, consistent with conduction in oxygen deficient VOx. 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 VOx. 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.