  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.
