  Reactive metal electrodes (Nb, Ti, Cr, Ta, and Hf) are shown to play an
important role in controlling the volatile switching characteristics of
metal/Nb2O5/Pt devices. In particular, devices are shown to exhibit stable
threshold switching under negative bias but to have a response under positive
bias that depends on the choice of metal. Three distinct responses are
highlighted: Devices with Nb and Ti top electrodes are shown to exhibit stable
threshold switching with symmetric characteristics for both positive and
negative polarities; devices with Cr top electrodes are shown to exhibit stable
threshold switching but with asymmetric hysteresis windows under positive and
negative polarities; and devices with Ta and Hf electrodes are shown to exhibit
an integrated threshold-memory (1S1M) response. Based on thermodynamic data and
lumped element modelling these effects are attributed to the formation of a
metal-oxide interlayer and its response to field-induced oxygen exchange. These
results provide important insight into the physical origin of the switching
response and pathways for engineering devices with reliable switching
characteristics.
