Raman evidence for absence of phase transitions in negative differential resistance thin film devices of niobium dioxide

We fabricate NbO2 thin films and measure their transport properties in simple devices. These could be potential components of future memristor devices because of peculiar conductivity variations observed as a function of device current. We find that threshold switching effects observed in the voltage control regime are better viewed in the current controlled regime where they can be understood in terms of a negative differential resistance phenomenon. No electronic or structural phase change is observed in the NbO2 thin films in this regime in the steady state, notably with in situ Raman measurements. In particular, both crystalline and amorphous films remain insulating since their resistance always decreases with an increase in temperature. However, a large decrease in resistivity corresponding to negative differential resistance is observed as current in the devices increases. Temperature is the parameter which induces this change in resistivity through thermal activation of carriers, confirming recent understanding of the phenomenon. Temperature changes are locally induced because of the power dissipated by the current in the device and the intrinsically low thermal conductivity of NbO2. This is confirmed by parameters extracted from the simulation of the phenomenon with different transport models. However, the simplest thermal activation model accounts for the observations in non-nanometric devices without the need for invoking more complex models. Finally, pulsed current can be used to provoke a structural, amorphous to crystalline phase transition in amorphous samples through a sudden local increase in temperature.