Frequency and percolation dependence of the observed phase transition in nanostructured and doped VO2 thin films

The response to applied electric fields of vanadium dioxide thin films above and below the phase transition depends on the size of grains if below similar to 200 nm across, and on aluminum doping above a critical concentration. T-c drops as doping level increases, but does not depend on grain size. The observed phase transition undergoes a remarkable qualitative shift as the applied field goes from optical to low frequencies. The expected insulator to metal transition is found at optical frequencies, but at low frequencies an insulator-to-insulator transition occurs. Optical switching at both T < T-c and T > T-c is nearly independent of doping level and grain size. In contrast dc properties in both phases are sensitive to both factors. The band gaps from optical and dc data differ, and densities of states change with doping level. Such behaviour can arise if there is a transient phase change. The way doping and grain size can support such a phase is discussed. Only individual nanograins need to switch phases coherently to explain data, not the whole sample. Resistance as a function of composition across the transition was derived using effective medium compositional analysis of optical data in the hysteresis zone. The percolation thresholds are not at the usual T-c values.