Phase analysis of VO2 thin film and the mechanism of the electrically triggered metal-insulator transition of VO2

Many researchers have produced various results for the mechanism of the metal-insulator transition (MIT) of VO₂. This seems to be because of the influence of various VO_x phases in the sample thin-film. VO_x has many phases, and VO₂ is in only a small window in the VO_x phase diagram. In this work, VO₂ thin films on Al₂O₃ substrates were prepared by a pulsed laser deposition method, and their I-V properties were measured over a temperature range from 50 K to 300 K. In the thin-films, not only the VO₂ phase, but also at least two other VO_x phases were present, including V₂O₃ and V₅O₉. For electrically triggered MIT, a space charge limit current (SCLC) appeared at high voltages while ohmic current appeared at low voltages, and MIT occurred when the space charge density became greater than the critical carrier density, nc. This is a current-driven device, basically, because MIT occurs after the charge carrier density, which is injected from the electrode becomes higher than nc. The switching time, which is the time for the whole sample to transition from insulator to metal because of bias application, depends on the charge carrier mobility in the insulator state. The switching speed of VO₂ due to electrical triggering must be slower than that of optical triggering because the mobility of the charge carrier in the insulating state is only 0.5 cm²/V•s. For a simple two terminal model, the switching speed is proportional to the square of the length between the two electrodes and the mobility of the charge carrier, and inversely proportional to the voltage. Therefore, a proper switching speed can be designed for one's use if the appropriate material and dimension of the device are used.