|
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 VOx phases in the sample thin-film. VOx has many phases, and VO2 is in
only a small window in the VOx 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 VOx 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.
|
