We investigated the origin of the variation in switching voltages in threshold-switching of VO2 thin films. When a triangular-waveform voltage signal was applied, the current changed abruptly at two switching voltages, i.e., VON (insulator-to-metal) and VOFF (metal-to-insulator). VON and VOFF were measured by changing the period of the voltage signal, the temperature of the environment, and the load resistance. We observed that either VON or VOFF varied significantly and had different dependences with respect to the external parameters. Based on the mechanism of the metal–insulator transition induced by Joule heating, numerical simulations were performed, which quantitatively reproduced all of the experimental results. From the simulation analysis, the variation in the switching voltages for threshold-switching was determined to be thermal in origin.

We report on multilevel switching behavior in the unipolar resistance switching of TiO2 thin films. Multiple metastable states were observed during the reset process by measuring I-V curves. As observed using a conducting atomic force microscope, the multilevel resistance switching was accompanied by decreases in area and in the conductance of the top surface conducting regions. These experimental observations at both the macroscopic and microscopic levels could be explained by using the “random circuit breaker network” model, which is a dynamic bond percolation model.