Nature of the electronic component of the thermal phase transition in VO$_2$ films

By the construction of a full grid of minor hysteresis loops and the step heating and cooling of vanadium dioxide films, it was found that the thermal semiconductor-metal phase transition in vanadium dioxide is a multistage process. It is shown that the transition consists of two inherently different transitions, i.e., electronic and structural ones. Both transitions occur simultaneously and trigger each other in a wide temperature range. More specifically, a continuous electronic transition occurs in the range 0–140$^\circ$C; in the range 20–80$^\circ$C, a series of discontinuous structural phase transformations occurs in film nanocrystallites at various fixed temperatures. The temperatures of the structural transformations are related to nanocrystallite sizes. An essentially new form of the elementary hysteresis loop of an individual nanocrystallite is constructed. The properties of minor hysteresis loops of the VO$_2$ film’s reflectance are analyzed; based on these results, the electronic component of the semiconductor-metal phase transition is identified and studied.