Dielectric spectroscopy and features of the mechanism of the semiconductor–metal phase transition in VO$_{2}$ films

In the range of 0.1–10$^6$ Hz, the temperature-induced transformation of the frequency dependences of the dielectric-loss tangent $\operatorname{tg}\delta(f)$ as well as the Cole–Cole diagrams for undoped vanadium-dioxide films are investigated. The measurements are carried out in the temperature range $T$ = 273–373 K. It is shown that the shape of the Cole–Cole diagrams for all films depends slightly on the temperature in the specified interval, while the frequencies $f_0$ corresponding to the peaks of the function $\operatorname{tg}\delta(f)$ increase with temperature. The thermal-hysteresis loops of the frequency positions $f_0(T)$ of the peaks are measured. When interpreting the data of dielectric spectroscopy, a complex equivalent electrical circuit of the sample is used; it makes it possible to detect the presence of two types of grains with different electrical properties in undoped VO$_{2}$ films. The presence of two types of grains determines the features of the semiconductor–metal phase-transition mechanism in VO$_{2}$ films.