Film thickness dependent electron transport and optical properties of thermochromic VO2

Abstract

Vanadium dioxide (VO2) thin films are very well known for their efficient thermochromic properties, i.e. temperature dependent reversible switching of their optical and electrical properties, rooted in a first order phase change at the transition temperature of 68 °C. Despite a vast amount of basic and applied research on VO2 thin films, a simultaneous study of the optical and electrical switching properties, especially as functions of film thickness is scarcely reported. Such a study may provide interesting insight into the basics of thermochromic switching and lead to a better designing of more efficient thermochromically switching optical and electrical interactive devices. In this work we have studied the film thickness dependence of the optical and electrical properties of sputter deposited VO2 thin films. The size dependent electrical behavior of these films is examined in the light of the existing theories. Applying Drude model which combines the optical and electrical behaviour, all the relevant parameters (plasma frequency and free electron density) are calculated for the semi-conducting and metallic phases of the VO2 thin films. All the obtained parameters and the strong thermochromic switching behaviour indicate that the films are predominantly made up of VO2. Importance of distinction between optical hysteresis derived transition temperature and the one derived from resistance hysteresis is emphasized. This comprehensive and in-depth study is expected to lead to a better understanding and design of switching devices based on VO2 thin films.