Thermochromic VO2 thin films studied by photoelectron spectroscopy

Thermochromic vanadium dioxide films which are discussed as ''intelligent'' window coatings were deposited by reactive rf-sputtering. The films are grown on quartz polycrystalline and unidirectional. Photoelectron spectroscopy (XPS, UPS) was applied to study the density of states distribution in the valence band regime below and above the phase transition temperature. It is shown that the band structure of the polycrystalline films bears resemblance to single-crystalline vanadium dioxide to a large extent, e.g, bandgap, valence band energy position, and valence band structure and width in the semiconducting phase. In contrast to single crystals, e.g. the metal-semiconductor transition in the films occurs in a broad temperature range of about 20 to 30 K, dependent mainly on deposition temperature. Correlations between optical, electronical, and structural properties of the film are evident.