Effect of structure and size on the electrical properties of nanocrystalline WO3 films.

Nanocrystalline WO3 films were grown by reactive magnetron sputter-deposition by varying the substrate temperature in the range of 303(RT)-673 K. The structure and electrical transport properties of WO3 films were evaluated using X-ray diffraction and dc electrical conductivity measurements. The effect of ultramicrostructure and grain-size was significant on the electrical properties of WO3 films. DC conductivity variation of the WO3 films measured in the temperature range of 120-300 K reveals their semiconducting nature. The temperature dependent electrical conductivity curves exhibit two distinct regions indicative of two different types of electrical transport mechanisms. Analysis of the conductivity indicates that the small polaron and variable-range-hopping mechanisms are operative in 180-300 K and 120-180 K temperature regions, respectively. The density of localized states at the Fermi level, N(EF), has been calculated and it was found to be ∼1×10(19) eV(-1) cm(-3) for all the films.