Effects of W doping on the metal–insulator transition in vanadium dioxide film

V1-xWxO2V1-xWxO2 thin films with various W concentrations were successfully deposited on sapphire by the sol–gel method to investigate the effects of W doping on the transition properties. All the films are grown to be (0 4 0)-preferred orientation and have well-formed grains. The transition temperature (TcTc) is significantly reduced with increasing W concentration and the rate of TcTc reduction reaches to 13.8 K per 1 at% W. The resistivity in the insulator state clearly decreases with doping amount due to the enhancement of the charge carriers. The doped-films have well-formed impurity level with an activation energy of 0.08 eV from the bottom of the conduction band. The excited charge carriers to the conduction band on the verge of the transition should play a role in the insulator-to-metal transition for the V1-xWxO2V1-xWxO2 film. The generated free carriers screen the Coulomb repulsion of the electrons and lead to the metallic state.