Electrical conduction mechanisms in thermally evaporated tungsten trioxide (WO3) thin films

Thin films of amorphous tungsten trioxide, a-WO3, have been thermally evaporated onto glass substrate held at 350 K. Annealing at 723 K caused the formation of polycrystalline tungsten trioxide, c-WO3, with a monoclinic structure. The dark DC electrical conductivity of both a-WO3 and c-WO3 was studied over a temperature range from 298 to 625 K in two environmental conditions (air and vacuum). A simple Arrhenius law, a polaron model and a variable range hopping model have been used to explain the conduction mechanism for a-WO3 films. Using the variable range hopping model, the density of localized states at the Fermi level, N(E-F), was found to be 1.08 x 10(19) eV(-1) cm(-3). The mechanism of electrical conduction in c-WO3 films is explained by means of the Seto model. The Seto model parameters were determined as the energy barrier (E-b = 0.15 eV), the energy of trapping states with respect to the Fermi level (E-t = 0.9 eV) and the impurity concentration (N-D = 4.05 x 10(15) eV(-1) cm(-3)). The thickness dependence of resistivity of c-WO3 films has been found to decrease markedly with increasing film thickness, which is explained on the basis of the effective mean free path model. Using this model, the mean free path of electrons in c-WO3 films was evaluated. The temperature dependence of the thermoelectric power for a-WO3 films reveals that our samples are n-type semiconductors.