Charge transport between localized states in lithium-intercalated amorphous tungsten oxide

We report on electron transport in amorphous tungsten oxide films, prepared by dc magnetron sputtering and subsequently electrochemically intercalated with lithium. The ion–electron intercalation process allows us to change the position of the Fermi level in a controlled way and to determine the density-of-states in the conduction band. We present a new method to determine the localization length of the electron states in disordered materials. The method is based on measurements of the electronic density-of-states together with electrical resistance in the variable range hopping regime. We find that the electronic states of amorphous tungsten oxide are localized up to about 1.3 eV into the conduction band, where an insulator-metal transition occurs. The localization length was determined on the insulating side of the transition and the estimated scaling exponent is consistent with the scaling theory of localization.