The thickness effect on the electrical conduction mechanism in titanium oxide thin films

In this paper we made a study on the effect of films thickness on the electrical conduction properties of nanostructured TiO2 thin films deposited by d.c. reactive sputtering. The deposition was performed on heated (573 K) glass substrates, using water vapor as reactive gas. The electrical conductivity of the films was investigated in the temperature range 13–320 K. The temperature dependence of electrical conductivity between 80 and 320 K indicated that electrical conductivity in the films was controlled by potential barriers caused by depletion of carriers at grain boundaries in the material. Values of grain barrier heights and surface trap density at the grain boundaries were extracted from the high temperature data for the investigated samples. Both the grain barrier heights and surface trap density at the grain boundaries were found to depend significantly on film thickness into the deposition process. The low-temperature (T < 80 K) conductivity of the films was described in terms of variable-range hopping (VRH) conduction. Characteristic parameters describing the low-temperature conductivity, such as the hopping distance, hopping energy and density of states were determined, and their values as a function of film thickness were discussed in the light of variable-range hopping conduction model.