Polaron variable range hopping in TiO2-δ(-0.04⩽δ⩽0.2) thin films

The mechanisms of electrical conduction in TiO2-δTiO2-δ(-0.04⩽δ⩽0.2)(-0.04⩽δ⩽0.2) thin films have been investigated. The samples were rf-diode reactive sputtered films deposited in O2+ArO2+Ar gas atmospheres where changes in δδ and film structure had been achieved by varying the O2O2 flow rate and the substrate temperature. The electrical transport properties of these samples were investigated by measuring the conductivity as a function of temperature between 17 K and room temperature. At the temperature range between 200 and 290 K the best fit to the experimental data was obtained assuming a dependence characteristic of adiabatic variable range hopping. At lower temperature the activation energy for the conductivity tends to zero. The results suggest that the conduction mechanism is adiabatic small polaron hopping, which switches to conduction in a polaron band at low temperatures.