Non-adiabatic small polaron hopping conduction in Nb-doped TiO2 thin film

Transport properties of Nb-doped titanium oxide (TiO2) thin film, obtained by a RF sputtering technique, have been investigated by means of the electrical conductivity and the Seebeck coefficient as a function of temperature (13-425 K). At high temperatures (T>325 K), temperature dependent behaviors of the electrical conductivity and the Seebeck coefficient confirm that the transport mechanism is the non-adiabatic small polaron hopping type. An excellent agreement between the theoretical and the experimental values of non-adiabatic polaron hopping energy (WH∼0.3 eV) is obtained. The conductivity follows the Mott's variable range hopping conduction (VRH) at the temperature range of 200-325 K, while it exhibits a temperature-independent behavior at low temperatures (T<200 K). Various physical parameters of the present sample such as polaron radius, effective dielectric constant, polaron hopping energy, density of states at the Fermi level, polaron band width and polaron coupling constant are determined using small polaron hopping model.