Electric conduction mechanisms and metal semiconductor transition in undoped amorphous ZnO thin films prepared by spray pyrolysis technique

In this work, the structural, electrical and optical properties of undoped ZnO amorphous thin films using Spray Pyrolysis deposition technique are presented. The structural measurements (XRD) show that for a film of thickness 20 nm, the material is in the amorphous phase. Therefore, this work is focused on the film of this thickness (20 nm). From the optical measurements, a transmission of 85 % as well as an energy-gap of 3.15 eV have been obtained. The electric conductivity and the Seebeck coefficient show a semiconductor-metal transition at around 305 K. Up to our knowledge, this transition is observed for the first time in amorphous ZnO thin films. It might be due to the oxygen adsorption above room temperature. In addition, three regimes are identified for the conduction mechanisms from the variation of both the electric conductivity and the Seebeck coefficient as a function of temperature. The standard transport model is used in order to interpret the behavior of the conductivity and the Seebeck coefficient. At low temperatures (T < 255 K), the conduction mechanism is dominated by the hopping between localized states. At temperatures 255 < T < 275 K, a brisk change in the conductivity and the Seebeck coefficient curves is observed; it might correspond to a transition zone of certain localized states close to the conduction band. At the end, for temperatures 275 < T < 305 K, the conduction mechanism is dominated by the non localized states.