Characteristics of Al-doped ZnO thin films obtained by ultrasonic spray pyrolysis: effects of Al doping and an annealing treatment

Transparent conducting Al-doped ZnO thin films were prepared on silica glass substrates by an ultrasonic spray pyrolysis method. The effects of Al doping and an annealing treatment on electrical and optical properties of ZnO thin films were investigated. Zinc acetate dihydrate, 2-methoxyethanol and aluminum chloride were used as a starting material, a solvent and a dopant source, respectively. The electrical conductivity of ZnO films was improved by Al doping and by annealing in a reducing atmosphere. The minimum electrical resistivity was obtained in the 3 at.% Al-doped film annealed at 500 °C in nitrogen with 5% hydrogen and its value was 1.71×10−2 Ω cm. The average optical transmittance of all films, regardless of a doping concentration and an annealing condition, was higher than 80% in the visible range. The optical direct band gap of films was dependent on the amount of a dopant and the annealing treatment in a reducing atmosphere. The optical direct band gap value of 3 at.% Al-doped films annealed at 500 °C in nitrogen were 3.33 eV.