Chemical states of gold doped in ZnO films and its effect on electrical and optical properties

Gold-doped ZnO (ZnO:Au) films were grown on quartz substrates at room temperature by radio frequency magnetron sputtering technique and then annealed in the temperature ranging from 350 to 800 °C in argon ambience. It is found that the Au substitute for Zn in positive univalence (AuZn1+) in the as-grown ZnO:Au film. When the ZnO:Au is annealed above 600 °C, some of the AuZn1+ begin to precipitate from the ZnO:Au and become metal Au existing in the grain boundaries of the ZnO:Au, implying that the solid solubility of Au in ZnO decreases with increasing temperature. p-type conductivity is obtained in the ZnO:Au annealed at 350 °C, but the conductivity changes gradually from p-type to n-type with increasing annealing temperature. The optical band-gap of the ZnO:Au shows red-shift compared to undoped ZnO. The ZnO:Au shows room temperature photoluminescence (PL) quenching as annealed below 800 °C, but has two weak PL peaks, located at 2.417 and 3.299 eV, respectively, as annealed at 800 °C. The electrical and optical properties of the ZnO:Au are affected by the chemical states and content of the Au doped, and the mechanism of the effects are suggested in the present paper.