Surface segregation of Ag and its effect on the microstructure, optical properties and conduction type of ZnO thin films

Abstract Ag doped ZnO thin films have potential applications in many electronic and optoelectronic devices. Effective control of the chemical state of Ag is important for adjusting the electrical and optical properties of Ag-doped ZnO materials. So far, the influence of annealing temperature on the physical properties of Ag-doped ZnO thin films prepared by sol-gel method has rarely been reported. In this work, Ag-doped ZnO thin films were prepared by spin-coating technique and the evolution of microstructure, optical properties and conduction type with the rise of annealing temperature were investigated. The experimental results show that with the increase of annealing temperature, the crystalline quality of Ag-doped ZnO thin film is gradually improved and the grains gradually grow up. The ultraviolet emission has been also enhanced step by step. When the annealing temperature exceeds 450 °C, the surface segregation of Ag is directly observed by a field emission scanning electron microscope. In addition, the diffraction peak of metallic Ag is also perceived on the XRD patterns. The X-ray photoelectron spectroscopy shows that the chemical valence of Ag has been changed from +1 to 0. The surface segregation of Ag leads to the conduction type of the ZnO thin film to change from P type to N type. The surface segregation of Ag is caused by the high annealing temperature combined with a rapid cooling treatment.