Structural And Optical Properties Of PbS And ZnS Mixed Thin Films

ZnS and PbS are widely used in various optoelectronic devices, especially the application in solar cells. So there are a lot of research reports about the preparation and application of ZnS and PbS. ZnS is a broad band gap semiconductor material, while PbS is a narrow band gap material. However, if these two kinds of materials are mixed, the optical properties of the mixed material will much differ from those of ZnS or PbS single material completely. Moreover, there are only few reports about the preparation of the ZnS and PbS mixed film. To this aim, this dissertation will be focused on the relationship between structure and optical properties of the ZnS and PbS mixed films under the different deposition conditions. The main content is arranged as follows:Firstly, ZnS and PbS mixed films deposited on quartz substrates by electron beam evaporation. The samples were subsequently annealed in a3×10-3Pa vacuum at temperatures ranging from100℃and600℃for1h. The experimental results show that when the annealing temperature is higher than500℃, PbS in the mixed films had been vaporized and PbS content in mixed films is very small. The as-deposited films are amorphous. The samples change into polycrystalline structure after annealing at300℃, and the crystalline performance enhances with the increase of the annealing temperature. When the annealing temperature increases from100℃to600℃, both the surface roughness and surface grain size of the mixed films decrease first, and then increase. Correspondingly, the transmittance of films increases first, then decreases, and increases last. The variation in transmittance of the annealed films is because of the changes of the film composition, crystalline performance, and surface morphology.Secondly, Zn-Pb-S mixied films with different composition were deposited on quartz substrates by electron beam evaporation. The weight ratio of PbS/(ZnS+PbS) in target changed from0%to60%. The experimental results show that when the Pb atomic percentage of the mixed film are greater than10%, and S atomic percentage of the sample are less than40%, which indicates that metallic state of Pb and Zn are in the films. When Pb atomic percentage of the sample is more than3%, the surface roughness and surface grain diameter increase with the content of Pb in the thin film. As the Pb content increases, maximum transmission rate decreases, and the extinction coefficient increases.The refractive index of the sample decreases with the increase of the content of Pb when Pb content is less than10%. As Pb content increases from0%to37%, the optical band gap of the samples decreases from3.31eV to1.87eV.Finally, five PbS films with thickness of220nm were deposited on quartz substrates by chemical bath deposition method. The samples were subsequently annealed in a10"3Pa vacuum at different temperatures for1h. The results show that the crystalline performance of the as-deposited film is poor. The crystalline performance of the films enhances with the increase of annealing temperature. When the annealing temperature increases, the surface roughness and surface particle size first decrease, then increase, and decrease finally. The maximum transmittance decreases, both the refractive index and extinction coefficient increase with increasing annealing temperature. As the annealing temperature increases from225℃to300℃, the optical band gap decreases from2.1eV to1.6eV.