Optical and Structural Properties of Nanocrystalline ZnS-SiO2 Composite Films

Composite films of ZnS and SiO2 were prepared by co-evaporation from two separately controlled sources. Pure ZnS films were found to consist of crystallites with mean diameter between 10 and 15 nm for films thicker than about 100 nm. The addition of SiO2 to ZnS seems to be very efficient in reducing crystallinity and the size of the ZnS grains. The annealing of the composite films initiates the formation of nanocrystallites in the films followed by growth of these crystallites. ZnS nanocrystallites within the films show a cubic structure with crystallite sizes depending on concentration and annealing time. The refractive index of the composite films increases continuously with increasing concentration of ZnS in the films from about 1·48 (the value of a pure SiO2 film) to 2.30 for a pure ZnS film. For the theoretical description of the composition dependence of the refractive index the Bruggeman effective medium approximation, the Maxwell-Garnett model, and a linear mixing model of the dielectic function ε give the best agreement with experimental values for higher concentrations. Optical absorption in the films was found to be increased and shifted to longer wavelengths compared with the absorption in the as-deposited state after the first and, to a lower extent, also after the subsequent annealing steps. A luminescence peak at room temperature at about 410 nm was measured. Its intensity was related to the grain size and depends on the ZnS concentration and the annealing time.