OPTICAL AND STRUCTURAL-PROPERTIES OF NANOCRYSTALLINE ZNS-SIO2 COMPOSITE FILMS

Composite films of ZnS and SiO2 were prepared by co-evaporation from t wo separately controlled sources. Pure ZnS films were found to consist of crystallites with mean diameter between 10 and 15 nm for films thi cker than about 100 nm. The addition of SiO2 to ZnS seems to be very e fficient in reducing crystallinity and the size of the ZnS grains. The annealing of the composite films initiates the formation of nanocryst allites in the films followed by growth of these crystallites. ZnS nan ocrystallites within tl-ie films show a cubic structure with crystalli te sizes depending on concentration and annealing time. The refractive index of the composite films increases continuously with increasing c oncentration 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 descriptio n of the composition dependence of the refractive index the Bruggeman effective medium approximation, the Maxwell-Garnett, model, and a line ar mixing model of the dielectic function epsilon give the best agreem ent with experimental values for higher concentrations. Optical absorp tion in the films was found to be increased and shifted to longer wave lengths compared with the absorption in the as-deposited state after t he first and, to a lower extent, also after the subsequent annealing s teps. A luminescence peak at room temperature at about 410 nm was meas ured. Its intensity was related to tile grain size and depends on the ZnS concentration and the annealing time.