A correlation between microstructure, composition and optical transparency of CVD-ZnS

The effects of deposition temperature, type of substrate and hot isostatic pressing (HIP) treatment on the optical transparency and dielectric constant of CVD-ZnS were studied. It was found that the increase in deposition temperature improves the optical transparency and decreases interface polarization. This behavior was correlated with the reduction in the density of pores and content of trapped gases in ZnS. The change in type of substrate from graphite to tantalum has negligible effect on the optical transparency and dielectric constant of CVD-ZnS, since the formation of pores is dominated by the kinetics of gas molecules' arrival to the substrates and not by the thermodynamic properties of the substrates. The density of pores decreases with increasing vertical distance from the substrate until a steady state is reached due to a decrease of the effect of substrate on the thermal conductivity and heat capacity of the growing film. The HIP treatment improves the optical transparency to about 72%, which is higher than the commercially required value for an optical window. In addition, the dielectric constant decreases and becomes independent of frequency. This behavior after hipping is well correlated with the closure of pores and cracks in ZnS.