A correlation between microstructure, composition and optical transparencyof CVD-ZnS

The effects of deposition temperature, type of substrate and hot isostatic pressing (HIP) treatment on the optical transparency and dielectric constan t of CVD-ZnS were studied. It was found that the increase in deposition tem perature improves the optical transparency and decreases interface polariza tion. This behavior was correlated with the reduction in the density of por es and content of trapped gases in ZnS. The change in type of substrate fro m graphite to tantalum has negligible effect on the optical transparency an d 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 cond uctivity and heat capacity of the growing film. The HIP treatment improves the optical transparency to about 72%, which is higher than the commerciall y required value for an optical window. In addition, the dielectric constan t decreases and becomes independent of frequency. This behavior after hippi ng is well correlated with the closure of pores and cracks in ZnS. (C) 2001 Published by Elsevier Science B.V.