Photochemical deposition of ZnS thin films by intermittent illumination

We prepared ZnS thin films by photochemical deposition (PCD) using continuous and intermittent illumination. The aqueous solution used for the deposition contains ZnSO4 and Na2S2O3, and H2SO4 for pH adjustment. The solution is illuminated with light from an ultrahigh-pressure mercury are lamp. When the illumination is continuous, the deposited film is black and not transparent because of excess zinc in the film. When the illumination is turned on and off periodically by a mechanical shutter, the deposited film becomes gradually transparent with increasing "off" time. The film deposited by the intermittent illumination has an optical absorption edge near 350 nm and shows photoconductivity.