Chemical bath deposition of ZnS thin films and modification by air annealing

We demonstrate the deposition of ZnS thin films with a thickness of 0.04 to 0.45 mu m at temperatures ranging from 25 to 80 degrees C from chemical baths comprising zinc sulphate, triethanolamine and thioacetamide at pH of about 10. The as-deposited films do not show crystallinity, are very resistive (conductivity 10(-9) Ohm(-1) cm(-1)) and possess no photosensitivity. Annealing of the films in air at 450 to 500 degrees C for 1-2 h leads to partial conversion of the ZnS films to ZnO films. This is accompanied by an increase in the photoconductivity by more than six orders of magnitude. The optical bandgap is > 3.85 eV in the as-prepared films; after annealing in air the value drops to about 3.7 eV.