Kinetic of growth of chemically deposited ZnS films near room temperature conditions

The growth mechanism of ZnS films deposited by chemical bath at temperatures ranging between 25 °C and 55 °C was investigated by a kinetic analysis. For the ZnS thin films deposition, zinc chloride, potassium hydroxide, ammonium nitrate, and thiourea were used as chemical reagents. The effects of the bath temperature, thiourea concentration and deposition time on the ZnS formation were investigated. The species distribution diagrams suggested that the growth mechanism involves bisulfide ions (HS−) for the sulfur species as well as zinc hydroxide complexes ( Zn ( OH ) 4 2 − and Zn ( OH ) 3 − ). Higher thickness and growth rates were observed for increased bath temperature. The activation energy of the system was estimated as 127 5 kJ/mole and the order of reaction with a value of 1.03 0.05. These results suggest that the growing mechanism of the ZnS films is a chemical process with formation and absorption of Zn(OH)2 in the substrate followed by an exchange reaction between OH− and HS− ions. Chemical, morphological and optical characterizations of the deposited ZnS thin films are reported. The formation of the ZnS and Zn(OH)2 compounds was identified by x-ray photoelectron spectroscopy technique. An increase in the [Zn]/[S] ratio from 0.367 at 25 °C to 0.906 at 55 °C, was obtained. The estimated bandgap energy of the deposited films ranged from 3.5 to 3.8 eV. Optical transmittance higher than 80% above 500 nm on films makes proper as optical window for thin film solar cells.