Effect of pH on the characteristics of nanocrystalline ZnS thin films prepared by CBD method in acidic medium

The structural, morphological, and optical properties of zinc sulfide (ZnS) thin films deposited using chemical bath deposition method are reported. These films were deposited on soda lime glass substrates using Na₂EDTA as a complexing agent, in acidic medium at 80 °C. The effect of the pH ranging from 5 to 6.5 on quality of ZnS thin films is investigated. Irrespective of the pH values used for the deposition all films are polycrystalline with a wurtzite (hexagonal) structure. Field emission scanning electron microscopy (FE-SEM) characterization shows that the surface of the sample deposited with pH 5 and 5.5 is compact and uniform. However, agglomerated clusters (∼100 nm) are observed in ZnS thin films grown with pH above 6. X-ray photoelectron spectroscopy results shows the presence of only Zn-S bonding for the ZnS thin film grown at pH = 5 while those films deposited with pH > 5.5, Zn-OH as well as Zn-S bonding have been observed. The composition is almost stoichiometric for a wide range of pH values of the solution (5-6.5). The transmission spectrum of ZnS thin films having thickness from 30 to 220 nm indicates a good transmission characteristic with an average transmittance of 70-85%. The absorption coefficient depends on the pH values of the solution used and are responsible to the changes observed in absorption edges. The band gap energy values of ZnS thin films decreased from 3.91 to 3.78 eV with increasing pH values of the solution.