Effect of the Sulfur Concentration on the Optical Band Gap Energy and Urbach Tail of Spray-Deposited ZnS Films

Zinc sulfide (ZnS) films were deposited through a simple and low cost spray pyrolytic technique using mixed aqueous solutions of zinc nitrate and thiourea. The structural and optical properties of these films were investigated as a function of initial (Zn:S) molar ratio in the precursor solution, which varied between (1:1) and (1:3). X-ray diffraction (XRD) analysis revealed that wurtzite zinc oxide (ZnO) and cubic ZnS phases formed in the film prepared by the equal molar ratio of zinc to sulfur ions and with increasing sulfur content in the precursor solution, only single cubic ZnS phase was appeared. The transmittance spectra measured by UV-Vis spectrophotometer indicated that with the increment of the sulfur content, the transmittance of the films increased in the visible and near infrared regions about 50% and the absorption edges shifted to shorter wavelengths. As a result, the band gap energy (Eg) increased from 3.43 to 3.72 eV and the band tail width (Eu) decreased from 553 to 259 meV, which is due to the phase composition and the decrement of structural defects. By extracting a linear relevance between the band gap energy and width of the band tail of ZnS, the optical band gap at Eu= 0 was estimated to be 3.977 eV.