Comprehensive optical studies on Ga-doped ZnS thin films synthesized by chemical bath deposition

ZnS thin films were prepared by chemical bath deposition on glass substrates using different concentrations of gallium (y=[Ga3+]/[Zn2+]) varying from 0 to 10 at% by a step of 2 at%. Samples were characterized using X-ray diffraction, spectrophotometric measurements and photoluminescence spectroscopy. The structural studies revealed that the ZnS thin films were poorly crystalline. The level of the residual strain and dislocation seemed to be reduced, respectively, to the values 0.18% and 2.62×109 lines.mm−2 for y=6 at%. Optical analysis by means of transmittance T(λ) and reflectance R(λ) measurements allowed us to determine the band gap energy value that varied in the range [3.60-3.76] eV, which promoted the use of this material as an optical window or buffer layer in solar cell devices. The refractive index dispersion was adequately described by the Wemple-DiDomenico model. The values of oscillator energy E0, dispersion energy Ed, high-frequency dielectric constant ε∞ as well as the ratio of the carrier concentration to the effective mass N/m* were estimated according to both Wemple-DiDomenico and Spitzer-Fan models. Photoluminescence spectroscopy was performed on ZnS:Ga thin films and the analysis revealed the quenching effect.