Physical properties of Cd-doped ZnS thin films

In this research, un- and Cd-doped ZnS nanostructures were synthesized using an electro-deposition method. The electrolyte contained 20 mM ZnCl2, 20 mM Na2S2O3 and the various amounts of CdCl2 solution. The X-ray diffraction (XRD) patterns of the as-deposited samples exhibited that Cd doping leads to decrease the crystallite size (D) of ZnS meanwhile the lattice strain, lattice stress, dislocation density (7), and stacking fault energy (SF) were increased. Field emission scanning electron microscopy (FESEM) images depicted very fine equiaxed grains (20-50 nm), in addition to big grains (200-400 nm) with the preferential orientation. The reflectance spectra of ZnS thin films indicated a decrease in the reflectance peaks and refractive index (n), after Cd doping. Based on the transmission spectra, the un- and Cd-doped ZnS samples showed an absorption edge between 328 and 357 nm. The band gap energy for undoped ZnS sample was estimated as 3.95 eV, which it was reduced to 3.63 eV for the Cd-doped ZnS samples owing to the presence of imperfections and crystalline disorder in the Cd-doped ZnS samples. Furthermore, the obtained dielectric constant of the Cd-doped ZnS thin films was smaller than undoped ZnS sample, which means that the Cd-doped ZnS samples were more appropriate than the undoped ZnS sample for application in the fast photodetectors. The photoluminescence (PL) spectra of the undoped ZnS thin film was presented six emission peaks at 340, 413, 536, 574, 748, and 878 nm as an outcome of near band emission (NBE) and crystalline defects such as zinc interstitials (I-zn), zinc vacancies (V-zn), sulfur interstitials (I-s), ZnS self-defect, and the defect states between the valence band (VB) and the conduction band (CB) of ZnS. Moreover, the PL emission of the Cd-doped ZnS samples demonstrated a red-shift relative to the undoped sample, which verified the UV-vis spectroscopy results.