Influence of chlorine doping on the optical & electrical properties of CdS thin films

A mixture of CdS and CdCl₂ powders with different CdCl₂ weight percentage was used to prepare chlorine doped CdS thin films onto glass substrates using HVE (High Vacuum Evaporation) technique with 1000 A¿ thickness. The influence of chlorine doping on the optical parameters of CdS:Cl films was studied using spectrophotometric measurements of the film transmittance in the wavelength range 200-1100 nm. All the chlorine doped films have sharp fall at the band edge, which is an indication of good crystallinity of the films, and exhibit high optical transmittance (70-90%) in the range of wavelengths above the absorption edge. It was found that as the CdCl₂ concentration increased the optical energy gap also increased while the refractive index decreased up to 0.10 concentration then stabilized for 0.15 and 0.20 concentrations. It is observed that as the CdCl₂ concentration is increased, the resistivity of the films decreased. The carrier concentration increases with the increase in CdCl₂ concentration which indicates that the chlorine impurity atoms serve as strong donor impurities. A rapid incremental behavior of mobility can be seen with increasing CdCl₂ concentration. This behavior begins to slow down as the doping concentration increases. AFM studies reveal the growth of nano crystalline film structure for pure CdS film as well as the film doped with 0.05 wt% CdCl₂ concentration. These significant results certainly encourage tailoring the CdS films with chlorine doping suitable as window materials required for heterojunction solar cells.