The Effect of Thermal Annealing on the Optical Band Gap of Cd1-xZnxS Thin Films Deposited by the Dip Technique

The effect of thermal annealing on the optical band gap of Cd1-xZnxS (x= 0.2, 0.4, 0.6, 0.8) thin films have been investigated. The films were deposited using the dip technique. The fundamental optical property which has been investigated here is the absorbance of light at room temperature, using the KLB Ultraspec II 4050 (UV/Visible) spectrophotometer over the wavelength range 300 – 900 nm. The energy band gap and absorption coefficient of the films were determined from the absorption spectrum. The optical band gap of the as-deposited films varied from 2.42 eV (x =0.2) to 3.61 eV (x =0.8), that is, the band gap increased with increasing Zn concentration of the alloy. These values compare favorably well with that obtained from literature of similar films prepared using other deposition techniques. The as-deposited samples were thermally annealed in air for an hour at temperatures of 100°C, 200°C and 300°C and the absorption spectra again recorded. It was observed that thermal annealing decreased the band gap of the samples; this may be due to improving crystallinity or alternatively, a phase transformation taking place in the samples as a result of the heat treatment.