Effect of film thickness on the energy band gap of nanocrystalline CdS thin films analyzed by spectroscopic ellipsometry

Nanocrystalline cadmium sulfide thin films were deposited by the radio frequency magnetron sputtering technique on Si and glass substrates with different particle sizes and thicknesses by varying deposition time. X-ray diffraction and transmission electron microscopic studies confirmed the nanocrystalline cubic CdS phase formation. TEM micrographs of the films revealed the manifestation of nano-CdS phase with average particle size of 3.6–7.3 nm for different films. These nanocrystalline films were analyzed by spectroscopic ellipsometry. The optical constants and the fitting constants were obtained by fitting the ellipsometric parameter del and psi data using Forouhi and Bloomer models in the wavelength range 248–825 nm. The optical band gaps of the films were also measured by spectrophotometric studies. It was found that the energy band gap decreased from 3.28 to 2.54 eV when the film thickness increased from 153 to 205 nm.