Optical and surface properties of spray-pyrolysed CdS thin films

Abstract

Variations in the real and imaginary parts of the refractive index of CdS films with preparation temperature were studied using variable-angle spectroscopic ellipsometry (VASE). The samples studied were prepared by the spray pyrolysis technique at various substrate temperatures in the range 200–360 °C. The VASE measurements were taken in the wavelength range 530–600 nm. Thicknesses of the samples were in the range 500–600 nm. Bruggeman's effective medium theory was used for surface roughness analysis. In the present study it was observed that the thickness of the rough layer decreases with increasing substrate temperature and reaches a minimum (~27 nm) in the range 280–300 °C. Thereafter it slowly increases. Scanning electron microscopy shows that the film prepared at 300 °C has a smooth texture. X-Ray diffraction patterns also indicate that samples prepared in this temperature range have better crystalline quality. The packing density of the films prepared at various temperatures also confirms this. The deposition rate of CdS thin films prepared by spray pyrolysis is also measured. The real part (n) of the refractive index of the CdS film increases with increasing substrate temperature. The n value is slightly less than that of the bulk sample. Similarly the imaginary part (k) of the refractive index also increases with increasing substrate temperature. The k value of the film is higher than that of the bulk medium. Resistivity measurements on the films show that the resistivity decreases with increasing substrate temperature.