Effect of substrates temperature on structural and optical properties of thermally evaporated CdS nanocrystalline thin films

Abstract

Nanocrystalline thin films of CdS were deposited by thermal evaporation technique under vacuum (p=2 × 10^-5 torr) on cleaned glass substrates maintained at different temperatures (300, 473 and 573 K). The effect of substrates temperature on structural and optical properties of CdS nanocrystalline thin films has been studied. X-ray diffraction (XRD), ultraviolet-visible (UV-Vis) and Scanning Electron Microscopy (SEM) were used to characterize the CdS nanocrystalline thin films. The optical and structural studies show that film deposited at 300 K was amorphous in nature and films deposited at higher temperatures were crystalline in nature. Optical constants, such as optical band gap was evaluated from these spectra. The optical band gap was found to be in the range 2.38-~2.51 eV.The study of structural and some physical properties of CdS films indicates that they are strongly dependent on the substrate temperature. Crystallinity levels became better at high substrate temperatures (especially for the film obtained at 573 K). Using the optical investigations including the band gap calculations, it was determined that the band gap of CdS films decreases with substrate temperature, due to the changes in grain size and crystallinity. There is a random effect of substrate temperature on band tail values.