Investigating the Optical Properties of Zinc Sulphide Thin Films Deposited from Chemical Acidic Baths

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A well adherent thin film of zinc sulphide (ZnS) has been deposited on silica glass substrates from acidic baths containing zinc chloride, thioacetamide and urea. Films of different thickness (684nm, 572nm, 523nm, and 271nm) were obtained. SEM micrograph of the as-deposited ZnS thin film, show the film to be uniform, dense, homogenous, and composed of largely irregular shaped grains of diameter in the range of 300 – 500 nm. These large grains are comprised of smaller spherical grains of 100 – 200 nm diameter. EDAX results are consistent with the formation of thin films of ZnS deposited on silica glass substrates. A UV mini Schimazu UV – VIS Spectrophotometer was used to determine the optical absorbance of the films as a function of wavelength at room temperature over the wavelength range 200 – 800 nm. The samples were then thermally annealed for one hour, at temperatures of 300°C, 400°C and 500°C, after which the absorbance of the films were again recorded. The absorbance of the films decreased with increasing annealing temperatures indicating an increase in transmittance with increasing annealing temperatures. The band gap values obtained varied between 3.64 eV and 3.97 eV, for as deposited films and increased between 3.98 eV and 4.00 eV after annealing. The increase in band gap with annealing temperature might be attributed to the improvement in crystallinity in the films. The high transmittance of the films together with their large band gap makes them good materials for application as a window layer for solar cells.
A thesis submitted to the Department of Physics, Kwame Nkrumah University of Science and Technology, in partial fulfillment of the requirement for the degree of MASTER OF SCIENCE (SOLID STATE PHYSICS) , APRIL, 2012