Direct Current Cylindrical Magnetron Sputtering Setup was used to deposit ZnO thin films on BK7 substrates. The effects of changing O 2 /Ar reactive gas mixtures on the structural and optical properties of films were studied. Crystallinity and structure of films were obtained by X-ray diffraction (XRD). Preferential crystalline growth orientation of ZnO films detected by XRD was always along the (002) orientation. The thickness of films was measured by surface profilometer which showed thickness increasing from 68.7 to 80.8 nm for 3-6% O 2 amount respectively. The morphology and roughness of the films were investigated by Atomic Force Microscopy (AFM). As oxygen gas amount was increased, the roughness and the grain size were decreased and the deposition rate was increased. The optical transmittance of ZnO films are obviously affected by the changing of O 2 /Ar reactive gas mixtures. All films exhibit a transmittance higher than 70% in the visible region. The optical band gap of films was measured by Tauc's method. The results show that by increasing the amount of O 2 in reactive gas mixture, the optical band gap of deposited films increases.

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