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Effect of annealing on ZnO thin films prepared by using the sol-gel method

ZnO thin films were spin-coated on Pt substrats (Pt/Ti/SiO2/Si) by using a sol-gel process and were characterized as a function of sintering temperature. The films were annealed at 600, 650, 700, and 750 °C to find the optimum annealing temperature in air. The crystalline structure, stress, and c-axis lattice constant were investigated by using X-ray diffraction (XRD). Strong (002) peaks were found in the XRD spectrum of the ZnO films, and the position of the peak changed from 34.48° ∼ 34.53° as the annealing temperature went from 600 °C to 750 °C. The stress decreased with increasing sintering temperature up to 700 °C and then increased until 750 °C. The FWHM for the (002) peak decreased to a minimum at 700 °C and then increased until 750 °C. Grain sizes were investigated by using scanning electron microscopy (SEM). The maximum grain sizes was obtained in the ZnO film sintered at 700 °C; its grains size was about 112 nm in diameter. However, green emissions were not found in ZnO films sintered at temperatures between 600 ∼ 750 °C. Therefore, additional experiments were carried out in order to find green emissions. Films were annealed at 800, 900, 1000, and 1100 °C. The intensities of the peaks in the ZnO film sintered at 1100 °C increased rapidly, and a green emission at about 519 nm was found. At temperatures above 900 °C, the X-ray diffraction peaks for Zn2SiO4 willemite were found. As the annealing temperature was increased, the relative amounts of silicon atoms and the ratio of oxygen to zinc (O/Zn) increased. This indicates that the origin of the green emissions and the Zn2SiO4 is related to the amounts of oxygen and silicon present.