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 degrees 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 degrees similar to 34.53 degrees as the annealing temperature went from 600 degrees C to 750 degrees C. The stress decreased with increasing sintering temperature up to 700 degrees C and then increased until 750 degrees C. The FWHM for the (002) peak decreased to a minimum at 700 degrees C and then increased until 750 degrees C. Grain sizes were investigated by using scanning electron microscopy (SEM). The maximum grain sizes was obtained in the ZnO film sintered at 700 degrees C; its grains size was about 112 nm in diameter. However, green emissions were not found in ZnO films sintered at temperatures between 600 similar to 750 degrees C. Therefore, additional experiments were carried out in order to find green emissions. Films were annealed at 800, 900, 1000, and 1100 degrees C. The intensities of the peaks in the ZnO film sintered at 1100 degrees C increased rapidly, and a green emission at about 519 nm was found. At temperatures above 900 degrees 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.