Abstract
The ZnO thin films have been prepared by sol-gel method at different annealing temperatures and aging time. The structural properties of ZnO thin films were investigated by X-ray diffraction (XRD) patterns and atomic force microscope (AFM) images. The results indicated that the film possess a hexagonal wurtzite structure with preferred orientation along the (002) direction. The crystalline quality of films improved with increasing post-annealing temperature, while gradually worsened with prolonging aging time. The optical properties of ZnO thin films were studied by the ultra-violet transmittance (UV-Vis) and photoluminescence (PL) spectra. The variations of UV-Vis transmittance and energy gap accorded well with the tendency presented in XRD patterns and AFM images. The PL spectra appeared the apparent ultraviolet emission and visible emissions. As the annealing temperature increased, the ultraviolet and visible emissions of films enhanced and decreased respectively. However, as the sol aging time prolonged, the PL emission presented an opposite tendency.
Highlights
Zinc Oxide as a new type of semiconductor material with wide band gap (3.37 eV) at room temperature has excellent electrical and optical properties in visible and ultraviolet region.[1]
After annealing 400 °C, comparing with these samples at same annealing temperature, both the grain size and X-ray diffraction (XRD) peaks intensity decrease with prolonging sol aging time, suggesting the decrease of crystalline quality of thin films, which is ascribed to following reasons (FIG. 3). (i) As everyone knows, ZnO crystal growth is accompanied with the grain reorganization and adjustment of internal structure
The higher annealing temperature led to better crystalline quality of thin films, the preferred orientation at (0 0 2) plane in XRD patterns were improved, the calculated grain size was larger, and the surface in atomic force microscope (AFM) images was smoother with compacter particles distribution
Summary
Zinc Oxide as a new type of semiconductor material with wide band gap (3.37 eV) at room temperature has excellent electrical and optical properties in visible and ultraviolet region.[1]. The main preparation methods of Zinc Oxide thin films are magnetron sputtering (MS), metal organic chemical vapor deposition (MOCVD), pulsed laser method (PLD), sol-gel method (Sol-Gel) and so on.[4,5,6,7] The sol-gel method has gradually become the main method for the preparation of ZnO thin films in the laboratory, because of its simple process, convenient doping, easy to form large area film and no need of vacuum equipment.[8] Both aging time and annealing can affect the quality of ZnO thin films prepared by sol-gel method but which was often studied with two separate aspects.[9,10] there are few studies about the combined effects of aging time and annealing temperature on the structural and optical properties of films. In this paper the work was to systematically discuss the combined effects of aging time and annealing temperature on the structural and optical properties of the films with physical mechanism analysis
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