Investigation of annealing-treatment on the optical and electrical properties of sol-gel-derived zinc oxide thin films

Highly preferential c-axis orientation ZnO thin films on Si(100) and quartz substrates have been achieved by the sol-gel method. Structural investigation including surface morphology and microstructure was carried out by XRD, SEM and AFM measurements. Also, optical properties were determined by photoluminescence, ellipsometry and UV-VIS spectrum analyses. XRD results indicated that an extremely sharp (002) peak will dominate under optimum annealing-treatment condition. Moreover, thin film quality and the morphology were improved by annealing treatment. The SEM images show that the grain sizes increased with increasing annealing temperature up to 750 degrees C, where the particle size was about 50 nm. Photoluminescence spectra revealed two main peaks centered at about 380 nm and 520 nm, corresponding to the band-edge and defect-related emission. The variation in UV emission intensity was attributed to the competition between the excitonic and nonradiative recombination. It was proposed that annealing temperature plays a key role in the formation of defects, which is strongly related to the nonradiative recombination centers. In addition, optical transmittance spectra demonstrated that these films are very transparent (similar to 90%) in the range of 380-800 nm wavelength, and optical band-gap was determined accordingly. The impact of the thermal treatment on the structural and optical properties was discussed in detail. (C) 2005 Elsevier Ltd. All rights reserved.