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 ^°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 (∼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.