Fabrication of highly oriented (002) ZnO film on glass by sol–gel method

Abstract

In this study high quality (002) ZnO films were deposited on glass substrate by a sol–gel spin coating process. The as-coated films were post-annealed at different temperatures in air to investigate the effect of annealing temperature in particular. The chemical composition of the precursor sol and the intermediates produced in the films heating process were analyzed by thermo gravimetric analysis/differential thermal analysis (TGA/DTA). The microstructure and its optical properties of ZnO films were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), ultraviolet–visible spectroscopy (UV–Vis) and photoluminescence. TGA/DTA showed that a significant weight loss occurred at around 200–300 °C and the weight stabilized at 300 °C. An extremely sharp (002) diffracted peak in XRD patterns indicated the high preference in crystallinity of these films. FESEM micrographs revealed that the films were filled with particulates with size ranging from 10 to 25 nm as post annealing temperature increased from 400 to 500 °C and turned into porous films at 600 °C. UV–Vis has shown that the films were highly transparent under visible light and had a sharp absorption edge in the ultraviolet region at 380 nm. The measured optical band gap values of the ZnO thin films were around 3.24–3.26 eV. Photoluminescence spectra revealed a strong UV emission centered at about 390 nm corresponding to the near-band-edge emission with a weak defect-related emission at about 520 nm. The intensity of UV emission increased with the annealing temperature. This may be attributed to a higher quality ZnO film while annealed at higher temperature.