Modification of surface morphology and lattice order in nanocrystalline ZnO thin films prepared by spin-coating sol–gel method

The surface morphology and structure of zinc oxide thin films play a key role in many applications such as chemical sensors and photocatalysts. In this study, ZnO thin films are prepared on Si/SiO 2 substrates by spin-coating sol–gel technique. New element in the films preparation is the application of drying with hot air flow ( T h = 90–95 °C), as first step of the drying procedure, followed by furnace drying as second step. It is shown that hot air drying has significant influence on the films properties. It reduces the internal stress, strongly affects the films surface morphology, gives assistance to the effusion of organic remains and results in a better crystallinity and lower defect density in the as-prepared films when compared with the films prepared by furnace drying only. Besides, first data on the modification of sol–gel ZnO films by post-deposition irradiation with a nanosecond infrared laser are obtained, giving an alternative to the standard furnace annealing. They indicate that laser irradiation of as-prepared ZnO films gives rise to certain improvement in crystal structure and slight increase in crystallite size, followed by the increase of micro-strain. It also reduces the number of defects playing role of non-radiative recombination centers, as well as the size and density of small cracks and pores on the surface of as-prepared films. Left: X-ray diffraction patterns of as-prepared and annealed at 400 °C ZnO films prepared by furnace drying only, the patterns are taken on non-irradiated and laser-irradiated films. Middle: Experimental and fitted E 2 high Raman mode in the spectra of non-irradiated and laser-irradiated samples prepared by two-step drying, as - the asymmetry fitting parameter. Right: Optical and AFM surface images of annealed ZnO films prepared by one-step drying; the AFM images were obtained at two different scales. Highlights Zinc oxide thin films of densely packed nanograins are prepared by sol–gel method. Hot air drying during films preparation and post-deposition infrared laser irradiation are applied. The hot air drying has strong influence on the internal strain and surface morphology. The laser irradiation reduces the size and density of pores and cracks on the surface. Defect density decreases but internal strain increases upon the laser annealing.