Preparation and characterization of ultrasonically sprayed zinc oxide thin films doped with lithium

Zinc oxide thin films doped with Li were deposited by ultrasonic spray pyrolysis (USP) at 350 ± 5°C on glass substrates from solutions of zinc acetate [Zn(CH COO) • 2H O] and lithium acetate [C H LiO • 2H O], in which the Li/Li + Zn ratios were 1 at.%, 3 at.%, and 5 at.%. The effects of the doping on the structural, optical, electrical, and morphological properties of the films were examined. X-ray diffraction patterns indicated that the undoped and Li-doped ZnO films had a polycrystalline hexagonal wurtzite structure with a (002) preferred orientation. The films showed optical transmission around 60-80% in the visible region of the spectrum. The films were found to be transparent in the wavelength range of 450-900 nm, with sharp ultraviolet absorption edges in the wavelength range of 350-450 nm. The absorption edge analysis revealed that the optical band gap energies for the films were between 3.24 eV and 3.29 eV, and the electronic transition was of the direct transition type. The width of the band tail states, which is connected to the localized states in the band gap, was estimated to be 82-113 meV by Urbach tail analysis. For study of the electrical properties of the films, Hall effect measurements, electrical conductivities, conductivity activation, and trap energies were investigated. The electrical measurements of the films were obtained in the dark, in vacuum, and in the temperature range of 10-300 K. Morphological studies for the films were carried out by scanning electron microscopy. © 2009 TMS.