Characterization of ZnO and ZnO:Al thin films deposited by the sol-gel dip-coating technique

Nanocrystalline zinc oxide films have been obtained by the sol-gel process. The films were deposited from precursor solutions by dip-coating on quartz substrates, and subsequently transformed into nanocrystalline pure or aluminium-doped ZnO films after a thermal treatment. The film microstructure and composition characterization Was Studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The optical properties were studied by transmittance spectroscopy. The water adsorption energy was measured by temperature programmed desorption (TPD) in the range 90-700 K. The optical transmittance in the UV region gives bandgap energy values of 3.27 eV for undoped samples, and higher than 3.30 eV for the Al-doped ones. The increase in bandgap energy in Al-doped samples may be explained by band-filling effects. The band edge absorption coefficient increases monotonically for the Al-doped samples but has a shoulder for the undoped ones, which may be assigned to room-temperature excitonic absorption. (C) 2008 Elsevier B.V. All rights reserved.