Synthesis and optical characterization of ZnO and ZnO:Al nanocrystalline films obtained by the sol-gel dip-coating process

Abstract

Zinc oxide is obtained by the sol-gel process. The precursor solution was deposited on quartz substrates by dip-coating. The as-deposited films are transformed into nanocrystalline pure or aluminium-doped ZnO after thermal treatment. The microstructure was studied by X-ray diffraction and the nanocrystallite size was estimated using Scherrer equation. The optical transmittance in the visible region is between 83 and 91%. A direct absorption edge is observed with bandgap energies between 3.27 and 3.31 eV for the undoped samples, and between 3.30 and 3.35 eV for the Al doped ZnO. These values correlate well with the nanocrystal sizes following two different laws for the undoped and doped samples, due to Moss-Burstein shift. The visible transmittance also increases for higher bandgap energies. The UV transmittance is greater than 50% due to the small thickness of the films, which for the undoped samples is proportional to the number of layers.