Deposition of thin films of ZnO by sol gel method and their characterization

Abstract

Thin films of zinc oxide have been deposited on Pyrex glass substrate by using sol gel process and characterized by means of different charactcrizatioll techniques including optical microscopy, X-ray diffractometry (XRD), thermogravimetry and spectrophotometry. The sol has been prepared by reacting zinc acetate and ethylene glycol and dissolving the resultant solution in npropanol. Glycerol was added to the sol to increase its stability. A proton acceptor like diethyl amille was added to assist hydrolysis of zinc acetate. Thin films of zinc oxide were cast on the substrate by dip coating method and gelled in humid air. The as deposited film was annealed at various temperatures ranging from 150 C to 450 C for 15 minutes at each set temperature. It has been found that the cleanliness of the glass substrate, the humidity during the gelation of the film as well as the time and temperature of the heal treatment affect the appearance and the morphology of the film. Cleaning of the glass substrate for the successful deposition of the film was achieved by the use of deionized water, acetic acid and absolute ethanol. Relative humidity less than 50% during the deposition of the film on to the glass substrate was found to be favorable for yielding good transparent film. It has been found that annealing of the film pyrolyzes gelled film to zinc oxide, drives out moistures and other volatiles and at higher temperatures decomposes the metalorganic complexes. The amorphous as-deposited film transforms to polycrystalline as a result of annealing above 275 C and the XRD peaks become sharper and more intense with the increase in annealing temperature, The films annealed at higher temperature, especially those annealed above 400 C, are denser and less porous and annealing leads to the increase in the size of the ZnO crystallites in them. The thin films of ZnO formed are randomly oriented without showing any preferred orientation. The ZnO crystallites have the hexagonal wurtdte structure, with e = 5.271 A and a = 3.285 A. The average thickness of the films from thermogravimetric analyses has been found to be about I ,.un after annealing at 450 C for 15 minutes and the film and the film exhibited about 80% transmission.