Microstructure evolution of zinc oxide films derived from dip-coating sol–gel technique:formation of nanorods through orientation attachment

Abstract

ZnO:Al thin films with Al incorporation of 0–20 at.% were deposited through the sol–geltechnique. Such a film undergoes a significant microstructure development, fromcolumnar to granular structures and then nanorod arrays with increasing Al content.The important role of Al incorporation level in the microstructure evolution wasdetermined using scanning electron microscopy, x-ray photoelectron spectroscopy andtransmission electron microscopy. At low Al level, the transition from columnar togranular grains can be attributed to the coarsening barrier resulting from theintroduction of Al into the matrix. However, oriented structures of ZnO nanorodarrays are formed at a high Al level. TEM investigation reveals that a nanorodwith smooth morphology at the top and rough morphology at the bottom has asingle-crystalline wurtzite structure, which is the aggregation of nanoparticles of a fewnanometers in size formed through the orientation attachment mechanism followed byepitaxial growth on the aggregated particles. Finally, the physical properties of theZnO films with different degrees of Al concentration are discussed. Such detailedmicrostructure studies may aid the understanding of the doping effect process on thegrowth of a film, which is essential to altering its physical or chemical properties.