ZnO thin films design: the role of precursor molarity in the spray pyrolysis process

Many investigations employ oxides grown by spray pyrolysis whose desired features depend on the synthesis parameters. This paper reports the influence of precursor dilution on the structural, morphological, and optical properties of ZnO films. For this purpose, the applied aqueous solutions using zinc acetate as precursor covered a wide range of molarities from 10 –3 to 1. The films were deposited on silicon and glass substrates and characterized by several techniques. X-ray diffraction showed that well-diluted conditions are essential for crystalline ordering. Atomic force microscopy and scanning electron microscopy confirmed extensive coating with rougher surfaces on silicon substrates. The optical transparency is remarkable, and the optical emissions measured by photoluminescence are predominantly in the ultraviolet range, related to near band edge transitions. X-ray photoelectron spectroscopy indicated surfaces with a high percentage of adventitious carbon. In contrast, at high molarities, the films are white and powder-like. Defect-level recombination dominates the optical emissions in the visible range. Moreover, the stoichiometric surfaces present lower levels of adventitious carbon. This investigation shows that the precursor molarity is crucial for ZnO films architecture, as demonstrated in their chromaticity characteristics by the redshift of UV emission and the raise of the visible band.