MOCVD of cobalt oxide thin films: dependence of growth, microstructure, and optical properties on the source of oxidation

Thin films of cobalt oxide were deposited on glass substrates by low-pressure chemical vapor deposition using cobalt (II) acetylacetonate as the precursor. The depositions were carried out using two different oxidant gases—(i) $O_2$ and (ii)$N_2O$, in the temperature range of 375–550C, under otherwise identical conditions, with the expectation that oxygen radicals released by $N_2O$ may alter the CVD process. The crystallinity and phase composition of films were examined by X-ray diffraction and transmission electron microscopy. The surface morphology and microstructure of the films were studied by scanning electron microscopy and atomic force microscopy. It is found that the identity of the oxidant gas affects the phase composition, growth kinetics, uniformity, as well as microstructure, of the films. Electron microscopy shows that growth in $O_2$ ambient results in films containing strongly faceted, uniform-sized grains of $Co_3O_4$ only, with a preference for the (111) orientation, even at relatively low temperatures. However, films grown in $N_2O$ ambient comprise poorly crystallized, randomly oriented grains of either $Co_3O_4$ or ($CoO+Co_3O_4$), depending on the deposition conditions. Optical properties of the films of the latter composition, deposited in the $N_2O$ ambient, studied by UV-visible spectrophotometry, show clearly the presence of two distinct bandgaps, corresponding, respectively, to CoO and $Co_3O_4$