Control of crystal structure, morphology and optical properties of ceria films by post deposition annealing treatments

In this paper, the effects of post-deposition annealing temperature and atmosphere on the properties of pulsed DC magnetron sputtered ceria (CeO2) thin films, including crystalline structure, grain size and shape and optical properties were investigated. Experimental results, obtained from X-ray diffraction (XRD), showed that the prepared films crystallised predominantly in the CeO2 cubic fluorite structure, although evidence of Ce2O3 was also seen and this was quantified by a Rietveld refinement. The anneal temperature and oxygen content of the Ar/O2 annealing atmosphere both played important roles on the size and shape of the nanocrystals as determined by atomic force microscopy (AFM). The average grain size (determined by an AFM) as well as the out of plane coherence length (obtained from XRD) varied with increasing oxygen flow rate (OFR) in the annealing chamber. In addition, the shape of the grains seen in the AFM studies transformed from circular to triangular as the OFR was raised from 20 sccm to 30 sccm during an 800 °C thermal anneal. X-ray photoelectron spectroscopy was used to measure near-surface oxidation states of the thin-films with varying OFR in the annealing chamber. The bandgap energies were estimated from the ultra-violet and visible absorption spectra and low-temperature photoluminescence. An extracted bandgap value of 3.04 eV was determined for as-deposited CeO2 films and this value increased with increasing annealing temperatures. However, no difference was observed in bandgap energies with variation of annealing atmosphere.