Thermal stability and surface behaviors of CeO2/Si films during in-situ vacuum annealing

CeO2 thin films were deposited on Si (1 1 1) substrates by laser molecular-beam epitaxy (LMBE). The thermal stability and surface behaviors of CeO2/Si samples were investigated by X-ray photoelectron spectroscopy during in-situ vacuum annealing. Temperature dependency of surface compositions was identified. At the temperature from 700 K to 1000 K, CeO2 was reduced gradually, but incomplete transformation of CeO2 to Ce2O3 was found even when the sample was annealed at 1000 K for an hour. When the sample was exposed to oxygen ambience, the enhanced thermal stability of the partially reduced surface was revealed by the slight variation of Ce3d and Ce4d spectra. Si2p peak at 102.7 eV was detected on the top surface after the whole heating treatments, while not for the sub-surface during sputtering till to the interface. The Si signal at the outmost surface is induced by surface segregation due to the decreasing solid solubility of Si atoms in CeO2 matrix with the decreasing temperature. At the interface, Si2p peaks at 99.9 eV with the presence of Ce4d peaks at 122.2 eV and 125.6 eV are attributed to the formation of cerium silicates with Ce-Si-O matrix during annealing due to interfacial reactions, which are supposed to stabilize the partially reduced ceria system.