Metal-organic chemical vapour deposition of lanthana-doped ceria layers at low temperatures

Non-porous, amorphous and nanocrystalline CeO₂–La₂O₃ layers were synthesized by means of MOCVD using Ce(tmhd)₄ and La(tmhd)₃ as reactants on tubular and planar quartz glass substrates. Different layers were synthesized in the temperature range 500–700 °C using various molar contents of La(tmhd)₃, i.e. 10, 15, 26 and 50%, respectively. A mixture of Ar and air was used as a carrier gas. Air was also an oxygen source necessary for carbon oxidation (a solid by-product of reactant pyrolysis). Therefore, the layers should only contain Ce⁴⁺ ions. When the synthesis process was controlled by the surface reaction rate, there was no correlation between the chemical composition of reactants and that of the layers. Such a dependence was obtained when the synthesis process was controlled by mass diffusion to the substrate. In addition, it was observed that La³⁺ ion segregation can occur during layer synthesis between surface areas with lower and higher amounts of a crystalline phase. The molar percent of La₂O₃ in areas exhibiting less crystallization with smaller grains was higher than in locations with larger grains, because, in this case, intergranular boundary content was lower. X-ray analysis indicates that layers deposited at 500 °C were amorphous, although SEM images show numerous small nuclei of a crystalline phase. It seems that the crystalline phase content was not high enough to be identified by X-ray analysis. Layers deposited at 600 °C consisted of a solid solution of La₂O₃ dissolved in CeO₂. From UV-VIS tests it follows that absorption is very strong up to a wavelength of approximately 320 nm.