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

Abstract

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.