Influence of the Initial Powder’s Specific Surface Area on the Properties of Sm-Doped Ceria Thin Films

Abstract

The influence of a specific surface area of evaporating powder on the properties of thin Sm-doped cerium (SDC) oxide films has not yet been sufficiently investigated. Therefore, SDC films were deposited by e-beam evaporation using Sm0.2Ce0.8O2-δ powders of 6.2 m2/g, 11.3 m2/g, and 201.3 m2/g specific surface area on SiO2, and Al2O3 substrates. X-Ray Diffraction (XRD) analysis showed that SDC thin films deposited on 600 °C SiO2 substrates changed their preferred orientation from (111) to (311), (200), and (220) when evaporating 6.2 m2/g and 11.3 m2/g powders and using 0.2 nm/s, 1.2 nm/s, and 1.6 nm/s deposition rates. However, thin films deposited by evaporating powder of 201.3 m2/g specific surface area do not change their preferred orientation. The crystallite size of the SDC thin films depends on the substrate temperature and specific surface area of the evaporating powder. It increases from 6.40 nm to 89.1 nm with increasing substrate temperature (50–600 °C). Moreover, crystallites formed by evaporating a powder of 201.3 m2/g specific surface area are 1.4 times larger than crystallites formed by evaporating a powder of 6.2 m2/g specific surface area. An impedance analysis revealed that the normalized resistance of “grains” is higher than the normalized resistance of grain boundaries. Moreover, a total conductivity depends on crystallite size. It changes from 4.4 × 10−7 S/cm to 1.1 × 10−2 S/cm (600 °C) when the crystallite sizes vary from 6.40 nm to 89.10 nm. In addition, the optical band gap becomes wider with increasing crystallite size proving that the Ce3+ concentration decreases with an increasing crystallite size.