Influence of samaria doping on the resistance of ceria thin films and its implications to the planar oxygen sensing devices
In order to evaluate and analyze the effect of samarium (Sm) doping on the resistance of cerium oxide, we have grown highly oriented samaria doped ceria (SDC) thin films on sapphire, Al2O3 (0 0 0 1) substrates by using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE). The film growth was monitored using reflection high-energy electron diffraction (RHEED) which shows two-dimensional growth throughout the deposition. Following growth, the thin films were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution X-ray diffraction (HRXRD), and Rutherford backscattering spectrometry (RBS). XPS depth-profile shows Sm atoms are uniformly distributed in ceria lattice throughout the bulk of the film. The valence states of Ce and Sm in doped thin films are found to be Ce4+ and Sm3+, respectively. HRXRD shows the samaria doped ceria films on Al2O3 (0 0 0 1) exhibit (1 1 1) preferred orientation. Ion-channeling in RBS measurements confirms high quality of the thin films. The resistance of the samaria doped ceria films, obtained by two probe measurement capability under various oxygen pressure (1 mTorr100 Torr) and temperatures (623973 K), is significantly lower than that of pure ceria under same conditions. The 6 at.% samaria doped ceria film is the optimum composition for highest conductivity. This is attributed to the increased oxygen vacant sites in fluorite crystal structure of the epitaxial thin films which facilitate faster oxygen diffusion through hopping process.