Pure and cobalt-doped SnO2(101) films grown by molecular beam epitaxy on Al2O3

Pure and Co-doped epitaxial SnO2 films grown by oxygen plasma assisted molecular beam epitaxy on r-cut α-alumina substrates were investigated by electron diffraction, X-ray photoelectron spectroscopy (XPS), and X-ray photoelectron diffraction (XPD). On hot alumina substrates (∼ 800 °C) only a submonolayer amount of Sn adsorbs, indicating a strong adhesion of the first monolayer of tin on the alumina surface. SnO2 films grown at ∼ 400-600 °C substrate temperature exhibit a SnO2(101)[010]‖Al2O3(-1012)[12-10] epitaxial relationship. Subtle differences in the XPD data of SnO2 films compared to measurements on SnO2(101) single crystal surfaces are consistent with the presence of a high density of stoichiometric antiphase domain boundaries in the film. These planar defects are introduced in the SnO2 film to compensate for the more than 10% lattice mismatch between the SnO2 films and the alumina substrate along the SnO2[-101] direction. CoxSn1−xO2 films with a Co-cation concentration of 5-15% were also grown. XPS indicates that Co is in a 2+ oxidation state and XPD shows that tin is replaced substitutionally by Co.