Quantitative Determination of Native Point-Defect Concentrations at the ppm Level in Un-Doped BaSnO3 Thin Films

The high-mobility, wide-bandgap perovskite oxide BaSnO3 is taken as a model system to demonstrate that the native point defects present in un-doped, epitaxial thin films grown by hybrid molecular beam epitaxy can be identified and their concentrations at the ppm level determined quantitatively. An elevated-temperature, multi-faceted approach is shown to be necessary: oxygen tracer diffusion experiments with secondary ion mass spectrometry analysis; molecular dynamics simulations of oxygen-vacancy diffusion; electronic conductivity studies as a function of oxygen activity and temperature; and Hall-effect measurements. The results indicate that the oxygen-vacancy concentration cannot be lowered below 10(17.3) cm(-3) because of a background level of barium vacancies (of this concentration), introduced during film growth. The multi-faceted approach also yields the electron mobility over a wide temperature range, coefficients of oxygen surface exchange and oxygen-vacancy diffusion, and the reduction enthalpy. The consequences of the results for the lowest electron concentration achievable in BaSnO3 samples, for the ease of oxide reduction and for the stability of reduced films with respect to oxidation, are discussed.