On the accurate measurement of oxygen self-diffusivities and surface exchange coefficients in oxides via SIMS depth profiling

Oxygen diffusivity in oxides is usually determined via gas/solid exchange experiments in an atmosphere enriched as to the concentration of the rare stable isotope 18O. The kinetics of the 16O and 18O isotope exchange at the surface is usually described by the surface exchange coefficient, K. The analytical solution of the diffusion problem suggests the definition of a characteristic time constant, τ≡D/K2. This time constant determines the duration that is necessary to reach equilibrium between the 18O gas concentration and the 18O concentration at the surface of the solid. The simultaneous determination of D and K by secondary ion mass spectrometry (SIMS) depth profiling works well in a large parameter range of D and K. However, at low temperatures, the simultaneous determination of D and K may be subject to severe errors. To overcome this problem, the possibility to determine D and K separately will be discussed.