A Critical Evaluation of Concentration Proxies in SIMS Diffusion Studies

Secondary Ion Mass Spectrometry (SIMS) is a powerful technique for investigating diffusion at and near material surfaces. It directly measures the intensity of secondary ions ejected upon ion beam bombardment of sample surfaces. In SIMS diffusion studies, three concentration proxies rather than intensities are often employed. Using the classical diffusion equation as the basis, we establish conditions under which each proxy is acceptable for extracting diffusivities. We demonstrate that these conditions depend on the temporal and spatial evolution of the intensity of the reference secondary ion. Further, these conditions may or may not be realized in experiments, regardless of the chemical potential landscape of the matrix. We advocate for reporting raw data as is, i.e., in terms of intensities of ejected ions.