Information on compositional depth profiles conveyed by angle‐resolved XPS

Publisher Website
Google Scholar
Abstract
The technique of retrieving depth profiles from angle‐resolved x‐ray photoelectron spectroscopy (ARXPS) data is analysed with respect to its depth resolution and its potential to differentiate compositional zones. The usable range of polar angles is shown to have an non‐obvious limitation at grazing electron escape owing to the peculiar way the ARXPS signal depends on the probing depth. This limitation along with the experimental error in the measurement introduces ambiguities in two respects. First, these experimental limitations lead to a considerable depth (broadening) uncertainty which confines the number of resolvable depth zones to about 10. Second, the space of functions (over the accessable range of probing depths), which includes all possible ARXPS signals, is of very low dimension. Within an assumed experimental error of 1%, the class of all possible ARXPS signal functions is shown to be contained in a subspace spanned by just five basis functions. Therefore, the information obtainable for each single ARXPS signal is confined to a maximum of five parameters. Almost all ARXPS signals of a single component (except for a few very special cases) can be interpreted in terms of a depth profile consisting of just three zones with constant composition and with two optimized interface positions. However, the number of resolvable compositional zones increases with the number of XPS components. A lower limit for this number can be assessed by a factor analysis that also yields unambiguous information on the composition of the zones. Elastic scattering effects and local variations of the electron escape length are expected to have little influence on these results, because they simply lead to a rescaling of the depth and the concentration scale of the retrieved profile.