XPS data were measured on a thermally oxidised Si wafer. The sample was totally oxidised and no elemental silicon could be detected. It was treated by low-pressure oxygen plasma to remove residual carbon and reach the stoichiometric Si:O = 1:2 ratio. After measuring the clean state of the sample, hydrocarbon contamination of increasing thickness was deposited by casting of polystyrene from benzene solution in subsequent steps, as illustrated by the black curve.
The compositions of the samples (atomic ratio) were calculated with different contamination correction methods. Without application of such corrections a substantial error is introduced.
When no correction is applied, only the clean (O plasma) sample gives the correct composition. The method of Evans is optimised for the 'averrage' contamination (As is) thus the clean sample is overcorrected. Mohai's method supplies the composition within the ±10% difference range for low and high levels of contamination.
M. Mohai, I. Bertóti:
Correction for Surface Contaminations in XPS: A Practical Approach.
in: ECASIA 95 (Eds. H. J. Mathieu, B. Reihl, D. Briggs), John Willey & Sons, Chichester (1995), p. 675.
XPS MultiQuant for Windows User's Manual.