Calcium and fluorine signals in HS-LEIS for CaF₂(111) and powder—Quantification of atomic surface concentrations using LiF(001), Ca, and Cu references

The powder of CaF₂ has been proposed as a practical reference for the quantitation of Ca and F in low energy ion scattering (LEIS) analysis. It is chemically inert, easy to clean, and inexpensive. LEIS is extremely surface sensitive. Thus, in contrast to surface analytic techniques such as x-ray photoelectron spectroscopy and secondary ion mass spectroscopy, the surface termination of a sample is clearly reflected in the LEIS results. It is, thus, unlikely that in LEIS, the F/Ca ratio for CaF₂ is 2.0. This paper supports the reliability of the CaF₂ powder reference by evaluating the calcium and fluorine atomic surface concentrations, roughness factor and shows that the surface termination of the powder is the same as that of CaF₂(111). The CaF₂ samples are treated by annealing at 725 K and measured at 625 K. The presented spectra are practically free of contamination. Ion scattering by LiF (001), an evaporated Ca layer, and a Cu foil are used as basic references for Ca and F. The atomic sensitivity factors and the relative sensitivity factors have been determined for F, Ca, and Cu (3 keV He⁺, 145°). The F/Ca atomic ratio is found to be the same (2.3 ± 0.1) for CaF₂(111) and its powder. For the powder, the Ca and F signals are reduced by a factor of 0.77 ± 0.03 in comparison with those for the single crystal.