Separation of bulk and surface-losses in low-loss EELS measurements in STEM

To identify major features in low electron energy loss spectra, the different excitations (bulk plasmons, interband transitions, surface plasmons, Cherenkov and surface guided modes) must be delineated from each other. In this paper, this process is achieved by noting the linear thickness dependence of bulk processes contrasted with the constant thickness behavior of surface excitations. An alternative approach of analyzing bulk plasmon-loss is also introduced. Using a new algorithm, the parameters of plasma generation-plasmon energy E_{P, 0}, a damping parameter Δ E_P and the coefficient of the dispersion relation γ were obtained from a single curve fitting on the example of Si. The ability to separate surface-losses from the rest of the data permitted identification of the fine structure of the surface-losses. The strong peak at 8.2 eV characteristic of non-radiative surface plasmon excitations was measured for Si. Analysis of surface excitations indicates that a 10 ÅSiO₂ surface coating layer is still present despite careful cleaning the specimen. Dielectric functions deduced from the EELS data prove to be considerably affected by the presence of the surface-losses for samples as thick as 800 Å.