Core-shell electronic absorption edges from thin specimens of silicon, α-silicon carbide, β-silicon nitride, and amorphous silica are studied by using electron-energy-loss spectroscopy in a transmission electron microscope. The elemental and chemical effects in the near-edge regions of the Si L2,3 and C, N, and O K edges are calculated by using some semiempirical models. The chemical effects in the region of the edges near-edge onset are due to valence-shell excited states, which we have modeled as linear combinations of atomic orbitals using the extended Hückel method, with the effects of translational periodicity in crystals included by using Bloch wave functions. Population analyses of valence-shell electronic structure and cross sections for bound→bound atomic transitions are used to interpret and calculate theoretical near-edge fine structure for direct comparison with experiment. The near-edge ionization region is calculated by using a plane-wave excited state to account for elemental effects. Chemical effects in the ionization region are accounted for by including contributions from the elastic backscattering of outgoing waves by the atoms that neighbor the excited atom. The elemental and chemical effects in the edges are shown to be separable to a large extent by using these models, and calculated cross sections are in good semiquantitative agreement with experimental results.
ASJC Scopus subject areas
- Physics and Astronomy(all)