Silicon valence states in calcium silicides: A SiL2,3VVAuger line-shape analysis

We present a Si ${\mathit{L}}_{2,3}$VV Auger line-shape analysis of the whole set of Ca silicides, ${\mathrm{Ca}}_2$Si, CaSi, and ${\mathrm{CaSi}}_2$. Measured spectra are compared, after data processing, to theoretical ones, computed within the framework of a one-electron picture. The overall good agreement found in this comparison allows us to understand the observed Si ${\mathit{L}}_{2,3}$VV spectral changes in terms of the Si valence states’ evolution across the whole family of Ca silicides. The narrowing of the pp spectral region and the appearance of a distinct sp shoulder observed on going from ${\mathrm{CaSi}}_2$ to ${\mathrm{Ca}}_2$Si are related, respectively, to the decreased width of the Si 3p band and to the narrowing of the Si 3s states together with their increased energy separation from the Si 3p states. A breakdown of the one-electron picture is observed in ${\mathrm{Ca}}_2$Si. We find that this effect is due to the quasiatomic configuration of the Si 3s states in this compound. The introduction of a correlation energy between two sp-like–final-state holes improves the comparison between theory and experiment, but does not appear to fully account for the Si ${\mathit{L}}_{2,3}$VV spectrum from ${\mathrm{Ca}}_2$Si. New theoretical effort is therefore needed to understand this spectrum.