S ' L core edge fine structure in an oxidation serijes of smcon compounds : A comparison of microelectron energy ~ oss spectra with theory

Transmission electron energy loss spectra were obtained from small (approximately loo-A diam) regions of a series of single-phase silicon-containing specimens at loo-keV incident beam energy, using a field emission source transmission electron microscope fitted with a magnetic sector spectrometer. The specimen foils were diamond cubic silicon, a-silicon carbide, a-silicon nitride, and amorphous silica. The SiL near-edge structure depends markedly upon the chemical environment of the silicon. In this paper we show that the changes in near-L-edge structure, including threshold onset energy shift and edge profile, result from bond-induced changes in the valence shell electronic structure of the specimen materials. Extended Huckel molecular orbital theory was used to calculate the valence shell electronic structure of five-atom tetrahedral clusters, with silicon as the central atom and the other atoms noted above in comer positions. Inelastic electron scattering cross sections for silicon 2p and 2s core shell transitions to valence shell excited states were then calculated using the first Born approximation.