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Surface-state transitions on ordered silicon (100) 21 and (111) 77 surfaces have been observed by electron energy-loss spectroscopy. The surface transitions were identified by comparing the clean ordered surfaces with clean disordered surfaces, and with disordered surfaces due to submonolayer quantities of absorbed oxygen. Three transitions are observed corresponding to surface states near the top, near the middle, and at the bottom of the valence band. These surface-state transitions are stronger on the (111) 77 surface than on the (100) 21 surface.
Rowe et al. (1973) studied this question.