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Two qualitatively different structural models are currently popular for the stable 21 superstructure characteristic of the Si (100) surface. We report the results of realistic self-consistent calculations of the electronic spectrum for both. Comparison with uv photoemission data clearly favors the pairing model over the vacancy model. Our results on surface Fermi-level pinning and bond saturation support this conclusion.
Appelbaum et al. (Mon,) studied this question.