Studies of Effects of Adsorption of Silicon or Germanium on the Electronic States of (100) GaAs Surfaces

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The electronic states of both bare Ga-terminated and As-terminated (100)-(1×1) GaAs surfaces and the same surfaces with adsorbed Si or Ge were studied by molecular orbital calculations using small cluster models and the discrete variational Xα method. The most stable adsorption sites for Si and Ge are bridge sites on the (100)-GaAs surface, regardless of whether they are Ga- or As-terminated. When Si or Ge atoms are adsorbed, the density of the surface states in the forbidden band of the Ga or As surface layers markedly decreases. This suggests that both the Ga-terminated and As-terminated surfaces have a clear band gap when either Si or Ge are adsorbed on the surfaces. These adsorbates have states in the lower half of the band gap or in the valence band.