First-principles investigation of geometric and electronic structures of aluminum adsorbed on silicon surfaces

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Abstract
The electronic structure of aluminum-adsorbed silicon (111) surfaces has been studied by the self-consistent-field Hartree-Fock cluster method with the aim of obtaining definitive assignments of the adsorption site(s). Calculations have been made for clusters representing adsorption at different high-symmetry sites, namely, substitutional, atop, open, and eclipsed. Besides the studies of adsorption at all these sites on the ideal surfaces, an adsorption study on a relaxed substrate has been made for the eclipsed site. Minimization of the total energies of the clusters with respect to the vertical distance of the adatom from the surface silicon layer leads to Si—Al bond lengths of 2.26, 2.37, 2.55, 2.68, and 2.44 Å for the substitutional, atop, open, eclipsed, and relaxed eclipsed sites, respectively, the corresponding binding energies being 9.50, 3.94, 1.96, 0.59, and 4.21 eV. Assuming that the surface vacancies are not abundant at low temperature (), so that substitutional adsorption is insignificant, it is proposed that there is coadsorption at the relaxed eclipsed and atop sites. This assignment is shown to provide successful explanations of low-energy electron diffraction and ultraviolet photoemission data and vibrational frequency data from high-resolution electron-energy-loss spectroscopy measurements. Vibrational amplitudes of the adatoms have been predicted. Also, it is shown that the results for the Si—Al bond distances modified to apply to the Si-Ga system explain the x-ray standing-wave results for Ga-adsorbed Si(111) surfaces.