First-principles calculations of energy and stress are performed for adatom-covered Si(111) and Ge(111) surfaces. The presence of adatoms was found to lower the surface energy, and caused a large change in surface stress. While the 1 x 1 surfaces were under a weak compressive stress, the √3 x √3 and 2 x 2 adatom-covered surfaces were under a strong tensile stress. Calculations at high plane-wave cut-off unambiguously identified the 2 x 2 top-site geometry as being the energetically preferred adatom configuration. Relaxed geometries were presented and compared with X-ray structural measurements of adatoms in the Si(111)-(7 x 7) structure. Vibrational mode frequencies and eigenvectors of the adatom unit were determined from a comprehensive set of frozen-phonon calculations for the Si 2 x 2 surface. Two symmetrical modes were found that were strongly localized at the surface; in agreement with electron energy-loss spectroscopic measurements. It was found that the 2 x 2 adatom-covered surfaces had 3 surface bands, and the dispersion relationships were calculated along symmetry directions in the surface Brillouin zone. The surface band structures were in good agreement with angle-resolved photo-electron spectroscopic data for the 7 x 7-Si(111) surface.

Adatoms on Si(111) and Ge(111) Surfaces. R.D.Meade, D.Vanderbilt: Physical Review B, 1989, 40, 3905-13