The surface reconstruction, 3 × 2, induced by Yb adsorption on a Ge (Si)(111) surface has been studied using first principles density-functional calculation within the generalized gradient approximation. The two different possible adsorption sites have been considered: (i) H3 (this site is directly above a fourth-layer Ge (Si) atom) and (ii) T4 (directly above a second-layer Ge (Si) atom). We have found that the total energies corresponding to these binding sites are nearly the same, indeed for the Yb/Ge (Si)(111)–(3 × 2) structure the T4 model is slightly energetic by about 0.01 (0.08) eV/unitcell compared with the H3 model. In particular for the Ge sublayer, the energy difference is small, and therefore it is possible that the T4, H3, or T4H3 (half of the adatoms occupy the T4 adsorption site and the rest of the adatoms are located at the H3 site) binding sites can coexist with REM/Ge(111)–(3 × 2). In contrast to the proposed model, we have not determined any buckling in the Ge = Ge double bond. The electronic band structures of the surfaces and the corresponding natures of their orbitals have also been calculated. Our results for both substrates are seen to be in agreement with the recent experimental data, especially that of the Yb/Si(111)–(3 × 2) surface.
