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We examine the Si(111) surface with 1/3 monolayer of group-IV elements within the ab initio density functional theory. We have considered two possible sites for adsorption atoms within the threefold-coordinated: (i) H3 site (this site lies directly above a fourth-layer Si atom and (ii) T4 site (directly above a second-layer Si atom). For the atoms C, Si, Ge, Sn, and Pb in a T4 position always gives the most stable configuration, comparing with the H3 site. The calculated energy difference between T4 and H3 for C, Si, Ge, Sn, and Pb is 0.65 eV, 0.61 eV, 0.59 eV, 0.41, and 0.32 eV, respectively. We have also presented their electronic band structures and orbital character of the surface states. |