Analysis of the adsorbates and determination of the adsorption sites on Si(1 1 1)-7×7 after a large amount of Ar exposure at room temperature

The surface structure of Si(1 1 1)-7×7 after exposure to a large dose of Ar at room temperature has been investigated by scanning tunneling microscopy and the adsorbates on the surface have been analyzed. It was found that Ar does not adsorb on the Si(1 1 1)-7×7 surface. The adsorption is dominated by the impurities, most likely to be H2O, in the Ar gas. The impurities dissociatively adsorb on the Si(1 1 1)-7×7 surface and appear as dark spots in both empty and filled state images due to the diminution of Fermi-level local density of states over the adsorption sites. The chemisorption is site selective and the ratio of reacted center sites vs. reacted corner sites is about 2:1, indicating that center-adatoms react with H2O twice as fast as corner-adatoms. After annealing the substrate to 660 °C, this ratio keeps invariant while the total number of dark sites is significantly decreased. The remaining dark spots on the Si(1 1 1)-7×7 surface after annealing at 660 °C are likely to be surface vacancies caused by the desorption of SiO.