STM observation of Sn overlayers on Si(111)

×4and 3×3, that appear at the coverages above 1 monolayer, have been observed in real space for the first time. It is found that the former consists of three or four one-dimensional columns of zigzag chains, while the latter consists of two-dimensional honeycomb structures. The two phases coexist as small domains on the terrace with three different orientations reflecting the substrate symmetry. After desorption of the Sn, Sn atoms are observed partly replacing the Si adatom sites of 7×7 reconstruction. A charge transfer from the surrounding Si adatoms of 7×7 is suggested by the voltage dependence of the STM images.