Tin atoms adsorbed on a Si(1 1 1)-(7×7) surface

Isolated adsorption of single-Sn atoms occurs equally on the faulted and unfaulted halves of the Si(1 1 1)-(7×7) surface at low coverage. These single-Sn atoms are mobile among the three center Si adatoms, but they do not diffuse out from the adsorbed half unit cells at room temperature. These single-Sn atoms are responsible for the noisy triangular feature of the STM image. When the faulted and unfaulted halves of a unit cell adsorb two single-Sn atoms in each, a noisy rhombic feature appears in the STM image. The two single-Sn atoms in the noisy rhombic unit cell are unstable at room temperature, and they form a bright Sn2-dimer spot in the faulted or unfaulted half. In addition, the two single-Sn atoms undergo replacement reaction with the center Si adatoms, and a Si4-tetramer is formed at the boundary of the faulted and unfaulted halves. These phenomena suggest that the potential barriers are markedly lowered by coming two single-Sn atoms in the adjacent halves.