Scanning tunneling microscopy is used to study the structure of Al-√3 × √3 domains on the Si(111)-7×7 surface and the atomic arrangement around the domain boundary. Al-√3 × √3 domains grow from kink sites of the 〈110〉 or 〈211〉 step and extend over the Si-7×7 terrace. Detailed investigation around the boundaries reveals that faulted sites face the boundaries on the Si-7×7 side, while on the Al-√3 × √3 side, Al adatoms occupy the ${\mathit{T}}_4$ sites except on the rows adjacent to the phase boundaries where Al (or Si) atoms are adsorbed on the Si adatom sites. The latter atoms play an important role in retaining the dimer structure at the boundaries.

• Received 11 May 1993

DOI:https://doi.org/10.1103/PhysRevB.48.5657