Real-space images of an incommensurate superlattice on a monolayer-gallium-covered silicon (111) surface have been obtained with the tunneling microscope. Large, internally ordered supercells which in turn form a lattice with discrete boundaries are observed. A graphitelike silicon-gallium top layer is suggested that is weakly bonded to the lattice below and stabilized by a periodic array of misfit dislocations. This model unifies reflection high-energy electron diffraction, x-ray standing-wave, and tunneling observations.

• Received 19 September 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.2867