  The tensile strained Si(111) layers grown on top of Ge(111) substrates are
studied by combining scanning tunneling microscopy, low energy electron
diffraction and first-principles calculations. It is shown that the layers
exhibit c(2x4) domains, which are separated by domain walls along <-110>
directions. A model structure for the c(2x4) domains is proposed, which shows
low formation energy and good agreement with the experimental data. The results
of our calculations suggest that Ge atoms are likely to replace Si atoms with
dangling bonds on the surface (rest-atoms and adatoms), thus significantly
lowering the surface energy and inducing the formation of domain walls. The
experiments and calculations demonstrate that when surface strain changes from
compressive to tensile, the (111) reconstruction converts from
dimer-adatom-stacking fault-based to adatom-based structures.
