Si(111) strained layers on Ge(111): Evidence for c ( 2×4 ) domains

The tensile-strained $\mathrm{Si}(111)$ layers grown on top of $\mathrm{Ge}(111)$ substrates are studied by combining scanning tunneling microscopy, low-energy electron diffraction, and first-principles calculations. It is shown that the layers exhibit $c(2\ifmmode\times\else\texttimes\fi{}4)$ domains, which are separated by domain walls along $\ensuremath{\langle}\overline{1}10\ensuremath{\rangle}$ directions. A model structure for the $c(2\ifmmode\times\else\texttimes\fi{}4)$ 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.