Ge(Si) ordering on a double-layer, stepped Si(Ge)(001) surface

Using the Car-Parrinello scheme, we study atomic ordering of Ge(Si) on a do uble-layer, stepped Si(Ge)(001) surface. We introduce a rebonded Dg step to find a stable absorption site of the Ge(Si) atoms. We find that the Si ato ms on a Ge surface favor subsurface sites near the step edge. The total ene rgy is lowered by the rebonded atom forming an asymmetry, which increases t he dehybridization on the rebonded atom. This leads to a compositional orde ring along the direction of the dimer row. We will also show that the simpl e Vegard law is not always obeyed at the interface, in particular, in the i nitial growth process.