Two-dimensional alloying on Si(111) surface: An ab initio study

We investigate the electronic, structural, and energetic properties of the 1/3 monolayer Sn1-xSix/Si(111) surface as a function of x by means of a density functional theory in the local-density approximation pseudopotential approach. The ideal (x=0) phase has a root3xroot3R30degrees reconstruction and does not show the 3x3 phase transition. The study, as a function of Si substitution, reveals that the two-dimensional alloys are stable against phase separation. Si impurities on the host alpha-Sn/Si(111) surface produces, in fact, relevant distortions of the substrate lattice up to the fourth layer; moreover, the Sn and Si surface states are decoupled with the Sn states always at lower energy with respect to Si with a decoupling of surface bands. This is confirmed by the experimental scanning tunnel microscope images and it produces a semiconducting phase at x=0.5.