Bi covered Si(111) surface revisited

Abstract

We have performed an ab initio study of the stability, atomic geometryand electronic structure of the Bi-covered (√3 ×√3)reconstructed Si(111) surface. We find that the energetically stable structurechanges from the milkstool model (for 1 monolayer (ML) coverage) to theT4model (for 1/3 ML coverage), without going through a stable structurefor the honeycomb model (2/3 ML coverage). Our theoretical scanningtunnelling microscopy (STM) simulation for the 1 ML coverage revealsthe formation of Bi trimers for occupied states, and a honeycomb imagefor empty states. This result, together with the energetically unstablestructure for 2/3 ML coverage, suggests that the experimentally observed STMimage in the form of the honeycomb structure does not mean that theminimum energy configuration corresponds to Bi coverage of 2/3 ML,but rather represents current tunnelling into the empty states localizedbetween Bi trimers for the milkstool model with 1 ML coverage.