Patterning of Si(001) with halogens: Surface structure as a function of the halogen chemical potential

Chlorine- and bromine-chemisorbed Si(001) surfaces at various coverages (formula presented) are studied using first-principles density functional calculations. Different stable reconstructions are obtained for the two halogen species. At (formula presented) Br stabilizes a (formula presented) reconstruction with adatoms only on next-nearest neighbor dimers, whereas Cl stabilizes a (formula presented) structure with alternating fully chlorinated and empty dimer rows. At (formula presented) a (formula presented) reconstruction consisting of dimer rows separated by missing atom rows is most stable for Br, whereas for Cl such a structure is degenerate with the conventional (formula presented) reconstruction. These findings are in agreement with recent scanning tunneling microscopy experiments. They can be rationalized on the basis of a stronger steric hindrance for Br.