  The determining factor of the bulk properties of doped Si is the column
rather than the row in the periodic table from which the dopants are drawn. It
is unknown whether the basic properties of dopants at surfaces and interfaces,
steadily growing in importance as microelectronic devices shrink, are also
solely governed by their column of origin. The common light impurity P replaces
individual Si atoms and maintains the integrity of the dimer superstructure of
the Si(001) surface, but loses its valence electrons to surface states. Here we
report that isolated heavy dopants are entirely different: Bi atoms form pairs
with Si vacancies, retain their electrons and have highly localized,
half-filled orbitals.
