  For the most precise incorporation of single impurities in silicon, which is
utilized to create quantum devices, a monolayer of adatoms on the Si(100)
surface and a dopant-containing molecule are used. Here we studied the
interaction of a phosphorus tribromide with a chlorine monolayer with mono- and
bivacancies in a scanning tunneling microscope (STM) at 77 K. The combination
of different halogens in the molecule and the adsorbate layer enabled
unambiguous identification of the structures after PBr3 dissociation on
Si(100)-Cl. A Cl monolayer was exposed to PBr3 in the STM chamber, which allows
us to compare the same surface areas before and after PBr3 adsorption. As a
result of this comparison, we detected small changes in the chlorine layer and
unraveled the molecular fragments filling mono- and bivacancies. Using density
functional theory, we found that the phosphorus atom occupies a bridge position
after dissociation of the PBr3 molecule, which primarily bonds to silicon in Cl
bivacancies. These findings provide insight into the interaction of a dopant
containing molecule with an adsorbate monolayer on Si(100) and can be applied
to improve the process of single impurities incorporation into silicon.
