  Renewed focus on the P-Si system due to its potential application in quantum
computing and self-directed growth of molecular wires, has led us to study
structural changes induced by P upon placement on Si(001)-$p(2\times 1)$. Using
first-principles density functional theory (DFT) based pseudopotential method,
we have performed calculations for P-Si(001) system, starting from an isolated
P atom on the surface, and systematically increasing the coverage up to a full
monolayer. An isolated P atom can favorably be placed on an {\bf M} site
between two atoms of adjacent Si dimers belonging to the same Si dimer row. But
being incorporated in the surface is even more energetically beneficial due to
the participation of the {\bf M} site as a receptor for the ejected Si. Our
calculations show that up to 1/8 monolayer coverage, hetero-dimer structure
resulting from replacement of surface Si atoms with P is energetically
favorable. Recently observed zig-zag features in STM are found to be consistent
with this replacement process. As coverage increases, the hetero-dimers give
way to P-P ortho-dimers on the Si dimer rows. This behavior is similar to that
of Si-Si d-dimers but are to be contrasted with the Al-Al dimers, which are
found between adjacent Si dimers rows and in a para-dimer arrangement. Unlike
Al-Si system P-Si does not show any para to ortho transition. For both systems,
the surface reconstruction is lifted at about one monolayer coverage. These
calculations help us in understanding the experimental data obtained using
scanning tunneling microscope.
