  This paper presents a DFT study for phosphine adsorption on a
Si(001)-2$\times$1 surface covered by a chlorine monolayer, including
adsorption on local defects, i.e. mono- and bivacancies in the adsorbate layer
(Cl, Cl$_2$), and combined vacancies with removed silicon atoms (SiCl,
SiCl$_2$). Activation barriers were found for the adsorbing PH$_3$ to
dissociate into PH$_2$+H and PH+H$_2$ fragments; it was also established that
phosphine dissociation on combined vacancies is possible at room temperature.
If there is a silicon vacancy on the surface, phosphorus settles in the Si(001)
lattice as PH (if the vacancy is SiCl) or as PH$_2$ (if the vacancy is
SiCl$_2$). This paper suggests a method to plant a separate phosphorus atom
into the silicon surface layer with atomic precision, using phosphine
adsorption on defects specially created on a Si(001)-2$\times$1-Cl surface with
an STM tip.
