  We use scanning tunneling microscopy (STM) and Auger electron spectroscopy to
study the behavior of adsorbed phosphine (PH$_{3}$) on Si(001), as a function
of annealing temperature, paying particular attention to the formation of the
Si-P heterodimer. Dosing the Si(001) surface with ${\sim}$0.002 Langmuirs of
PH$_{3}$ results in the adsorption of PH$_{x}$ (x=2,3) onto the surface and
some etching of Si to form individual Si ad-dimers. Annealing to 350$^{\circ}$C
results in the incorporation of P into the surface layer to form Si-P
heterodimers and the formation of short 1-dimensional Si dimer chains and
monohydrides. In filled state STM images, isolated Si-P heterodimers appear as
zig-zag features on the surface due to the static dimer buckling induced by the
heterodimer. In the presence of a moderate coverage of monohydrides this static
buckling is lifted, rending the Si-P heterodimers invisible in filled state
images. However, we find that we can image the heterodimer at all H coverages
using empty state imaging. The ability to identify single P atoms incorporated
into Si(001) will be invaluable in the development of nanoscale electronic
devices based on controlled atomic-scale doping of Si.
