  Doping of Si using the scanning probe hydrogen depassivation lithography
technique has been shown to enable placing and positioning small numbers of P
atoms with nanometer accuracy. Several groups have now used this capability to
build devices that exhibit desired quantum behavior determined by their
atomistic details. What remains elusive, however, is the ability to control the
precise number of atoms placed at a chosen site with 100% yield, thereby
limiting the complexity and degree of perfection achievable. As an important
step towards precise control of dopant number, we explore the adsorption of the
P precursor molecule, phosphine, into atomically perfect dangling bond patches
of intentionally varied size consisting of 3 adjacent Si dimers along a dimer
row, 2 adjacent dimers, and 1 single dimer. Using low temperature scanning
tunneling microscopy, we identify the adsorption products by generating and
comparing to a catalog of simulated images, explore atomic manipulation after
adsorption in select cases, and follow up with incorporation of P into the
substrate. For 1-dimer patches we demonstrate that manipulation of the adsorbed
species leads to single P incorporation in 12 out of 12 attempts. Based on the
observations made in this study, we propose this 1-dimer patch method as a
robust approach that can be used to fabricate devices where it is ensured that
each site of interest has exactly one P atom.
