  Diborane (B$_2$H$_6$) is a promising molecular precursor for atomic precision
p-type doping of silicon that has recently been experimentally demonstrated [T.
{\v{S}}kere{\v{n}}, \textit{et al.,} Nature Electronics (2020)]. We use density
functional theory (DFT) calculations to determine the reaction pathway for
diborane dissociating into a species that will incorporate as electrically
active substitutional boron after adsorbing onto the Si(100)-2$\times$1
surface. Our calculations indicate that diborane must overcome an energy
barrier to adsorb, explaining the experimentally observed low sticking
coefficient ($< 10^{-4}$ at room temperature) and suggesting that heating can
be used to increase the adsorption rate. Upon sticking, diborane has an $\sim
50\%$ chance of splitting into two BH$_3$ fragments versus merely losing
hydrogen to form a dimer such as B$_2$H$_4$. As boron dimers are likely
electrically inactive, whether this latter reaction occurs is shown to be
predictive of the incorporation rate. The dissociation process proceeds with
significant energy barriers, necessitating the use of high temperatures for
incorporation. Using the barriers calculated from DFT, we parameterize a
Kinetic Monte Carlo model that predicts the incorporation statistics of boron
as a function of the initial depassivation geometry, dose, and anneal
temperature. Our results suggest that the dimer nature of diborane inherently
limits its doping density as an acceptor precursor, and furthermore that
heating the boron dimers to split before exposure to silicon can lead to poor
selectivity on hydrogen and halogen resists. This suggests that while diborane
works as an atomic precision acceptor precursor, other non-dimerized acceptor
precursors may lead to higher incorporation rates at lower temperatures.
