  The homoepitaxial growth of Si on Si(100) covered by a resist mask is a
necessary technological step for the fabrication of donor-based quantum devices
with scanning tunneling microscope lithography. In the present work, the
chlorine monolayer is selected as the resist. Using density functional theory,
we investigated the adsorption of a single silicon atom on
Si(100)-2$\times$1-Cl as the starting process of Si epitaxy. The incorporation
of a silicon atom under a Cl monolayer proved to be the most energetically
favorable process. Our results show that chlorine segregates on the surface
during Si deposition and does not incorporate into homoepitaxial layers. In
addition, we found that SiCl$_2^{\ast}$, SiCl$_3^{\ast}$, and SiCl$_4^{\ast}$
clusters can be formed above a Si(100)-2$\times$1-Cl surface while Si is
adsorbed. SiCl$_2^{\ast}$ clusters are bound weakly to the substrate, and their
desorption leaves the silicon surface free of chlorine. To check whether the Si
epitaxy is possible on the chlorine resist, we compare our results with the
well-studied case of a hydrogen resist. We find the two processes to be
similar; moreover, epitaxy on chlorine resist appears to have an advantage.
