  We investigate the band structure and topological phases of silicene embedded
on halogenated Si(111) surface, by virtue of density functional theory and
tight-binding calculations.Our results show that the Dirac character of low
energy excitations in silicene is almost preserved in the presence of silicon
substrate passivated by various halogens. Nevertheless, the combined effects of
charge transfer into the substrate, stretching of bonds between silicon atoms,
and symmetry breaking which originates from van der Waals interaction, result
in a gap $E_{g1}$ in the spectrum of the embedded silicene. We further take the
spin-orbit interaction into account and obtain its strength and the resulting
enhancement in the gap $E_{g2}=2\lambda$. Both $E_{g1}$ and $E_{g2}$ which
contribute to the total gap, vary significantly when different halogen atoms
are used for the passivation of the Si surface and for the case of iodine, they
have very large values of $70$ and $23$ meV, respectively. To examine the
topological properties, we calculate the projected band structure of silicene
from which the tight binding parameters of the low-energy effective Hamiltonian
are obtained by fitting. Our results based on Berry curvature and
$\mathbb{Z}_2$ invariant reveals that silicene on halogenated Si substrates has
a topological insulating state which can survive even at room temperature for
the substrate with iodine and bromine at the surface. Similar to the free
standing silicene, by applying a perpendicular electric field and at a certain
critical value which again depends on the type of halogens, the gap closes and
silicene undergoes a transition to a trivial insulating state. As a key
finding, we see that the presence of halogenated substrate except for the case
of fluorine enhances the robustness of the topological phases against the
vertical electric field and most probably other external perturbations.
