The electronic properties of ultra-thin Si layers have been investigat
ed. Using the linear muffin tin orbital (LMTO) method, we have studied
band gap and interface states for supercells formed by Si(111) layers
of variable thickness (1-7 double layers) embedded in a CaF2 host cry
stal, a system with interesting analogies to porous silicon. Our resul
ts show that the band gap opening is dominated by the valence band whi
ch follows the effective mass confinement picture, while the conductio
n band is found to flatten out and shift very modestly. The latter beh
aviour is due to hybridization effects between Si and Ca states which
lead to bonding-antibonding interface states in the gap. For 1 and 2 d
ouble layers the bonding Si-Ca states emerge from the Si valence band
and lead to an almost direct gap at finite wavevectors. This behaviour
could account for efficient visible luminescence in this system. The
relevance of these results for Si-based low-dimensional structures is
discussed.