GAP OPENING IN ULTRATHIN SI LAYERS - ROLE OF CONFINED AND INTERFACE STATES

We present first principle calculations of ultrathin silicon (111) lay ers embedded in CaF2, a lattice matched insulator. Our all electron ca lculation allows a check of the quantum confinement hypothesis for the Si band gap opening as a function of thickness. We find that the gap opening is mostly due to the valence band while the lowest conduction band states shift very modestly due to their pronounced interface char acter. The latter states are very sensitive to the sample design. We s uggest that a quasidirect band gap can be achieved by stacking Si laye rs of different thickness.