First-principle investigation of the electronic structure of Si- based layered structures

The electronic properties of ultra-thin Si layers have been investigated. 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 crystal, a system with interesting analogies to porous silicon. Our results show that the band gap opening is dominated by the valence band which follows the effective mass confinement picture, while the conduction band is found to flatten out and shift very modestly. The latter behaviour is due to hybridization effects between Si and Ca states which lead to bonding-antibonding interface states in the gap. For 1 and 2 double 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.