Ab initio study of quantum confined unpassivated ultrathin Si films

Abstract

An ab initio study of the electronic structure of the one-dimensional quantum-confined crystalline ultrathin Si (100) films both with the unreconstructed p(1×1) and the reconstructed c(2×2) film surfaces in a large thickness range of 2.7–29.7 Å has been made after employing a self-consistent full potential linear muffin tin orbital method along with the density functional theory in local approximation. A relatively large unit cell of 28 layers has been employed. In the case of the unreconstructed p(1×1) surface, the dangling bond states fill the fundamental energy gap completely and these states are weakly localized, i.e., the wave functions of these states die out in the inner region of the film slowly. After the c(2×2) surface reconstruction, the bonding and the antibonding dangling bond states are seen to lie near the valence band maximum and the conduction band minimum in the fundamental energy gap, respectively, and are again weakly localized. There remain dangling bond states within the gap after reconstruction. The separation between the two surface Si atoms in a dimer shows quite an appreciable change of about 36–40%. The band gaps of the various films remain practically unchanged after surface reconstruction; the changes lie well within 6%. The present results give convincing theoretical evidence for the occurrence of the weakly localized dangling bond states and the absence of too much localized states in quantum-confined systems, which may be responsible for the occurrence of luminescence observed in a comparatively low energy region in the quantum-confined nanostructures, e.g., in the infrared region in porous silicon. The presently predicted exponential rise of the band gap with the decrease in the quantum-confined size in the ultrathin film region is in sharp contrast to an approximate linear rise in the band gap observed in the spectroellipsometric measurements.