CONFINEMENT, SURFACE, AND CHEMISORPTION EFFECTS ON THE OPTICAL-PROPERTIES OF SI QUANTUM WIRES

We have used the empirical pseudopotential method to study the electro nic and optical properties of [001] Si quantum wires with (110)-(($) o ver bar 110) square cross sections ranging from 4x4 to 14x14 monolayer s (7.7x7.7 to 26.9x26.9 Angstrom, respectively). We present energy lev els, band gaps, oscillator-strength, and charge-density distributions. To understand-the electronic structure of these systems we calculate their properties in a stepwise process, considering (1) wires with a f ree surface but without hydrogen and (2) wires with hydrogen chemisorp tion on the surface. We find that (i) in both cases, the band gap betw een bulklike states increases as the wire size is;reduced (due to quan tum confinement). However,(ii) hydrogen chemisorption acts to reduce t he gap. (iii) Whereas the low-energy states near the valence-band mini mum are effective-mass-like, the near-band-gap states with or without H on the surface can be decisively non-effective-mass-like. The lowest conduction states are pseudodirect, not direct. (iv) The calculated e nergy dependence of the transition lifetimes is too strong to explain the observed low-energy ''slow'' emission band in porous Si purely in terms of transitions in an ideal wire. However, an alternative model, which introduces a mixture of wires and boxes, can account for the exp erimental slope.