THE ELECTRONIC-ENERGY LEVELS OF SI-BASED NANOCRYSTALLINE MATERIALS - THEORY COMPARED WITH EXPERIMENT

Since the discovery of the efficient luminescence of porous Si, a lot of effort has been devoted to the understanding of this phenomenon. Re cently other Si-based luminescent materials have been synthesised, suc h as ''nanocrystalline'' Si/CaF2 multi-quantum wells (MQWs). A common feature of these nanocrystalline materials is that they contain small Si grains, passivated by hydrogen and/or oxygen. X-ray absorption meas urements have recently suggested that the dimensions of the grains in the Si/CaF2 MQWs are smaller than 15 Angstrom. We have calculated, by linear combination of atomic orbitals, the electronic structure of dif ferent types of nanostructures, in order to compare our theoretical re sults to optical experiments performed on the MQWs, but also on porous Si. Here we present results on spherical Si clusters, Si (111) layers , and ''nanocrystalline'' layers, i.e. Si (111) layers divided into sm all grains. These nanocrystalline layers should model at least qualita tively the Si wells in the Si/CaF2 MQWs. The main result of our calcul ations is that the interaction between the wave functions of electrons belonging to adjacent grains in nanocrystalline materials decreases t he band gap considerably, and has therefore to be included in the calc ulations. (C) Elsevier Science S.A.