Engineering Quantum Confined Silicon Nanostructures: Ab-Initio Study of the Structural, Electronic and Optical Properties

Silicon nanostructures, in the form of nanodots, nanowires and nanoslabs have attracted a lot of interest in recent years. Quantum confinement effects play an important role with respect to both the electronic and optical properties. We review and summarize here our results concerning the properties of semiconductor nanocomplexes. Total energy calculations within the density functional theory have been carried out in order to investigate the structural, electronic and optical properties of undoped and doped Si nanosystems of different dimensionality, size and surface termination. Single-particle and Many- Body perturbation theory calculations have been carried out in order to study the optical properties, both in ground and excited state configuration, of Si nanodots. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order both, to understand the physical properties of these systems, and to provide an explanation for both the Stokes shift and the near-visible PL experimentally observed. For Si nanocrystals embedded in a SiO2 matrix, the strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in good agreement with the experimental results. Concerning the doping, we consider B and P single- and co-doped Si nanoclusters. The neutral impurities formation energies are calculated and their dependence on the impurity position within the nanocrystal is discussed. In the case of co-doping the formation energy is strongly reduced, favoring this process with respect to the single doping. Moreover the band gap and the optical threshold are clearly red-shifted with respect to that of the pure crystals, showing the possibility of an impurity based engineering of the absorption and luminescence properties of Si nanocrystals. We also discuss here the case of multiple doping. In the case of one-dimensional systems we have calculated the structural, electronic and optical properties of hydrogenated Ge and Si nanowires of different sizes and different spatial orientations. We have analyzed how the geometrical relaxation affects the optoelectronic properties. Moreover for the smallest structures, we have calculated the electronic and optical properties overcoming the one-particle approach. Large self-energy corrections, compared to the bulk ones, have been found together with strong excitonic effects. In particular in the case of Si nanowires the calculated electronic and optical gaps compare well with the available experimental data. Indeed we show that freshly etched porous Si is better described as a distribution of interacting nanowires than as an ensemble of isolated nanoparticles. Concerning the two-dimensional nanoslab systems we show how the calculated optoelectronic properties of Si superlattices and multiple quantum wells, where CaF2 and SiO2 are the barrier mediums, are in good agreement with the experimental outcomes and we discuss the comparison between Si and Ge nanofilms. © 2009, Elsevier Ltd. All rights reserved.