Marcello Luppi

Multiple Si=O bonds at the silicon cluster surface

A first-principle investigation of the effects of multiple Si=O bonds at the surface of silicon-based clusters with different sizes has been carried out. Total-energy pseudopotential calculations within density functional theory have been applied varying systematically the number of Si=O bonds at the clusters surface. A nonlinear reduction of the energy gap with the Si=O bond number is found. A sort of saturation limit is displayed, providing a consistent interpretation of the photoluminescence redshift observed in oxidized porous silicon samples. Moreover, our results help to clarify the very recent findings on the single silicon quantum dot photoluminescence bandwidth.

Gain Theory And Models In Silicon Nanostructures

The main goal in the information technology is to have the possibility of integrating low-dimensional structures showing appropriate optoelectronic properties with the well established and highly advanced silicon microelectronics present technology [1]. Therefore, after the initial impulse given by the work of Canham on visible luminescence from porous Si [5], nanostructured Si has received extensive attention both from experimental and theoretical point of view during the last ten years (for review see Refs. [6]). This activity is mainly centered on the possibility of getting relevant optoelectronic properties from nanocrystalline Si, which in the bulk crystalline form is an indirect band gap semiconductor, with very inefficient light emission in the infrared. Although some controversial interpretations of the visible light emission from low-dimensional Si structures still exist, it is generally accepted that the quantum confinement, caused by the restricted size, and the surface passivation are essential for this phenomenon [12].

From Undulating Si Quantum Wires to Si Quantum Dots: A Model for Porous Silicon

Freshly etched porous silicon shows the structure of a crystalline silicon skeleton with a connected undulating-wire morphology; in aged porous silicon samples the presence of Si dots is predominant. In this paper we present, for the first time, ab-initio results of the electronic and optical properties of undulating Si quantum wires, moreover, the transition from Si quantum wires to Si quantum dots is also discussed

Symmetry and passivation dependence of the optical properties of nanocrystalline silicon structures

Abstract The electronic and optical properties of Si-based quantum wells (QWs) are studied ab initio by means of the linear-muffin-tin-orbital (LMTO) method in order to investigate their dependence on the symmetry of the lattice and on the passivating species that saturates the Si dangling bonds. We find that the symmetry of the lattice changes the nature of the gap that is indirect in the Si–H(111) saturated QWs and becomes direct in the Si–H(001) saturated QWs. The saturating species play instead an important role in the formation of interface states that can occupy or leave free the band gap so improving or making worse the optical properties of the material. Studying the Si–SiO2(001) superlattice we found that oxygen related defects play an important role in the determination of the optoelectronic properties of the material.

Oxygen role on the optoelectronic properties of silicon nanodots

Abstract The optoelectronic properties of Si nanodots have been investigated using ab initio total energy calculations within the density functional theory. Structural relaxations have been considered. We have studied two types of nanodots: isolated clusters covered by H, studying the substitution of SiH bonds with different SiO bonds, and nanocrystals embedded in SiO 2 matrix. In the first case, we find that the optoelectronic properties strongly depend on the type and the number of SiO bonds, especially for the gap value and the arrangement of the energy levels. In the second case, the close interplay between chemical and structural effects is pointed out.

Si nanostructures embedded in SiO2: electronic and optical properties

We present ab initio results for the structural, electronic and optical properties of silicon nanostructures confined by silicon dioxide. We investigate the role of the dimension, symmetry and bonding situations at the interfaces. In particular we consider Si/SiO2 superlattices and Si nanocrystals embedded in SiO2 matrix. In the case of Si/SiO2 superlattices the presence of oxygen defects at the interface and the dimensionality are the key points in order to explain the experimental outcomes concerning photoluminescence. For Si nanocrystals embedded in SiO2 we show, in agreement with experimental results, the close interplay between chemical and structural effects on the electronic and optical properties.

Surface and confinement effects on the optical and structural properties of silicon nanocrystals

In this work we investigate, by first-principles calculations, the structural, electronic and optical properties of: (1) oxygenated silicon-based nanoclusters of different sizes in regime of multiple oxidation at the surface, and (2) hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations within density functional theory. In the first case we have varied systematically the number of Si=O bonds at the cluster surface and found a nonlinear reduction of the energy gap with the Si=O bond number. A saturation limit is reached, which allows us to provide a consistent interpretation of the photoluminescence (PL) redshift observed in oxidized porous silicon samples. Our results help also to explain some very recent findings on the single silicon quantum dot photoluminescence bandwidth. In the second case, after a preliminary study of the clusters stability, the properties of the ground and excited states have been compared varying the cluster dimensions from 1 to 29 Si atoms. Ab-initio calculations of the Stokes shift as a function of the cluster dimension will be presented. A structural model linked to the four level scheme recently invoked to explain the experimental outcomes relative to the observed optical gain in Si-nc embedded in a SiO2 matrix will be also suggested.

Electronic and Optical Properties of Silicon Nanocrystals: Structural Effects

The aim of this work is to investigate the structural, electronic and optical properties of hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations. Recent results about ab-initio calculations of Stokes shift as a function of the cluster dimension and of optical gain will be presented here. A structural model that can be linked to the four level scheme recently invoked to explain the experimental outcomes relative to the observed optical gain in Si-nc embedded in a SiO2 matrix will be suggested too.