Electronic and Optical Properties of Silicon Nanocrystals: Structural Effects
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Electronic and Optical Properties of Silicon Nanocrystals: Structural Effects E. Degolia, S. Ossicinia, M. Luppib, E. Luppib, R. Magrib, G. Cantelec, D. Ninnoc INFM- Research Center for nanoStructures and bioSystems at Surfaces(S 3) Dipartimento di Scienze e Metodi dell’Ingegneria, Università di Modena e Reggio Emilia, via Allegri 13, 42100 Reggio Emilia, Italy b INFM-Research Center for nanoStructures and bioSystems at Surfaces(S 3) Dipartimento di Fisica, Università di Modena e Reggio Emilia, via Campi 213/A, 41100 Modena, Italy c INFM and Universita' di Napoli "Federico II" - Dipartimento di Scienze Fisiche, Compl. Univ. M. S. Angelo, Via Cintia, I-80126 Napoli, Italy a
ABSTRACT 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. INTRODUCTION The main goal in the information technology is to have the possibility of integrating lowdimensional structures showing appropriate optoelectronic properties with the well established and highly advanced silicon microelectronics technology. Therefore, after the initial impulse given by the work of Canham on visible luminescence from porous Si, nanostructured Si has received extensive attention both from experimental and theoretical point of view during the last years (for review see Refs. [1]). 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. Understanding and improving the properties of these novel semiconductor materials requires to learn more about the interplay between their structural, electronic and optical properties. 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, is essential for this phenomenon [1] but actually not sufficient; other mechanisms have to act in order to explain the new physical effects that dominate the optoelectronic properties of these nanometric structures. Moreover optical gain has been recently observed in ion implanted Si-nc [2] and in Si-nc formed by plasma enhanced chemical vapour deposition and annealing treatments [3]. An effective rate equation model based on a four level system has been proposed to explain the experimental findings on these systems [3,4]. Through the theoretical r
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