Electronic and Optical Properties of Si/SiO 2 Superlattices from First Principles: Role of Interfaces.
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Electronic and Optical Properties of Si/SiO2 Superlattices from First Principles : Role of Interfaces. Pierre Carrier1, Gilles Abramovici1,2, Laurent J. Lewis1 and M. W. C. Dharma-wardana3 1 Département de Physique et Groupe de Recherche en Physique et Technologie des Couches Minces (GCM), Université de Montréal, Case Postale 6128, Succursale Centre-Ville, Montréal, Québec, Canada H3C 3J7 2 Laboratoire de Physique des Solides, associé au C.N.R.S., Université Paris Sud, Centre d'Orsay, 91405 Orsay, France 3 Institute for Microstructural Sciences, National Research Council, Ottawa, Canada K1A 0R6 ABSTRACT The observation of intense luminescence in Si/SiO2 superlattices (SLs) has lead to new theoretical research on silicon-based materials. We have performed first-principles calculations using three Si/SiO2 SL models in order to examine the role of interfaces on the electronic structure and optical properties. The first two models are derived directly from crystalline structures and have simple interfaces. These models have been studied using the full-potential, linearized-augmented-plane-wave method, in the local-density-approximation (LDA). The optical absorption within the interband transition theory (excluding excitonic effects) have been deduced. The Si(001)-SiO2 interface structure is shown to affect the optical behaviour. Following these observations, we have considered a more realistic, fully-relaxed model. The projector-augmented-wave method under the LDA is used to perform the structural relaxation as well as band structure and optical calculations. The role of confinement on the energy gap is studied by inserting additional silicon slabs into the supercell. Direct energy gaps are observed and the energy gap is found to decrease with increasing silicon slab thickness, as observed experimentally. The role of the interface has been considered in more details by studying the contribution to the energy gap of Si atoms having different oxidation patterns; partially oxidized Si atoms at the interface, as well as Si atoms inside the Si layer, are shown to contribute to the transitions at the energy gap. INTRODUCTION Lu et al [1], and many others [2] have reported enhanced luminescence in the visible part of the spectrum of Si/SiO2 superlattices (SLs). A shift of the energy spectrum towards the blue is observed when the thickness of the Si layers is reduced from 6 to 2 nm. In contrast to porous silicon [3], Si/SiO2 SLs present the advantage of being stable, with no significant decrease of the luminescence observed with time. From this experiment, it can be inferred that confinement plays a major role on the optical enhancement of SLs. However, confinement cannot explain the whole optical mechanism. It has been observed in experiments by Kanemitsu [4], and from first-principles calculations performed by Kageshima [5], that the Si(001)-SiO2 interface plays as well, a non-negligible role on the optical properties of the SLs. It is worth noticing that Si and SiO2 are both already standard components of MOSFETs and other d
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