Composition and Size Effects on the Optical Properties of Isolated Silicon-Germanium Nanowires
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Composition and Size Effects on the Optical Properties of Isolated Silicon-Germanium Nanowires Houssem Kallel 1,2,3 , Abdallah Chehaidar 3, Arnaud Arbouet 1,2 , Thierry Baron4, Alexis Potié4, Bassem Salem4, and Vincent Paillard 1,2 1 Université Paul Sabatier, Université de Toulouse, 118 route de Narbonne, 31062 Toulouse cedex 9, France. 2 CEMES-CNRS, 29 rue Jeanne Marvig, 31055 Toulouse Cedex 4, France. 3 Département de Physique, Faculté des Sciences de Sfax, Université de Sfax, B.P. 1171, 3000 Sfax, Tunisie. 4 LTM, CNRS-CEA-LETI, 17 rue des Martyrs, 38054 Grenoble cedex 9, France.
ABSTRACT Silicon and Germanium nanowires (NWs) have shown a strong ability to enhance both the absorption and scattering of light. Tailoring the optical properties of Si or Ge NWs can be obtained by adjusting the nanowire diameter. Another parameter that can be used is the chemical composition of silicon-germanium (Si1-xGex-NWs) alloys. In this work, we perform a numerical study on the optical properties of single Si1-xGex-NWs based on the Lorenz-Mie theory. The effects of Ge composition, light polarization and angle of incidence on the nanowire optical properties are investigated. INTRODUCTION Semiconductor nanowires are promising novel materials for the next-generation of nanoelectronics CMOS transistors, as well as for photovoltaic devices such as photodetectors and solar cells. Among all semiconducting nanostructures, silicon nanowires are nowadays the most studied because of the supremacy of silicon in CMOS technology or low cost solar cells [1-3]. Compared to their bulk counterparts, they show an elastic light scattering enhancement, Raman efficiency enhancement, or increased photoconductivity adjustable by tuning of nanowire diameter. These phenomena are due to morphology-dependent resonances so-called Mie resonances [4-10]. Some theoretical and experimental studies of the optical properties have been performed on pure silicon [4-7] or germanium nanowires [8-10]. The results showed a branched enhancement of absorption and/or scattering resulting from multiple wave interferences inside nanowires of diameters above a given wavelength threshold, and an apparently conventional behavior dominated by direct electronic transitions below this threshold. For pure Si-NWs or Ge-NWs, the resonance phenomena, leading to electric-field enhancement, occur in a limited spectral region. We propose in this paper, a theoretical study on the optical properties of individual Si1-xGex NWs, which offer the chemical composition x as supplementary tunable parameter. Our calculations show that this parameter leads to high efficiencies over the whole wavelength range of sunlight from near ultraviolet to near infrared.
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THEORY We used the Lorenz-Mie theory [11] applied to the geometry of a nanowire which is described as an infinitely long cylinder of radius a, immersed in vacuum, as illustrated schematically in Fig. 1. The plane of incidence is defined by the incident wave vector and the nanowire axis, and is the incidence angle.
Figure 1- Schematic geo
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