Raman Spectroscopy in Group IV Nanowires and Nanowire Axial Heterostructures
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Raman Spectroscopy in Group IV Nanowires and Nanowire Axial Heterostructures J. Anaya1, A. Torres1, J. Jiménez1, A. Rodríguez2, T. Rodríguez2, C. Ballesteros3 Optronlab Group, Departamento de Física de la Materia Condensada, Centro I+D, Universidad de Valladolid, Paseo de Belén 1, 47011 Valladolid, Spain 2 Departamento de Tecnología Electrónica, E.T.S.I. de Telecomunicación, Universidad Politécnica de Madrid, 28040 Madrid, Spain 3 Departamento de Física, E.P.S., Universidad Carlos III, 28911 Leganés (Madrid), Spain
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ABSTRACT The control of the SiGe NW composition is fundamental for the fabrication of high quality heterostructures. Raman spectroscopy has been used to analyse the composition of SiGe alloys. We present a study of the Raman spectrum of SiGe nanowires and SiGe/Si heterostructures. The inhomogeneity of the Ge composition deduced from the Raman spectrum is explained by the existence of a Ge-rich outer shell and by the interaction of the NW with the electromagnetic field associated with the laser beam. INTRODUCTION Semiconductor NWs are very promising low dimensional structures due to their unique electronic and optical properties, which make them suitable for a new generation of devices, as nanotransistors, sensors, photovoltaic cells [1], thermoelectric converters [2], and ligth emitting devices (LEDs) [3], among others. The group IV semiconductor NWs are the most intensively studied because of their full compatibility with complementary metal oxide semiconductor (CMOS) technology. The growth of alloyed NWs is receiving increasing attention, because of the continuous tunability of the physical properties by the intrinsic effect of the alloy composition, which opens the possibility of fabricating a wide range of heterostructured NWs, necessary for the design of advanced devices. In particular, SiGe alloys present a huge interest, because of the excellent properties of this alloy for high frequency devices, as well as the possibilities offered in the modulation of the band gap, and the electric and thermal transports. The growth of SiGe NWs is more challenging than that of single NWs, either Si or Ge, because the solubilities of Si and Ge in the metal catalysts are different, which can introduce instabilities that affect the shape and composition of the NWs [4]. The control of the NW composition is fundamental for the fabrication of high quality heterostructured NWs. Therefore, the assessement of the composition and homogeneity of SiGe Nws is a crucial issue for making high quality heterostructured NWs. Raman scattering has been frequently used as a characterization tool for alloyed semiconductors [5]. However, in the case of NWs, the measurement of the Raman spectrum is challenging because of the low efficiency of the Raman scattering, which results in very weak signals for small sampling volumes; for this reason, bundles of SiGe NWs have been analyzed by other authors [6]. However, that study averages the composition over several NWs, therefore, the study of individual NWs is more suitable for the asse
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