Raman spectrum of group IV nanowires: influence of temperature
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Raman spectrum of group IV nanowires: influence of temperature J.Anaya1, C.Prieto1, J.Souto1, J. Jiménez1, A.Rodríguez2, J.Sangrador2, T.Rodríguez2 Optronlab Group, Dpto. Física de la Materia Condensada, Centro I+D, Univ. de Valladolid, Paseo de Belén 1, 47011 Valladolid, Spain 1
2
Tecnología Electrónica, E.T.S.I.T., Universidad Politécnica de Madrid, 28040 Madrid,
Spain ABSTRACT Group IV semiconductor nanowires are characterized by Raman spectroscopy. The results are analyzed in terms of the heating induced by the laser beam on the nanowires. By solving the heat transport equation one can simulate the temperature reached by the NWs under the exposure to a laser beam. The results are illustrated with Si and Si1-xGex nanowires. Both bundles of nanowires and individual nanowires are studied. The main experimental conditions contributing to the nanowire heating are discussed. INTRODUCTION Nanoscaled semiconductor structures present a high potential for the fabrication of new devices because of their unique optical and electrical properties (1). In particular nanowires (NWs) will be the building blocks for future advanced electronic, and optoelectronic nanodevices (1). A great deal of characterization effort has been devoted to the study of the NWs properties, which present a strong dependence with its dimension. One of the main issues regarding the NWs concerns its thermal management. In fact, NWs are systems with lower thermal conductivity than their bulk counterparts (2), on the other hand they are usually standing in air; therefore, the thermal dissipation is very slow constituting a serious drawback for the practical use of these structures. The study of the thermal behaviour of semiconductor NWs is a crucial issue for the future application of the NWs. In this sense, Raman spectroscopy is a very powerful tool to study the properties of NWs, since it can provide information about phonon confinement associated with the reduced size (3,4), but also it is very sensitive to temperature (5). Both effects, phonon confinement and temperature, need to be separated in order to give a reliable interpretation of the Raman spectrum of NWs. The analysis of the Raman spectrum of nanosized structures is based on the phenomenological confinement model introduced by Ley et al (3), further extended to NWs by Campbell et al (4). In the case of Si NWs there is a great amount of controversy because of the discrepancies between the expected and measured lineshapes of the one phonon bands, which does not permit to provide accurate values for the NWs diameter (6-10). The one phonon Raman lineshape can be contributed by other factors, e.g. dispersion of NW diameters (6), Fano broadening from photogenerated carriers (7), and laser induced heating of the NWs (8-10). Usually, Raman spectra are acquired with an optical microscope attached to the Raman spectrometer, under such conditions the excitation is done with a relative high power laser density, the energy absorbed by the NWs cannot be efficiently dissipated and the NWs can reach eleva
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