Laser Induced Heating of Group IV Nanowires
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Laser Induced Heating of Group IV Nanowires J.Anaya1, A. Torres1, A.Martin-Martín1, J.Jiménez1, A.Rodríguez2, T.Rodríguez2 1GdS Optronlab, Ed I+D, Universidad de Valladolid, P. de Belén, 1, 47011 Valladolid, Spain 2Tecnología Electrónica, ETSIT, Universidad Politécnica de Madrid, 28040 Madrid, Spain. ABSTRACT Semiconductor nanowires (NWs) are fundamental structures for nanoscale devices. The excitation of NWs with laser beams results in thermal effects that can substantially change the spectral shape of the spectroscopic data. In particular, the interpretation of the Raman spectrum is greatly influenced by excitation induced temperature. A study of the interaction of the NWs with the excitation laser beam is essential to interpret the spectra. We present herein a finite element analysis of the interaction between the laser beam and the NWs. The resultas are applied to the interpretation of the Raman spectrum of bundles of NWs. INTRODUCTION Semiconductor NWs are promising structures for advanced nanodevices, due to their unique optical and electrical properties [1]. One of the main issues regarding these nanostructures is the thermal transport. In spite of the lower power supply required for driving the nanodevices the heat per unit area dissipated is still very high. Optical characterization of semiconductors, and in particular of NWs, is necessary for the understanding the properties of the NWs, in order to develop performing and reliable devices. The optical characterization is usually carried out under excitation with laser beams focused through microscope lenses, e.g. microRaman (μ-R) spectroscopy, and microphotoluminescence (μ-PL). The NW diameters range from a few nm to a hundred nms; these sizes are below the diffraction limit; therefore, one does not know the exact position of the NWs inside the laser beam, which is not uniform but has a gaussian power distribution. Generally, the measurements are carried out with bundles of NWs, which can have different dimensions; therefore, the spectroscopic analysis is done on the basis of averaging over a certain number of NWs, or using complex size distribution functions (2,3). Therefore, the excitation conditions are uncertain in both Raman and PL measurements, because of the non ability to localize the NWs with respect to the laser beam. On the other hand, NWs are systems with reduced thermal conductivity, orders of magnitude below the bulk thermal conductivity (4,5); furthermore, they stand in air, which makes very difficult the evacuation of heat, which is extensible to devices under operation. NWs under the excitation with a focused laser beam can reach non negligible temperatures, which is crucial for interpreting the Raman and PL spectra (6, 7). These techniques can be used as contactless probes for measuring the thermal thermal conductivity of NWs; X.F.Liu et al measured the thermal conductivity of CdS NWs using microPL spectroscopy (8), and M. Soini et al. measured the thermal conductivity of GaAs NWs using micro-Raman spectroscopy (9). Therefore, the study of
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