Dependence of the Raman Spectrum of Silicon Nanowires on the Wire Environment
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0963-Q16-03
Dependence of the Raman Spectrum of Silicon Nanowires on the Wire Environment H. Scheel1, S. Reich2, and C. Thomsen1 1 Inst. f¸r Festkˆrperphysik, Technische Universit‰t Berlin, Hardenbergstrasse 36, Berlin, 10623, Germany 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139-4307
ABSTRACT The large surface to volume ratio in nanometer sized wire structures causes a strong dependence of the optical Raman mode on the thermal conductivity of a surrounding medium. On the basis of optical measurements on silicon nanowires as a function of excitation laser power we explain the very large red-shifted Raman spectra observed already for moderate laser powers. This thermal effect is enhanced by a silicon oxide sheath, rendering a reduced thermal contact of the wires to the substrate. The intrinsic red shift due to spatial confinement in silicon nanowires is found to be smaller than 2 cm-1. INTRODUCTION Semiconductor structures with dimensions of only a few nanometers have attracted continued attention. Nanowells, wires or dots show very unique low dimensional properties, leading to enhanced device properties. The research on carbon nanostructures generated an enormous variety of publications in the last decade.[1] The basic material of present information technology, however, is still silicon. While silicon structures have been intensively investigated for some time, silicon nanowires (SiNWs) are still only partially understood, facing a lack of standard, non-destructive methods for the characterization. Here we report on the large Raman shift and broadening of the LO vibrational mode, which has been attributed to quantum confinement, laser heating, or Fano interference. [2][3] Our detailed analysis of the Raman spectrum of SiNWs and its dependence on the laser power used for excitation shows a remarkable connection to the thermal conductivity of the ambient gas surrounding the SiNW sample. Conversely we are able to determine the surrounding gas by the amount of the Raman red-shift for a particular incident laser power. EXPERIMENT The SiNWs used for this study were produced by vapor transport growth,[4][5] which allows high-yield SiNWs production. Si/SiO2 powders are evaporated at ~1200∞ C in a horizontal tube furnace. The Si vapor then condenses at ~ 800∞C on a substrate (average production ~10 mg per run). The average wire diameter is 15 nm, consisting of an outer SiO2 shell of 2-3 nm, and a crystalline core. [5][6] For Raman measurements the SiNWs are sonicated in isopropanol and dispersed on Ag coated Cu substrates, establishing a small number (2-5) of SiNW-spots. Each sample-spot is one to two
millimeters in diameter and approximately 20-50 µm thick, as observed by atomic force microscopy. Within the SiNW spot the wires are randomly arranged, this can be clearly seen in Fig.1a). The Scanning electron micro graph shows wires, several microns in length. The wires are in contact to other wires, but well separated from the substrate. We analyzed our sa
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