Micro-Raman spectroscopy of Si nanowires: Influence of diameter and temperature

Abstract

Raman spectroscopy provides nondestructive information about nanoscaled semiconductors by modeling the phonon confinement effect. However, the Raman spectrum is also sensitive to the temperature, which can mix with the size effects borrowing the interpretation of the Raman spectrum. We present an analysis of the Raman spectra of Si nanowires (NWs). The influence of the excitation conditions and the temperature increase in the NWs are discussed. The interpretation of the data is supported by the calculation of the temperature inside the NWs with different diameters.

Raman spectrum of group IV nanowires: influence of temperature

“…In previous reports was demonstrated that heating induced by the Raman beam has a relevant contribution to the Raman spectrum of NWs (8)(9)(10). The Raman spectrum can be acquired over a bundle of NWs, or can be acquired on individual NWs conveniently separated from the substrate, deposited and localized in the grid holder in the scanning electron microscope (SEM).…”

“…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)(7)(8)(9)(10). The one phonon Raman lineshape can be contributed by other factors, e.g.…”

“…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)(9)(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 elevated temperatures that broaden and downshift the one phonon Raman bands, inducing misinterpretation of the Raman spectrum if one only considers the phonon confinement.…”

Si and SixGe1‐x NWs studied by Raman spectroscopy

“…1 has been adjusted considering four NWs with diameters of 20, 30, 40 and 100 um (see the inset in Fig. 1), which according to our calculations reach temperatures of 400, 325, 250 and 150°C, respectively, obtaining a reasonable fitting of the experimental data [12].…”

Laser Induced Heating of Group IV Nanowires

“…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 contact less 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).…”