Raman-active phonon line profiles in semiconducting nanowires (original) (raw)
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Raman spectrum of Si nanowires: temperature and phonon confinement effects
Applied Physics A, 2013
The Raman spectrum of Si nanowires (NWs) is a matter of controversy. Usually, the one-phonon band appears broadened and shifted. This behaviour is interpreted in terms of phonon confinement; however, similar effects are observed for NWs with dimensions for which phonon confinement does not play any relevant role. In this context, the temperature increase induced by the laser beam is recognized to play a capital role in the shape of the spectrum. The analysis of the Raman spectrum, under the influence of the heating induced by the laser beam, is strongly dependent on the excitation conditions and the properties of the NWs. We present herein an analysis of the Raman spectrum of Si NWs based on a study of the interaction between the laser beam and the NWs, for both ensembles of NWs and individual NWs, taking account of the temperature increase in the NWs under the focused laser beam and the dimensions of the NWs.
Applied Physics A, 2006
Raman scattering is shown to be an effective probe of optical and surface optical phonons in highly crystalline semiconducting nanowires (SNWs). We show that the confinement model of Richter et al. well describes the nanowire diameter dependence of the asymmetric broadening of the onephonon band in Si nanowires observed at ∼ 520 cm −1. We also show that the use of high laser flux (∼ 0.1 mW/µm 2) leads to a second mechanism that can asymmetrically broaden the 520 cm −1 Raman band. This broadening has nothing to do with confinement, and can qualitatively be understood in terms of inhomogeneous laser heating. A model is presented that supports this explanation. The production of SNWs via the vapor-liquidsolid growth mechanism leads, in many cases, to an instability in the nanowire diameter or cross-sectional area. In the second part of this review, we show that this instability activates the surface optical (SO) phonon Raman scattering. Examples of this phenomenon are shown for GaP and ZnS nanowires. The former and latter have, respectively, cylindrical and rectangular cross sections. We show that the cross-sectional shape of the nanowire is important for a quantitative analysis of these SO modes.
Size dependence of Raman line-shape parameters due to confined phonons in silicon nanowires
Advances in Materials and Processing Technologies, 2020
A comparison of experimentally observed Raman scattering data with Raman line-shapes, generated theoretically using phonon confinement model, has been carried out to understand the sensitivity of different Raman spectral parameters on quantum confinement effect. Size dependent variations of full width at half maximum (FWHM), Raman peak position and asymmetry ratio have been analyzed to establish the sensitivity of their corresponding physical counterparts (phonon life time and dispersion) in confined systems. The comparison has been done in three different confinement regimes namely, weakly, moderately and strongly. Proper reasoning has been assigned for such a variation after validation of the theoretical analysis with the experimental observations. A moderately confined system was created by preparing 6 nm sized Si NSs using metal induced etching. An asymmetrically broadened and red-shifted Raman line-shape was observed which established that all the parameters get affected in moderately confined system. Sensitivity of a given Raman spectral parameters has been shown to be used as a tool to understand the role of external perturbations in a material.
Micro-Raman spectroscopy of Si nanowires: Influence of diameter and temperature
Applied Physics Letters, 2010
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 ͑NWs͒. 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.
Probing Phonons in Nonpolar Semiconducting Nanowires with Raman Spectroscopy
Journal of Nanotechnology, 2012
We present recent developments in Raman probe of confined optical and acoustic phonons in nonpolar semiconducting nanowires, with emphasis on Si and Ge. First, a review of the theoretical spatial correlation phenomenological model widely used to explain the downshift and asymmetric broadening to lower energies observed in the Raman profile is given. Second, we discuss the influence of local inhomogeneous laser heating and its interplay with phonon confinement on Si and Ge Raman line shape. Finally, acoustic phonon confinement, its effect on thermal conductivity, and factors that lead to phonon damping are discussed in light of their broad implications on nanodevice fabrication.
Temperature dependence of Raman spectra for individual silicon nanowires
Physical Review B, 2009
The temperature dependence of the Stokes first-order optical phonon frequency has been measured for individual silicon nanowires with diameters between 33 to 180 nm in the temperature range of 20 to 300°C. The nanowires were synthesized via both the vapor-liquid-solid method and electrochemical etching of bulk silicon. Significant laser-induced local heating was avoided by using a laser power of 0.5 mW or less, corresponding to fluxes of Յ0.7 mW/ m 2. For both types of nanowires the slope of Raman frequency vs temperature closely matches the value of bulk Si, ͑ d dT ͒ = −0.022Ϯ .001 cm −1°C−1 , across the entire diameter range, indicating no change in lattice anharmonicity. These results have important implications for understanding nanowire lattice thermal conductivity and extending the domain for Raman thermometry of silicon nanostructures.
Journal of Applied Physics, 2011
Engineering of the cross-section shape and size of ultra-scaled Si nanowires (SiNWs) provides an attractive way for tuning their structural properties. The acoustic and optical phonon shifts of the free-standing circular, hexagonal, square and triangular SiNWs are calculated using a Modified Valence Force Field (MVFF) model. The acoustic phonon blue shift (acoustic hardening) and the optical phonon red shift (optical softening) show a strong dependence on the cross-section shape and size of the SiNWs. The triangular SiNWs have the least structural symmetry as revealed by the splitting of the degenerate flexural phonon modes and The show the minimum acoustic hardening and the maximum optical hardening. The acoustic hardening, in all SiNWs, is attributed to the decreasing difference in the vibrational energy distribution between the inner and the surface atoms with decreasing cross-section size. The optical softening is attributed to the reduced phonon group velocity and the localization of the vibrational energy density on the inner atoms. While the acoustic phonon shift shows a strong wire orientation dependence, the optical phonon softening is independent of wire orientation.
Journal of Raman Spectroscopy, 2015
Arrays of single crystalline Si nanowires (NWs) decorated with arbitrary shaped Si nanocrystals (NCs) are grown by a metal assisted chemical etching (MACE) process using silver (Ag) as the noble metal catalyst. The MACE grown Si NWs exhibit strong photoluminescence (PL) emission in the visible and near infra-red region at room temperature. Quantum confinement (QC) of carriers in the Si NCs is believed to be primarily responsible for the observed PL emission. Raman spectra of the Si NCs decorated on Si NWs exhibit a red shift and an asymmetric broadening of first order Raman peak as well as the other multi phonon modes when compared with that of the bulk Si. Quantitative analysis of confinement of phonons in the Si NCs is shown to account for the measured Raman peak shift and asymmetric broadening. To eliminate the laser heating effect on the phonon modes of the Si NWs/NCs, the Raman measurement were performed at extremely low laser power.
Lineshape analysis of Raman scattering from LO and SO phonons in III-V nanowires
Journal of Applied Physics, 2009
Micro-Raman spectroscopy is employed to study the phonon confinement in Au-and Mn-catalyzed GaAs and InAs nanowires. The phonon confinement model is used to fit the LO phonon peaks, which also takes into account the contribution to the asymmetry of the line shape due to the presence of surface optical ͑SO͒ phonons and structural defects. This also allows us to determine the correlation lengths in these wires, that is the average distance between defects and the defect density in these nanowires. Influence of these defects on the SO phonon is also investigated. A good agreement between the experimental results and the calculations for the SO phonon mode by using the dielectric continuum model is also obtained.