Raman Spectroscopy of Group IV Nanostructured Semiconductors: Influence of Size and Temperature
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Raman Spectroscopy of Group IV Nanostructured Semiconductors: Influence of Size and Temperature A. Torres1, O. Martínez1, C. Prieto1, J. Jiménez1, A. Rodríguez2, J. Sangrador2, T. Rodríguez2 1 GdS Optronlab, Edificio I+D, Universidad de Valladolid, Paseo de Belén, 1, 47011 Valladolid, Spain 2 Departamento de Tecnología Electrónica, E.T.S.I. de Telecomunicación, Universidad Politécnica de Madrid, 28040 Madrid, Spain. ABSTRACT Group IV nanostructures have attracted a great deal of attention because of their potential applications in optoelectronics and nanodevices. Raman spectroscopy has been extensively used to characterize nanostructures since it provides non destructive information about their size, by the adequate modeling of the phonon confinement effect. However, the Raman spectrum is also sensitive to other factors, as stress and temperature, which can mix with the size effects borrowing the interpretation of the Raman spectrum. We present herein an analysis of the Raman spectra obtained for Si nanowires; the influence of the excitation conditions and the heat dissipation media are discussed in order to optimize the experimental conditions for reliable spectra acquisition and interpretation. INTRODUCTION The potential applications of nanoscaled semiconductor structures have raised a great deal of characterization effort. In particular, nanosized structures of group IV semiconductors have been the object of many studies. The properties of those nanostructures depend on their nanostructure and dimension; therefore, a great effort is paid to investigate simple ways to determine their dimension. Raman spectroscopy is a very powerful non destructive tool suitable to such study, because of the spectral changes related to the phonon confinement in reduced size structures [1-8]. The study of the lineshape of the first order phonon lines of the Raman spectrum may supply valuable information about the nanostructure size. The Raman spectrum is also sensitive to temperature and stress [9]; therefore, the Raman study can provide information about the thermal properties, such as the thermal expansion and the anharmonicity, which are crucial to model the heat dissipation and the thermal induced stresses, which are critical issues to design devices with optimal mechanical and heat transport properties. As a consequence of the break down of the translational symmetry for reduced dimensions, the phonon correlation length becomes finite and the momentum selection rule is relaxed allowing scattering by phonons out of the zone center [1, 2]. The result of this is a softening and asymmetric broadening of the phonon modes. However, other factors induce similar changes in the Raman spectrum, which might mask the assessment of the nanostructure features. The phonon confinement effect can also occur in the presence of extended defects that breakdown the translational symmetry; in other words, the Raman spectra of structures with defects can induce erroneous estimations of sizes. Moreover, temperature plays a relevant role
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