Inhomogeneous laser heating and phonon confinement in silicon nanowires: A micro-Raman scattering study

Results of a systematic set of micro-Raman experiments on the changes in the line shape of the ∼520 cm-1 one-phonon band in Si nanowires with laser flux Φ are presented. A complicated dependence of the 520 cm-1 Raman band asymmetry (A) with Φ is observed that depends both on the nanowire diameter and on the thermal anchoring of the wires to an indium foil substrate. With increasing power density in a ∼1 μ focal spot common to micro-Raman spectroscopy, we see a clear growth in A that has nothing to do with phonon confinement. In fact, we can explain the complex changes in A(Φ) by extending the model to include an inhomogeneous heating in the Raman volume. The effects we observe in Si nanowires should be common to all semiconducting nanostructures and underscores the importance of demonstrating a flux-independent line shape when studying pure phonon confinement effects by Raman scattering.