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 $\sim 520{\mathrm{cm}}^{-1}$ one-phonon band in Si nanowires with laser flux $\mathrm{\Phi }$ are presented. A complicated dependence of the $520{\mathrm{cm}}^{-1}$ Raman band asymmetry $\left(A\right)$ with $\mathrm{\Phi }$ 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 $\sim 1\mathrm{\mu }$ 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 $A\left(\mathrm{\Phi }\right)$ by extending the model [H. Richter, Z. P. Wang, and Y. Ley, Solid State Commun. 39, 625 (1981)] 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.