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

Abstract

Results of a systematic set of micro-Raman experiments on the changes in the line shape of the $\ensuremath{\sim}520\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ one-phonon band in Si nanowires with laser flux $\mathrm{\ensuremath{\Phi}}$ are presented. A complicated dependence of the $520\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ Raman band asymmetry $(A)$ with $\mathrm{\ensuremath{\Phi}}$ 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 $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}$ 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 $A(\mathrm{\ensuremath{\Phi}})$ 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.