Effects of temperature on strain and lattice anharmonicity in silicon nanowires as investigated by power-dependent Raman spectroscopy
We performed power-dependent Raman spectroscopy in the backscattering geometry to analyze the dependence of temperature on the lattice anharmonicity and strain in silicon nanowires (Si NWs). Vertically-aligned Si NWs were grown on p-type (100) Si by electroless metal-assisted chemical etching. The Si NWs were grown at 30 min and 60 min etching times to vary the NW length. Both arrays of NWs have a nominal diameter of 150 nm. Power-dependent Raman spectroscopy was done to increase the local temperature of the Si NWs and observe the heating-induced Raman peak shifts in the Si NWs. Measurements showed that for the Si NWs etched at 30 min, the peak position at 520 cm-1 decreases linearly with power, which implies that the lattice anharmonicity, and strain induced by thermal expansion is comparable with bulk Si. For the NWs etched at 60 min, a second peak arises due to the Raman scattering from a second distribution of NWs that have a higher calculated local temperature. We attributed the changes in the lineshape to the thermal expansion coefficient and multiphonon scattering in the Si NWs. The contribution from thermal expansion and anharmonic multiphonon effects were also investigated. The results suggest that Raman spectroscopy is a good tool to quantify the thermal properties of bulk materials and nanostructures.