Temperature-dependent Raman scattering studies in nanocrystalline silicon and finite-size effects

A comparative study of the temperature-dependent Raman scattering of nanocrystalline and bulk silicon is presented. The nanocrystalline silicon samples were made by a cw laser annealing process, and the characteristic dimensions were determined with a phenomenological phonon confinement model. Experimental results indicate a higher degree of anharmonicity in nanocrystals compared to that in the bulk. The anharmonic constants are found to be highly size dependent and increase greatly with decreasing dimensions. The phonon lifetimes have two contributions, one temperature dependent and the other temperature independent, both decreasing rapidly with decreasing nanocrystal size. The temperature-dependent term ${\tau }_0$ is important for larger nanocrystals, while the temperature-independent term ${\tau }_1$ becomes dominant for nanocrystals of sizes less than 4 nm.