Raman shift and broadening in stress-minimized Ge nanocrystals in silicon oxide matrix

Abstract

Ge nanocrystals (nc-Ge) embedded in silicon oxide films were synthesized using RF magnetron sputtering and post-annealing procedure. To minimize the stress effect and inhomogeneity, we intentionally lower the cooling rates and reduce the temperature gradient during annealing. Significant Raman shifts ranging from 2.0 to 5.8 cm-1 have been observed from samples annealed at different temperatures. The size-dependent shift and broadening is found to be in good agreement with the phonon confinement mode together with the Gaussian weighting function, and the isotropic TO2 phonon dispersion relation introduced by Sasaki et al. The Raman spectra can also be well-fitted using peaks calculated from the phonon confinement model. The inhomogeneous Raman peak broadening from our samples annealed at lower temperatures are attributed to the non-Gaussian size distribution of Ge nanocrystals.