Si–Si optical phonon behavior in localized Si clusters of SixGe1−x alloy nanocrystals

Raman spectra acquired from SixGe1−x-nanocrystal-embedded SiO2 films show dependence of the Si–Si optical phonon frequency on Si content. The frequency upshifts, and peak intensity increases as the silicon concentration increases. For a given Si content, the frequency remains unchanged with annealing temperature. Spectral analysis and density functional theory calculation reveal that the optical Si–Si phonon is related to the formation of localized Si clusters surrounded by Si/Ge atomic layers in the SixGe1−x nanocrystals and the intensity enhancement arises from the larger cluster size. The synergetic effect of surface tensile stress and phonon confinement determines the Si–Si optical phonon behavior.