Atomic mass dependency of a localized phonon mode in SiGe alloys

Using molecular dynamics, the effect of an atomic mass difference on a localized phonon mode in SiGe alloys was investigated. Phonon dispersion relations revealed that a change in atomic mass causes the optical and acoustic modes to shift frequency. The results indicate that the local mode is sensitive to both Si and Ge atomic mass changes; reducing the Si atomic mass shifts the local mode to higher frequencies, and increasing the Ge atomic mass shifts the local mode to lower frequencies. Furthermore, the results suggest that the local mode originates from the Si-Ge bond vibration. Although the Si-Si, Si-Ge, and Ge-Ge optical mode frequencies are well approximated by the two-body harmonic oscillator model, a much heavier effective mass than that of the Si-Ge pair must be assumed to reproduce the local mode frequency. A plausible interpretation of the local mode is a collective vibration of Ge clusters embedded within the Si lattice.