High-resolution (±0.1 ${\mathrm{cm}}^{\mathrm{-}1}$) first-order Raman spectra of c-${\mathrm{Ge}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ alloys show unexpected features of the Ge-Ge vibrational modes for very low concentrations of Si (x≤0.03). The corresponding phonon frequency is above that in pure Ge, and an asymmetry on the high-energy side of the Raman line appears. Supercell calculations with a Keating model for the lattice dynamics are in good agreement with the observed concentration dependence of the phonon energy. A possible reason for the asymmetric line shape is discussed.

• Received 12 August 1991

DOI:https://doi.org/10.1103/PhysRevB.44.13120