Raman spectra of c -$\mathrmSi_1\mathrm-\mathrmx$$\mathrmGe_\mathrmx$ alloys
We observe several weak features between 420 and 470 cm-1 in addition to the normally observed Si-Si (∼500 cm-1), Si-Ge (∼400 cm-1), and Ge-Ge (∼300 cm-1) optic modes in the Raman spectra of Si1-xGex (0.28≤x≤0.77) single crystal layers grown by liquid-phase epitaxy (LPE). The quasiequilibrium LPE-growth process rules out the type of long-range ordering recently observed in Si1-xGex/Si strained-layer superlattices as the origin for these peaks. Calculations of the first-order Raman spectra of random 216-atom c-Si1-xGex alloys reproduce these weak features, which proves that they are not due to second-order Raman processes. Normal-mode analysis shows that they are due to localized Si-Si motion in the neighborhood of one or more Ge atoms. Implications for strain-induced long-range ordering in c-Si1-xGex alloys are discussed.