Raman spectra of c-Si1-xGex alloys.

Abstract

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