CARBON DEPENDENCE OF RAMAN MODE FREQUENCIES IN SI1-X-YGEXCY ALLOYS

The frequencies of the Si-Si, Si-Ge, and Ge-Ge Raman modes in Si1-x-yG exCy alloys with y less than or equal to 0.03 have been measured. All frequencies are found to increase as a function of the carbon concentr ation, Within experimental error, this dependence is linear with a lar ge slope, which can be explained qualitatively in terms of the large b ond distortions caused by the lattice mismatch between Si1-xGex and di amond. Good numerical agreement with theoretical predictions is obtain ed if the mode frequencies are plotted as a function of the substituti onal carbon fraction determined from Rutherford backscattering studies . For the ''Si-Si'' mode, the magnitude of the carbon contribution to the bond-distortion dependence of the Raman frequency shift is found t o be 15 times larger than the Ge contribution. This disagrees with the 8-to-1 strain compensation ratio predicted form Vegard's law, but agr ees well with theoretical predictions of this ratio.