Raman spectroscopy has been used to study the bond structure of Si1-x-yGexCy alloys. An anharmonic model for the force constants is applied to interpret the results. It is well established that the Ge and C concentrations in these alloys can be chosen so that their average lattice constants may be equal to, or even smaller than that of the Si lattice, Our Raman results show that the C local mode vibrational energy increases rapidly with increasing C concentration, al a rate of 3.2 cm(-1)/at%. Additional fine structure is observed near the Si-C main peak in samples grown by rapid thermal chemical vapor deposition (RTCVD). This fine structure is absent in samples grown by pulsed laser induced epitaxy (PLIE). In agreement with previous studies, the satellite structure of the Si-C peak is interpreted as an indication for short-range order. Our analysis shows that pronounced differences exist in the microscopic structures of SiGeC alloys grown by different growth techniques.