We report on incorporation of carbon in Si1-x-yGexCy alloys' by ultrahigh vacuum chemical vapor deposition and on thermal relaxation properties of Si1-x-yGexCy alloys with low carbon levels. Si1-x-yGexCy alloys have been grown at temperatures between 550 and 650 degrees C using silane, germane and methylsilane as precursor gases. For levels of less than 1% total C the layers are of excellent quality. The total carbon level was found to be independent of the Ge fraction and growth temperature. However, the Ge fraction in the alloys was observed to increase when carbon was added to the alloys, suggesting that C alters the sticking probabilities of silane and germane. We also studied the thermal stability of Si1-x-yGexCy alloys with low levels of carbon and found that adding even 0.2% C significantly improves the thermal stability when compared to SiGe alloys of similar strain and thickness.