Si1-x-yGexCy crystals were grown by ultra-high-vacuum chemical-vapor deposition and their thermal-annealing behaviors were studied by infrared (IR) absorption measurements. The present samples exhibited absorption peaks originating from local vibrational modes of substitutional C atoms with two different atomic configurations; one is a C atom with four Si neighbors (Si4 configuration) and the other is with one Ge and three Si neighbors (Si3Ge1 configuration). As the annealing temperature was increased from 700 to 950 °C, the absorption peak of the C atoms with Si3Ge1 configuration increased in intensity. This clearly demonstrated that thermal annealing leads to the formation of Ge–C bonds. Moreover, the composition dependence of the IR spectra suggested that Ge–C bonds are formed to reduce the strain energy of the Si1-x-yGexCy crystals. © 2001 American Institute of Physics.