We synthesized SiGeC layers by the high dose ion implantation of Ge and C into a Si substrate and the subsequent annealing. Although the end-of-range defect band resulting from high dose Ge ion implantation was found in transmission electron microscopy (TEM) observation, the ideality factor of forward bias characteristics in a heterojunction p(+)/n diode was almost one in every sample. We, found that the I-V characteristics of diodes shifted to the lower bias voltage with an increase in C dose. From comparison with the theoretical calculations, we attribute this shift to the reduced bandgap energy in unstrained SiGeC. This shows that SiGeC can be used as a narrow bandgap material suitable for bandgap engineering in sub-100 nm metal-oxide-semiconductor field effect transistors (MOSFETs).