Epitaxial growth of Si-C alloy films on Si(100) were achieved in the C fraction range up to about 5at% by surface reaction of SiH4 and CH4 under low-energy Ar plasma irradiation without substrate heating in electron-cyclotron-resonance (ECR) plasma chemical-vapor deposition (CVD). Moreover, it was found that the Si-C alloy (C fraction of 1.4at%) with an about 1%-larger vertical lattice constant than unstrained Si could be epitaxially grown on Si(100) under perfect lattice matching, which was different from the generally-reported results of tensile-strained Si-C alloy epitaxy on Si(100) at relatively higher temperatures. It was also found that deposition interruption effectively improved crystal quality of the film with an increased strain.