Pseudomorphic Gel-yCy and Sil-yCy alloy layers have been synthesized by solidsource molecular beam epitaxy on Si (001) substrates. High quality short-period Gel-y Cy/Si superlattice structures with a carbon content up to about 5 % are grown at low substrate temperature. The partial compensation of strain within Ge964C036 layers of 7 Å thickness improves the thermal stability against lattice relaxation, compared to pure Ge layers. Band-edge related photoluminescence is observed from pseudomorphic Sil-yCy/Si multiple quantum well structures at low temperature. The two predominant luminescence lines are attributed to no-phonon transitions and Si-Si TO phonon replicas of bound excitons confined within the Sil-yCy alloy quantum well layers. The tensile strain within the Sil-yCy layers shifts the twofold degenerate Δ(2) conduction band valley down in energy and is mainly responsible for the linear band gap reduction which is observed for increasing C content.