High-quality pseudomorphic Si1-yCy and Si1-x-yGexCy alloy layers with a carbon concentration up to 7% are prepared by solid-source molecular beam epitaxy, Near band-edge photoluminescence (PL) is observed from Si/Si1-yCy multiple quantum well (MQW) structures. The bandgap in the pseudomorphic films is reduced by about 65 meV per percent C. The data from Si/Si1-yCy MQWs indicate a type-I heterostructure with the band offset being mainly in the conduction band. In Si1-yGexCy MQWs compressive strain caused by Ce is partially compensated by C alloying and the bandgap increases with y. PL measurements from closely spaced Si1-yCy/Si(1-x)Gc(x) layers show a lower transition energy than that of isolated Si1-yCy and Si1-xGex reference samples. This is attributed to spatially indirect PL transitions between the electrons confined in the Si1-yCy layers and the heavy holes located in the Si1-xGex layers. The PL is dominated by no-phonon recombination. Electrical properties of n-type doped thick Si1-yCy layers and modulation doped p-type Si/Si1-x-yGexCy quantum well structures are presented. No carrier capture by C or C-related defects is observed at room temperature. A significant mobility enhancement is measured for n-type doped strained Si0.996C0.004 layers at temperatures below 180 K, which is attributed to the splitting of the Delta valleys in the conduction band. In a modulation doped p-type Si0.49Ge0.49C0.02 QW we observe an improved hole mobility at room temperature and 77 K compared to a corresponding sample without C, which is a consequence of the reduced strain in the layer due to substitutional C.