We have prepared hydrogenated amorphous silicon/silicon-carbon alloy compositional superlattices($a-Si:H/a-Si_{1-x}C_x:H$) by rf glow discharge decomposition of silane gas mixtures and investigated their novel electrical and optical properties. As the thickness of a-Si:H well layer is decreased, the optical gap of superlattice is increased and the electrical conductivity is decreased, which are attributed to the quantum size effects. The persistent photoconductivity(PPC) was also examined in $a-Si:H/a-Si_{1-x}C_x:H$ superlattices. The PPC effect in these multilayers is very small and anneals away at relatively low temperatures compared with $a-Si:H/a-Si_{1-x}N_x:H$ multilayers. The PPC effect in $a-Si:H/a-Si_{1-x}C_x:H$ superlattices might be explained by the deep trap of electron at the interface between a-Si:H and $a-Si_{1-x}C_x:H$
