We have studied the influence of carbon codoping in Si1-yCy:Er layers on the photoluminescence efficiency of the 1.54 mu m wavelength Er emission. All samples were prepared by molecular beam epitaxy (MBE) with carbon concentrations between y = 0.08 and 0.4%. Maximum photoluminescence output at low temperature T = 5 K could be realized for growth temperatures around 430 degrees C and an erbium to carbon content ratio of about one ([Er] = 4.5 x 10(19) cm(-3), y = 0.1%). The efficiency could be further enhanced by annealing and is comparable to our best Si:Er:O samples. A decrease in photoluminescence intensity at 1.54 mu m was observed for increasing sample temperature. It decreases stronger for carbon codoped Si1-yCy:Er layers than for Si:Er:O samples. High resolution photoluminescence spectra show both a difference in the spectral position of the main erbium line as well as in the fine structure for oxygen and carbon codoping.