Pseudomorphic Si1-yCy alloys on silicon (100) were grown by molecular beam epitaxy using a single effusion source of silicon contained in a graphite crucible, producing carbon concentrations of y=0.008. The behavior of carbon incorporation using this source was studied as a function of growth temperature using x-ray diffraction and infrared spectroscopy, and was compared to previous studies, where Si1-yCy was grown from separate silicon and graphite sources. An increased energy barrier for the surface diffusion of carbon was observed using the single silicon–graphite source. An infrared absorption mode near 725 cm-1, observed for growth temperatures up to 700 °C, was attributed to a transitional phase between the loss of substitutional carbon and the formation of silicon carbide precipitates. © 1998 American Institute of Physics.