In situ n-type doping was investigated for Ge1-yCy/Si heteroepitaxial layers (y-0.001) for a potential optoelectronic material compatible with Si. Using a solid GaP sublimation source for phosphorus doping, epitaxial Ge1-yCy films were in situ doped on Si(100) substrates during solid source molecular beam epitaxy and we compare their electrical and optical properties with those of epitaxial Ge on Si. Infrared absorption revealed red shifts in the absorption of visible light with increasing P doping for both Ge1-yCy and Ge. The index of refraction decreases for Ge1-yCy layers compared with Ge. Free carrier absorption increased with increasing phosphorus concentrations, following a wavelength dependence of similar to lambda(3.1) in the region of 10-20 mu m for heavily doped material. Addition of C did not affect the incorporation of P donors in the grown layers or the electrical activation of the donors. An increase in the electron mobility for heteroepitaxial Ge1-yCy layers compared with Ge was observed for the doping levels studied.