Ge1−xCx/Si heterostructure photodiodes with nominal carbon percentages(0⩽x⩽0.02),which exceed the solubility limit, were grown by solid source molecular beam epitaxy onn-type (100) Si substrates. Thep-Ge1−xCx/n-Siphotodiodes were fabricated and tested. Thep-Ge1−xCx/n-Sijunction exhibits diode rectification with a reverse saturation current of about 10 pA/&mgr;m2at −1 V and high reverse breakdown voltage, up to −80 V. A significant reduction in diode reverse leakage current was observed by adding C to Ge, but these effects saturated with more C. Photoresponsivity was observed from these Si-basedp-Ge1−xCx/n-Siphotodiodes at a wavelength of ⩾1.3 &mgr;m, compatible with fiber optic wavelengths. External quantum efficiency of these thin surface-normal photodetectors was measured up to 2.2&percent;, which decreased as the carbon percentage was increased. ©1998 American Institute of Physics.