Photoconductivity (PC) spectroscopy is used to determine the optical absorption and behavior of free carriers of Si/Ge structures and (Si6Ge4)100superlattices grown on Si0.4Ge0.6alloys. Negative photoconductivity is observed at a temperature of 67 K. Some samples are additionally doped by nuclear transmutation. These samples exhibit a strong negative PC signal even at room temperature, which cannot be suppressed by a bias illumination. ©1995 American Institute of Physics.