The effects of hydrogen dilution of up to 54:1 (=H2:SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) was investigated at a low substrate temperature, while keeping the optical gap (Eopt) constant. It was found that deterioration of the film properties, when substrate temperature decreases, can be compensated by the high hydrogen dilution method. As the substrate temperature decreases from 230 to 180 °C, the high photoconductivity, high photosensitivity, and low silicon dihydride content of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties becomes worse with conventional low hydrogen dilution ratios. Probably, hydrogen radicals substitute for the surface reaction energy lost by decreasing the temperature. Besides, a-SiGe:H films deposited under higher hydrogen dilution have more germanium and less hydrogen content than those of the conventional films, despite having the same Eopt. One possible explanation for why Eopt can be kept constant is the suppression of the formation of Ge–Ge bonds at the growing surface by the energy supplied by the hydrogen radicals. © 1997 American Institute of Physics.