We suggest that the role of hydrogen dilution in the improvement of the quality of hydrogenated amorphous silicon germanium alloys is to alter the relative contribution of the short and long lifetime precursors to film growth which are deleterious and beneficient, respectively. In order to circumvent the preferential depletion of germane when mixed with silane, we have used low power disilane–germane discharges and thereby obtained films with an optical gap <1.5 eV having similar Urbach tail width and defect absorption, with and without hydrogen dilution. ©1995 American Institute of Physics.