The deposition of hydrogenated and fluorinated silicon-germanium alloys (Si1−xGex:H.F) by glow discharge decomposition of silicon tetrafluoride (SiF4) and germane (GeH4) mixtures has been studied. Optical emission spectroscopy (OES), for the analysis of the emitting species in plasma phase, and mass spectroscopy (MS) for the analysis of the stable species, are used for the plasma diagnostics. In addition, in situ measurements of the deposition rate by laser interferometry are performed. A series of alloys with germanium content ranging from 0 to 55% has been prepared by varying the gas compositional ratio. Data on the optical gap (Ec), sub-gap absorption, and photo-to-dark conductivity ratio (δσ/σ) are used to evaluate the quality of the materials. An alloy a–Si0.75Ge0.25: H, F having Ec δ 1.5 eV and δσ/σ = 104 has been prepared by adding 1% of GeH4 to SiF4 in the feed.