It was noted that the alloying of hydrogenated amorphous Si films with Ge permitted the optical band-gap to be varied between 1.7 and 1.0eV. But, although Si and Ge exhibited minimal differences in bond length and ionicity, considerable changes in the electronic density of states of tails and defects were caused by alloying to more than a critical Si content of about 70%. Due to the lower bonding energy of Ge, as compared with Si, the total density of dangling bond defects around the mid-gap increased markedly as the band gap shrank. The exact effects of bond strength, composition, H content and density of the weak bond reservoir remained unclear, but it was accepted that an increase in dangling-bond density of about 2 orders of magnitude (in going from amorphous hydrogenated Si to amorphous hydrogenated Ge) appeared to be unavoidable.

G.H.Bauer: Solid State Phenomena, 1995, 44-46, 365-84