The compositional dependence of the optical and electrical properties of hydrogenated amorphous Si-Ge films prepared by co-sputtering

Hydrogenated amorphous Si-Ge alloy films were prepared by co-sputtering from two half-disc targets. The films are about 1 μm thick and 12.5 cm long. The composition along the length of the film varies continuously from 70 to 5 at.% Si. The optical absorption and photoconductivity as a function of wavelength were measured. The energy gap as evaluated form these measurements decreases linearly from 1.65 eV for 70 at.% Si to 1.1 eV for 7 at.% Si. The photoconductance response ημτ varies markedly with composition, where η is the quantum efficiency for photocarrier generation, μ the mobility and τ the lifetime of the photocarriers. At the silicon-rich end ημτ = 2 × 10^-6 cm² V^-1, decreases to 2 × 10^-9 cm² V^-1 at around 40 at.% Si and increases again by one order of magnitude at the germanium-rich end. On the whole the characteristics of our sputtered films are comparable with those reported for similar alloys prepared by glow discharge. An important advantage of the sputtered films, however, is that it is possible to prepare a whole range of alloy concentrations in one run.