URBACH EDGES AND ANGULAR DISTORTIONS IN A-SI-H AND RELATED ALLOYS

A general discussion is proposed on the experimental correlation between the valence band tail slope parameter kBTv and the rms deviation ΔΘb of the Si-Si bond angle distribution which has been observed by various authors in doped and undoped a-Si:H, as well as in a-SiGe:H and a-SiN:H alloys. In the simplified case of a Gaussian distribution of bond angle values with a single harmonic bending potential, an exponential distribution is expected for the elastic energy fluctuations, with a decay parameter scaling with ΔΘb2. For Si-centered tetrahedra with at least two Si nearest neighbors, the Keating formalism yields realistic estimates of the elastic potential site-to-site fluctuations along the Si-Si skeleton of Si-based alloys.