Remarks on the nature of Urbach absorption tail in glassy semiconductors

Exponential absorption edge of chalcogenide glassy alloys with sulfur content in the region of the topological transition is studied. It is shown that the slope of the tail (1/Δ) depends on the mean coordination number (r¯), the mean volume occupied by an atom (V) and the stoichiometry (Ω) of the materials, i. s. Δ=f(r¯, V, Ω). Analysis of the experimental results for alloys As-S-Ge as well as for other amorphous materials, including the elemental ones indicates that the topological disorder controlled by r¯, Ω and V is the main reason in formation of the exponential absorption tail