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//spl Delta/) depends on the mean coordination number (r~), the mean volume occupied by an atom (V) and the stoichiometry (/spl Omega/) of the materials, i. s. /spl Delta/=f(r~, V, /spl Omega/). 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~, /spl Omega/ and V is the main reason in formation of the exponential absorption tail.