The Exponential Shape of the Optical Absorption Edge Tail

A simple model of light absorption for photon energy smaller than the width of the forbidden band‐gap Eg is proposed. Assuming that the shape of the fundamental absorption edge is influenced by elastic fluctuations in Eg caused by both, permanent and temperature lattice disorder, a general expression for the exponential shape of the optical tail is obtained, which is valid for both, amorphous and crystalline solids. The so‐called Urbach rule (exponential dependence on energy and temperature) represents an asymptotic relation of this formula when the permanent disorder is neglected. The proposed model also predicts quantitatively the values of the steepness parameter for several materials which show the Urbach shape of the absorption edge tail.