It is argued that the topological disorder in a small region of an amorphous solid can be described by the local strain field related to the local reference crystal. A localized state spread only in one distorted region can be viewed as the consequence of superposition among some Bloch waves and its scattering waves caused by the disorder. A semiclassical approximation is used to calculate the phase shift of Bloch waves in the amorphous solid. The inverse participation ratio and the mobility edge positions in the band tails are formulated in terms of the parameters of the disorder potential. The dependence of the band tail decay rates on static and thermal disorders is derived. The model is applied to $a\text{-Si}$, though conceptually it can be implemented to a wide range of disordered systems. The ab initio calculations on $a\text{-Si}$ and the experimental results on $a\text{-Si}$:H are compared to the predictions of our model.

• Received 6 October 2008

DOI:https://doi.org/10.1103/PhysRevB.78.195208