How glasses are able to show both direct “band gap” and indirect “band gap” transitions? Despite that the concepts of direct and indirect band gaps cannot be applied to amorphous materials such as glasses because of the loss of long range periodicity in material and hence the destruction of Brillouin zone which used for band structure calculation.???
Hi,
You can find the explanation in the following paper
Jasprit Singh, Influence of disorder on the electronic structure of amorphous silicon, Physical Review B, 23,4156 (1981)
The dihedral-angle disorder was found to be important at the valence-band edge and responsible for the observed features near the top of the valence band. Topological disorder was found to have important consequences in the bulk of the bands as well as at the conduction-band edge. Apart from the effects of the bond-length and bond-angle disorder, the states at the band edges are confined within regions closely approaching the crystalline structure locally, where they have the same form as in the crystal, but do not extend through the entire structure.
Hello, Horchani:
Maybe "Indirect". Check the book of Davis and Mott, Electronic processes in Non-crystalline Materials. It is not so easy to tell that it is like that or like that. The problem is more complex and dependent on the band model. His description is very complicated in amorphous materials (mobility edges) comparing to crystalline.
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