What is the phenomena for the glasses to possess both direct and indirect band gaps?
HI. Generally all the materials possess both direct and indirect transitions across the bands in electronic structure. If the material consists of different crystallographic orientations, then emission from one band gets absorbed in other and the resultant emission we see is the integration of all reinforced emission from one particular crystallographic plane. In glasses, if one change the chemical composition its even possible to obtain direct and indirect gaps in same material.
Best luck.
@Abhay Sagade
Then how to observe them. ie. how to differentiate them?
A complete materials characterization needs to be used. Such as UV-Vis spectroscopy and Tauc's plot, ARPES, etc.
The nature of the transition depends on the structure of material, it is likely that the structure of glass is amorphous and therefore the electronic transfer not be simple, but by the density of the states in the energy band gap.
The same question in link: https://www.researchgate.net/post/How_are_glasses_able_to_show_both_direct_band_gap_and_indirect_band_gap_transitions?.
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