How can I explain optical band gap from electronic band structure?
My calculated Electronic band gap (1.3 eV) is different from optical band gap (2.1eV). I have seen in some literatures where they briefly talked about spin selection rule, orbital selection rule, symmetry forbidden transition etc to explain the difference.
Can someone help me to understand the difference.
Electrical band gap is the minimal energy required to create an electron hole pair in a semiconductor. It is simply given as the difference between the top of the valence band, and the bottom of the conduction band.
Whereas optical band gap is the exciton energy which determines the onset of vertical interband transitions. An exciton is a bound state of an electron and hole which are held together by the electrostatic Coulomb force. An exciton forms when a photon is absorbed by a semiconductor, so the optical band gap is the threshold for photons to be absorbed.
it is known that electronic band gap is larger than optical band gap
Dear
Abdullah
my calculated optical direct band gap is very close to experimental optical direct band gap. Now I am trying to explain the transitions from electronic band structure. I have read that structure with inversion center blocks transitions from HOMO to LUMO so transition starts from below maximum of valance band. It will be great you add more information at this point.
Perhaps this excellent perspective by Prof. Baerends & coworkers is of help:
"The Kohn–Sham gap, the fundamental gap and the optical gap: the physical meaning of occupied and virtual Kohn–Sham orbital energies"
https://doi.org/10.1039/C3CP52547C
This set of slides is also informative: https://www.scm.com/wp-content/uploads/TCCM--Orbitals_TD-DFT--Baerends.pdf
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