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Here it is shown that silicon absorbs visible light:

Absorption

but the band gap energy for direct transitions is 3.6 eV which does not match to this absorption coefficient. Instead, the band gap energy for an indirect transition is 1.12 eV. Hence, I assume, the visible light must excite silicon through indirect transitions. Is that correct? If so, how does this work? Why is visible light being able to lift an electron from the valence up into the conduction band?

Band gaps

Source

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  • $\begingroup$ Because phonon-assisted processes allow an electron to go from the top of the valence band to the bottom of the conduction band (not via the steps shown in your second figure, but directly over). $\endgroup$ – Jon Custer Aug 19 at 13:17
  • $\begingroup$ @user47014 Afaik this is pure silica $\endgroup$ – Ben Aug 19 at 13:22
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    $\begingroup$ The lattices has phonons flying around already since the temperature is above 0K. In Raman scattering, one gets both photons back at higher energy (having absorbed a phonon) and lower energy (having generated a phonon). $\endgroup$ – Jon Custer Aug 19 at 13:27
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    $\begingroup$ I highly recommend the book 'Optical Processes in Semiconductors' by Jacques Pankove. Old but good. He devotes 50+ pages to the many and various ways photons can be absorbed in direct and indirect semiconductors. But, yes, phonon-assisted absorption is a big one. $\endgroup$ – Jon Custer Aug 19 at 17:58
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    $\begingroup$ Note that 'silica' and 'silicon' are two different things (one is SiO2, the other is Si). And, yes, they introduced a degenerately doped impurity band. $\endgroup$ – Jon Custer Aug 19 at 18:31

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