Absorption of photons with less energy than the band gap

[Lambduh]

Hi guys i have a question about how semiconductor materials absorb photons at less than the band gap energy. Particularly in an indirect material like silicon.

Looking at absorption spectra for silicon i expected for it to immediately drop off at lower than the band gap energy ~1100 nm (1.12 eV) but it definitely does not from the graphs that I've seen. Is this something to do with phonon assisted absorption (even then there doesn't seem to be enough energy, or momentum, to get absorbed? It just seems to me that for any photon with energy less than the bandgap the material should be transparent. Is this due to "traps" or some defects in the material? Thanks!

Mark

[Dr Transport]

Dopants have levels in the gap. Another way is the phonon assist mechanism.

[genneth]

There are also excitonic effects, where the created electron and hole are in a bound state, ala a hydrogen atom; that might not be very strong in silicon though. At the end of the day, remember that all experiments are limited in resolution, and reality is always smooth, never any sharp jumps.

[cmos]

Even without the assist, there could be absorption "into" the phonon spectrum.

[Minich]

1. If temperature >0 then there are electrons in the conductunce band, so they may absorb photons in greate number.

2. Laser photons are permitted be absorbed by electrons even at T=0 with multiphoton processes.

3. Of course nobody can neglect phonons.