# Bandgaps of Silicon and their application in active optical elements

I know that silion has an indirect bandgap at $E = 1.12 \, eV$ or $\lambda = 1.107 \, \mu m$ and I have read that active optical processes like absorption and emission have a decreased likelihood to occur in silicon due to an additional phonon that is required to satisfy the conservation of momentum. I also know that for this reason silicon is usually not used to build light emitters and solar cells on silicon base are only used for other technical reasons.

What I do not understand is ...

1. ... why does silicon still has a quantum efficiency of up to 90%, which is even higher than some direct semiconductors.

2. Moreover, the indirect bandgap seems to coincide with the cutoff frequency (cf. figure) - a point where I would not expect any absorption or emission. What does that mean?

3. Is there an asymmetry between the emission and the absorption efficiency of silicon? And if so, why?

4. I also read that there is even a direct bandgap in silicon at $E = 4.1 \, eV$ or $\lambda = 0.302 \, \mu m$. So I would expect a high quantum efficieny there, but the figure indicates otherwise.

I really think I got something completely mixed up. Any ideas? I would really appreciate a hint. Thank you!