Why do electrons emit phonons instead of photons?

Question

Why do electrons emit phonons when they "relax" into the minimum energy level of the conduction band after getting into it from the valence band by absorbing a photon with an energy higher than their bandgap? Why don't they simply emit a photon with an energy equivalent to the energy of the phonon emitted? In other words, why a phonon and not a photon?

Answer 1

"To conserve the k-vector." To find a place in the band diagram, an electron should have right k and right energy E. Please see any E-k diagram of the conduction band. Emitting a photon will only lower the energy of the electron with unaltered k value. But if it emits a phonon, both k and E are reduced such that it finds a suitable place in the E-k plot.