The discussion centers on the momentum conservation of electrons transitioning between energy levels in materials with an indirect band gap. When an electron descends from a higher energy level, it emits a photon; however, due to the nature of indirect band gaps, this process necessitates the emission of phonons to conserve momentum. The key takeaway is that in indirect band gap materials, photons cannot carry crystal momentum, necessitating the involvement of phonons to balance the momentum difference between the electron and hole.
PREREQUISITESStudents and researchers in solid-state physics, materials scientists, and anyone interested in the behavior of electrons in semiconductor materials.
In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice.
...photons cannot carry crystal momentum,...in an indirect band gap material, the process must also involve the absorption or emission of a phonon, where the phonon momentum equals the difference between the electron and hole momentum.
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