Insensitivity to selection rules of two-phonon luminscence spectra

Analysis of newly detected luminescence lines in germanium indicates that two-phonon recombination processes in indirect-gap semiconductors are insensitive to selection-rule constraints, although such restrictions generally play a crucial role in the optical properties of semiconductors. The two-phonon replicas of electron-hole recombination lines are found to be constituted from well-defined groups of phonons whose momenta differ from points of high symmetry in the Brillouin zone.