Two-Phonon Assisted Indirect Exciton Luminescence in α-ZnP ₂

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Luminescence spectra of tetragonal ZnP₂ crystal are measured at various temperatures. Below the indirect exciton threshold, about 30 intrinsic indirect exciton luminescence bands are confirmed. Amidst the broad bound exciton luminescence band, several emission bands appear even at 2 K. Intensities of these bands increase with increasing temperature, especially in the high energy part of the band. They are also intrinsic exciton luminescence caused through the indirect exciton transition processes accompanied by two phonons at k_1 and k_2 conserving the exciton wavevector; k_ex = k_1 + k_2.Energy difference between the lowest edges of one- and two-phonon luminescence bands 59.4 meV is equal to the largest Raman shift energy. Phonons participating in the luminescence are mainly momentum conserving phonons appearing in the one-phonon luminescence bands and the ones at the Γ point of the Brillouin zone.