2-PHONON ASSISTED INDIRECT EXCITON LUMINESCENCE IN ALPHA-ZNP2

Luminescence spectra of tetragonal ZnP2 crystal are measured at variou s temperatures. Below the indirect exciton threshold, about 30 intrins ic indirect exciton luminescence bands are confirmed. Amidst the broad bound exciton luminescence band, several emission bands appear even a t 2 K. Intensities of these bands increase with increasing temperature , especially in the high energy part of the band. They are also intrin sic exciton luminescence caused through the indirect exciton transitio n processes accompanied by two phonons at k(1) and k(2) conserving the exciton wavevector; k(ex) = k(1) + k(2). Energy difference between th e lowest edges of one- and two-phonon luminescence bands 59.4 meV is e qual to the largest Raman shift energy. Phonons participating in the l uminescence are mainly momentum conserving phonons appearing in the on e-phonon luminescence bands and the ones at the Gamma point of the Bri llouin zone.