I think these materials are semiconductors, but why not all exciton recombinations with respect to different materials all show photons coming, so called quantum dots? Do the efficiencies of phonon and photon relaxations compete with each other?
this is depend on its energy gap with aspect and with another depend on cohesive energy among its bonds require to electrons movement
First of all, they have indirect bandgap so they need phonon assistance to emit photons, which is not very probable under normal recombination process of electron-hole pairs. Other words, recombination process to emit photons in the direct bandgap materials for example GaAs, InP, ZnS, CdSe, AlN, and etc is naturally permitted because the momentum change during the recombination is conserved-like the selection rule of quantum mechanics.
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