Bi-exciton annihilation as a possible origin of both M and P luminescence bands in CdS and CdSe at low temperatures

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Abstract
By means of temperature-, intensity- and magnetic-field-dependent luminescence and excitation spectroscopy it is proven that the prominent groups of P and M high-density emission bands in CdS and CdSe are exclusively caused by bi-exciton recombination processes, and cannot be explained by exciton-exciton collision or by multi-exciton annihilation. The energy balances, the similarity of the excitation spectra of the M and P bands, their temperature dependence and their behaviour in a magnetic field confirm a model that assigns the M luminescence to bi-exciton decay leaving one of the two excitons within the complex in the lowest polariton state, while P is explained by assuming the second-lowest polariton state to be the final state of the remaining polariton. The nature of this state is characteristically dependent on the compound under investigation. Neither in CdS nor in CdSe is there evidence for the occurrence of luminescence due to exciton-exciton collision at low temperatures.