Exciton dynamics in Ni-activated CdS

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Abstract
The recombination processes of excitonic complexes in undoped and Ni-activated CdS crystals are investigated by means of time-resolved photoluminescence spectroscopy at liquid-He temperatures. The free-exciton recombination is found to be dominated by trapping at impurities, making the free-exciton lifetime a sensitive measure of crystal purity. Bound-exciton lifetimes in the ps region are observed which increase with increasing binding energy of the complex and are altered by the Ni doping. Calculations on the basis of the model of Rashba and Gurgenishvili show that competing nonradiative Auger-recombination processes can be neglected in undoped CdS. In Ni-doped samples a pronounced reduction of the excitonic lifetimes is found and attributed to energy-transfer processes. The deep Ni center offers an additional nonradiative recombination channel for bound-exciton complexes, which is assigned to ‘‘deep-center Auger processes’’ exciting charge-transfer transitions of Ni. Additionally, a bound-exciton complex with a binding energy of 13 meV is found to be related to the Ni doping. This complex is attributed to an exciton bound to a Ni-related neutral acceptor formed by the deep substitutional Ni acceptor paired with a shallow donor. The results demonstrate the possibility of the formation of shallow bound-exciton complexes at transition-metal centers.

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