Spin-dependent donor-acceptor pair recombination in ZnS crystals showing the self-activated emission

Abstract

Donor-acceptor recombination has been investigated in ZnS by optically detected magnetic resonance (ODMR). Resonances due to both donors and A-centre acceptors have been observed in hexagonal and cubic ZnS containing group IIIB and group VIIB dopants. Spectral dependence measurements show that both resonances are associated with the blue self-activated emission, which lies at 2.70 eV in ZnS:Cl and 2.60 eV in ZnS:Al,I. The resonances are observed as increases of order 0.4% in the total emission intensity and are accounted for in terms of a donor-acceptor recombination process. It is found that the signals are predominantly from donor-acceptor pairs that are sufficiently close for recombination to occur before thermalisation is achieved but which are sufficiently well separated for the exchange interactions and associated line broadening to be small. The investigation confirms that the self-activated emission is due to radiative recombination of an electron from a shallow donor with a hole on an A-centre.