EFFECT OF DISORDER ON EXCITON DYNAMICS IN CATION-SUBSTITUTED ZNXCD1-XS MIXED-CRYSTALS

Time-integrated and time-resolved (temporal resolution approximate to 20 ps) excitonic luminescence of cation-substituted ZnxCd1-xS (0 less than or equal to x less than or equal to 0.6) platelet mixed crystals has been studied at T = 1.8 K. It is shown that the evolution of excit on spectra with the increase of x is not simply governed by ''disorder -induced broadening'' (spread of exciton resonance frequency). Lumines cence transients are analyzed to clarify the relative importance of tu nneling (usually associated with the localization of excitons by disor der that is known to dominate in the model anion-substituted system Cd SeyS1-y). Tunneling in the range of Zn content x less than or equal to 0.3 is ruled out whereas the indisputable evidences of it (though on a substantially shorter timescale than in CdSeyS1-y) are found for the samples with x = 0.34, and 0.41. Both at small and large Zn contents different temporal behavior is observed compared to CdSeyS1-y implying some persistence of impurity-bound excitons. It is suggested that in cation-substituted ZnxCd1-xS the inhomogeneously broadened levels of i mpurity-bound excitons also participate in the tunneling and relaxatio n processes whereas the role of disorder-induced fluctuations of tile crystal potential is less important than in anion-substituted CdSeyS1- y.