Effect of degree of localization and confinement dimensionality of excitons on their recombination process in CdSe/ZnSe/ZnSxSe1-x single quantum wellstructures

We have studied the recombination dynamics of localized excitons in CdSe/Zn Se/ZnSxSe1-x single quantum well structures (SQWS) with the CdSe well layer of 1, 2, anti 3 monolayers (MLs). A time-resolved photoluminescence (TRPL) measurement and a nonlinear photoluminescence (NLPL) measurement were used . A time-integrated photoluminescence (TIPL) measurement at 20 K showed tha t the photoluminescence (PL) from the CdSe layers was observed in the range from the blue spectral region to the green one with increasing CdSe layer width, and that the PL linewidths were about 20-50 meV with an increase in the CdSe laver width. This is resultant from the quantum energy shift and t he interfacial roughness between the CdSe and ZnSe layers, respectively. Th e TRPL measurement showed that excitons were localized at the tail states c aused by the interfacial fluctuation. Decay times were measured and found t o increase with increasing monitored photon energy, and their values increa sed with increasing CdSe layer width. It was found that the degree of local ization of excitons was larger and the radiative lifetimes of excitons were longer with an increase in the CdSe layer width, indicating that the energ y depth in the tail stares was smaller and the oscillator strength of excit ons was larger for the SQW with a thinner well layer. A NLPL measurement wa s performed for the 1 and 2 ML SOWS, which revealed that the many-body effe ct involving localized excitons occurred in the two samples, and that such an effect could result from the localized biexcitons for the 1 ML SQW and f rom the localized exciton-exciton inelastic scattering for the 2 ML SQW. Th e difference in the optical properties observed in the two samples is proba bly due to the difference in the inhomogeneity of the potentials felt by ex citons. In order to study the effect of such inhomogeneity on the optical p roperties of excitons, we investigated the temperature dependence of the TR PL measurement for the three CdSe SQW samples, by which the radiative and t he nonradiative lifetimes of localized excitons in the three SQW samples we re obtained. These systematic measurements have enabled us to clarify the r elationship of localized excitons to both the localization effect and the q uantum confinement one. It has been deduced from these results that for a 1 ML SQW, the quantum size effect of excitons is enhanced and the biexcitoni c emission is observed. and for 2 and 3 ML SOWS, the degree of localization of excitons becomes larger compared with that for the :I ML SQW and it pre vents excitons from encountering the nonradiative recombination centers.