Effect of degree of localization and confinement dimensionality of excitons on their recombination process inCdSe/ZnSe/ZnSxSe1−xsingle quantum well structures

Abstract

We have studied the recombination dynamics of localized excitons in ${\mathrm{C}\mathrm{d}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ single quantum well structures (SQWS) with the CdSe well layer of 1, 2, and 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 that 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 layer width. This is resultant from the quantum energy shift and the interfacial roughness between the CdSe and ZnSe layers, respectively. The TRPL measurement showed that excitons were localized at the tail states caused by the interfacial fluctuation. Decay times were measured and found to increase with increasing monitored photon energy, and their values increased with increasing CdSe layer width. It was found that the degree of localization of excitons was larger and the radiative lifetimes of excitons were longer with an increase in the CdSe layer width, indicating that the energy depth in the tail states was smaller and the oscillator strength of excitons was larger for the SQW with a thinner well layer. A NLPL measurement was performed for the 1 and 2 ML SQWS, which revealed that the many-body effect 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 from the localized exciton-exciton inelastic scattering for the 2 ML SQW. The difference in the optical properties observed in the two samples is probably due to the difference in the inhomogeneity of the potentials felt by excitons. In order to study the effect of such inhomogeneity on the optical properties of excitons, we investigated the temperature dependence of the TRPL measurement for the three CdSe SQW samples, by which the radiative and the nonradiative lifetimes of localized excitons in the three SQW samples were obtained. These systematic measurements have enabled us to clarify the relationship of localized excitons to both the localization effect and the quantum 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 biexcitonic emission is observed, and for 2 and 3 ML SQWS, the degree of localization of excitons becomes larger compared with that for the 1 ML SQW and it prevents excitons from encountering the nonradiative recombination centers.