Electronic states and optical gain in strained CdS/ZnS quantum structures

Ultrathin, coherently strained CdS layers have been grown epitaxially on ZnS with nominal thicknesses below the critical value for strain relaxation. These CdS/ZnS quantum structures, which show efficient photoluminescence and optical gain in the deep blue to ultraviolet spectral range, have been analyzed with respect to the dimensionality of the electronic states. It has been found that in wide-gap II-VI quantum structures small monolayer fluctuations result in such strong localization of excitons that the localization depth reaches energies around 100 meV. Consequently, the luminescence and gain can well be explained by optical transitions from an ensemble of spatially distributed exciton states in which the deepest and decoupled states can be considered as individual, three-dimensionally confined excitons.