Electronic structure and optical properties of ultrathin CdS/ZnS quantum wells grown by molecular-beam epitaxy

We investigate in detail the optical properties of ultrathin, highly strained, cubic CdS/ZnS single and multiple quantum well structures using mainly photoluminescence, photoluminescence-excitation, and absorption spectroscopy. An effective model within the envelope function approximation is presented, taking into account strain and excitonic effects. Using an improved parameter set for cubic CdS this model successfully describes the observed exciton transition energies. A fit to experimental data taking into account the Stokes shift between luminescence and absorption yields some information about the model parameters, namely, the CdS/ZnS band alignment and the tetragonal deformation potential of CdS in the zinc-blende modification. Finally, we investigate the influence of localization effects due to well width fluctuations on the dimensionality of exciton confinement. A strong enhancement of the excitonic exchange interaction is found for deeply localized states in extremely narrow quantum wells, suggesting a significant lateral confinement and the formation of quasi-zero-dimensional states.