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Monolayer fluctuations in ultrathin, coherently strained CdS/ZnS quantum-well structures result in a very strong localization of excitons. The deepest localized excitons can be considered as individual, decoupled and three-dimensionally confined. Consequently, the optical properties can be well explained by transitions from an ensemble of spatially distributed, quasi-zero-dimensional excitonic states. The efficient photoluminescence (PL) and the optical gain in the deep-blue spectral range exhibit unusually broad and unstructured spectra. In μ-PL measurements of a mesa-etched SQW sample (10 μm × 10 μm mesas down to 100 nm × 100 nm mesas) single, ultranarrow luminescence lines can be resolved when the number of luminescing states is reduced.

The optical properties of CdSe/ZnSe quantum structures with nominal thickness of the CdSe layer between 1 and 4 monolayers (ML) have been studied. The transition from two- to zero-dimensional quantum confinement can be monitored by photoluminescence (PL), photoluminescence-excitation (PLE), and time-resolved PL experiments. The influence of different degrees of localisation on the relaxation process of excitons is investigated. Experimental results for the excitonic transition energies are in good agreement with theory when strain-, confinement- and alloy-fluctuation-effects are included. The observation of multiple phonon modes indicates the formation of a Zn1−xCdxSe alloy with varying composition as well as the accumulation of CdSe in small islands.