II–VI widegap superlattices

Abstract

We review our recent results of the excitonic properties in ZnSeZnS and Cd xZn 1−xSZnS strained-layer superlattices (SLSs). The most important physical insights in the II–VI widegap superlattices are to understand the relationship between the optical properties of quasi-two-dimensional exciton and strain because the well layer frequently receives biaxial compression or tension. The strain thus causes the significant shifts of the bandgap and splitting of the valence band. Semi-quantative calculations lead to an expectation that ZnSeZnS SLS always exhibits a type I band lineup within 100 Å thicknesses of the ZnSe well at a constant ZnS barrier width of several tens angstrom. This is in good agreement with the experimental results of exciton absorption and its luminescence excitation spectra. The Cd 0.3Zn 0.7SZnS SLSs with a range of well widths can produce intense excitonic emissions around 3.4 eV at room temperature due to the quantum confinement of excitons in the ternary CdZnS well. In order to elucidate localisation and relaxation processes of excitons, we have for the first time reported a multiple-LO-phonon emission process in the excitation spectra. The electric-field studies suggest that the concomitant decrease in intensity and the energy downshift of the exciton line may originate from the quantum confined Stark effect.