Spectroscopy of single excitons localized at defects

Summary form only given. Spectroscopy at single, localized excitons in semiconductor quantum structures is presently a very interesting field of research. Much attention is paid to single-dot spectroscopy on quasi-0D excitons, which are localized e.g., at well width fluctuations in ultrathin quantum wells or within self-assembled islands. The microscopic origin of three-dimensional quantum confinement is a strong local potential fluctuation given by growth conditions. In a similar way, impurities ties and extended defects can provide such local potentials. In bulk semiconductors, excitons or electrons bound at some defects often represent well-defined, discrete two-level systems. Optical spectroscopy at single impurity states has to solve the problem of how to optically address an exciton localized at any defect. At very low impurity concentrations, imaging spectroscopy allows spatially and spectrally to separate single emission centers at micrometer length scales. In the contribution we study impurity-related single exciton emission in ZnSe layers homoepitaxially grown on ZnSe substrates. Well-known defects in ZnSe are donor-bound excitons, donor-acceptor pair transitions and the Y-defect, a structural defect connected with dislocations. By imaging spectroscopy we analysed the spatially resolved emission. The emission at the free-exciton energy (FE) is characterized by a homogeneously distributed signal intensity extended over the whole detection area of the CCD. No effects of localization are seen as expected for a free exciton emission.