ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons (Y lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the Y 0 transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers (I lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all Y lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II-VI and III-V semiconductors.

In this letter, we demonstrate that self-organized InGaAs quantum dots ͑QDs͒ grown on GaAs ͑111͒ substrate using droplet epitaxy have great potential for the generation of entangled photon pairs. The QDs show spectrally sharp luminescence lines and low spatial density. A second order correlation value of g ͑2͒ ͑0͒ Ͻ 0.3 proves single-photon emission. By comparing the power dependence of the luminescence from a number of QDs we identify a typical luminescence fingerprint. In polarization dependent microphotoluminescence studies a fine-structure splitting ranging Յ40 eV down to the determination limit of our setup ͑10 eV͒ was observed.

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Direct observation of large permanent dipole moments of excitonic complexes in InGaN/GaN quantum dots is reported. Characteristic traces of spectral diffusion, observed in cathodoluminescence of InGaN/GaN quantum dots, allow deducing the magnitude of the intrinsic dipole moment. Our experimental results are in good agreement with realistic calculations of quantum dot transition energies for position-dependent external electric fields.

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Site-controlled growth of quantum dots (QDs) for single photon emitters (SPEs) is achieved applying a buried stressor approach. Theoretical and experimental analysis shows that site-controlled QD growth on buried oxide stressor-layers benefits enormously from a defect-free growth interface. Laterally modulated strain fields at GaAs(001) growth surfaces are used to tailor surface morphologies at the centre of prescribed mesa structures for subsequent QD growth. Suitable morphologies for site-controlled QD growth such as nano-hillocks and nanoholes are identified. Site-controlled QD growth appears above the boundaries between the oxidised layer and the non-oxidised semiconductor layer. Through fine tuning of wetting layer thickness and growth interruption high selectivity for QD nucleation is achieved. Thus, growth of single QDs at the centre of a current-injection limiting aperture is demonstrated. Moreover, the QD growth on a defect-free surface yields high quality optical properties in terms of narrow emission linewidth and temporal stability with no discernible difference to QDs grown on planar substrates. The technological simplicity of the buried stressor approach and the inherent integration of a current aperture for efficient carrier injection into site-selected QDs enable mass production of SPEs on large substrate sizes.

Optical properties of multi-stacked InGaAs/GaNAs quantum dot solar cell fabricated on GaAs (311)B substrate J. Appl. Phys. 112, 064314 (2012) Optical polarization properties of a nanowire quantum dot probed along perpendicular orientations Appl. Phys. Lett. 101, 111112 (2012) Electroluminescence from quantum dots fabricated with nanosphere lithography Appl. Phys. Lett. 101, 103105 (2012) Nucleation features and energy levels of type-II InAsSbP quantum dots grown on InAs(100) substrate Appl. Phys. Lett. 101, 093103 (2012)Highly luminescing multi-shell semiconductor nanocrystals InP/ZnSe/ZnS Appl.

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