The optical properties of excitonic recombinations in bulk, n-type ZnO are investigated by photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements. At liquid helium temperature in undoped crystals the neutral donor bound excitons dominate in the PL spectrum. Two electron satellite transitions (TES) of the donor bound excitons allow to determine the donor binding energies ranging from 46 to 73 meV. These results are in line with the temperature dependent Hall effect measurements. In the as-grown crystals a shallow donor with an activation energy of 30 meV controls the conductivity. Annealing annihilates this shallow donor which has a bound exciton recombination at 3.3628 eV. Correlated by magnetic resonance experiments we attribute this particular donor to hydrogen. The Al, Ga and In donor bound exciton recombinations are identified based on doping and diffusion experiments and using secondary ion mass spectroscopy. We give a special focus on the recombination around 3.333 eV, i.e. about 50 meV below the free exciton transition. From temperature dependent measurements one obtains a small thermal activation energy for the quenching of the luminescence of 10 ± 2 meV despite the large localization energy of 50 meV. Spatially resolved CL measurements show that the 3.333 eV lines are particularly strong at crystal irregularities and occur only at certain spots hence are not homogeneously distributed within the crystal contrary to the bound exciton recombinations. We attribute them to excitons bound to structural defects (Y-line defect) very common in II-VI semiconductors. For the bound exciton lines which seem to be correlated with Li and Na doping we offer a different interpretation. Li and Na do not introduce any shallow acceptor level in ZnO which otherwise should show up in donor -acceptor pair recombinations. Nitrogen creates a shallow acceptor level in ZnO. Donor -acceptor pair recombination with the 165 meV deep N-acceptor is found in nitrogen doped and implanted ZnO samples, respectively. In the best undoped samples excited rotational states of the donor bound excitons can be seen in low temperature PL measurements. At higher temperatures we also see the appearance of the excitons bound to the B-valence band, which are approximately 4.7 meV higher in energy.

Carrier relaxation and recombination in self-organized InAs/GaAs quantum dots ͑QD's͒ is investigated by photoluminescence ͑PL͒, PL excitation ͑PLE͒, and time-resolved PL spectroscopy. We demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism. Multiphonon processes involving up to four LO phonons from either the InAs QD's, the InAs wetting layer, or the GaAs barrier are resolved. The observation of multiphonon resonances in the PLE spectra of the QD's is discussed in analogy to hot exciton relaxation in higher-dimensional semiconductor systems and proposed to be intricately bound to the inhomogeneity of the QD ensemble in conjunction with a competing nonradiative recombination channel observed for the excited hole states. Carrier capture is found to be a cascade process with the initial capture into excited states taking less than a few picoseconds and the multiphonon ͑involving three LO phonons͒ relaxation time of the first excited hole state being 40 ps. The ͉001͘ hole state presents a relaxation bottleneck that determines the ground-state population time after nonresonant excitation. For the small self-organized InAs/GaAs QD's the intradot carrier relaxation is shown to be faster than radiative ͑Ͼ1 ns͒ and nonradiative ͑Ϸ100 ps͒ recombination explaining the absence of a ''phonon bottleneck'' effect in the PL spectra. ͓S0163-1829͑97͒09340-5͔

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The structure of ZnO quantum dots prepared via the wet chemical method was studied. By introducing an annealing treatment (150 °C–500 °C), we also investigated the effect of the change in the structure of the dots on their luminescence properties. Our studies revealed that the surface of the as-prepared dots is passivated by a thin layer of Zn(OH)2, thus, the dots consist of a ZnO/Zn(OH)2 core-shell structure. We present evidence that the weak excitonic transition of ZnO quantum dots is strongly correlated with the presence of the surface shell of Zn(OH)2. When Zn(OH)2 is present, the excitonic transition is quenched.

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