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As-grown ZnO bulk crystals and crystals annealed in vacuum, oxygen, or zinc vapour were characterized by electrical, optical and magnetic resonance spectroscopy. The experiments show that the residual carrier concentration is caused by residual H, Al, Ga and oxygen vacancies (VO) in the material. Annealing the samples in O2 at about 1000 °C (2 atm, 20 h) reduces the H and VO donor concentration by typically one order of magnitude. The photoluminescence and deep level transient spectroscopy (DLTS) results suggest a correlation between the broad unstructured emission at 2.45 eV (“green band”) and a donor level 530 meV below the conduction band, it is attributed to the VO 0/++transition. By using DLTS experiments with optical excitation it is possible to observe a metastable level 140 meV below the conduction band which is assigned to the VO 2+/+ recharging. The results give evidence for the “negative-U” properties of the oxygen vacancy defects predicted by recent theoretical calculations. [ABSTRACT FROM AUTHOR]

Vapor-phase grown ZnO crystals were investigated by means of DLTS measurements. The generation of defect center E4 subsequent to annealing in different ambients was monitored. By conducting electron irradiations with energies, where either both the Zn- and O-sublattice are damaged or according to [1] only the Zn-lattice, a chemical assignment to the defect centers E4 and E3 could be accomplished. DLTS investigations of ZnO samples under illumination give evidence that E4 is a negative-U center. [ABSTRACT FROM AUTHOR]

This work investigates cobalt doped ZnO nanoparticles prepared by using wet chemical methods. The nanoparticles have a typical size of 3–8 nm. The electronic structure as well as the optical and magnetic properties of Co2+ have been characterized. X-ray diffraction spectra of the powder show wurtzite ZnO with no secondary Co phases. In the energy range below the bandgap, the optical absorption spectra show the internal d–d transitions related to Co2+ incorporated on the Zn lattice site in ZnO. Low temperature photoluminescence measurements confirm these results. Based on the analysis of the g-valuesfor bulk ZnO:Co., electron paramagnetic resonance measurements coincide with the simulation of Co-doped ZnO powder. Thus far, no evidence for ferromagnetism has been obtained. [ABSTRACT FROM AUTHOR]

We present our investigations on the absorption and emission properties of the neutral oxygen vacancy in ZnO. Temperature-dependent photoluminescence (PL) experiments allow to determine the zero-phonon-energy of the emission and its phonon-coupling parameters. The absorption to the singlet excited state is observed in PL-excitation experiments at energies above 3.1 eV. Relaxation drives the system from the singlet, diamagnetic, excited state to the emissive, paramagnetic triplet state. Applying the Mollwo–Ivey relation for the energetical position of the F-centre absorption as a function of the anion–cation distance, we find that the oxygen vacancies in ZnO fit perfectly in the line given by MgO, CaO, SrO and BaO. [Copyright &y& Elsevier]