High-quality single crystals of ZnO in the as-grown and N + ion-implanted states have been investigated using a combination of three experimental techniques-namely, positron lifetime/slow positron implantation spectroscopy accompanied by theoretical calculations of the positron lifetime for selected defects, temperaturedependent Hall ͑TDH͒ measurements, and deep level transient spectroscopy ͑DLTS͒. The positron lifetime in bulk ZnO is measured to be ͑151± 2͒ ps and that for positrons trapped in defects ͑257± 2͒ ps. On the basis of theoretical calculations the latter is attributed to Zn+ O divacancies, existing in the sample in neutral charge state, and not to the Zn vacancy proposed in previous experimental work. Their concentration is estimated to be 3.7ϫ 10 17 cm −3 . From TDH measurements the existence of negatively charged intrinsic defects acting as compensating acceptors is concluded which are invisible to positrons-maybe interstitial oxygen. This view is supported from TDH results in combination with DLTS which revealed the creation of the defect E1, and an increase in concentration of the defect E3 after N + ion implantation, and peculiarities in the observation of the defect E4.

We report on nonpolar GaN quantum dots embedded in AlN, grown on (11-20) 6H–SiC by plasma-assisted molecular-beam epitaxy. These dots are aligned in the growth plane and present a constant aspect ratio of 10. Their optical properties were studied as a function of GaN coverage. Especially, the variation of their emission energy as compared to that of (0001) GaN quantum dots is a clear fingerprint of the reduced internal electric field present in these nonpolar nanostructures. Time-resolved spectroscopy confirmed this result by revealing lifetimes in the few 100 ps range in contrast to the much longer ones obtained for the (0001) GaN quantum dots.

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The effect of annealing of Co∕ZnO(0001) was studied by scanning tunneling microscopy, low energy electron diffraction, and Auger electron spectroscopy. At room temperature, Co forms islands on ZnO. Annealing up to 940K leads to coalescence of the islands. At 970K, Co diffuses into ZnO where it partially replaces Zn. A model of the Auger intensities, based on exponential attenuation with thickness and including correction for matrix effects, confirms this interpretation and suggests that the fraction of Zn replaced by Co is near 50% or higher.

We report on A-plane GaN quantum dots in AlN, grown on A-plane 6H SiC substrates by plasma-assisted molecular-beam epitaxy. AFM imaging revealed a strong alignment of the dots along the [1 100] direction that we correlated with the anisotropic morphology of the AlN buffer layer. A vertical correlation of these dots was evidenced by high resolution transmission electron microscopy on superlattice samples with an AlN spacer thickness of 5 nm. Time-resolved spectroscopy performed on both C-plane and A-plane samples revealed much shorter radiative lifetimes for the A-plane dots, indicating a strong reduction of the internal electric field with respect to the one present in their C-plane counterparts.