We present a comprehensive study of vacancy and vacancy-impurity complexes in InN combining positron annihilation spectroscopy and ab initio calculations. Positron densities and annihilation characteristics of common vacancy-type defects are calculated using density functional theory, and the feasibility of their experimental detection and distinction with positron annihilation methods is discussed. The computational results are compared to positron lifetime and conventional as well as coincidence Doppler broadening measurements of several representative InN samples. The particular dominant vacancy-type positron traps are identified and their characteristic positron lifetimes, Doppler ratio curves, and line-shape parameters determined. We find that indium vacancies (V In ) and their complexes with nitrogen vacancies (V N ) or impurities act as efficient positron traps, inducing distinct changes in the annihilation parameters compared to the InN lattice. Neutral or positively charged V N and pure V N complexes, on the other hand, do not trap positrons. The predominantly introduced positron trap in irradiated InN is identified as the isolated V In , while in as-grown InN layers V In do not occur isolated but complexed with one or more V N . The number of V N per V In in these complexes is found to increase from the near-surface region toward the layer-substrate interface.

The symmetry ordering of the valence bands in ZnO is derived from high-resolution magneto-optical measurements of bound excitons. We report on the experimental observation of a hole state related fine splitting for bound excitons in the Voigt configuration. This splitting is related to a nonzero Landé g value g Ќ for hole states from the A valence band. Based on theoretical considerations, the symmetry of the uppermost valence band is doubtlessly identified as ⌫ 7 . This attribution is confirmed by polarization and angular resolved magnetophotoluminescence spectroscopy.

We study the existence of Li-related shallow and deep acceptor levels in Li-doped ZnO nanocrystals using electron paramagnetic resonance ͑EPR͒ and photoluminescence ͑PL͒ spectroscopy. ZnO nanocrystals with adjustable Li concentrations between 0% and 12% have been prepared using organometallic precursors and show a significant lowering of the Fermi energy upon doping. The deep Li acceptor with an acceptor energy of 800 meV could be identified in both EPR and PL measurements and is responsible for the yellow luminescence at 2.2 eV. Additionally, a shallow acceptor state at 150 meV above the valence band maximum is made responsible for the observed donor-acceptor pair and free electron-acceptor transitions at 3.235 and 3.301 eV, possibly stemming from the formation of Li-related defect complexes acting as acceptors.

We report on the magnetic properties of GaN:Gd layers grown by molecular beam epitaxy. A poor reproducibility with respect to the magnetic properties is found in these samples. Our results show strong indications that defects with a concentration of the order of 10 19 cm −3 might play an important role for the magnetic properties. Positron annihilation spectroscopy does not support a direct connection between the ferromagnetism and the Ga vacancy in GaN:Gd. Oxygen codoping of GaN:Gd promotes ferromagnetism at room temperature and points to a role of oxygen for mediating ferromagnetic interactions in Gd-doped GaN.

The reproducible preparation of p-type ZnO represents an important task that might be realized via the partial substitution of Zn 2+ by Li + in the ZnO lattice. Consequently, it is necessary to understand the effect of Li incorporation into ZnO in detail. A systematic approach employing molecular organometallic precursors for the preparation of ZnO materials containing different amounts of Li is presented. The materials have been characterized with various analytical techniques including X-ray absorption, micro-Raman, and low-temperature photoluminescence spectroscopy; high-resolution transmission electron microscopy; powder X-ray diffraction; and electron paramagnetic resonance. It could be seen that Li incorporation is successful only below 12% Li. Already at ∼1% Li, a maximum in lattice substitution seems to be reached. At higher Li contents, more and more interstitial sites become occupied.