This work provides a consistent picture of the structural, optical and electronic properties of Fe doped GaN. A set of high-quality GaN crystals doped with Fe at concentrations ranging from 5×10 17 cm −3 to 2×10 20 cm −3 is systematically investigated by means of electron paramagnetic resonance and various optical techniques. Fe 3+ is shown to be a stable charge state at concentrations from 1×10 18 cm −3 . The fine structure of its mid-gap states is successfully established including an effective-mass-like state consisting of a hole bound to Fe 2+ with a binding energy of 50±10 meV. A major excitation mechanism of the Fe 3+ ( 4 T 1 -6 A 1 ) luminescence is identified to be the capture of free holes by Fe 2+ centers. The holes are generated in a two step process via the intrinsic defects involved in the yellow luminescence. The Fe 3+/2+ charge transfer level is found 2.863±0.005 eV above the valence band, suggesting that the internal reference rule does not hold for the prediction of band off-sets of heterojunctions between GaN and other III-V materials. The Fe 2+ ( 5 E-5 T 2 ) transition is observed around 390 meV at any studied Fe concentration by means of Fourier transform infra red spectroscopy. Charge transfer processes and the effective-mass-like state involving both Fe 2+ states are observed. At Fe concentrations from 1×10 19 cm −3 , additional lines occur in EPR and PL spectra which are attributed to defect complexes involving Fe 3+ . With increasing Fe concentration, the Fermi level is shown to move from near the conduction band to the Fe 3+/2+ charge transfer level, where it stays pinned for concentrations from 1×10 19 cm −3 . Contrary to cubic II-VI and III-V materials, both electronic states are effected by only a weak Jahn-Teller interaction.

Post-growth annealing and electron beam irradiation during cathodoluminescence were used to determine the chemical origin of the main optical emission lines in moderately and heavily Mg-doped GaN. The 3.27 eV donor-acceptor pair ͑DAP͒ emission line that dominates the emission spectrum in moderately Mg-doped ͑p-type͒ GaN was found to be strongly reduced by electron irradiation and of different chemical origin than the DAP at a similar energetic position in Si-doped ͑n-type͒ GaN. These results suggest that the acceptor responsible for the 3.27 eV DAP emission in Mg-doped GaN is Mg and that the donor ͑20-30 meV͒ is hydrogenrelated, possibly a (V N-H) complex. This complex is dissociated either by electron irradiation or thermal annealing in N 2 or O 2 atmosphere. We found that upon electron irradiation, a deeper emission line ͑centered at 3.14 eV͒ emerged, which was assigned to a DAP consisting of the same Mg acceptor level and a deeper donor ͑100-200 meV͒ with a similar capture cross section as the donor in the 3.27 eV emission. Moreover, two different deep donor levels at 350Ϯ30 and 440Ϯ40 meV were identified as being responsible for the blue band ͑2.8-3.0 eV͒ in heavily Mg-doped GaN. The donor level at 350Ϯ30 meV was strongly affected by electron irradiation and attributed to a H-related defect.

An identification of shallow bound exciton centers in ZnO is presented based on magneto-optical measurements and diffusion experiments. The thermalization behavior of the Zeeman split components confirms that the I 4 , I 6 , I 8 and I 9 exciton lines stem from donor bound exciton complexes. The results are supported by theoretical analysis of shallow bound exciton complexes revealing the Γ 7 symmetry of the upper valence band. The presence of two-electron satellites related to the respective transitions is further evidence for the donor bound complexes and enabled the determination of donor binding energies. Hydrogen, aluminum, gallium and indium were identified to origin the I 4 , I 6 , I 8 and I 9 lines by doping, diffusion and annealing experiments combined with photoluminescence and secondary ion mass spectrometry.

The effect of low-energy electron-beam irradiation (LEEBI) on native defects and residual impurities in metalorganic-vapor-phase-epitaxy-grown, lightly Mg-doped, p-type GaN was studied by temperature-resolved and excitation power density-resolved cathodoluminescence spectroscopy. Following the LEEBI treatment, the ubiquitous shallow donor–acceptor-pair emission at 3.27 eV decreased, while a deeper DAP emission at ∼3.1 eV dramatically increased in intensity, and a broad yellow luminescence band centered at 2.2 eV evolved. The results clearly indicate that the centers involved in the 3.27 eV transition are not stable during irradiation by low-energy electrons. Further, we report that the LEEBI-treatment not only dissociates neutral Mg-H complexes as intended, but simultaneously dissociates other hydrogenated defect complexes, giving rise to additional radiative recombination channels.