Many theoretical and experimental studies deal with the realization of room‐temperature ferromagnetism in dilute magnetic semiconductors (DMS). However, a detailed quantitative understanding of the electronic properties of transition metal doped semiconductors has often been neglected. This article points out which issues concerning electronic states and charge transfers need to be considered using Fe as an example. Methods to address these issues are outlined, and a wealth of data on the electronic properties of Fe doped III–V and II–VI compound semiconductors that have been obtained over a few decades is reviewed thoroughly. The review is complemented by new results on the effective‐mass‐like state consisting of a hole bound to Fe2+ forming a shallow acceptor state.The positions of established Fe3+/2+ and Fe2+/1+ charge transfer levels are summarized and predictions on the positions of further charge transfer levels are made based on the internal reference rule. The Fe3+/4+ level has not been identified unambiguously in any of the studied materials. Detailed term schemes of the observed charge states in tetrahedral and trigonal crystal field symmetry are presented including hyperfine structure, isotope effects and Jahn–Teller effect. Particularly, the radiative transitions Fe3+(4T1 → 6A1) and Fe2+(5E → 5T2) are analyzed in great detail.An effective‐mass‐like state [Fe2+, h] consisting of a hole bound to Fe2+ is of great significance for a potential realization of spin‐coupling in a DMS. New insights on this shallow acceptor state could be obtained by means of stress dependent and temperature dependent absorption experiments in the mK range. The binding energy and effective Bohr radius were determined for GaN, GaP, InP and GaAs and a weak exchange interaction between the hole and the Fe2+ center was detected.With regard to the Fe3+ ground state, 6A1, in GaP and InP, the hyperfine structure level Γ8 was found to be above the Γ8 level.All results are discussed with respect to a potential realization of a ferromagnetic spin‐coupling in DMSs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

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.

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