Structural and electronic properties of Fe3+ and Fe2+ centers in GaN from optical and EPR experiments

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Physical Review B - Condensed Matter and Materials Physics, 2006, 74 (16)
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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× 1017 cm-3 to 2× 1020 cm-3 is systematically investigated by means of electron paramagnetic resonance and various optical techniques. Fe3+ is shown to be a stable charge state at concentrations from 1× 1018 cm-3. The fine structure of its midgap states is successfully established, including an effective-mass-like state consisting of a hole bound to Fe2+ with a binding energy of 50±10 meV. A major excitation mechanism of the Fe3+ (T14 → A16) luminescence is identified to be the capture of free holes by Fe2+ centers. The holes are generated in a two-step process via the intrinsic defects involved in the yellow luminescence. The Fe3+/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 offsets of heterojunctions between GaN and other III-V materials. The Fe2+ (E5 → T25) transition is observed around 390 meV at any studied Fe concentration by means of Fourier transform infrared spectroscopy. Charge-transfer processes and the effective-mass-like state involving both Fe2+ states are observed. At Fe concentrations from 1× 1019 cm-3, additional lines occur in electron paramagnetic resonance and photoluminescence spectra which are attributed to defect complexes involving Fe3+. With increasing Fe concentration, the Fermi level is shown to move from near the conduction band to the Fe3+/2+ charge-transfer level, where it stays pinned for concentrations from 1× 1019 cm-3. Contrary to cubic II-VI and III-V materials, both electronic states are effected by only a weak Jahn-Teller interaction. © 2006 The American Physical Society.
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