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 $5\times {10}^{17}{\mathrm{cm}}^{-3}\text{to}2\times {10}^{20}{\mathrm{cm}}^{-3}$ is systematically investigated by means of electron paramagnetic resonance and various optical techniques. ${\mathrm{Fe}}^{3+}$ is shown to be a stable charge state at concentrations from $1\times {10}^{18}{\mathrm{cm}}^{-3}$. The fine structure of its midgap states is successfully established, including an effective-mass-like state consisting of a hole bound to ${\mathrm{Fe}}^{2+}$ with a binding energy of $50\pm 10\mathrm{meV}$. A major excitation mechanism of the ${\mathrm{Fe}}^{3+}({}^{4}T_1\to {}^{6}A_1)$ luminescence is identified to be the capture of free holes by ${\mathrm{Fe}}^{2+}$ centers. The holes are generated in a two-step process via the intrinsic defects involved in the yellow luminescence. The ${\mathrm{Fe}}^{3+∕2+}$ charge-transfer level is found $2.863\pm 0.005\mathrm{eV}$ 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 ${\mathrm{Fe}}^{2+}({}^{5}E\to {}^{5}T_2)$ transition is observed around $390\mathrm{meV}$ at any studied Fe concentration by means of Fourier transform infrared spectroscopy. Charge-transfer processes and the effective-mass-like state involving both ${\mathrm{Fe}}^{2+}$ states are observed. At Fe concentrations from $1\times {10}^{19}{\mathrm{cm}}^{-3}$, additional lines occur in electron paramagnetic resonance and photoluminescence spectra which are attributed to defect complexes involving ${\mathrm{Fe}}^{3+}$. With increasing Fe concentration, the Fermi level is shown to move from near the conduction band to the ${\mathrm{Fe}}^{3+∕2+}$ charge-transfer level, where it stays pinned for concentrations from $1\times {10}^{19}{\mathrm{cm}}^{-3}$. Contrary to cubic II-VI and III-V materials, both electronic states are effected by only a weak Jahn-Teller interaction.

• Received 26 May 2006

DOI:https://doi.org/10.1103/PhysRevB.74.165202