Internal 5E→5T2 transition of Fe2+ in GaN

We present Fourier transform infrared transmission measurements on Fe-doped GaN. A set of freestanding high-quality GaN:Fe samples with varying Fe concentrations shows a rich absorption structure around 390 meV which is unambiguously assigned to the internal E-5 -> T-5(2) transition of the Fe2+ center. Up to now, the T-5(2) state was believed to be degenerate with the conduction band. By means of temperature- and polarization-dependent measurements the observed lines can be assigned to single transitions between E-5 and T-5(2) sublevels. As a result a quantitative term scheme of the Fe2+ center, under the influence of the C-3V-symmetric crystal field, is established. Only a weak Jahn-Teller interaction seems to effect the Fe center.