In the spectral region around 690nm, a richly structured luminescence was observed in undoped high-quality ZnO crystals. By means of emission, excitation, and magneto-optical spectroscopy, this luminescence was unambiguously assigned to the 4T1(G)-6A1(S) transition of isolated Fe3+ ions on Zn2+ lattice sites. Basic arguments were the six-fold degeneracy of the ground state with an isotropic g factor of 2.020, a long lifetime of 25.2ms, and the fine structure of the 6A1(S) ground state. The observed fine structure of the excited 4T1(G) state indicates an intermediate Jahn-Teller coupling instead of the strong coupling usually observed for iso-electronic centers in II-VI and III-V compound semiconductors. The excitation mechanism was described by an energy transfer to Fe2+ centers by free holes. The holes were photogenerated at deep acceptors with ionization energies above 2.25eV. The 4T1(G)-6A1(S) transition energy was found to shift +39μeV/nucleon by an isotope effect induced by Fe isotopes and to shift 365 and 222μeV, respectively, by the presence of one 18O ion among the 16O ions of the Fe3+O42- cluster, depending upon its location. The isotope shifts were interpreted in the framework of mass-dependent local modes, which contributed to the total energy of the transition-metal states. Here, the Jahn-Teller interaction as well as the C3v distortion of the Fe3+O42- cluster was taken into account.

Fe3+ Center in ZnO. Heitz, R., Hoffmann, A., Broser, I.: Physical Review B, 1992, 45[16], 8977–88