Fe3+ center in ZnO.

Abstract

In the spectral region around 690 nm, a richly structured luminescence is observed in undoped high-quality ZnO crystals. By means of emission, excitation, and magneto-optical spectroscopy, this luminescence is unambiguously assigned to the $^{4}$${\mathit{T}}_{1}$(G)${\mathrm{\ensuremath{-}}}^{6}$${\mathit{A}}_{1}$(S) transition of isolated ${\mathrm{Fe}}^{3+}$ ions on ${\mathrm{Zn}}^{2+}$ lattice sites. Basic arguments are the sixfold degeneracy of the ground state with an isotropic g factor of 2.020\ifmmode\pm\else\textpm\fi{}0.015, a long lifetime of 25.2 ms, and the fine structure of the $^{6}$${\mathit{A}}_{1}$(S) ground state. The observed fine structure of the excited $^{4}$${\mathit{T}}_{1}$(G) state indicates an intermediate Jahn-Teller coupling instead of the strong coupling usually observed for isoelectronic centers in II-VI and III-V compound semiconductors. The excitation mechanism is described by an energy transfer to ${\mathrm{Fe}}^{2+}$ centers by free holes. The holes are photogenerated at deep acceptors with ionization energies above 2.25\ifmmode\pm\else\textpm\fi{}0.05 eV. The $^{4}$${\mathit{T}}_{1}$(G)${\mathrm{\ensuremath{-}}}^{6}$${\mathit{A}}_{1}$(S) transition energy is found to shift +39\ifmmode\pm\else\textpm\fi{}3 \ensuremath{\mu}eV/nucleon by an isotope effect induced by Fe isotopes and to shift 365 and 222 \ensuremath{\mu}eV, respectively, by the presence of one $^{18}\mathrm{O}$ ion among the $^{16}\mathrm{O}$ ions of the ${\mathrm{Fe}}^{3+}$${\mathrm{O}}_{4}^{2\mathrm{\ensuremath{-}}}$ cluster, depending on its location. The isotope shifts are interpreted in the framework of mass-dependent local modes, which contribute to the total energy of the transition-metal states. Here, the Jahn-Teller interaction as well as the ${\mathit{C}}_{3\mathit{v}}$ distortion of the ${\mathrm{Fe}}^{3+}$${\mathrm{O}}_{4}^{2\mathrm{\ensuremath{-}}}$ cluster is taken into account.