Ground and excited states of iron centers in ZnO: Pulse-EPR and magneto-optical spectroscopy

We report on the ground- and excited-state properties of Fe3+ centers in hydrothermally and chemical-vapor-transport grown single ZnO crystals studied by continuous-wave electron-paramagnetic resonance (EPR) under dark and laser-illuminated conditions, pulsed-EPR and magneto-photoluminescence. By use of EPR experiments, the fine-structure parameters of the Fe3+ spin Hamiltonian are determined. Three types of charge-compensated Fe3+ centers are identified and the charge conversion from Fe2+ to Fe3+ is highlighted. The magneto-optical studies of the Zeeman components of the spin-forbidden electric-dipole transitions from excited T14(G) to ground A16(S6) states of the Fe3+ center indicate the trigonal symmetry of the fine structure of the lowest Γ8(T14) excited state. The energy positions of the Zeeman components are measured in the external magnetic field of 8 T rotated in (12¯10) and (0001) crystal planes. The angular variation of the Zeeman lines exhibits two magnetically nonequivalent Fe3+ centers. These features result from the contribution of high-rank Zeeman terms of dimension BJ3 in the spin Hamiltonian. For the electron spin S=5/2 system of the trigonal Fe3+ ion, we further demonstrate the tuning of one-photon Rabi oscillations by means of electron spin-echo measurements.