Study of charge transfer of Fe³⁺ in Ga₂O₃ by photo-induced electron paramagnetic resonance

We investigate charge transfer of a point defect, Fe³⁺, by electron paramagnetic resonance (EPR) spectroscopy in Ga₂O₃ single crystal. Two Czochralski grown Ga₂O₃ single crystals: one doped with 10¹⁸ cm^-3 Mg and 10¹⁷ cm^-3 unintentional Fe, and the other with 5 x 10¹⁷ cm^-3 Fe were studied by photo-induced EPR. The technique detects a specific charge state of a point defect such as Fe³⁺ and the intensity of EPR signal is proportional to the amount of Fe³⁺, therefore, a change in the EPR intensity suggests a change in charge state of the point defect. Steady state photo-EPR was performed by illuminating the crystals with light of 0.8-4.9 eV. The amount of Fe³⁺ decreased at approximately 2.5 eV for the Mg- and Fe-doped crystals. The observed threshold is surprisingly higher than the Fe-defect level, which is reported to be within 1.0 eV of band edges. Therefore, rather than direct ionization, we suggest that the amount of Fe³⁺ may decrease via two-step process: ionization of a defect Ir³⁺, which is reported to be at 2.25 eV below conduction band edge, and capture of the free electron by Fe³⁺. However, we note that Fe³⁺ acts as a compensator in Ga₂O₃, so one should not disregard the possibility that the 2.5 eV mid-gap threshold represents the Fe-defect level.