Optical transition of neutral Mg in Mg-doped β-Ga₂O₃

Mg-doped gallium oxide (β-Ga₂O₃) is semi-insulating and can be useful for power electronic devices. Understanding defects and their charge-transition levels is critical for the success of Ga₂O₃-based devices. Here, we investigate optical transitions of Mg⁰ using photoinduced electron paramagnetic resonance (photo-EPR), an absorption spectroscopy in which charge state changes are detected by the EPR intensity. Photo-EPR measurements are carried out at 130 K by illuminating Mg-doped Ga₂O₃ crystals, grown by Czochralski or floating zone method. Steady-state results show an onset of the charge transition of Mg⁰ near 1.6 eV, larger than the Mg^-/0 level predicted by our density functional theory (DFT) calculations. However, the optical cross section spectrum derived from time-dependent measurements agrees well with results from a model using the DFT-calculated relaxation energy. We conclude that the observed neutral-to-negative transition of Mg involves an electron transition from the valence band to the Mg⁰ and the Mg^-/0 level lies ~1.35 eV above the valence band maximum, with a relaxation energy of ~1.10 eV.