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Ultrawide bandgap semiconductors (UWBGs) provide great promise for optical devices operating in the near to deep ultraviolet, and recently they have become a viable semiconducting material for high power electronics. From the power grid to electronic vehicles, the intention is to replace massively awkward components with the convenience of a solid state electronic “chip.” Unfortunately, the challenges faced by wide bandgap electronic materials, such as GaN and SiC, increase as the bandgap increases. A point defect, for example, can take on more charge states and energy configurations. This perspective describes a method to investigate the many charge states and their associated transitions—photo-induced electron paramagnetic resonance (photo-EPR) spectroscopy. Although not new to the study of defects in semiconductors, photo-EPR studies can probe the entire ultrawide bandgap given the appropriate light source for excitation. Examples provided here cover specific defects in UWBGs, AlN, and Ga2O3. The discussion also reminds us how the rapid pace of discovery surrounding this newest class of semiconductors is due, in part, to fundamental research studies of the past, some as far back as a century ago and some based on very different materials systems. 

Gallium oxide when doped with Mg becomes semi-insulating and can be useful for power electronic devices. The present work investigates optical transitions of neutral Mg (MgGa0) using photoinduced electron paramagnetic resonance spectroscopy, a variation of the traditional optical absorption. Steady-state and time-dependent measurements are carried out at 130 K by illuminating the samples with photon energies from 0.7 to 4.4 eV. Interpretation of the data using a model that incorporates electron–phonon coupling yields a defect transition level that is consistent with the MgGa−/0 level obtained from hybrid density functional theory calculations. We conclude that the neutral to negative transition of MgGa that we observe involves an electron transition from the valence band to the defect, and the MgGa−/0 level is located 1.2 eV above the valence band maximum, with a relaxation energy of 1.3 eV. 

Gallium oxide (Ga2O3), an ultra-wide bandgap semiconductor with potential applications in power devices, may be doped with Mg to control the native n-type conductivity. The charge transitions associated with Mg in Mg-doped β-Ga2O3 crystals are studied using photoinduced electron paramagnetic resonance (photo-EPR) spectroscopy to understand the mechanisms that produce stable semi-insulating substrates. The steady state photo-EPR measurements are performed at 130 K by illuminating the samples with photon energy from 0.7 to 4.7 eV. Our results show that there are two transitions associated with Mg in the bandgap: onset of quenching of neutral Mg at 1.5 eV and excitation at 3.0 eV. The quenching threshold is consistent with several DFT predicted values for Mg-/0 level. Therefore, we suggest the quenching is due to transition of an electron from the valence band to the neutral Mg. For photoexcitation, hole capture is the only viable process due to polaronic nature of neutral Mg in Ga2O3. The measurements demonstrate that electron excitation to impurities, such as Fe and Ir, does not contribute to creation of the holes. Further, gallium vacancies must not participate since their characteristic EPR spectrum is never seen. Thus, we speculate that the defects responsible for the hole formation and consequent excitation of the neutral Mg are oxygen vacancies.