The influence of heat treating [Formula: see text]-type bulk [Formula: see text]-Ga[Formula: see text]O[Formula: see text] in hydrogen (H[Formula: see text]) and argon (Ar) gases on the presence of the defect level commonly labeled as [Formula: see text] was studied. Fourier transform-infrared spectroscopy confirms that hydrogen (H) is incorporated into [Formula: see text]-Ga[Formula: see text]O[Formula: see text] during H[Formula: see text] annealing at 900 °C. Deep-level transient spectroscopy measurements reveal that the concentration of the [Formula: see text] level is promoted by the introduction of H, in contrast to what is observed in samples heat-treated in an Ar flow. We further find the [Formula: see text] level to be stable against heat treatments at 650 K, both with and without an applied reverse-bias voltage. Potential candidates for the defect origin of [Formula: see text] are investigated using hybrid-functional calculations, and three types of defect complexes involving H are found to exhibit charge-state transition levels compatible with [Formula: see text], including substitutional H at one of the threefold coordinated O sites, Ga-substitutional shallow donor impurities passivated by H, and certain configurations of singly hydrogenated Ga–O divacancies. Among these types, only the latter exhibit H binding energies that are consistent with the observed thermal stability of [Formula: see text].

The electric field dependence of emission rate of the deep traps with level near Ec−0.6 eV, so-called E1 traps, was studied by means of deep level transient spectroscopy measurements over a wide range of applied voltages. The traps were initially introduced by 900 °C ampoule annealing in molecular hydrogen. The results indicate the activation energy of the centers and the ratio of high-field to low-field electron emission rates at a fixed temperature scale as the square root of electric field, suggesting that the centers behave as deep donors. The possible microscopic nature of the centers in view of recent theoretical calculations is discussed. The most likely candidates for the E1 centers are SiGa1–H or SnGa2–H complexes.

Defects in pulsed-laser deposition grown have been investigated using thermal admittance spectroscopy and secondary ion mass spectrometry (SIMS). A film was grown on either a tin-doped indium oxide or an aluminum-doped zinc oxide buffer layer on a sapphire substrate functioning as back contact layer in vertical diode structures. In both sample types, a distinct signature in the capacitance signal was observed in the temperature range of 150–260 K. The corresponding defect charge-state transition level, labeled , was found to exhibit an activation energy of 0.21 eV. Potential candidates for the level were investigated using a combination of SIMS and hybrid-functional calculations. SIMS revealed the main impurities in the sample to be tin, silicon, and iron. The hybrid-functional calculations predict the acceptor levels of substitutional iron to lie 0.7–1.2 eV below the conduction band minimum depending on Ga-site, making an unlikely candidate for the level. Furthermore, Si as well as Sn substituting on the sixfold coordinated Ga2 site and the fivefold coordinated Ga3 and Ga4 sites are all shallow donors in -, similar to that of -. Sn substituting on the fourfold Ga1 site is, however, predicted to have levels in the bandgap at 0.15 and 0.24 eV below the conduction band minimum, in accordance with the extracted activation energy for . Thus, we tentatively assign as the origin of the level.