Hydrogen has a strong influence on the electrical properties of transparent conducting oxides where it can give rise to shallow donors and can passivate deep compensating defects. We have carried out infrared absorption experiments on H- and D-doped β-Ga2O3 that involve temperature- and polarization-dependent effects as well as relative H- and D-concentrations to probe the defect structures that hydrogen can form. The results of analysis of these data, coupled with detailed theoretical calculations, show that the dominant O-H vibrational line observed at 3437 cm−1 for hydrogenated Ga2O3 is due to a relaxed VGa-2H center.

β-Ga2O3 is a transparent conducting oxide with a wide bandgap (4.9 eV) whose properties are generating widespread interest. It has been found that hydrogen can play a key role in the conductivity of Ga2O3 by passivating deep defects and acting as a shallow donor. Recent vibrational spectroscopy experiments have found a dominant hydrogen center with a polarized O-H line at 3437 cm−1. These experiments along with theoretical analysis assign this line to a defect consisting of two equivalent H atoms trapped at a relaxed Ga vacancy. An expansion of this research has involved annealing treatments as well as measurements at different crystal orientations. These results have discovered a reservoir of “hidden” hydrogen in Ga2O3 whose identification involves a variety of hydrogen centers associated with the Ga vacancy, as well as other possible species.

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Conductive rutile TiO2 has received considerable attention recently due to multiple applications. However, the permittivity in conductive, reduced or doped TiO2 appears to cause controversy with reported values in the range 100–10,000. In this work, we propose a method for measurements of the permittivity in conductive, n-type TiO2 that involves: (i) hydrogen ion-implantation to form a donor concentration peak at a known depth, and (ii) capacitance–voltage measurements for donor profiling. We cannot confirm the claims stating an extremely high permittivity of single crystalline TiO2. On the contrary, the permittivity of conductive, reduced single crystalline TiO2 is similar to that of insulating TiO2 established previously, with a Curie–Weiss type temperature dependence and the values in the range 160–240 along with the c-axis.

Deep-level transient spectroscopy measurements are conducted on β-Ga 2 O 3 thin-films implanted with helium and hydrogen (H) to study the formation of the defect level E 2 ∗ ( E A = 0.71 eV) during heat treatments under an applied reverse-bias voltage (reverse-bias annealing). The formation of E 2 ∗ during reverse-bias annealing is a thermally-activated process exhibiting an activation energy of around 1.0 eV to 1.3 eV, and applying larger reverse-bias voltages during the heat treatment results in a larger concentration of E 2 ∗ . In contrast, heat treatments without an applied reverse-bias voltage (zero-bias annealing) can be used to decrease the E 2 ∗ concentration. The removal of E 2 ∗ is more pronounced if zero-bias anneals are performed in the presence of H. A scenario for the formation of E 2 ∗ is proposed, where the main effect of reverse-bias annealing is an effective change in the Fermi-level position within the space-charge region, and where E 2 ∗ is related to a defect complex involving intrinsic defects that exhibits several different configurations whose relative formation energies depend on the Fermi-level position. One of these configurations gives rise to E 2 ∗ , and is more likely to form if the Fermi-level position is further away from the conduction band edge. The defect complex related to E 2 ∗ can become hydrogenated, and the corresponding hydrogenated complex is likely to form when the Fermi level is close to the conduction band edge. Di-vacancy defects formed by oxygen and gallium vacancies (V O −V G a ) fulfill several of these requirements, and are proposed as potential candidates for E 2 ∗ .