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 ∗ .

Schottky barrier diodes (SBDs) were fabricated by depositing Pd, Pt or Ni on single crystal, conductive n-type rutile TiO2 using e-beam evaporation. As-grown and nominally undoped rutile TiO2 single crystals are semi-insulating, and were heat-treated in forming gas flow, N2 flow or H2 gas to obtain conductive n-type crystals displaying electrical conductivities in the range of ( 0.5 − 8 ) × 10 − 2 Ω − 1 cm − 1 . Additionally, SBDs were deposited on Nb-doped conductive n-type rutile TiO2 with a conductivity of around 0.25 Ω − 1 cm − 1 . Generally, SBDs displaying a rectification of up to eight orders of magnitude were obtained, when comparing the current under reverse and forward bias. The extracted ideality factors were in the range of 1.1 − 4.0 . From Capacitance-Voltage measurements, the built-in voltage was derived to be around 1.2 V – 1.9 V , depending on the doping concentration of the specific TiO2 single crystal. Series resistances as low as 19 Ω were achieved. A considerable variation in the electrical characteristics of different SBDs deposited on the same crystal was found, regardless of the metal or doping strategy used. Moreover, the SBD characteristics change over time, particularly seen as a degradation in rectification, mainly related to an increase in the current under reverse bias. Additional surface treatments such as boiling in H2O2 and etching in HF do not have a significant effect on the quality of the SBDs. Clear indications for poor adhesion between TiO2 and Pd are shown. In conclusion, we demonstrate the fabrication of SBDs which are suitable for studying the fundamental properties of metal/TiO2 junctions and the characteristics of electrically-active defects in TiO2 using space-charge spectroscopy.

This work systematically explores 19 unique configurations of the close-associate Ga-O divacancies (V Ga V O ) in β-Ga 2 O 3 , including their complexes with H impurities, using hybrid functional calculations. Interestingly, most configurations are found to retain the negative-U behavior of V O , as they exhibit a thermodynamic (−/3−) charge-state transition level energetically located in the upper part of the band gap, where the 3− charge state is associated with the formation of a Ga-Ga dimer. The energy positions of the thermodynamic (−/3−) chargestate transition levels divide the divacancy configurations into three different groups, which can be understood from the three possible Ga-Ga dimerizations resulting from the tetrahedral and octahedral Ga sites. The relative formation energies of the different divacancy configurations, and hence the electrical activity of the divacancies, is found to depend on the Fermi-level position, and the energy barriers for transformation between different divacancy configurations are explored from nudged elastic band calculations. Hydrogenation of the divacancies is found to either passivate their negative-U charge-state transition levels or shift them down in Fermi level position, depending on whether the H resides at V O or forms an O-H bond at V Ga , respectively. Finally, the divacancy is discussed as a potential origin of the so-called E * 2 center previously observed by deep-level transient spectroscopy.

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].

We report on electrically-active defects located between 0.054 and 0.69 eV below the conduction band edge in rutile T i O 2 single crystals subjected to reducing and hydrogenating heat treatments. Deep-level transient spectroscopy measurements recorded on T i O 2 samples subjected to different heat treatments are compared. In samples annealed in H 2 gas, three defect levels are commonly observed. One of these levels, E 192 , located 0.43 eV below the conduction band edge is tentatively assigned to a hydrogen-impurity complex. Two levels at 0.054 and 0.087 eV below the conduction band edge, which were present after all different heat treatments, are tentatively assigned as being related to O vacancies or Ti self-interstitials. Deep-level transient spectroscopy spectra of samples heat-treated in N 2 display a larger number of defect levels and larger concentrations compared to samples heat-treated in H 2 gas. N 2 treatments are performed at considerably higher temperatures. Four energy levels located between 0.28 and 0.69 eV, induced by annealing in N 2 , are tentatively attributed to O vacancy- or Ti interstitial-related complexes with impurities.