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Gallium oxide (Ga2O3) exists in different polymorphic forms, including the trigonal (α), monoclinic (β), cubic (γ), and orthorhombic (κ) phases, each exhibiting distinct structural and electronic properties. Among these, β-Ga2O3 is the most thermodynamically stable and widely studied for high-power electronics applications due to its ability to be grown as high-quality bulk crystals. However, metastable phases such as α-, γ-, and κ-Ga2O3 offer unique properties, including wider bandgap or strong polarization and ferroelectric characteristics, making them attractive for specialized applications. This paper summarizes the radiation hardness of these polymorphs by analyzing the reported changes in minority carrier diffusion length (LD) and carrier removal rates under various irradiation conditions, including protons, neutrons, alpha particles, and gamma rays. β-Ga2O3 demonstrates high radiation tolerance with LD reductions correlated to the introduction of electron traps (E2*, E3, and E4) and gallium–oxygen vacancy complexes (VGa–VO). α-Ga2O3 exhibits slightly better radiation hardness similar to κ-Ga2O3, which also shows minimal LD changes postirradiation, likely due to suppressed defect migration. γ-Ga2O3 is the least thermodynamically stable, but surprisingly is not susceptible to radiation-induced damage, and is stabilized under Ga-deficient conditions. The study highlights the role of polymorph-specific defect dynamics, doping concentrations, and nonuniform electrical properties in determining radiation hardness. We also discuss the effect of radiation exposure on the use of NiO/Ga2O3 heterojunction rectifiers that provide superior electrical performance relative to Schottky rectifiers. The presence of NiO does change some aspects of the response to radiation. Alloying with Al2O3 further modulates the bandgap of Ga2O3 and defect behavior, offering potentially tunable radiation tolerance. These findings provide critical insights into the radiation response of Ga2O3 polymorphs, with implications for their use in aerospace and radiation-hardened power electronics. Future research should focus on direct comparisons of polymorphs under identical irradiation conditions, defect identification, and annealing strategies to enhance radiation tolerance. 

Abstract 17 MeV proton irradiation at fluences from 3–7 × 10¹³cm⁻²of vertical geometry NiO/β-Ga₂O₃heterojunction rectifiers produced carrier removal rates in the range 120–150 cm⁻¹in the drift region. The forward current density decreased by up to 2 orders of magnitude for the highest fluence, while the reverse leakage current increased by a factor of ∼20. Low-temperature annealing methods are of interest for mitigating radiation damage in such devices where thermal annealing is not feasible at the temperatures needed to remove defects. While thermal annealing has previously been shown to produce a limited recovery of the damage under these conditions, athermal annealing by minority carrier injection from NiO into the Ga₂O₃has not previously been attempted. Forward bias annealing produced an increase in forward current and a partial recovery of the proton-induced damage. Since the minority carrier diffusion length is 150–200 nm in proton irradiated Ga₂O₃, recombination-enhanced annealing of point defects cannot be the mechanism for this recovery, and we suggest that electron wind force annealing occurs. 

The effect of doping in the drift layer and the thickness and extent of extension beyond the cathode contact of a NiO bilayer in vertical NiO/β-Ga2O3 rectifiers is reported. Decreasing the drift layer doping from 8 × 1015 to 6.7 × 1015 cm−3 produced an increase in reverse breakdown voltage (VB) from 7.7 to 8.9 kV, the highest reported to date for small diameter devices (100 μm). Increasing the bottom NiO layer from 10 to 20 nm did not affect the forward current–voltage characteristics but did reduce reverse leakage current for wider guard rings and reduced the reverse recovery switching time. The NiO extension beyond the cathode metal to form guard rings had only a slight effect (∼5%) in reverse breakdown voltage. The use of NiO to form a pn heterojunction made a huge improvement in VB compared to conventional Schottky rectifiers, where the breakdown voltage was ∼1 kV. The on-state resistance (RON) was increased from 7.1 m Ω cm2 in Schottky rectifiers fabricated on the same wafer to 7.9 m Ω cm2 in heterojunctions. The maximum power figure of merit (VB)2/RON was 10.2 GW cm−2 for the 100 μm NiO/Ga2O3 devices. We also fabricated large area (1 mm2) devices on the same wafer, achieving VB of 4 kV and 4.1 A forward current. The figure-of-merit was 9 GW  cm−2 for these devices. These parameters are the highest reported for large area Ga2O3 rectifiers. Both the small area and large area devices have performance exceeding the unipolar power device performance of both SiC and GaN. 

NiO/β-Ga 2 O 3 vertical rectifiers exhibit near-temperature-independent breakdown voltages ( V B ) of >8 kV to 600 K. For 100 μm diameter devices, the power figure of merit ( V B ) 2 / R ON , where R ON is the on-state resistance, was 9.1 GW cm −2 at 300 K and 3.9 GW cm −2 at 600 K. By sharp contrast, Schottky rectifiers fabricated on the same wafers show V B of ∼1100 V at 300 K, with a negative temperature coefficient of breakdown of 2 V K −1 . The corresponding figures of merit for Schottky rectifiers were 0.22 GW cm −2 at 300 K and 0.59 MW cm −2 at 600 K. The on–off ratio remained >10 10 up to 600 K for heterojunction rectifiers but was 3 orders of magnitude lower over the entire temperature range for Schottky rectifiers. The power figure of merit is higher by a factor of approximately 6 than the 1-D unipolar limit of SiC. The reverse recovery times were ∼26 ± 2 ns for both types of devices and were independent of temperature. We also fabricated large area, 1 mm 2 rectifiers. These exhibited V B of 4 kV at 300 K and 3.6 kV at 600 K. The results show the promise of using this transparent oxide heterojunction for high temperature, high voltage applications. 

Vertical geometry NiO/β n-Ga2O/n+ Ga2O3 heterojunction rectifiers with contact sizes from 50 to 200 μm diameter showed breakdown voltages (VB) up to 7.5 kV for drift region carrier concentration of 8 × 1015 cm−3. This exceeds the unipolar 1D limit for SiC and was achieved without substrate thinning or annealing of the epi layer structure. The power figure-of-merit, VB2/RON, was 6.2 GW cm−2, where RON is the on-state resistance (9.3–14.7 mΩ cm2). The average electric field strength was 7.56 MV/cm, approaching the maximum for β-Ga2O3. The on–off ratio switching from 5 to 0 V was 2 × 1013, while it was 3 × 1010–2 × 1011 switching to 100 V. The turn-on voltage was in the range 1.9–2.1 V for the different contact diameters, while the reverse current density was in the range 2 × 10−8–2 × 10−9 A cm−2 at −100 V. The reverse recovery time was 21 ns, while the forward current density was >100 A/cm2 at 5 V. 

In this study, the response to a heavy-ion strike and the resulting single effect burnout on beta-Ga 2 O 3 Schottky diodes with biased field rings is investigated via TCAD. The model used to simulate the device under high-reverse bias is validated using experimental current-voltage (I-V) curves. A field ring configuration for the device demonstrates an improved charge removal after simulated heavy-ion strikes. If the time scale for charge removal is faster than single event burnout, this can be an effective mechanism for reducing the effect of single ion strikes. This study explores various configurations of the termination structure and shows the impact of different design parameters in terms of a transient response after the ion strike. 

The energy and beam current dependence of Ga⁺focused ion beam milling damage on the sidewall of vertical rectifiers fabricated on n-type Ga₂O₃was investigated with 5–30 kV ions and beam currents from 1.3–20 nA. The sidewall damage was introduced by etching a mesa along one edge of existing Ga₂O₃rectifiers. We employed on-state resistance, forward and reverse leakage current, Schottky barrier height, and diode ideality factor from the vertical rectifiers as potential measures of the extent of the ion-induced sidewall damage. Rectifiers of different diameters were exposed to the ion beams and the “zero-area” parameters extracted by extrapolating to zero area and normalizing for milling depth. Forward currents degraded with exposure to any of our beam conductions, while reverse current was unaffected. On-state resistance was found to be most sensitive of the device parameters to Ga⁺beam energy and current. Beam current was the most important parameter in creating sidewall damage. Use of subsequent lower beam energies and currents after an initial 30 kV mill sequence was able to reduce residual damage effects but not to the point of initial lower beam current exposures. 

The wide bandgap semiconductors SiC and GaN are commercialized for power electronics and for visible to UV light-emitting diodes in the case of the GaN/InGaN/AlGaN materials system. For power electronics applications, SiC MOSFETs (metal–oxide–semiconductor field effect transistors) and rectifiers and GaN/AlGaN HEMTs and vertical rectifiers provide more efficient switching at high-power levels than do Si devices and are now being used in electric vehicles and their charging infrastructure. These devices also have applications in more electric aircraft and space missions where high temperatures and extreme environments are involved. In this review, their inherent radiation hardness, defined as the tolerance to total doses, is compared to Si devices. This is higher for the wide bandgap semiconductors, due in part to their larger threshold energies for creating defects (atomic bond strength) and more importantly due to their high rates of defect recombination. However, it is now increasingly recognized that heavy-ion-induced catastrophic single-event burnout in SiC and GaN power devices commonly occurs at voltages ∼50% of the rated values. The onset of ion-induced leakage occurs above critical power dissipation within the epitaxial regions at high linear energy transfer rates and high applied biases. The amount of power dissipated along the ion track determines the extent of the leakage current degradation. The net result is the carriers produced along the ion track undergo impact ionization and thermal runaway. Light-emitting devices do not suffer from this mechanism since they are forward-biased. Strain has also recently been identified as a parameter that affects radiation susceptibility of the wide bandgap devices. 

Neutrons generated through charge-exchange⁹Be (p; nⁱ)⁹Be reactions, with energies ranging from 0–33 MeV and an average energy of ∼9.8 MeV were used to irradiate conventional Schottky Ga₂O₃rectifiers and NiO/Ga₂O₃p-n heterojunction rectifiers to fluences of 1.1–2.2 × 10¹⁴cm⁻². The breakdown voltage was improved after irradiation for the Schottky rectifiers but was highly degraded for their NiO/Ga₂O₃counterparts. This may be a result of extended defect zones within the NiO. After irradiation, the switching characteristics were degraded and irradiated samples of both types could not survive switching above 0.7 A or 400 V, whereas reference samples were robust to 1 A and 1 kV. The carrier removal rate in both types of devices was ∼45 cm⁻¹. The forward currents and on-state resistances were only slightly degraded by neutron irradiation. 

BCl 3 is an attractive plasma etchant for oxides because it is a Lewis acid used to scavenge native oxides on many semiconductors due to the strong B–O bonding. We investigated BCl 3 -based dry etching of the NiO/Ga 2 O 3 heterojunction system. BCl 3 /Ar Inductively Coupled Plasmas produced maximum etch rates for NiO up to 300 Å.min −1 and 800 Å.min −1 for β -Ga 2 O 3 under moderate plasma power conditions suitable for low damage pattern transfer. The selectivity for NiO: Ga 2 O 3 was <1 under all conditions. The ion energy threshold for initiation of etching of NiO was between 35–60 eV, depending on the condition and the etch mechanism was ion-driven, as determined by the linear dependence of etch rate on the square root of ion energy incident on the surface. By sharp contrast, the etching of Ga 2 O 3 had a stronger chemical component, without a well-defined ion energy threshold. The as-etched NiO and Ga 2 O 3 surfaces show chlorine residues, which can be removed on both materials by the standard 1NH 4 OH: 10H 2 O or 1HCl: 10H 2 O rinses used for native oxide removal. According to the location of the Cl 2p 3/2 peak, the Cl is ionically bonded.