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As a viable alternative to the challenging fabrication of robust β-Ga2O3 p–n homojunctions, this study investigates the variable-temperature photocurrent of p-NiO/n-Ga2O3 heterojunction photodiodes under zero-bias conditions. The device's built-in electric field is utilized to achieve efficient separation of non-equilibrium photogenerated carriers. To support the experimental findings, computer simulations of the electric field distribution at the heterointerface were performed and correlated with experimental current–voltage and capacitance–voltage measurements. The photocurrent measurements confirm the narrowing of the n-Ga2O3 bandgap with increasing temperature, consistent with predictions from the Varshni equation. The observed decrease in photocurrent amplitude at lower temperatures is attributed to bandgap widening, which results in a smaller number of non-equilibrium carriers generated by the excitation wavelength. 

This study investigates minority electron diffusion length and carrier recombination phenomena in p-type 300 nm-thick Ga2O3 films homoepitaxially grown over a (001) tin-doped β-Ga2O3 conductive substrate. This research is novel due to its systematic and near-simultaneous measurements in the top layer of a p-Ga2O3/n-Ga2O3 structure using independent electron beam-induced current and cathodoluminescence techniques. Previous work primarily focused on heteroepitaxial architectures or gallium oxide grown over insulating substrates of the same material. In this work, the activation energies related to point defects in gallium oxide were extracted from temperature-dependent incremental electron beam irradiation experiments to gain insight into the defect landscape and its influence on minority carrier transport and recombination dynamics. 

Abstract Lateral Schottky or heterojunction rectifiers were irradiated with 10 MeV protons and neutrons. For proton irradiation, the forward current of both types of rectifiers decreased by approximately an order of magnitude, with a corresponding increase in on-state resistance. The resultant on/off ratio improved after irradiation because of the larger decrease in reverse current compared to forward current. Both types of rectifiers displayed a shift in forward current and RON curves to lower voltages after irradiation. This could be due to defects created by neutron irradiation introducing deep energy levels within the bandgap of AlN. These deep levels can trap charge carriers, reducing their mobility and increasing the on-state resistance. Transmission electron microscopy showed disorder created at the AlN/NiO interface by neutron irradiation. TCAD simulation was used to study the effects of irradiation with both protons and neutrons. The results confirmed that the irradiation caused a significant reduction in electron concentration and a small increase in the recombination rate. Neutron irradiation can also introduce interface states at the metal or oxide-semiconductor junction of the rectifier. These interface states can modify the effective Schottky barrier height, affecting the forward voltage drop and on-state resistance. 

Minority carrier diffusion length in undoped p-type gallium oxide was measured at various temperatures as a function of electron beam charge injection by electron beam-induced current technique in situ using a scanning electron microscope. The results demonstrate that charge injection into p-type β-gallium oxide leads to a significant linear increase in minority carrier diffusion length followed by its saturation. The effect was ascribed to trapping of non-equilibrium electrons (generated by a primary electron beam) on metastable native defect levels in the material, which in turn blocks recombination through these levels. While previous studies of the same material were focused on probing a non-equilibrium carrier recombination by purely optical means (cathodoluminescence), in this work, the impact of charge injection on minority carrier diffusion was investigated. The activation energy of ∼0.072 eV, obtained for the phenomenon of interest, is consistent with the involvement of Ga vacancy-related defects. 

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. 

It has recently been demonstrated that electron beam injection into p-type β-gallium oxide leads to a significant linear increase in minority carrier diffusion length with injection duration, followed by its saturation. The effect was ascribed to trapping of non-equilibrium electrons (generated by a primary electron beam) at meta-stable native defect levels in the material, which in turn blocks recombination through these levels. In this work, in contrast to previous studies, the effect of electron injection in p-type Ga2O3 was investigated using cathodoluminescence technique in situ in scanning electron microscope, thus providing insight into minority carrier lifetime behavior under electron beam irradiation. The activation energy of ∼0.3 eV, obtained for the phenomenon of interest, is consistent with the involvement of Ga vacancy-related defects. 

Forward bias hole injection from 10-nm-thick p-type nickel oxide layers into 10-μm-thick n-type gallium oxide in a vertical NiO/Ga2O3 p–n heterojunction leads to enhancement of photoresponse of more than a factor of 2 when measured from this junction. While it takes only 600 s to obtain such a pronounced increase in photoresponse, it persists for hours, indicating the feasibility of photovoltaic device performance control. The effect is ascribed to a charge injection-induced increase in minority carrier (hole) diffusion length (resulting in improved collection of photogenerated non-equilibrium carriers) in n-type β-Ga2O3 epitaxial layers due to trapping of injected charge (holes) on deep meta-stable levels in the material and the subsequent blocking of non-equilibrium carrier recombination through these levels. Suppressed recombination leads to increased non-equilibrium carrier lifetime, in turn determining a longer diffusion length and being the root-cause of the effect of charge injection. 

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. 

Neutrons generated through charge-exchange 9 Be (p; n i ) 9 Be reactions, with energies ranging from 0–33 MeV and an average energy of ∼9.8 MeV were used to irradiate conventional Schottky Ga 2 O 3 rectifiers and NiO/Ga 2 O 3 p-n heterojunction rectifiers to fluences of 1.1–2.2 × 10 14 cm −2 . The breakdown voltage was improved after irradiation for the Schottky rectifiers but was highly degraded for their NiO/Ga 2 O 3 counterparts. 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.