Introduction

Monoclinic β-Ga 2 O 3 has recently garnered significant attention for power switching devices and solar-blind UV photodetectors [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Notable advancements have been made in NiO/β-Ga 2 O 3 power rectifiers, surpassing the performance of GaN [15][16][17][18][19][20][21][22][23][24][25]. Experimental investigations have revealed maximum breakdown voltages above 8 kV, corresponding to critical electric fields > 8 MV cm -1 [23][24][25]. The intended applications for Ga 2 O 3 power electronics include next-generation power, GHz switching and RF applications, including the traction inverter, dc-dc converter and on-board charger for electric vehicles, as well as distributed energy resource systems and uni-and bi-directional power converters for renewable energy systems [2,6,8,13].

The lack of p-type doping for Ga 2 O 3 initially limited power devices to unipolar Schottky Barrier Diode (SBD) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) [2,5]. However, the implementation of p-type NiO to form heterojunctions with n-type Ga 2 O 3 has led to demonstrations of vertical rectifiers with excellent high-temperature operation [15][16][17][18][19][20][21][22][23][24][25]. The wide bandgap of Ga 2 O 3 means less charge is deposited during radiation exposure than for narrower gap materials and combined with the higher bond strength and high rate of dynamic annealing, indicates that Ga 2 O 3 may be well-suited to high radiation environments [26][27][28][29][30][31]. Ga 2 O 3 demonstrates notable resilience against total ionizing dose (TID) effects [27,28]. Recent investigations have reported single-event burnout (SEB) in Ga 2 O 3 rectifiers [32,33]. Strategies involving field management have been proposed to partially mitigate single-event effects [32][33][34][35].

The minimization of displacement damage in semiconductors is key to enhancing their survivability in environments where they are subject to high fluences of ionizing particles, such as satellites, defense systems and nuclear reactor control electronics. While shielding or turning off the devices if it is known that a radiation event will occur are some of the methods employed for reducing the effects of radiation damage, at a device level, various types of annealing may be possible. For example, in similar NiO/Ga 2 O 3 rectifiers to those studied here, annealing at 400 °C was partially successful in restoring the initial properties [36]. While the reverse leakage increased due to reaction of the contacts, the forward current and the initial carrier density in the drift region were significantly restored by this thermal annealing [36].

There are three basic carrier-driven mechanisms known to produce near-athermal annealing of defects in semiconductors. Firstly, Recombination Enhanced Defect Reactions (REDR) involve vibrational energy locally deposited at defects by nonradiative electronic transitions [37,38]. This additional energy will increase defect annealing or reaction rates [38]. This mechanism requires a pn junction and a large enough minority carrier diffusion length to affect traps within a device structure. This has been shown to cause recombination-enhanced annealing of point defects in numerous semiconductors [37][38][39][40][41][42][43][44][45]. Recombination-enhanced annealing of defects is well-established in Ga 2 O 3 , with both optical and electrical injection of minority carriers leading to increased carrier diffusion length and lifetime. Chernyak et al [39][40][41][42][43][44][45] have shown the transport properties of many other semiconductors, including ZnO and GaN, can be improved by minority carrier injection, attributed to increased minority carrier lifetime due to trapping of carriers on native point defects.

Secondly, during ion irradiation, there can be ionization enhanced-energy transfer from ions to binding electrons in the atomic structure via electron-phonon coupling [46,47]. This electronic energy transfer can also lead to annealing of damage and is reported in many semiconductor systems [46,47]. For example, in Ga 2 O 3 , Azarov et al [48] have shown that implanting ions at 300 °C effectively suppresses the defect formation. By sharp contrast, in order to reach similar crystalline quality in the samples implanted at room temperature, postirradiation anneals in excess of 900 °C are necessary. It plays a role in the so-called dynamic annealing that is significant in Ga 2 O 3 [27,48].

Thirdly, Electron Wind Force (EWF) annealing occurs by electron momentum transfer to enhance defect annealing kinetics [49][50][51][52][53][54][55][56][57][58][59][60]. The mechanical force originates from the use of a high current density at a low duty cycle to suppress heat accumulation. Rasel et al [49] demonstrated that short, high current density pulses can mobilize and anneal defects produced by 60 Co gamma irradiated of GaN high electron mobility transistors, as well as existing defects in SiC Schottky barrier diodes [52]. The EWF mechanism is well-established in electromigration and electroplasticity studies [55][56][57][58][59][60], but its application to annealing defects in electronic devices can potentially be a powerful method to reverse the degradation of devices. It is generally accepted that the athermal impact of electromigration, in addition to the thermal effect, causes the atomic diffusion to be accelerated by the electric current treatment [55][56][57][58][59][60]. Electric current can induce annealing in aluminum alloys, employing a mechanism that is not Joule heating [60].

In this paper, we report the effects of forward-biased carrier injection of carriers on damage created by 17 MeV protons in NiO/ β-Ga 2 O 3 vertical rectifiers. The displacement damage from the protons reduces the carrier concentration in the drift region of the rectifiers and reduces forward current density [44]. Partial recovery of the proton-induced is observed by carrier injection.

Experimental

The vertical geometry rectifier structures were constructed with drift region that was 10 μm thick, grown by Hydride Vapor Phase Epitaxy. This was grown on a conducting Ga 2 O 3 single crystal substrate [23][24][25]. The free carrier concentrstion in the drift region was 6.7 × 10 15 cm -3 , while the substrate was heavily doped (10 19 cm -3 ) with a (001) surface orientation. To form the heterojunction, 20 nm of rf sputtered (13.56 MHz) NiO was then deposited at low power (70 W) and a working pressure of 3 mTorr from twin targets. The Ar/O 2 ratio was varied to produce p-typ doping in the range 10 18 -2.6 × 10 19 cm -3 . The device fbrication was concluded by depositing of Ti/Au on the back of the substrate and Ni/Au to the front surface where the NiO had been deposited. A schematic representation of the device structure is provided in figure 1. More details are given in reference [36].

The iiradiation with protons was carried out on a cyclotron at a fixed energy of 17 MeV and 10 nA beam current. The fluences were set at 3, 5 and 7 × 10 13 cm -2 A. The theoretical ion profiles were obtained from the standard Stopping and Range of Ions in Matter (SRIM) code [61]. This showed the projected range of the 17 ,eV protons was ∼760 μm (figure 2). This penetration depth is much larger than the substrate thickness. Therefore the protons create damage along their path through the NiO and the Ga 2 O 3 drift region. The SR-NIEL simulator [62] was also used to estimate both the nuclear and electronic stopping energy losses. The non-ionizing energy, or nuclear stopping loss at this energy was calculated to be 5.7 × 10 -3 MeV.cm 2 g -1 . By contrast, the electronic stopping energy loss was significantly higher at 18 MeV.cm 2 g -1 (figure 3). The latter represents energy loss due to ionization, which dissipates as heat and dies not induce lattice damage. The device DC characteristics were measured with an HP 4156 parameter analyzer, while capacitance-voltage measurements were conducted with an Agilent 4284 A Precision LCR Meter. Post-irradiation annealing was performed at 300 K by forward biasing at voltages ranging from 4-10 V for durations of 1-4 h.

Results and discussion

The first measurement was that of the forward current density-voltage (J-I) characteristics. The data from HJDs after irradiation at different fluences are illustrated in figure 4. The corresponding on-state resistance, R ON , determined from the slope of these characteristics are also illustrated. Irradiation reduces this forward current density by up to two orders of magnitude. The current density is a function of both the carrier mobility and density. Therefore the creation of trapping states within the bandgap of the Ga 2 O 3 reuduces the carrier concentration in the drift region and the intridction these scatring centrars also reudces the electron mobility. Both of thes effects lead to a smaller forward current. The R ON of the rectifiers exhibit a corresponding two orders of magnitude increase at the 7 × 10 13 cm -2 fluence. The linear plot in figure 4 elucidates the trends in forward current density with increasing fluence. As proton fluence rises, the slope of the J-V characteristics decreases, indicating an increase in R ON . Additionally, the reverse current in the low bias voltage region as a function of fluence is presented in figure 4. The reverse current density increases by more than an order of magnitude due to the introduction of generation-recombination centers by proton damage. However, at the highest fluence, the current decreases again, as the competing mechanism of carrier loss to trap sites in the drift region becomes more significant.

The reduction in electron concentration in the lightly doped drift layer was determined from the C -2 -V characteristics, as depicted in figure 5 (top). The bottom portion of figure 5 presents the carrier distribution profiles obtained from the C-V data. This illustrates the higher rate of loss of carriers in the drift region for higher fluences. The carrier removal rates were found to be in the range of 120-150 cm -1 for the investigated fluence range. This aligns with studies for both NiO/Ga 2 O 3 HJDs and Ni/ Ga 2 O 3 Schottky diodes with the NiO layer [26,27]. Figure 6 compiles the reported values for electron removal rate in Ga 2 O 3 using protons of varying energies. The spread in literature values reflects the dependence of the carrier removal rate on the type of compensating impurities and pre-existing defects, as well as the startingelectron density in the drift region. At high fluences, the electron removal rate will saturate due to the fact all the carriers are trapped and additional introduction of defect states will not affect the electron density [36].

In order to study the effect of carrier injection on forward-biased devices, various bias levels and durations were tested. Recombination enhancement is a phenomenon where vibrational energy localized at a defect, and induced by a nonradiative electronic transition, accelerates the reaction rate. When the electronic energy fully transitions into vibrational excursions along the reaction pathway, the reaction rate is maximally augmented, corresponding to an effective reduction in barrier height-namely, the energy difference between the free and trapped electronic configurations.

Figure 7 shows improvements up to 20% in forward current density for biasing up to 10 V for a fixed duration of 30 min. These increases get larger with the magnituide of the applied bias voltage. For bias voltages beyond 10 V, an irreversible decrease in current density was evident, indicating potential degradation of device interfaces. Gong et al [63] reported that high carrie density injection conditions could lead to increased interface state densities at the top contact. The observed improvement in current density at moderate currents aligns with results reported by Rasel et al [49,52]. That work invoked the electron wind force mechanism. The ame basic trends were observed for the reverse current, which was made smaller by up to 80%. This is shown at the bottom of figure 7.

The results in figure 8 (top and center) report the time dependence of forward bias voltage injection at 10 V. This forward current density was larger by almost a factor of 10 for carrier I njection for 1 h. This is roughly 50% of the reducton die to the initial damage induced by the irradtion with protons. The trend was that the reverse current density was reduced monotonically with injection time (figure 8, bottom) . By sharp contrast, the forward current density was found to tdegrade for long injection periods. This was also observed in previous studies [63]. It is important to point out that conventional thermal annealing ,ust be carried out at temperatures of at least 300 °C-400 °C in order to observe significant reduxtion in the damage for similar fluence and energies to the conditions eompluyed in this work [36]. The temperatuire of devices in our experiments did not rise above ∼50 °C during carrier injection, so thermal damage is not present. To understand the mechanism for improvement of the transport properties upon carrier I jection, it is important to note that the minority carrier diffusion lengths in unirradated Ga 2 O 3 range from 350-400 nm. When subject to proton irradiation, these are effectively reduced by approximately one half their intial value [46]. This demonstrates the influence of point defect introduction on minority carrier transport; however it precludes this mechanism from mitigating damage throughout the thick drift layer of the rectifiers. C-V profiling on the partially recovered irradiated devices indicated there was a uniform increase in electron density after the carrier injection (figure 5). Since an increase would be expected only within a few minority carrier diffusion lengths of the contact if recombination-enhanced annealing were occurring, we propose that an alternative carrier-induced mechanism is present During forward biasing, holes are injected from the p-type NiO, while electrons flow toward the top contact from the heavility doped substrate. This process is analgous to the EWF mechanism, wherein the momentum of the electrons facilitates defect annealing [49][50][51][52][53][54][55][56][57][58][59][60]. This mechanism is almost athermal, as evidenced by precise measurements of the device temperature during the application of pulsed currents at low duty cycles near room temperature [50]. Future work will focus on similar studies to examine changes in lattice strain and defect density in Ga 2 O 3 rectifiers after pulsed current annealing cycles. Investigating the long-term stability will also be a priority.

Table 1 tabulates the relative changes in forward and reverse current densities, in addition to the changes in on-state resistance as a result of proton irradiation and subsequent thermal [36] or current injection annealing. Thermal annealing induces a significantly greater recovery of forward current and on-state resistance but degrades the reverse current due to reactions between the metal contacts on the Ga 2 O 3 rectifiers [36]. In contrast, the improvements observed with carrier injection are more modest, however this method does not cause thermal degradation of the device metallization.

Summary and conclusions

In synopsis, proton irradiation significantly impacts the performance of vertical geometry NiO/β-Ga 2 O 3 heterojunction rectifiers, primarily manifesting as a decrease in forward current density, thereby elevating the RON of the devices. This effect stems from diminished carrier density and mobility within the drift layer. Concurrently, the reverse leakage current experiences an approximate twofold increase with higher fluence. The removal rates of carriers within the Ga 2 O 3 layer range between 120 and 150 for the applied fluence range, akin to Schottky rectifiers lacking the NiO layer.

Following proton-induced damage, subsequent carrier injection experiments indicate partial recovery, discounting recombination-enhanced annealing due to the limited carrier diffusion length in Ga 2 O 3 . The predominant mechanism likely involves electron wind force annealing induced by momentum transfer from electronics to defect sites.However, there exists a threshold for injected charge beyond which benefits diminish. Under very heavy carrier injection conditions (forward bias of 13 V for 10 ks), the on-state current density decreases by 60%, while the reverse current density experiences a four-order-of-magnitude increase. This phenomenon is attributed to heightened interface states at the anode contact [63,64].

Table 1. Comparison of relative changes in select device parameters of proton irradiated NiO/Ga 2 O 3 rectifiers as a result of subsequent thermal annealing at 400 °C or carrier injection near room temperature. I F is the forward current density at 3 V bias, R ON the on-state resistance and I R the reverse current at 100 V.

Sample

I F (3 V) I R (100 V) R ON Proton irradiated (7-10 × 10 13 cm -2 )

↓100× ↑10× ↑100×

Thermal anneal 400 °C ↑10× ↑10× ↓10× Carrier injection (10 V) ↑20% 5× ↓20%