Deep traps in Ga₂O₃ Schottky diodes induced by soft electric breakdown

Abstract

We report the electrical characteristics and deep trap behavior of vertical Ga₂O₃ Schottky diodes with diameters ranging from 200 to 380 μm, fabricated on commercial n/n⁺ epistructures. There was a significant inverse relationship between diode diameter and breakdown voltage (V_B). Specifically, the V_B for a 200 μm diameter diode was 330 V, decreasing to 225 V for 300 μm, and further to 200 V for 380 μm diameter diodes. This scaling is attributed to pre-existing extended defects and those forming under high voltage stress, which act as localized electric field concentrators and enhance current leakage. Electron beam induced current (EBIC) mapping post-breakdown revealed new bright spots and lines within the diodes, indicative of increased local current gain due to defect agglomerates. Following breakdown, a pronounced increase in reverse current was observed at low voltages, which is explained by Poole–Frenkel hopping. This hopping mechanism is predominantly mediated by deep centers with energy levels near E_c −0.7 eV, identified as the dominant E2* traps from pre-breakdown deep trap spectra. While capacitance deep level transient spectroscopy (DLTS) showed only slight changes in trap concentrations, current deep level transient spectroscopy (CDLTS) exhibited a strong signal for E1 and E2* traps after breakdown, suggesting a non-uniform formation of these centers in the vicinity of extended defects, leading to enhanced localized leakage.

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Introduction

The thermodynamically stable polymorph β-Ga₂O₃ is a promising material for next generation power electronic devices because of the commercial availability of high electrical and structural quality, large diameter native substrates grown from the melt.^1,2 In addition, there is a strong experience base with epitaxial films prepared by various techniques, and rapidly maturing processing techniques for fabrication of power devices with impressive performance.^1,2 Currently substrates with up to 6-inch diameter prepared by edge-defined film-fed growth (EFG) or Bridgman techniques are commercially available, while epitaxial films of Ga₂O₃ can be grown by halide vapor phase epitaxy (HVPE), metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE).^1–10 Crystals and films can be doped n-type with concentrations between 10¹⁵ cm⁻³ and 10¹⁹ cm⁻³, or made semi-insulating with high resistivity by Fe, N or Mg doping.² The absence of viable p-type doping in Ga₂O₃ has been circumvented by growth of p–n heterojunctions of n-Ga₂O₃ with naturally p-type wide-bandgap compounds among which the most prominent are heterojunctions (HJs) of p-NiO/n-Ga₂O₃. Efforts to master p-type formation in β-Ga₂O₃ by using complexes of Ga vacancy acceptors with hydrogen or by capitalizing on hopping conductivity via deep acceptors have also shown promise.¹¹ Ohmic and Schottky contact formation, preparation of Metal–Insulator–Semiconductor (MIS) structures, ion implantation and annealing, dry etching and passivation processes are also well advanced.^1–10

There have been numerous theoretical and experimental studies of defects in Ga₂O₃, their introduction by irradiation and their transformation upon annealing.^12–19 This has aided development of Ga₂O₃-based power devices with promising performance such as power rectifiers with breakdown voltages of about 10 kV^20,21 resistant to high power surges²² or power Field Effect Transistors (FETs) with breakdown voltages of 10 kV²³ and high-power FETs with oscillation frequencies of 35 GHz.²⁴

Breakdown voltage is an important characteristic of power devices. An extensive general treatment of the factors determining the values of breakdown voltage in wide-bandgap semiconductors including Ga₂O₃²⁵ underlines the difficulties in achieving robust avalanche breakdown in wide-bandgap semiconductors. Point and extended defects are important in the processes leading to early breakdown via the formation of defect leakage paths through extended defects. Deep level defects determining the mechanisms of current flow in high electric fields through changing the charge occupation of defect centers and their possible migration, thus influencing the electric field strength and tunneling probabilities of charge carriers.

Recently multiple experimental papers have treated various aspects of defect transformations as affecting the electric breakdown and current instabilities in Ga₂O₃ devices. The structural changes observed in Ga₂O₃ power Schottky diodes were reported in a number of ex situ and in situ transmission electron microscopy studies^26–29 that revealed formation of vacancy clusters and amorphized inclusions under electrical stress that could be transformed into dislocations and stacking faults and lead to premature breakdown.

Photoluminescence (PL) studies of trench Ga₂O₃ structures subjected to high electrical stress revealed nanometer-scale movement of defects responsible for 3.2 eV and 3.6 eV PL bands attributed to self-trapped-holes (STH) states formed at two different oxygen atoms sites O1 and O2.³⁰ This has been attributed to the local difference in the oxygen vacancies formation and movement consequently changing the local relative concentrations of the O1 and O2 sites.

Monitoring of changes caused by prolonged forward bias stress and application of high reverse bias on the shape of current–voltage I–V characteristics of NiO/Ga₂O₃ HJ rectifiers at different temperatures³¹ showed a shift of the turn-on voltage ΔV_on and emergence of shoulders in the forward I–V characteristics. Those were accompanied by an increase in reverse current. Both the ΔV_on and the reverse current recovered with time after the stress. The stress time was changed from 1 s to 10 000 s at three different temperatures. From the dependence of ΔV_on on stress and recovery times, an activation energy of 0.46 eV was deduced and attributed to the operation of electron trap with that energy. Such stress led to a strong decrease of the breakdown voltage from 1.9 kV to 1.2 kV.

In a similar stress test, Sijie Bu et al.³² monitored I–V characteristics changes of Ga₂O₃ Schottky diodes induced by forward current stress at 8V for 50 s, 500 s and 5000 s and observed a marked increase in both forward current and reverse current with the emergence of a low voltage step in forward current. Simultaneously, it was observed that the ideality factor in the forward current branch increased from η = 1 to >4 after stress. Analyzing the temperature dependence of reverse current showed it was described by the Poole–Frenkel^32,33 mechanism assuming hopping of injected electrons via deep traps with the activation energy of 0.76 eV. This is very close to the predominant deep electron trap detected in deep level transient spectroscopy (DLTS) measurements ascribed to the so-called E2* electron trap associated with gallium vacancies V_Ga or V_Ga–V_O divacancies.³⁴ After the electrical stress, the concentration of this E2* center increased in the region adjacent to the Schottky diode with the current flow determined by the trap-assisted tunneling mechanism.³⁵

Alternative explanations of the nature of deep traps determining the changes of electrical properties of NiO/Ga₂O₃ p–n HJs and Ga₂O₃ Schottky diodes have also appeared.^36,37 In the first of these, the decrease of forward current of the HJ and increase of the series resistance has been attributed to the emergence of deep hole traps at E_v + 1.3 eV attributed to a complex of Ga vacancy V_Ga acceptors with hydrogen. It was postulated the emergence of this feature is due to increase in V_Ga density and the release of free hydrogen available for complexing. In the second paper, a correlation was noted between the decrease of breakdown voltage in Ga₂O₃ Schottky diodes with the difference in density of E3 electron traps near E_c −1.1 eV tentatively associated with oxygen vacancies V_O. This was demonstrated by I–V measurements and DLTS spectra measurements performed on Schottky diodes prepared on commercial epitaxial structure that contained the E3 centers only in the ∼180 nm of the sample adjacent to the Schottky diode. The breakdown voltage remarkably increased once this E3-rich layer had been removed by polishing. The forward bias stressing caused reverse breakdown degradation and thermally induced failure on both the Ni/Au Schottky contact and the epitaxial layer due to the low thermal conductivity of Ga₂O₃.³⁶ The temperature profile at the surface during the rectifier turn-on period showed a strong dependence with crystalline orientation. The maximum junction temperature rise occured at the center of the metal contact and was in the range 270–350 °C. Stabilization of Schottky rectifiers under thermal aging is enhanced by formation of PtOx interfacial layers.³⁷

The question of which deep traps in Ga₂O₃ affect or are correlated with breakdown voltage is currently not clear.³⁸ In this paper, we present electrical breakdown measurements and deep trap analysis performed on Ni Schottky diodes prepared on commercial epistructure of the type now commonly used for fabrication of high-performance power rectifiers. Because no edge termination techniques were used in our structure, the measured breakdown voltages are lower than in more refined devices, but the observations are still relevant to understanding the nature of underlying processes.

Experimental

The vertical Schottky diode rectifiers were prepared on commercial n-Ga₂O₃ (shallow Si donor concentration N_d = 2 × 10¹⁶ cm⁻³) 10-μm-thick epitaxial structures grown by halide vapor phase epitaxy (HVPE) on (001)-oriented 600-μm thick n⁺-Ga₂O₃ Sn doped (N_d = 3 × 10¹⁸ cm⁻³) substrates fabricated by edge-defined film-fed (EFG) method (Tamura/Novel Crystal Technologies, Japan). The top epitaxial surface was chemo-mechanically polished and epi-ready, the back substrate surface was mechanically polished. The back full-area Ohmic contact was prepared by deposition of Ti/Au (40 nm/80 nm). A set of Ni/Au (50 nm/350 nm) Schottky diodes with diameters 200, 300 μm or 380 μm was prepared by photolithography. No mesa etching and passivation was applied in preparation of these structurers. Samples were mounted on the gold-plated Kovar alloy plate with conductive glue. Diodes were wired using thermocompression with 18 μm gold wires. The diode structure and a photograph of the sample are shown in Fig. 1(a and b).

Fig. 1 Ga₂O₃-based Schottky diode. (a) – diode structure, (b) – sample photo.

The fabricated diodes were characterized by current–voltage (I–V) measurements from 80–500 K using either Agilent B1500 semiconductor parameter analyzer (USA) and Linkam HFSX350-PB4 Probe Stage (Great Britain) or B2902A current–voltage source/meter (Keysight Technologies, USA) and cold-finger liquid nitrogen cryostat (Cryotrade, Russia).³⁹ Capacitance–voltage (C–V) profiling in the frequency range 20 Hz–20 MHz was performed to obtain the charge concentration profiles and the Schottky barrier height versus temperature in the temperature range 80–500 K. These measurements were done using either Agilent B1500 semiconductor parameters analyzer (USA) or E4980A LCR meter (Keysight Technologies, USA). These devices were also used to measure the temperature dependence of capacitance and AC conductance at different frequencies (so called Admittance Spectra (AS)⁴⁰ and to measure Deep Level Transient Spectra (DLTS).^39,40 Such measurements were complemented by Current Deep Level Transient Spectra⁴⁰ using B2902A current–voltage source/meter (Keysight Technologies, USA). Experimental setups are described elsewhere.^41–43 Breakdown voltage (V_B) and high-voltage IV measurements were performed using Keysight B2987B source-meter (Keysight Technologies, USA) at room temperature. When measuring BV, samples were connected to source in series with protection resistor 20 MΩ, so that the current was limited and breakdown was non-destructive (“soft” breakdown). V_B measurements were provided using pulse method: high negative voltage was pulsed from zero bias for 50 ms with 1 s pause between pulses.

In addition, the samples were characterized by electron beam induced current (EBIC) mapping using a scanning electron microscope JSM-840 SEM (JEOL) at room temperature with beam energy E_b varying from 10 keV to 30 keV and beam current of 0.1 nA. A Keithley 428 Keithley, USA) current amplifier was used. The mapping was performed with and without application of reverse bias. The reverse bias was changed from 0 V to −5 V. EBIC images were obtained with dwell time 5 μs per pixel and 1024 × 1024 pixel per frame. The experimental setup is described elsewhere.^44,45

Results and discussion

Room temperature I–V characteristics of diodes as a function of diameter are presented in Fig. 2(a). The reverse current at voltages below 10 V was extremely low and close to the detection limit. At elevated temperatures, the reverse current linearly increased with the ratio of the diode area S to diode perimeter P, indicating prominent perimeter-related leakage (Fig. 2(c)).⁴⁰ The turn-on voltage was about 0.8 V if assessed, as is usual, this value to be corresponding to the current density exceeding 10⁻³ A cm⁻² (note that Fig. 2(a) presents the current dependence on voltage, not the current density dependence). The specific series resistance at room temperature was 10 mΩ cm² and the ideality factor close to 1. The saturation current in forward direction increased with an activation energy of 1.1 eV. The reverse-recovery time from forward current 20 mA was 6 ns.

Fig. 2 (a) Room temperature I–V characteristics measured for diode diameters 200, 300 and 380 μm before and after electric breakdown; (b) reverse current voltage characteristics at high voltages for different diode diameters, the dashed line shows the current limited by the 20 MΩ resistor in series; (c) current density at 250 °C at −5 V as a function of the ratio of diodes area S to diodes perimeter P.

In C–V profiling, the 1/C²versus voltage dependences were linear, indicating a uniform shallow donor density distribution for depths up to at least 0.7 μm (Fig. 3(a)). The measured carrier concentration was not dependent on temperature, indicating a low contribution of deep traps, while the built-in voltage was 1 V at room temperature and changed slightly with temperature (Fig. 3(b)).

Fig. 3 (a) Room temperature charge concentration profiles calculated from C–V measurements before breakdown (black line) and after breakdown (red line); (b) 1/C²versus voltage plots for the 380 μm diameter diode at different temperatures.

DLTS spectra measurements revealed the presence of dominant electron traps with level at E_c −0.72 eV, electron capture cross-section σ_n = 2.4 × 10⁻¹⁵ cm², so called E2* trap often attributed to Ga vacancy–nitrogen vacancy (V_Ga–V_O) complexes.³³ Two other centers detected were so called E2 trap near E_c −0.8 eV (σ_n = 2.5 × 10⁻¹⁴ cm²) associated with Fe acceptors on Ga site,³³ and electron trap E1 with level near E_c −0.6 eV (σ_n = 1.4 × 10⁻¹⁴ cm²) assigned to complexes of Si donors with hydrogen.³³Fig. 4(a) shows the spectra of ΔC = C(t₁) − C(t₂) normalized by steady-state capacitance C, with t₁ and t₂ being the time windows.⁴⁰ We did not see a measurable signal from the electron traps E3 with level near E_c −1.1 eV³³ that were reported to be very prominent in some Ga₂O₃ rectifiers and associated with lowering of the observed breakdown voltages in rectifiers.³⁹ The carrier concentration profiles and the deep traps concentration profiles obtained by measurements with reverse bias voltages up to 30 V (the depth of 1.5 μm) showed that both are fairly uniform.

Fig. 4 (a) DLTS spectra measured for the 380 μm diameter diode before (blue line) and after (red line) breakdown, measurements were done with voltage pulsing from −5 V to 1 V (pulse length t_p = 50 ms), the spectra are shown for time windows 150 ms/1500 ms; (b) CDLTS spectra measured for the 380 μm diameter diode before (blue line) and after (red line) breakdown with reverse bias −40 V, forward pulsing to 1 V for 1 s, the signal ΔI is normalized by the steady-state current at −40 V, the spectra are shown for the time windows 60 ms/1200 ms.

CDLTS measurements were performed by keeping the sample at reverse bias −40 V and pulsing voltage to 1 V for 1 s. Fig. 4(b) shows the spectra of ΔI = I(t₁) − I(t₂) normalized to the steady-state current I_ss at −40 V. The sensitivity to deep traps of CDLTS is lower than in capacitance DLTS and no measurable spectra could be observed before the breakdown.

Fig. 2(b) presents the results of the “soft” breakdown voltage measurements for the diodes with different diameters. (We here call the breakdown “soft” in the technical sense that the series resistance switched in series with the Schottky diode limits the maximal current and prevents the irreversible catastrophic breakdown.) The breakdown voltage increases considerably as the diode diameter decreases. Specifically, the BV was 330 V for 200 μm diameter diode, 225 V for 300 μm diode, and 200 V for 380 μm diode. Such strong dependence of the breakdown voltage on diameter has been observed previously²⁰ and attributed to the presence of certain extended defects. Indeed, the existence of such defects can be inferred from EBIC mapping.

After such a soft breakdown, the samples were not irreparably damaged, but their reverse current increased considerably, as shown in Fig. 2(a). The temperature dependence of reverse current for such damaged diodes with diameter 380 μm is shown in Fig. 5(a). The reverse current was found to increase with voltage proportionally to E^1/2 (E being the electric field), which is the hallmark feature of the Poole–Frenkel current flow via hopping between the deep centers in the space charge region.³² This leakage mechanism is described by the following equation:

where C is a constant, E – electric field, q – elementary charge, ϕ – Poole–Frenkel emission potential barrier height, ε_s – the relative dielectric constant, ε₀ – the permittivity of vacuum, k_B – Boltzmann constant, and T – temperature.

Fig. 5 (a) I–V characteristics measured at different temperatures after the breakdown for the 380 μm diameter diode; (b) the temperature dependence of coefficient B in Poole–Frenkel equation, used to calculate the height of the barrier ϕ.

Then the current dependence on electric field is described by the expression (2):

where is the curve slope, is the offset.

The temperature dependence of the B coefficient thus obtained is presented in Fig. 5(b). The dependence is not strictly linear, with the low temperature region showing the activation barrier 0.65 eV compatible with the energy of the dominant E2* center. Above room temperature the barrier is higher, 1.2 eV, possibly due to another deep center (possibly E3) not detected in our DLTS/CDLTS spectra.

The ideality factor in forward I–V characteristics became much higher than 1 at temperatures below 160 K, which is often attributed to the predominance of trap assisted tunneling (TAT) mechanism due to tunneling via deep traps,^31,46 presumably the dominant E2* centers³¹ or possibly via E3 centers at higher temperatures. These observations suggest that the concentration of the said traps increases after the breakdown. However, the charge concentration profiles and DLTS spectra of such damaged samples are changed only slightly as can be seen from Fig. 3 and 4. In the CDLTS spectra in Fig. 4(b) one observes emergence of strong features due to the E2* and E1 traps not present in the pristine spectra. The explanation we propose is that the distribution of the centers is highly nonuniform and contributes to strong changes in the current flowing through regions with locally strongly enhanced concentration of deep traps. Note also that the Poole–Frenkel analysis also suggests the emergence of the E3 defect centers after the breakdown. These centers, however, are not clearly seen in CDLTS spectra because the reverse current at high voltages is too high at high temperatures necessary to detect this trap.

It should be mentioned that CDLTS spectra shown in Fig. 4(b) are depicted for time windows shorter than for DLTS spectra in Fig. 4(a) and for much higher quiescent reverse bias. The necessity to use shorter time windows comes from CDLTS sensitivity being lower than in DLTS. However, in contrast to DLTS, the signal in CDLTS increases in magnitude with shorter time window t₁ as 1/t₁,⁴⁰ hence the use of shorter time windows, although even with the same time windows as used in DLTS we still see a measurable signal in CDLTS spectra, albeit with much lower magnitude.

EBIC mapping supports this explanation, as can be seen from Fig. 6 and 7. At 0 V bias we observe in diodes not subjected to breakdown, that there are featureless images in EBIC mode (Fig. 6(a)). With increasing bias, bright contrast appears on the diode rim (Fig. 6(b)). At 5 V reverse bias, a few bright points can be also seen in the EBIC image (marked with arrows in Fig. 6(c)). In contrast, for the diodes after the soft breakdown, one bright spot can be seen even at 0 bias (marked with arrows in Fig. 7(a)). New bright dots inside the diode and horizontal lines appear already at 1 V with the brightest one associated with the defect seen at 0 V (Fig. 7(b)). With increasing bias, both bright dots and lines number and their brightness increases (Fig. 7(c–e)). Similar behavior has been already observed in κ-Ga₂O₃.⁴⁷ The bright dots in the EBIC mode are due to local enhancement of collected current, i.e. current gain. At the current high enough the amplifier is overloaded, and its relaxation leads to appearance of bright lines, with the width increasing with the local gain. Gain in the Schottky barriers increases with electric field.^47,48 Thus, the appearance of bright spots and increase of both their number and intensity shows that some extended defects or/and point defect agglomerates are present in the crystal, which enhance electric field near metal/Ga₂O₃ interface. After soft breakdown, the density of such defects increases.

Fig. 6 EBIC maps of the 300 μm diode before the breakdown as obtained with different applied reverse voltage.

Fig. 7 EBIC maps of the 300 μm diode after breakdown for different applied reverse voltages.

Transmission electron microscope studies^27,28 indicate that, even in high-quality epilayers, defects, such strings of Ga vacancies and amorphized regions, are formed during electric breakdown and give rise to excessive leakage paths. Recently, conductive interstitial Ga-related line defects were observed in β-Ga₂O₃.⁴⁹ All such defects could increase local electric field and hole trapping on them could lead to the local collected current gain, and in turn to the bright EBIC contrast. Recently, it has also been suggested that bunches of split Ga vacancies can be formed, which results in excess leakage.⁵⁰ Our CDLTS spectra measured after breakdown could point to generation of high densities of E1 and E2* centers in the vicinity of such agglomerates.

Another noteworthy fact that is that near the breakdown, repeated measurements of I–V characteristics show definite trends. Fig. 8 shows the I–V characteristics in one of the 200 μm diameter diodes measured after several sweeps bringing the voltage close to breakdown but not exceeding it. The shape of the I–V characteristics strongly implies that, at high voltages, the current flow is determined by the space-charge-limited current (SCLC).²⁴ The region of rapid increase of current with voltage of the form I–V^α with high α values shows a steady shift to higher threshold voltage values with each consecutive voltage sweep, presumably because of the increased thickness of the layer in which traps are filled after previous voltage sweeps.²⁴ When changing the direction of voltage sweep in this voltage region, one observes strong hysteresis of the I–V characteristics because of the changes of the filling of the traps. The effect is more pronounced after the soft breakdown. The width of the hysteresis reflects the change in the accumulated charge on the traps and is less pronounced at high temperatures. As seen in Fig. 8(b) the integrated charge is much higher after the breakdown and the width of the loop decreases at elevated temperature (the data are presented for two sweep directions indicated by arrows and for two temperatures, 290 K (black lines) and 400 K (red lines)).

Fig. 8 (a) Current–voltage characteristics measured on 200 μm diameter diode in several consecutive tries; (b) reverse current hysteresis before (solid lines) and after (dashed lines) breakdown; measurements are shown for 300 K (black lines) and 400 K (red lines), the arrows indicate the direction of the voltage sweep.

Conclusions

We have studied electrical properties and deep trap spectra before and after soft electric breakdown for different diameter Schottky diode rectifiers prepared on commercial n/n⁺ Ga₂O₃ epistructures. Such structures show low reverse leakage current at room temperature, and the deep traps spectra in them are dominated by E2* centers (E_c −0.72 eV) associated with V_Ga–V_O. divacancies.³³ Two other deep traps detected were the E2 (E_c −0.8 eV) acceptors due to substitutional Fe on Ga site and the E1 (E_c −0.6 eV) acceptors attributed to Si–H complexes.³³ At high reverse voltage, the I–V characteristics are determined by the SCLC mechanism.²⁴ The soft breakdown voltage (i.e. breakdown mitigated by a high resistance in series with the source thus preventing the sample destruction) strongly increases with decreasing the diodes diameter in the order 330 V/225 V/200 V for diameters 200 μm/300 μm/380 μm, respectively. It could be mentioned that the breakdown can also justifiably be called as “soft” in the sense assigned to the term in ref. 51 where it is called because the breakdown is to large extent caused by the changes induced in the Schottky barrier height but the deep centers. As we have seen above, that seems to be a plausible explanation of the observed phenomena. After the breakdown, the reverse current strongly increased and, at moderate reverse voltages, was determined by Poole–Frenkel type hopping via deep traps in which the E2* traps played a prominent role. This would suggest a strong increase in density of the deep traps participating in hopping. However, the increase in capacitance DLTS and changes in concentration profiles calculated from C–V measurements were small. However, in current DLTS, a strong signal due to the E1, E2* traps emerged, suggesting that the traps determining the current increase are located near extended defects created (or transformed) at high reverse voltages. EBIC mapping of the samples after the breakdown indicates the appearance of extended defects giving rise to enhanced leakage. The presence of such defects explains the dependence of the breakdown voltage on the diodes area.^52,53 The role of different deep traps in premature breakdown of Ga₂O₃ Schottky diodes shows these being the E2*(E_c −0.76 eV),³¹ E3 (E_c −1.1 eV),³⁶ E1-like E_c −0.46 eV³⁰ centers. The E3 centers have not been detected in the Schottky diodes in the present work. The E2* and E1 centers concentrations increased judging by CDLTS spectra measurements, but not in capacitance DLTS, which suggests that the density of these centers increases locally near some extended defects both inside the diodes and at the diodes boundaries where the electric field is strong. EBIC mapping reveals the presence of such defects emerging after the breakdown and serving as high leakage sites for increasing reverse voltage. The exact atomic structure of these defects needs further investigation. In view of the results presented here, one might consider whether the different deep traps found to be correlated with the observed premature breakdown voltages are indeed the cause of these premature breakdowns or the markers indicating that the centers in question are segregated near the extended defects actually causing the premature breakdown.

Author contributions

A. Y. Polyakov: conceptualization, methodology, validation, investigation, writing, supervision. I. V. Shchemerov: methodology, validation, investigation, writing. E. B. Yakimov: methodology, validation, investigation, writing. A. V. Chernykh: methodology, writing. S. V. Chernykh: methodology, writing. A. A. Vasilev: methodology, writing. N. R. Matros: methodology, validation. A. A. Romanov: methodology, validation. L. A. Alexanyan: methodology, validation. E. E. Yakimov: methodology, validation, investigation, writing. S. J. Pearton: methodology, validation, investigation, writing.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data for this article are available within the text.

Acknowledgements

The work at NUST MISIS was supported initially by a grant from the Ministry of Science and Higher Education of Russian Federation (Agreement No. 075-15-2022-1113) and then by a grant from the Russian Science Foundation (Agreement 25-72-10020). The work at IMT RAS was supported by the State Task No 075-00295-25-00. The work at UF was performed as part of the Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA), sponsored by the Department of the Defense, Defense Threat Reduction Agency, under Award No. HDTRA1-20-2-0002, monitored by Jacob Calkins.

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