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Abstract Large-density functional theory (DFT) databases are a treasure trove of energies, forces, and stresses that can be used to train machine-learned interatomic potentials for atomistic modeling. Herein, we employ structural relaxations from the AFLOW database to train moment tensor potentials (MTPs) for four carbide systems: CHfTa, CHfZr, CMoW, and CTaTi. The resulting MTPs are used to relax ~6300 random symmetric structures, and are subsequently improved via active learning to generate robust potentials (RP) that can relax a wide variety of structures, and accurate potentials (AP) designed for the relaxation of low-energy systems. This protocol is shown to yield convex hulls that are indistinguishable from those predicted by AFLOW for the CHfTa, CHfZr, and CTaTi systems, and in the case of the CMoW system to predict thermodynamically stable structures that are not found within AFLOW, highlighting the potential of the employed protocol within crystal structure prediction. Relaxation of over three hundred (Mo_1−xW_x)C stoichiometry crystals first with the RP then with the AP yields formation enthalpies that are in excellent agreement with those obtained via DFT. 

Ultra-high temperature ceramics (UHTCs) are refractory transition-metal carbides, nitrides, and borides with the highest melting temperatures known materials, making them prime candidates for applications in aerospace and hypersonic vehicles. Of the UHTCs, tantalum carbide (TaC) and hafnium carbide (HfC) feature the highest melting temperatures. We investigated the binderless consolidation of HfC/TaC powder blends using Field Assisted Sintering Technology (FAST). Powders consisting of 90/10, 50/50, and 10/90 vol% HfC:TaC were sintered to high densities (>94 %). Bulk and nanomechanical, chemical, and microstructural characterization revealed substantially greater strength, hardness, and stiffness for ternary alloys. Mechanical properties correlated with physiochemical analysis indicated trace oxygen phases, solid-solution strengthening, and nonstoichiometric carbon were the key mechanisms driving the peak property enhancement of the 50 vol% solid-solution sample, despite lower densities. This study provides insight into optimizing the compositional design of HfC-TaC alloys by balancing influences from solid solution strengthening and the thermodynamic effects of oxygen/carbon stoichiometry. 

Abstract In this study, we explore the rejuvenation of a Zener diode degraded by high electrical stress, leading to a leftward shift, and broadening of the Zener breakdown voltage knee, alongside a 57% reduction in forward current. We employed a non-thermal annealing method involving high-density electric pulses with short pulse width and low frequency. The annealing process took <30 s at near-ambient temperature. Raman spectroscopy supports the electrical characterization, showing enhancement in crystallinity to explain the restoration of the breakdown knee followed by improvement in forward current by ∼85%. 

Thermal annealing is a widely used strategy to enhance semiconductor device performance. However, the process is complex for multi-material multi-layered semiconductor devices, where thermoelastic stresses from lattice constant and thermal expansion coefficient mismatch may create more defects than those annealed. We propose an alternate low temperature annealing technique, which utilizes the electron wind force (EWF) induced by small duty cycle high density pulsed current. To demonstrate its effectiveness, we intentionally degrade AlGaN/GaN high electron mobility transistors (HEMTs) with accelerated OFF-state stressing to increase ON-resistance ∼182.08% and reduce drain saturation current ∼85.82% of pristine condition at a gate voltage of 0 V. We then performed the EWF annealing to recover the corresponding values back to ∼122.21% and ∼93.10%, respectively. The peak transconductance, degraded to ∼76.58% of pristine at the drain voltage of 3 V, was also recovered back to ∼92.38%. This recovery of previously degraded transport properties is attributed to approximately 80% recovery of carrier mobility, which occurs during EWF annealing. We performed synchrotron differential aperture x-ray microscopy measurements to correlate these annealing effects with the lattice structural changes. We found a reduction of lattice plane spacing of (001) planes and stress within the GaN layer under the gate region after EWF annealing, suggesting a corresponding decrease in defect density. Application of this low-temperature annealing technique for in-operando recovery of degraded electronic devices is discussed. 

High-power electronics, such as GaN high electron mobility transistors (HEMTs), are expected to perform reliably in high-temperature conditions. This study aims to gain an understanding of the microscopic origin of both material and device vulnerabilities to high temperatures by real-time monitoring of the onset of structural degradation under varying temperature conditions. This is achieved by operating GaN HEMT devices in situ inside a transmission electron microscope (TEM). Electron-transparent specimens are prepared from a bulk device and heated up to 800 °C. High-resolution TEM (HRTEM), scanning TEM (STEM), energy-dispersive x-ray spectroscopy (EDS), and geometric phase analysis (GPA) are performed to evaluate crystal quality, material diffusion, and strain propagation in the sample before and after heating. Gate contact area reduction is visible from 470 °C accompanied by Ni/Au intermixing near the gate/AlGaN interface. Elevated temperatures induce significant out-of-plane lattice expansion at the SiNx/GaN/AlGaN interface, as revealed by geometry-phase GPA strain maps, while in-plane strains remain relatively consistent. Exposure to temperatures exceeding 500 °C leads to almost two orders of magnitude increase in leakage current in bulk devices in this study, which complements the results from our TEM experiment. The findings of this study offer real-time visual insights into identifying the initial location of degradation and highlight the impact of temperature on the bulk device’s structure, electrical properties, and material degradation. 

Strain plays an important role in the performance and reliability of AlGaN/GaN high electron mobility transistors (HEMTs). However, the impact of strain on the performance of proton irradiated GaN HEMTs is yet unknown. In this study, we investigated the effects of strain relaxation on the properties of proton irradiated AlGaN/GaN HEMTs. Controlled strain relief is achieved locally using the substrate micro-trench technique. The strain relieved devices experienced a relatively smaller increase of strain after 5 MeV proton irradiation at a fluence of 5 × 1014 cm−2 compared to the non-strain relieved devices, i.e., the pristine devices. After proton irradiation, both pristine and strain relieved devices demonstrate a reduction of drain saturation current (Ids,sat), maximum transconductance (Gm), carrier density (ns), and mobility (μn). Depending on the bias conditions the pristine devices exhibit up to 32% reduction of Ids,sat, 38% reduction of Gm, 15% reduction of ns, and 48% reduction of μn values. In contrast, the strain relieved devices show only up to 13% reduction of Ids,sat, 11% reduction of Gm, 9% reduction of ns, and 30% reduction of μn values. In addition, the locally strain relieved devices show smaller positive shift of threshold voltage compared to the pristine devices after proton irradiation. The less detrimental impact of proton irradiation on the transport properties of strain relieved devices could be attributed to reduced point defect density producing lower trap center densities, and evolution of lower operation related stresses due to lower initial residual strain. 

Abstract Radiation susceptibility of electronic devices is commonly studied as a function of radiation energetics and device physics. Often overlooked is the presence or magnitude of the electrical field, which we hypothesize to play an influential role in low energy radiation. Accordingly, we present a comprehensive study of low-energy proton irradiation on gallium nitride high electron mobility transistors (HEMTs), turning the transistor ON or OFF during irradiation. Commercially available GaN HEMTs were exposed to 300 keV proton irradiation at fluences varying from 3.76 × 10¹²to 3.76 × 10¹⁴cm², and the electrical performance was evaluated in terms of forward saturation current, transconductance, and threshold voltage. The results demonstrate that the presence of an electrical field makes it more susceptible to proton irradiation. The decrease of 12.4% in forward saturation and 19% in transconductance at the lowest fluence in ON mode suggests that both carrier density and mobility are reduced after irradiation. Additionally, a positive shift in threshold voltage (0.32 V and 0.09 V in ON and OFF mode, respectively) indicates the generation of acceptor-like traps due to proton bombardment. high-resolution transmission electron microscopy and energy dispersive x-ray spectroscopy analysis reveal significant defects introduction and atom intermixing near AlGaN/GaN interfaces and within the GaN layer after the highest irradiation dose employed in this study. According toin-situRaman spectroscopy, defects caused by irradiation can lead to a rise in self-heating and a considerable increase in (∼750 times) thermoelastic stress in the GaN layer during device operation. The findings indicate device engineering or electrical biasing protocol must be employed to compensate for radiation-induced defects formed during proton irradiation to improve device durability and reliability.