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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Remote Detection of Radioactive Materials Using Long Wavelength Infrared Laser Driven Avalanche Breakdown [Slides]

The yearly goals of the "Remote Detection of Radioactive Materials Using Long Wavelength Infrared Laser Driven Avalanche Breakdown" experiment are broken into two parts. Year 1: Perform proof-of-concept scaling experiments for detection of radioactive materials at 10m standoff distance, characterizing avalanche breakdowns with the 9.2μm long pulse beam line and optimizing diagnostics. Years 2-3: Extend technique to 30m detection distance, characterizing the effect of long propagation paths on sensitivity, improving diagnostic sensitivity and resolution, and characterizing the effects of turbulence and aerosols.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2022

Distributed polarization-doped GaN p–n diodes with near-unity ideality factor and avalanche breakdown voltage of 1.25 kV

Polarization-induced (Pi) distributed or bulk doping in GaN, with a zero dopant ionization energy, can reduce temperature or frequency dispersions in impurity-doped p–n junctions caused by the deep-acceptor-nature of Mg, thus offering GaN power devices promising prospects. Before comprehensively assessing the benefits of Pi-doping, ideal junction behaviors and high-voltage capabilities should be confirmed. In this work, we demonstrate near-ideal forward and reverse I–V characteristics in Pi-doped GaN power p–n diodes, which incorporates linearly graded, coherently strained AlGaN layers. Hall measurements show a net increase in the hole concentration of 8.9 × 1016 cm−3 in the p-layer as a result of the polarization charge. In the Pi-doped n-layer, a record-low electron concentration of 2.5 × 1016 cm−3 is realized due to the gradual grading of Al0-0.72GaN over 1 μm. The Pi-doped p–n diodes have an ideality factor as low as 1.1 and a 0.10 V higher turn-on voltage than the impurity-doped p–n diodes due to the increase in the bandgap at the junction edge. A differential specific on-resistance of 0.1 mΩ cm2 is extracted from the Pi-doped p–n diodes, similar with the impurity-doped counterpart. The Pi-doped diodes show an avalanche breakdown voltage of ∼1.25 kV, indicating a high reverse blocking capability even without an ideal edge-termination. This work confirms that distributed Pi-doping can be incorporated in high-voltage GaN power devices to increase hole concentrations while maintaining excellent junction properties.

Nomoto, Kazuki↗
DOE OSTI2022

Observation of breakdown wave mechanism in avalanche ionization produced atmospheric plasma generated by a picosecond CO 2 laser

Understanding the formation and long-timescale evolution of atmospheric plasmas produced by ultrashort, long-wavelength infrared (LWIR) pulses is an important but partially understood problem. Of particular interest are plasmas produced in air with a peak laser intensity ~10 12 W/cm 2 , the so-called clamping intensity observed in LWIR atmospheric guiding experiments where tunneling and multi-photon ionization operative at near-IR or shorter wavelengths are inoperative. We find that avalanche breakdown on the surface of aerosol (dust) particles can act to seed the breakdown of air observed above the 200 GW/cm 2 threshold when a train of 3 ps 10.6 μm laser pulses separated by 18 ps is used. The breakdown first appears at the best focus but propagates backward toward the focusing optic as the plasma density approaches critical density and makes forward propagation impossible. The velocity of the backward propagating breakdown can be as high as 10 9 cm/s, an order of magnitude greater than measured with ns pulse-produced breakdown, and can be explained rather well by the so-called breakdown wave mechanism. Transverse plasma expansion with a similar velocity is assisted by UV photoionization and is observed as a secondary longitudinal breakdown mechanism in roughly 10% of the shots. When a cm-size, TW power beam is propagated, interception of aerosol particles is guaranteed and several (40 cm -3 ) breakdown sites appear, each initially producing a near-critical density plasma. On a 10 ns–1 μs timescale, shockwaves from each site expand radially and coalesce to produce a large hot gas channel. The radial velocity of the expansion agrees well with the prediction of the blast wave theory developed for ultrafast atmospheric detonations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1991

High-voltage 6H-SiC p-n junction diodes

A chemical vapor deposition (CVD) process has been used to produce device structures of n- and p-type 6H-SiC epitaxial layers on commercially produced single-crystal 6H-SiC wafers. Mesa-style p-n junction diodes were successfully fabricated from these device structures using reactive ion etching, oxide passivation, and electrical contact metallization techniques. When tested in air, the 6H-SiC diodes displayed excellent rectification characteristics up to the highest temperature tested, 600 C. To observe avalanche breakdown of the p-n junction diodes, testing under a high-electrical-strength liquid was necessary. The avalanche breakdown voltage was 1000 V representing the highest reverse breakdown voltage to be reported for any CVD-grown SiC diode.

Matus, L. G.↗
NASA NTRS2011

Reliability Evaluation of Base-Metal-Electrode Multilayer Ceramic Capacitors for Potential Space Applications

Base-metal-electrode (BME) ceramic capacitors are being investigated for possible use in high-reliability spacelevel applications. This paper focuses on how BME capacitors construction and microstructure affects their lifetime and reliability. Examination of the construction and microstructure of commercial off-the-shelf (COTS) BME capacitors reveals great variance in dielectric layer thickness, even among BME capacitors with the same rated voltage. Compared to PME (precious-metal-electrode) capacitors, BME capacitors exhibit a denser and more uniform microstructure, with an average grain size between 0.3 and 0.5 m, which is much less than that of most PME capacitors. BME capacitors can be fabricated with more internal electrode layers and thinner dielectric layers than PME capacitors because they have a fine-grained microstructure and do not shrink much during ceramic sintering. This makes it possible for BME capacitors to achieve a very high capacitance volumetric efficiency. The reliability of BME and PME capacitors was investigated using highly accelerated life testing (HALT). Most BME capacitors were found to fail with an early avalanche breakdown, followed by a regular dielectric wearout failure during the HALT test. When most of the early failures, characterized with avalanche breakdown, were removed, BME capacitors exhibited a minimum mean time-to-failure (MTTF) of more than 105 years at room temperature and rated voltage. Dielectric thickness was found to be a critical parameter for the reliability of BME capacitors. The number of stacked grains in a dielectric layer appears to play a significant role in determining BME capacitor reliability. Although dielectric layer thickness varies for a given rated voltage in BME capacitors, the number of stacked grains is relatively consistent, typically around 12 for a number of BME capacitors with a rated voltage of 25V. This may suggest that the number of grains per dielectric layer is more critical than the thickness itself for determining the rated voltage and the life expectancy of the BME capacitor. The leakage current characterization and the failure analysis results suggest that most of these early avalanche failures are due to the extrinsic minor construction defects introduced during fabrication of BME capacitors. The concentration of the extrinsic defects must be reduced if the BME capacitors are considered for high reliability applications. There are two approaches that can reduce or prevent the occurrence of early failure in BME capacitors: (1) to reduce the defect concentration with improved processing control; (2) to prevent the use of BME capacitors under harsh external stress levels so that the extrinsic defects will never be triggered for a failure. In order to do so appropriate dielectric layer thickness must be determined for a given rated voltage.

Liu, David (Donhang)↗
DOE OSTI2022

Demonstration of >6.0-kV Breakdown Voltage in Large Area Vertical GaN p-n Diodes With Step-Etched Junction Termination Extensions

Vertical gallium nitride (GaN) p-n diodes have garnered significant interest for use in power electronics where high-voltage blocking and high-power efficiency are of concern. In this article, we detail the growth and fabrication methods used to develop a large area (1 mm 2 ) vertical GaN p-n diode capable of a 6.0-kV breakdown. We also demonstrate a large area diode with a forward pulsed current of 3.5 A, an 8.3-mΩ$\cdot$cm 2 differential specific ON-resistance, and a 5.3-kV reverse breakdown. In addition, we report on a smaller area diode (0.063 mm 2 ) that is capable of 6.4-kV breakdown with a differential specific ON-resistance of 10.2 mΩ$\cdot$cm 2 , when accounting for current spreading through the drift region at a 45° angle. Finally, the demonstration of avalanche breakdown is shown for a 0.063-mm 2 diode with a room temperature breakdown of 5.6 kV. In this work, these results were achieved via epitaxial growth of a 50-μm drift region with a very low carrier concentration of <1×10 15 cm –3 and a carefully designed four-zone junction termination extension.

42 ENGINEERING↗
NASA NTRS2008

Current Leakage Evolution in Partially Gate Raptured Power MOSFETs

It has been observed that power MOSFETs can experience an SEGR and continue to function with altered parameters. We propose that there are three different types of SEGR modes; the micro-break, the thermal runaway, and the avalanche breakdown. Data that demonstrates these stages of device failure are presented as well as a proposed model for the micro-break. Brief discussions of the other modes, based on analysis combined with our interpretations of the older literature, are also given.

metal oxide semiconductor field-effect transistors↗
DOE OSTI2025

Robust Avalanche (1.5 kV, 2 kA/cm²) in Vertical GaN Diodes on Patterned Sapphire Substrate

The lack of avalanche capability is a key limitation of current lateral GaN devices. Despite the report of avalanche in vertical GaN-on-GaN devices, the high wafer cost hinders device commercialization. Here, in this work, we demonstrate a circuit-level avalanche in vertical GaN diodes on low-cost patterned sapphire substrate (PSS), with the avalanche voltage (1.57 kV) and avalanche current density (>2 kA/cm 2 ) both being the highest reported in GaN devices on foreign substrates. The PSS enables a lower dislocation density than conventional sapphire substrate and is employed in high-voltage GaN devices for the first time. The avalanche voltage in the circuit test reaches 98% of the parallel-plane limit, further affirming that near-ideal avalanche breakdown can be realized on GaN devices on foreign substrates. These results show the promise of the GaN-on-PSS platform for low-cost, robust power devices.

42 ENGINEERING↗
DOE OSTI2025

Heating of Cs2Te photocathode via field emission and radiofrequency pulsed heating: Implication toward breakdown

The occurrence of radiofrequency (RF) breakdown limits operational electromagnetic gradients in accelerator structures. Experimental evidence often suggests that breakdown events are associated with temperature and dark current spikes on the surface of RF devices. In the past decade, there has been increased interest in unveiling the mechanism behind breakdown initiation in metal copper and copper alloys; however, efforts regarding a breakdown phenomenon in photocathode-relevant semiconductors have been more limited. In this work, we explore field-emission-assisted heating via Nottingham and Joule processes, as a possible candidate for breakdown initiation. For this, field emission from an intrinsic Cs2Te ultrathin film coated on a copper substrate was modeled within the Stratton–Baskin–Lvov–Fursey formalism, describing the processes and effects in the bulk and on the surface of a photocathode exposed to high RF electromagnetic fields. It is shown that a field-emission characteristic deviates significantly from the classical Fowler–Nordheim (FN) theory, whereby predicting that dark current is orders of magnitude lower than the one expected by FN law. Conventional pulsed heating was also found to impose negligible heating to the photocathode. Both conclusions suggest that a Cs2Te photocathode coated on a metal substrate would be insensitive to catastrophic thermal-material runaway breakdown, unlike what is observed for metal surfaces. Finally, a few unconventional breakdown candidate scenarios are identified and discussed, including thermoelastic deformation and avalanche breakdown.

Shinohara, Ryo (ORCID:0000000276995732)↗
DOE OSTI2023

Atmospheric effects on the laser-driven avalanche-based remote detection of radiation

Here, the effect of realistic atmospheric conditions on mid-IR (λ = 3.9 µm) and long-wave-IR (λ = 10 µm) laser-induced avalanche breakdown for the remote detection of radioactive material is examined experimentally and with propagation simulations. Our short-range in-lab mid-IR laser experiments show a correlation between increasing turbulence level and a reduced number of breakdown sites associated with a reduction in the portion of the focal volume above the breakdown threshold. Simulations of propagation through turbulence are in excellent agreement with these measurements and provide code validation. We then simulate propagation through realistic atmospheric turbulence over a long range (0.1–1 km) in the long-wave-IR regime (λ = 10 µm). The avalanche threshold focal volume is found to be robust even in the presence of strong turbulence, only dropping by ~50% over a propagation length of ~0.6 km. We also experimentally assess the impact of aerosols on avalanche-based detection, finding that, while background counts increase, a useful signal is extractable even at aerosol concentrations 10 5 times greater than what is typically observed in atmospheric conditions. Our results show promise for the long-range detection of radioactive sources under realistic atmospheric conditions.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

20 kV Gallium Nitride pn Diode Electro-Magnetic Pulse Arrestor for Grid Reliability (Final Scientific/Technical Report)

This project developed vertical Gallium Nitride (GaN) pn diodes under two main thrusts: (1) A focus on relatively higher-voltage devices for use as fast EMP arrestors to protect the electric grid; and (2) A focus on a Foundry effort to establish the manufacturability of relatively lower voltage devices. For the first thrust, the aim was to develop devices that go into avalanche breakdown to clamp the voltage across sensitive grid equipment subject to voltage transients induced by electromagnetic pulses (EMPs). Devices with breakdown voltages exceeding 6.5 kV were achieved, and breakdown times shorter than 1 ns were demonstrated, which is sufficiently fast to protect against the fast component of an EMP-induced signal. Key challenges included the epitaxial growth of thick (50 µm or more), low-doped (low 10 15 cm -3 range) GaN layers comprising the drift regions of the diodes, as well as the design and fabrication of edge termination structures (step-etched junction termination extensions) to prevent premature breakdown. Midway through the project, an additional emphasis was put on large-area, high-current devices, and forward currents of approximately 400 A were achieved in composite devices towards the end of the project. Experimental and theoretical studies of impact ionization and avalanche ruggedness were also conducted. For the second thrust (the Foundry), the focus was primarily on 1.2-kV-class devices, although towards the end of the project outstanding results on 3.3-kV class devices were also achieved. The aim of the Foundry was to develop a high-yield, reliable, and economic vertical GaN pn diode process. The Foundry conducted characterization of incoming epitaxial material and correlated this information with the yield and performance of fully processed devices. Wafer maps of diode characteristics such as forward and reverse current as well as capacitance-voltage curves were measured using auto-probing. Several mask sets comprising different-area devices as well as multiple edge termination designs (implanted junction termination extensions, guard rings, and combinations thereof) were studied, and machine-learning-based approaches were utilized to analyze the data. Packaging and reliability efforts were also undertaken for the Foundry diodes, which are necessary for a viable commercial process.

20KV↗
DOE OSTI2023

20 kV Gallium Nitride pn Diode Electro-Magnetic Pulse Arrestor for Grid Reliability (Final Report)

This project developed vertical Gallium Nitride (GaN) pn diodes under two main thrusts: (1) A focus on relatively higher-voltage devices for use as fast EMP arrestors to protect the electric grid; and (2) A focus on a Foundry effort to establish the manufacturability of relatively lower-voltage devices. For the first thrust, the aim was to develop devices that go into avalanche breakdown to clamp the voltage across sensitive grid equipment subject to voltage transients induced by electromagnetic pulses (EMPs). Devices with breakdown voltages exceeding 6.5 kV were achieved, and breakdown times shorter than 1 ns were demonstrated, which is sufficiently fast to protect against the fast component of an EMP-induced signal. Key challenges included the epitaxial growth of thick (50 um or more), low-doped (low 10 15 cm -3 range) GaN layers comprising the drift regions of the diodes, as well as the design and fabrication of edge termination structures (step-etched junction termination extensions) to prevent premature breakdown. Midway through the project, an additional emphasis was put on large-area, high-current devices, and forward currents of approximately 400 A were achieved in composite devices towards the end of the project. Experimental and theoretical studies of impact ionization and avalanche ruggedness were also conducted. For the second thrust (the Foundry), the focus was primarily on 1.2-kV-class devices, although towards the end of the project outstanding results on 3.3-kV-class devices were also achieved. The aim of the Foundry was to develop a high-yield, reliable, and economic vertical GaN pn diode process. The Foundry conducted characterization of incoming epitaxial material and correlated this information with the yield and performance of fully processed devices. Wafer maps of diode characteristics such as forward and reverse current as well as capacitance-voltage curves were measured using auto-probing. Several mask sets comprising different-area devices as well as multiple edge termination designs (implanted junction termination extensions, guard rings, and combinations thereof) were studied, and machine-learning-based approaches were utilized to analyze the data. Packaging and reliability efforts were also undertaken for the Foundry diodes, which are necessary for a viable commercial process.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2023

Implanted Guard Ring Edge Termination With Avalanche Capability for Vertical GaN Devices

Edge termination is the key building block in power devices to enable near-ideal, avalanche breakdown voltage (BV). Here, this work presents the design, fabrication, and physics of a GaN guard ring (GR) edge termination formed by selective-area nitrogen implantation through an epitaxial p-GaN layer. The fabrication of this termination only includes a single implantation step that does not require precise control of implant depth, rendering a large process latitude. The selective-area implantation produces p-GaN rings that are separated by the implanted, semi-insulating regions. The number and spacing of the p-type rings are found to determine the BV of the vertical GaN p-n diode. The 16-ring structure enables a BV of 1800 V, being 88% of the theoretical 1-D parallel-plane limit. Avalanche characteristics are observed in devices with a large variety of GR designs. Finally, we present a comprehensive survey on the efficiency, fabrication complexity, real estate, and avalanche capability of various edge termination techniques that have been reported in vertical GaN devices. The high efficiency (among the highest reported in avalanche-capable GaN terminations), simple and robust fabrication process, and uniform avalanche capability make this implanted GR a promising edge termination for high-voltage GaN devices.

42 ENGINEERING↗
DOE OSTI2022

A discussion on various experimental methods of impact ionization coefficient measurement in GaN

Impact ionization coefficients play a critical role in semiconductors. In addition to silicon, silicon carbide and gallium nitride are important semiconductors that are being seen more as mainstream semiconductor technologies. As a reflection of the maturity of these semiconductors, predictive modeling has become essential to device and circuit designers, and impact ionization coefficients play a key role here. Recently, several studies have measured impact ionization coefficients. We dedicated the first part of our study to comparing three experimental methods to estimate impact ionization coefficients in GaN, which are all based on photomultiplication but feature characteristic differences. The first method inserts an InGaN hole-injection layer, the accuracy of which is challenged by the dominance of ionization in InGaN, leading to possible overestimation of the coefficients. The second method utilizes the Franz–Keldysh effect for hole injection but not for electrons, where the mixed injection of induced carriers would require a margin of error. The third method uses complementary p–n and n–p structures that have been at the basis of this estimation in Si and SiC and leans on the assumption of a constant electric field, and any deviation would require a margin of error. In the second part of our study, we evaluated the models using recent experimental data from diodes demonstrating avalanche breakdown.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗