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Edge termination is the enabling building block of power devices to exploit the high breakdown field of wide bandgap (WBG) and ultra-wide bandgap (UWBG) semiconductors. This work presents a heterogeneous junction termination extension (JTE) based on p-type nickel oxide (NiO) for gallium oxide (Ga2O3) devices. Distinct from prior JTEs usually made by implantation or etch, this NiO JTE is deposited on the surface of Ga2O3 by magnetron sputtering. The JTE consists of multiple NiO layers with various lengths to allow for a graded decrease in effective charge density away from the device active region. Moreover, this surface JTE has broad design window and process latitude, and its efficiency is drift-layer agnostic. The physics of this NiO JTE is validated by experimental applications into NiO/Ga2O3 p–n diodes fabricated on two Ga2O3 wafers with different doping concentrations. The JTE enables a breakdown voltage over 3.2 kV and a consistent parallel-plate junction field of 4.2 MV/cm in both devices, rendering a power figure of merit of 2.5–2.7 GW/cm2. These results show the great promise of the deposited JTE as a flexible, near ideal edge termination for WBG and UWBG devices, particularly those lacking high-quality homojunctions.

Recently, gallium oxide (Ga2O3) has attracted great interest as material for efficient power devices. Yet, experimental studies rather concentrate on the investigation of bare dies and lack the analysis of devices in industry standard packages. Furthermore, while Ga2O3 trench MOS Schottky barrier diodes (SBDs) appear to be promising candidates, temperature-dependent measurements previously revealed inhomogeneous junctions. In this work, a vertical β-Ga2O3 trench MOS Schottky barrier diode is presented. The diode exhibits a homogeneous Schottky junction and nearly ideal thermionic current flow. This indicates better junction properties than previously observed for trench as well as non-trench Ga2O3 SBDs, with only a slight influence of interface states at high temperatures. As a next step towards application, the chip is successfully bonded in an industry-standard TO-247 package. The molded discrete is operational at low temperatures of -50 °C and up to high temperatures of 150 °C while exhibiting a lower increase in on-resistance with rising temperature than SiC Schottky diodes.

Cellular engraftment onto an engineered scaffold is of significant importance for successful implantation. Herein, we propose the use of three-dimensional (3D) silica nonwoven fabrics (SNFs, Cellbe...

Trihalide vapor phase epitaxy (THVPE) is a new type of halide vapor phase epitaxy (HVPE) that uses GaCl3 as a group III source, enabling Ga2O3 growth without particle generation, although the growth rate is low. In this study, β-Ga2O3 is grown by THVPE using solid GaCl3 as a group III precursor. The growth rate increases linearly with increasing partial pressure of the precursor. The dependence of the growth rate on the VI/III ratio is revealed on sapphire substrates, with the growth rate reaching a maximum at a VI/III ratio of 95. We have also obtained a growth rate of 32.2 μm h−1 on β-Ga2O3 (001) substrates with no particle generation, crystal quality equivalent to that of the substrate, and high purity equivalent to that of HVPE.

We propose using gallium suboxide, Ga2O, as a Ga source for the growth of high-purity Ga2O3 by vapor phase epitaxy. It is shown in a thermochemical analysis that the suboxide can be generated effectively in the reaction between Ga2O3 and Ga and subsequently be utilized for the epitaxial growth of Ga2O3. A demonstration of Ga2O3 crystal growth was carried out on β-Ga2O3 (001) substrates with Ga2O and O2 used as the gaseous precursors, resulting in high-purity epitaxial layers. No possible donor impurities from the sources or growth environment, such as Si or Sn, were detected in the grown layers.

We demonstrate high-performance normally-off multi-fin β-Ga2O3 vertical transistors with a wide fin width from 1.0 to 2.0 μm by using a nitrogen-doped β-Ga2O3 high-resistive layer grown by halide vapor phase epitaxy. Normally-off operation was achieved with a threshold voltage of ≥1.3 V, a specific on-resistance of 2.9 mΩ·cm2 and a current density of 760 A cm−2 at a gate voltage of +10 V. The estimated MOS channel field effect mobility was ∼100 cm2 V−1 s−1. These findings offer important insights on the development of Ga2O3 MOSFETs and show the great promise of Ga2O3 vertical power devices.

The growth of large-diameter high-resistivity β-Ga2O3 (010) substrates is important for the low-cost production of lateral Ga2O3 devices. We grew a 2 inch diameter Fe-doped high-resistivity β-Ga2O3 (010) single crystal by using the vertical Bridgman (VB) method, which is expected to grow large-diameter β-Ga2O3 crystals with various crystal orientations. Two-inch substrates were prepared from the obtained crystals, and their crystallinity, concentration of Fe dopants, and electrical properties were investigated. Consequently, a 2 inch β-Ga2O3 (010) substrate, which is comparable to the largest size of (010) substrate prepared using the Czochralski method, was successfully fabricated with the VB method. The in-plane distribution of the X-ray rocking curve from 020 diffraction of the fabricated 2 inch substrate showed that the full widths at half maximums were less than 35 arcsec at almost all measurement points, indicating high crystallinity and high in-plane uniformity. In addition, the crystals contain Fe concentrations in the range of 3.5 × 1018–1.9 × 1019 cm−3, indicating that impurity Si donors are sufficiently compensated by the Fe dopants. Therefore, substrates prepared using the VB method exhibited high resistivities of 6 × 1011–9 × 1012 Ω·cm at room temperature.

High uniformity of the thickness and net donor concentrations (N d-N a) in 100 mm diameter β-Ga2O3 (001) epitaxial wafer prepared by halide vapor phase epitaxy was demonstrated. An epitaxial wafer grown on a substrate with a miscut angle near 0° generated a specific epitaxial region with a lower growth rate and a higher N d-N a than the rest of the region, which deteriorated the wafer uniformity. In contrast, epitaxial wafers prepared on substrates with a miscut angle of –0.1° suppressed the occurrence of the specific epitaxial region, and an average thickness after chemical mechanical polishing of 10.6 μm with 5% tolerance and an average N d-N a without intentionally doping of 1.1 × 1016 cm−3 within 7% tolerance were successfully obtained.

Benefitted from progress on the large-diameter Ga2O3 wafers and Ga2O3 processing techniques, the Ga2O3 power device technology has witnessed fast advances toward power electronics applications. Recently, reports on large-area (ampere-class) Ga2O3 power devices have emerged globally, and the scope of these works have gone well beyond the bare-die device demonstration into the device packaging, circuit testing, and ruggedness evaluation. These results have placed Ga2O3 in a unique position as the only ultra-wide bandgap semiconductor reaching these indispensable milestones for power device development. This paper presents a timely review on the state-of-the-art of the ampere-class Ga2O3 power devices (current up to >100 A and voltage up to >2000 V), including their static electrical performance, switching characteristics, packaging and thermal management, and the overcurrent/overvoltage ruggedness and reliability. Exciting research opportunities and critical technological gaps are also discussed. National Science Foundation [ECCS-2230412]; Center for Power Electronics Systems High Density Integration Industry Consortium; [ECCS-2100504] Published version We appreciate the in-person discussions with Ga2O3 researchers in IWGO2022. The work at Virginia Tech is in part supported by National Science Foundation under Grants ECCS-2100504 and ECCS-2230412 and in part by the Center for Power Electronics Systems High Density Integration Industry Consortium.

This study elucidates the dislocation responsible for the leakage current in a halide vapor phase epitaxial (001) β-Ga2O3 Schottky barrier diode. A high reverse leakage current of −0.98 μA was observed at −100 V as an emission pattern via ultrahigh-sensitive emission microscopy. This was attributed to the presence of dislocation. Moreover, the Burgers vector of this dislocation was determined to be ⫽ 〈1 3 ¯ 2〉 from the g·b invisibility criteria via synchrotron X-ray topography observation. This dislocation was speculated to be induced by the relaxation of the strain field surrounding the void and subsequently propagated to the surface. Cross-sectional scanning transmission electron microscopy revealed multiple threading dislocations along the (100) plane.

Activation of implanted donors into a highly-resistive, nitrogen-doped homoepitaxial β-Ga2O3 has been investigated. Nitrogen acceptors with the concentration of ∼1017 cm−3 were incorporated during epitaxial growth yielding low-doped (net donor concentration 14 cm−3) films subsequently implanted with Si, Ge, and Sn. Upon Ohmic contact formation to the implanted regions, sheet resistance values of 314, 926, and 1676 Ω/sq were measured at room temperature for the Si-, Ge-, and Sn-implanted samples, respectively. Room temperature Hall measurements resulted in sheet carrier concentrations and Hall mobilities of 2.13 × 1014 /93, 8.58 × 1013/78, and 5.87 × 1013/63 cm2/(V s), respectively, for these three donor species. Secondary ion mass spectroscopy showed a volumetric dopant concentration of approximately 2 × 1019 cm−3 for the three species, resulting in carrier activation efficiencies of 64.7%, 40.3%, and 28.2% for Si, Ge, and Sn, respectively. Temperature-dependent Hall effect measurements ranging from 15 to 300 K showed a nearly constant carrier concentration in the Si-implanted sample, suggesting the formation of an impurity band indicative of degenerate doping. With a bulk carrier concentration of 1.3 × 1019 cm−3 for the Si implanted sample, a room temperature mobility of 93 cm2/(V s) is among the highest reported in Ga2O3 with a similar carrier concentration. The unimplanted Ga2O3:N regions remained highly resistive after the surrounding areas received implant and activation anneal. These results open the pathway for fabricating Ga2O3 devices through the selective n-type doping in highly resistive epitaxial Ga2O3.

International audience

This letter demonstrates vertical Ga2O3 Schottky barrier diodes (SBDs) with a novel edge termination, the small-angle beveled field plate (SABFP), fabricated on thinned $\text{G}_{{2}}\text{O}_{{3}}$ substrates. Non-punch-though design is used for the drift region with a donor concentration of ${3}\sim {3.5} \times {10} ^{{16}}$ cm−3, rendering a device differential ON-resistance of $\sim 2~\text{m}\Omega ~\cdot $ cm2. A new wet-etch technique is developed by using a bi-layer mask, which consists of spin-on-glass (SOG) and plasma-enhanced chemical vapor deposited (PECVD) SiO2, to fabricate a very small bevel angle (∼ 1°) in the mesa and field plates. This SABFP structure facilitates the electric field spreading at device edges, rendering a breakdown voltage of 1100 V, a peak electric field of 3.5 MV/cm in Ga2O3 at the Schottky contact edge, and an averaged electric field over 3.4 MV/cm underneath the contact. Our device demonstrates a Baliga’s figure of merit of 0.6 GW/cm2, which is among the highest in all reported Ga2O3 power devices and comparable to the state-of-the-art GaN SBDs. These results show the great potential of Ga2O3 SBDs for future power applications.

For solid oxide fuel cells (SOFCs), durability to carbon deposition is a common problem to use hydrocarbon fuels from the city gas and the biogas. Identification of degradation factors and locations leads to development of diagnosis method and optimized operation control for prevention of failure and accident. Electrochemical data by the degradation give useful parameters for health diagnosis. In this study, we are aiming to understand spatial degradation, so that we prepared an anode-supported planar cell in which the cathode layer was segmented to three parts. We fed dry methane gas to the anode as accelerated aging tests and measured the current distribution directly among the upstream, midstream and downstream parts along the anode flow channels. Electrochemical impedance spectroscopy analysis was carried out in each segment as well. Upstream part of the cell exhibited larger overpotentials with more enhanced carbon deposition than the other parts with the dry methane fuel.

$\beta $ -Ga2O3 is an attractive material for high-voltage applications and has the potential for monolithically integrated RF devices. A combination of Raman nano-particle thermometry measurement and thermal simulation has been used to measure the peak channel temperature due to self-heating in $\beta $ -Ga2O3 MOSFETs. The peak channel thermal resistance measured at the gate surface in the device center was $88~mm\,\! \cdot \, K/W$ . This value is higher than what has been previously reported using electrical methods, which determine an average temperature over the whole device area. Experimentally validated thermal simulations have been used to propose possible thermal management mitigation approaches.

Dislocation responsible for leakage current in a halide vapor phase epitaxial (HVPE) (001) β-Ga2O3 Schottky barrier diode (SBD) was explored via ultrahigh-sensitive emission microscopy, synchrotron X-ray topography, and scanning transmission electron microscopy (STEM). Light emission due to the reverse leakage current was observed at the dislocation position with a leakage current of −0.98 μA at −100 V. Moreover, the dislocation is visible for the g vectors of 605 and 225; however, it is invisible for 2 ¯ 24 and 10 ¯ 05 . The b was determined to be ⫽ 〈 1 3 ¯ 2 〉 from the g·b invisible criteria. This dislocation was determined mixed-type.

In this study, we modified Lennard-Jones (LJ) parameters and point-charge parameters of the DREIDING force field (the modified force-field model is named DREIDING-UT). While the original LJ parameters of DREIDING were derived through an analytical formula to reproduce the potential depths and the equilibrium lengths of the Buckingham potentials of DREIDING/X6, the modified LJ parameters were derived through the least square fitting of the Buckingham potentials. Because the Gasteiger-Marsili (GM) charges of the original DREIDING underestimated electrostatic interactions, we replaced it with the restrained electrostatic potential (RESP) charges calculated from the ab initio wavefunctions, taking the dynamic electron correlation and solvation effects into account. To confirm how the modified force field works, we conducted molecular dynamics (MD) simulations of typical liquids. It was found that the densities and self-diffusion coefficients of the DREIDING-UT model agreed with the experimental ones much better than those of the original model.

Using bright-field x-ray topography based on anomalous transmission (AT), we have demonstrated the first large-area total-thickness imaging of dislocations in β-Ga2O3 at the substrate scale. The dislocation images were acquired from the entire 10 mm × 15 mm × 680 μm (001)-oriented substrate prepared by edge-defined film-fed growth (EFG) by stitching together hundreds of topographic images, each recorded with the forward-diffracted beam in the Laue geometry for g = 020, 0–20, 022, and 400, under the conditions in which AT occurred. Dislocations distributed over the entire crystal volume were imaged as long as their Burgers vectors ( b) were not orthogonal to the g-vectors. The results of the g· b analysis of the dislocation contrasts clearly revealed three major dislocation types that were numerically dominant in the EFG crystal: (i) b-axis screw-type dislocations with b∥ ξ∥[010] ( ξ is the unit vector of line direction), (ii) b-axis edge-type dislocations with b∥[001] and ξ∥[010], and (iii) curved mixed-type dislocations lying on the (001) planes with b∥[010]. Based on their b - and ξ-vectors, types (i) and (ii) were attributed to dislocations that propagated during EFG pulling up along the [010] direction, while type (iii) was attributed to dislocations generated through glide in the [010](001) slip system under stress. The extent to which AT can manifest itself is explained by using the effective absorption coefficient calculated for the above g-vectors based on dynamical x-ray diffraction theory.

International audience; Ultra-wide-bandgap gallium oxide (Ga2O3) devices have recently emerged as promising candidates for power electronics; however, the low thermal conductivity (kT) of Ga2O3 causes serious concerns about their electrothermal ruggedness. This work presents the first experimental demonstrations of large-area Ga2O3 Schottky barrier diodes (SBDs) packaged in the bottom-side-cooling and double-side-cooling configurations, and for the first time, characterizes the surge current capabilities of these packaged Ga2O3 SBDs. Contrary to popular belief, Ga2O3 SBDs with proper packaging show high surge current capabilities. The double-side-cooled Ga2O3 SBDs with a 3x3 mm2 Schottky contact area can sustain a peak surge current over 60 A, with a ratio between the peak surge current and the rated current superior to that of similarly-rated commercial SiC SBDs. The key enabling mechanisms for this high surge current are the small temperature dependence of on-resistance, which strongly reduces the thermal runaway, and the double-side-cooled packaging, in which the heat is extracted directly from the Schottky junction and does not need to go through the low-kT bulk Ga2O3 chip. These results remove some crucial concerns regarding the electrothermal ruggedness of Ga2O3 power devices and manifest the significance of their die-level thermal management.