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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. 

The characteristics of NiO/β-(Al0.21Ga0.79)2O3/Ga2O3 heterojunction lateral geometry rectifiers with the epitaxial layers grown by metal organic chemical vapor deposition were measured over a temperature range from 25 °C–225 °C. The forward current increased with temperature, while the on-state resistance decreased from 360 Ω.cm²at 25 °C to 30 Ω.cm²at 225 °C. The forward turn-on voltage was reduced from 4 V at 25 °C to 1.9 V at 225 °C. The reverse breakdown voltage at room temperature was ∼4.2 kV, with a temperature coefficient of −16.5 V K⁻¹. This negative temperature coefficient precludes avalanche being the breakdown mechanism and indicates that defects still dominate the reverse conduction characteristics. The corresponding power figures-of-merit were 0.27–0.49 MW.cm⁻². The maximum on/off ratios improved with temperature from 2105 at 25 °C to 3 × 107 at 225 °C when switching from 5 V forward to 0 V. The high temperature performance of the NiO/β-(Al0.21Ga0.79)2O3/Ga2O3 lateral rectifiers is promising if the current rate of optimization continues.

 

Power semiconductor devices are fundamental drivers for advances in power electronics, the technology for electric energy conversion. Power devices based on wide-bandgap (WBG) and ultra-wide bandgap (UWBG) semiconductors allow for a smaller chip size, lower loss and higher frequency compared with their silicon (Si) counterparts, thus enabling a higher system efficiency and smaller form factor. Amongst the challenges for the development and deployment of WBG and UWBG devices is the efficient dissipation of heat, an unavoidable by-product of the higher power density. To mitigate the performance limitations and reliability issues caused by self-heating, thermal management is required at both device and package levels. Packaging in particular is a crucial milestone for the development of any power device technology; WBG and UWBG devices have both reached this milestone recently. This paper provides a timely review of the thermal management of WBG and UWBG power devices with an emphasis on packaged devices. Additionally, emerging UWBG devices hold good promise for high-temperature applications due to their low intrinsic carrier density and increased dopant ionization at elevated temperatures. The fulfillment of this promise in system applications, in conjunction with overcoming the thermal limitations of some UWBG materials, requires new thermal management and packaging technologies. To this end, we provide perspectives on the relevant challenges, potential solutions and research opportunities, highlighting the pressing needs for device–package electrothermal co-design and high-temperature packages that can withstand the high electric fields expected in UWBG devices.

 

NiO/β-(Al x Ga 1− x ) 2 O 3 /Ga 2 O 3 heterojunction lateral geometry rectifiers with diameter 50–100  μm exhibited maximum reverse breakdown voltages >7 kV, showing the advantage of increasing the bandgap using the β-(Al x Ga 1− x ) 2 O 3 alloy. This Si-doped alloy layer was grown by metal organic chemical vapor deposition with an Al composition of ∼21%. On-state resistances were in the range of 50–2180 Ω cm 2 , leading to power figures-of-merit up to 0.72 MW cm −2 . The forward turn-on voltage was in the range of 2.3–2.5 V, with maximum on/off ratios >700 when switching from 5 V forward to reverse biases up to −100 V. Transmission line measurements showed the specific contact resistance was 0.12 Ω cm 2 . The breakdown voltage is among the highest reported for any lateral geometry Ga 2 O 3 -based rectifier. 

We report the first experimental demonstration of a vertical superjunction device in GaN. P-type nickel oxide (NiO) is sputtered conformally in 6μm deep n-GaN trenches. Sputter recipe is tuned to enable 1017 cm −3 level acceptor concentration in NiO, easing its charge balance with the 9×1016 cm −3 doped n-GaN. Vertical GaN superjunction p-n diodes (SJ-PNDs) are fabricated on both native GaN and low-cost sapphire substrates. GaN SJ-PNDs on GaN and sapphire both show a breakdown voltage (BV) of 1100 V, being at least 900 V higher than their 1-D PND counterparts. The differential specific on-resistance (RON,SP) of the two SJ-PNDs are both 0.3mΩ⋅ cm 2 , with the drift region resistance (RDR,SP) extracted to be 0.15mΩ⋅ cm 2 . The RON,SP∼BV trade-off is among the best in GaN-on-GaN diodes and sets a new record for vertical GaN devices on foreign substrates. The RDR,SP∼BV trade-off exceeds the 1-D GaN limit, fulfilling the superjunction functionality in GaN. 

Halide vapor phase epitaxial (HVPE) Ga2O3 films were grown on c-plane sapphire and diamond substrates at temperatures up to 550 °C without the use of a barrier dielectric layer to protect the diamond surface. Corundum phase α-Ga2O3 was grown on the sapphire substrates, whereas the growth on diamond resulted in regions of nanocrystalline β-Ga2O3 (nc-β-Ga2O3) when oxygen was present in the HVPE reactor only during film growth. X-ray diffraction confirmed the growth of α-Ga2O3 on sapphire but failed to detect any β-Ga2O3 reflections from the films grown on diamond. These films were further characterized via Raman spectroscopy, which revealed the β-Ga2O3 phase of these films. Transmission electron microscopy demonstrated the nanocrystalline character of these films. From cathodoluminescence spectra, three emission bands, UVL′, UVL, and BL, were observed for both the α-Ga2O3/sapphire and nc-Ga2O3/diamond, and these bands were centered at approximately 3.7, 3.2, and 2.7 eV.

 

β-Ga2O3 is an emerging ultra-wide bandgap semiconductor, holding a tremendous potential for power-switching devices for next-generation high power electronics. The performance of such devices strongly relies on the precise control of electrical properties of β-Ga2O3, which can be achieved by implantation of dopant ions. However, a detailed understanding of the impact of ion implantation on the structure of β-Ga2O3 remains elusive. Here, using aberration-corrected scanning transmission electron microscopy, we investigate the nature of structural damage in ion-implanted β-Ga2O3 and its recovery upon heat treatment with the atomic-scale spatial resolution. We reveal that upon Sn ion implantation, Ga2O3 films undergo a phase transformation from the monoclinic β-phase to the defective cubic spinel γ-phase, which contains high-density antiphase boundaries. Using the planar defect models proposed for the γ-Al2O3, which has the same space group as β-Ga2O3, and atomic-resolution microscopy images, we identify that the observed antiphase boundaries are the {100}1/4 ⟨110⟩ type in cubic structure. We show that post-implantation annealing at 1100 °C under the N2 atmosphere effectively recovers the β-phase; however, nano-sized voids retained within the β-phase structure and a γ-phase surface layer are identified as remanent damage. Our results offer an atomic-scale insight into the structural evolution of β-Ga2O3 under ion implantation and high-temperature annealing, which is key to the optimization of semiconductor processing conditions for relevant device design and the theoretical understanding of defect formation and phase stability.