Showing papers by "Marko J. Tadjer published in 2023"

2.5 kV Vertical Ga2O3 Schottky Rectifier With Graded Junction Termination Extension

Abstract: This work demonstrates vertical Ga2O3 Schottky barrier diodes (SBDs) with a novel junction termination extension (JTE) comprising multiple layers of sputtered p-type nickel oxide (NiO). The NiO layers have the varied lengths to enable a graded decrease in effective charge density away from the main junction. The fabrication of this JTE obviates the etch or implantation and shows good throughput. The fabricated Ga2O3 SBDs exhibit a forward voltage below 1.9 V at the current density of 100 A/cm2, a differential specific on-resistance of 5.9 $\text{m}\Omega \cdot $ cm2, and a breakdown voltage over 2.5 kV. The Baliga’s figure of merit (FOM) exceeds 1 GW/cm2 and is among the highest in multi-kilovolt Ga2O3 SBDs. Besides, the capacitance of the JTE region is extracted, allowing for evaluation of the capacitance, charge, and switching FOM of 1.7 kV-class Ga2O3 SBDs with varied current ratings. The results show good promise of Ga2O3 SBDs for kilovolt power electronics.

NiO junction termination extension for high-voltage (>3 kV) Ga2O3 devices

Abstract: 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.

Electrothermal performance of AlGaN/GaN lateral transistors with >10 μm thick GaN buffer on 200 mm diameter engineered substrates

Abstract: Herein, the electrical and thermal performance of lateral AlGaN/GaN high electron mobility transistors (HEMTs) and metal-insulator-semiconductor field effect transistors (MISFETs) fabricated with 11 μm thick GaN buffer layers on 200 mm diameter Qromis Substrate Technology (QST) substrates are investigated. The QST substrate has a polycrystalline core engineered to be coefficient of thermal expansion (CTE)-matched to GaN to minimize wafer bow and residual stress in the GaN film as a result of epitaxial growth. Raman spectroscopy is used to determine the biaxial residual stress in the GaN buffer of the as-fabricated devices. Electrical characterization is demonstrated on the HEMTs including DC and pulsed output characteristics, DC transfer characteristics, Hall mobility, carrier concentration, sheet resistance, median transition frequency, and maximum stable gain. Finally, the thermal performance of the AlGaN/GaN MISFET is assessed via thermoreflectance thermal imaging at DC power densities up to 19 W mm−1. The thermal resistance of the MISFET, calculated using the peak temperature rises on the gate electrode for DC power densities <10 W mm−1, is measured to be 15.4 mm K W−1, which is comparable with state-of-the-art GaN-on-Si lateral transistors.

NiO Junction Termination Extension for Ga2O3 Devices: High Blocking Field, Low Capacitance, and Fast Switching Speed

Abstract: This work investigates the blocking electric field, capacitance, and switching speed of the p-type NiO based junction termination extension (JTE) for vertical Ga2O3 devices. The JTE comprises multiple NiO layers sputtered on the surface of Ga2O3 drift region, the acceptor concentration and length of which are carefully optimized. This NiO JTE enabled a breakdown voltage over 3 kV in vertical Ga2O3 diodes with a parallel-plate junction field of 4.2 MV/cm. Large-area Ga2O3 p-n diodes with a current over 1 A were fabricated to evaluate the JTE's capacitance and switching characteristics. The JTE accounts for only, ~11 % of the junction capacitance of this 1 A diode, and the percentage is expected to be even smaller for higher-current diodes. The turn-ON/OFF speed and reverse recovery time of the diode are comparable to commercial SiC Schottky barrier diodes. These results show the good promise of NiO JTE as an effective edge termination for Ga2O3 power devices.

Transient Thermal and Electrical Co-Optimization of BEOL Top-Gated ALD In2O3 FETs Toward Monolithic 3-D Integration

Abstract: In this work, the transient thermal and electrical characteristics of top-gated (TG), ultrathin, atomic-layer-deposited (ALD), back-end-of-line (BEOL) compatible indium oxide (In2O3) transistors on various thermally conductive substrates are co-optimized by visualization of the self-heating effect (SHE) utilizing an ultrafast high-resolution (HR) thermo-reflectance (TR) imaging system and overcome the thermal challenges through substrate thermal management and short-pulse measurement. At the steady-state, the temperature increase ( $\Delta {T}$ ) of the devices on highly resistive silicon (HR Si) and diamond substrates are roughly 6 and 13 times lower than that on a SiO2/Si substrate, due to the much higher thermal conductivities ( $\kappa $ ) of HR Si and diamond. Consequently, the ultrahigh drain current ( ${I}_{D}$ ) of 3.7 mA/ $\mu \text{m}$ at drain voltage ( ${V}_{\text {DS}}$ ) of 1.4 V with direct current (dc) measurement is achieved with TG ALD In2O3 FETs on a diamond substrate. Furthermore, transient thermal study shows that it takes roughly 350 and 300 ns for the devices to heat-up and cool-down to the steady-states, being independent of the substrate. The extracted thermal time constants of heat-up ( $\tau _{h}$ ) and cool-down ( $\tau _{c}$ ) processes are 137 and 109 ns, respectively. By employing electrical short-pulse measurement with a pulsewidth ( ${t}_{\text {pulse}}$ ) shorter than $\tau _{h}$ , the SHE can be significantly reduced. Accordingly, a higher ${I}_{D}$ of 4.3 mA/ $\mu \text{m}$ is realized with a 1.9-nm-thick In2O3 FET on HR Si substrate after co-optimization. Besides, to integrate BEOL-compatible ALD In2O3 transistors on the front-end-of-line (FEOL) devices with the maintenance of the satisfactory heat dissipation capability, a FEOL-interlayer-BEOL structure is proposed where the interlayer not only electrically isolates the FEOL and BEOL devices but also serves as a thermally conductive layer to alleviate the SHE.

Effect of GaN/AlGaN buffer thickness on the electrothermal performance of AlGaN/GaN HEMTs on engineered substrates

Abstract: AlGaN/GaN high electron mobility transistors on QST® engineered substrates were grown with different GaN/AlGaN buffer layer thickness. The as-grown heterostructures were evaluated for their structural quality via atomic force microscopy, high-resolution X-ray diffraction, Raman spectroscopy, and steady-state thermoreflectance. Transistor devices were fabricated and evaluated via DC and pulsed electrical techniques, as well as thermoreflectance imaging. We report that buffer layer thickness of at least 10 µm can result in lateral HEMTs with simultaneously high GaN quality, low stress, good DC electrical performance, low current collapse, and low thermal resistance. This article is protected by copyright. All rights reserved.

(Invited) NiO/ β-(Al x Ga1-x )2O3 /Ga2O3 Heterojunction Lateral Rectifiers with Reverse Breakdown Voltage > 7kV

Abstract: NiO/ β-(Al x Ga1-x )2O3 /Ga2O3 heterojunction lateral geometry rectifiers with diameter 50-100 µm exhibited maximum reverse breakdown voltages >7kV, showing the advantage of increasing the bandgap using the β-(Al x Ga1-x )2O3 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 50-2180 Ω.cm2, leading to power figures-of-merit up to 0.72 MW.cm-2. The forward turn-on voltage was in the range 2.3-2.5 V, with maximum on/off ratios >700 when switching from 5V forward to reverse biases up to -100V. Transmission line measurements showed the specific contact resistance was 0.12 Ω.cm2. The breakdown voltage is among the highest reported for any lateral geometry Ga2O3-based rectifier.

NiO/β-(AlxGa1−x)2O3/Ga2O3 heterojunction lateral rectifiers with reverse breakdown voltage >7 kV

Abstract: NiO/β-(Al xGa1− x)2O3/Ga2O3 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 xGa1− x)2O3 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 cm2, 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 cm2. The breakdown voltage is among the highest reported for any lateral geometry Ga2O3-based rectifier.

Efficient Activation and High Mobility of Ion-Implanted Silicon for Next-Generation GaN Devices

Abstract: Selective area doping via ion implantation is crucial to the implementation of most modern devices and the provision of reasonable device design latitude for optimization. Herein, we report highly effective silicon ion implant activation in GaN via Symmetrical Multicycle Rapid Thermal Annealing (SMRTA) at peak temperatures of 1450 to 1530 °C, producing a mobility of up to 137 cm2/Vs at 300K with a 57% activation efficiency for a 300 nm thick 1 × 1019 cm−3 box implant profile. Doping activation efficiency and mobility improved alongside peak annealing temperature, while the deleterious degradation of the as-grown material electrical properties was only evident at the highest temperatures. This demonstrates efficient dopant activation while simultaneously maintaining low levels of unintentional doping and thus a high blocking voltage potential of the drift layers for high-voltage, high-power devices. Furthermore, efficient activation with high mobility has been achieved with GaN on sapphire, which is known for having relatively high defect densities but also for offering significant commercial potential due to the availability of cheap, large-area, and robust substrates for devices.

N2O grown high Al composition nitrogen doped β-(AlGa)2O3/β-Ga2O3 using MOCVD

Abstract: We report on the MOCVD growth of smooth (010) (AlxGa1–x)2O3 and (100) (AlyGa1–y)2O3 epitaxial films on β-Ga2O3 substrates with (010) and (100) orientations, respectively, using N2O for oxidation. High resolution x-ray diffraction was used to evaluate the phase purity and strain characteristics of the (AlGa)2O3 layers and estimate the Al composition. The incorporation efficiency of Al into the (AlGa)2O3 films depends on process conditions, including chamber pressure, growth temperature, and gas phase Al concentration. Layers grown at lower reactor pressure and substrate temperature and higher gas phase Al concentration showed higher Al incorporation. Pure beta phase (AlGa)2O3 films with a record high Al composition of x = 30% for a film grown on a (010) β-Ga2O3 substrate and with an Al composition of up to y = 45% on the (100) β-Ga2O3 substrate was realized by introducing ∼18% Al mole fraction into the reactor. N2O grown β-(AlGa)2O3/β-Ga2O3 superlattice structures with an Al composition of 5% were also demonstrated on both substrate orientations. When higher gas phase Al concentration is introduced into the reactor, pure γ-phase (AlxGa1–x)2O3 is grown on (010) β-Ga2O3 substrates. In contrast, on the (100) β-Ga2O3 substrate, the (AlyGa1–y)2O3 layers are β-phase, but with two separate Al compositions owing to the local Al segregation. The nitrogen doping of (010) β-(AlxGa1–x)2O3 with [N] ranging 6 × 1017–2 × 1019 cm−3 was achieved using N2O. Higher Al composition and lower substrate temperature lead to higher N incorporation. The results show that using N2O as an oxygen source can lead to the growth of high Al content β-(AlGa)2O3, which paves the way for the realization of efficient power devices, such as modulation-doped field effect transistors.