Hsiao-Hsuan Wan

University of Florida

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F. Ren

University of Florida

Jian-Sian Li

University of Florida

S. J. Pearton

University of Florida

Chao-Ching Chiang

University of Florida

Xinyi Xia

University of Florida

Yu‐Te Liao

National Yang Ming Chiao Tung University

Fikadu Alema

Agnitron Technology (United States)

Timothy Jinsoo Yoo

University of Florida

Honggyu Kim

University of Florida

A. Osinsky

Agnitron Technology (United States)

Cooperative Institutions

University of Florida

National University of Science and Technology

Korea University

Pennsylvania State University

United States Naval Research Laboratory

Physico-Technical Institute

Institute of Microelectronics Technology and High Purity Materials

National Academies of Sciences, Engineering, and Medicine

National Yang Ming Chiao Tung University

Ioffe Institute

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Switching of kV-class Ga2O3 heterojunction vertical rectifiers

Journal of Vacuum Science & Technology A Vacuum Surfaces and Films (2024)

Jian-Sian Li Chao-Ching Chiang Hsiao-Hsuan Wan Meng-Hsun Yu Yiting Lin

Switching of vertical 6.1 kV/4A NiO/Ga2O3 rectifiers from voltages up to 1.45 kV showed reverse recovery times of 75 ns, current slew rate of 39.0 A/μs, and energy loss of ∼105 μW. These are the highest switching voltages reported for Ga2O3 rectifiers. To place the results in context, commercial 3.3 kV/5 A SiC merged PiN Schottky diodes showed reverse recovery times of 20 ns, current slew rate of 47.5 A/μs, and energy loss of ∼67 μW. The validity of comparing unpackaged experimental Ga2O3 diodes with commercial Si or SiC diodes without considering their differences in chip size and consequently in capacitive charge and ON-resistance is restricted. However, the results show the rapid progress in these devices when compared to commercial SiC rectifiers.

Slew rate

10.1116/6.0003839

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15 MeV proton damage in NiO/β-Ga₂O₃vertical rectifiers

Journal of Physics Materials (2023)

Jian-Sian Li Chao-Ching Chiang Xinyi Xia Hsiao-Hsuan Wan Jihyun Kim

Abstract 15 MeV proton irradiation of vertical geometry NiO/ β -Ga 2 O 3 heterojunction rectifiers produced reductions in reverse breakdown voltage from 4.3 kV to 3.7 kV for a fluence of 10 13 ions·cm −2 and 1.93 kV for 10 14 ions·cm −2 . The forward current density was also decreased by 1–2 orders of magnitude under these conditions, with associated increase in on-state resistance R ON . These changes are due to a reduction in carrier density and mobility in the drift region. The reverse leakage current increased by a factor of ∼2 for the higher fluence. Subsequent annealing up to 400 °C further increased reverse leakage due to deterioration of the contacts, but the initial carrier density of 2.2 × 10 16 cm −3 was almost fully restored by this annealing in the lower fluence samples and by more than 50% in the 10 14 cm −2 irradiated devices. Carrier removal rates in the Ga 2 O 3 were in the range 190–1200 for the fluence range employed, similar to Schottky rectifiers without the NiO.

Non-blocking I/O

Reverse leakage current

10.1088/2515-7639/acef98

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Lateral NiO/AlN Heterojunction Rectifiers with Breakdown Voltage >11 kV

ECS Advances (2024)

Hsiao-Hsuan Wan Jian-Sian Li Chiao-Ching Chiang Md Hafijur Rahman Aman Haque

Lateral NiO/AIN heterojunction diodes (HJDs) with breakdown voltage up to 11.6 kV and Ni/Au/AlN Schottky barrier diodes (SBDs) with V B of 8.6 kV were fabricated on layers grown on sapphire substrates by metalorganic chemical vapor phase deposition. The power figure-of-merits V B 2 /R ON where R ON is the on-resistance were 0.31 MW·cm −2 for HJD and 0.16 MW·cm −2 for SBD. The lowest turn-on voltages were ∼2.03 and 1.91 V for HJDs and SBDs, respectively, with ON/OFF ratios up to 10 2 . The maximum field before breakdown was 0.45 MV·cm −1 in HJDs and 0.31 MV·cm −1 in SBDs. These correspond to <3% of the critical field in AlN of ∼15 MV·cm −1 . This work demonstrates there is still significant optimization to be done in the overall quality of the AlN, including purity, crystal perfection, and defect density to realize the potential of this material as an ultra-wide bandgap semiconductor for efficient multi-kV power switching applications. Our results also demonstrate the promise of NiO as a p-type conducting oxide for forming heterojunctions with AlN.

Non-blocking I/O

10.1149/2754-2734/ad7867

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Selective and nonselective plasma etching of (Al0.18Ga0.82)2O3/ Ga2O3 heterostructures

Journal of Vacuum Science & Technology A Vacuum Surfaces and Films (2024)

Hsiao-Hsuan Wan Chao-Ching Chiang Jian-Sian Li F. Ren Fikadu Alema

The addition of CHF3 to Cl2/Ar inductively coupled plasmas operating at low dc self-biases (<100 V, corresponding to incident ion energies <125 eV) leads to etch selectivity for Ga2O3 over (Al0.18Ga0.82)2O3 of >30, with a maximum value of 55. By sharp contrast, without CHF3, the etching is nonselective over a large range of source and rf chuck powers. We focused on low ion energy conditions that would be required for device fabrication. This result has a direct application to selective removal of Ga2O3 contact layers to expose underlying (Al0.18Ga0.82)2O3 donor layers in high-electron-mobility transistor structures. It is expected that formation of nonvolatile AlF3 species helps produce this selectivity. X-ray photoelectron spectroscopy does detect F residues on the etched surface for the Cl2/Ar/ CHF3 plasma chemistry.

Plasma Etching

10.1116/6.0003400

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

Journal of Vacuum Science & Technology A Vacuum Surfaces and Films (2023)

Hsiao-Hsuan Wan Jian-Sian Li Chao-Ching Chiang Xinyi Xia F. Ren

NiO/β-(AlxGa1−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-(AlxGa1−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.

Non-blocking I/O

Rectifier (neural networks)

10.1116/6.0002393

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Achievement of low turn-on voltage in Ga2O3 Schottky and heterojunction hybrid rectifiers using W/Au anode contact

AIP Advances (2024)

Chao-Ching Chiang Jian-Sian Li Hsiao-Hsuan Wan F. Ren S. J. Pearton

The use of the low work function (4.5 eV) tungsten (W) as a rectifying contact was studied to obtain low on-voltages in W/Ga2O3 Schottky rectifiers and NiO/Ga2O3 heterojunction rectifiers that were simultaneously fabricated on a single wafer. The devices were produced with varying proportions of relative areas and diameters, encompassing a spectrum from pure Schottky Barrier Diode (SBD) to pure Heterojunction Diode (HJD) configurations. The turn-on voltages with W contacts ranged from 0.22 V for pure Schottky rectifiers to 1.50 V for pure HJDs, compared to 0.66 and 1.77 V, respectively, for Ni/Au contacts. The reverse recovery times ranged from 31.2 to 33.5 ns for pure Schottky and heterojunction rectifiers. Switching energy losses for the SBD with W contacts were ∼20% of those for HJDs. The reverse breakdown voltages ranged from 600 V for SBDs to 2400 V for HJDs. These are the lowest reported turn-on voltage values for 600/1200 V-class Ga2O3 rectifiers that extend the range of applications of these devices down to the voltages of interest for electric vehicle charging applications.

Turn (biochemistry)

Metal–semiconductor junction

10.1063/5.0227210

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Cryogenic temperature operation of NiO/Ga2O3 heterojunction and Ni/Au Schottky rectifiers

AIP Advances (2024)

Hsiao-Hsuan Wan Chao-Ching Chiang Jian-Sian Li Madani Labed J. Park

Vertical geometry NiO/Ga2O3 heterojunction (HJ) rectifiers and Ni/Au Schottky rectifiers fabricated on the same wafer and each with the same diameter (100 μm) were operated at 77–473 K to compare their capabilities in space-like environments. The HJ rectifiers suffer a 4-order reduction in forward current at 77 K due to the freeze-out of acceptors in the NiO, leading to MIS-type operation. By sharp contrast, the Schottky rectifiers have higher forward current and lower on-resistance at 77 K compared to 298 K due to improved carrier mobility. The on/off ratio of Schottky rectifiers at 77 K becomes similar to HJ rectifiers at 298–473 K. The reverse recovery time of Schottky rectifiers is reduced from 20 ns at 273 K to 16 ns at 77 K, while HJ rectifiers cannot be switched at this temperature. While the latter are superior for high-temperature applications, the former are better suited to cryogenic operation.

Non-blocking I/O

10.1063/5.0233627

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β-Ga2O3 orientation dependence of band offsets with SiO2 and Al2O3

Journal of Vacuum Science & Technology A Vacuum Surfaces and Films (2023)

Hsiao-Hsuan Wan Jian-Sian Li Chao-Ching Chiang Xinyi Xia David C. Hays

Two of the most common dielectrics for β-Ga2O3 are SiO2 and Al2O3 because of their large bandgaps, versatility of preparation, and thermal stability. However, because of the anisotropic properties of the β-polytype, it is necessary to understand differences in band alignment for the different crystal orientation. Using x-ray photoelectron spectroscopy, we performed a comparative study of the band alignment of SiO2/β-Ga2O3 and Al2O3/ β-Ga2O3 heterojunctions with different β-Ga2O3 orientations of (001), (010), and (2¯01). The bandgaps were determined to be 4.64, 4.71, and 4.59 eV for the (2¯01), (001), and (010) oriented β-Ga2O3 substrates, respectively. The valence band offsets for SiO2 on these three orientations were 1.4, 1.4, and 1.1 eV, respectively, while for Al2O3, the corresponding values were 0.0, 0.1, and 0.2 eV, respectively. The corresponding conduction band offsets ranged from 2.59 to 3.01 eV for SiO2 and 2.26 to 2.51 eV for Al2O3.

10.1116/6.0003039

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Citations (1)

Comparison of Ti/Au, Ni/Au, and Sc/Au ohmic contact metal stacks on (Al0.18Ga0.82)2O3

Journal of Materials Science (2024)

Hsiao-Hsuan Wan Chao-Ching Chiang Jian-Sian Li F. Ren Fikadu Alema

Ohmic contact

10.1007/s10853-024-10330-2

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Effect of drift layer doping and NiO parameters in achieving 8.9 kV breakdown in 100 μm diameter and 4 kV/4 A in 1 mm diameter NiO/β-Ga2O3 rectifiers

Journal of Vacuum Science & Technology A Vacuum Surfaces and Films (2023)

Jian-Sian Li Chao-Ching Chiang Xinyi Xia Hsiao-Hsuan Wan F. Ren

The effect of doping in the drift layer and the thickness and extent of extension beyond the cathode contact of a NiO bilayer in vertical NiO/β-Ga2O3 rectifiers is reported. Decreasing the drift layer doping from 8 × 1015 to 6.7 × 1015 cm−3 produced an increase in reverse breakdown voltage (VB) from 7.7 to 8.9 kV, the highest reported to date for small diameter devices (100 μm). Increasing the bottom NiO layer from 10 to 20 nm did not affect the forward current–voltage characteristics but did reduce reverse leakage current for wider guard rings and reduced the reverse recovery switching time. The NiO extension beyond the cathode metal to form guard rings had only a slight effect (∼5%) in reverse breakdown voltage. The use of NiO to form a pn heterojunction made a huge improvement in VB compared to conventional Schottky rectifiers, where the breakdown voltage was ∼1 kV. The on-state resistance (RON) was increased from 7.1 m Ω cm2 in Schottky rectifiers fabricated on the same wafer to 7.9 m Ω cm2 in heterojunctions. The maximum power figure of merit (VB)2/RON was 10.2 GW cm−2 for the 100 μm NiO/Ga2O3 devices. We also fabricated large area (1 mm2) devices on the same wafer, achieving VB of 4 kV and 4.1 A forward current. The figure-of-merit was 9 GW cm−2 for these devices. These parameters are the highest reported for large area Ga2O3 rectifiers. Both the small area and large area devices have performance exceeding the unipolar power device performance of both SiC and GaN.

Non-blocking I/O

Figure of Merit

Saturation current

10.1116/6.0002722

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Citations (22)