IWGO 2026 Session IWGO-TuA1: Heterogeneous Integration and other WBG and UWBG Oxides Beyond Ga₂O₃

Gallium Oxide (Ga₂O₃) devices offer potential to outcompete SiC technology though have still to prove their true potential including device reliability, We demonstrate pathways to enable heterogenous Ga₂O₃ growth on alternative substrates including Si, furthermore challenges and solutions to overcome in vertical device technology, including mitigation of trap states for improve device reliability.

We report the fabrication and characterization of vertical NiO_x/β-Ga₂O₃ heterojunction diodes (HJDs) on 20-μm-thick/low-doped HVPE-grown (011) β-Ga₂O₃ with breakdown voltage beyond 10 kV, differential specific on-resistance (R_on,sp) at 43 mΩ∙cm², and power figure of merit (PFOM) exceeding 2.3 GW/cm² on 60-μm dia. device. The parallel-plane breakdown electric field (E_Br,||) is extracted to be >5.3 MV/cm.

The NiO_x/(011) HVPE β-Ga₂O₃ HJD fabrication begins with a backside Ti/Au (50/350 nm) Ohmic metallization on n⁺ β-Ga₂O₃ bulk substrate using e-beam evaporation followed by a 60-seconds rapid thermal annealing (RTA) at 470 °C in N₂. A ~28 nm of p^- NiO_x is deposited on HVPE β-Ga₂O₃ drift region with a pre-patterned photoresist liftoff mask by optical lithography. Following the first layer NiO_x deposition, a self-aligned p⁺⁺ NiO_x (~20 nm) contact layer is reactively sputtered. Then, a Ni/Au/Ni (50/100/150 nm) anode cap/metal hard mask stack is deposited via e-beam evaporation. The fabricated HJDs are later dry-etched ~2 μm into the β-Ga₂O₃ drift region below the heterojunction interface using BCl₃ inductively coupled plasma (ICP) at 200 W for edge termination. A ~1.5-μm-thick SiO₂ field plate oxide is sputtered conformally on the HJDs, then a Ni/Au (50/300 nm) field plate metal stack a with 20-μm edge extension (L_FP) is deposited on the oxide via e-beam evaporation to conclude the device fabrication.

The mesa-isolated/field plated (MI/FP) HJD with 60-μm dia. dimension shows rectifying behavior with an on/off ratio of 10¹⁰ and turns on at ~2 V forward bias with an on-state current density >60 A/cm². The capacitance-voltage (C-V) characteristics of the HJD reveals a built-in potential of 2.2 V and an average apparent charge density at 1.8×10¹⁵ cm^-3. The 60-μm dia. device exhibits a breakdown voltage > 10 kV with a corresponding E_Br,|| >5.3 MV/cm. MI/FP HJDs with 100-μm dia. also exhibits breakdown voltages at 6.5-7.3 kV. The R_on,sp of the 60-μm dia. device is at 43 mΩ∙cm², giving rise to a PFOM > 2.3 GW/cm², which approaches the material limit of 4H-SiC. The HJDs across various diode dimensions (60-μm dia. to 1-mm dia.) show consistent R_on,sp values which indicates that there is no current spreading and the extracted R_on,sp is accurate. The pulsed current-voltage (I-V) measurements with <1% duty cycle show that the 1-mm dia. HJD accomplishes >1.5 A absolute current value at 9 V on thick and low-doped HVPE-grown β-Ga₂O₃, which is comparable to the I-V characteristics of co-fabricated Schottky barrier diode (SBD) counterpart, indicating that the vertical MI/FP HJD is able to fully exploit the on-state potential of the epitaxial drift region.

β-Ga₂O₃ is one of the most promising ultra-wide band gap semiconductor materials due to its combination of intrinsic semiconductor properties and manufacturing scalability. Considering the difficulties associated with p-type doping of β-Ga₂O₃, alternative strategies involving the heteroepitaxial growth of p-type oxide layers have been explored as potential substitutes. For example, the use of p-NiO contacts to β-Ga₂O₃ has led to a demonstration of vertical power diodes with breakdown voltages over 10 kV. Our recent work has shown that the NiO/Ga₂O₃ may become unstable above 400 °C temperature, leading to uncontrolled formation of NiGa₂O₄ at the interface. Such high temperatures may be present due to intentional heating for NiO growth or unintentional hot spots during high-power operation, exacerbated by low thermal conductivity. However, the NiGa₂O₄ structure and properties remains relatively unknown due to the thinness of this naturally formed interphase.

This presentation will report on the intentional epitaxial growth and semiconductor properties of NiGa₂O₄ spinel layers that naturally form at Ga₂O₃/NiO interfaces used in electronic devices. Cubic spinel NiGa₂O₄ films of 10-50 nm thicknesses and low surface roughness (~ 2 nm) were grown using pulsed laser deposition at the α-Al₂O₃ and β-Ga₂O₃ substrate temperatures in the 300-900 °C. The optical absorption onset (3.6-3.9 eV) and thermal conductivity (4-9 W m^-1 K^-1) vary systematically with substrate temperature due to varying Ni and Ga cation ordering on the spinel lattice. The valence band offset between NiGa₂O₄ and β-Ga₂O₃ is determined to be 1.8 eV. The NiGa₂O₄-based p-n heterojunction devices on Ga₂O₃ (001) substrates with 10⁸ rectification ratio and 1.4 V turn-on voltage maintaining this diode behavior up to 600 ºC operating temperature. These results highlight the potential of NiGa₂O₄ as a p-type interlayer to increase the performance and reliability of Ga₂O₃-based devices in high-power and high-temperature applications.

Establishing a new material as a viable semiconductor platform can be done by meeting three fundamental criteria: the demonstration of controllable doping with mobile carriers, a measurable bandgap, and the successful fabrication of rectifying junctions or observation of transistor action. In this work, we demonstrate that Si-doped α-(Al_xGa_1-x)₂O₃ grown by suboxide molecular-beam epitaxy on sapphire fulfills these three criteria [1-3]. Hall effect measurements verify semiconductor behavior through controllable n-type conduction. Vacuum UV ellipsometry confirms a channel bandgap of 6.71

Leveraging this platform, we realize the first CBG “AlphaDiodes” and “AlphaFETs.” The diodes exhibit an on-off ratio > 10⁷ and a lateral breakdown field of ~12 MV/cm - among the highest values recorded for any semiconductor. The Pd-gated AlphaFETs demonstrate enhancement-mode (normally-off) operation with a threshold voltage of ~1 V, proving that the channel potential can be effectively modulated via electrostatic gating despite the extreme bandgap. While current density is currently limited by contact and access resistance at the regrown interfaces, the combined evidence of Hall transport, optical bandgap, and transistor action establishes α-(Al_xGa_1-x)₂O₃ as a practical platform for high-voltage, high-efficiency electronics.

Crucially, this platform utilizes m-plane sapphire substrates. Sapphire offers a unique trifecta of benefits: it is an abundant, low-cost material produced at a massive industrial scale (with crystals reaching diameters up to 640 mm) [4], and its high structural quality allows for the growth of high-purity epitaxial layers. By achieving these device milestones on a native, readily available substrate, we break the historical bottleneck of small-scale, high-cost synthesis that has previously hindered ultra-wide bandgap materials like diamond and c-BN, providing a clear path for the scalable adoption of colossal bandgap semiconductors.

[1] Steele, J., et al. (2025).APL Materials, 13, 101117

[2] Steele, J., et al. (2024). APL Materials, 12, 041113

[3] Jinno, R., et al. (2021). Science Advances, 7, eabd5891

[4] Kang Sen, et al. (J. Synth. Cryst.), 50, 1397–1401 (2021)

We demonstrate successful fabrication of single-crystalline β-Ga₂O₃ composite substrates on either 4H-SiC or diamond for thermal management of high-power electronic devices. Direct wafer bonding, without the use of bonding interlayers, at room temperature with ~kPa of applied pressure is employed to heterogeneously integrate β-Ga₂O₃. Prior to bonding, β-Ga₂O₃ substrates are implanted with light-atoms to induce exfoliation and transfer of single-crystalline β-Ga₂O₃ films to a handle substrate such as 4H-SiC and diamond upon annealing. Leveraging our previous efforts on β-Ga₂O₃ exfoliation [1-3] and CMP [4], we continue to improve and expand key processing steps towards pristine thin-film β-Ga₂O₃ composite substrates for different β-Ga₂O₃ orientations including, but not limited to, (001), (010), and (100). We also develop post-bond wafer-stock removal (lapping and polishing) processes for achieving β-Ga₂O₃ composite substrates that require thicker β-Ga₂O₃ films that currently may not be commercially feasible via ion implantation (i.e., > 1 MeV). Thermal transport measurements comparing bulk β-Ga₂O₃ vs composite substrates subjected to the same heating conditions show a ~5× lower temperature rise and an effective thermal conductivity ~5 to ~10 times higher than bulk β-Ga₂O₃ depending on the film thickness. Thermal boundary conductance values for the β-Ga₂O₃|4H-SiC and β-Ga₂O₃|diamond interfaces are ~120 MW/(m²K) and ~75 MW/(m²K), respectively, which are on the same order achieved via β-Ga₂O₃ heteroepitaxy reported in the current literature. Unlike heteroepitaxy, which limits the β-Ga₂O₃ orientation typically to twinned (201) or polycrystalline depending on the substrate, wafer bonding bypasses limitations associated with heteroepitaxy to achieve structures with control over material combination and crystal orientation without sacrificing crystalline quality. To test robustness, thermal cycling of our β-Ga₂O₃composite substrates from room temperature up to 1000 °C show no degradation in the thermal transport across the interface. Subsequent homoepitaxial growth of device layers on these composite substrates will be presented.

The authors acknowledge the support from Air Force SBIR Phase II program. TPOC Dr. Thaddeus Asel.

References

[1] M. E. Liao, et al., ECS J. Solid State Sci. Technol., 8(11), P673 (2019).

[2] M. E. Liao, et al., J. Vac. Sci. Technol. A, 41, 063203 (2023).

[3] M. E. Liao, et al., ECS Trans., 112(3), 269 (2023).

[4] M.E. Liao, et al., J. Vac. Sci. Technol. A, 41, 013205 (2023).