【Member Papers】Band Alignment, Thermal Transport Property, and Electrical Performance of High-Quality β-Ga₂O₃/AlN Schottky Barrier Diode Grown via MOCVD

      Researchers from the Fudan University and Suzhou Institute of Nano-Tech and Nano-Bionics,CAS have published a dissertation titled "Band Alignment, Thermal Transport Property, and Electrical Performance of High-Quality β-Ga₂O₃/AlN Schottky Barrier Diode Grown via MOCVD" in ACS Applied Materials & Interfaces.

Project Support

      This work is supported by the National Key R&D Program of China (2023YFB4606300), National Natural Science Foundation of China (No. 62474049), the Science and Technology Innovation Plan of Shanghai Science and Technology Commission (Nos. 21DZ1100800, 23ZR1405300, 20501110700, and 20501110702), and the Joint Research Plan of sci-tech innovation community in Yangtze River Delta (No.2023CSJG0600). The authors also acknowledge Autinst Technology Co., Ltd for providing Autinst PIONEER-ONE TDTR measurements and technological help regarding this study. Part of the sample fabrication and experimental work was performed at Fudan Nanofabrication Laboratory.

Background

      In recent years, the ultrawide-bandgap semiconductor β-phase gallium oxide (β-Ga₂O₃) has garnered significant attention due to its exceptional properties, including a wider bandgap (4.5− 4.9 eV) compared to Si, GaN, and SiC, an extremely high critical breakdown field (∼8 MV/cm), an outstanding Baliga’s figure of merit (BFOM ∼2800), and a high electron saturation velocity (∼2 × 10⁷ cm/s). β-Ga₂O₃ has a promising future to be applied to high-voltage power devices. Among all crystal structures of gallium oxide (α, β, γ, δ, and ε phases), β-phase gallium oxide exhibits the highest thermal and chemical stability. Additionally, the production cost of large-size bulk crystals of β-Ga₂O₃ is significantly lower than that of other wide-bandgap materials. The β-Ga₂O₃ unit cell adopts a monoclinic crystal structure (C2/m space group) with lattice parameters of a = 12.23 Å, b = 3.04 Å, and c = 5.80 Å. Its structure comprises GaO₆ octahedra and GaO₄ tetrahedra.

      Experimental measurements, including angle-resolved photoelectron spectroscopy (ARPES) and low-energy electron diffraction (LEED), combined with first-principles calculations, have confirmed that β-Ga₂O₃ is a direct bandgap semiconductor. Its electronic structure features a flat conduction band and a fluctuating valence band, indicating a small electron effective mass (∼0.28 m₀) and a large hole effective mass, which results in low hole mobility and significant challenges for p-type doping.

      Several power devices with superior electrical performance have been developed based on β-Ga₂O₃. For instance, the p-NiO_x/n-Ga₂O₃ heterojunction PN diode achieved a recordbreaking BV² /R_on,sp value, surpassing the 1-D unipolar limit of SiC and GaN. By employing anode engineering techniques, the breakdown voltage of β-Ga₂O₃ Schottky barrier diodes has been improved to exceed 10 kV, while maintaining a low turnon voltage of 1 V. Moreover, a lateral field-plated β-Ga₂O₃ MOSFET was vacuum annealed and processed with reactive ion etching (RIE), resulting in enhanced current recovery and significantly reduced R_on values.

      However, β-Ga₂O₃ faces challenges such as low electron mobility (∼180 cm² /(V s)) and ultralow thermal conductivity (＜27 W/mK), which hinder its industrialization. Its poor heat dissipation capability exacerbates self-heating effects, compromising the reliability and performance of the β-Ga₂O₃-based devices. Addressing these thermal issues is critical for advancing the development and application of β-Ga₂O₃ in high-performance devices.

Abstract

      β-phase gallium oxide (β-Ga₂O₃)/aluminum nitride (AlN) heterojunctions hold significant potential for high-power and microwave device applications. In this study, we systematically investigated the properties of the β-Ga₂O₃/AlN heterostructure grown via metal–organic chemical vapor deposition (MOCVD). High-resolution X-ray diffraction (HRXRD) and Raman spectroscopy revealed the crystal structures and demonstrated the high-crystalline quality of both films. Atomic force microscopy (AFM) scans displayed a smooth β-Ga₂O₃ surface with a root-mean-square (RMS) roughness of 3.6 nm. Scanning electron microscopy (SEM) images showed a flat surface with distinct heterostructure boundaries. Elemental distributions across the interface were mapped by using energy-dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) analysis characterized the chemical components of the sample and confirmed a type-II band alignment in the heterojunction, which facilitates electron accumulation. Furthermore, the thermal conductivity of β-Ga₂O₃ was measured at 4.2 W/(m·K), and the thermal boundary conductivity at the β-Ga₂O₃/AlN interface was determined to be 118.6 MW/(m²·K) using the time-domain thermoreflectance (TDTR) method. Temperature-dependent electrical performance of the β-Ga₂O₃/AlN SBD, including a low turn-on voltage of 0.1 V, ideality factor of 4.22, modified Richardson constant of 48.5 A/cm² K², and high breakdown voltage of 1260 V, was obtained. All of these values are competitive among β-Ga₂O₃-based heterostructures. The findings highlight the excellent interface quality, superior heat dissipation capability, and decent SBD performance of the β-Ga₂O₃/AlN integration, offering a promising platform for developing β-Ga₂O₃-based power devices capable of operating at high temperatures.

Summary

      In this study, a high-quality β-Ga₂O₃/AlN heterostructure was successfully grown by using the MOCVD method. The physical and optical properties, band alignment, and thermal characteristics of the heterojunction were thoroughly analyzed. The as-grown heterostructure exhibited high crystal quality, a low surface roughness of 3.6 nm, and a well-integrated boundary. XPS survey spectra confirmed the presence of distinct components within the sample, while core-level, VBM, and energy loss spectral measurements revealed a type-II band alignment of the β-Ga₂O₃/AlN interface. The thermal conductivity of β-Ga₂O₃ was measured at 4.2 W/(m·K), and the thermal boundary conductivity at the β-Ga₂O₃/AlN interface was found to be 118.6 MW/(m²·K), highlighting the efficient heat dissipation enabled by the AlN interlayer and the high-quality growth of the heterostructure. The β-Ga₂O₃/ AlN Schottky barrier diode was fabricated and the electrical performances of a low turn-on voltage of 0.1 V, ideality factor of 4.22, modified Richardson constant of 48.5 A/cm² K², and a high breakdown voltage over 1.2 kV at high temperature were measured, showing good capability and potential of the β-Ga₂O₃/AlN SBD to operate under extreme thermal environments.

      These findings demonstrate the potential of β-Ga₂O₃/AlN heterostructures as an innovative solution for power devices that require enhanced thermal management, especially in hightemperature environments. The results open the door to the development of advanced power devices with improved performance and reliability. Future research will focus on fabricating a new β-Ga₂O₃/AlN power device with lower on resistance, ideality factor, and barrier inhomogeneities while maintaining high heat-sinking capability and excellent breakdown characteristics. Its practical applications in power and RF electronics are also of great interest.

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