Fabrication of a Nb-Doped β-Ga2O3 Nanobelt Field-Effect Transistor and Its Low-Temperature Behavior

Progress in Gallium Oxide Field-Effect Transistors for High-Power and RF Applications

Power electronics are becoming increasingly more important, as electrical energy constitutes 40% of the total primary energy usage in the USA and is expected to grow rapidly with the emergence of electric vehicles, renewable energy generation, and energy storage. New materials that are better suited for high-power applications are needed as the Si material limit is reached. Beta-phase gallium oxide (β-Ga2O3) is a promising ultra-wide-bandgap (UWBG) semiconductor for high-power and RF electronics due to its bandgap of 4.9 eV, large theoretical breakdown electric field of 8 MV cm-1, and Baliga figure of merit of 3300, 3-10 times larger than that of SiC and GaN. Moreover, β-Ga2O3 is the only WBG material that can be grown from melt, making large, high-quality, dopable substrates at low costs feasible. Significant efforts in the high-quality epitaxial growth of β-Ga2O3 and β-(AlxGa1-x)2O3 heterostructures has led to high-performance devices for high-power and RF applications. In this report, we provide a comprehensive summary of the progress in β-Ga2O3 field-effect transistors (FETs) including a variety of transistor designs, channel materials, ohmic contact formations and improvements, gate dielectrics, and fabrication processes. Additionally, novel structures proposed through simulations and not yet realized in β-Ga2O3 are presented. Main issues such as defect characterization methods and relevant material preparation, thermal studies and management, and the lack of p-type doping with investigated alternatives are also discussed. Finally, major strategies and outlooks for commercial use will be outlined.

Fabrication of β-Ga2O3 Nanotubes via Sacrificial GaSb-Nanowire Templates

β-Ga2O3 nanostructures are attractive wide-band-gap semiconductor materials as they exhibit promising photoelectric properties and potential applications. Despite the extensive efforts on β-Ga2O3 nanowires, investigations into β-Ga2O3 nanotubes are rare since the tubular structures are hard to synthesize. In this paper, we report a facile method for fabricating β-Ga2O3 nanotubes using pre-synthesized GaSb nanowires as sacrificial templates. Through a two-step heating-treatment strategy, the GaSb nanowires are partially oxidized to form β-Ga2O3 shells, and then, the residual inner parts are removed subsequently in vacuum conditions, yielding delicate hollow β-Ga2O3 nanotubes. The length, diameter, and thickness of the nanotubes can be customized by using different GaSb nanowires and heating parameters. In situ transmission electron microscopic heating experiments are performed to reveal the transformation dynamics of the β-Ga2O3 nanotubes, while the Kirkendall effect and the sublimation process are found to be critical. Moreover, photoelectric tests are carried out on the obtained β-Ga2O3 nanotubes. A photoresponsivity of ~25.9 A/W and a detectivity of ~5.6 × 1011 Jones have been achieved with a single-β-Ga2O3-nanotube device under an excitation wavelength of 254 nm.

Electrical and Gas Sensor Properties of Nb(V) Doped Nanocrystalline β-Ga2O3

A flame spray pyrolysis (FSP) technique was applied to obtain pure and Nb(V)-doped nanocrystalline β-Ga₂O₃, which were further studied as gas sensor materials. The obtained samples were characterized with XRD, XPS, TEM, Raman spectroscopy and BET method. Formation of GaNbO₄ phase is observed at high annealing temperatures. Transition of Ga(III) into Ga(I) state during Nb(V) doping prevents donor charge carriers generation and hinders considerable improvement of electrical and gas sensor properties of β-Ga₂O₃. Superior gas sensor performance of obtained ultrafine materials at lower operating temperatures compared to previously reported thin film Ga₂O₃ materials is shown.

Effective Suppression of MIS Interface Defects Using Boron Nitride toward High-Performance Ta-Doped-β-Ga2O3 MISFETs

β-Ga₂O₃ is considered an attractive candidate for next-generation high-power electronics due to its large band gap of 4.9 eV and high breakdown electrical field of 8 MV/cm. However, the relatively low carrier concentration and low electron mobility in the β-Ga₂O₃-based device limit its application. Herein, the high-quality β-Ga₂O₃ single crystal with high doping concentration of ∼3.2 × 10¹⁹ cm^-3 was realized using an optical float-zone method through Ta doping. In contrast to the SiO₂/β-Ga₂O₃ gate stack structure, we used hexagonal boron nitride as the gate insulator, which is sufficient to suppress the metal-insulator-semiconductor (MIS) interface defects of the β-Ga₂O₃-based MIS field-effect transistors (FETs), exhibiting outstanding performances with a low specific on-resistance of ∼6.3 mΩ·cm², a high current on/off ratio of ∼10⁸, and a high mobility of ∼91.0 cm²/(V s). Our findings offer a unique perspective to fabricate high-performance β-Ga₂O₃ FETs for next-generation high-power nanoelectronic applications.

A Heterostructured Graphene Quantum Dots/β-Ga2O3 Solar-Blind Photodetector with Enhanced Photoresponsivity

The superior optical and electronic characteristics of quasi-two-dimensional β-Ga₂O₃ make it suitable for solar-blind (200-280 nm) photodetectors (PDs). The metal-semiconductor-metal (MSM) PDs commonly suffer from low photoresponsivity, slow response speed, and a narrow detection wavelength range despite their simple fabrication process. Herein, we report a high-performance MSM PD by integrating exfoliated β-Ga₂O₃ flakes with zero-dimensional graphene quantum dots (GQDs), which exhibits the advantages of enhancing the photoresponsivity, shortening the photoresponse time, and stimulating a broad range of photon detection. The hybrid GQDs/β-Ga₂O₃ heterostructure PD is sensitive to deep-ultraviolet (DUV) light (250 nm) with an ultrahigh responsivity (R of ∼2.4 × 10⁵ A/W), a large detectivity (D* of ∼4.3 × 10¹³ Jones), an excellent external quantum efficiency (EQE of ∼1.2 × 10⁸%), and a fast photoresponse (150 ms), which is superior to the bare β-Ga₂O₃ PD. These improvements result from effective charge transfer due to the introduction of GQDs, which enhance the light absorption and the generation of electron-hole pairs. In addition, the hybrid GQDs/β-Ga₂O₃ PD also exhibits better photoelectric performance than the bare β-Ga₂O₃ PD at a 1000 nm wavelength. As a conclusion, the hybrid GQDs/β-Ga₂O₃ DUV photodetector shows potential applications in commercial optoelectronic products and provides an alternative solution for the design and preparation of high-performance photodetectors.

A Selective Etching Route for Large-Scale Fabrication of β-Ga2O3 Micro-/Nanotube Arrays

In this paper, based on the different etching characteristics between GaN and Ga₂O₃, large-scale and vertically aligned β-Ga₂O₃ nanotube (NT) and microtube (MT) arrays were fabricated on the GaN template by a facile and feasible selective etching method. GaN micro-/nanowire arrays were prepared first by inductively coupled plasma (ICP) etching using self-organized or patterning nickel masks as the etching masks, and then the Ga₂O₃ shell layer converted from GaN was formed by thermal oxidation, resulting in GaN@Ga₂O₃ micro-/nanowire arrays. After the GaN core of GaN@Ga₂O₃ micro-/nanowire arrays was removed by ICP etching, hollow Ga₂O₃ tubes were obtained successfully. The micro-/nanotubes have uniform morphology and controllable size, and the wall thickness can also be controlled with the thermal oxidation conditions. These vertical β-Ga₂O₃ micro-/nanotube arrays could be used as new materials for novel optoelectronic devices.

Single-Crystalline All-Oxide α-γ-β Heterostructures for Deep-Ultraviolet Photodetection

Recent advancements in gallium oxide (Ga₂O₃)-based heterostructures have allowed optoelectronic devices to be used extensively in the fields of power electronics and deep-ultraviolet photodetection. While most previous research has involved realizing single-crystalline Ga₂O₃ layers on native substrates for high conductivity and visible-light transparency, presented and investigated herein is a single-crystalline β-Ga₂O₃ layer grown on an α-Al₂O₃ substrate through an interfacial γ-In₂O₃ layer. The single-crystalline transparent conductive oxide layer made of wafer-scalable γ-In₂O₃ provides high carrier transport, visible-light transparency, and antioxidation properties that are critical for realizing vertically oriented heterostructures for transparent oxide photonic platforms. Physical characterization based on X-ray diffraction and high-resolution transmission electron microscopy imaging confirms the single-crystalline nature of the grown films and the crystallographic orientation relationships among the monoclinic β-Ga₂O₃, cubic γ-In₂O₃, and trigonal α-Al₂O₃, while the elemental composition and sharp interfaces across the heterostructure are confirmed by Rutherford backscattering spectrometry. Furthermore, the energy-band offsets are determined by X-ray photoelectron spectroscopy at the β-Ga₂O₃/γ-In₂O₃ interface, elucidating a type-II heterojunction with conduction- and valence-band offsets of 0.16 and 1.38 eV, respectively. Based on the single-crystalline β-Ga₂O₃/γ-In₂O₃/α-Al₂O₃ all-oxide heterostructure, a vertically oriented DUV photodetector is fabricated that exhibits a high photoresponsivity of 94.3 A/W, an external quantum efficiency of 4.6 × 10⁴%, and a specific detectivity of 3.09 × 10¹² Jones at 250 nm. The present demonstration lays a strong foundation for and paves the way to future all-oxide-based transparent photonic platforms.