Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

Building printable electronics demands flexible and transparent electrodes that can be fabricated on plastic substrates. Here, Kang et al. report a polymer-metal hybrid electrode that has a small bending radius of 95%.

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Polymer-metal hybrid transparent electrodes for flexible electronics.

We report 30-nm-gate-length InAlN/AlN/GaN/SiC high-electron-mobility transistors (HEMTs) with a record current gain cutoff frequency (fT) of 370 GHz. The HEMT without back barrier exhibits an extrinsic transconductance (gm.ext) of 650 mS/mm and an on/off current ratio of 106 owing to the incorporation of dielectric-free passivation and regrown ohmic contacts with a contact resistance of 0.16 Ω·mm. Delay analysis suggests that the high fT is a result of low gate-drain parasitics associated with the rectangular gate. Although it appears possible to reach 500-GHz fT by further reducing the gate length, it is imperative to investigate alternative structures that offer higher mobility/velocity while keeping the best possible electrostatic control in ultrascaled geometry.

InAlN/AlN/GaN HEMTs With Regrown Ohmic Contacts and $f_{T}$ of 370 GHz

Due to its significant applications in many relevant fields, light detection in the solar-blind deep-ultraviolet (DUV) wavelength region is a subject of great interest for both scientific and industrial communities. The rapid advances in preparing high-quality ultrawide-bandgap (UWBG) semiconductors have enabled the realization of various high-performance DUV photodetectors (DUVPDs) with different geometries, which provide an avenue for circumventing numerous disadvantages in traditional DUV detectors. This article presents a comprehensive review of the applications of inorganic UWBG semiconductors for solar-blind DUV light detection in the past several decades. Different kinds of DUVPDs, which are based on varied UWBG semiconductors including Ga2O3, MgxZn1−xO, III-nitride compounds (AlxGa1−xN/AlN and BN), diamond, etc., and operate on different working principles, are introduced and discussed systematically. Some emerging techniques to optimize device performance are addressed as well. Finally, the existing techniques are summarized and future challenges are proposed in order to shed light on development in this critical research field.

Recent Progress in Solar-Blind Deep-Ultraviolet Photodetectors Based on Inorganic Ultrawide Bandgap Semiconductors

Solar-blind photodetectors are of great interest to a wide range of industrial, civil, environmental, and biological applications. As one of the emerging ultrawide-bandgap semiconductors, gallium oxide (Ga2O3) exhibits unique advantages over other wide-bandgap semiconductors, especially in developing high-performance solar-blind photodetectors. This paper comprehensively reviews the latest progresses of solar-blind photodetectors based on Ga2O3 materials in various forms of bulk single crystal, epitaxial films, nanostructures, and their ternary alloys. The basic working principles of photodetectors and the fundamental properties and synthesis of Ga2O3, as well as device processing developments, have been briefly summarized. A special focus is to address the physical mechanism for commonly observed huge photoconductive gains. Benefitting from the rapid development in material epitaxy and device processes, Ga2O3-based solar-blind detectors represent to date one of the most prospective solutions for UV detection technology towards versatile applications.

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Review of gallium-oxide-based solar-blind ultraviolet photodetectors

We have developed a novel AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor using a stack gate HfO2/Al2O3 structure grown by atomic layer deposition. The stack gate consists of a thin HfO2 (30-A) gate dielectric and a thin Al2O3 (20- A) interfacial passivation layer (IPL). For the 50-A stack gate, no measurable C-V hysteresis and a smaller threshold voltage shift were observed, indicating that a high-quality interface can be achieved using a Al2O3 IPL on an AlGaN substrate. Good surface passivation effects of the Al2O3 IPL have also been confirmed by pulsed gate measurements. Devices with 1- mum gate lengths exhibit a cutoff frequency (fT) of 12 GHz and a maximum frequency of oscillation (f MAX) of 34 GHz, as well as a maximum drain current of 800 mA/mm and a peak transconductance of 150 mS/mm, whereas the gate leakage current is at least six orders of magnitude lower than that of the reference high-electron mobility transistors at a positive gate bias.

AlGaN/GaN MOS-HEMT With $\hbox{HfO}_{2}$ Dielectric and $\hbox{Al}_{2}\hbox{O}_{3}$ Interfacial Passivation Layer Grown by Atomic Layer Deposition

In this letter, we report on demonstrating high-performance field-plated lateral $\beta $ -Ga2O3 Schottky barrier diode (SBD) on a sapphire substrate with a reverse blocking voltage of more than 3 kV and a low dc specific ON-resistance ( ${R}_{{ \mathrm{\scriptscriptstyle ON}, \text {sp}}}$ ) of 24.3 $\text{m}\Omega \cdot $ cm2 at an anode–cathode spacing ( ${L}_{\text {AC}}$ ) of $24~\mu \text{m}$ . To the best of our knowledge, this lateral breakdown voltage (BV) >3 kV with dc power figure-of-merit (FOM) >370 MW/cm2 is the highest BV achieved among all the $\beta $ -Ga2O3 SBDs. Meanwhile, lateral $\beta $ -Ga2O3 SBD with ${L}_{\text {AC}} = \text {16}\,\,\mu \text{m}$ also demonstrates a high BV of 2.25 kV and low ${R}_{{ \mathrm{\scriptscriptstyle ON}, \text {sp}}} = \text {10.2}\,\,\text{m}\Omega \cdot $ cm2, yielding a record high dc power FOM of 500 MW/cm2. Combining with 109 high-current ON/OFF ratio, 1.15-eV Schottky barrier height and 1.25 ideality factor, $\beta $ -Ga2O3 SBD with field-plate structure shows its great promise for power electronics applications.

Field-Plated Lateral $\beta$ -Ga 2 O 3 Schottky Barrier Diode With High Reverse Blocking Voltage of More Than 3 kV and High DC Power Figure-of-Merit of 500 MW/cm 2

Hf0.5Zr0.5O2(HZO)-based ferroelectric-field-effect transistor (FeFET) with a ZrO2 seed layer was demonstrated. It was found that the ZrO2 seed layer could effectively improve the ferroelectric properties of the (hafnium zirconium oxide) HZO thin film. The remanent polarization and the coercive voltage of the metal–ferroelectric–insulator–semiconductor (MFIS) structure with the ZrO2 seed layer were larger than those without the seed layer. Moreover, the FeFET with the ZrO2 seed layer showed wider counterclockwise hysteresis loops in the transfer characteristics than that without the seed layer, achieving a large memory window of about 2.8 V. These results validate the advantages of the ZrO2 seed layer in promotion of FeFET performance and thus warrant further study.

Performance Improvement of Hf 0.5 Zr 0.5 O 2 -Based Ferroelectric-Field-Effect Transistors With ZrO 2 Seed Layers

Single crystallinity (AlGa)2O3 solar-blind photodetectors are epitaxially grown on sapphire. Oxygen pressure during the growth has a great impact on the Al composition in (AlGa)2O3, which is investigated utilizing X-ray photoelectron spectroscopy and X-ray diffraction measurements. Measured transmittance spectra and responsivity demonstrate that (AlGa)2O3 photodetectors achieve a wider bandgap compared to a Ga2O3 device. An (Al0.12Ga0.88)2O3 device obtains 10 times higher photocurrent Iphoto than a Ga2O3 photodetector. However, as Al composition increases, significant Iphoto degradation is observed in an (Al0.35Ga0.65)2O3 photodetector. Meanwhile, an (Al0.35Ga0.65)2O3 device exhibits the stronger persistent photoconductivity compared to the Ga2O3 control device. Analysis shows that defect states in a bandgap of (AlGa)2O3 might be associated with the performance change in (AlGa)2O3 photodetectors.

(AlGa)_2O_3 solar-blind photodetectors on sapphire with wider bandgap and improved responsivity

This work reports on implementing Mg implanted Edge Termination (ET), a simple but very useful technique to increase the breakdown voltage (BV) of the vertical $\beta $ -Ga2O3 Schottky barrier diode (SBD). With this ET, vertical $\beta $ -Ga2O3 SBD demonstrates a reverse blocking voltage of 1.55 kV and low specific on-resistance ( ${R} _{ \mathrm{\scriptscriptstyle ON},\textrm {sp}}$ ) of 5.1 $\text{m}\Omega \cdot \textrm {cm}^{\textrm {2}}$ at a lightly doped $\beta $ -Ga2O3 layer with epitaxial thickness of $10~\mu \text{m}$ , yielding a high power figure-of-merit (P-FOM) of 0.47 GW/cm2. Combined with high forward current on/off ratio of 108 ~ 109, Schottky barrier height of 1.01 eV, and ideality factor of 1.05, vertical $\beta $ -Ga2O3 Schottky Diode with implanted ET verifies its great potential for future power rectifiers.

Qian Feng

Papers

AlGaN/GaN MOS-HEMT With $\hbox{HfO}_{2}$ Dielectric and $\hbox{Al}_{2}\hbox{O}_{3}$ Interfacial Passivation Layer Grown by Atomic Layer Deposition

Field-Plated Lateral $\beta$ -Ga 2 O 3 Schottky Barrier Diode With High Reverse Blocking Voltage of More Than 3 kV and High DC Power Figure-of-Merit of 500 MW/cm 2

Performance Improvement of Hf 0.5 Zr 0.5 O 2 -Based Ferroelectric-Field-Effect Transistors With ZrO 2 Seed Layers

(AlGa)_2O_3 solar-blind photodetectors on sapphire with wider bandgap and improved responsivity

High-Performance Vertical $\beta$ -Ga 2 O 3 Schottky Barrier Diode With Implanted Edge Termination