Ultra-wide Bandgap Semiconductor Research Articles

Progress and challenges in the development of ultra-wide bandgap semiconductor α-Ga2O3 toward realizing power device applications

Ultra-wide-bandgap (UWBG) semiconductors, such as Ga2O3 and diamond, have been attracting increasing attention owing to their potential to realize high-performance power devices with high breakdown voltage and low on-resistance beyond those of SiC and GaN. Among numerous UWBG semiconductors, this work focuses on the corundum-structured α-Ga2O3, which is a metastable polymorph of Ga2O3. The large bandgap energy of 5.3 eV, a large degree of freedom in band engineering, and availability of isomorphic p-type oxides to form a hetero p–n junction make α-Ga2O3 an attractive candidate for power device applications. Promising preliminary prototype device structures have been demonstrated without advanced edge termination despite the high dislocation density in the epilayers owing to the absence of native substrates and lattice-matched foreign substrates. In this Perspective, we present an overview of the research and development of α-Ga2O3 for power device applications and discuss future research directions.

Near-infrared light excitation of h-BN ultra-wide bandgap semiconductor

We demonstrate a method to faithfully excite an ultra-wide bandgap semiconductor hexagonal boron nitride (h-BN) by using optical frequency upconversion technology. By means of Yb3+ and Tm3+ as dual bridging sensitizers, NaYF4:Yb3+, Tm3+, and Gd3+ microcrystals were excited by near-infrared light and generated high-energy (>6 eV) excited states. We fabricated a photoelectric conversion device by attaching the microcrystals to the surfaces of the h-BN thin film. When the device was irradiated with 980-nm near-infrared light, the Gd3+ ions in the microcrystals were populated to the high-energy excited states 5GJ through an internal 7-photon process, emitting 205 nm deep ultraviolet fluorescence and 195.3 nm vacuum ultraviolet fluorescence, which provided enough energy for h-BN photoexcitation. Dynamic analysis showed that Förster resonance energy transfer played a very important role in the optical excitation, and populating Gd3+ ions to high-energy excited states was the technical key.

Perspectives on MOVPE-grown (100) β-Ga2O3 thin films and its Al-alloy for power electronics application

Beta gallium oxide (β-Ga2O3) is a promising ultra-wide bandgap semiconductor with attractive physical properties for next-generation high-power devices, radio frequency electronics, and solar-blind ultraviolet radiation detectors. Here, we present an overview and perspective on the development of MOVPE-grown (100) β-Ga2O3 thin films and its role in supplementing high-power electronics. We review the development path of the growth process on (100) β-Ga2O3 thin films with a discussion regarding the solved and remaining challenges. The structural defect formation mechanism, substrate treatment strategies, and different growth windows are analyzed to optimize the grown film to fulfill the requirements for device fabrication. Toward industrial applications, MOVPE-grown β-Ga2O3 thin films are evaluated in two aspects: thick layers with smooth surface roughness and the electrical properties in terms of high carrier mobility and low doping concentration. Based on the reviewed results, we propose strategies in substrate preparation treatments and supportive tools such as the machine learning approaches for future growth process optimization and envision the rising interest of the β-Ga2O3-related alloy, β-(AlxGa1−x)2O3.

500 °C operation of β-Ga2O3 field-effect transistors

We demonstrated 500 °C operation of field-effect transistors made using ultra-wide bandgap semiconductor β-Ga2O3. Metal–semiconductor field-effect transistors were fabricated using epitaxial conductive films grown on an insulating β-Ga2O3 substrate, TiW refractory metal gates, and Si-implanted source/drain contacts. Devices were characterized in DC mode at different temperatures up to 500 °C in vacuum. These variable-temperature measurements showed a reduction in gate modulation of the drain current due to an increase in gate leakage across the gate/semiconductor Schottky barrier. Devices exhibited a reduction in transconductance with increasing temperature; despite this, drain current increased with temperature due to a reduction in threshold voltage caused by the de-trapping of electrons from deep-level traps. Devices also showed negligible change in semiconductor epitaxy and source/drain contacts, hence demonstrated recovery to their room-temperature electrical properties after the devices were tested intermittently at different high temperatures in vacuum. The mechanism of gate leakage was also explored, which implicated the presence of different conduction mechanisms at different temperatures and gate electric fields.

Numerical Modelling for the Experimental Improvement of Growth Uniformity in a Halide Vapor Phase Epitaxy Reactor for Manufacturing β-Ga2O3 Layers

The development of growth processes for the synthesis of high-quality epitaxial layers is one of the requirements for utilizing the ultrawide band gap semiconductor Ga2O3 for high-voltage, high-power electronics. A halide vapor phase epitaxy (HVPE) process used to grow β-Ga2O3 layer was optimized by modifying the gas inlet, resulting in improved growth uniformity. A conventional tube acting as an inlet for the Ga precursor GaCl gas was replaced with a shower head with four outlets at 45 degrees to the horizontal axis of the reactor. The modification was performed based on numerical calculations of the three-dimensional distribution of gases inside the growth chamber with different designs of the GaCl precursor inlet. It was shown that variation in the Ga/O ratio over the substrate holder was ~10% for a shower head compared with ~40% for a tube. In addition, growth with a tube leads to the film thickness varying by a factor of ~4 depending on the position on the holder, whereas when using a shower head, the thickness of the grown Ga2O3 layers became much more uniform with a total spread of just ~30% over the entire substrate holder.

Comparative Studies of c- and m-Plane AlN Seeds Grown by Physical Vapor Transport.

The ultra-wide bandgap semiconductor AlN has attracted a great deal of attention owing to its wide application potential in the field of electronics and optoelectronic devices. In this report, based on the mechanism of the physical vapor transport (PVT) growth of AlN crystal, the c- and m-plane AlN seed crystals were prepared simultaneously through special temperature field design. It is proved that AlN crystals with different orientations can be obtained at the same temperature field. The structure parameter of AlN crystal was obtained through the characteristic evaluations. In detail, XPS was used to analyze the chemical states and bonding states of the surface of seed crystals. The content of oxygen varied along with distinct orientations. Raman spectrum documented a small level of compressive stress on these crystal seeds. Tested results confirmed that the prepared AlN crystal seeds had high quality.

A review of thermoreflectance techniques for characterizing wide bandgap semiconductors’ thermal properties and devices’ temperatures

Thermoreflectance-based techniques, such as pump–probe thermoreflectance (pump–probe TR) and thermoreflectance thermal imaging (TTI), have emerged as the powerful and versatile tools for the characterization of wide bandgap (WBG) and ultrawide bandgap (UWBG) semiconductor thermal transport properties and device temperatures, respectively. This Review begins with the basic principles and standard implementations of pump–probe TR and TTI techniques, illustrating that when analyzing WBG and UWBG materials or devices with pump–probe TR or TTI, a metal thin-film layer is often required. Due to the transparency of the semiconductor layers to light sources with sub-bandgap energies, these measurements directly on semiconductors with bandgaps larger than 3 eV remain challenging. This Review then summarizes the general applications of pump–probe TR and TTI techniques for characterizing WBG and UWBG materials and devices where thin metals are utilized, followed by introducing more advanced approaches to conventional pump–probe TR and TTI methods, which achieve the direct characterizations of thermal properties on GaN-based materials and the channel temperature on GaN-based devices without the use of thin-film metals. Discussions on these techniques show that they provide more accurate results and rapid feedback and would ideally be used as a monitoring tool during manufacturing. Finally, this Review concludes with a summary that discusses the current limitations and proposes some directions for future development.

Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications

Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Bandgap Semiconductor Devices for RF and Power Applications"

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Bandgap Semiconductor Devices for RF and Power Applications"

Impact of electron-phonon interactions on phonon transport in diamond and c-BN

Diamond and cubic polymorph of boron nitride (c-BN) are two promising next-generation ultrawide bandgap semiconductor materials owning superior thermal properties. Although lattice vibration is the dominant mechanism of heat conduction in semiconductors, electron-phonon interactions exist and may affect phonon transport in doped semiconductors; yet such effects in wide bandgap materials have not received much attention. In this study, we explore the effects of electron-phonon interactions on the lattice thermal conductivity and phonon transport in electron-doped Si, diamond, and c-BN under various electron concentrations and in a wide temperature range from 300 K to 900 K based on the first-principles calculation. It is found that the electron-phonon interactions will bring down the thermal conductivity of doped materials, and the depletion impact increases as the electron concentration increases but decreases as the temperature increases. This depression effect in ultrawide bandgap diamond and c-BN is apparent, though it is not as severe as in Si. At room temperature with a high electron concentration of 10 21 cm −3 , the reduction of thermal conductivity reaches 36 % in Si, and 17.4 % and 16.1 % in diamond and c-BN, respectively. • Thermal effect of electron-phonon interactions in doped c-BN and diamond is revealed. • Temperature and electron concentration dependent lattice thermal conductivity is quantified. • Thermal depletions exist in doped c-BN and diamond, but not as severe as in doped Si. • Depletion lessens as temperature rises, but increases with increasing electron concentration. • Ultrawide bandgap materials have great potentials in high-power high-temperature electronics.

Full-band Monte Carlo simulation of two-dimensional electron gas in (AlxGa1−x)2O3/Ga2O3 heterostructures

β -Gallium oxide (Ga2O3) is an extensively investigated ultrawide-bandgap semiconductor for potential applications in power electronics and radio frequency switching. The room temperature bulk electron mobility (∼200cm2V−1s−1) is comparatively low and is limited by the 30 phonon modes originating from its 10-atom primitive cell. The theoretically calculated saturation velocity in bulk is 1–2×107cms−1 (comparable to GaN) and is limited by the low field mobility. This work explores the high field electron transport (and hence the velocity saturation) in the 2DEG based on the first principles calculated parameters. A self-consistent calculation on a given heterostructure design gives the confined eigenfunctions and eigenenergies. The intrasubband and the intersubband scattering rates are calculated based on the Fermi’s golden rule considering longitudinal optical (LO) phonon–plasmon screening. The high field characteristics are extracted from the full-band Monte Carlo simulation of heterostructures at 300 K. The overall system is divided into a 2D and a 3D region mimicking the electrons in the 2DEG and the bulk, respectively. The electron transport is treated through an integrated Monte Carlo program which outputs the steady state zone population, transient dynamics, and the velocity–field curves for a few heterostructure designs. The critical field for saturation does not change significantly from bulk values, however, an improved peak velocity is calculated at a higher 2DEG density. The velocity at low 2DEG densities is impacted by the antiscreening of LO phonons which plays an important role in shaping the zone population. A comparison with the experimental measurements is also carried out and possible origins of the discrepancies with experiments is discussed.

Theoretical study of the interface engineering for H-diamond field effect transistors with h-BN gate dielectric and graphite gate

Diamond has compelling advantages in power devices as an ultrawide-bandgap semiconductor. Using first-principles calculations, we systematically investigate the structural and electronic properties of hydrogen-terminated diamond (H-diamond) (111) van der Waals (vdW) heterostructures with graphite and hexagonal boron nitride (h-BN) layers. The graphite/H-diamond heterostructure forms a p-type ohmic contact and the p-type Schottky barrier decreases as the number of graphite layers increases. In contrast, the h-BN/H-diamond heterostructure exhibits semiconducting properties and a tunable type-II band alignment. Moreover, the charge transfer is concentrated at the interface with a large amount of charge accumulating on the C–H bonds on the H-diamond (111) surface, indicating the formation of a highly conductive two-dimensional hole gas (2DHG) layer. In a similar vein, the promising structural and electronic properties of graphite, h-BN, and H-diamond (111) in the graphite/h-BN/H-diamond (111) vdW heterostructure are well preserved upon their contact, while such heterostructure exhibits flexible band offset and Schottky contacts. These studies of interface engineering for H-diamond heterostructures are expected to advance the application of 2D materials in H-diamond field effect transistors, which is an important development in the design of surface transfer doping for 2DHG H-diamond devices.

Toward gallium oxide power electronics.

Ultrawide-bandgap semiconductors show promise for high-power transistors.

Full-momentum exciton landscape and macroscopic quantum phase diagram in two-dimensional gallium oxides

High-temperature exciton macroscopic quantum phases have a wide application prospect in many fields such as ultrahigh power photonics and quantum information, and are extremely difficult to realize due to the strong thermal dissociation effect. In this study, the exciton dispersions and macroscopic quantum phase diagrams of two-dimensional (2D) ultrawide bandgap semiconductors I–Ga2O3 and II-Ga2O3 are investigated for the first time. Both I–Ga2O3 and II-Ga2O3 possess the complex excitonic composition and morphology, resulting from their highly degenerate and flat valence bands. For the exciton B1 (B1') with an ultra-large binding energy of 1.14 (1.03) eV in I–Ga2O3, the phase transition temperature from free exciton to Bose-Einstein condensation (TBEC) is as high as 356 (679) K, respectively. The critical carrier densities for the formation of electron-hole liquid at room temperature (NEHL) are 1.3 × 1013 cm−2 and 1.8 × 1013 cm−2, respectively. For the free exciton B1 with an ultra-large binding energy of 1.49 eV in II-Ga2O3, TBEC reaches 365 K and NEHL is 5.9 × 1013 cm−2. Our findings extend the material basis for the research of exciton macroscopic quantum phases and will boost their applications in novel optoelectronic devices.

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers: Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Application

Call for Papers: Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Application

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers for a Special Issue of IEEE Transactions on Electron Devices on "Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications"

Call for Papers: Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications

Call for Papers: Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications

Toward AlGaN channel HEMTs on AlN: Polarization-induced 2DEGs in AlN/AlGaN/AlN heterostructures

Due to its high breakdown electric field, the ultra-wide bandgap semiconductor AlGaN has garnered much attention recently as a promising channel material for next-generation high electron mobility transistors (HEMTs). A comprehensive experimental study of the effects of Al composition x on the transport and structural properties is lacking. We report the charge control and transport properties of polarization-induced 2D electron gases (2DEGs) in strained AlGaN quantum well channels in molecular-beam-epitaxy-grown AlN/AlxGa1−xN/AlN double heterostructures by systematically varying the Al content from x = 0 (GaN) to x = 0.74, spanning energy bandgaps of the conducting HEMT channels from 3.49 to 4.9 eV measured by photoluminescence. This results in a tunable 2DEG density from 0 to 3.7 × 1013 cm2. The room temperature mobilities of x ≥ 0.25 AlGaN channel HEMTs were limited by alloy disorder scattering to below 50 cm2/(V.s) for these 2DEG densities, leaving ample room for further heterostructure design improvements to boost mobilities. A characteristic alloy fluctuation energy of ≥1.8 eV for electron scattering in AlGaN alloy is estimated based on the temperature dependent electron transport experiments.