Ultra-wide Bandgap Semiconductor Research Articles

Wet Chemical Synthesis of Ultrathin γ-Ga2O3 Quantum Wires Enabling Far-UVC Photodetection with Ultrahigh Selectivity and Sensitivity.

As compared to solar-blind ultraviolet (UV) photodetectors (PDs), far-UVC PDs not only show some irreplaceable advantages but also are more challenging to be developed. To solve this challenge, we report herein a soft template-assisted solvothermal route to synthesize ultrathin γ-Ga2O3 quantum wires (UQWs) with diameters down to 1-2 nm. These UQWs all exhibit a cluster-like absorption feature with a strong peak located between 190 and 230 nm and an edge below 250 nm, allowing highly selective absorption to far-UVC light. Notably, their normalized absorption coefficients were experimentally and theoretically confirmed to increase obviously with decreasing their diameters. Self-powered photoelectrochemical-type PDs based on Ga2O3 QWs of 1.7 nm diameter were therefore fabricated, exhibiting an excellent far-UVC detection performance with an unprecedented ultrahigh spectral selectivity (R210 nm/R250 nm = 452). As a proof of concept, this paper offers a new idea for developing ultrawide bandgap semiconductor materials and devices by leveraging a strong quantum confinement effect.

Reliable operation of Cr2O3:Mg/β-Ga2O3 p–n heterojunction diodes at 600 °C

Beta gallium oxide (β-Ga2O3)-based semiconductor heterojunctions have recently demonstrated improved performance at high voltages and elevated temperatures and are, thus, promising for applications in power electronic devices and harsh environment sensors. However, the long-term reliability of these ultra-wideband gap (UWBG) semiconductor devices remains barely addressed and may be strongly influenced by chemical reactions at the p–n heterojunction interface. Here, we experimentally demonstrate operation and evaluate the reliability of Cr2O3:Mg/β-Ga2O3 p–n heterojunction diodes during extended operation at 600 °C, as well as after 30 repeated cycles between 25 and 550 °C. The calculated pO2-temperature phase stability diagram of the Ga-Cr-O material system predicts that Ga2O3 and Cr2O3 should remain thermodynamically stable in contact with each other over a wide range of oxygen pressures and operating temperatures. The fabricated Cr2O3:Mg/β-Ga2O3 p–n heterojunction diodes show room-temperature on/off ratios >104 at ±5 V and a breakdown voltage (VBr) of −390 V. The leakage current increases with increasing temperature up to 600 °C, which is attributed to Poole–Frenkel emission with a trap barrier height of 0.19 eV. Over the course of a 140-h thermal soak at 600 °C, both the device turn-on voltage and on-state resistance increase from 1.08 V and 5.34 mΩ cm2 to 1.59 V and 7.1 mΩ cm2, respectively. This increase is attributed to the accumulation of Mg and MgO at the Cr2O3/Ga2O3 interface as observed from the time-of-flight secondary ion mass spectrometry analysis. These findings inform future design strategies of UWBG semiconductor devices for harsh environment operation and underscore the need for further reliability assessments for β-Ga2O3-based devices.

Prediction of phonon properties of cubic boron nitride with vacancy defects and isotopic disorders by using a neural network potential

Cubic boron nitride (c-BN) is a promising ultra-wide bandgap semiconductor for high-power electronic devices. Its thermal conductivity can be substantially modified by controlling the isotope abundance and by the quality of a single crystal. Consequently, an understanding of the phonon transport in c-BN crystals, with both vacancy defects and isotopic disorders at near-ambient temperatures, is of practical importance. In the present study, a neural network potential (NNP) for c-BN has been developed, which has facilitated the investigation of phonon properties under these circumstances. As a result, the phonon dispersion and the three- and four-phonon scattering rates that were predicted with this NNP were in close agreement with those obtained from density-functional theory (DFT) calculations. The thermal conductivities of the c-BN crystals were also investigated, with boron (B) vacancies ranging from 0.0% to 0.6%, by using equilibrium molecular dynamics simulations based on the Green-Kubo formula. These simulations accurately capture vacancy-induced phonon softening, localized vibration modes, and phonon localization effects. As has previously been experimentally prepared, four isotope-modified c-BN samples were selected for analyses in the evaluation of the impact of isotopic disorders. The calculated thermal conductivities aligned well with the DFT benchmarks. In addition, the present study was extended to include a c-BN crystal with a natural abundance of B atoms, which also contained B vacancies. Reasonable thermal conductivities and vibrational characteristics, within the temperature range of 250–500 K, were then obtained.

Reduced temperature in lateral (AlxGa1−x)2O3/Ga2O3 heterojunction field effect transistor capped with nanocrystalline diamond

The low thermal conductivity of β-Ga2O3 is a significant concern for maximizing the potential of this ultra-wide bandgap semiconductor as a power switching device technology. Here, we report on the use of nanocrystalline diamond (NCD) deposited via microwave plasma enhanced chemical vapor deposition (MP-CVD) as a top-side, device-level thermal management solution on a lateral β-Ga2O3 transistor. NCD was grown via MP-CVD on β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures prior to the gate formation of the field-effect transistor. A reduced growth temperature of 400 °C and a SiNx barrier layer were used to protect the oxide semiconductors from etching in the MP-CVD H2 plasma environment. Raman spectroscopy showed a highly sp3-bonded NCD film was obtained at 400 °C, with grain size of about 50–100 nm imaged via atomic force microscopy. The incorporation of the NCD heat-spreading layer resulted in a β-(AlxGa1−x)2O3/β-Ga2O3 heterostructure field-effect transistor showing a decrease in the total thermal resistance at the gate by 42%. The fabrication process, including the NCD etch in the gate region, will need to be improved to minimize the impact of these processes on important device characteristics (i.e., drain current, threshold voltage, and leakage current).

Highly Polarization-Deep-Ultraviolet-Sensitive β-Ga2O3 Epitaxial Films by Disrupting Rotational Symmetry and Encrypted Solar-Blind Optical Communication Application.

Ultrawide bandgap semiconductor β-Ga2O3 (4.9 eV), with its monoclinic crystal structure, exhibits distinct anisotropic characteristics both optically and electrically, making it an ideal material for solar-blind polarization photodetectors. In this work, β-Ga2O3 epitaxial films were deposited on sapphire substrates with different orientations, and the mechanisms underlying the anisotropy of these epitaxial films were investigated. Compared to c-plane sapphire, the lattice mismatch between m- or r-plane sapphire and β-Ga2O3 is more pronounced, disrupting the rotational symmetry of the films and rendering them anisotropic. Thanks to the improved anisotropy, the polarization ratio of the photodetector based on β-Ga2O3 films grown on r-plane substrates is 0.24, nearly ten times higher than that on c-plane substrates. Finally, by utilizing these polarization-sensitive photodetectors, we developed an encrypted solar-blind ultraviolet optical communication system. Our work provides a new approach to facilitate the fabrication and application of high-performance polarization-sensitive solar-blind photodetectors.

ZnS and CdS counterparts of biphenylene lattice: A density functional theory prediction

This study presents four novel inorganic structures based on the biphenylene network (BPN): planar BPN-ZnS (p-BPN-ZnS) and BPN-CdS (p-BPN-CdS) and buckled BPN-ZnS (b-BPN-ZnS) and BPN-CdS (b-BPN-CdS). Cohesive energy analyses reveal that structural buckling enhances stability, and a perturbation is necessary to obtain planar lattices. Phonon dispersion and ab initio molecular dynamics (AIMD) simulations demonstrated the dynamical and thermal stabilities. ZnS-based structures exhibited more pronounced charge accumulation and depletion than CdS. All evaluated structures are ultra-wideband gap semiconductors, with band gap energy values of 4.33, 5.34, 3.59, and 4.30 eV (at HSE06 level) for p-BPN-ZnS, b-BPN-ZnS, p-BPN-CdS, and b-BPN-CdS, respectively. p-BPN-ZnS demonstrated the highest Young modulus (Yx/Yy = 25.295/34.874 N/m), followed by p-BPN-CdS (Yx/Yy = 15.286/21.339 N/m). On the other hand, b-BPN-ZnS and b-BPN-CdS exhibit lower Young Modulus, Yx/Yy = 4.135/14.709 and 11.842/8.218 N/m, respectively. Notably, b-BPN-ZnS displayed a particular characteristic, a negative Poisson ratio (νx/νy = -0.008/-0.030), being an auxetic material. This report is expected to stimulate both theoretical and experimental researchers in the prediction and development of new inorganic materials based on the biphenylene network.

Preparation of ZnGa2O4-based deep ultraviolet photodetector with high photodetectivity by magnetron sputtering

Ultra-wide bandgap semiconductors are frequently utilized materials in the fabrication of deep ultraviolet photodetectors (DUVPDs). However, it is imperative to enhance both the photoresponsivity and response speed of these detectors. Herein, we deposited high-quality (111)-oriented ZnGa2O4 films with a bandgap of approximately 4.75 eV onto c-plane sapphire (0001) substrates using magnetron sputtering. Our findings illuminate the pivotal role of pressure in shaping their structural properties, chemical compositions, and photoelectric characteristics. Importantly, DUVPDs based on ZnGa2O4 films exhibited a photo/dark current ratio of 1.46 × 104. Furthermore, we observed the highest responsivity to be 72.2 A/W, complemented by a photodetectivity of 4.21 × 1014 Jones, an external quantum efficiency of 4.04 × 104 %, and rise and decay times of 2.32 s and 0.055 s, respectively. This study underscores the commendable photoresponsivity and rapid response time of the ZnGa2O4 photodetector, positioning it as a compelling candidate for the advancement of deep ultraviolet devices.

Bandgap engineering of spinel-structured oxide semiconductor alloys

We investigated bandgap engineering of spinel-structured MgB 2O4 (B = Al, Ga, In) alloys. The trend of bandgap change was tunable from approximated 7.8–3.6 eV by substituting group III cation atoms in MgB 2O4. To evaluate the doping possibility, we have calculated the natural band alignment and doping pinning energy of the normal and inverse spinel structures of MgB 2O4 alloys. The calculated doping pinning energies show that spinel-structured MgB 2O4 alloys can potentially control the electron concentration, but hole doping is difficult. Our computational results are expected to provide new insights into the doping control of ultra-wide bandgap semiconductors.

Experimental determination of the band offsets at the UWBG p-LiGa5O8/Ga2O3 interface

LiGa5O8, a recently discovered ultrawide bandgap semiconductor exhibiting p-type conductivity at room temperature, is grown on (010) β-Ga2O3 substrate. Utilizing a mist chemical vapor deposition method, LiGa5O8 thin film grown on insulating Ga2O3 substrate exhibits hole concentration ∼2.31 × 1018 cm−3 and hole mobility ∼2.07 cm2/V s. The resulting heterostructure is characterized comprehensively. A smooth, uniform film growth was observed by scanning electron microscopy and atomic force microscopy imaging. Cross-sectional scanning transmission electron microscopy and x-ray diffraction confirm the growth of spinel cubic crystal structure of LiGa5O8 along the [110] direction. Moreover, the valence and conduction band offsets at p-LiGa5O8/β-Ga2O3 heterointerface were determined via x-ray photoelectron spectroscopy. The determined band alignments at LiGa5O8/β-Ga2O3 interface reveal the formation of a type-II (staggered) heterojunction. The experimental measurements provide valuable parameters of the band offsets at the heterointerface, addressing a crucial aspect for future power electronic device design and fabrication based on β-Ga2O3 ultrawide bandgap semiconductors.

High-performance a-Ga2O3 solar-blind photodetectors by pulsed magnetron sputtering deposition

Solar-blind photodetectors (SBPDs) based on the ultrawide-bandgap semiconductor Ga2O3 have gained attention due to their potential applications in both military and civilian domains. As technology advances, photodetectors are being improved to achieve better energy efficiency, smaller size, and better performance. Solar-blind photodetectors based on a metal-semiconductor-metal structure of amorphous gallium oxide (a-Ga2O3) films were fabricated by pulsed magnetron sputtering deposition (PSD). The photodetector based on amorphous gallium oxide has a responsivity of 71.52 A/W, a fast rising and falling response time of less than 200 ms, a photo-to-dark current ratio (PDCR) of 6.52 × 104, and an external quantum efficiency of 34 526.62%. PSD-prepared gallium oxide SBPDs demonstrate a cost-effective room temperature method for growing gallium oxide and show the advantages of growing gallium oxide.

Strategy to solve the thermal issue of ultra-wide bandgap semiconductor gallium oxide field effect transistor

Gallium Oxide (Ga2O3) holds significant potential for the next generation of electronic devices following SiC and GaN due to its ultra-wide bandgap of approximately 4.5 eV - 4.9 eV and high theoretical critical breakdown field strength of 8 MV/cm. Nonetheless, Ga2O3 has a naturally low thermal conductivity, resulting in limited device output performance and hindering Ga2O3 devices from reaching their full theoretical potential. In this study, we demonstrate that the appropriate thermal management strategy can solve the above challenges. By comparing the thermal control schemes including the Ga2O3 FET devices on the original substrate, the thinned Ga2O3 substrate, the high thermal conductivity substrate, the heat sink packaging, and the flip-chip packaging, it is demonstrated that the flip-chip model is the most effective strategy to improve the heat dissipation performance of the Ga2O3 device. By utilizing the appropriate carrier in flip-chip packaging, the temperature elevation of the device at 2 W/mm power density will be diminished by around 91% in contrast to the initial basic device. Furthermore, the output performance of the device demonstrates significant enhancement. This thermal management technique successfully resolves the severe heat dissipation issue prevalent in Ga2O3 devices and eliminates primary obstacles concerning the industrialization of Ga2O3 RF and power devices.

Polar iodate BiO(IO3): A two-dimensional ultrawide-bandgap semiconductor with high carrier mobility and robust piezoelectricity

Ultrawide-bandgap (UWBG) semiconductors, surpassing GaN with a bandgap of 3.4 eV, offer distinct advantages in high-temperature, high-frequency, and radiation-resistant applications. Specifically, two-dimensional (2D) UWBG materials play a crucial role in the miniaturization of practical devices. Here, employing first-principles calculations, we explore the properties of the polar iodate BiO(IO3) in its 2D monolayer configuration, boasting a bandgap of 3.61 eV. Our calculations confirm the feasibility of deriving 2D BiO(IO3) monolayer from its bulk counterpart while retaining structural stability at room temperature. The UWBG BiO(IO3) monolayer exhibits remarkable ultraviolet absorption, mechanical flexibility, and favorable electronic transport behavior. Notably, the estimated electron mobility reaches an impressive 1353.13 cm2 V−1 s−1. Importantly, the 2D structure of BiO(IO3) displays robust in-plane piezoelectricity without the odd–even effect commonly observed in other 2D piezoelectric materials. The piezoelectric coefficients d21 and d22 of monolayer reach high values of 13.87 and 16.66 pm V−1, respectively, surpassing or closely approaching those of most well-studied 2D systems. The direct stacking configuration enables 2D BiO(IO3) materials to maintain robust piezoelectricity at different thicknesses. Charge injection simulations validate the electromechanical conversion process, aligning well with its piezoelectric properties. This suggests the promising application potential of 2D BiO(IO3) in devices such as micro-electro-mechanical systems.

Tutorial: Microscopic properties of O–H centers in β-Ga2O3 revealed by infrared spectroscopy and theory

β-Ga2O3 is an ultrawide bandgap semiconductor that is attracting much attention for applications in next-generation high-power, deep UV, and extreme-environment devices. Hydrogen impurities have been found to have a strong effect on the electrical properties of β-Ga2O3. This Tutorial is a survey of what has been learned about O–H centers in β-Ga2O3 from their vibrational properties. More than a dozen, O–H centers have been discovered by infrared absorption spectroscopy. Theory predicts defect structures with H trapped at split configurations of a Ga(1) vacancy that are consistent with the isotope and polarization dependence of the O–H vibrational spectra that have been measured by experiment. Furthermore, O–H centers in β-Ga2O3 have been found to evolve upon thermal annealing, giving defect reactions that modify conductivity. While much progress has been made toward understanding the microscopic properties and reactions of O–H centers in β-Ga2O3, many questions are discussed that remain unanswered. A goal of this Tutorial is to inspire future research that might solve these puzzles.

From wide to ultrawide-bandgap semiconductors for high power and high frequency electronic devices

Abstract Wide and ultrawide-bandgap (U/WBG) materials have garnered significant attention within the semiconductor device community due to their potential to enhance device performance through their substantial bandgap properties. These exceptional material characteristics can enable more robust and efficient devices, particularly in scenarios involving high power, high frequency, and extreme environmental conditions. Despite the promising outlook, the physics of UWBG materials remains inadequately understood, leading to a notable gap between theoretical predictions and experimental device behavior. To address this knowledge gap and pinpoint areas where further research can have the most significant impact, this review provides an overview of the progress and limitations in U/WBG materials. The review commences by discussing Gallium Nitride, a more mature WBG material that serves as a foundation for establishing fundamental concepts and addressing associated challenges. Subsequently, the focus shifts to the examination of various UWBG materials, including AlGaN/AlN, Diamond, and Ga2O3. For each of these materials, the review delves into their unique properties, growth methods, and current state-of-the-art devices, with a primary emphasis on their applications in power and radio-frequency electronics.

Carbon-related donor–acceptor pair transition in the infrared in h-BN

Experimental studies of intentionally doped impurities for the understanding of conductivity control in hexagonal boron nitride (h-BN) ultrawide bandgap (UWBG) semiconductor are limited but are highly desired for emerging applications of h-BN. We report synthesis by hydride vapor phase epitaxy and comparison photoluminescence (PL) emission spectroscopy studies of intentionally carbon (C)-doped and undoped h-BN semi-bulk crystals. In addition to the well-known C-related emission lines observed previously, a C-impurity-related transition near 1.31 eV consisting of multiple phonon replicas has been observed in C-doped h-BN at room temperature. Phonon replicas involved in the 1.31 eV emission have been identified using polarization resolve PL spectroscopy as the transverse acoustic (TA)/longitudinal acoustic (LA) and out-of-plane optical phonon (ZO) modes at the middle point, T, between the Γ- and K-points in the first Brillouin zone. Based on the agreement between the spectral peak position of the observed dominant emission line at 1.31 eV and the calculated energy-level separation between CB donor (carbon replacing boron) and Ci acceptor (carbon interstitial), the observed IR emission line can be decisively assigned to the donor–acceptor pair (DAP) transition involving the CB donor and Ci acceptor assisted by the intervalley (Κ → Μ) scattering processes. The results reinforce the perception that C impurities form deep-level centers and provided an improved understanding of C impurities in h-BN.

Guest Editorial: Special Issue on Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications

Guest Editorial: Special Issue on Wide and Ultrawide Band Gap Semiconductor Devices for RF and Power Applications

Effects of low fluence 212 MeV Ge swift heavy ion irradiation on the structural and optical properties of β-Ga2O3 epitaxial layers

Considering the future applications of the ultra-wide bandgap semiconductor β-Ga2O3 in high-performance electronic devices and aerospace, the effects of low fluence 212 MeV Ge swift heavy ion (SHI) irradiation on the structural and optical properties of β-Ga2O3 epitaxial layers have been studied in this work. The alternations in Raman scattering properties of the epitaxial layer after Ge SHI irradiation indicate the generation of lattice defects. The increased concentration of oxygen vacancies after irradiation leading to an enhanced area ratio of the yellow-red emission band in the PL spectrum of the β-Ga2O3 epitaxial layer. Simultaneously, the PL intensity quenches as the irradiation fluence increases, attributed to the generation of non-radiative recombination centers. Furthermore, HRTEM analysis provides direct evidence of lattice damage caused by 212 MeV Ge SHI irradiation to the samples. The results demonstrate that the spectral behaviors of β-Ga2O3 epitaxial layers can be modulated by low fluence Ge SHI irradiation with less damage caused to the overall crystalline quality and structure.

Enhancement-Mode Ambipolar Thin-Film Transistors and CMOS Logic Circuits using Bilayer Ga2O3/NiO Semiconductors.

Recent advancements in power electronics have been driven by Ga2O3-based ultrawide bandgap (UWBG) semiconductor devices, enabling efficient high-current switching. However, integrating Ga2O3 power devices with essential silicon CMOS logic circuits for advanced control poses fabrication challenges. Researchers have introduced Ga2O3-based NMOS and pseudo-CMOS circuits for integration, but these circuits may either consume more power or increase the design complexity. Hence, this article proposes Ga2O3-based CMOS realized using heterogeneous 3D-stacked bilayer ambipolar transistors. These ambipolar transistors consist of HfO2/NiO/Ga2O3/NiO/HfO2 heterostructures that are wrapped around by the Ti/Au gate electrode, resulting in record high electron and hole current on/off ratios of 109 and 107. The threshold voltage, subthreshold swing, and current density measured from 100 ambipolar devices (across 5 batches) are around -7.99 ± 0.92 V (p-channel) and 7.81 ± 0.81 V (n-channel), 0.59 ± 0.07 V/dec (p-channel) and 0.61 ± 0.06 V/dec (n-channel), and 0.99 ± 0.26 mA/mm (p-channel) and 58.23 ± 12.99 mA/mm (n-channel), respectively. All the 100 ambipolar devices showed decent long-term stability over a period of 200 days, exhibiting reliable electrical performance. The threshold voltage shift (ΔVTH) after negative bias stressing for a period of 3500 s is around 11.52 V (p-channel) and 10.21 V (n-channel), respectively. Notably, the n-channels exhibit ∼2 orders higher on/off ratio than the best Ga2O3 unipolar transistors at 300 °C. Moreover, the polarities of ambipolar transistors are reconfigurable into p- or n-MOS, which are integrated to demonstrate CMOS inverter, NOR, and NAND logic gates. The switching periods from "0" to "1" and from "1" to "0" of NOR are 0.12 and 0.17 μs, and those of NAND are 0.16 and 0.13 μs. This work lays the foundation of oxide-semiconductor-based CMOS for future integrated electronics.

Photo-induced electron paramagnetic resonance: A means to identify defects and the defect level throughout the bandgap of ultrawide bandgap semiconductors

Ultrawide bandgap semiconductors (UWBGs) provide great promise for optical devices operating in the near to deep ultraviolet, and recently they have become a viable semiconducting material for high power electronics. From the power grid to electronic vehicles, the intention is to replace massively awkward components with the convenience of a solid state electronic “chip.” Unfortunately, the challenges faced by wide bandgap electronic materials, such as GaN and SiC, increase as the bandgap increases. A point defect, for example, can take on more charge states and energy configurations. This perspective describes a method to investigate the many charge states and their associated transitions—photo-induced electron paramagnetic resonance (photo-EPR) spectroscopy. Although not new to the study of defects in semiconductors, photo-EPR studies can probe the entire ultrawide bandgap given the appropriate light source for excitation. Examples provided here cover specific defects in UWBGs, AlN, and Ga2O3. The discussion also reminds us how the rapid pace of discovery surrounding this newest class of semiconductors is due, in part, to fundamental research studies of the past, some as far back as a century ago and some based on very different materials systems.

Improved heteroepitaxy of κ-Ga2O3 on c-plane sapphire by initial mist flow stabilization during mist chemical vapor deposition

Kappa-phase gallium oxide (κ-Ga2O3) is a metastable gallium oxide polymorph and an ultrawide bandgap semiconductor with potential applications in various electronic and optoelectronic devices. In this study, we present the effect of mist flow stabilization during the initial stages of mist chemical vapor deposition process on the crystal quality of heteroepitaxial thin films of κ-Ga2O3 grown on c-plane sapphire. Two κ-Ga2O3 films were grown using the same growth parameters with the only differing factor being the stabilization of the initial mist flow. Structural and optical properties of the films were characterized. Both processes resulted in epitaxial growth; however, controlling the initial mist flow leads to an improved Ga2O3/Al2O3 interface and superior crystal quality. The film grown with regulated mist flow exhibited an optical bandgap of 4.93 eV, a smooth surface with a root mean square roughness of 1.56 nm and a rocking curve full-width half-maximum value of 0.17°. Also, Ti/Au deposition on κ-Ga2O3 resulted in an interface resembling that of β-Ga2O3 and Ti/Au, explaining previous reports of ohmic behavior between κ-Ga2O3 and Ti/Au contacts.