Ultra-wide Bandgap Semiconductor Research Articles

Interfacial Optimization for AlN/Diamond Heterostructures via Machine Learning Potential Molecular Dynamics Investigation of the Mechanical Properties.

AlN/diamond heterostructures hold tremendous promise for the development of next-generation high-power electronic devices due to their ultrawide band gaps and other exceptional properties. However, the poor adhesion at the AlN/diamond interface is a significant challenge that will lead to film delamination and device performance degradation. In this study, the uniaxial tensile failure of the AlN/diamond heterogeneous interfaces was investigated by molecular dynamics simulations based on a neuroevolutionary machine learning potential (NEP) model. The interatomic interactions can be successfully described by trained NEP, the reliability of which has been demonstrated by the prediction of the cleavage planes of AlN and diamond. It can be revealed that the annealing treatment can reduce the total potential energy by enhancing the binding of the C and N atoms at interfaces. The strain engineering of AlN also has an important impact on the mechanical properties of the interface. Furthermore, the influence of the surface roughness and interfacial nanostructures on the AlN/diamond heterostructures has been considered. It can be indicated that the combination of surface roughness reduction, AlN strain engineering, and annealing treatment can effectively result in superior and more stable interfacial mechanical properties, which can provide a promising solution to the optimization of mechanical properties, of ultrawide band gap semiconductor heterostructures.

Engineering shallow and deep level defects in κ-Ga2O3 thin films: comparing metal-organic vapour phase epitaxy to molecular beam epitaxy and the effect of annealing treatments

Orthorhombic gallium oxide (κ-Ga2O3) is an ultra-wide bandgap semiconductor with great potential in new generation electronics. Its application is hindered at present by the limited physical understanding of the relationship between synthesis and functional properties. This work discusses the effects of growth method (metal-organic vapour phase epitaxy and molecular beam epitaxy) as well as annealing treatments in different atmospheres (O2, H2) on point defects in κ-Ga2O3 layers epitaxially grown on c-plane sapphire. Comprehensive experimental characterization by X-ray diffraction, photo current-as well as photoluminescence excitation spectroscopy, and X-ray photo electron spectroscopy is combined with first principles calculations of the point defects’ formation and complex-dissociation energies. We demonstrate that for κ-Ga2O3 the concentration of shallow and deep level defects can be sensitively controlled through annealing treatments at temperatures (T = 500 °C) well below the thermal stability threshold of this polymorph. In particular, our results suggest that hydrogen-related defects (e.g., H-interstitials, Ga-vacancies—H complexes) play a key role in this process. While we provide direct exemplary implications of our results for the performances of κ-Ga2O3 based photodetectors, these findings are predicted to impact further application fields of κ-Ga2O3, such as high electron mobility transistors or memory devices.

Status of h-BN quasi-bulk crystals and high efficiency neutron detectors

III-nitrides have fomented a revolution in the lighting industry and are poised to make a huge impact in the field of power electronics. In the III-nitride family, the crystal growth and use of hexagonal BN (h-BN) as an ultrawide bandgap (UWBG) semiconductor are much less developed. Bulk crystals of h-BN produced by the high-temperature/high-pressure and the metal flux solution methods possess very high crystalline and optical qualities but are impractical to serve as substrates or for device implementation as their sizes are typically in millimeters. The development of crystal growth technologies for producing thick epitaxial films (or quasi-bulk or semi-bulk crystals) in large wafer sizes with high crystalline quality is a prerequisite for utilizing h-BN as an UWBG electronic material. Compared to traditional III-nitrides, BN has another unique application as solid-state neutron detectors, which however, also require the development of quasi-bulk crystals to provide high detection efficiencies because the theoretical efficiency (ηi) relates to the detector thickness (d) by ηi=1−e−dλ, where λ denotes the thermal neutron absorption length which is 47 μm (237 μm) for 10B-enriched (natural) h-BN. We provide an overview and recent progress toward the development of h-BN quasi-bulk crystals via hydride vapor phase epitaxy (HVPE) growth and the attainment of thermal neutron detectors based on 100 μm thick 10B-enriched h-BN with a record efficiency of 60%. The thermal neutron detection efficiency was shown to enhance at elevated temperatures. Benchmarking the crystalline and optical qualities of h-BN quasi-bulk crystals with the state-of-the-art mm-sized bulk crystal flakes and 0.5 μm thick epitaxial films identified that reducing the density of native defects such as vacancies remains the most critical task for h-BN quasi-bulk crystal growth by HVPE.

Synthesis and characteristics of a monocrystalline GaAs/β-Ga2O3p-n heterojunction

Beta phase-gallium oxide (β-Ga2O3) is an emerging ultrawide bandgap semiconductor but lacks efficient p-type doping, which hinders development of high-performance bipolar heterojunction devices. This study introduces the synthesis and characterization of a monocrystalline GaAs/β-Ga2O3p-n heterojunction, employing advanced semiconductor grafting technology. The heterojunction was innovatively fabricated by grafting a p-type GaAs single crystal nanomembrane to an Al2O3-coated n-type β-Ga2O3 epitaxial substrate. The synthesized heterojunction was comprehensively characterized. The band alignment of grafted GaAs/β-Ga2O3p-n heterojunction was experimentally constructed through X-ray photoelectron spectroscopy (XPS), and the result is in accordance with the theorical prediction based on the electron affinity rule. The resulting GaAs/β-Ga2O3p-n diodes built on this heterojunction exhibit remarkable performance metrics, including an ideality factor of 1.23, a high rectification ratio of 8.04 × 109 at ± 4 V, and a turn on voltage at 2.35 V. Furthermore, at + 5 V, the diode displays a large current density of 2500 A/cm2 along with a low ON resistance of 2 mΩ∙cm2. This study marks the first successful demonstration of a high-quality grafted GaAs/β-Ga2O3p-n heterojunction, establishing a critical building block for the further development of bipolar Ga2O3-based device applications such as diodes, transistors, and thyristors.

The fabrication of ultra-wide bandgap GeO2 thin films by DC magnetron sputtering: The impacts of growth temperature and post-annealing process

Ultra-wide bandgap semiconductors (UWBSs), possessing the bandgap greater than 4 eV, have emerged as an advanced platform for optoelectronic device applications. The exploration of novel UWBSs becomes compelling research focus on the solar blind photodetectors (SBPDs). Here, we have deposited ultra-wide bandgap GeO2 films on c-plane sapphire substrates using direct-current (DC) magnetron sputtering method and investigated the effects of the growth and annealing temperatures on surface morphologies, crystal structure, oxygen vacancies, and bandgap energies of GeO2 films. The high quality GeO2 film with the hexagonal phase can be obtained at the growth temperature of 500 °C, which transforms to the mixture of hexagonal and tetragonal phases after annealing at 900 °C. By analyzing the energy loss spectra of O 1s peaks, GeO2 films exhibit the ultra-wide bandgap ranging from 5.78 to 5.90 eV, which is verified by optical transmittance measurements. Moreover, 200 nm SBPDs based on GeO2 films have been fabricated and an obvious improvement in the photoresponse current of the annealed device has been found due to the induced additional electrons by the hole capture of oxygen vacancies. Our findings on the development of ultra-wide bandgap GeO2 films pave a way towards the realization of robust and high-performance SBPDs.

Monolithic n‐Type Metal–Oxide–Semiconductor Inverter Integrated Circuits Based on Wide and Ultrawide Bandgap Semiconductors

Wide bandgap (WBG) and ultrawide bandgap (UWBG) semiconductors in n‐type metal–oxide–semiconductor (n‐MOS) integrated circuits (ICs) are increasingly being explored for their potential applications in the rapidly developing field of electronics. This review comprehensively examines the role of n‐MOS inverters underpinned by WBG and UWBG semiconductors and their application possibilities. It delves into various n‐MOS inverter topologies, including resistive, enhancement or diode‐load, depletion‐load, and pseudo‐complementary MOS inverter topologies. Each topology's operational principles, unique advantages, and potential performance are elucidated in detail. Finally, these topologies are simulated using the Advanced Design System software for a fair comparison between various topologies. The literature and simulation results show that the pseudo‐D topology has the best gain and improved noise margin. The review methodology involves an extensive exploration of WBG/UWBG n‐MOS inverters to advance the current understanding of WBG/UWBG n‐MOS‐based ICs.

Properties of photocurrent and metal contacts of highly resistive ultrawide bandgap semiconductors

Properties of photocurrent and metal contacts of highly resistive ultrawide bandgap semiconductors

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.