Ultra-wide Bandgap Research Articles

Transmission electron microscopic study on rutile-type GeO2 film on TiO2 (001) substrate

Rutile-type GeO2 (r-GeO2) with an ultrawide bandgap of ∼4.7 eV has emerged as a promising material for next-generation power-electronic and optoelectronic devices. We performed transmission electron microscopy (TEM) observation to analyze the structural properties of r-GeO2 film on r-TiO2 (001) substrate at an atomic level. The r-GeO2 film exhibits a threading dislocation density of 3.6 × 109 cm−2 and there exist edge-, screw-, and mixed-type dislocations in the film as demonstrated by two-beam TEM. The edge-type dislocations have Burgers vectors of [100] and/or [110]. The bandgap of the r-GeO2 film is 4.74 ± 0.01 eV as determined by electron energy loss spectroscopy.

Design and research of high voltage β-Ga2O3/4H-SiC heterojunction LDMOS

Abstract In recent years, gallium oxide (Ga2O3), one of the ultra-wide band-gap semiconductor materials, has been regarded as one of the most promising materials in the field of high-voltage and high-power electronic devices in the future because of its unique electrical properties and low preparation cost. However, it is difficult for β-Ga2O3 materials to form effective P-type doping, so the PN junction is not formed in most of its power devices, which greatly limits the improvement of its voltage resistance. A novel lateral double-diffused metal-oxide-semiconductor field-effect transistor (LDMOS) is proposed to realize a junction β-Ga2O3 power device and improve the voltage resistance performance of β-Ga2O3 power devices. In this device, a β-Ga2O3 power device with a junction is realized by using P-type 4H-SiC to form a PN junction with N-type β-Ga2O3, and the voltage resistance of the β-Ga2O3 power device is improved. Sentaurus TCAD software was used to simulate the device structure and electrical performance. The device is optimized by adjusting the length of the drift zone, drift zone concentration, SiC channel concentration, and gate oxide thickness. Optimized, the device exhibits a positive threshold voltage of 3.42 V, a breakdown voltage of 2203 V, a specific on-resistance of 7.80 mΩ·cm2, and a power figure of merit of 622 MW cm−2. The results show that the heterojunction device is significant for realizing high-performance junction β-Ga2O3 power devices.

Photoluminescence and Raman spectroscopy of wide bandgap semiconductors damaged by deep-UV laser irradiation

The effects of a pulsed, focused, deep-UV (4.66 eV) laser on wide and ultra-wide bandgap semiconductors were investigated with photoluminescence (PL) and Raman spectroscopy. Three semiconductor single crystals were studied: silicon carbide (6H-SiC), gallium nitride (GaN), and gallium oxide (β-Ga2O3). Atomic emission lines from neutral Ga or Si were observed during the laser-damage process. For all three semiconductors, PL mapping (3.49 eV laser excitation) of the damaged material revealed visible emission bands in the 2.6–2.8 eV range, attributed to point defects. Raman spectra (2.33 eV excitation) showed a reduction in the Raman peak intensities in the damaged region, along with weak PL bands around 1.9–2.1 eV.

Zr-Doping Strategy of High-Quality Cu2O/β-Ga2O3 Heterojunction for Ultrahigh-Performance Solar-Blind Ultraviolet Photodetection.

β-Ga2O3, as an ultrawide band gap semiconductor, has emerged as the most promising candidate in solar-blind photodetectors. The practical application of β-Ga2O3, however, suffers from intrinsic defects and suboptimal crystal quality within the devices. In this work, high-quality β-Ga2O3 was successfully synthesized by employing the Zr-doping strategy, which has facilitated the development of ultrahigh-performance solar-blind photodetectors based on Cu2O/β-Ga2O3 heterostructures. Structural analyses indicate that the strong Zr-O covalent bond effectively stabilizes the material, thereby eliminating oxygen vacancy defects. The Cu2O/β-Ga2O3 heterostructure photodetector demonstrates an ultrahigh responsivity and detectivity coupled with an external quantum efficiency. Furthermore, the device exhibits a photocurrent-to-dark current ratio of 3 × 105, showcasing its superior capability in detecting low-intensity deep ultraviolet signals, markedly surpassing previous heterostructure ultraviolet photodetectors. These exceptional performances are attributed to the effective elimination of oxygen vacancy defects in β-Ga2O3 and the variation of band alignment at the interface, which facilitate rapid separation of photogenerated electron-hole pairs under reverse bias. This study not only provides an enhanced and easy route to mitigate oxygen vacancy defects in oxide materials but also propels further exploration into the next generation of flexible, high-performance, solar-blind ultraviolet photodetectors.

Excellent Wavelength Selectivity of p-AlGaN/n-Ga2O3 Solar-Blind UVC PD.

Due to the advantages of ultrawide bandgap, chemical stability, self-powered, and low cost, gallium oxide (Ga2O3) based photodetectors (PDs) are considered as one of the most promising solar-blind ultraviolet PDs, having garnered significant attention in fields such as missile warning and flame arc detection. High selective ratios and excellent responsivity are important to reduce the false alarm rate in solar-blind detection. However, due to the lack of p-type Ga2O3, existing Ga2O3-based PN PDs utilized heterostructures with narrower bandgap p-type semiconductors (e.g., p-GaN, p-SiC, etc.), inducing poor selective ratios with visible light and ultraviolet-A/B (UVA/B). Therefore, a high Al content AlGaN was used in this study to form a p-AlGaN/n-Ga2O3 solar-blind ultraviolet-C (UVC) PD. Since the bandgap of p-AlGaN aligned with the UVC region, the device exhibited exceptionally high selective ratios with R247nm/R360nm = 3.71 × 105 and R247nm/R300nm = 1.47 × 105 at 0 V, which surpassed the reported Ga2O3-based PN PDs. The formation of the type-II heterojunction facilitated the effective separation and transport of photogenerated carriers, resulting in high responsivity (0.36 A/W) under zero bias. Overall, the high selective ratios and high responsivity of the p-AlGaN/n-Ga2O3 PD in this paper provided a reliable pathway for self-powered solar-blind PDs with a low false alarm rate.

Impact of Temperature and Substrate Type on the Optical and Structural Properties of AlN Epilayers: A Cross-Sectional Analysis Using Advanced Characterization Techniques

AlN, with its ultra-wide bandgap, is highly attractive for modern applications in deep ultraviolet light-emitting diodes and electronic devices. In this study, the surface and cross-sectional properties of AlN films grown on flat and nano-patterned sapphire substrates are characterized by a variety of techniques, including photoluminescence spectroscopy, high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and Raman spectroscopy. The results indicate that different sapphire substrates have minimal impact on the photoluminescence spectrum of the epitaxial films. As the temperature increased, the radius of curvature of the AlN films increased, while the warpage decreased. The AlN films grown on nano-patterned substrates exhibited superior quality with less surface oxidation. During the growth of AlN thin films on different types of substrates, slight shifts in the energy bands occurred due to differences in the introduction of carbon-related impurities and intrinsic defects. The Raman shift and full width at half maximum (FWHM) of the E2(low), A1(TO), E2(high), E1(TO), and E1(LO) phonon modes for the cross-sectional AlN films varied with the depth and temperature. The stress state within the film was precisely determined with specific depths and temperatures. The FWHM of the E2(high) phonon mode suggests that the films grown on nano-patterned substrates exhibited better crystalline quality.

Tunable magnetism by nonmagnetic-doping in 2D flexible alkaline-earth metal halofluoride XYF (X = Ca/Sr/Ba, Y = Cl/Br/I) with ultra-wide bandgap

Tunable magnetism by nonmagnetic-doping in 2D flexible alkaline-earth metal halofluoride XYF (X = Ca/Sr/Ba, Y = Cl/Br/I) with ultra-wide bandgap

Machine Learning Enabled High‐Throughput Screening of 2D Ultrawide Bandgap Semiconductors for Flexible Resistive Materials

AbstractThe 2D ultrawide bandgap (UWBG) semiconductors have attracted great attentions for the next generation of electronics and optoelectronics, owing to their superiority on material flexibility, device stability, and power consumption. However, few 2D UWBG semiconductors have been discovered, impeding their prosperous developments and widespread applications. Here, a high‐throughput workflow is constructed to screen 2D UWBG semiconductors assisted by machine learning, and 507 potential candidates are obtained. Moreover, by learning, predicting, and screening Young's modulus and Poisson's ratio, 31 flexible 2D UWBG semiconductors are identified. Then the generation and the diffusion of anion vacancies, as well as the corresponding electronic properties are investigated by using the first‐principles calculations, and 3 of them are demonstrated as the most promising candidates for the flexible resistive materials. The facile interface tunneling and the increased material conductance caused by the anion vacancies will contribute to the transition from high resistive state to low resistive state. This work provides an efficient high‐throughput screening protocol to enrich the family of 2D UWBG semiconductors and is expected to foster their practical applications.

Huge photosensitivity gain combined with long photocurrent decay times in various polymorphs of Ga2O3: effects of carriers trapping with deep centers

Abstract The materials system of ultra-wide bandgap Ga2O3 has already shown great promise in the field of solar-blind photodetectors with high photoresponsivity, high photoresponsivity gain and low dark current. These promising results have been achieved on Ga2O3 films of different polymorphs and by different methods, often not with particularly high crystalline quality. In fact, it would often seem the case that the lower the crystalline quality of the films, the higher the photosensitivity and its gain. This, however, is in most cases accompanied by unusually long photocurrent build-up and decay times. We show that the experimental results can be explained by models in which the high photosensitivity gain is related to the effects of holes being trapped by deep states that, in Schottky diodes, results in the decrease of the Schottky barrier height with consequent increase of electron current, and in Metal-Semiconductor-Metal (MSM) structures additionally gives rise to the usual gain increase due to the increased concentration and lifetime of electrons. We present and discuss the models describing the effects in Ga2O3 Schottky diodes, MSM structures, unipolar and bipolar heterojunctions and propose possible candidates for the role of the hole traps in different Ga2O3 polymorphs. We also discuss the existing results for the photocurrent build-up and decay times and offer possible explainations of the observed temperature dependences of the characteristic times where such data are present.

Deep-ultraviolet transparent conducting SrSnO3 via heterostructure design.

Exploration and advancements in ultrawide bandgap (UWBG) semiconductors are pivotal for next-generation high-power electronics and deep-ultraviolet (DUV) optoelectronics. Here, we used a thin heterostructure design to facilitate high conductivity due to the low electron mass and relatively weak electron-phonon coupling, while the atomically thin films ensured high transparency. We used a heterostructure comprising SrSnO3/La:SrSnO3/GdScO3 (110), and applied electrostatic gating, which allow us to effectively separate charge carriers in SrSnO3 from dopants and achieve phonon-limited transport behavior in strain-stabilized tetragonal SrSnO3. This led to a modulation of carrier density from 1018 to 1020 cm-3, with room temperature mobilities ranging from 40 to 140 cm2 V-1 s-1. The phonon-limited mobility, calculated from first principles, closely matched experimental results, suggesting that room temperature mobility could be further increased with higher electron density. In addition, the sample exhibited 85% optical transparency at a 300-nm wavelength. These findings highlight the potential of heterostructure design for transparent UWBG semiconductor applications, especially in DUV regime.

Si/AlN p-n heterojunction interfaced with ultrathin SiO2

Ultra-wide bandgap (UWBG) materials offer significant potential for high-power RF electronics and deep ultraviolet photonics. Among these, AlxGa1−xN stands out due to its tunable bandgap (3.4 eV to 6.2 eV) and excellent material properties. However, achieving efficient p-type doping in high aluminum composition AlGaN remains a challenge. This study presents a novel approach by fabricating a p+Si/n-AlN/n+AlGaN heterojunction using semiconductor grafting. Atomic force microscopy (AFM) revealed smooth surfaces for AlN and the nanomembrane, with roughness values of 1.96 nm and 0.545 nm, respectively. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) confirmed a sharp and well-defined Si/AlN interface with minimal defects and strong chemical bonding. X-ray photoelectron spectroscopy (XPS) measurements identified a type-I heterojunction with a valence band offset (ΔEv) of 2.73–2.84 eV and a conduction band offset (ΔEc) of 2.22–2.11 eV. The pn diode devices exhibited linear current–voltage (I-V) characteristics, an ideality factor of 1.92, and a high rectification ratio of 3.3 × 104, with a turn-on voltage of 3.9 V. Temperature-dependent I-V measurements showed stable operation up to 90 °C. The heterojunction’s high-quality interface and impressive electrical performance underscore its potential for advanced AlGaN-based optoelectronic and electronic applications.

Effect of Substrate Thinning on Temperature Rise in Ga2O3 Rectifiers

The low thermal conductivity of β-Ga2O3 is a concern for the high-power switching applications envisaged for this ultra-wide bandgap semiconductor. In this work, we examine the effect of substrate thinning to reduce the temperature rise in rectifiers under high power conditions and also reduce the on-resistance. The Ga2O3 substrates on which the rectifiers were fabricated were thinned from the original thickness of 630 μm to a lowest value of 50 μm and transferred to a brass heat sink. Experimentally, we observed that the on-resistance was reduced from 5.66 to 5.17 mΩ.cm2 when thinning to 50 μm, in excellent agreement with simulations. The calculated peak temperature rise was roughly halved for rectifiers on such thin substrates over a broad range of power densities (500–1500 W.cm2), a result supported by thermal imaging.

Two-dimensional Sc2O3 monolayer with tunable ultrawide bandgap for solar-blind ultraviolet photodetector

A novel 2D scandia (Sc2O3) monolayer was identified using an evolutionary algorithm-based crystal structure prediction method. The Sc2O3 monolayer exhibits a P3‾m1 symmetry and excellent energetic, thermal, dynamical, and mechanical stability, as well as good mechanical flexibility. The monolayer possesses an ultrawide indirect band gap of 6.246 eV. The monolayer is transparent in the visible light zone, while the large exciton effect leads to significant absorption in the solar-blind and vacuum ultraviolet regions. The band gap of this monolayer can be reduced monotonically by external biaxial tensile strain, resulting in the absorption spectrum covering the entire solar blind spectral region when the load reaches 5.0 %. Additionally, the monolayer has an ultra-high in-plane dielectric constant of approximately 50. The superior stability, flexibility, and strain-tunable electronic and optical properties, as well as the ultra-high dielectric constant, suggest its potential application in a solar-blind photodetector in harsh environments.

NPDC structure double-channel N-polar E-mode GaN HEMTs: Innovations in enhancing RF and DC performance and mitigating trap effects

—High-electron-mobility transistors (HEMTs) based on gallium nitride (GaN) are favored for their exceptional performance in high-power and high-frequency applications. However, developing dual-channel HEMTs presents challenges due to spontaneous polarization effects and design complexities. This study introduces a novel N-polar E-mode GaN HEMT with a dual-channel structure, featuring recessed drain-source regions and non-alloyed ohmic contacts. TCAD simulations provide theoretical insights and optimization strategies for these structures, focusing on the combination of N-polar GaN and ultra-wide bandgap AlN barrier layers. The optimized device demonstrates a threshold voltage of +1.08 V and a 45.9 % increase in current density. The N-polar double-channel HEMT (NPDC-HEMT) exhibited a 55.75 % improvement in peak transconductance, with Ft increasing from 13.3 GHz to 30.1 GHz and Fmax from 63.7 GHz to 65.9 GHz. Further enhancement was achieved in the T-gate variant (NPDC-T-HEMT), with Ft reaching 52.6 GHz and Fmax 94.4 GHz. The dual-channel design effectively mitigated short-channel effects and current collapse, demonstrating the potential for high-power RF applications.

Enhanced Laser Damage Threshold in Optically Addressable Light Valves via Aluminum Nitride Photoconductors

AbstractOptically addressable light valves (OALVs) are specialized optical components utilized for spatial beam shaping in various laser‐based applications, including optics damage mitigation, and enhanced functionality in diode‐based additive manufacturing requiring high intensities. Current state‐of‐the‐art OALVs employ photoconductors such as Bismuth Silicon Oxide (BSO) or Bismuth Germanium Oxide (BGO), which suffer from limited laser‐induced damage thresholds (LiDT) and inadequate thermal conductivities, thus restricting their use in high peak and average power applications. Aluminum nitride (AlN), an emerging ultra‐wide band gap (UWBG) III–V semiconductor, offers promising optoelectronic properties and superior thermal conductivity (>300 Wm−1K−1 at 298° K, compared to BSO's 3.29 Wm−1K−1). In this study, the first AlN‐based OALVs are designed, fabricated, and experimentally demonstrated using commercially available single‐crystal AlN substrates. These AlN‐based OALVs have shown clear superiority over BSO and BGO‐based devices. Design considerations for OALVs incorporating UWBG photoconductors are discussed, and the photoresponsivity from defect‐mediated sub‐bandgap absorption in AlN crystals is verified as sufficient for OALVs operating under high light fluences. The optimum driving voltage for the AlN‐based OALV is determined to be ≈ 45 Vpp at 100 Hz, achieving a transmittance of 91.3%, an extinction ratio (ER) of more than 100, and a 51:1 image contrast.

2D Embedded Ultrawide Bandgap Devices for Extreme Environment Applications.

Ultrawide bandgap semiconductors such as AlGaN, AlN, diamond, and β-Ga2O3 have significantly enhanced the functionality of electronic and optoelectronic devices, particularly in harsh environment conditions. However, some of these materials face challenges such as low thermal conductivity, limited P-type conductivity, and scalability issues, which can hinder device performance under extreme conditions like high temperature and irradiation. In this review paper, we explore the integration of various two-dimensional materials (2DMs) to address these challenges. These materials offer excellent properties such as high thermal conductivity, mechanical strength, and electrical properties. Notably, graphene, hexagonal boron nitride, transition metal dichalcogenides, 2D and quasi-2D Ga2O3, TeO2, and others are investigated for their potential in improving ultrawide bandgap semiconductor-based devices. We highlight the significant improvement observed in the device performance after the incorporation of 2D materials. By leveraging the properties of these materials, ultrawide bandgap semiconductor devices demonstrate enhanced functionality and resilience in harsh environmental conditions. This review provides valuable insights into the role of 2D materials in advancing the field of ultrawide bandgap semiconductors and highlights opportunities for further research and development in this area.

Composition analysis of β-(In x Ga1-x )2O3 thin films coherently grown on (010) β-Ga2O3 via mist CVD

ABSTRACT This study investigates the compositional analysis and growth of β-(In x Ga1-x )2O3 thin films on (010) β-Ga2O3 substrates using mist chemical vapor deposition (CVD), including the effects of the growth temperature. We investigated the correlation between In composition and b-axis length in coherently grown films, vital for developing high-electron-mobility transistors and other devices based on β-(In x Ga1-x )2O3. Analytical techniques, including X-ray diffraction (XRD), reciprocal space mapping, and atomic force microscopy, were employed to evaluate crystal structure, strain relaxation, and surface morphology. The study identified a linear relationship between In composition and b-axis length in coherently grown films, facilitating accurate composition determination from XRD peak positions. The films demonstrated high surface flatness with root-mean-square roughness below 0.6 nm, though minor relaxation and granular features emerged at higher In compositions (x = 0.083) at the growth temperature of 750°C. XRD results revealed that lattice relaxation were observed at a growth temperature of 700°C despite low In composition. In contrast, at 800°C, the In composition was higher than at 750°C, and coherent growth was achieved. The surface morphology was the flattest at 750°C. These findings indicate that the growth temperature plays a crucial role in the mist CVD growth of β-(In x Ga1-x )2O3 thin films. This study offers insights into the relationship between In composition and lattice parameters in coherently grown β-(In x Ga1-x )2O3 films, as well as the effect of growth conditions, contributing to the advancement of ultra-wide bandgap semiconductor device development.

A Radial Distribution Function Based Recognition Algorithm of Point Defects in Large-Scale β-Ga2O3 Systems.

The atomic configurations and concentrations of intrinsic defects profoundly influence the electrical and optical properties of the semiconductor materials. This influence is particularly significant in the case of β-Ga2O3, which is a highly promising ultrawide bandgap semiconductor characterized by highly complex intrinsic defect configurations. Despite its importance, there is a notable absence of an accurate method to recognize these defects in large-scale atomistic computational modeling. We design an effective algorithm for the explicit identification of various intrinsic point defects in the β-Ga2O3 lattice, which constitutes the integration of the particle swarm optimization (PSO) and K-means clustering (K-MC) methods. Our algorithm attains the recognition accuracy exceeding 95%. Finally, the algorithm is applied to dynamic simulations, where the feasibility of dynamic real-time detection is explored.

Demonstration of aluminum nitride metal oxide semiconductor field effect transistor on sapphire substrate

Abstract Aluminum nitride (AlN) is a promising ultrawide bandgap material with significant advantages for power electronics and optoelectronic applications due to its high breakdown voltage, mobility, and thermal conductivity. AlN Schottky barrier diodes (SBDs) and metal semiconductor field effect transistors (MESFETs) have shown potential but are limited by issues such as high off-state leakage current and complex structures to achieve ohmic contacts. To address these challenges, we report on the fabrication and characterization of an AlN metal oxide semiconductor field effect transistor (MOSFET) with a recessed gate structure. The source and drain contacts were fabricated on n-doped AlN epitaxy using Ti-based contacts with a Ti/Al/Ti/Au metal stack. To evaluate the performance of these contacts, a circular transmission line model (CTLM) was employed, and contacts were annealed at various temperatures ranging from 750 to 950 °C in a nitrogen ambient. Our results reveal that unannealed Ti-based contacts on AlN showed no current conduction. However, annealing these contacts at 950 °C for 30 s significantly reduced the specific contact resistance to 0.148 Ω·cm2, achieving an ~80% reduction compared to samples annealed at 750 °C. Utilizing these optimized contact conditions, we fabricated, to the best of our knowledge, the first AlN MOSFET. The fabricated AlN MOSFET exhibits a threshold voltage of −10.91 V, an effective mobility of 2.95 cm2/Vs, an on-off current ratio spanning two orders of magnitude, and a reverse breakdown voltage of approximately ~250 V in air without a field plate. 

Interfacial Reaction Boosts Thermal Conductance of Room‐Temperature Integrated Semiconductor Interfaces Stable up to 1100 °C

AbstractOverheating has emerged as a primary challenge constraining the reliability and performance of next‐generation high‐performance (ultra)wide bandgap (WBG or UWBG) electronics. Advanced heterogeneous bonding of high‐thermal‐conductivity WBG thin films and substrates not only constitutes a pivotal technique for fabricating these electronics but also offers potential solutions for thermal management. This study presents the integration of 3C‐silicon carbide (SiC) thin films and diamond substrates through a surface‐activated bonding technique. Notably, following annealing, the interfaces between 3C‐SiC and diamond demonstrate an enhancement in thermal boundary conductance (TBC), reaching up to ≈300%, surpassing all other grown and bonded heterointerfaces. This enhancement is attributed to interfacial reactions, specifically the transformation of amorphous silicon into SiC upon interaction with diamond, which is further corroborated by picosecond ultrasonics measurements. After annealing at 1100 °C, the achieved TBC (150 MW m−2 K−1) is among the highest among all bonded diamond interfaces. Additionally, the visualization of large‐area TBC, facilitated by femtosecond laser‐based time‐domain thermoreflectance measurements, shows the uniformity of the interfaces which are capable of withstanding temperatures as high as 1100 °C. The research marks a significant advancement in the realm of thermally conductive WBG/substrate bonding, which is promising for enhanced cooling of next‐generation electronics.