Progresses and Frontiers in Ultrawide Bandgap Semiconductors

Abstract

Ultrawide bandgap (UWBG) semiconductors are paving the way for a new era of high-performance electronic and photonic devices. Characterized by their large bandgaps, UWBG materials can withstand higher electric fields, operate at elevated temperatures, and achieve greater efficiencies compared to more established semiconductors like silicon and GaAs. These unique properties position UWBG semiconductors as crucial materials for next-generation power electronics, deep-ultraviolet (UV) photodetectors, and high-frequency communication systems.

In recent years, materials such as gallium oxide (Ga₂O₃), aluminum gallium nitride (Al(Ga)N), and diamond have demonstrated remarkable potential in applications requiring high voltage, high power, and extreme environmental stability. Advances in processing techniques, defect management, and heterostructure design are driving this field forward, enabling devices that are more robust, efficient, and scalable. However, achieving the full potential of UWBG materials still presents significant challenges, including the need for improved material quality, better surface processing techniques, and innovative device architectures.

This special issue of Progresses and Frontiers in Ultrawide Bandgap Semiconductors brings together seven outstanding contributions that address these challenges and highlight recent breakthroughs in the field. The papers in this issue cover a range of topics, including advanced processing techniques, novel device fabrication methods, defect characterization, and the development of heterostructures for enhanced performance. Together, these works provide a comprehensive snapshot of the state-of-the-art in UWBG semiconductor research and offer insights that will guide future developments.

The following summaries highlight each of these contributions, illustrating the diversity of approaches and the depth of innovation in UWBG semiconductor research.

Brianna Klein and her team from Sandia National Lab and the Ohio State University present an innovative approach in their paper, “Al-Rich AlGaN Transistors with Regrown p-AlGaN Gate Layers and Ohmic Contacts.” This work focuses on fabricating Al-rich AlGaN high electron mobility transistors (HEMTs) with enhancement-mode operation. By employing a deep gate recess etch and epitaxial regrowth of p-AlGaN gate structures, they achieve a large positive threshold voltage (V_TH = +3.5 V) and negligible gate leakage. Additionally, low-resistance Ohmic contacts are realized using regrown, heavily doped, reverse compositionally graded n-type structures, achieving a specific contact resistance as low as 4 × 10⁻⁶ Ω cm². These advancements provide a viable pathway for developing high-current, low-leakage, enhancement-mode AlGaN-based ultra-wide bandgap transistors, crucial for future high-power and high-frequency applications.

Xuanyi Zhao and her colleagues from Shandong University present a comprehensive review on “Advance Chemical Mechanical Polishing Technique for Gallium Nitride Substrate.” This paper explores recent developments in chemical mechanical polishing (CMP) of GaN, a critical substrate material known for its excellent mechanical properties and thermal stability. The review highlights both conventional and enhanced CMP techniques, focusing on strategies to improve material removal rates and surface quality. It discusses auxiliary systems that incorporate optical, electrical, magnetic, and plasma enhancements to optimize oxidation and mechanical removal processes. Key challenges, such as subsurface damage and achieving atomically smooth surfaces, are outlined, along with future opportunities for commercial application. This work offers valuable insights and inspiration for advancing GaN processing technology.

In a technical paper entitled “Enhanced Laser Damage Threshold in Optically Addressable Light Valves via Aluminum Nitride Photoconductors”, Soroush Ghandiparsi and colleagues from the Lawrence Livermore National Laboratory have demonstrated the potential of aluminum nitride (AlN) photoconductors to significantly enhance the performance of optically addressable light valves (OALVs) for high-intensity laser applications. Conventional OALVs, based on photoconductors like Bismuth Silicon Oxide (BSO) and Bismuth Germanium Oxide (BGO), suffer from low laser-induced damage thresholds and poor thermal conductivity. By leveraging AlN's superior thermal conductivity (over 300 Wm⁻¹ K⁻¹) and higher laser damage resistance, the researchers designed and fabricated the first AlN-based OALVs. These devices achieved a 91.3% transmittance, an extinction ratio greater than 100, and a 51:1 image contrast. The AlN OALVs demonstrated minimal read beam leakage (0.9%) and effective photoresponsivity under various wavelengths. Addressing fabrication challenges like liquid crystal uniformity and alignment layer quality could further enhance image contrast and device performance, paving the way for practical applications in laser optics and high-power systems.

Pengxiang Sun and colleagues introduce a novel method in their paper, “Laser Writing of GaN/Ga₂O₃ Heterojunction Photodetector Arrays.” This study demonstrates the fabrication of GaN/Ga₂O₃ heterojunctions by laser processing, where GaN surfaces are oxidized to form Ga₂O₃. The resulting photodetectors show excellent performance characteristics, including a responsivity of 110.22 mA W⁻¹, a detection rate of 5.56 × 10¹¹ Jones, and an external quantum efficiency of 42.34%. Notably, these devices operate under zero bias and achieve a high light-to-dark current ratio of 10⁴. The team successfully fabricates an 8 × 8 photodetector array, showcasing its capability for ultraviolet imaging. This work offers a scalable and versatile laser-based approach for producing large-area photodetector arrays, paving the way for advancements in UV detection and imaging applications.

Alexander Polyakov and his team explore the “Properties of κ-Ga₂O₃ Prepared by Epitaxial Lateral Overgrowth.” This study investigates the structural and electrical characteristics of undoped and Sn-doped κ-Ga₂O₃ layers grown on TiO₂/sapphire substrates using stripe and point masks. The epitaxial lateral overgrowth method significantly enhances the crystalline quality compared to conventional planar growth. The undoped films exhibit semi-insulating behavior with the Fermi level and deep electron trap level characterized. Low Sn doping achieves net donor concentrations of ∼10¹³ cm⁻³, while hydrogen plasma treatment increases donor density near the surface to ∼10¹⁹ cm⁻³. The study also observes strong persistent photocapacitance and photoconductivity, suggesting DX-like centers. Notably, thin κ-Ga₂O₃ films grown on GaN/sapphire templates exhibit unexpected p-type-like behavior, possibly due to the formation of a 2D hole gas at the κ-Ga₂O₃/GaN interface. This work provides valuable insights for optimizing κ-Ga₂O₃ for electronic and optoelectronic applications.

Zbigniew Galazka and his team from Leibniz-Institut für Kristallzüchtung and other institutions explore the “Solid-Solution Limits and Thorough Characterization of Bulk β-(Al_xGa_1–x)₂O₃ Single Crystals Grown by the Czochralski Method.” This study investigates the maximum Al content that can be incorporated into the β-Ga₂O₃ lattice while maintaining a single-crystalline, monoclinic phase, concluding a limit of 40 mol% Al (35 mol% in the melt). TEM analysis shows a random distribution of Al across both octahedral and tetrahedral sites. The authors report an increase in electrical resistivity for β-(Al_xGa_1–x)₂O₃:Mg compared to β-Ga₂O₃:Mg, while dielectric constant, refractive index, and thermal conductivity decrease with higher Al content. Notably, thermal conductivity drops to one-third of pure β-Ga₂O₃ at 30 mol% Al. Raman spectra indicate that low-energy Ag(3) phonon modes contribute to reduced electron mobility. Additionally, Ir incorporation decreases with Al content, with Ir⁴⁺ Raman peaks disappearing above 15 mol% Al. These findings suggest that β-(Al_xGa_1–x)₂O₃ crystals are well-suited for lateral power devices, offering a balance of properties advantageous for electronic applications.

Jith Sarker and colleagues from University at Buffalo-SUNY and other institutions provide an in-depth study in their paper, “Microscopic and Spectroscopic Investigation of (Al_xGa_1–x)₂O₃ Films: Unraveling the Impact of Growth Orientation and Aluminum Content.” This work investigates the structural and spectroscopic properties of (Al_xGa_1–x)₂O₃ films grown on (100), (201), and (010) substrates with Al concentrations of 20% and 50%. Using scanning transmission electron microscopy (STEM), atom probe tomography (APT), and first-principle calculations (DFT), the authors reveal that low Al-content films exhibit chemical homogeneity, while high Al-content films show inhomogeneities, particularly in (100) and (010) orientations. APT analysis maps Ga─O and Al─O bond lengths, showing trends influenced by growth orientation, consistent with DFT predictions. The study also identifies an inverse relationship between bond energy and bond length for different orientations. These findings provide critical insights into optimizing (Al_xGa_1–x)₂O₃ films for high-power transistors and deep-UV photodetectors, highlighting the potential of (100) and (201) orientations alongside the established (010) films. This work expands APT's capabilities and deepens the understanding of the structure-property relationships in these ultrawide-bandgap materials.

Mengen Wang and colleagues from University of California Santa Barbara present a comprehensive investigation in their paper, “First-Principles Study of Twin Boundaries and Stacking Faults in β-Ga₂O₃.” Using density functional theory (DFT) calculations, the study examines the energetics and electronic structures of twin boundaries (TBs) and stacking faults (SFs) in monoclinic β-Ga₂O₃. The results indicate that the (100)A twin boundary has a remarkably low formation energy of 0.01 Jm⁻², consistent with experimental observations. Similarly, type-I stacking faults on the (100) plane exhibit low formation energy (0.03 Jm⁻²), making planar-defect formation more likely during growth on (100) substrates. Despite their higher formation energies, TBs and SFs on the (010) and (001) planes are also observed experimentally, often due to growth direction changes or domain coalescence. Importantly, the study finds that TBs and SFs on the (100) plane do not introduce defect states in the bandgap, thereby preserving the material's electrical properties. These insights clarify the origins of planar defects in β-Ga₂O₃ and provide guidance for optimizing growth techniques to minimize defects, enhancing the material's potential for high-performance electronic applications.

Together, these contributions offer a comprehensive view of the latest advances and ongoing challenges in UWBG semiconductor research. As we continue to explore new materials and innovative fabrication techniques, the insights presented in this special issue will help drive the development of high-performance devices for a wide range of applications, from power electronics to UV detection. We hope this collection inspires further research and collaboration in this exciting and rapidly evolving field.