Browsing by Author "Zhao, Yuji"
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Item A review of ultrawide bandgap materials: properties, synthesis and devices(Oxford University Press, 2022) Xu, Mingfei; Wang, Dawei; Fu, Kai; Mudiyanselage, Dinusha Herath; Fu, Houqiang; Zhao, YujiUltrawide bandgap (UWBG) materials such as diamond, Ga2O3, hexagonal boron nitride (h-BN) and AlN, are a new class of semiconductors that possess a wide range of attractive properties, including very large bandgap, high critical electric field, high carrier mobility and chemical inertness. Due to these outstanding characteristics, UWBG materials are promising candidates to enable high-performance devices for power electronics, ultraviolet photonics, quantum sensing and quantum computing applications. Despite their great potential, the research of UWBG semiconductors is still at a nascent stage and represents a challenging interdisciplinary research area of physics, materials science and devices engineering. In this review, the material properties, synthesis methods and device applications of UWBG semiconductors diamond, Ga2O3, h-BN and AlN will be presented and their recent progress, challenges and research opportunities will be discussed.Item Characterizations of two-photon absorption process induced by defects in aluminum nitride using Z-scan method(IOP Publishing, 2024) Zhou, Jingan; Li, Tao; Zhao, Xuan; Zhang, Xiang; Doumani, Jacques; Xu, Mingfei; He, Ziyi; Luo, Shisong; Mei, Zhaobo; Chang, Cheng; Robinson, Jacob T.; Ajayan, Pulickel M.; Kono, Junichiro; Zhao, Yuji; Smalley-Curl InstituteIn this work, we reported two-photon absorption (TPA) measurements for aluminum vacancies in Aluminum nitride single crystals. We measured the linear transmission and identified the defect levels. Using the Z-scan method, we measured the TPA coefficients of the transitions between defect levels from 380 nm to 735 nm. The transition occurs between the aluminum vacancies defect levels. Furthermore, the power dependence shows good linear fitting, confirming the TPA mechanism. These results will be helpful for the design and fabrication of ultra-low loss waveguides and integrated photonics in the ultraviolet spectral range.Item First Demonstration of beta-phase Gallium Oxide Optical Waveguide in Visible-UV Spectrum(2023-04-17) Zhou, Jingan; Zhao, YujiWe report the first demonstration of beta-phase gallium oxide (β-Ga2O3) as optical waveguides on sapphire substrates grown by metal-organic chemical vapor deposition (MOCVD). Their propagation losses in the visible spectrum were comprehensively studied via experiments and simulations of the finite difference method by varying the dimensions of the waveguides. The fabrication process of the waveguides was shown and their propagation losses were measured by collecting the top scattered optical power along the propagation direction using a charge-coupled device (CCD) camera. The minimum measured loss was 3.7 dB/mm at a wavelength of 800 nm depending on the dimensions of the waveguides. In addition, we compared the losses of waveguides with different widths, heights, wavelengths, and polarizations. It is revealed that the total propagation loss is mainly contributed by the bulk and sidewall scattering. Combined with theoretical simulations, various loss mechanisms from two-photon absorption, sidewall scattering, top surface scattering, and bulk scattering, were discussed for β-Ga2O3 waveguides, and their contributions to the total optical loss were estimated. After all these mechanism analyses on absorption and scattering, a large improvement and optimization were applied to the material quality and etching recipe, which improves the top surface and sidewall roughness respectively. This work provides valuable information for the fabrication of optical devices based on Ga2O3 material, which is promising for applications in on-chip high-speed interconnections and UV–NIR nonlinear optics.Item Gallium Oxide as Photonic Integrated Platforms in UV–Visible Spectrum(2023-12-01) Zhou, Jingan; Zhao, YujiWe report the gallium oxide (Ga2O3) as a photonic integrated platform and its nonlinear optical effects in the UV–visible spectra. The β-Ga2O3 as optical waveguides on sapphire substrates grown by metal-organic chemical vapor deposition (MOCVD). For linear properties, their propagation losses in the visible spectrum were comprehensively studied via experiments and simulations of the finite difference method by varying the dimensions of the waveguides. The fabrication process of the waveguides was shown and their propagation losses were measured by collecting the top scattered optical power along the propagation direction using a charge-coupled device (CCD) camera. The minimum measured loss was 3.7 dB/mm at a wavelength of 800 nm depending on the dimensions of the waveguides. For nonlinear properties, by focusing a pulsed laser beam onto a polished ε-Ga2O3 thin film, we collected the generated second harmonic photons with an ultra-sensitive femtowatt photodetector, obtaining effective second-order nonlinear optical susceptibility from visible to UV spectra. Two different measurement systems collecting reflected and transmitted photons were applied separately to make the result more convincing. The wavelength dependence from 790 nm to 900 nm and polarization dependence from TM mode to TE mode were measured as well. In addition, we compared the losses of waveguides with different widths, heights, wavelengths, and polarizations. It is revealed that the total propagation loss is mainly contributed by the bulk and sidewall scattering. Combined with theoretical simulations, various loss mechanisms from two-photon absorption, sidewall scattering, top surface scattering, and bulk scattering, were discussed for β-Ga2O3 waveguides, and their contributions to the total optical loss were estimated. After all these mechanism analyses on absorption and scattering, a large improvement and optimization were applied to the material quality and etching recipe, which improves the top surface and sidewall roughness respectively. This work provides valuable information for the fabrication of optical devices.Item High-surface-area corundum nanoparticles by resistive hotspot-induced phase transformation(Springer Nature, 2022) Deng, Bing; Advincula, Paul A.; Luong, Duy Xuan; Zhou, Jingan; Zhang, Boyu; Wang, Zhe; McHugh, Emily A.; Chen, Jinhang; Carter, Robert A.; Kittrell, Carter; Lou, Jun; Zhao, Yuji; Yakobson, Boris I.; Zhao, Yufeng; Tour, James M.; Smalley-Curl Institute; NanoCarbon Center; Welch Institute for Advanced MaterialsHigh-surface-area α-Al2O3 nanoparticles are used in high-strength ceramics and stable catalyst supports. The production of α-Al2O3 by phase transformation from γ-Al2O3 is hampered by a high activation energy barrier, which usually requires extended high-temperature annealing (~1500 K, > 10 h) and suffers from aggregation. Here, we report the synthesis of dehydrated α-Al2O3 nanoparticles (phase purity ~100%, particle size ~23 nm, surface area ~65 m2 g−1) by a pulsed direct current Joule heating of γ-Al2O3. The phase transformation is completed at a reduced bulk temperature and duration (~573 K, < 1 s) via an intermediate δʹ-Al2O3 phase. Numerical simulations reveal the resistive hotspot-induced local heating in the pulsed current process enables the rapid transformation. Theoretical calculations show the topotactic transition (from γ- to δʹ- to α-Al2O3) is driven by their surface energy differences. The α-Al2O3 nanoparticles are sintered to nanograined ceramics with hardness superior to commercial alumina and approaching that of sapphire.Item Investigation into the Fabrication and Challenges of GaN/BN Vertical FinFETs(2024-06-11) Mei, Zhaobo; Veeraraghavan, Ashok; Zhao, YujiThis work studies the fabrication and challenges of GaN vertical power fin-structure field effect transistors (FinFETs) using BN as dielectric for high temperature applications. GaN etching technique and planarization methods to achieve the fabrication of vertical FinFETs have been systematically studied. A device using Al2O3 as gate dielectric material exhibits an on/off ratio of ~ 106 and a specific on-resistance of 4.2mΩcm2 confirming the successful development of fabrication process. BN thin films are directly grown on the GaN fins by chemical vapor deposition (CVD) and pulsed laser deposition (PLD). The presence of BN layer on flat GaN surface is confirmed by X-ray photoelectron spectroscopy (XPS). BN layer on sidewall GaN exhibits significantly reduced current blocking capabilities as compared to that in planar MIS structure. GaN/BN vertical FinFETs is demonstrated for the first time while the device performance still needs to be improved. The high-resolution transmission electron microscopy (HRTEM) reveals the local absence of BN after device fabrication, which can be ascribed to the adhesion issue or difficulty of nucleation on GaN non-polar sidewalls. A further investigation into the device’s high temperature performance is looking for study. This work serves as an important reference for future studies on BN-based gate dielectrics for GaN vertical transistors.Item Power Electronics Based on Ultrawide Bandgap Semiconductors: from Material Engineering to Device Applications(2023-11-16) Xu, Mingfei; Zhao, YujiThis work demonstrates high-performance BN/β-Ga2O3 metal-insulator-semiconductor Schottky barrier diodes (MIS SBDs). The BN layer is directly grown on the β-Ga2O3 by pulsed laser deposition (PLD). The presence of a ~2.8 nm BN layer is confirmed by a series of techniques, including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The MIS SBDs show an on/off ratio of ~ 107 and an increased on-resistance due to the insertion of the BN layer. An increased Schottky barrier height is observed from capacitance-voltage (C-V) measurements. Temperature-dependent measurements suggest the existence of an inhomogeneous Schottky barrier. The breakdown voltage is enhanced from 732 V for a regular SBD to 1045 V for a MIS SBD with the ultrathin BN layer, which can be ascribed to the increased Schottky barrier height and reduced leakage currents. This work provides a promising way to optimize the performance of β-Ga2O3-based devices for power electronics.Item Topological Photonic Devices in the UV-visible Spectrum Based on the III-N Wide Bandgap Semiconductor Platform(2024-04-19) Li, Tao; Zhao, Yuji; Huang, Shengxi; Chen, SongtaoTopological photonics, renowned for the edge/interface states resistant to local defects and back-scattering, can be a promising solution for ensuring the stability in integrated photonic platforms and has already found applications in lasers and quantum photonic circuits. However, existing topological photonic demonstrations have primarily operated in the microwave or near-infrared spectrum due to material and nanofabrication limitations. In this thesis, we break through this wavelength barrier and extend the limit into UV-visible spectrum by implementing topological photonics on the III-N wide bandgap semiconductor platform. In the first part of the thesis, we devise a 1D topological photonic cavity fabricated from a gallium nitride on silicon (GaN-on-Si) wafer. The designed cavity has a single resonance mode around the wavelength of 800 nm and shows a simulated quality factor (Q) around 1600. Based on the non-zero second-order susceptibility of the GaN, we further demonstrate the second harmonic generation (SHG) from the 1D topological photonic cavity and reveal the power dependence and polarization dependence of the cavity-based SHG. The second part of the thesis focuses on the design of topological photonic routing devices in the visible spectrum based on 2D photonic crystals (PC) made of hexagonal boron nitride (h-BN). Interfacing 2D h-BN PCs with distinct topological phases gives rise to topological edge states supporting polarization-resolved unidirectional propagation. Through meticulous design of the interfaces’ shape, we demonstrate ultra-compact topological photonic routers. These routers feature 6 input/output ports within a 10 µm × 10 µm footprint and showcase a simulated crosstalk extinction ratio exceeding 15 dB. The results from this thesis underpin the UV-visible topological photonics based on the III-N wide bandgap semiconductor platform and can potentially benefit the design of high-performance integrated photonic devices in the UV-visible spectrum by leveraging the unique properties of photonic topology.Item β-Ga2O3-Based Heterostructures and Heterojunctions for Power Electronics: A Review of the Recent Advances(MDPI, 2024) Herath Mudiyanselage, Dinusha; Da, Bingcheng; Adivarahan, Jayashree; Wang, Dawei; He, Ziyi; Fu, Kai; Zhao, Yuji; Fu, HouqiangDuring the past decade, Gallium Oxide (Ga2O3) has attracted intensive research interest as an ultra-wide-bandgap (UWBG) semiconductor due to its unique characteristics, such as a large bandgap of 4.5–4.9 eV, a high critical electric field of ~8 MV/cm, and a high Baliga’s figure of merit (BFOM). Unipolar β-Ga2O3 devices such as Schottky barrier diodes (SBDs) and field-effect transistors (FETs) have been demonstrated. Recently, there has been growing attention toward developing β-Ga2O3-based heterostructures and heterojunctions, which is mainly driven by the lack of p-type doping and the exploration of multidimensional device architectures to enhance power electronics’ performance. This paper will review the most recent advances in β-Ga2O3 heterostructures and heterojunctions for power electronics, including NiOx/β-Ga2O3, β-(AlxGa1−x)2O3/β-Ga2O3, and β-Ga2O3 heterojunctions/heterostructures with other wide- and ultra-wide-bandgap materials and the integration of two-dimensional (2D) materials with β-Ga2O3. Discussions of the deposition, fabrication, and operating principles of these heterostructures and heterojunctions and the associated device performance will be provided. This comprehensive review will serve as a critical reference for researchers engaged in materials science, wide- and ultra-wide-bandgap semiconductors, and power electronics and benefits the future study and development of β-Ga2O3-based heterostructures and heterojunctions and associated power electronics.