受賞区分：国内外の国際的学術賞  受賞国：アメリカ合衆国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：スウェーデン王国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：グレートブリテン・北アイルランド連合王国(英国)

受賞国：グレートブリテン・北アイルランド連合王国(英国)

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

受賞国：日本国

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

We investigated the hole mobility limiting factors in dopant-free p-type distributed polarization-doped (DPD) AlGaN layers by an experimental method. p-DPD AlGaN exhibited a higher hole mobility than GaN:Mg with a similar room temperature hole concentration across all temperature ranges owing to the absence of ionized impurity scattering. In addition, unlike in n-DPD AlGaN, alloy scattering was not always critical in p-DPD AlGaN. The extracted alloy scattering potential was only 0.3 eV, which resulted in a reduced effect of alloy scattering and originated from a small valence band offset in the GaN/AlN heterojunction system. The results suggest that DPD principally enables the fabrication of low-resistance p-type nitride semiconductor thin films as a result of the high hole concentration and high hole mobility.

DOI： 10.1063/5.0155363

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

We demonstrate high, up to 30% In content InGaN sub-micrometer platelets on GaN by metalorganic vapor phase epitaxy. These InGaN platelets were selectively grown on flat GaN seeds formed in sub-micrometer-scale openings in a SiNx mask. The platelets were highly uniform without any dislocations or pits, with an atomically flat (0001) surface. The typical height was ∼120 nm, which significantly exceeded the normal critical layer thickness of a c-plane InGaN film. The strain state was comprehensively characterized by microbeam x-ray diffraction and transmission electron microscopy. Due to a gradual elastic relaxation of strain, the In content increased almost linearly from bottom to top because of the strong strain-dependent In incorporation. These platelets can serve as high-quality strain-relaxed templates for long wavelength micro-light-emitting diodes.

DOI： 10.1063/5.0120723

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

The precise control of Mg concentration ([Mg]) in p-type GaN layers from 2.3 × 1016 to 2.0 × 1019 cm-3 was demonstrated by halide vapor phase epitaxy (HVPE) on n-type GaN (0001) freestanding substrates. [Mg] in GaN layers could be controlled well by varying the input partial pressure of MgCl2 formed by a chemical reaction between MgO solid and HCl gas under the thermodynamic equilibrium condition. In the sample with [Mg] of 2.0 × 1019 cm-3, a step-bunched surface was observed because the surface migration of Ga adatoms was enhanced by the surfactant effect of Mg atoms. The samples show high structural qualities determined from x-ray rocking curve measurements. The acceptor concentration was in good agreement with [Mg], indicating that almost all Mg atoms act as acceptors. The compensating donor concentrations in the samples were higher than the concentrations of Si, O, and C impurities. We also obtained the Mg acceptor level at a sufficiently low net acceptor concentration of 245 ± 2 meV. These results show that the HVPE method is promising for fabricating GaN vertical power devices, such as n-channel metal-oxide-semiconductor field-effect transistors.

DOI： 10.1063/5.0122292

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Reports  

As a newly developed technique to slice GaN substrates, which are currently very expensive, with less loss, we previously reported a laser slicing technique in this journal. In the previous report, from the perspective of GaN substrate processing, we could only show that the GaN substrate could be sliced by a laser and that the sliced GaN substrate could be reused. In this study, we newly investigated the applicability of this method as a device fabrication process. We demonstrated the thinning of GaN-on-GaN high-electron-mobility transistors (HEMTs) using a laser slicing technique. Even when the HEMTs were thinned by laser slicing to a thickness of 50 mm after completing the fabrication process, no significant fracture was observed in these devices, and no adverse effects of laser-induced damage were observed on electrical characteristics. This means that the laser slicing process can be applied even after device fabrication. It can also be used as a completely new semiconductor process for fabricating thin devices with thicknesses on the order of 10 mm, while significantly reducing the consumption of GaN substrates.

DOI： 10.1038/s41598-022-10610-4

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

We demonstrated continuous-wave lasing of an AlGaN-based ultraviolet laser diode, fabricated on a single-crystal AlN substrate when operating at 5 °C. The threshold current density and device series resistance were reduced by improvements to the epitaxial structure and electrode arrangement. A peak wavelength of 274.8 nm was observed for lasing at a drive current over 110 mA, which corresponded to a threshold current density of 3.7 kA cm-2. The operating voltage at the threshold current was as low as 9.6 V.

DOI： 10.35848/1882-0786/ac6198

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

We have demonstrated a fabrication process for the Ohmic contact on low-doping-density p-type GaN with nitrogen-annealed Mg. An Ohmic contact with a contact resistance of 0.158 ω cm2 is realized on p-GaN ([Mg] = 1.3 × 1017 cm-3). The contact resistance of p-type GaN with higher Mg concentration ([Mg]=1.0 × 1019 cm-3) can also be reduced to 2.8 × 10-5 ω cm2. A localized contact layer is realized without any etching or regrowth damage. The mechanism underlying this reduced contact resistance is studied by scanning transmission electron microscopy with energy dispersive x-ray spectroscopy and secondary ion mass spectrometry, representing a mutual diffusion of Ga and Mg atoms on the interface. Reductions in the barrier height and surface depletion width with the nitrogen-annealed Mg layer are confirmed by XPS and Hall effect measurements qualitatively.

DOI： 10.1063/5.0076764

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Reports  

We have investigated the possibility of applying lasers to slice GaN substrates. Using a sub-nanosecond laser with a wavelength of 532 nm, we succeeded in slicing GaN substrates. In the laser slicing method used in this study, there was almost no kerf loss, and the thickness of the layer damaged by laser slicing was about 40 µm. We demonstrated that a standard high quality homoepitaxial layer can be grown on the sliced surface after removing the damaged layer by polishing.

DOI： 10.1038/s41598-021-97159-w

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Crystal Growth  

Results of a thermodynamic study of Sn doping and fabrication of a Sn-doped GaN freestanding layer with high structural quality by halide vapor phase epitaxy (HVPE) are described in this paper. Thermodynamic analysis revealed that SnCl2 and/or SnCl act as Sn precursors through the reaction between Sn metal and HCl gas. The equilibrium partial pressures of SnCl2 and SnCl increase with the input HCl partial pressure. To generate Sn precursors effectively, it is desirable that the reaction between Sn metal and HCl gas occurs in the inert gas ambient. On the basis of results of the thermodynamic study, the Sn-doped GaN freestanding layer with a Sn concentration of 5.7 × 1019 cm−3 is fabricated by removing the GaN seed substrate after HVPE growth. The Sn-doped GaN freestanding layer has high crystal quality, and the lattice constants along the c- and a-axes of the Sn-doped GaN freestanding layer are larger than those of the GaN seed substrate because of the high electron density and the size effect of Sn atoms.

DOI： 10.1016/j.jcrysgro.2024.127923

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

We report on the growth of AlPN on GaN/sapphire templates by metalorganic vapor phase epitaxy using tertiarybutylphosphine (tBP) and NH3 as group-V precursors. P is easy to incorporate into the group-III lattice site, forming PAl anti-site defects and shrinking lattice constants that are even beyond AlN since Al is larger than P. We found that higher temperatures favor P incorporation on the N-sublattice, forming AlPyN1−y, while growth temperatures below 1000 °C result in dominant P incorporation on the Al-sublattice, forming PAl anisites. Similarly, larger NH3 flows stabilize GaN, leading to flat interfaces, but favor the formation of PAl. Furthermore, the P incorporation into AlPyN1−y is non-linear. At very low tBP flows, it initially increases to reach a maximum. Further increasing the tBP flow increases mostly the incorporation of P on the Al-sublattice, and the c-lattice constant decreases again. This leaves a small window of low V/III ratios below 5 and low P/N ratios of 1% or smaller, leading up to ∼4% P incorporation at typical growth temperatures of GaN. However, at such low V/III ratios, GaN is not stable even with N2 carrier gas and requires optimized switching sequences to minimize its decomposition and preserve flat interfaces. Eventually, a 10 nm coherent layer of AlP0.01N0.99 could be reproducibly grown on top of GaN channels with a smooth surface, an abrupt AlPN/GaN interface, and a two-dimensional electron gas with an electron mobility of ∼675 cm2/V s and a sheet carrier density of 1.5 × 1013 cm−2 at room temperature.

DOI： 10.1063/5.0225115

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

This Letter reports an investigation of hole transport in p-GaN/AlGaN/GaN heterostructures through experimental and theoretical analyses under varied conditions. Highly non-linear current-voltage (I-V) characteristics, obtained via the linear transmission line method measurements, are utilized for this study. At low bias voltage, the transport can be ascribed to the Schottky nature of the contact, while at high bias, the conduction is observed to be governed by space-charge limited current (SCLC). The Schottky characteristics (Schottky barrier height and non-ideality factor) and the SCLC exponent were analyzed for devices with varying contact spacings and at different high temperatures. The SCLC exponent, m, is in the range of 2 ≤ m ≤ 4 depending on the applied voltage range, revealing the existence of the trap states in the channel region. The findings of this work indicate that the charge injection, field-induced ionization, and trap states in the p-GaN channel are critical factors in the current transport of p-GaN/AlGaN/GaN heterostructure.

DOI： 10.1063/5.0203344

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Reviews  

Display technology has developed rapidly in recent years, with III-V system-based micro-light-emitting diodes (μLEDs) attracting attention as a means to overcome the physical limitations of current display systems related to their lifetime, brightness, contrast ratio, response time, and pixel size. However, for μLED displays to be successfully commercialized, their technical shortcomings need to be addressed. This review comprehensively discusses important issues associated with μLEDs, including the use of the ABC model for interpreting their behavior, size-dependent degradation mechanisms, methods for improving their efficiency, novel epitaxial structures, the development of red μLEDs, advanced transfer techniques for production, and the detection and repair of defects. Finally, industrial efforts to commercialize μLED displays are summarized. This review thus provides important insights into the potential realization of next-generation display systems based on μLEDs.

DOI： 10.1063/5.0177550

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission ( τ PL RT ) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τ PL RT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3-4, in ammonothermal GaN. The values of τ PL RT in n-GaN samples are compared with those of p-GaN, in which τ PL RT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

DOI： 10.1063/5.0201931

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Physics Condensed Matter  

Metastability of Aln/12Ga1−n/12N (n = 2–10: integer) with the 1–2 monolayer (ML) in-plane configuration towards the c [0001] direction has been demonstrated recently. To theoretically explain the existence of these metastable structures, relatively large calculation cells are needed. However, previous calculations were limited to the use of small calculation cell sizes to estimate the local potential depth (∆σ) of ordered Al1/2Ga1/2N models. In this work, we were able to evaluate large calculation cells based on the interaction energies between proximate Al atoms (δEAl–Al) in AlGaN alloys. To do this, δEAl–Al values were estimated by first-principles calculations (FPCs) using a (5a1 × 5a2 × 5c) cell. Next, a survey of the possible ordered configurations using various large calculation cell models was performed using the estimated δEAl–Al values and the Monte-Carlo method. Then, various ∆σ values were estimated by FPCs and compared with the configurations previously reported by other research groups. We found that the ordered configuration obtained from the (4a1 × 2a2 × 1c) calculation cell (C42) has the lowest ∆σ of −9.3 meV/cation and exhibited an in-plane configuration at the c(0001) plane having (–Al–Al–Ga–Ga–) and (–Al–Ga–) sequence arrangements observed along the m{11̄00} planes. Hence, we found consistencies between the morphology obtained from experiment and the shape of the primitive cell based on our numerical calculations.

DOI： 10.1088/1361-648X/ad1137

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

We investigated the C-related complexes in highly C-doped GaN by electron spin resonance (ESR) spectroscopy, Fourier transform IR spectroscopy (FTIR), and minority carrier transient spectroscopy (MCTS) measurements. In the ESR spectra, two resonances with g values of 2.02 and 2.04 were found to be assigned by (0/−) deep acceptor and (+/0) charge transition levels of carbon substituting for nitrogen site (CN). In the FTIR spectra, two local vibrational modes positioned at 1679 and 1718 cm−1 were confirmed to be associated with tri-carbon complexes of CN-CGa-CN (basal) and CN-CGa-CN (axial), respectively. In the MCTS spectra, we observed the hole trap level of E v + 0.25 ± 0.1 eV associated with the tri-carbon complexes, which are the dominant C-related defects, suggesting that these complexes affect the electronic properties in the highly C-doped GaN.

DOI： 10.35848/1347-4065/ad3b54

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Electron Devices  

In this work, we report a low-cost methodology to increase the barrier height of N-polar GaN Schottky diodes. Physical vapor deposition (PVD) of Mg followed by a thermal diffusion anneal at 800 °C leads to the formation of N-polar GaN Camel diode offering a larger barrier height than the conventional N-polar GaN Schottky diodes. The increase in barrier height after the Mg diffusion process is validated using current-voltage ({I} - {V}) and capacitance-voltage (CV) measurements. A barrier height of 0.7 eV and a near-unity ideality factor observed in the N-polar GaN Camel diode confirms that the proposed Mg diffusion process is an alternative method for improving the performance of future N-polar GaN diodes.

DOI： 10.1109/TED.2023.3341831

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physica Status Solidi (A) Applications and Materials Science  

The degradation of an AlGaN-based deep-ultraviolet laser diode during operation is studied. A rapid increase in threshold current without any facet mirror damage under direct current stress below the threshold current is observed, which is considered to limit the lifetime of continuous wave lasing to the level of seconds. The evolution of current–light and current–voltage characteristics suggests that the dominant mechanism of degradation is the increased loss of carriers before they can reach the active layer, i.e., the decrease in carrier injection efficiency. The decrease in emission intensity, which is more pronounced at lower current densities, and subsequent increase in sub-threshold current indicate the increase in defect density under current stress, which is similar to the well-analyzed degradation mechanism found in AlGaN-based light-emitting diodes.

DOI： 10.1002/pssa.202300946

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Crystal Growth  

A Sn-doped n-type GaN layer with a high electron density of 2 × 1020 cm−3 and a low resistivity of 8.7 × 10−4 Ω∙cm was grown by halide vapor phase epitaxy (HVPE). Sn doping was performed through the reaction between Sn metal and HCl gas. The Sn concentration markedly increased with decreasing growth temperature and the activation energy of Sn desorption from the GaN surface was found to be 4.1 eV. Smooth surfaces were obtained by introducing the Sn precursor even though the samples were grown at a low temperature of 905 °C, suggesting that Sn atoms act as surfactants and promote the migration of Ga adatoms. Almost all the Sn atoms act as donors in GaN below the Sn concentration of 2 × 1020 cm−3. These results indicate that using the Sn donor is promising for fabricating low-resistivity n-type GaN substrates by HVPE.

DOI： 10.1016/j.jcrysgro.2023.127529

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Crystal Growth  

DOI： 10.1016/j.jcrysgro.2023.127552

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

Increasing the injection efficiency, a critical factor constraining the reduction in threshold current in AlGaN-based deep-ultraviolet laser diodes, represents one of the paramount remaining technical challenges. In this study, the impact of compositionally graded layers that were unintentionally formed at the interface between the p-cladding and the core layer on carrier injection efficiency was analyzed. Experimental evaluations using laser diodes have shown that the elimination of an unintentionally formed layer increases the injection efficiency above the threshold current, from the conventional 3% to 13%. It has been postulated that the electron overflow toward the p-side exerts a substantial deleterious effect on the injection efficiency. An improvement in this aspect is achieved by increasing the electron-blocking capability due to the improved interface abruptness between the p-cladding layer and the core layer. The lasing threshold was strongly reduced, and characteristic temperature increased from 76 to 107 K for the improved devices.

DOI： 10.1063/5.0184543

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms  

Lanthanide (Ln)-implanted gallium nitride (GaN) semiconductors have a variety of potential applications as light-emitting devices and quantum light sources, but their optical properties are not well understood. In this study, we investigate the room temperature optical properties of praseodymium (Pr) ion implanted single-crystal GaN and their changes upon ultrahigh pressure annealing (UHPA) up to 1480 °C. Photoluminescence (PL) spectra, luminescence transition lifetime, and excitation cross section of the implanted Pr ions are analyzed. In addition, the recovery of implantation-induced damage and the thermal diffusion of implanted Pr ions by UHPA are investigated by X-ray diffraction, Raman spectroscopy, and secondary ion mass spectrometry. The results show that the implantation damage is recovered by annealing at temperatures above 1200 °C, but Pr ions thermally diffuse to the surface as the annealing temperature increases. The annealing temperature at which a maximum PL intensity is obtained increases with increasing the implantation dose. However, the PL intensity decreases in all cases after annealing at 1480 °C, indicating that a quenching factor is dominant in this temperature range.

DOI： 10.1016/j.nimb.2023.165181

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

The development of gallium nitride (GaN) vertical-type metal-oxide-semiconductor field-effect transistors and p-i-n diode devices has gathered increasing attention. These devices require an n-type drift layer with a low doping level of 1016 cm−3 or less, minimized point defects inhibiting electron conduction, and a layer approximately 10 μm thick. Therefore, a practical method with a growth rate of at least several tens of μm/h and impurity concentrations of less than 1015 cm−3, except for that of dopants, is necessary. Halogen-free vapor-phase epitaxy (HF-VPE) has a high growth rate suitable for fabricating thick drift layers and utilizes a simple reaction between Ga vapor and ammonia gas (without a corrosive halogen gas), resulting in lower impurity levels. Herein, we eliminated the quartz content from the high-temperature zone to reduce the excess unintentional Si doping and identified that the nitrile gloves used for the growth preparation are other impurity contamination sources. We obtained a lightly n-type ([Si]=∼1016 cm−3) GaN layer, in which C, O, B, Fe, Mg, Al, Ca, Cr, Zn, Ni, Mn, and Ti impurity contents were below the detection limits of secondary ion mass spectrometry. Deep-level transient spectroscopy revealed that electron traps at EC − 0.26 and at EC − 0.59 eV were 2.7 × 1013 and 5.2 × 1014 cm−3, respectively. Moreover, the Hall effect analysis showed the acceptor-type defect-compensating donor content as approximately 2.7 × 1015 cm−3, resulting in a high electron mobility of HF-VPE GaN in the 30-710 K temperature range. Furthermore, we identified the Ca impurity as a deep acceptor, another killer defect leading to mobility collapse.

DOI： 10.1063/5.0191774

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Semiconductor Science and Technology  

In this letter, we investigate the impact of periphery, width, length and area on the external quantum efficiency (EQE) of stripe-type InGaN-based red micro-light-emitting diodes (µLEDs). A longer periphery resulted in a higher light extraction efficiency ( η e ) via the sidewall regardless of the area of the µLEDs. However, as the injection current increased a somewhat larger efficiency droop was observed at the longer periphery due to current crowding. Additionally, larger µLEDs experienced more self-heating than smaller ones, resulting in a red shift of wavelengths and a larger efficiency droop. When the current density exceeded 100 A cm−2, the EQE ratio of smaller-area μLEDs to larger-area ones increased significantly due to the difference in efficiency droop. Besides, a short light propagation length and a long emission width yielded a higher η e . Hence, the periphery, width, length and area of the µLEDs determine EQE, which provides insight into the pixel design of µLED displays.

DOI： 10.1088/1361-6641/ad0b88

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Sensors and Materials  

Group-III nitride semiconductors, such as gallium nitride (GaN), have been proposed as novel materials for radiation detection owing to their wide bandgap and their ability to operate at high temperatures. In this study, the radiation detection properties of GaN PIN diode detectors were evaluated at high temperatures (~573 K). The energy spectrum peak profiles of241Am α-particles were obtained at different temperatures, confirming the operation of GaN PIN diodes up to 573 K. The peak positions shifted toward the lower-energy side and the full width at half maximum (FWHM) of the detection energy peak improved with increasing temperature. Furthermore, the variation in electron carrier mobility–lifetime product (μeτe) between 293 and 573 K was not significant. These results indicate the potential high-temperature operation of group-III nitride semiconductors. Additionally, the variation in each detection characteristic was caused by increasing the atmospheric temperature, which affected the mobility, lattice scattering, bandgap, and built-in potential differently.

DOI： 10.18494/SAM4647

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ACS Photonics  

Neuromorphic computing, inspired by biological synapses, has emerged as a pivotal approach to overcome the limitations of von Neumann architecture. In this work, we employ a three-terminal III-nitride metal-oxide-semiconductor field-effect transistor (MOSFET) structure based on the GaN-on-silicon platform, characterized by ultralow power consumption, to emulate artificial synapses through light stimulation. Remarkably, our device exhibits a minimal power consumption of 7 fJ per synaptic event, surpassing that of biological synapses (∼10 fJ). Notably, we observe that the paired pulse facilitation (PPF) index can be modulated by the external light intensity, pulse width, and light pulse interval. By manipulating the light intensity and pulse width, we achieve a PPF index exceeding 300% in our device. Furthermore, our devices demonstrate gate-tunable synaptic plasticity, enabling electric/light cooperative control and increasing the tuning freedom. Finally, acceleration of photopic adaption and a 91.28% handwritten digit recognition accuracy are achieved, which provide strong support in potential visual sensory applications.

DOI： 10.1021/acsphotonics.4c01038

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

Observation of high aspect ratio (HAR) structures is a difficult challenge in metrology and inspection in semiconductor manufacturing. In imaging HAR trenches using a scanning electron microscope (SEM), obtaining SEM images without information loss due to whiteouts and blackouts is challenging. One reason for the difficulty is that the probe current is constant in conventional SEM imaging. Suppose the probe current is increased to detect more secondary electrons from the bottom of the trench. In that case, excessive secondary electron emission from the top of the trench will result in a whiteout. The SEM equipped with a photocathode electron gun (PC-SEM) can change the probe current on a pixel-by-pixel basis by applying a pulsed electron beam. In this study, we propose two methods of SEM observation for HAR trenches. The first method uses a lower probe current at the top of the trench and a higher probe current at the bottom. With this method, the top and bottom of the trench could be observed simultaneously without any whiteout or blackout. Another method is to adjust the probe current so that the SEM image is in a constant grayscale. In this case, information about the sample appears in the probe current. The image of the probe current captured the trench bottom more clearly than the conventional SEM image under equivalent conditions.

DOI： 10.1117/12.3010733

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

Semiconductor photocathodes are electron beam sources with versatile electron beam performance such as pulsed structure as well as high beam current with high monochromaticity. Photocathode using GaN semiconductor material has solved the durability problem, resulting in the development of a compact photocathode electron gun suitable for industrial technology. The photocathode electron gun can be retrofitted to existing electron microscopes, has the same brightness as a cold field emitter cathode, and the pulsed beam not only brings selective beam irradiation to arbitrary area in the field of view in SEM imaging, but also allows blur-free TEM imaging of moving samples.

DOI： 10.1117/12.3010730

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

Scanning electron microscopy (SEM) is used for metrology and inspection in semiconductor manufacturing. In addition, electrical defects such as short circuits and unintentional insulation appear as contrast differences called voltage contrast (VC) in SEM under low acceleration voltage conditions. Moreover, by using pulsed electron beams from a photocathode, the probe current can be arbitrarily changed by pixel in the SEM image. Using this technology, we succeeded in observing the change in the VC of the drain in the metal-oxide-semiconductor field effect transistor (MOSFET) by changing in electron beam irradiation on the gate only. In this study, to estimate the threshold voltage of n-type MOSFET (nMOS) from VC, we investigated quantitative changes in the specimen current of the drain (Id) and the gate (Ig) due to gate e-beam irradiation ON/OFF during SEM imaging. The landing energy of the electron beam was set to 0.8 keV, the probe current was 6.3 pA, and the e-beam was irradiated onto only the gate and drain electrodes. Id and Ig, which showed a positive value at the beginning, decreased with time, and saturated at negative values. When the electron beam irradiation to the gate was turned OFF, the Id decreased further and reached saturation. When the gate e-beam irradiation was turned ON again, Ig recovered to a positive and then saturated again to a negative value. On the other hand, the drain Id increased when the gate irradiation was turned ON and returned to the same value as before it was turned OFF.

DOI： 10.1117/12.3009947

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

The anisotropic hole transport along [0001] and [1120] in the p-doped (1010) GaN layer was compared for layers grown on bulk (1010) GaN substrates and on (1010) sapphire. The sheet resistance along [0001] was 1.1 times larger on GaN substrates and even 1.2 times larger on sapphire than that along [1120]. The anisotropic hole transport on bulk GaN substrates is due to the anisotropy of the hole’s effective mass and the different contribution of carriers in different bands, whereas the larger anisotropy for GaN on sapphire is also due to additional scattering at stacking faults. The annealing process of metal Mg applied to the m-plane p-type GaN successfully results in a robust p-type ohmic contact, functioning as a p++ layer.

DOI： 10.1063/5.0177681

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

Minority carrier properties in dopant-free p-type distributed polarization doped (DPD) AlGaN layers were investigated on the basis of the forward-biased current density-voltage (J-V) characteristics of p-n+ diodes. The fabricated p-DPD AlGaN/n+-AlGaN:Si diodes exhibited ideal electrical characteristics despite the absence of acceptor atoms in the p-type layer. The extracted Shockley-Read-Hall lifetime exceeded 300 ps, which was longer than that reported for p-GaN:Mg on GaN substrates with a similar acceptor concentration (20-50 ps). Moreover, the electron diffusion coefficient was about 20 cm2 s−1 at any temperature, which was convincing in terms of the electron mobility in DPD layers. The results suggest that p-DPD AlGaN has more desirable minority carrier properties than conventional p-GaN:Mg, particularly for bipolar device applications.

DOI： 10.1063/5.0180062

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

In this letter, we show that low-cost physical vapor deposition of Mg followed by a thermal diffusion annealing process increases the effective barrier height at the metal/Ga-polar GaN Schottky interface. Thus, for the first time, GaN Camel diodes with improved barrier height and turn-on voltage were realized compared to regular GaN Schottky barrier diodes. Temperature-dependent current-voltage characteristics indicated a near-homogeneous and near-ideal behavior of the GaN Camel diode. The analysis performed in this work is thought to be promising for improving the performance of future GaN-based unipolar diodes.

DOI： 10.35848/1882-0786/ad0db9

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

One approach to improving the output power of ultraviolet (UV-C) light-emitting diodes (LEDs) is to adopt an electron-blocking layer (EBL) with a high barrier. However, the intended effect may not be realized because of the composition pulling effect, which is the unintended occurrence of a gradient layer at an AlGaN/AlGaN hetero-interface with substantial differences in the Al composition. Here, we demonstrate that low-temperature growth (i.e., <1000 °C) can be used to control the unintentional gradient layer at an AlN/AlGaN hetero-interface between a barrier layer and AlN-EBL with a difference in Al compositions of more than 30%. LEDs with an emission wavelength of 265 nm were fabricated, and an AlN-EBL was grown at low temperature to realize an abrupt interface. At an applied current of 100 mA, growing the EBL under low-temperature conditions improved the forward voltage by 0.5 V and remarkably improved the peak luminous intensity by 1.4-1.6 times. Our results can be used to realize UV-C LEDs with a steep EBL and further improve their device characteristics.

DOI： 10.1063/5.0183320

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Optics Letters  

III-nitride optoelectronic chips have tremendous potential for developing integrated computing and communication systems with low power consumption. The monolithic, top–down approaches are advantageous for simplifying the fabrication process and reducing the corresponding manufacturing cost. Herein, an ultraviolet optical interconnection system is investigated to discover the way of multiplexing between emission and absorption modulations on a monolithic optoelectronic chip. All on-chip components, the transmitter, monitor, waveguide, modulator, and receiver, share the same quantum well structure. As an example, two bias-controlled modulation modes are used to modulate video and audio signals in the experiment presented in this Letter. The results show that our on-chip optoelectronic system works efficiently in the near ultraviolet band, revealing the potential breadth of GaN optoelectronic integration.

DOI： 10.1364/OL.503429

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PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Laser and Photonics Reviews  

Nonradiative recombination rate that consists of dislocation-related nonradiative recombination rate (A0) and surface recombination rate (As) is one of the major parameters determining the performance of microlight-emitting diodes (µLEDs). Recent demonstrations improving the efficiency of blue InGaN or red AlGaInP µLEDs using specific methods such as atomic layer deposition or chemical treatment confirm the suppression of As. However, it is hardly found that those methods effectively improve the efficiency of red InGaN µLEDs so far. Here, it is discovered that the dislocation leads to an ineffective As. First, an intrinsic As degrades the external quantum efficiency (EQE) of blue InGaN µLEDs, resulting in EQE decreases with shrinking size. Second, panchromatic cathodoluminescence finds evidence that most of the carriers can be trapped before reaching the sidewall due to high A0. This results in shortened diffusion length of carriers and reduces the number of carriers reaching the sidewall. Consequently, the opposite trend of increasing EQE with shrinking size occurs in the case of red InGaN µLEDs due to an ineffective As. Furthermore, an 8.3 nm quantum well of InGaN with 13% Indium content that can reach a ≈690 nm wavelength at the low current is shown.

DOI： 10.1002/lpor.202300199

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

DOI： 10.1063/5.0174270

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

We attempted to grow (10-13) semi-polar GaN on graphene to confirm the possibility of a remote epitaxy of semi-polar GaN. Single crystalline (10-13) GaN was obtained on an optimized template using optimized growth conditions. However, (10-13), (0002), and other GaN orientations were found under the same growth conditions on a graphene-coated template. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy revealed that multi-domain GaN and (0002) GaN occurred in areas with a damaged graphene interfacial layer and intact graphene, respectively. Raman spectroscopy confirmed that graphene could survive under the growth conditions used here. Using cross-sectional scanning electron microscopy, we propose a simple approach to distinguish damaged graphene. Although the remote epitaxy of semi-polar GaN has not been demonstrated despite the usage of an optimized template and growth conditions, our results confirm the importance of the interfacial state in determining the crystallinity of the overgrown layer.

DOI： 10.1063/5.0157588

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physica Status Solidi (A) Applications and Materials Science  

AlN-based field-effect transistors (FETs) enable high-breakdown voltage, high drain current, and high-temperature operation. To realize high-frequency devices, N-polar AlGaN/AlN heterostructure FETs are focused on. N-polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN FET is fabricated using metal–organic vapor-phase epitaxy, and its electrical characteristics are evaluated. An N-polar AlN layer is grown on a sapphire substrate with a misorientation angle of 2.0° toward m-axis, on which a 20 nm thick Al0.9Ga0.1N base layer and a 20 nm Al0.1Ga0.9N channel layer are grown. The static FET operation is confirmed to exhibit an n-channel and pinch-off. Normally, during operation with a turn-on voltage of −3.2 V, a high operating breakdown voltage of 620 V and high operating temperature of 280 °C are also confirmed.

DOI： 10.1002/pssa.202200871

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Environment International  

A non-filter virus inactivation unit was developed that can control the irradiation dose of aerosolized viruses by controlling the lighting pattern of a 280 nm deep-UV (DUV)-LED and the air flowrate. In this study, the inactivation properties of aerosolized SARS-CoV-2 were quantitatively evaluated by controlling the irradiation dose to the virus inside the inactivation unit. The RNA concentration of SARS-CoV-2 remained constant when the total irradiation dose of DUV irradiation to the virus exceeded 16.5 mJ/cm2. This observation suggests that RNA damage may occur in regions below the detection threshold of RT-qPCR assay. However, when the total irradiation dose was less than 16.5 mJ/cm2, the RNA concentration monotonically increased with a decreasing LED irradiation dose. However, the nucleocapsid protein concentration of SARS-CoV-2 was not predominantly dependent on the LED irradiation dose. The plaque assay showed that 99.16% of the virus was inactivated at 8.1 mJ/cm2 of irradiation, and no virus was detected at 12.2 mJ/cm2 of irradiation, resulting in a 99.89% virus inactivation rate. Thus, an irradiation dose of 23% of the maximal irradiation capacity of the virus inactivation unit can activate more than 99% of SARS-CoV-2. These findings are expected to enhance versatility in various applications. The downsizing achieved in our study renders the technology apt for installation in narrow spaces, while the enhanced flowrates establish its viability for implementation in larger facilities.

DOI： 10.1016/j.envint.2023.108022

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PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

The stress relaxation with increasing thickness of metal-organic vapor phase epitaxy grown Al0.19Ga0.81N on quasi-bulk (101 ¯0) m-plane GaN substrates was investigated by x-ray diffraction. The anisotropic in-plane stress leads to an orthorhombic distortion of the lattice, which requires special mathematical treatment. Extending earlier works, we developed a method to calculate the distortion along [12 ¯10], [0001], and [101 ¯0] and obtained the lattice parameters, Al content, and strain values. The stress relaxation along the two in-plane directions involves two different mechanisms. First, the stress along [12 ¯10] relaxes by the onset of misfit dislocations through the { 10 1 ¯ 0 } ⟨ 1 2 ¯ 10 ⟩ slip system while for thicker layers the stress along [0001] relaxes by crack formation. Comparing the cathodoluminescence emission at room temperature with the expected bandgap showed that both tensile in-plane strains along [12 ¯10] and [0001] decrease the bandgap.

DOI： 10.1063/5.0149838

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Advanced Optical Materials  

The sidewall condition is a key factor determining the performance of micro-light emitting diodes (µLEDs). In this study, equilateral triangular III-nitride blue µLEDs are prepared with exclusively m-plane sidewall surfaces to confirm the impact of sidewall conditions. It is found that inductively coupled plasma-reactive ion etching (ICP-RIE) causes surface damages to the sidewall and results in rough surface morphology. As confirmed by time-resolved photoluminescence (TRPL) and X-ray photoemission spectroscopy (XPS), tetramethylammonium hydroxide (TMAH) eliminates the etching damage and flattens the sidewall surface. After ICP-RIE, 100 µm2-µLEDs yield higher external quantum efficiency (EQE) than 400 µm2-µLEDs. However, after TMAH treatment, the peak EQE of 400 µm2-µLEDs increases by ≈10% in the low current regime, whereas that of 100 µm2-µLEDs slightly decreases by ≈3%. The EQE of the 100 µm2-µLEDs decreases after TMAH treatment although the internal quantum efficiency (IQE) increases. Further, the IQE of the 100 µm2-µLEDs before and after TMAH treatment is insignificant at temperatures below 150 K, above which it becomes considerable. Based on PL, XPS, scanning transmission electron microscopy, and scanning electron microscopy results, mechanisms for the size dependence of the EQE of µLEDs are explained in terms of non-radiative recombination rate and light extraction.

DOI： 10.1002/adom.202203128

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

We have demonstrated the fabrication process for a lateral p-type Schottky barrier diode (SBD) with the annealed Mg ohmic contact layer on a MOVPE-grown p-GaN wafer and measured the electrical characteristic of the diode. Because of the selective-area ohmic contact, the interface between the Schottky electrode and p-type GaN is well protected from any damage introduced by dry-etching or regrowth. The ideality factor of the forward current-voltage characteristic is as low as 1.09 at room temperature and an on-off ratio above 109 is also achieved. Various metals are deposited as the Schottky electrode and the work function dependence of the Schottky barrier height is confirmed with a pinning factor of 0.58. The temperature dependence of the current-voltage characteristic indicates that the GaN p-type SBD still fits the thermionic emission mode at 600 K with an ideality factor of 1.1. The reverse current of the p-SBD is also studied with the Poole-Frenkel emission model, and the trap energy level in the p-GaN is confirmed.

DOI： 10.1063/5.0146080

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：AIP Advances  

When a quantum well (QW) diode is biased with a forward voltage and illuminated with an external shorter-wavelength light, the device simultaneously emits and detects light, with the injected current and the induced current mixed inside the wells. Separating these superimposed and dynamic electrical signals is useful for the development of multifunctional displays that can simultaneously transmit and receive light signals. By utilizing the unique overlap between the electroluminescence and detection spectra, we establish a wireless optical communication system using two AlGaInP diodes that have identical QW structures. The communication distance is 25 m, with one diode functioning as the transmitter and the other as the receiver. In particular, at the receiver end, the QW diode demonstrates simultaneous light emission and reception ability, and the mixed signals can be efficiently extracted, suggesting great potential for applications from light communication to advanced displays.

DOI： 10.1063/5.0142093

Web of Science

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.3390/nano13061077

PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Crystals  

Conventional deep-ultraviolet (UV) light-emitting diodes (LEDs) based on AlGaN crystals have extremely low light-emission efficiencies due to the absorption in p-type GaN anode contacts. UV-light-transparent anode structures are considered as one of the solutions to increase a light output power. To this end, the present study focuses on developing a transparent AlGaN homoepitaxial tunnel junction (TJ) as the anode of a deep-UV LED. Deep-UV LEDs composed of n+/p+-type AlGaN TJs were fabricated under the growth condition that reduced the carrier compensation in the n+-type AlGaN layers. The developed deep-UV LED achieved an operating voltage of 10.8 V under a direct current (DC) operation of 63 A cm−2, which is one of the lowest values among devices composed of AlGaN tunnel homojunctions. In addition, magnesium zinc oxide (MgZnO)/Al reflective electrodes were fabricated to enhance the output power of the AlGaN homoepitaxial TJ LED. The output power was increased to 57.3 mW under a 63 A cm−2 DC operation, which was 1.7 times higher than that achieved using the conventional Ti/Al electrodes. The combination of the AlGaN-based TJ and MgZnO/Al reflective contact allows further improvement of the light output power. This study confirms that the AlGaN TJ is a promising UV-transmittance structure that can achieve a high light-extraction efficiency.

DOI： 10.3390/cryst13030524

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Materials  

With the ammonothermal method, one of the most promising technologies for scalable, cost-effective production of bulk single crystals of the wide bandgap semiconductor GaN is investigated. Specifically, etch-back and growth conditions, as well as the transition from the former to the latter, are studied using a 2D axis symmetrical numerical model. In addition, experimental crystal growth results are analyzed in terms of etch-back and crystal growth rates as a function of vertical seed position. The numerical results of internal process conditions are discussed. Variations along the vertical axis of the autoclave are analyzed using both numerical and experimental data. During the transition from quasi-stable conditions of the dissolution stage (etch-back process) to quasi-stable conditions of the growth stage, significant temperature differences of 20 K to 70 K (depending on vertical position) occur temporarily between the crystals and the surrounding fluid. These lead to maximum rates of seed temperature change of 2.5 K/min to 1.2 K/min depending on vertical position. Based on temperature differences between seeds, fluid, and autoclave wall upon the end of the set temperature inversion process, deposition of GaN is expected to be favored on the bottom seed. The temporarily observed differences between the mean temperature of each crystal and its fluid surrounding diminish about 2 h after reaching constant set temperatures imposed at the outer autoclave wall, whereas approximately quasi-stable conditions are reached about 3 h after reaching constant set temperatures. Short-term fluctuations in temperature are mostly due to fluctuations in velocity magnitude, usually with only minor variations in the flow direction.

DOI： 10.3390/ma16052016

Scopus

PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

We demonstrated nanoplatelet In x Ga1−x N pseudosubstrates with In content varying from 0 to 0.3 on low-dislocation-density GaN substrates. These nanoplatelets efficiently relax in-plane strain, thus allowing for the use of a thick active layer to reduce built-in polarization. The 15 nm thick InGaN active layers grown under the same conditions on these nanoplatelets showed a remarkable cathodoluminescence redshift from 460 to 617 nm, suggesting enhanced In incorporation efficiency in InGaN nanoplatelets with higher In content. Moreover, the 617 nm-emitting sample presented an imperceptible blueshift under excitation-power-dependent photoluminescence, indicating a weak polarization field introduced by the high-In-content pseudosubstrates and the thick active layer.

DOI： 10.35848/1347-4065/acb74c

Web of Science

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本機械学会  

DOI： 10.1299/jsmemecj.2023.s054-05

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本機械学会  

DOI： 10.1299/jsmecmd.2023.36.os-1406

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physica Status Solidi (A) Applications and Materials Science  

The metal stop laser drilling of GaN-on-GaN devices is demonstrated using a UV sub-nanosecond laser as a light source. By monitoring the Bremsstrahlung emission at the drilling point, metal stops with a precision higher than 1 μm are realized for vias with a depth of 100 μm. From in situ laser-induced breakdown spectroscopy measurements, it is shown that endpoint detection is realized with high signal-to-noise ratio owing to the difference in the emission process between the strongly excited semiconductor and the metal. Herein, a through-substrate electrode with a resistance of less than 5 mΩ on GaN-on-GaN high-electron-mobility transistor (HEMT) wafers is demonstrated. The fabrication of through-substrate electrodes by this technique provides a simple process that does not require lithography or other complex processes. This process is expected to be useful in the fabrication of future GaN-on-GaN devices, including very thin GaN-on-GaN HEMTs.

DOI： 10.1002/pssa.202200739

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

The scanning electron microscope (SEM) with photocathode technology was launched by retrofitting the photocathode electron gun to a commercial-based SEM system. In this SEM system, the excitation laser for photoelectron generation from the photocathode is synchronized to the scanning signal. SEM images were obtained by high-speed modulation of the photoelectron beam current using the photocathode SEM, where the location in the field of view and its irradiation current were arbitrarily selected on a pixel-by-pixel basis (Selective e-Beaming technology). As a demonstration experiment contributing to non-contact electrical inspection, low-voltage SEM imaging of MOS-FET structures in 3D-NAND flash memory was performed using this selective e-beam technology. As a result, changes in the voltage contrast of the drain electrode were observed in response to on/off selective electron beam irradiation to the gate electrode in the MOS-FET structure. As an extension of the selective electron beaming technology, a Yield Controlled e-beaming (YCeB) technology was invented to control the secondary electron yield generated in the entire field of view of the SEM image by feedback control of the laser power irradiating the photocathode to the intensity of each pixel in the SEM image. The YCeB image, in which the laser power intensity corresponding to the probe intensity is modulated so that the secondary electron yield generated in the entire field of view of the SEM image is constant, is a clearer image with less noise than the original SEM image.

DOI： 10.1117/12.2657853

Web of Science

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

An InGaN photocathode with a negative electron affinity (NEA) surface is suitable for industrial use because of features such as a long quantum efficiency lifetime, availability with a visible laser as an excitation light source, and the presence of a transmission-type structure. The first objective is the development of an InGaN photocathode electron gun that can be mounted on a scanning electron microscope (SEM) and the evaluation of the electron beam size at the emission point, maximum emission current, and transverse energy of the electron beam, which are important factors for realizing a high probe current in the SEM. The second objective is the evaluation of emission current stability, while the third objective is the generation of a pulsed electron beam and multi-electron beam from the InGaN photocathode. The parameters of the electron beam from the photocathode electron gun were an emission beam radius of 1 µm, transverse energy of 44 meV, and an emission current of up to 110 µA. Using a high beam current with low transverse energy from the photocathode, a 13 nA probe current with 10 nm SEM resolution was observed with 15 µA emission. At 15 µA, the continuous electron beam emission for 1300 h was confirmed; at 30 µA, the cycle time between the NEA surface reactivations was confirmed to be 90 h with 0.043% stability. Moreover, a 4.4 ns pulsed e-beam with a 4.7 mA beam current was generated, and a 5 × 5 multielectron beam with 12% uniformity was then obtained. The advantages of the InGaN photocathode, such as high electron beam current, low transverse energy, long quantum efficiency lifetime, pulsed electron beam, and multi-electron beam, are useful in industries including semiconductor device inspection tools.

DOI： 10.1117/12.2657032

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

To reduce the operating voltage, we analyzed the p-n junction of an aluminum gallium nitride (AlGaN) homojunction tunnel junction (TJ) deep-ultraviolet light-emitting diode using phase-shifting electron holography. We obtained a phase image reflecting the band alignment of the p-n homojunction and derived a depletion layer width of approximately 10 nm. We found the AlGaN homojunction TJ forms a p-n junction. Furthermore, the operating voltage reached 8.8 V at 63 A cm-2 by optimizing the structural characteristics of the AlGaN TJ, such as the thickness and impurity concentration, where the thickness of the TJ was 23 nm. We found that the TJ thickness should be at least the same as the depletion layer width at the AlGaN TJ.

DOI： 10.1117/12.2646757

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Technical Digest - International Electron Devices Meeting, IEDM  

Nearly ideal AlN-based vertical p-n diodes are demonstrated on an AIN substrate utilizing dopant-free distributed-polarization doping (DPD). Capacitance-voltage measurements revealed that the effective doping concentration agreed well with the designed DPD charge concentration. The fabricated devices exhibited a low tum-on voltage of 6.5 V, a low differential specific ON-resistance of 3 mO cm2, and an ideality factor of 2 for a wide range of temperatures (room temperature-573 K). Moreover, the breakdown electric field was 7.3 MV/cm, which was almost twice as high as the reported critical electric field of 4H-SÌC and GaN. These results clearly demonstrate the usefulness of DPD in the fabrication of high-performance AlN-based power devices.

DOI： 10.1109/IEDM45741.2023.10413866

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

Until recently, lack of stable p-type doping source in HVPE hindered its use to the application of III-nitride light emitting devices. Recently, K. Ohnishi discovered the stable MgO source for p-type doping in GaN HVPE. This has enabled the use of HVPE for light emitting devices as well as electron devices such as vertical MOSFET. In this article, we will review the current HVPE technology of p-GaN HVPE, multi-junction AlInGaP/InGaP/GaAs solar cell, InGaN HVPE by tri-chlorides, and discuss the challenge and opportunities of III-nitride HVPE in terms of epitaxial layer design and the remaining issues of the growth of the low temperature buffer, MQWs as well as the source supply design to grow multilayer structures.

DOI： 10.1117/12.2647336

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：AIAA Aviation and Aeronautics Forum and Exposition, AIAA AVIATION Forum 2023  

We demonstrated static characteristics testing and Double-Pulse-Testing (DPT) at 77 K using GaN-Polarization Super Junction (PSJ) transistors, commercialized GaN HEMTs and commercialized SiC-MOSFETs. In the results of SiC-MOSFETs, total switching losses increased at 77 K due to the increase in on-resistance (Ron). On the other hand, in the results of GaN-PSJ transistors, total switching losses decreased at 77 K due to the increase in channel mobilities. In the case of GaN-PSJ transistors, energy losses which are induced self-charging and discharging (Eqoss) in the upper side DUT increased at 77 K. From the measurement of Vds dependence of Cds, Cds increased at 77 K compared with room temperature conditions. However, total switching losses decreased at low temperatures because Eqoss accounts for about 10% of the switching losses.

DOI： 10.2514/6.2023-4538

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Reports  

Lanthanoid-doped Gallium Nitride (GaN) integrated into nanophotonic technologies is a promising candidate for room-temperature quantum photon sources for quantum technology applications. We manufactured praseodymium (Pr)-doped GaN nanopillars of varying size, and showed significantly enhanced room-temperature photon extraction efficiency compared to unstructured Pr-doped GaN. Implanted Pr ions in GaN show two main emission peaks at 650.3 nm and 651.8 nm which are attributed to 3P0-3F2 transition in the 4f-shell. The maximum observed enhancement ratio was 23.5 for 200 nm diameter circular pillars, which can be divided into the emitted photon extraction enhancement by a factor of 4.5 and the photon collection enhancement by a factor of 5.2. The enhancement mechanism is explained by the eigenmode resonance inside the nanopillar. Our study provides a pathway for Lanthanoid-doped GaN nano/micro-scale photon emitters and quantum technology applications.

DOI： 10.1038/s41598-022-25522-6

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PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Environment International  

A general-purpose virus inactivation unit that can inactivate viruses was developed using deep ultraviolet (DUV) LEDs that emit DUV rays with a wavelength of 280 nm. The inside of the virus inactivation unit is a rectangular conduit with a sharp turn of 180° (sharp-turned rectangular conduit). Virus inactivation is attempted by directly irradiating the air passing through the conduit with DUV rays. The flow characteristics of air and virus particles inside the virus inactivation unit were investigated using numerical simulations. The air was locally accelerated at the sharp turn parts and flowed along the partition plate in the sharp-turned rectangular conduit. The aerosol particles moving in the sharp-turned rectangular conduit were greatly bent in orbit at the sharp turn parts, and then rapidly approached the partition plate at the lower part of the conduit. Consequently, many particles collided with the partition plates behind the sharp-turn parts. SARS-CoV-2 virus was nebulized in the virus inactivation unit, and the RNA concentration and virus inactivation rate with and without the emission of DUV-LEDs were measured in the experiment. The concentration of SARS-CoV-2 RNA was reduced to 60% through DUV-LED irradiation. In addition, SARS-CoV-2 passing through the virus inactivation unit was inactivated below the detection limit by the emission of DUV-LEDs. The virus inactivation rate and the value of the detection limit corresponded to 99.38% and 35.36 TCID50/mL, respectively.

DOI： 10.1016/j.envint.2022.107580

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PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Vacuum Science and Technology B  

Pulsed electron beams from a photocathode using an InGaN semiconductor have brought selectively scanning technology to scanning electron microscopes, where the electron beam irradiation intensity and area can be arbitrarily selected within the field of view in SEM images. The p-type InGaN semiconductor crystals grown in the metalorganic chemical vapor deposition equipment were used as the photocathode material for the electron beam source after the surface was activated to a negative electron affinity state in the electron gun under ultrahigh vacuum. The InGaN semiconductor photocathode produced a pulsed electron beam with a rise and fall time of 3 ns, consistent with the time structure of the irradiated pulsed laser used for the optical excitation of electrons. The InGaN photocathode-based electron gun achieved a total beam operation time of 1300 h at 15 μA beam current with a downtime rate of 4% and a current stability of 0.033% after 23 cycles of surface activation and continuous beam operation. The InGaN photocathode-based electron gun has been installed in the conventional scanning electron microscope by replacing the original field emission gun. SEM imaging was performed by selective electron beaming, in which the scanning signal of the SEM system was synchronized with the laser for photocathode excitation to irradiate arbitrary regions in the SEM image at arbitrary intensity. The accuracy of the selection of regions in the SEM image by the selective electron beam was pixel by pixel at the TV scan speed (80 ns/pix, 25 frame/s) of the SEM.

DOI： 10.1116/6.0002111

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Communications Engineering  

Optical fiber curvature sensors have been considered as a promising option for human motion detection due to its good toughness, bending flexibility and anti-electromagnetic interference. However, for wearable devices, the miniature configuration is preferred, and a high integration of the light emitter, receiver and guided fiber is essential to configure the miniaturized sensing system. Here, we present a miniaturized curvature sensing system by integrating a GaN-based optoelectronic chip with the plastical optical fiber (POF). The light emitter and detector are fabricated on a GaN-on-sapphire wafer to form a tiny chip sized at 2.5 × 1.5 mm2. The on-chip photodetector (PD) effectively senses the reflected light intensity, extracting information on the fiber bending deformation. A compact curvature sensing system is demonstrated for finger motion detection with movement angles of 30–90° and frequencies of 0.4, 1, and 1.6 Hz. The results show that the monolithically integrated LED and PD chip can be combined with the POF with reliable operation. The demonstration of the monolithically integrated optoelectronic device suggests a promising potential technology for future wearable fiber optical sensor system.

DOI： 10.1038/s44172-022-00049-w

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

Although the pulsed operation of AlGaN-based laser diodes at UV-C wavelengths has been confirmed in the previous studies, continuous oscillation without cooling is difficult because of the high operating voltage. In this study, the temperature dependence of key parameters was investigated and their impact on achieving continuous-wave lasing was discussed. A reduction in the threshold voltage was achieved by tapering the sides of the laser diode mesa and reducing the lateral distance between the n- and p-electrodes. As a result, continuous-wave lasing at room temperature was demonstrated at a threshold current density of 4.2 kA / cm 2 and a threshold voltage of 8.7 V.

DOI： 10.1063/5.0124480

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

Previously reported UV-C laser diode (LD) structures have been subject to design constraints owing to dark line defects at the edge of the mesa stripe after device fabrication. To address this issue, a detailed analysis revealed that the dark line defects were dislocations generated by local residual shear stresses associated with mesa formation on highly strained epitaxial layers. A technique for controlling the local concentration of shear stress using a sloped mesa geometry was proposed based on insight gained by modeling the stress distribution at the edge of the mesa stripe. Experimental results showed that this technique succeeded in completely suppressing the emergence of dark-line defects. This technique will be useful in improving the performance of pseudomorphic AlGaN/AlN-based optoelectronic devices, including UV-C LDs.

DOI： 10.1063/5.0124512

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ECS Journal of Solid State Science and Technology  

We report the fabrication of high-barrier-height and thermally reliable Schottky contacts to n-Al0.6Ga0.4N by using an Ag-Pd-Cu (APC) alloy. The Schottky barrier heights (SBHs) and ideality factors computed using the current-voltage (I-V) model ranged from 0.82 to 0.97 eV and from 3.15 to 3.44, respectively. The barrier inhomogeneity model and capacitance-voltage (C-V) method yielded higher SBHs (1.62-2.19 eV) than those obtained using the I-V model. The 300 °C-annealed APC sample exhibited more uniform electrical characteristics than the 500 °C-annealed Ni/Au Schottky samples (each with the best Schottky behavior). Furthermore, the scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) results indicated that the APC Schottky contacts were more thermally stable than the Ni/Au contacts. On the basis of the X-ray photoemission spectroscopy (XPS) results, the improved Schottky characteristics of the APC alloy contacts are described and discussed.

DOI： 10.1149/2162-8777/aca1df

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

We evaluated Mg-diffusion into GaN from GaN/Mg mixture. The diffusion depth of Mg increased with diffusion temperature from 1100 °C to 1300 °C, whereas the Mg concentration remained constant at 2-3 × 1018 cm−3 independent of temperature. The estimated activation energy for Mg diffusion was 2.8 eV, from which the substitutional diffusion mechanism was predicted. Mg-diffused GaN samples showed p-type conductivity with a maximum hole mobility of 27.7 cm2 V−1 s−1, suggesting that substitutional diffusion contributes to Mg activation. This diffusion technique can be used to easily form p-type GaN and has potential as a p-type selective doping technique.

DOI： 10.35848/1882-0786/ac9c83

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

Photocathodes with negative electron affinity (NEA) characteristics have various advantages, such as small energy spread, high spin polarization, and ultrashort pulsing. Nitride semiconductors, such as GaN and InGaN, are promising materials for NEA photocathodes because their lifetimes are longer than those of other materials. In order to further prolong the lifetime, it is important to better understand the deterioration of NEA characteristics. The adsorption of residual gases and back-bombardment by ionized residual gases shorten the lifetime. Among the adsorbed residual gases, H2O has a significant influence. However, the adsorption structures produced by the reaction with H2O are not comprehensively studied so far. In this study, we investigated adsorption structures that deteriorated the NEA characteristics by exposing InGaN and GaAs to an H2O environment and discussed the differences in their lifetimes. By comparing the temperature-programmed desorption curves with and without H2O exposure, the generation of CsOH was confirmed. The desorption of CsOH demonstrated different photoemission behaviors between InGaN and GaAs results. InGaN recovered its NEA characteristics, whereas GaAs did not. Considering the Cs desorption spectra, it is difficult for an NEA surface on InGaN to change chemically, whereas that for GaAs changes easily. The chemical reactivity of the NEA surface is different for InGaN and GaAs, which contributes to the duration of photoemission. We have attempted to prolong the lifetime of InGaN by recovering its NEA characteristics. We found that InGaN with NEA characteristics can be reused easily without thermal treatment at high temperatures.

DOI： 10.1063/5.0125344

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

GaN-based devices have grown rapidly in recent decades, due to their important research value and application prospects. There is a desire to monolithically integrate different GaN devices into a single chip for the development of future optoelectronic systems with low power consumption. In addition to improved multifunctional performance, a miniature integrated system can result in a significant reduction in material costs, processing costs, and packaging costs. In view of such prospects, we propose monolithic, top-down approaches to build III-nitride transmitter, modulator, waveguide, beam splitter, receiver, and monitor as a single unit onto a conventional GaN-on-silicon wafer without involving regrowth or postgrowth doping. Data communication among these components is realized through light propagation, opening up horizons for GaN optoelectronic systems on a chip.

DOI： 10.1063/5.0125324

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Surface Science  

A high quantum efficiency (QE) can be obtained on negative electron affinity (NEA) surfaces. It is well-known that NEA surfaces can be formed on semiconductor materials such as GaAs by the alternating supply of cesium (Cs) and oxygen (O2), which is called the yo-yo method. While GaN and related compounds such as InGaN are expected to realize an NEA photocathode with a long lifetime, the surface reactions between GaAs and nitride semiconductors are completely different with respect to the increasing rate of QE induced by the supply of O2. In addition, the surface processes of photoemission from NEA nitride semiconductors have not yet been elucidated. In the present study, a higher QE was achieved in InGaN by simultaneously supplying Cs and O2 instead of using the conventional yo-yo method. The possible Cs adsorption states in relation to the photoemission are also discussed based on the QE tendencies and the temperature programmed desorption (TPD) spectra of the NEA surfaces formed under elevated temperature conditions. This study suggests that the Cs oxide species, which is one of the key compounds for imparting the NEA nature, decomposes at approximately 350 °C and that the InGaN-Cs2O2 structure is a possible candidate for NEA photocathodes.

DOI： 10.1016/j.apsusc.2022.153882

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Materials  

For the fundamental understanding and the technological development of the ammonothermal method for the synthesis and crystal growth of nitrides, an in situ monitoring technique for tracking mass transport of the nitride throughout the entire autoclave volume is desirable. The feasibility of using high-energy computed tomography for this purpose was therefore evaluated using ex situ measurements. Acceleration voltages of 600 kV were estimated to yield suitable transparency in a lab-scale ammonothermal setup for GaN crystal growth designed for up to 300 MPa operating pressure. The total scan duration was estimated to be in the order of 20 to 40 min, which was sufficient given the comparatively slow crystal growth speed in ammonothermal growth. Even shorter scan durations or, alternatively, lower acceleration voltages for improved contrast or reduced X-ray shielding requirements, were estimated to be feasible in the case of ammonoacidic growth, as the lower pressure requirements for this process variant allow for thinned autoclave walls in an adapted setup designed for improved X-ray transparency. Promising nickel-base and cobalt-base alloys for applications in ammonothermal reactors with reduced X-ray absorption in relation to the maximum operating pressure were identified. The applicability for the validation of numerical simulations of the growth process of GaN, in addition to the applicability of the technique to further nitride materials, as well as larger reactors and bulk crystals, were evaluated.

DOI： 10.3390/ma15176165

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PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

We studied indium incorporation into InGaN/GaN quantum wells grown by metal-organic vapor phase epitaxy by systematically varying of gallium and indium precursor flows on (0001), (10 1 ¯3), (11 2 ¯2), and (10 1 ¯0) orientations. The layer thickness and indium composition obtained from x-ray diffraction analysis were correlated with a model based on indium and gallium incorporation efficiencies. In the model, the indium incorporation efficiency is reproduced by the Langmuir surface coverage of gallium, indicating that indium atoms close to gallium atoms can be incorporated preferably.

DOI： 10.1063/5.0088908

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Crystal Growth  

The effects of halide vapor phase epitaxial (HVPE) growth conditions such as input V/III ratio and substrate off-cut angle on the surface morphology of n-type GaN layers grown on GaN (0001) freestanding substrates were investigated to develop a model for growing smooth surfaces. The spiral hillock density increased with increasing input V/III ratio and/or decreasing off-cut angle. The critical off-cut angle between the spiral growth and the step-flow growth depended on the vapor supersaturation calculated by thermodynamic analysis. To understand the transition of the growth mode, we proposed a Burton–Cabrera–Frank-theory-based model considering the effect of spiral growth, which was utilized to explain the obtained experimental results. The developed growth model can be effective for predicting the HVPE growth mode between the spiral growth and the step-flow growth.

DOI： 10.1016/j.jcrysgro.2022.126749

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Alloys and Compounds  

In this study, chlorine (Cl) treatment was carried out on p-AlGaN to enhance the performance of ultraviolet-C light emitting diodes (UVC LEDs) by modifying ITO work function and hence reducing the contact resistance of ITO/Al reflector. The Cl-treated UVC LEDs exhibit the forward voltage of 6.88 V at 20 mA, whereas the reference samples show 7.50 V. The light output power and relative wall plug efficiency (WPE) of the Cl-treated UVC LEDs are enhanced by 17.1% at 500 mW and 19.5% at 100 mA, respectively, as compared to the reference. Additionally, the Cl-treated LEDs also display reduction in both the leakage current and ideality factor. Further, the photoluminescence (PL) intensity of AlGaN micro-disks is also enhanced by the Cl-treatment. X-ray photoemission spectroscopy (XPS) results indicate the formation of Cl-ITO at the ITO/p-AlGaN interface and the passivation of the surface states of AlGaN by Cl radicals. Based on the XPS results, a possible mechanism for the improved performance of Cl-treated UVC AlGaN-based LEDs is described and discussed.

DOI： 10.1016/j.jallcom.2022.164895

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

Energy-dispersive X-ray signals calibrated by Rutherford backscattering indicated the generation of Al13/24Ga11/24N in Ga-rich stripes in a nonflat Al0.58Ga0.42N layer. Also, the CL peak wavelengths of ∼259 and 272 nm also showed the generation of Al15/24Ga9/24N and Al13/24Ga11/24N in Al-rich zones and Ga-rich stripes, respectively. The wavelength of a strong CL peak at ∼246 nm, which was observed from the Al0.7Ga0.3N layer in our previous study, is also considered to correspond to the near-band-emission wavelengths of Al17/24Ga7/24N. In particular, the stronger reproducibility of metastable Al15/24Ga9/24N generation was confirmed, in agreement with the computed predictions by other research groups.

DOI： 10.35848/1882-0786/ac79a1

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physica Status Solidi - Rapid Research Letters  

Herein, the operation of dopant-free GaN-based p-n junctions formed by distributed polarization doping (DPD) is experimentally demonstrated and their space charge profiles and carrier transport properties are investigated. The device exhibits ideal space charge profiles explained by polarization effects and demonstrates the excellent controllability of DPD. In addition, it shows rectification and electroluminescence under forward-biased conditions. The carrier transport properties could be explained by the conventional recombination/diffusion model used for impurity-doped p-n junctions. Repeatable breakdowns are also observed in all devices and the temperature-dependent breakdown voltages reveal that the breakdowns are caused by avalanche multiplication, which is also the same as those reported in impurity-doped GaN p-n diodes. These results indicate that DPD is a promising doping technology for GaN-based power devices overcoming any issues associated with conventional impurity doping.

DOI： 10.1002/pssr.202200127

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Physics D: Applied Physics  

The metastability of Al n/12Ga1-n/12N (n = 2, 3, and 4) was investigated by the statistical analysis of electroluminescence (EL) spectra having dual peaks with a peak-to-peak distance (pp) of >10 nm generated from nonflat Al x Ga1-x N (x ∼0.2) quantum wells (QWs) fabricated on c(0001) sapphire substrates with a miscut of 1.0° towards the m[1 1ˉ 00] axis. To explain the origins of the dual-peak EL (DPEL) spectra, which are often observed for AlGaN-QWs with Ga content of greater than 0.7, a nonflat QW model incorporating two metastable compositions, Al(n-1)/12Ga1-(n-1)/12N and Al n/12Ga1-n/12N (n: integer), is proposed. By the statistical analysis of peak wavelengths in DPEL spectra and the verification of EL spectral shapes, two series of featured EL peak wavelengths with intervals of 2-3 nm were obtained from five out of six LED wafers. The two series of featured EL peak wavelengths were assigned by comparison with the calculated EL wavelengths. Then, Al2/12Ga10/12N and Al3/12Ga9/12N were determined to be the origins of peaks with the longer and shorter wavelengths in the DPEL, respectively, in addition to the metastable Al n/12Ga1-n/12N (n= 4-9) compositions observed in our previous studies. When DPEL (pp> 10 nm) appeared, the difference in QW thickness between Al2/12Ga10/12N and Al3/12Ga9/12N tended to be larger than one monolayer (ML), indicating a significant amount of Ga or GaN mass transport. Furthermore, the Al2/12Ga10/12N and Al3/12Ga9/12N QWs are considered to have thicknesses of m ML (m: consecutive integers), suggesting the 1 ML configuration of Al and Ga atoms on the c(0001) plane. In addition, the DPEL obtained from nonflat Al x Ga1-x N (x ∼0.25) QWs by another research group was shown to be related to two metastable Al n/12Ga1-n/12N (n = 3, 4), similarly to our one exceptional LED wafer, which also agreed with the model proposed in this work.

DOI： 10.1088/1361-6463/ac5d03

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1038/s41598-022-12628-0

PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Optics Letters  

This Letter describes the impact of shape on micro light-emitting diodes (µLEDs), analyzing 400 µm2 area µLEDs with various mesa shapes (circular, square, and stripes). Appropriate external quantum efficiency (EQE) can yield internal quantum efficiency (IQE) which decreases with increasing peripheral length of the mesas. However, light extraction efficiency (ηe) increased with increasing mesa periphery. We introduce analysis of Jpeak (the current at peak EQE) since it is proportional to the non-radiative recombination. Etching the sidewalls using tetramethylammonium hydroxide (TMAH) increased the peak EQE and decreased the sidewall dependency of Jpeak. Quantitatively, the TMAH etching reduced non-radiative surface recombination by a factor of four. Hence, shrinking µLEDs needs an understanding of the relationship between non-radiative recombination and ηe, where analyzing Jpeak can offer new insights.

DOI： 10.1364/OL.456993

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PubMed

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

As the size of micro light-emitting diodes (μLEDs) decreases, μLEDs encounter etching damage especially at the sidewalls that critically affects their properties. In this study, we investigated the influence of etching bias power (Pbias) on the performance of μLEDs and found that the current-voltage and light output-current characteristics of μLEDs were enhanced when Pbias was reduced. It was shown that at low Pbias, the chemical reaction between etching gas and gallium nitride, rather than ion sputtering, dominated the etching process, leading to low plasma damage and rough surface morphology. Additionally, to understand the etching-induced surface roughening behaviors, various substrates with different threading dislocation densities were treated at low Pbias. It was found that for the sample (with p-contact size of 10 × 10 μm2), the efficiency droop was approximately 20%, although the current reached 10 mA due most probably to the suppressed polarization effect in the quantum well. It was further observed that the external quantum efficiency (EQE) was dependent on Pbias, where the lowest Pbias yielded the highest maximum EQE, indicating that the plasma damage was mitigated by reducing Pbias. Optimization of dry etching and polarization-suppression conditions could pave the way for realizing high-performance and brightness μLEDs for next-generation displays.

DOI： 10.1063/5.0085384

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

An AlGaN/GaN heterojunction bipolar transistor (HBT) with N-p-n configuration was fabricated by the "regrowth-free"method, resulting in a contamination-free emitter-base AlGaN/GaN heterojunction. The low-bias-power-based low-damage inductively coupled plasma-reactive ion etching was employed in this study for emitter mesa definition instead of the conventional selective-area-regrowth technique. The method successfully minimized the etching-induced damage in the p-GaN base layer and the contamination at the emitter-base AlGaN/GaN heterojunction. Consequently, the fabricated device exhibited a high current gain of 25, the highest current density of 15.0 kA cm-2, and the lowest on-state voltage offset of 0.75 V ever reported for AlGaN/GaN HBTs.

DOI： 10.35848/1882-0786/ac6197

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

The working mechanism of the anti-parasitic-reaction (APR) catalyst of tungsten carbide (WC) coating on graphite in hydride vapor phase epitaxy GaN growth were examined. During NH3 annealing, the surface of WC is reduced as well as nitrided. The W2N topmost layer was found to work as an APR-active catalyst to suppress the formation of GaN polycrystals during high-rate HVPE-GaN growth, while the regions covered with thick pyrolytic graphite residues were catalytically inert. The formation of an additional W2C top layer on the WC underlayer was demonstrated to exhibit superior APR activity, i.e. complete suppression of GaN polycrystal formation.

DOI： 10.35848/1882-0786/ac5ba4

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

In this study, we enhanced the emission power of AlGaN-based tunnel junction deep-ultraviolet LEDs (TJ LEDs) by using a MgZnO and aluminum stacked structure as UV reflective electrodes on the anode side. The emission wavelength of the fabricated TJ LED was 284 nm, and the emission power was 57.3 mW at 63 A cm-2. The polycrystalline MgZnO enabled both conductivity and UV transmittance to be achieved, approximately doubling with a reflectance of the fabricated TJ LED at 284 nm compared to conventional TJ LED with Ti/Al. These factors contributed to the increased emission power of TJ LEDs.

DOI： 10.35848/1882-0786/ac5acf

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

We demonstrated the structural optimization of AlGaN tunnel junction (TJ) deep-ultraviolet light-emitting diodes by changing the thickness and impurity concentrations of p+-type and n+-type AlGaN constituting the TJ. By decreasing the total thickness of the TJ to 23 nm, the operating voltage reached a minimum of 8.8 V at 63 A cm-2. Further decrease in TJ thickness markedly increases the operating voltage. This finding implies that the depletion layer width becomes greater than the TJ thickness if it is smaller than 12 nm. Therefore, we conclude that the TJ thickness must be greater than the depletion layer width.

DOI： 10.35848/1882-0786/ac60c7

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

In this work, a deliberate etching-based top-down approach is proposed to fabricate the GaN nanorod (NR) Schottky barrier diode (SBD). As a key step during the fabrication, the impact of the wet-etching process on device performance is systematically studied. By virtue of the reduced surface states at the sidewall, the performance of NR SBD with the wet-etching process is substantially improved, delivering a forward turn-on voltage of 0.65 V, a current density of ∼10 kA/cm2 at 3 V, an ideality factor of 1.03, an ON/OFF current ratio of ∼1010, and no severe current collapse, along with a reverse breakdown voltage of 772 V.

DOI： 10.1063/5.0083194

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ECS Journal of Solid State Science and Technology  

For micro-light-emitting diode (LED)-based display applications, such as virtual reality and augmented reality, high-performance Ohmic contacts (namely, the improvement of current injection efficiency) is vital to the realization of high-efficiency micro-LEDs. The surface Fermi level pinning characteristics could be comprehended in terms of the relation between work function of metals (M) and Schottky barrier height (SBH, B). In this study, we have investigated the surface Fermi level pinning characteristics of (001) n-AlInP surfaces by employing Schottky diodes with different metals. With an increase in the temperature, B increases linearly and ideality factors (n) decreases. This behavior is related to the barrier height inhomogeneity. Inhomogeneity-modelbased B is evaluated to be in the range of 0.86 1.30 eV, which is dependent on the metal work functions and are similar to those measured from capacitance-voltage relation. Further, The S-parameter, the relation between B and M (d B/d M), is 0.36. This is indicative of the partial pinning of the surface Fermi level at the surface states placed at 0.95 eV below the conduction band. Furthermore, it is also shown that (NH4)2S-passivation results in an increases the mean SBH and the S-parameter (e.g., 0.52).

DOI： 10.1149/2162-8777/ac5d66

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

We analyzed the p-n junction of an aluminum gallium nitride (AlGaN) homojunction tunnel junction (TJ) deep-ultraviolet light-emitting diode by phase-shifting electron holography. We clearly obtained a phase image reflecting the band alinement of the p-n homojunction and derived a depletion layer width of approximately 10 nm. In addition, the observed depletion layer width for the AlGaN TJ was in good agreement with the simulated one reflecting the diffusion profile of Mg and Si, thus enabling a discussion on the electrical conduction mechanism for an AlGaN p-n junction.

DOI： 10.35848/1882-0786/ac53e2

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

We evaluated the beam current dependence of defect formation during Mg ion implantation into GaN at a high temperature of 1100 °C with two beam currents. Photoluminescence spectra suggest that low-beam-current implantation reduced the vacancy concentration and activated Mg to a greater extent. Moreover, scanning transmission electron microscopy analysis showed that low-beam-current implantation reduced the density of Mg segregation defects with inactive Mg and increased the density of intrinsic dislocation loops, suggesting decreases in the densities of Ga and N vacancies. The formation of these defects depended on beam current, which is an important parameter for defect suppression.

DOI： 10.35848/1882-0786/ac481b

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

Advances in obtaining untwinned (10 1 ¯3)-oriented semi-polar GaN enable a new crystal orientation for the growth of green and red LED structures. We present a scanning electron microscopy study that combines the structural characterization of electron channeling contrast imaging with the optical characterization of cathodoluminescence hyperspectral imaging on a (10 1 ¯ 3) GaN layer. An extensive defect analysis revealed that the dominant defects consist of basal plane stacking faults (BSFs), prismatic stacking faults, partial dislocations, and threading dislocations. With a defect density of about an order of magnitude lower than in comparable. The optical properties of the defects have been characterized from 10 to 320 K, showing BSF luminescence at room temperature indicating a reduced density of non-radiative recombination centers in the as-grown samples compared to established semi- and non-polar orientations. Our findings suggest that growth along (10 1 ¯3) has the potential for higher radiative efficiency than established semi-polar orientations.

DOI： 10.1063/5.0077084

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

Heterogeneous integration is important to create multi-functionality in future electronic devices. However, few thermal studies of the interfaces formed in these integrated devices have been reported before. Recently, integrated interfaces by surface-Activated bonding were found to have high thermal boundary conductance, which provides a solution for heat dissipation of GaN and β-Ga2O3-based power electronics. Here, we review the recent progress on the interfacial thermal transport across heterogeneously integrated interfaces, including transferred van der Waals force bonded interfaces, surface-Activated bonded interfaces, plasma bonded interfaces, and hydrophilic bonded interfaces. This Perspective specifically focuses on applications of thermal management strategies of electronics, especially power electronics. Finally, the challenges, such as high-Throughput thermal measurements of buried interfaces, thermal property-structure relations of interfaces bonded under different conditions, theoretical understanding of interfacial thermal transport, and device demonstrations, are pointed out.

DOI： 10.1063/5.0077039

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

The presence of hexagonal-pyramid-shaped hillocks (HPHs) in AlGaN epitaxial films affects device characteristics; this effect is significant in DUV laser diodes (LDs) on AlN substrates, where the presence of HPHs under the p-electrode increases the threshold current density and inhibits the lasing. In this study, we investigated the difference between the lasing characteristics of LDs with and without HPHs. It was found that in the presence of HPHs, the threshold excitation power density increased and the slope efficiency decreased by optical excitation. To investigate the cause of these phenomena, we performed structural, optical, and electrical analyses of the HPHs. Various imaging techniques were used to directly capture the characteristics of the HPHs. As a result, we concluded that HPHs cause the degradation of LD characteristics due to a combination of structural, optical, and electrical factors.

DOI： 10.35848/1347-4065/ac3a1d

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Electron Device Letters  

We demonstrated the formation of excellent Ohmic contact to p-type GaN (including the plasma etching-damaged p-type GaN which otherwise exhibited undetectable current within ±5 V) by the post-growth diffusion of magnesium. The specific contact resistivity on the order of 10-4Omega cm2 (extracted at V=0 V) was achieved on the plasma-damaged p-GaN with linear current-voltage characteristics by the transfer length method (TLM) measurement. The improvement in current by a factor of over 109 was also obtained on the plasma-damaged p-n junction diode after the same Mg-treatment. These experimental results indicate a great potential of post-growth diffusion of Mg to overcome the bottleneck of forming a good Ohmic contact to p-GaN.

DOI： 10.1109/LED.2021.3131057

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

ISAMU AKASAKI, crystal grower, a pioneer of GaN-based blue light-emitting diodes (LEDs), and a Nobel Laureate in Physics, passed away because of pneumonia on April 1, 2021 at the age of 92. According to the Nobel Foundation, the LED lamp holds considerable promise for improving the quality of life of over 1.5 billion people worldwide who lack access to electricity grids. Owing to its low power requirements, it can be powered by cheap local solar energy. In this article, his pursuit of blue LEDs for 20 years is described.

DOI： 10.1117/12.2619005

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Journal of the Electron Devices Society  

In this paper, a technique to develop small signal circuit (SSC) models of AlGaN/GaN high electron mobility transistors (HEMTs) using dependent current sources is presented. In this technique, experimentally measured broadband Y-parameters of AlGaN/GaN HEMTs are mathematically modeled as a sum of pole-residue terms. By representing each pole-residue term as a dependent current source, it is possible to develop an accurate SSC models for HEMTs which otherwise may not be possible using passive resistive-inductive-capacitive elements. The accuracy of the proposed SSC model is validated against the conventional SSC model using a 2nd, 3rd and 4th order rational function representation of the admittance branches of AlGaN/GaN HEMTs. Therefore, the proposed SSC model turns out to be highly robust in nature and can take care of any form of the transfer functions of the admittance branches between the gate, drain, and source terminal of an AlGaN/GaN HEMT.

DOI： 10.1109/JEDS.2022.3208028

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：2022 International Conference on Compound Semiconductor Manufacturing Technology, CS MANTECH 2022  

Exciting high performance power electronic devices have been widely demonstrated and manufactured using GaN epitaxial layers, but the majority of these devices have been fabricated on lattice mismatched substrates, including SiC, Si, or sapphire. Unfortunately, using lattice mismatched substrates inevitably introduces high concentrations of point defects and dislocations into the GaN epilayers, and these defects degrade the electrical performance of the fabricated GaN devices. Also, the mismatch of lattice constant and coefficient of thermal expansion cause strain and wafer bowing in the GaN epi, which further degrade the quality material for device fabrication. Using lattice matched GaN substrates provides solutions to these problems. In 2014, the Ministry of the Environment launched a national project to develop the required technology and to prove the superiority of GaN on GaN devices in real systems, with more than 10 partnerships from academia and industry. The project included work in several areas, including GaN substrate growth, GaN epitaxy, material characterization, and fabrication of devices and ICs for application in various systems. Large area GaN substrates have been grown with low defect densities, which has enabled fabrication of new types of vertical and horizontal GaN devices. The GaN devices have been used in servers, solar cell power conditioners, microwave ICs, distribution transformers, electric vehicle power converters. The performance improvements were compared with conventional approaches in each case. An “ALL GaN vehicle” has also been demonstrated, in which GaN devices are used in all power components. In this talk, we will present these results which show the great potential of GaN on GaN devices in the industry.

Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本機械学会  

DOI： 10.1299/jsmemecj.2022.s055-10

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本機械学会  

DOI： 10.1299/jsmemecj.2022.s055-11

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本機械学会  

DOI： 10.1299/jsmefed.2022.os03-22

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Advanced Engineering Materials  

The integration of III-nitride electronics and photonics is of great interest toward future computing systems with low power consumption. Multifunctioning multiple quantum well (MQW) diodes can address the challenging issue for on-chip integration of a light source, which is a key component to drive the photonic circuits. Herein, a transmitter, waveguide, modulator, and receiver are monolithically integrated on a III-nitride-on-silicon platform to perform light emission, transmission, modulation, and detection simultaneously. Both the receiver and modulator exhibit sufficient sensitivity to optical signals from the transmitter, which has an identical InGaN/AlGaN multiple quantum well (MQW) structure because the III-nitride diode provides spectral overlap between the emission and absorption spectra. On-chip data communication among these optical components is achieved using light, and the effective wavelength range is from 365 to 385 nm, in which multifunctional devices can be operated.

DOI： 10.1002/adem.202100582

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Physics D: Applied Physics  

When nonflat Al x Ga1-x N quantum wells (QWs) for producing 285 nm light emitting diodes (LEDs) were fabricated on n-AlGaN on AlN templates with dense macrosteps on c(0001) sapphire substrates with a 1.0 miscut relative to the m[1-100] axis, composite electroluminescence (EL) spectra from both inclined and terrace zones in Al x Ga1-x N QWs (x∼ 1/3) were generated owing to compositional and thickness modulations. The shoulder or main peaks in composite EL spectra tended to locate at fixed discrete wavelengths of ∼287, ∼292, and ∼296 nm from 12 nonuniform 285 nm LED wafers that were involved in nonnegligible run-to-run drift, even though these wafers were fabricated using the same source gas flow parameters for metal-organic vapor phase epitaxy. The discrete wavelengths of ∼287, ∼292, and ∼296 nm were attributed to EL from Al1/3Ga2/3N QWs with thicknesses of 8, 9, and 10 monolayers (ML), respectively, by referring to the results of cathodoluminescence (CL) analysis. Also, when nonflat Al x Ga1-x N QWs (x∼ 1/2) for 265 nm LEDs were grown, single-peak-like EL spectra were mainly generated from the inclined zones in nonflat QWs. The EL spectra taken from four nonuniform 265 nm LED wafers tended to show weak structures or main peaks at ∼257, ∼261, ∼266, and ∼271 nm, which were also attributed to emissions from Al1/2Ga1/2N QWs with thicknesses of 5, 6, 7, and 8 ML, respectively, by referring to CL analysis results. The creation of Al1/3Ga2/3N and Al1/2Ga1/2N in nonflat QWs in this work was in agreement with the results of our previous studies that indicated the creation of metastable Al n/12Ga1-n/12N (n: consecutive natural numbers). Furthermore, QW thicknesses of consecutive n ML may imply that Al1/3Ga2/3N and Al1/2Ga1/2N have 1 ML configurations of Al and Ga atoms on a c(0001) plane.

DOI： 10.1088/1361-6463/ac2065

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics  

In this study, we generated 25 multielectron beam (MEB) using an InGaN photocathode with a negative electron affinity state irradiating with 25 multilaser beam. The uniformity of the MEB and the total electron beam current were evaluated. A laser beam was split into 25 laser beams using a spatial light modulator. The coefficient of variation (CV) of laser power was 20%. The CV of quantum efficiency was 1.1%. The CV of electron beam current was 12%, and the total current was about 1.2 μA. These results will enhance the development of the MEB-defect inspection using the InGaN photocathode.

DOI： 10.1116/6.0001272

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Electron Devices  

Conventional lumped small signal circuit (SSC) models of AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOS-HEMTs) are derived from the low frequency portion of the experimentally measured ${Y}$ -parameters. Consequently, these models cannot faithfully capture the high frequency behavior of the device. In this work, modified SSC models of AlGaN/GaN MOS-HEMTs are developed by fitting the entire broadband measured ${Y}$ -parameters of the device with rational functions. These rational functions are then physically realized using additional passive circuit elements added to the conventional SSC model. The accuracy of the proposed SSC models can be improved by increasing the order of the rational functions. The proposed models are demonstrably more accurate than the conventional SSC models in estimating the higher order poles and zeroes present in the experimentally measured ${Y}$ -parameters of AlGaN/GaN MOS-HEMTs.

DOI： 10.1109/TED.2021.3119528

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

After the paper was published,1we found errors in the manuscript shown in page 152102-2. In the original published article, the SIMS detec-tion limits of [O] and [H] were written as 7 × 1015and 3 × 1015cm-3, respectively. The corrected detection limits of [O] and [H] are 6 × 1015and 3 × 1016cm-3, respectively. The SIMS depth profiles shown in Fig. 2(b) and the conclusions of the paper are not affected by this erratum.

DOI： 10.1063/5.0077364

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

A vertical GaN p+-n junction diode with an ideal breakdown voltage was grown by halide vapor phase epitaxy (HVPE). A steep p+-n interface was observed even with the use of the HVPE method. No Si-accumulating layer was formed at the p+-n interface because of the continuous HVPE growth from the n-type drift layer to the p-type layer. This method provides improved electrical properties compared with the regrowth of p-type GaN layers. The minimum ideality factor of approximately 1.6 was obtained. The breakdown voltage increased from 874 to 974 V with the increase in the temperature from 25 to 200 °C, which suggests that avalanche multiplication causes the breakdown. The temperature-dependent breakdown voltage was in good agreement with the breakdown voltage calculated using the ideal critical electric field. These results indicate that HVPE is promising for the fabrication of vertical GaN power devices.

DOI： 10.1063/5.0066139

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

In this study, vertical-type thick BGaN PIN diodes were successfully fabricated to improve the neutron capture rate of BGaN diodes by improving the BGaN epitaxial growth technique. In this technique, 5-μm-thick epitaxial growth was achieved using trimethylboron as a B metal-organic source, which suppressed the gas-phase reaction. The α-particle energy spectrum, the neutron pulse signal, and the residual energy of particles emitted from a neutron capture reaction were measured using the fabricated BGaN diodes by performing radiation detection measurements. The detected signal position in the neutron detection signal spectrum was similar to that of 2.3 MeV α-particles. These results indicate that vertical-type BGaN diodes can be used as effective neutron detectors.

DOI： 10.1063/5.0051053

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ECS Journal of Solid State Science and Technology  

We have investigated the performance of AlGaInP-based red micro-light-emitting diodes (micro-LEDs) with different n-type contact schemes as functions of current, ambient temperature, and chip size. The samples with AuGe/Ni/Au contact revealed wider full width at half maximum of electroluminescence than that with the Pd/Ge contact. All samples also exhibited broad peaks at wavelengths between ∼632 and ∼640 nm, whose intensity depended on the type of contact schemes and temperature. Regardless of the contact schemes, the 10 μm-size samples showed a larger temperature-dependent reduction in the output power at current density of <50 A cm-2 than the 100 μm-size ones. Above 100 A cm-2, however, both samples showed similar temperature dependence. Irrespective of the contact schemes, the main peak of the 100-μm samples was red-shifted, whereas no red-shift was detected in the 10-μm samples. The third peak of the AuGe-based contact samples became more dominant at 700 A cm-2 than the main peak, whereas that of the PdGe contact samples became more dominant at 1000 A cm-2. Based on the chip size, current, contact scheme, and temperature dependence, the performance degradation of the red micro-LEDs is described and discussed.

DOI： 10.1149/2162-8777/ac2029

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Materials Today Physics  

The minimization of electronics makes heat dissipation of related devices an increasing challenge. When the size of materials is smaller than the phonon mean free paths, phonons transport without internal scattering and laws of diffusive thermal conduction need to be revisited. This work reports the temperature dependent thermal conductivity of doped epitaxial 6H–SiC and monocrystalline porous 6H–SiC below room temperature probed by time-domain thermoreflectance. Strong quasi-ballistic thermal transport was observed in these samples, especially at low temperatures. Doping and structural boundaries were applied to tune the quasi-ballistic thermal transport since dopants selectively scatter high-frequency phonons while boundaries scatter phonons with long mean free paths. Exceptionally strong phonon scattering by boron dopants are observed, compared to nitrogen dopants. Furthermore, orders of magnitude reduction in the measured thermal conductivity was observed at low temperatures for the porous 6H–SiC compared to the epitaxial 6H–SiC. Finally, first principles calculations and a simple Callaway model are built to understand the measured thermal conductivities. Our work sheds light on the fundamental understanding of thermal conduction in technologically-important wide bandgap semiconductors such as 6H–SiC and will impact applications such as thermal management of 6H–SiC-related electronics and devices.

DOI： 10.1016/j.mtphys.2021.100462

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics A: Materials Science and Processing  

Gallium nitride (GaN)-based devices surpass the traditional silicon-based power devices in terms of higher breakdown voltage, faster-switching speed, higher thermal conductivity, and lower on-resistance. However, heteroepitaxial GaN growths like GaN on sapphire are not suitable for power devices due to the threading dislocation densities as high as 108/cm2. Recently, homoepitaxial GaN growth has become possible thanks to the native GaN substrates with dislocation densities in the order of 104/cm2 but the extremely high cost of the GaN substrates makes the homoepitaxy method unacceptable for industrial applications, and the slicing of wafers for reusing them is an effective solution for cost reduction. In this study, we will investigate a route for slicing the GaN single crystal substrate by controlling the laser pulse energy and changing the distance between each laser shot. The 2D and 3D crack propagations are observed by a multiphoton confocal microscope, and the cross section of samples is observed by a scanning electron microscope (SEM). The results showed that two types of radial and lateral cracking occurred depending on the pulse energy and shot pitch, and controlling them was of importance for attaining a smooth GaN substrate slicing. Cross-sectional SEM images showed that at suitable pulse energy and distance, crack propagation could be controlled with respect to the irradiation plane.

DOI： 10.1007/s00339-021-04808-y

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Electron Device Letters  

We demonstrated a prototype Gated Lateral power bipolar junction transistor (GLP-BJT) on wide bandgap semiconductor. The device combined the intrinsic advantages of high current gain of a Gated Lateral-BJT and good current handling and voltage blocking capabilities of GaN material. As a result, the common-emitter current gain remained over 300 at a high collector current density of 2 kA/cm2 despite a wide p-base region of 2 &#x03BC;m. The open base breakdown voltage BVCEO was over 300 V corresponding to a high critical field of 2.5 MV/cm. These figures of merit show great promise of GaN-based GLP-BJT in power applications and also shed light on the development of state-of-the-art bipolar transistors based on other wide bandgap semiconductors.

DOI： 10.1109/LED.2021.3099982

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Alloys and Compounds  

The stable electrical performance of micro-light-emitting diodes (micro-LEDs) is critical to display application. We investigated the effect of the interface morphologies of contacts to n-AlInP on the electrical stability of AlGaInP-based red micro-LEDs. Regardless of chip sizes (100 µm or 10 µm-size), micro-LEDs with Pd/Ge contacts gave lower and stable forward voltages than those with AuGe/Ni/Au contacts. When annealed at 450 °C, the AuGe/Ni/Au contact underwent seriously inhomogeneous interfacial reactions, resulting in a large variation of interfacial morphologies across the whole contact/AlInP interface. However, the Pd/Ge contact exhibited similar morphologies across the whole interface when annealed. Further, when operated at 800 A/cm2, micro-LEDs with the Pd/Ge contacts underwent less electrical degradation than the ones with the AuGe/Ni/Au contacts. Based on the electrical and scanning transmission electron microscope (STEM) results, the unstable electrical behavior of red micro-LEDs with the AuGe/Ni/Au contact is discussed.

DOI： 10.1016/j.jallcom.2021.159629

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

In this study, we investigated the relationship of light output power with the optical thickness of the p-layers in AlGaN-based deep ultraviolet light-emitting diodes with a transparent high-Al-composition p-AlGaN clad layer, a thin p-GaN contact layer, and a reflective p-type electrode. By adjusting the thickness of the transparent high-Al-composition p-AlGaN clad layer, we observed a marked change in light output power. A maximum light output power of 385 mW at 1500 mA, a maximum external quantum efficiency of 15.7% at 10 mA, and a maximum wall-plug efficiency of 15.3% at 10 mA were obtained at an emission wavelength of 275 nm.

DOI： 10.35848/1882-0786/ac154c

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

We reduced the operating voltage of AlGaN homojunction tunnel junction (TJ) deep-ultraviolet (UV) light-emitting diodes (LEDs) by two approaches: the suppression of carbon incorporation and the doping of a high concentration of silicon in an n+-AlGaN layer. The AlGaN homojunction TJ deep-UV LEDs had a significantly reduced forward voltage upon suppressing the incorporation of carbon in the n+-AlGaN layer. The suppression of electron compensation by carbon in nitrogen sites and the doping of a high concentration of silicon in an n+-AlGaN layer are important for reducing the operating voltage of AlGaN homojunction TJ deep-UV LEDs.

DOI： 10.35848/1882-0786/ac0fb6

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

The interface properties of GaN metal-insulator-semiconductor (MIS) structures with a gate electrode metal deposited by electron beam (EB) or resistive heating evaporation were investigated. Also, the impact of the interface properties on the channel mobility in GaN MIS field-effect transistors was investigated. It was confirmed that interface charges including both interface states and positive fixed charges were introduced to an Al2O3/GaN interface by the formation of a gate electrode by EB evaporation. Consequently, the introduced interface charges degraded the electron mobility in the MIS channel.

DOI： 10.35848/1882-0786/ac0ffa

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Crystal Growth  

The electrical properties and structural defects of p-type GaN layers with Mg concentrations from 8.0 × 1018 to 8.3 × 1019 cm−3 grown by halide vapor phase epitaxy (HVPE) were investigated. In all samples, p-type conduction was confirmed at room temperature. The hole concentration at room temperature decreased in a heavily Mg-doped sample. By analyzing the results of Hall-effect measurements at various temperatures, the acceptor concentration decreased in a heavily Mg-doped sample, whereas the compensating donor concentration increased. These results affect the decrease in the hole concentration. The hole mobility decreased with increasing acceptor concentration. In the heavily Mg-doped sample, pyramidal inversion domains (PIDs) were formed. The size of each PID in an HVPE-grown sample is in good agreement with that Mg-doped GaN layers grown by metalorganic vapor phase epitaxy (MOVPE). Thus, the formation mechanism of PIDs in HVPE-grown samples is possibly the same as that in MOVPE-grown samples. Energy-dispersive X-ray spectroscopy shows that Mg atoms accumulate in PIDs, which suggests that Mg atoms in PIDs are electrically inactive, inhibiting the increase in the acceptor concentration. These results are useful guidelines for fabricating p-type GaN layers with higher hole concentrations by HVPE.

DOI： 10.1016/j.jcrysgro.2021.126173

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：公益社団法人 応用物理学会  

DOI： 10.11470/oubutsu.90.7_455

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本物理学会  

<p>追悼</p><p>赤﨑　勇先生を偲んで</p>

DOI： 10.11316/butsuri.76.7_478

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

An optimized top-down approach was utilized to fabricate vertical GaN-on-GaN nanowire Schottky barrier diodes (NWSBDs) in this letter. The suppressed reverse leakage current and enhanced breakdown voltage (BV) of the vertical GaN NWSBDs are attributed to the reduced electric field at the interface of the Schottky junction achieved through the dielectric reduced surface field technique. As-fabricated NWSBD delivers a low turn-on voltage of 0.80 V, a near-unity ideality factor of 1.04, along with a soft BV of 480 V. The measured soft BV is comparable with the avalanche BV of the p-n diode with a similar net doping concentration in the drift region.

DOI： 10.35848/1347-4065/ac06b5

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/pssa.202100329

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

We have demonstrated non-polar GaN power diodes (Schottky barrier diode and p-n junction diode) on foreign substrates featuring the true-lateral p-n and metal-semiconductor junctions. The diodes were fabricated on GaN islands laterally overgrown on the mask-patterned sapphire and Si substrates by metalorganic vapor phase epitaxy. The anode and cathode were formed on the opposed a-plane sidewalls of the island, making the device architecture essentially like the 90° rotation of the desired true-vertical power diodes. The ideality factor of the Schottky barrier diode remained 1.0 (from 1.00 to 1.05) over 7 decades in current. Specifically, a high critical electric field of 3.3 MV/cm was demonstrated on the p-n junction diode with avalanche capability. These performances reveal a strong potential of non-polar GaN with the true-lateral junctions for high power applications.

DOI： 10.1063/5.0051552

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Chemical Science  

A challenging approach, but one providing a key solution to material growth, remote epitaxy (RE)—a novel concept related to van der Waals epitaxy (vdWE)—requires the stability of a two-dimensional (2-D) material. However, when graphene, a representative 2-D material, is present on substrates that have a nitrogen atom, graphene loss occurs. Although this phenomenon has remained a hurdle for over a decade, restricting the advantages of applying graphene in the growth of III-nitride materials, few previous studies have been conducted. Here, we report the stability of graphene on substrates containing oxygen or nitrogen atoms. Graphene has been observed on highly decomposed Al2O3; however, graphene loss occurred on decomposed AlN at temperatures over 1300 °C. To overcome graphene loss, we investigated 2-D hexagonal boron nitride (h-BN) as an alternative. Unlike graphene on AlN, it was confirmed that h-BN on AlN was intact after the same high-temperature process. Moreover, the overgrown AlN layers on both h-BN/AlN and h-BN/Al2O3could be successfully exfoliated, which indicates that 2-D h-BN survived after AlN growth and underlines its availability for the vdWE/RE of III-nitrides with further mechanical transfer. By enhancing the stability of the 2-D material on the substrate, our study provides insights into the realization of a novel epitaxy concept.

DOI： 10.1039/d1sc01642c

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

Inductively coupled plasma–reactive ion etching (ICP–RIE)-induced damage in heavily Mg-doped p-type GaN ([Mg] = 2 × 1019 cm−3) was investigated by low-temperature photoluminescence (PL) and depth-resolved cathodoluminescence (CL) spectroscopy. From PL measurements, we found broad yellow luminescence (YL) with a maximum at around 2.2–2.3 eV, whose origin was considered to be isolated nitrogen vacancies (VN), only in etched samples. The depth-resolved CL spectroscopy revealed that the etching-induced YL was distributed up to the electron-beam penetration depth of around 200 nm at a high ICP–RIE bias power (Pbias). Low-bias-power (low-Pbias) ICP–RIE suppressed the YL and its depth distribution to levels similar to those of an unetched sample, and a current–voltage characteristic comparable to that of an unetched sample was obtained for a sample etched with Pbias of 2.5 W.

DOI： 10.35848/1347-4065/abd538

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

The electroluminescence (EL) uniformity of AlGaN-based deep UV laser diodes on AlN substrate was analyzed by using the EL imaging technique. Although nonuniform EL patterns were observed, the uniformity was improved by changing the position of the p-electrode. The threshold current density was also reduced by suppressing the inhomogeneity of the EL. Cathodoluminescence analysis revealed that the cause of the non-uniformity is the degradation of the active layer and the nonuniformity emission formed by rapid thermal annealing at high temperature after mesa structure formation.

DOI： 10.35848/1882-0786/abf443

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Laser and Photonics Reviews  

Typical light-emitting diodes (LEDs) have a form factor >(300 × 300) µm2. Such LEDs are commercially mature in illumination and ultralarge displays. However, recent LED research includes shrinking individual LED sizes from side lengths >300 µm to values <100 µm, leading to devices called micro-LEDs. Their advent creates a number of exciting new application spaces. Here, a review of the principles and applications of micro-LED technology is presented. In particular, the implications of reduced LED size in necessitating mitigation strategies for nonradiative device edge damage as well as the potential for higher drive current densities are discussed. The opportunities to integrate micro-LEDs with electronics, and into large-scale arrays, allow pixel addressable scalable integrated displays, while the small micro-LED size is ideal for high-speed modulation for visible light communication, and for integration into biological systems as part of optogenetic therapies.

DOI： 10.1002/lpor.202000133

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

Ga-rich zones created along macrosteps in n-AlGaN plausibly function as electron pathways of AlGaN-based deep-ultraviolet (DUV) LEDs fabricated on AlN templates using 1.0°-miscut c(0001) sapphire substrates toward the m[1-100] axis. This work was performed to clarify AlN mole fractions (xAl) of Ga-rich zones. xAl ≃ (7/12, 6/12, and 5/12) was observed in Ga-rich zones in AlαGa1−αN (α ≃ 0.63, 0.55, and 0.43, respectively) by the method proposed in our previous article in which we showed that Ga-rich zones of Al8/12Ga4/12N were created in Al0.7Ga0.3N. xAl in the Ga-rich zones obtained from an energy-dispersive x-ray signal by scanning transmission electron microscopy calibrated by Rutherford backscattering well agreed with xAl obtained by cross-sectional cathodoluminescence (CL) spectroscopy using scanning electron microscopy. A weak CL shoulder peak corresponding to Al4/12Ga8/12N was also observed for Al0.43Ga0.57N. In addition, xAl ≃ n/12 (n = 6-9) in Al-rich zones appeared in the rest of the Ga-rich zones. Furthermore, nanobeam electron diffraction patterns of the Ga-rich zones indicated a high possibility of a regular configuration of Ga and Al atoms on the c(0001) plane in our samples. Consequently, xAl values in nonflat AlGaN layers with macrosteps were often determined to be near n/12 (n: integer). Thus, Ga-rich zones (xAl = n/12: n = 4-8) formed in our nonflat AlGaN layers, which originated from the macrosteps along [11-20] edgelines normal to the m[1-100] axis, are suggested to be metastable. The creation of discrete xAl in Ga-rich zones should contribute to the stable production of DUV-LEDs using high-miscut sapphire substrates.

DOI： 10.1063/5.0042036

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

We report the first experimental demonstration of microwave oscillation in GaN impact ionization avalanche time transit (IMPATT) diodes at the X-band. The device used in this study is a single drift diode with a p+–n simple abrupt junction and vertical mesa termination. The reverse I–V characteristic of the diode shows low leakage current, clear avalanche breakdown, and high avalanche capability, as required for IMPATT operation. Microwave testing is performed in an X-band waveguide circuit with a reduced-height waveguide resonant cavity. Oscillations are observed at 9.52 GHz at a power of ∼56 mW.

DOI： 10.35848/1882-0786/abe3dc

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ECS Journal of Solid State Science and Technology  

We optimized Ni/Ag-based p-type reflectors for the improvement of efficiency of 273 nm deep ultraviolet (DUV) AlGaN-based flip-chip light emitting diodes (FCLEDs). The Ni(3 nm)/Ag(5-15 nm)/Al/Ni and Ni(25-50 nm)/Ag/Ni contacts exhibited higher reflectance (36.4-39.5%) at 273 nm than reference Ni(5 nm)/Au(5 nm)/Al/Ni contact (26.1%). The Ni(3 nm)/Ag/Al(200 nm)/Ni(20 nm) and Ni/Ag(200 nm)/Ni(20 nm)-based FCLEDs gave forward voltages in the rage of 6.93-7.11 V and 5.5-6.28 V at 20 mA, respectively, whereas the Ni/Au-based sample showed 6.35 V. Further, the Ni(3 nm)/Ag(10 nm)/Al/Ni-based and Ni(50 nm)/Ag(200 nm)/Ni-based samples exhibited 4.85% and 13.4% larger output power at 1.2 W than the reference sample. The Ni(3 nm)/Ag(10 nm)-based and Ni(50 nm)/Ag(200 nm)/Ni-based samples produced 5.6% and 8.5% higher peak external quantum efficiency than the reference sample. It was further shown that the Ni(3 nm)/Ag(10 nm)/Al/Ni-based and Ni(50 nm)/Ag(200 nm)/Ni-based samples experienced less efficiency droop (namely, 27.9 and 26.4%, respectively) than the reference sample (31.4%). Based on the scanning transmission electron microscopy and X-ray photoemission spectroscopy results, the ohmic formation mechanism is described and discussed.

DOI： 10.1149/2162-8777/abf49b

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Crystals  

Numerical simulations are a valuable tool for the design and optimization of crystal growth processes because experimental investigations are expensive and access to internal parameters is limited. These technical limitations are particularly large for ammonothermal growth of bulk GaN, an important semiconductor material. This review presents an overview of the literature on simulations targeting ammonothermal growth of GaN. Approaches for validation are also reviewed, and an overview of available methods and data is given. Fluid flow is likely in the transitional range between laminar and turbulent; however, the time-averaged flow patterns likely tend to be stable. Thermal boundary conditions both in experimental and numerical research deserve more detailed evaluation, especially when designing numerical or physical models of the ammonothermal growth system. A key source of uncertainty for calculations is fluid properties under the specific conditions. This originates from their importance not only in numerical simulations but also in designing similar physical model systems and in guiding the selection of the flow model. Due to the various sources of uncertainty, a closer integration of numerical modeling, physical modeling, and the use of measurements under ammonothermal process conditions appear to be necessary for developing numerical models of defined accuracy.

DOI： 10.3390/cryst11040356

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Reports  

GaN nanorods (NRds) with axial InGaN/GaN MQWs insertions are synthesized by an original cost-effective and large-scale nanoimprint-lithography process from an InGaN/GaN MQWs layer grown on c-sapphire substrates. By design, such NRds exhibit a single emission due to the c-axis MQWs. A systematic study of the emission of the NRds by time-resolved luminescence (TR-PL) and power dependence PL shows a diameter-controlled luminescence without significant degradation of the recombination rate thanks to the diameter-controlled strain tuning and QSCE. A blueshift up to 0.26 eV from 2.28 to 2.54 eV (543 nm to 488 nm) is observed for 3.2 nm thick InGaN/GaN QWs with an In composition of 19% when the NRds radius is reduced from 650 to 80 nm. The results are consistent with a 1-D based strain relaxation model. By combining state of the art knowledge of c-axis growth and the strong strain relieving capability of NRds, this process enables multiple and independent single-color emission from a single uniform InGaN/GaN MQWs layer in a single patterning step, then solving color mixing issue in InGaN based nanorods LED devices.

DOI： 10.1038/s41598-021-86139-9

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Crystal Growth and Design  

To improve the properties of semiconductors, it is necessary to construct an integrated crystal growth model that covers all elementary processes of metal-organic vapor phase epitaxy (MOVPE). Although there are several theoretical models that can reproduce any elemental growth process, they are inadequate for controlling semiconductor epitaxy: the elementary processes of (1) the vapor phase reaction, (2) the surface reaction, and (3) incorporation are entangled with each other. That is, sequential analyses of elementary growth processes from upstream (1) to downstream (3) are indispensable for an understanding of the entire process of MOVPE. In this Review, the recent progress of theoretical models based on calculations from first-principles calculations are summarized. The possibility of predicting carbon concentrations in GaN grown by MOVPE are explored as an example. The results of calculations using a model that integrates (1) → (2) → (3) reproduce the experimental tendencies of carbon incorporation. Calculations show that the contribution of each elementary growth process to a change in carbon concentration can be discussed separately, but the relationship between the input parameters and the resulting outputs can only be determined through experiment. Although this examination explores a special case, the development of a precise integrated crystal growth model would greatly contribute to innovation in semiconductor manufacturing.

DOI： 10.1021/acs.cgd.0c01564

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

Photo-electrochemical (PEC) etching is a promising technique for fabricating GaN microelectromechanical systems devices. In this study, we demonstrate the fabrication of GaN cantilevers by the bandgap-selective PEC etching of an InGaN superlattice sacrificial layer. By using an InGaN superlattice as a sacrifice layer, we found the PEC etching rate became higher than using a normal InGaN layer. As a result, the InGaN superlattice was completely etched and we fabricated GaN-based cantilevers whose resonance characteristics were measured. The Young’s modulus of GaN was determined from the resonance characteristics of GaN cantilevers to be the same as the highest value reported previously.

DOI： 10.35848/1882-0786/abe657

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Crystals  

Thermal boundary conditions for numerical simulations of ammonothermal GaN crystal growth are investigated. A global heat transfer model that includes the furnace and its surroundings is presented, in which fluid flow and thermal field are treated as conjugate in order to fully account for convective heat transfer. The effects of laminar and turbulent flow are analyzed, as well as those of typically simultaneously present solids inside the autoclave (nutrient, baffle, and multiple seeds). This model uses heater powers as a boundary condition. Machine learning is applied to efficiently determine the power boundary conditions needed to obtain set temperatures at specified locations. Typical thermal losses are analyzed regarding their effects on the temperature distribution inside the autoclave and within the autoclave walls. This is of relevance because autoclave wall temperatures are a convenient choice for setting boundary conditions for simulations of reduced domain size. Based on the determined outer wall temperature distribution, a simplified model containing only the autoclave is also presented. The results are compared to those observed using heater-long fixed temperatures as boundary condition. Significant deviations are found especially in the upper zone of the autoclave due to the important role of heat losses through the autoclave head.

DOI： 10.3390/cryst11030254

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Quantum Beam Science  

The development of pulsed electron sources is applied to electron microscopes or electron beam lithography and is effective in expanding the functions of such devices. The laser photocathode can generate short pulsed electrons with high emittance, and the emittance can be increased by changing the cathode substrate from a metal to compound semiconductor. Among the substrates, nitride-based semiconductors with a negative electron affinity (NEA) have good advantages in terms of vacuum environment and cathode lifetime. In the present study, we report the development of a photocathode electron gun that utilizes photoelectron emission from a NEA-InGaN substrate by pulsed laser excitation, and the purpose is to apply it to material nanofabrication and high-speed observation using a pulsed transmission electron microscope (TEM) equipped with it.

DOI： 10.3390/qubs5010005

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

The leakage current caused by the Si pileup at the regrowth interface of AlGaN/GaN high electron mobility transistors (HEMTs) is significantly suppressed by the semi-insulating Mg-doped GaN layer. Mg is unintentionally doped and can be originated from the graphite susceptor of metal organic vapor phase epitaxy. Before regrowth of the AlGaN/GaN heterostructure, the GaN template is treated with hydrochloric acid (HCl) and hydrogen peroxide/potassium hydroxide (H2O2/KOH) solution to promote the adsorption of Mg on the GaN surface. The Mg-doped GaN channel layer is highly resistive due to the passivation of hydrogen. The p-n junction formed by the weakly p-type Mg-doped GaN layer and the n-type Si pileup layer depletes the excess electrons at the regrowth interface. As a result, the off-state drain leakage current of the HEMT device can be decreased by two orders of magnitude at 40 V.

DOI： 10.1063/5.0034584

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Optical Materials Express  

We correct an error in the Eq. (1).

DOI： 10.1364/OME.420328

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：2021 5th IEEE Electron Devices Technology and Manufacturing Conference, EDTM 2021  

We reported the first-time utilization of ELO (epitaxial lateral overgrowth) GaN (gallium nitride) for power diodes. The undesired stage of coalescence related to ELO is avoided by virtue of a novel 3D device architecture built on the ELO GaN islands on foreign substrate which features pure-lateral p-n and n+ -n-junctions and electrodes lying on the opposing sidewalls of the island. Excellent electrical performance was demonstrated, revealing a strong potential of ELO GaN with 3D device architecture for power switching applications.

DOI： 10.1109/EDTM50988.2021.9420859

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Journal of the Electron Devices Society  

Traditional lumped small signal equivalent circuit models of AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOS-HEMTs) are made up of constant valued circuit elements. Such models are unable to capture the high frequency behavior (above 20 GHz) of the device. In this work, a modified small signal equivalent circuit model of AlGaN/GaN MOS-HEMTs is presented. The key feature of the proposed model is that the values of the different circuit elements in the model are considered to be frequency dependent in nature and not constants. The frequency dependent value of each circuit element is mathematically represented using polynomial functions where the coefficients of the functions are determined via a least-square curve fitting approach. This frequency dependent attribute of the circuit element values ensures that the proposed model is very accurate at high frequencies without sacrificing the compactness of the model topology. The accuracy of the proposed model has been verified up to 50 GHz using experimentally measured Y-parameters of AlGaN/GaN MOS-HEMTs having a different gate dielectric and gate length.

DOI： 10.1109/JEDS.2021.3081463

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

DOI： 10.1117/12.2575872

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

DOI： 10.1117/12.2578134

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/5.0028516

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Physics D: Applied Physics  

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm - due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.

DOI： 10.1088/1361-6463/aba64c

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

To clarify the behavior of the AlGaN in 20 nm wide Ga-rich current pathways in an n-AlGaN layer, which assists carrier localization in AlGaN-based light-emitting diodes, we performed a detailed analysis using an n-Al0.7Ga0.3N layer on AlN with dense macrosteps on a 1.0 miscut sapphire substrate. Energy-dispersive X-ray spectra, obtained using cross-sectional scanning transmission electron microscopy calibrated by Rutherford backscattering and cross-sectional cathodoluminescence spectra, indicated that AlN mole fraction in the Ga-rich current pathways was nearly ∼2/3. This result is consistent with those of other research groups, suggesting that metastable Al2/3Ga1/3N is created in Ga-rich current pathways.

DOI： 10.35848/1882-0786/abcb49

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Surfaces and Interfaces  

III-V compound semiconductor-based light emitting devices (LEDs) operating in ultraviolet to visible wavelength ranges are greatly important for their applications, including displays, solid-state lighting, biomedical applications, optogenetics, and high bandwidth visible light communications. The performance of such LEDs is unavoidably affected by the surface characteristics of semiconductor layers. In particular, for LEDs, plasma-etching processes are inevitably adopted to define mesas, via holes and waveguides. The presence of surface defects is exceedingly detrimental to the device performance. It is therefore vital to understand the passivation mechanisms and approaches in order to control and so to maximize the efficiency. Here, we review recent progress in the surface treatment and passivation of III-V compound semiconductors-based LEDs. We show how different types of passivation approaches, including treatments with aqueous solutions, dielectric materials, or combination of both, affect the surface states and plasma-induced defects and hence the electrical and optical performance of LEDs.

DOI： 10.1016/j.surfin.2020.100765

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Reports  

Core–shell nanorods (NRs) with InGaN/GaN quantum wells (QWs) are promising for monolithic white light-emitting diodes and multi-color displays. Such applications, however, are still a challenge because intensity of the red band is too weak compared with blue and green. To clarify this problem, we measured photoluminescence of different NRs, depending on power and temperature, as well as with time resolution. These studies have shown that dominant emission bands come from nonpolar and semipolar QWs, while a broad yellow-red band arises mainly from defects in the GaN core. An emission from polar QWs located at the NR tip is indistinguishable against the background of defect-related luminescence. Our calculations of electromagnetic field distribution inside the NRs show a low density of photon states at the tip, which additionally suppresses the radiation of polar QWs. We propose placing polar QWs inside a cylindrical part of the core, where the density of photon states is higher and the well area is much larger. Such a hybrid design, in which the excess of blue radiation from shell QWs is converted to red radiation in core wells, can help solve the urgent problem of red light for many applications of NRs.

DOI： 10.1038/s41598-020-76042-0

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Semiconductor Science and Technology  

We show evidence that tunnel junctions (TJs) in GaN grown by metal-organic vapor phase epitaxy are dominated by defect level-assisted tunneling. This is in contrast with the common belief that highly doped layers (>1020 cm−3) are required to narrow the TJ space charge region and promote the band-to-band tunneling. Our conclusion stems from the study and the review of the major doping limitations of carefully optimized p++ and n++ layers. The secondary ions mass spectroscopy profiles of GaN based TJ LEDs show a strong oxygen concentration located close to the p++/n++ interface, typical for three dimensional growth. In addition, considering the doping limitation asymmetry and Mg carry-over, our simulations indicate a depletion region of more than 10 nm which is buried in a rough and defective n++ layer. However, decent electrical characteristics of the studied TJ based LEDs are obtained, with a low penalty voltage of 1.1 V and a specific differential resistance of about 10-2 Ω.cm2 at 20 mA. This indicates that a common TJ could be greatly optimized by using a moderate doping (∼1019 cm−3) while intentionally introducing local defects within the TJ.

DOI： 10.1088/1361-6641/abad73

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Photonics Research  

In this work, we analyze and model the effect of a constant current stress on an ultraviolet light-emitting diode with a nominal wavelength of 285 nm. By carrying out electrical, optical, spectral, and steady-state photocapacitance (SSPC) analysis during stress, we demonstrate the presence of two different degradation mechanisms. The first one occurs in the first 1000 min of stress, is ascribed to the decrease in the injection efficiency, and is modeled by considering the defect generation dynamics related to the de-hydrogenation of gallium vacancies, according to a system of three differential equations; the second one occurs after 1000 min of stress and is correlated with the generation of mid-gap defects, for which we have found evidence in the SSPC measurements. Specifically, we detected the presence of deep-level states (at 1.6 eV) and mid-gap states (at 2.15 eV), indicating that stress induces the generation of non-radiative recombination centers.

DOI： 10.1364/PRJ.401785

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ACS applied materials &amp; interfaces  

Hexagonal boron nitride (hBN) and diamond are promising materials for next-generation electronics and optoelectronics. However, their combination is rarely reported. In this study, we for the first time demonstrate the success to direct growth of two-dimensional (2D) hBN crystal layers on diamond substrates by metalorganic vapor phase epitaxy. Compared with the disordered growth we found on diamond (100), atomic force microscopy, X-ray diffraction, and transmission electron microscopy results all support 2D hBN with highly oriented lattice formation on diamond (111). Also, the epitaxial relationship between hBN and diamond (111) substrate is revealed to be [0 0 0 1]hBN // [1 1 1]diamond and [1 0 1̅ 0]hBN // [1 1 2̅]diamond. The valence band offset at hBN/diamond (111) heterointerface determined by X-ray photoelectron spectroscopy is 1.4 ± 0.2 eV, thus yielding a conduction band offset of 1.0 ± 0.2 eV and type II staggered band alignment with a bandgap of 5.9 eV assumed for hBN. Furthermore, prior thermal cleaning of diamond in a pure H2 atmosphere smoothens the surface for well-ordered layered hBN epitaxy, while thermal cleaning in a mixed H2 and NH3 atmosphere etches the diamond surface, creating many small faceted pits that destroy the following epitaxy of hBN.

DOI： 10.1021/acsami.0c11883

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

The space charge density profile of the nondoped AlGaN-based p-type cladding layer for UV-C laser diodes realized by distributed polarization doping is examined theoretically and experimentally. The analysis of the capacitance-voltage measurement revealed that the average effective acceptor density of 4.2×1017 cm-3 is achieved even without impurity doping, and it is in good agreement with the theoretical prediction from the measured Al composition profile. This result suggests that the cladding layer is ideal for UV-C LDs because it provides sufficient hole injection while potentially avoiding internal losses due to impurity doping.

DOI： 10.1063/5.0027789

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Optical Materials Express  

Wide bandgap semiconductors are increasingly important for bioimaging applications, as they can possess good biocompatibility and host a large range of fluorescent defects spanning the visible to infrared. Gallium nitride is one promising host for photostable fluorophores. In particular, neodymium (Nd)-doped gallium nitride (GaN) shows bright near-infrared fluorescence and narrow room temperature linewidth and is therefore a candidate material for fluorescent probes for bioimaging. To explore the conditions necessary to generate biomarkers based on Nd:GaN, this paper reports the room temperature photoluminescence (PL) properties of small ensembles of Nd ions implanted into the nanoscale regions of GaN epilayers. The minimum volume of Nd-implanted GaN that can be optically detected in this study is about 8×104 nm3 and the minimum detected ensemble of Nd ions is about 4×103, although not all of implanted Nd ions activate as luminescence centers. We show from the PL excitation spectra that the strongest resonant excitation appears at 619 nm, attributed to the 4I9/2 → 4G5/2 (4G7/2) transition in the 4f -shell. We measure the luminescence lifetime to be several tens of microseconds. We also identify the presence of a different excitation mechanism from the resonant excitation when excited below 510 nm (above 2.43 eV).

DOI： 10.1364/OME.401765

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

Multiple-quantum-well diodes (MQW diodes) naturally have key functionalities of light emission, transmission, modulation and detection all at the same time, enabling new forms of information and energy conversion between photons and electrons. When approximately biased and illuminated at the same time, a multifunctional MQW diode allows simultaneous light emission and detection within a single diode. A single-chip imaging system that uses a 4 × 4 MQW diode array is experimentally demonstrated here that can simultaneously transmit light and capture an image, showing the great development potential of advanced monolithic III-nitride information systems.

DOI： 10.35848/1882-0786/abb786

Scopus

担当区分：筆頭著者   記述言語：日本語   掲載種別：研究論文（学術雑誌）  

その他リンク： https://www.journal.ieice.org/bin/pdf_link.php?fname=k103_10_1016&lang=J&year=2020

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms  

Since Rare-Earth (RE) doped Gallium Nitride (GaN) is expected to be used as electrically driven single photon source at room temperature, low dose RE-ion implantation and their activation as luminescent centers are of interest. This paper reports photoluminescence (PL) properties of Praseodymium (Pr) implanted GaN at different temperatures ranging from room temperature to 1200 °C. All the Pr-implanted GaN samples are thermally annealed after implantation and show strong PL emissions at 650.2 nm and 652.0 nm, originated from 3P0→3F2 transition in 4f-shell of Pr3+ ions. It is shown that the PL intensity originating from Pr3+ ions is reduced as the implantation temperature increases for the Pr-implanted samples annealed at 1200 °C. This result suggests that Pr3+ ions quench due to the formation of complex defects and defect clusters. The effect of high temperature implantation on the recovery of GaN crystallinity is discussed in terms of critical dose and displacement damage.

DOI： 10.1016/j.nimb.2020.06.007

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

Ni/Al2O3/GaN structures with vicinal GaN surfaces from the c- or m-plane were formed. Then, electrical interface properties of the structures were systematically investigated. It was found that interface state density (Dit) at the Al2O3/GaN interface for the c-plane is higher than that for the m-plane, and post-metallization annealing is quite effective to reduce Dit for both c- and m-planes. As a result, the low Dit value of ∼3 × 1010 eV-1 cm-2 was demonstrated for both planes.

DOI： 10.1063/5.0010774

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

We present an optical modeling and characterization study of prototype ultraviolet laser diode (LD) structures grown on single-crystal AlN substrates, with focus on the reduction of modal loss caused by optical mode coupling to the absorptive layers on the p-side (i.e. p-contact and p-metal layers). The transparent AlN substrates enabled optical pumping for measuring modal loss without requiring functioning LDs. The modal loss measured in this way was in good agreement with electrically evaluated results of processed LDs, and both results were consistent with optical modeling predictions. By using 0.32 μm thick p-side cladding, we were able to suppress the modal loss of the designed LD structure to 8.4 cm-1, where the contribution from the absorptive p-contact layers was less than 3 cm-1.

DOI： 10.35848/1347-4065/abaac6

Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevApplied.14.024018

Web of Science

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

The halide vapor phase epitaxy (HVPE) of Mg-doped GaN using solid MgO is investigated. Thermodynamic analysis of the reactions amongst MgO, HCl and N2 is performed based on first-principles calculations. It is found that the equilibrium partial pressure of MgCl2 is the highest amongst magnesium related molecules at 900 °C. By increasing the input partial pressure of HCl, the pressure of MgCl2 is increased, which agrees well with recently reported experiments. From these results, it is concluded that MgCl2 is the key molecule which plays the most important role for Mg-doping in fabricating p-type GaN using HVPE with MgO.

DOI： 10.35848/1347-4065/aba0d5

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

We report the electrical and optical properties of gallium nitride grown by halogen-free vapor phase epitaxy (HF-VPE). The electron mobility of the HF-VPE-GaN layers was found to be comparable to or better than the GaN layers obtained using MOCVD. The positron annihilation spectroscopy analyses revealed that the density of the electroneutral or negatively charged vacancy-type defects in the HF-VPE-GaN layers was below the detection limit (≤1015 cm-3), equivalent to that of a defect-free hydride vapor phase epitaxy (HVPE)-GaN reference sample. Our study shows that the HF-VPE technique can be employed to achieve high-quality and cost-effective bulk crystal and epitaxial layer growth for GaN devices.

DOI： 10.35848/1882-0786/aba494

Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Materials Chemistry C  

The impacts of pulsed-flow growth on aluminium incorporation in polar (0001), semipolar (1013) and (1122), as well as nonpolar (1010) AlGaN layers have been investigated. The layers were grown simultaneously on differently oriented AlN/sapphire templates by metal-organic vapour phase epitaxy. The AlN mole fraction (0 < xAlN ≤ 0.85) of the layers was varied by simply changing the supply time of the aluminium precursor while keeping nitrogen and gallium precursors constant. Phase separation has been observed for the (0001) and (1122) layers by X-ray diffraction, and is attributed to their different surface reconstructions during growth. In contrast, no phase separation has been observed for the (1010) and (1013) layers, attributed to their stable surfaces during growth. The AlN mole fraction of the differently oriented layers generally follows the order: (1122) < (0001) < (1013) ≤ (1010), attributed to their different surface dangling-bond densities. By means of room-temperature photoluminescence measurements, high carbon-incorporation has quantitatively been found in all the layers. This journal is

DOI： 10.1039/d0tc01369b

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/5.0010664

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

Current-voltage-temperature characteristics (0 °C to 150 °C) of SBDs on highly compensated 15 μm and 30 μm n-type GaN drift layer were measured for voltages up to-300 V and up to-800 V, respectively. When the temperature is between 75 °C and 100 °C, both SBDs exhibited a similar change in conduction mechanism from thermionic field emission (TFE) to thermionic emission (TE) due to the activation of N-vacancies (VN) (Ea =-1.67 ± 0.02 eV). However, at high voltages when the temperature is >100 °C, the conduction mechanism changes from TE to TFE due to the de-trapping of electrons from activated carbon-traps (Ea = +0.69 eV) in the grown drift layers.

DOI： 10.35848/1882-0786/ab93a0

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記述言語：日本語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physica Status Solidi (A) Applications and Materials Science  

The properties of phase-separated MgZnO induced by heat treatment are investigated, because the electrical conductivity of MgZnO deposited by sputtering can be improved by heat treatment. The absorption edge shows a red shift, but the transmittance in the ultraviolet (UV)-C region increases after heat treatment. This is due to the formation of wurtzite (WZ) MgZnO with a high Zn content and a rock salt (RS) structure with a low Zn content induced by phase separation. This suggests that the current path is formed by the WZ crystals with low resistivity, and the transmittance in the UV region is increased by the RS crystals with high transmittance. It is confirmed that the RS structure is partially mixed with the WZ crystal in the in-plane direction. In contrast, different distributions of crystals are observed in the stacking direction. A WZ structure is formed after RS-MgZnO is formed on the substrate interface. It is found that Mg is incorporated into the RS structure by heat treatment, and Mg-rich MgZnO is formed.

DOI： 10.1002/pssa.201900955

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Crystal Growth  

Lattice bow generated by 40 µm thick HVPE homoepitaxial layers on commercial free-standing, ammonothermal and HVPE GaN wafers was studied. While a change in lattice bow was measured for all wafers, the additional bow on the ammonothermal GaN wafers was minimal. The main driving force for the observed increase in the lattice bow for HVPE wafers was related to stress in the films generated by the elongation of dislocations via climb and generation of new dislocations at the homoepitaxial interface. Lattice bow is a crucial wafer parameter as it determines the variation of the offcut across the surface. If an offcut variation of 0.1° is allowed for desired control surface morphology, composition of alloys, and uniformity of doping on this surface, the measured bow on the two HVPE GaN wafers and one ammonothermal GaN wafer limits their uniformity-diameter to ~0.5″, 1″ and >4″, respectively.

DOI： 10.1016/j.jcrysgro.2020.125643

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

Halide vapor phase epitaxy of p-Type GaN:Mg films was realized by using solid MgO as the Mg source. The Mg concentration was controlled by supplying HCl gas in a MgO source zone. Mg-related photoluminescence peaks were observed at around 3.3 and 2.9 eV. For a sample with a Mg concentration of 2.8 × 1019 cm-3, the Hall-effect measurement showed p-Type conduction with a hole concentration and a hole mobility of 1.3 × 1017 cm-3 and 9.1 cm2 V-1 s-1, respectively, at room temperature. The Mg acceptor level was 232 15 meV, which is in good agreement with the previous report.

DOI： 10.35848/1882-0786/ab9166

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記述言語：日本語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physica Status Solidi - Rapid Research Letters  

The oxygen incorporation kinetics of vicinal m(10−10) gallium nitride (GaN) growth during metal-organic vapor phase epitaxy is clarified using a diffusion equation-based approach that incorporates diffusion potentials obtained by large-scale density functional theory (DFT) calculations. A diffusion model based on the Burton, Cabrera and Frank (BCF) theory is proposed, and then, the oxygen concentration in the epitaxial films is calculated quantitatively. The calculation results agree with the experimental tendency that the oxygen concentration in the −c 5° off m-GaN epilayers is lower than that in the +c 5° off m-GaN epilayers. Then, the off-angle dependence of oxygen incorporation in vicinal m-GaN growth is predicted.

DOI： 10.1002/pssr.202000142

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.3390/mi11050519

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PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ECS Journal of Solid State Science and Technology  

We investigated the effect of atomic layer deposition (ALD) Al2O3 (50 nm)/plasma-enhanced chemical vapour deposition (PECVD) SiO2 (250 nm) and PECVD SiO2 (300 nm) passivation layers on the leakage current and efficiency of InGaN-based micro-LEDs with different sizes. Regardless of passivation layers, the leakage current increased with decreasing LED size and increasing reverse bias. Emission microscopy examination showed that with increasing reverse bias, the number of defect-related emission spots and their intensities increased. For the micro-LEDs <50 μm, the emission spots were mainly located at the sidewall regions. Above-10 V, the single PECVD SiO2 passivation layer gave higher leakage current than the double ALD-Al2O3/PECVD-SiO2 layer. The micro-LEDs with the single passivation layer gave the ideality factors of about 2.0, while that with the double layer exhibited values smaller than 2.0. The micro-LEDs with the double passivation layer exhibited external quantum efficiency peaks at lower current density compared to those with the single layer. It was shown that smaller micro-LEDs were more sensitively dependent on the types of the passivation layers. These results exhibit that the ALD-Al2O3/PECVD-SiO2 passivation layer is more effective in suppressing the sidewall damage-induced current than the PECVD-SiO2 layer.

DOI： 10.1149/2162-8777/ab915d

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

Magnesium (Mg) ion implantation into gallium nitride (GaN) at 1000 °C is proposed. Since ion implantation and annealing occur simultaneously in high-Temperature implantation, it is considered that Mg ions can be introduced at an appropriate position upon their implantation. GaN vertical diodes implanted with Mg ions were fabricated and current-voltage measurements were performed. As a result, clear rectifying properties were confirmed in a sample that was implanted with Mg ions at 1000 °C and annealed after implantation. However, the sample subjected to Mg ion implantation at RT and annealed after implantation showed no clear rectification. These results show that high-Temperature implantation of Mg ions at 1000 °C reduces Mg ion implantation-induced damage and simultaneously activates Mg ions upon their implantation.

DOI： 10.35848/1347-4065/ab8b3d

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Express  

Using density functional calculations, we clarify the oxygen incorporation mechanism in vicinal m-GaN growth by metal-organic vapor-phase epitaxy. We first identify reconstructed structures of 5° off m-GaN toward the ±c directions. Next, we explore preferable sites for oxygen substitution near step edges. We find that oxygen prefers the lower nitrogen site of the step edge on the +c 5° off m-GaN substrate compared with that on the-c 5° off m-GaN substrate. This tendency agrees with recent experimental findings that the oxygen concentration in-c 5° off m-GaN epilayers is lower than that in +c 5° off m-GaN epilayers.

DOI： 10.35848/1882-0786/ab8723

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1149/2162-8777/ab8b6f

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevMaterials.4.044602

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1039/c9ce01971e

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Photonics Technology Letters  

We investigated how the performance and reliability of AlGaInP-based red (620 nm) micro-light-emitting diodes (micro-LEDs) (25×17 μ m2) were influenced by the use of n-AlInP ohmic layer and distributed Bragg reflector (DBR). The AlGaAs-based DBR showed reflectivity of 94.9% at 622 nm with a 14 nm-stopband width. The micro-LEDs with n-GaAs gave slightly lower forward voltages by 0.013 - 0.021 V than those with n-Al0.5In0.5P/ n-Al0.6Ga0.4As and nAl0.5In0.5P/DBR. However, the micro-LEDs with n-Al0.5In0.5P/ n-Al0.6Ga0.4As and n-Al0.5In0.5P/DBR gave 61% and 125% higher light output power at 20μ A compared with that with n-GaAs. It was shown that after annealing at 120 °C for 2,000 h, the forward voltage and the light output power at 4.7 A/cm2 of the micro-LEDs with n-Al0.5In0.5P/DBR were degraded by 0.09% and 6.33%, respectively, with reference to those before annealing.

DOI： 10.1109/LPT.2020.2977376

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Crystal Growth  

The integration of gallium nitride with low-cost, transparent substrates such as glass and crystallographically incompatible substrates such as Si(0 0 1) has been long desired for III-N electronics. Here the authors demonstrate how GaN growth on uniformly 〈1 1 1〉 oriented silicon seed layers fabricated by aluminum-induced crystallization provide a straightforward method for realizing large area GaN growth on these substrate by adopting well-developed GaN on Si(1 1 1) growth processes. Critical factors for promoting high quality GaN growth are identified, and remaining steps necessary for realizing GaN devices are discussed.

DOI： 10.1016/j.jcrysgro.2020.125522

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physica Status Solidi (B) Basic Research  

Herein, a newly developed birefringence microscope is used to observe dislocations in gallium nitride (GaN) substrates. The comparison results of the observation method using this microscope with other observation methods, such as X-ray topography and Raman microscopy, confirms that dislocations in a GaN substrate can be detected with a birefringence microscope. In addition, the observation can be carried out as easily as with an optical microscope. It is also found that under certain observation conditions, the direction of the edge component of dislocations can be determined.

DOI： 10.1002/pssb.201900553

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Applied Physics Letters  

We have demonstrated an on-wafer fabrication process for AlGaN-based UV-C laser diodes (LDs) with etched mirrors and have achieved lasing for 100 ns pulsed current injection at room temperature. A combined process of dry and wet etching was employed to achieve smooth and vertical AlGaN (1 1 &macr;00) facets. These etched facets were then uniformly coated with a distributed Bragg reflector by atomic layer deposition. A remarkable reduction of the lasing threshold current density to 19.6 kA / cm 2 was obtained owing to the high reflectivity of the etched and coated mirror facets. The entire laser diode fabrication process was carried out on a whole 2-in. wafer. We propose this mirror fabrication process as a viable low-cost AlGaN-based UV-C LD production method that is also compatible with highly integrated optoelectronics based on AlN substrates.

DOI： 10.1063/1.5145017

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1149/2162-8777/ab7c40

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1093/jmicro/dfz037

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PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1149/2162-8777/ab74c3

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ECS Journal of Solid State Science and Technology  

We have investigated how different types of the reflectors affected the optical and electrical performance of AlGaInP-based micro-LEDs. Simulations showed that the AlGaAs-based epitaxial distributed Bragg reflector (DBR) had a stopband at the 610–624 nm region with reflectivity of 90%, the SiO2/TiO2 dielectric DBR gave a stopband at the 580–770 nm range with a maximum reflectivity of 99%, and the ITO/Ag metal reflector exhibited reflectivity of 90% across the 400–800 nm region. All micro-LEDs gave forward voltages of 1.895–1.960 V at 20 μA. The micro-LEDs with the dielectric DBR and metal reflector yielded 31% and 13% higher light output at 20 μA than that with the epitaxial DBR, respectively. All of the micro-LEDs contained a shoulder peak at approximately 615 nm in their electroluminescence spectra. Ray-tracing simulations exhibited that the micro-LEDs with the dielectric DBR and metal reflector produced 26% and 22% higher total light output power than the one with the epitaxial DBR, respectively. It was also shown that for the micro-LEDs with the metal reflector, some of the micro-LEDs were detached from the metal reflectors due to the interfacial voids induced as a result of agglomeration of Ag layer during fabrication process.

DOI： 10.1149/2162-8777/ab74c3/pdf

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Microelectronic Engineering  

The quantum efficiency (QE) of an InGaN photocathode as a function of InGaN layer thickness (240, 100, and 70 nm) was investigated. To activate the sample surface, Cs and O were deposited on the surface in an ultrahigh–vacuum chamber. The QE for different optical power densities was measured by irradiating excitation light from the front and back sides of each sample. The QEs for InGaN layer thickness of 240, 100, and 70 nm with back-side irradiation were 0.9, 9.8, and 7.5%, respectively. For the thicknesses of 70 and 100 nm, the QEs were higher for back-side irradiation than for front-side irradiation, whereas for the thickness of 240 nm, the QE was higher for front-side irradiation. The InGaN layer thickness dependence of the QEs for back- and front-side irradiations was calculated using a continuous equation considering processes such as excitation, diffusion, recombination, and escape of electrons from the surface of the photocathode. The tendency of the experimental results, where QE was maximum at 100–120 nm, corresponded to that of the calculated results.

DOI： 10.1016/j.mee.2020.111229

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1361-6463/ab52d0

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1149/2162-8777/ab709a

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1149/2162-8777/ab709a

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1149/2162-8777/aba914

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

Vertical Schottky barrier diodes (SBD) were fabricated on 1a 5 μm thick GaN drift layer (DL) with and without Mg-compensation grown by metal organic chemical vapour deposition on free-standing hydride vapour phase epitaxy grown substrate. The SBDs with Mg-compensated DL exhibited ∼3.4 × higher breakdown voltage (V bd) than the SBDs with conventional DL. The activation energy of 0.43 eV from the SBD with Mg-compensated DL can be correlated to the presence of Mg. The reverse current conduction mechanism of SBDs with Mg-compensated DL and conventional DL was dominated by thermionic field emission (TFE) and barrier modified TFE, respectively.

DOI： 10.7567/1347-4065/ab65cd

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1116/1.5120417

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：2D Materials  

We studied the nucleation and growth of hexagonal BN (h-BN) on AlN template on c-plane sapphire by metalorganic vapor phase epitaxy as functions of growth temperature, deposition time, and triethylboron (TEB) partial pressure. A lateral growth rate of about 25 nm min-1 for h-BN nuclei was obtained by atomic force microscopy and a nucleation activation energy of 2.1 eV was extracted from the temperature dependence of the nucleation density. A large TEB flow rate strongly enhances the formation of h-BN nuclei. At a reduced TEB flow rate, we observed a significantly decreased nuclei density and a delay in nucleation due to TEB desorption. By fine tuning the growth parameters, single-crystalline multilayer h-BN was successfully formed on AlN surface, as confirmed by x-ray diffraction and transmission electron microscopy (TEM). The epitaxial relationship between h-BN and AlN was [0 0 0 1]h-BN || [0 0 0 1]AlN and [1 0 -1 0]h-BN || [1 1 -2 0]AlN from TEM and electron backscatter diffraction measurements. In addition, TEM showed that the initial h-BN layers are not parallel and tend to form half-domes. On those half-domes (cap-shaped-like) a 2D lateral growth sets on, resulting in a well-oriented 2D multilayer observed in TEM. Thus, the surface topography further develops to form a relatively flat surface without wrinkles and finally a typical hexagon-like wrinkled surface at thicker h-BN layers. Particularly, the small h-BN nuclei have dangling bonds at their periphery that can interact with the substrate, forming actual bonds with AlN. Hence the choice on the substrate is important, despite the basal planes of multilayer h-BN are bonded by a weak van der Waals force.

DOI： 10.1088/2053-1583/ab46e6

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Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ECS Journal of Solid State Science and Technology  

DOI： 10.1149/2.0452001JSS

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1361-6641/ab63f1

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.35848/1347-4065/ab6fb0

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Photonics Technology Letters  

We investigated the effect of plasma-etching on the electrical and optical performance of lateral AlGaInP-based red micro-LEDs as a function of etching time, where plasma-etched Ag particles were used as masks. For the sample etched for 12 s, around 33% of the sample surface were randomly etched, whereas for the sample etched for 24 s, nearly 60% were randomly etched with additional nanoscale hillocks. Regardless of etching times, all samples exhibited similar forward voltages of 1.944-1.929 V at $20~\mu \text{A}$ and reverse leakage currents of $1\times 10^{-8}$ A at -10 V. The micro-LEDs fabricated with 12 s-etched and 24 s-etched $p$ -GaP gave 26.2% and 42.3% higher light output powers at $20~\mu \text{A}$ , respectively, than the one with unetched $p$ -GaP. The $S$ parameter decreased with increasing forward current and etching time. The electroluminescence (EL) peak intensities of the micro-LEDs were consistent with their light output performance and included shoulder peaks at 612 nm. Emission images showed that the micro-LEDs with the 24 s-etched $p$ -GaP revealed the most intense and uniform emission area among the three samples.

DOI： 10.1109/LPT.2020.3010820

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Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：日本結晶成長学会  

<p>  The microscopic structural and optical characteristics of AlGaN light-emitting diodes (LEDs) fabricated on the AlN templates with dense macrosteps are shown to clarify the origin of their high internal quantum efficiency of radiation (IQE). The cross-sectional transmission electron microscopy observations under the high-angle annular dark field scanning mode and microscopic energy dispersive X-ray spectroscopy revealed that the AlGaN cladding layer under the AlGaN quantum well (QW) layer had a microscopic compositional modulation, which originates from the macrosteps at the AlN template surface. Moreover, Ga-rich portions in the cladding layer behaved as current micropaths, and the micropaths are connected with the carrier localization structure formed in QWs. The in-plane cathodoluminescence (CL) spectroscopy showed a significant inhomogeneity of the CL characteristics. The gentle slopes at the sample surface showed brighter emissions with a lower peak photon energy, confirming the carrier localization. This carrier localization structure in the QWs combined with the current micropaths in the cladding layer can increase the IQE as well as external quantum efficiency of the AlGaN LEDs.</p>

DOI： 10.19009/jjacg.47-3-04

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of the International Symposium on Power Semiconductor Devices and ICs  

In this work, gallium nitride (GaN) nanowire (NW) Schottky barrier diodes was fabricated using well-optimized top-down approach. As-fabricated $100 \times 800$-nm-diameter NWs SBD with high current density over 1kA/cm2 at a forward bias of 2.2V, a low differential specific ON-resistance of $0.15\mathrm{m} \Omega \cdot cm^{2}$ are demonstrated. By the virtue of dielectric Reduced Surface Field (RESURF) effect, the device also delivers a breakdown voltage of 515V, leading to a competitive Baliga's Figure of merit of 1.76 GW/cm2.

DOI： 10.1109/ispsd46842.2020.9170101

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of SPIE - The International Society for Optical Engineering  

AlGaN-based UltraViolet Light Emitting Diodes (UV LEDs) are promising devices for replacing the conventional UV lamps, which contain toxic substances like mercury, in order to have smaller devices, lower operating voltage and the possibility of tuning the emission wavelength by changing the Al and Ga content in the alloy. However, UV-LEDs may suffer from a relatively fast degradation of electrical and optical characteristics, that can be due to the generation of defects that increase the Shockley-Read-Hall (SRH) recombination components. The aim of this paper is to study the behavior of UV-B LEDs submitted to a constant current stress, through electrical, optical and spectral characterization, and capacitance deep-level transient spectroscopy (C-DLTS). The results of this analysis demonstrate that UV-B LEDs show a decrease in the driving voltage, probably correlated with the increased activation of the Mg dopant, and an increase in subthreshold forward current, ascribed to the generation of mid-gap defects caused by the stress. We also found a strong optical degradation at low current levels, that indicates the increase in SRH recombination, probably due to the increased density of mid-gap defects. To investigate on the origin of the defects, we carried out C-DLTS measurements; the results indicate the presence of Mg-related defects and/or intrinsic defects related to the GaN growth. Moreover, after stress we notice the appearance of a peak that is strictly related to the increase of mid-gap defects generated during the stress.

DOI： 10.1117/12.2544704

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本機械学会  

DOI： 10.1299/jsmemecj.2020.s16306

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 レーザー学会  

DOI： 10.2184/lsj.48.8_427

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ECS Journal of Solid State Science and Technology  

To enhance the light output of blue InGaN-based light emitting diodes (LEDs), a buried stripe-type n-electrode, expanded stripe-type p-electrode, and reflective p-bonding pad were employed. Flip-chip (FC) LEDs with the expanded p-electrode gave forward voltages of 2.99-3.11 V at 100 mA and series resistances of 3.28-3.94 Ω. The expanded p-electrode FCLED fabricated with 375 nm-thick window and TiO2 adhesion layers produced 22.7% higher light output at 21 A/cm2 than conventional FCLEDs. The expanded p-electrode FCLEDs revealed better current spreading efficiency than the c-FCLED, indicating the importance of the use of an optimised window and TiO2 adhesion layers.

DOI： 10.1149/2.0462001JSS

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Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

AlGaN-based deep-ultraviolet light-emitting diodes (LEDs) incorporating uneven multiple quantum wells (MQWs) with inclined and terrace zones, which were fabricated on an AlN template with dense macrosteps, have exhibited a high internal quantum efficiency (IQE). To investigate the microscopic structure of uneven MQWs, cathodoluminescence (CL) mapping characterization was carried out, and the maps of the CL intensity at 300 K relative to that at 38 K were obtained for uneven MQWs that targeted 265 and 285 nm LEDs. At an electron beam current of less than 1.0 nA, the signals from inclined and terrace zones of the uneven MQWs were confirmed to satisfy the nonsaturated excitation condition at 300 K. Nonradiative recombination (NR) was insufficiently frozen even at 38 K, specifically on the terraces in the 265 nm MQW, suggesting high concentrations of NR centers due to point defects (PDs). In contrast, NR in the 285 nm MQW at 38 K was closer to freeze-out. The concentration of PDs in the 285 nm MQW was likely to be lower than that in the 265 nm MQW. Finally, the ratios of the CL intensity at 300 K to those at 38 K were mapped, demonstrating an approach to creating an approximate map of IQE. The values in the CL intensity ratio maps are discussed by considering the analytical error factors. The results support the model of localized current injection through Ga-rich stripe zones in the n-AlGaN cladding layer.

DOI： 10.1063/1.5125623

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/pssb.201900554

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.7567/1882-0786/ab50e0

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1361-6641/ab4d2c

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Sensors (Switzerland)  

A low voltage (–20 V) operating high-energy (5.48 MeV) α-particle detector with a high charge collection efficiency (CCE) of approximately 65% was observed from the compensated (7.7 × 1014 /cm3) metalorganic vapor phase epitaxy (MOVPE) grown 15 µm thick drift layer gallium nitride (GaN) Schottky diodes on free-standing n+-GaN substrate. The observed CCE was 30% higher than the bulk GaN (400 µm)-based Schottky barrier diodes (SBD) at –20 V. This is the first report of α–particle detection at 5.48 MeV with a high CCE at –20 V operation. In addition, the detectors also exhibited a three-times smaller variation in CCE (0.12 %/V) with a change in bias conditions from –120 V to –20 V. The dramatic reduction in CCE variation with voltage and improved CCE was a result of the reduced charge carrier density (CCD) due to the compensation by Mg in the grown drift layer (DL), which resulted in the increased depletion width (DW) of the fabricated GaN SBDs. The SBDs also reached a CCE of approximately 96.7% at –300 V.

DOI： 10.3390/s19235107

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PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1149/2.0332001JSS

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1038/s41598-019-52067-y

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.7567/1882-0786/ab45d1

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/1.5114866

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Microelectronics Reliability  

Within this paper, we present an extensive analysis of the degradation of UV-B light-emitting diodes (LEDs) submitted to constant current stress. The study is based on combined electrical, optical and spectral characterization, and capacitance deep-level transient spectroscopy (C-DLTS). The results of this analysis demonstrate that the decrease in the optical power during the stress is stronger at low measuring current levels, indicating that the degradation is related to the increase in Shockley-Read-Hall (SRH) recombination. The electrical characterization shows a decrease in the driving voltage, probably due to an increased activation of the Mg dopant, and an increase in the sub-threshold forward current, that suggest a generation of mid-gap states during the stress. C-DLTS measurements were carried out to study the variation in defects concentration after stress; the most relevant traps were ascribed to the presence of Mg doping and/or to intrinsic defects related to the GaN growth.

DOI： 10.1016/j.microrel.2019.113418

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ECS Journal of Solid State Science and Technology  

We investigated the effect of via-holes in the lead-frame on the reliability of blue flip-chip light-emitting diodes (FCLEDs). For bonding, conventional Sn-Ag(3.0%)-Cu(0.5%) solder (SAC) and Ag sintered paste were adopted. Lateral LED, conventional FCLED (c-FCLED), and via-hole FCLED gave efficacies of 218.5, 221.6 and 225 lm/W at 30 mA. The lateral LED sample (bonding material: silicone), c-FCLED sample (SAC), via-hole-type sample (Ag sinter), and via-hole-type sample (SAC) exhibited total thermal resistance of 15.24, 10.19, 8.95, and 11.79 K/W, respectively. The humidity/temperature/H2S gas reliability examinations showed that unlike the via-hole-type samples, the lumen of the c-FCLED was fallen by 15.0% after exposure for 500 h. Furthermore, the c-FCLED exhibited a 19.6% higher forward voltage than the via-hole FCLED after 1,000 thermal-cycles. SEM results showed that unlike the via-hole type FCLED, the c-FCLED underwent cracking after thermal cycle of 1000 times.

DOI： 10.1149/2.0171909jss

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.jallcom.2019.05.070

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.3390/ma12162583

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/admi.201900821

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

We fabricated a GaInN/GaInP/GaInAs/Ge four-junction solar cell by wafer bonding a GaInN solar cell and a GaInP/GaInAs/Ge three-junction solar cell. We performed our wafer bonding at high pressure (500 N) and temperature (450 °C) by using p-type GaN and n-type GaAs. The open-circuit voltage (V OC), J SC, and fill factor of our four-junction solar cell under the condition of AM 1.5 G, i.e., 1 sun are 2.85 V, 0.219 mA cm-2, and 0.74, respectively. The improved V OC of our four-junction solar cells was confirmed by a series connection. The J SC is almost comparable to the theoretical value (itself based on the assumption of an ideal series junction), but the V OC is approximately 1 V less than that predicted by theory. Our research will improve the solar cell efficiency and help meet future energy needs.

DOI： 10.7567/1347-4065/ab26ad

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

This study investigated the change in carrier concentration near the surface of a silicon substrate during gallium nitride (GaN) growth with an aluminum nitride (AlN) buffer layer. It was observed that aluminum, gallium, and carbon diffused into the silicon substrate during the growth process and that the carrier concentration increased with increasing concentration of aluminum and gallium impurities. The gallium that diffused into the silicon substrate was identified as having originated from the gallium that decomposed on the reactor wall during the growth process and the gallium introduced onto the silicon substrate during GaN growth. In contrast, the amount of aluminum that diffused into the substrate was influenced by the duration of the trimethylaluminum (TMAl) flow: A long duration of the TMAl flow step before AlN growth led to a high aluminum concentration near the substrate surface.

DOI： 10.7567/1347-4065/ab2657

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.jcrysgro.2019.03.025

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/1.5097767

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

In order to make low cost bulk GaN single crystal with a high growth rate, we have built a vertical HVPE reactor with a showerhead configuration. The flow from the showerhead of the reactor is independently controlled by inner/outer two sets of gas supply lines. This special showerhead design makes the flow model different from the conventional showerhead reactors. We have employed a finite element-based simulator to study the fluid dynamic and crystal growth of this inner/outer flow independently controlled showerhead. Two simulation experiments were performed. One experiment has demonstrated that both growth rate and Ga yield can be improved without compromising the uniformity by controlling the different input parameters of inner and outer flow on the showerhead, as compared with the conventional showerhead. The other experiment has demonstrated that the showerhead can be further optimized by changing the inner/outer area ratio and input parameters.

DOI： 10.7567/1347-4065/ab124e

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

We report on the growth, processing and optical characterization of monolithically integrated tricolor micro-LEDs. The 100 × 100 μm2 active area of the devices is composed of independent subpixels emitting in the blue, green and yellow-orange range with color saturation of over 90% for all bands. The gamut of the device is recorded by both digital and analog dimming, i.e. by pulse width modulation or by varying the current density. Results indicate color mixing performed by both methods leads to a rotated or distorted gamut significantly different from the one predicted by the CIE color model. We explain our findings in terms of quantum-confined Stark effect screening and efficiency droop at high current density, which modify the expected hue and brightness of mixed colors.

DOI： 10.7567/1347-4065/ab06ae

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

Recessed-gate AlGaN/GaN metal-insulator-semiconductor (MIS) high electron mobility transistors (HEMTs) were fabricated by utilizing a photoelectrochemical (PEC) etching and a post-metallization annealing (PMA) process. In order to demonstrate the device performance and reliability, both electrical and optical electroluminescence (EL) properties were evaluated. The capacitance-voltage (C-V) characteristic showed that the PEC etching and subsequent PMA process enhanced the gate control of two-dimensional electron gas density. The PEC-etched-gate AlGaN/GaN MIS-HEMT showed the smallest sub-threshold slope of all the samples including planar-gate and inductively coupled plasma-etched-gate devices. Furthermore, the PEC-etched devices showed an extremely low 10-11 A mm-1 gate leakage current with no spot-like EL. These results indicated that the PEC etching and subsequent PMA process improved the electrical properties of the Al2O3/AlGaN interface, resulting in enhanced device performance of the AlGaN/GaN MIS-HEMTs.

DOI： 10.7567/1347-4065/ab06b9

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

A vertical p-n diode with a simple edge termination structure on a GaN free-standing substrate is demonstrated. The edge of this device is terminated simply by etching a drift layer deeply and vertically. A device simulation revealed that the electric field at the device edge was more relaxed and uniformly applied by etching the mesa deeper than the depletion region. The fabricated device showed low leakage current and avalanche capability, and its breakdown characteristics could be reproduced many times. By emission microscopy observation, we found that there was no leakage current at the side wall of the device and that avalanche breakdown occurred throughout the inside of the device. This indicates that the electric field crowding at the side wall of the device was completely suppressed and a uniform electric field distribution was obtained by this structure.

DOI： 10.7567/1347-4065/ab106c

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

We fabricated p-n diodes under different growth pressures on free-standing GaN substrates of the same quality and observed a noteworthy difference in the reverse leakage current. A large reverse leakage current was generated by nanopipes, which were formed from screw dislocations in the homoepitaxial layer. There were two types of screw dislocation observed in this study. The first type already existed in the substrate and the other was newly generated in the epilayer by the coalescence of edge and mixed dislocations. An increase in the growth pressure suppressed the transformation of screw dislocations into nanopipes, which led to a reduction in the reverse leakage current. To reduce the leakage current further, it is necessary to apply growth conditions that do not transform screw dislocation into nanopipes and to use a free-standing substrate without threading dislocations, that become nanopipes.

DOI： 10.7567/1347-4065/ab1250

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japanese Journal of Applied Physics  

We have studied the impact on the surface orientation of different sputtered Al layers followed by AlN layers sputtered on m-plane sapphire. These initial layers were then overgrown by AlN and followed by GaN using metal-organic vapor phase epitaxy (MOVPE). By increasing sputtering time of the initial Al layer from 0 s to 15 s, we obtained single phase (10-10), (10-13) and (10-14) GaN/AlN layers. The thickness of the initial Al layer was estimated by optical transmission measurements to be about 0.5-1 nm for the (10-13) orientation, and >1 nm for the (10-14) orientation. After MOVPE growth, no trace of metallic Al was found by transmission electron microscopy, indicating that this layer was fully converted to AlN.

DOI： 10.7567/1347-4065/ab1252

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.7567/1882-0786/ab21a9

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/pssb.201800648

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.jcrysgro.2019.02.058

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.jcrysgro.2019.02.013

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.jcrysgro.2019.02.020

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1038/s41566-019-0359-9

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/1.5087491

Web of Science