875 results on '"Isamu Akasaki"'
Search Results
2. Demonstration of electron beam laser excitation in the UV range using a GaN/AlGaN multiquantum well active layer
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Takafumi Hayashi, Yuta Kawase, Noriaki Nagata, Takashi Senga, Sho Iwayama, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, Isamu Akasaki, and Takahiro Matsumoto
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Medicine ,Science - Abstract
Abstract This study investigated electron beam laser excitation in the UV region using a GaN/AlGaN multiquantum well (MQW) active layer. Laser emission was observed when the GaN/AlGaN MQW was excited by an electron beam, with a wavelength of approximately 353 nm and a threshold power density of 230 kW/cm2. A comparison of optical pumping and electron beam pumping demonstrated that the rate of generation of electron-hole pairs when using electron beam excitation was approximately one quarter that of light excitation.
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- 2017
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3. Voltage-Controlled Anodic Oxidation of Porous Fluorescent SiC for Effective Surface Passivation
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Kosuke Yanai, Weifang Lu, Yoma Yamane, Dong-Pyo Han, Haiyan Ou, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, and Isamu Akasaki
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fluorescent SiC ,anodic oxidation ,porous structures ,photoluminescence ,surface passivation ,Chemistry ,QD1-999 - Abstract
This study investigated the fabrication of porous fluorescent SiC using a constant voltage-controlled anodic oxidation process. The application of a high, constant voltage resulted in a spatial distinction between the porous structures formed inside the fluorescent SiC substrates, due to the different etching rates at the terrace and the large step bunches. Large, dendritic porous structures were formed as the etching process continued and the porous layer thickened. Under the conditions of low hydrofluoric acid (HF) concentration, the uniformity of the dendritic porous structures through the entire porous layer was considerably improved compared with the conditions of high HF concentration. The resulting large uniform structure offered a sizable surface area, and promoted the penetration of atomic layer-deposited (ALD) Al2O3 films (ALD–Al2O3). The emission intensity in the porous fluorescent SiC was confirmed via photoluminescence (PL) measurements to be significantly improved by a factor of 128 after ALD passivation. With surface passivation, there was a clear blueshift in the emission wavelength, owing to the effective suppression of the non-radiative recombination rate in the porous structures. Furthermore, the spatial uniformity of emitted light was examined via PL mapping using three different excitation lasers, which resulted in the observation of uniform and distinctive emissions in the fluorescent SiC bulk and porous areas.
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- 2020
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4. Development of Monolithically Grown Coaxial GaInN/GaN Multiple Quantum Shell Nanowires by MOCVD
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Kazuma Ito, Weifang Lu, Naoki Sone, Yoshiya Miyamoto, Renji Okuda, Motoaki Iwaya, Tetsuya Tekeuchi, Satoshi Kamiyama, and Isamu Akasaki
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monolithic growth ,multi-color emission ,coaxial MQS nanowires ,In incorporation ,Chemistry ,QD1-999 - Abstract
Broadened emission was demonstrated in coaxial GaInN/GaN multiple quantum shell (MQS) nanowires that were monolithically grown by metalorganic chemical vapor deposition. The non-polar GaInN/GaN structures were coaxially grown on n-core nanowires with combinations of three different diameters and pitches. To broaden the emission band in these three nanowire patterns, we varied the triethylgallium (TEG) flow rate and the growth temperature of the quantum barriers and wells, and investigated their effects on the In incorporation rate during MQS growth. At higher TEG flow rates, the growth rate of MQS and the In incorporation rate were promoted, resulting in slightly higher cathodoluminescence (CL) intensity. An enhancement up to 2–3 times of CL intensity was observed by escalating the growth temperature of the quantum barriers to 800 °C. Furthermore, decreasing the growth temperature of the quantum wells redshifted the peak wavelength without reducing the MQS quality. Under the modified growth sequence, monolithically grown nanowires with a broaden emission was achieved. Moreover, it verified that reducing the filling factor (pitch) can further promote the In incorporation probability on the nanowires. Compared with the conventional film-based quantum well LEDs, the demonstrated monolithic coaxial GaInN/GaN nanowires are promising candidates for phosphor-free white and micro light-emitting diodes (LEDs).
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- 2020
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5. Enhanced Device Performance of GaInN-Based Green Light-Emitting Diode with Sputtered AlN Buffer Layer
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Seiji Ishimoto, Dong-Pyo Han, Kengo Yamamoto, Ryoya Mano, Satoshi Kamiyama, Tetsuya Takeuchi, Motoaki Iwaya, and Isamu Akasaki
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light-emitting diode ,green gap ,efficiency droop ,piezoelectric field ,tunneling leakage ,internal quantum efficiency ,Technology ,Engineering (General). Civil engineering (General) ,TA1-2040 ,Biology (General) ,QH301-705.5 ,Physics ,QC1-999 ,Chemistry ,QD1-999 - Abstract
In this study, we compared the device performance of GaInN-based green LEDs grown on c-plane sapphire substrates with a conventional low temperature GaN buffer layer to those with a sputtered-AlN buffer layer. The light output power and leakage current characteristics were significantly improved by just replacing the buffer layer with a sputtered-AlN layer. To understand the origin of the improvement in performance, the electrical and optical properties were compared by means of electro-reflectance spectroscopy, I⁻V curves, electroluminescence spectra, L⁻I curves, and internal quantum efficiencies. From the analysis of the results, we concluded that the improvement is mainly due to the mitigation of strain and reduction of the piezoelectric field in the multiple quantum wells active region.
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- 2019
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6. Tuning the Resonant Frequency of a Surface Plasmon by Double-Metallic Ag/Au Nanoparticles for High-Efficiency Green Light-Emitting Diodes
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Ryoya Mano, Dong-Pyo Han, Kengo Yamamoto, Seiji Ishimoto, Satoshi Kamiyama, Tetsuya Takeuchi, Motoaki Iwaya, and Isamu Akasaki
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light-emitting diode ,surface plasmon ,resonant frequency ,internal quantum efficiency ,nanoparticle ,Technology ,Engineering (General). Civil engineering (General) ,TA1-2040 ,Biology (General) ,QH301-705.5 ,Physics ,QC1-999 ,Chemistry ,QD1-999 - Abstract
Currently, the internal quantum efficiency (IQE) of GaInN-based green light-emitting diodes (LEDs) is still low. To overcome this problem, surface plasmon (SP)-enhanced LEDs have been intensively studied for the last 15 years. For an SP effect in green LEDs, Au and Ag are typically employed as the plasmonic materials. However, the resonance wavelength is determined by their material constants, which are theoretically fixed at ~537 nm for Au and ~437 nm for Ag. In this study, we aimed to tune the SP resonant wavelength using double-metallic nanoparticles (NPs) composed of Au and Ag to match the SP resonance wavelength to the LED emission wavelength to consequently improve the IQE of green LEDs. To form double-metallic NPs, Au/Ag multilayers were deposited on a GaN layer and then thermally annealed. We changed the thicknesses of the multilayers to control the Ag/Au ratio in the NPs. We show that the SP resonant wavelength could be tuned using our approach. We also demonstrate that the enhancement of the IQE in SP-enhanced LEDs was strongly dependent on the SP resonant wavelength. Finally, the highest IQE was achieved by matching the SP resonant wavelength to the LED emission wavelength.
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- 2019
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7. Internal quantum efficiency enhancement of GaInN/GaN quantum-well structures using Ag nanoparticles
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Daisuke Iida, Ahmed Fadil, Yuntian Chen, Yiyu Ou, Oleksii Kopylov, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, Isamu Akasaki, and Haiyan Ou
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Physics ,QC1-999 - Abstract
We report internal quantum efficiency enhancement of thin p-GaN green quantum-well structure using self-assembled Ag nanoparticles. Temperature dependent photoluminescence measurements are conducted to determine the internal quantum efficiency. The impact of excitation power density on the enhancement factor is investigated. We obtain an internal quantum efficiency enhancement by a factor of 2.3 at 756 W/cm2, and a factor of 8.1 at 1 W/cm2. A Purcell enhancement up to a factor of 26 is estimated by fitting the experimental results to a theoretical model for the efficiency enhancement factor.
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- 2015
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8. Morphology Control and Crystalline Quality of p-Type GaN Shells Grown on Coaxial GaInN/GaN Multiple Quantum Shell Nanowires
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Isamu Akasaki, Motoaki Iwaya, Koji Okuno, Naoki Sone, Kazuma Ito, Tetsuya Takeuchi, Weifang Lu, Satoshi Kamiyama, Yoshiya Miyamoto, Sae Katsuro, and Nanami Nakayama
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Materials science ,business.industry ,Doping ,Nanowire ,Shell (structure) ,Chemical vapor deposition ,chemistry.chemical_compound ,chemistry ,Scanning transmission electron microscopy ,Optoelectronics ,Partial dislocations ,General Materials Science ,Coaxial ,Trimethylgallium ,business - Abstract
The morphology and crystalline quality of p-GaN shells on coaxial GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) were investigated using metal-organic chemical vapor deposition. By varying the trimethylgallium (TMG) flow rate, Mg doping, and growth temperature, it was verified that the TMG supply and growth temperature were the dominant parameters in the control of the p-GaN shape on NWs. Specifically, a sufficiently high TMG supply enabled the formation of a pyramid-shaped NW structure with a uniform p-GaN shell. The ratio of the growth rate between the c- and m-planes on the NWs was calculated to be approximately 0.4545. High-angle annular dark-field scanning transmission electron microscopy characterization confirmed that no clear extended defects were present in the n-GaN core and MQS/p-GaN shells on the sidewall. Regarding the p-GaN shell above the c-plane MQS region, only a few screw dislocations and Frank-type partial dislocations appeared at the interface between the serpentine c-plane MQS and the p-GaN shell near the tips. This suggested that the crystalline quality of the MQS structure can trigger the formation of screw dislocations and Frank-type partial dislocations during the p-GaN growth. The growth mechanism of the p-GaN shell on NWs was also discussed. To inspect the electronic properties, a prototype of a micro light-emitting diode (LED) with a chip size of 50 × 50 μm2 was demonstrated in the NWs with optimal growth. By correlating the light output curve with the electroluminescence spectra, three different emission peaks (450, 470, and 510 nm) were assignable to the emission from the m-, r-, and c-planes, respectively.
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- 2021
9. Emission characteristics of GaInN/GaN multiple quantum shell nanowire-based LEDs with different p-GaN growth conditions
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Weifang Lu, Nanami Nakayama, Koji Okuno, Isamu Akasaki, Sae Katsuro, Satoshi Kamiyama, Naoki Sone, Motoaki Iwaya, Tetsuya Takeuchi, and Kazuma Ito
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Materials science ,current injection ,business.industry ,Physics ,QC1-999 ,Multiple quantum ,Shell (structure) ,Nanowire ,p-gan shell ,Atomic and Molecular Physics, and Optics ,Electronic, Optical and Magnetic Materials ,law.invention ,nanowires ,gainn/gan multiple-quantum-shells ,law ,Optoelectronics ,teg flow rate ,Electrical and Electronic Engineering ,business ,nw-leds ,Biotechnology ,Light-emitting diode - Abstract
Improving current injection into r- and m-planes of nanowires (NWs) is essential to realizing efficient GaInN/GaN multiple quantum shell (MQS) NW-based light-emitting diodes (LEDs). Here, we present the effects of different p-GaN shell growth conditions on the emission characteristics of MQS NW-LEDs. Firstly, a comparison between cathodoluminescence (CL) and electroluminescence (EL) spectra indicates that the emission in NW-LEDs originates from the top region of the NWs. By growing thick p-GaN shells, the variable emission peak at around 600 nm and degradation of the light output of the NW-LEDs are elaborated, which is attributable to the localization of current in the c-plane region with various In-rich clusters and deep-level defects. Utilizing a high growth rate of p-GaN shell, an increased r-plane and a reduced c-plane region promote the deposition of indium tin oxide layer over the entire NW. Therefore, the current is effectively injected into both the r- and m-planes of the NW structures. Consequently, the light output and EL peak intensity of the NW-LEDs are enhanced by factors of 4.3 and 13.8, respectively, under an injection current of 100 mA. Furthermore, scanning transmission electron microscope images demonstrate the suppression of dislocations, triangular defects, and stacking faults at the apex of the p-GaN shell with a high growth rate. Therefore, localization of current injection in nonradiative recombination centers near the c-plane was also inhibited. Our results emphasize the possibility of realizing high efficacy in NW-LEDs via optimal p-GaN shell growth conditions, which is quite promising for application in the long-wavelength region.
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- 2021
10. GaN-Based p-n Junction Blue-Light-Emitting Devices.
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Isamu Akasaki
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- 2013
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11. Correlation between Optical and Structural Characteristics in Coaxial GaInN/GaN Multiple Quantum Shell Nanowires with AlGaN Spacers
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Weifang Lu, Tetsuya Tekeuchi, Naoki Sone, Motoaki Iwaya, Kazuma Ito, Satoshi Kamiyama, Yoshiya Miyamoto, Isamu Akasaki, and Renji Okuda
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010302 applied physics ,Materials science ,business.industry ,Nanowire ,02 engineering and technology ,Chemical vapor deposition ,Substrate (electronics) ,021001 nanoscience & nanotechnology ,01 natural sciences ,Evaporation (deposition) ,law.invention ,law ,0103 physical sciences ,Scanning transmission electron microscopy ,Optoelectronics ,General Materials Science ,Quantum efficiency ,0210 nano-technology ,business ,Quantum well ,Light-emitting diode - Abstract
High crystalline quality coaxial GaInN/GaN multiple quantum shells (MQSs) grown on dislocation-free nanowires are highly in demand for efficient white-/micro-light-emitting diodes (LEDs). Here, we propose an effective approach to improve the MQS quality during the selective growth by metal-organic chemical vapor deposition. By increasing the growth temperature of GaN barriers, the cathodoluminescent intensity yielded enhancements of 0.7 and 3.9 times in the samples with GaN and AlGaN spacers, respectively. Using an AlGaN spacer before increasing the barrier temperature, the decomposition of GaInN quantum wells was suppressed on all planes, resulting in a high internal quantum efficiency up to 69%. As revealed by scanning transmission electron microscopy (STEM) characterization, the high barrier growth temperature allowed to achieve a clear interface between GaInN quantum wells and GaN quantum barriers on the c-, r-, and m-planes of the nanowires. Moreover, the correlation between the In incorporation and structure features in MQS was quantitatively assessed based on the STEM energy-dispersive X-ray spectroscopy mapping and line-scan profiles of In and Al fractions. Ultimately, it was demonstrated that the unintentional In incorporation in GaN barriers was induced by the evaporation of predeposited In-rich particles during low-temperature growth of GaInN wells. Such residual In contamination was sufficiently inhibited by inserting low Al fraction (∼6%) AlGaN spacers after each GaInN well. During the growth of AlGaN spacers, AlN polycrystalline particles were deposited on the surrounding dummy substrate, which suppressed the evaporation of the predeposited In-rich particles. Thus, the presence of AlGaN spacers certainly improved the uniformity of In fraction through five GaInN quantum wells and reduced the diffusion of point defects from n-core to MQS active structures. The superior coaxial GaInN/GaN MQS structures with the AlGaN spacer are supposed to improve the emission efficiency in white-/micro-LEDs.
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- 2020
12. Photoluminescence Characterization of Fluorescent Sic with High Boron and Nitrogen Concentrations
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Tetsuya Takeuchi, Motoaki Iwaya, Satoshi Kamiyama, Daiki Tanaka, Weifang Lu, and Isamu Akasaki
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010302 applied physics ,Materials science ,Photoluminescence ,Mechanical Engineering ,chemistry.chemical_element ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Fluorescence ,Nitrogen ,Characterization (materials science) ,chemistry ,Chemical engineering ,Mechanics of Materials ,0103 physical sciences ,General Materials Science ,Quantum efficiency ,0210 nano-technology ,Boron - Abstract
The dependence of optical properties on impurity concentrations and excitation power was explored. In particular, it was found that the peak shift of photoluminescence (PL) is proportional to the boron concentration. This might be due to donor–acceptor pair (DAP) recombination via double deep acceptor levels (D-centers), where the occupancy of the D*-center increased with the B concentration, and the recombination via the D*-center for longer wavelengths became dominant. Moreover, the relative constants B and C were calculated by BC model fitting from the internal quantum efficiency (IQE) curve as a function of excitation power. The theoretical extrapolation based on BC model predicted that high impurity concentrations are sufficient to suppress the droop phenomenon of efficiency induced by the increased excitation power.
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- 2020
13. Improved Reverse Leakage Current in GaInN-Based LEDs With a Sputtered AlN Buffer Layer
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Satoshi Kamiyama, Weifang Lu, Ryoya Mano, Tetsuya Takeuchi, Dong-Pyo Han, Isamu Akasaki, Motoaki Iwaya, and Seiji Ishimoto
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Threading dislocations ,Materials science ,business.industry ,Gallium nitride ,Atomic and Molecular Physics, and Optics ,Buffer (optical fiber) ,Electronic, Optical and Magnetic Materials ,law.invention ,Reverse leakage current ,chemistry.chemical_compound ,chemistry ,law ,Optoelectronics ,Electrical and Electronic Engineering ,business ,Diode ,Light-emitting diode - Abstract
In this study, the improvement of reverse leakage current characteristics with a sputtered (SP) -AlN buffer layer in GaInN-based green light-emitting diodes (LEDs) has been presented for the first time. To understand the origin of the improvement, a detailed review and careful analysis of reverse leakage current characteristics were performed. The review and analysis identified that the improvement was primarily caused by the suppression of variable-range-hopping process obtained by replacing conventional low-temperature GaN buffer. Verification that threading dislocations and V- defects can enhance the variable-range-hopping process has been received. We believe that this study will contribute to the realization of green LEDs with advantages of high reliability, a long lifetime, and electrical robustness.
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- 2019
14. Structural and optical impacts of AlGaN undershells on coaxial GaInN/GaN multiple-quantum-shells nanowires
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Hedeki Murakami, Nanami Goto, Tetsuya Tekeuchi, Motoaki Iwaya, Mizuki Terazawa, Kazuyoshi Iida, Satoshi Kamiyama, Naoki Sone, Isamu Akasaki, Weifang Lu, and Dong-Pyo Han
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010302 applied physics ,internal quantum efficiency (iqe) ,Materials science ,business.industry ,Physics ,QC1-999 ,Multiple quantum ,Nanowire ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Electronic, Optical and Magnetic Materials ,Nanomaterials ,algan undershell ,point defect diffusion ,0103 physical sciences ,Optoelectronics ,nanowires (nws) ,Electrical and Electronic Engineering ,Coaxial ,gainn/gan mqs ,0210 nano-technology ,business ,Biotechnology - Abstract
The superior crystalline quality of coaxial GaInN/GaN multiple-quantum shell (MQS) nanowires (NWs) was demonstrated by employing an AlGaN undershell during metal-organic chemical vapor deposition. Scanning transmission electron microscopy (STEM) results reveal that the NW structure consists of distinct GaInN/GaN regions on different positions of the NWs and the cores were dislocation-free. High-resolution atomic contrast STEM images verified the importance of AlGaN undershells in trapping the point defects diffused from n-core to MQSs (m-planes), as well as the improvement of the grown crystal quality on the apex region (c-planes). Time-integrated and time-resolved photoluminescence (PL) measurements were performed to clarify the mechanism of the emission within the coaxial GaInN/GaN MQS NWs. The improved internal quantum efficiency in the NW sample was attributed to the unique AlGaN undershell, which was able to suppress the point defects diffusion and reduce the dislocation densities on c-planes. Carrier lifetimes of 2.19 ns and 8.44 ns were derived from time-resolved PL decay curves for NW samples without and with the AlGaN undershell, respectively. Hence, the use of an AlGaN undershell exhibits promising improvement of optical properties for NW-based white and micro light-emitting diodes.
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- 2019
15. Influence of Ohmic Contact Resistance on Transconductance in AlGaN/GaN HEMT.
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Yoshikazu Hirose, Akira Honshio, Takeshi Kawashima, Motoaki Iwaya, Satoshi Kamiyama, Michinobu Tsuda, Hiroshi Amano, and Isamu Akasaki
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- 2006
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16. Crystal Growth and Characterization of n-GaN in a Multiple Quantum Shell Nanowire-Based Light Emitter with a Tunnel Junction
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Kazuma Ito, Weifang Lu, Motoaki Iwaya, Yoshiya Miyamoto, Kazuyoshi Iida, Koji Okuno, Naoki Sone, Renji Okuda, Isamu Akasaki, Tetsuya Takeuchi, Koichi Mizutani, Satoshi Kamiyama, and Masaki Ohya
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Materials science ,Tunnel junction ,Quantum dot ,Scanning transmission electron microscopy ,Doping ,Analytical chemistry ,Nanowire ,General Materials Science ,Crystal growth ,Metalorganic vapour phase epitaxy ,Epitaxy - Abstract
Here, we systematically investigated the growth conditions of an n-GaN cap layer for nanowire-based light emitters with a tunnel junction. Selective-area growth of multiple quantum shell (MQS)/nanowire core-shell structures on a patterned n-GaN/sapphire substrate was performed by metal-organic vapor phase epitaxy, followed by the growth of a p-GaN, an n++/ p++-GaN tunnel junction, and an n-GaN cap layer. Specifically, two-step growth of the n-GaN cap layer was carried out under various growth conditions to determine the optimal conditions for a flat n-GaN cap layer. Scanning transmission electron microscopy characterization revealed that n++-GaN can be uniformly grown on the m-plane sidewall of MQS nanowires. A clear tunnel junction, involving 10-nm-thick p++-GaN and 3-nm-thick n++-GaN, was confirmed on the nonpolar m-planes of the nanowires. The Mg doping concentration and distribution profile of the p++-GaN shell were inspected using three-dimensional atom probe tomography. Afterward, the reconstructed isoconcentration mapping was applied to identify Mg-rich clusters. The density and average size of the Mg clusters were estimated to be approximately 4.3 × 1017 cm-3 and 5 nm, respectively. Excluding the Mg atoms contained in the clusters, the remaining Mg doping concentration in the p++-GaN region was calculated to be 1.1 × 1020 cm-3. Despite the lack of effective activation, a reasonably low operating voltage and distinct light emissions were preliminarily observed in MQS nanowire-based LEDs under the optimal n-GaN cap growth conditions. In the fabricated MQS-nanowire devices, carriers were injected into both the r-plane and m-plane of the nanowires without a clear quantum confinement Stark effect.
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- 2021
17. Impact of low-temperature buffer layers on nitride-based optoelectronics.
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Hiroshi Amano, Satoshi Kamiyama, and Isamu Akasaki
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- 2002
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18. Identification of multi-color emission from coaxial GaInN/GaN multiple-quantum-shell nanowire LEDs
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Tetsuya Takeuchi, Satoshi Kamiyama, Sae Katsuro, Isamu Akasaki, Motoaki Iwaya, Kazuma Ito, Weifang Lu, Koichi Mizutani, Renji Okuda, Naoki Sone, and Nanami Nakayama
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Materials science ,Photoluminescence ,business.industry ,General Engineering ,Nanowire ,Bioengineering ,General Chemistry ,Electroluminescence ,Epitaxy ,Atomic and Molecular Physics, and Optics ,law.invention ,law ,Sapphire ,Optoelectronics ,General Materials Science ,business ,Luminous efficacy ,Quantum well ,Light-emitting diode - Abstract
Multi-color emission from coaxial GaInN/GaN multiple-quantum-shell (MQS) nanowire-based light-emitting diodes (LEDs) was identified. In this study, MQS nanowire samples for LED processes were selectively grown on patterned commercial GaN/sapphire substrates using metal–organic chemical vapor deposition. Three electroluminescence (EL) emission peaks (440, 540, and 630 nm) were observed, which were primarily attributed to the nonpolar m-planes, semipolar r-planes, and the polar c-plane tips of nanowire arrays. A modified epitaxial growth sequence with improved crystalline quality for MQSs was used to effectively narrow the EL emission peaks. Specifically, nanowire-based LEDs manifested a clear redshift from 430 nm to 520 nm upon insertion of AlGaN spacers after the growth of each GaInN quantum well. This demonstrates the feasibility of lengthening the EL emission wavelength since an AlGaN spacer can suppress In decomposition of the GaInN quantum wells during ramping up the growth temperature for GaN barriers. EL spectra showed stable emission peaks as a function of the injection current, verifying the critical feature of the non-polarization of GaN/GaInN MQSs on nanowires. In addition, by comparing EL and photoluminescence spectra, the yellow-red emission linked to the In-fluctuation and point defects in the c-plane MQS was verified by varying the activation annealing time and lowering the growth temperature of the GaInN quantum wells. Therefore, optimization of MQS nanowire growth with a high quality of c-planes is considered critical for improving the luminous efficiency of nanowire-based micro-LEDs/white LEDs.
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- 2021
19. Modified Shockley Equation for GaInN-Based Light-Emitting Diodes: Origin of the Power- Efficiency Degradation Under High Current Injection
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Tetsuya Takeuchi, Dong-Soo Shin, Motoaki Iwaya, Jong-In Shim, Satoshi Kamiyama, Isamu Akasaki, and Dong-Pyo Han
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Materials science ,business.industry ,02 engineering and technology ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,law.invention ,020210 optoelectronics & photonics ,law ,0202 electrical engineering, electronic engineering, information engineering ,Radiative transfer ,Optoelectronics ,Spontaneous emission ,Quantum efficiency ,Electrical and Electronic Engineering ,business ,Quantum well ,Voltage drop ,Voltage ,Light-emitting diode ,Diode - Abstract
As an attempt to elucidate the origin of the power-efficiency (PE) degradation at high current injection for GaInN-based light-emitting diodes, the injection current is quantitatively separated to radiative and nonradiative current components as a function of applied voltage. It is found that the conventional Shockley equation for the current-voltage curve of a Si pn diode is not adequate for the LED since the carrier transport and recombination processes are quite different from those of the Si pn diode. Hence, we propose a diode equation for an LED where the radiative and nonradiative currents are separately expressed as a function of applied voltage. By analyzing the proposed diode equation, it is concluded that the PE degradation at high injection currents is due to the increase of the junction voltage and the decrease of the internal quantum efficiency at the same time. The phenomena can be understood by the insufficient recombination rate in the active quantum wells.
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- 2019
20. Study on N and B Doping by Closed Sublimation Growth Using Separated Ta Crucible
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Hiroaki Kurokawa, Daiki Tanaka, Tetsuya Takeuchi, Isamu Akasaki, Satoshi Kamiyama, and Motoaki Iwaya
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Materials science ,Mechanics of Materials ,Mechanical Engineering ,Doping ,Analytical chemistry ,General Materials Science ,Sublimation (phase transition) ,Condensed Matter Physics - Abstract
Boron (B) doping sources and crucible materials for stable, reproducible and high concentration B doping in fluorescent SiC (f-SiC) were investigated. When a Ta crucible was used with BN powder as a B doping source were used, B doping did not occur owing to too low C/Si ratio. On the other hand, when a C crucible and suitable Ta components inside the crucible were used, a high B concentration of 1.58 × 1019 cm-3 was obtained, owing to the high C/Si ratio. The results indicate that a C crucible with optimal Ta components and BN powder are suitable for high concentration B doping.
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- 2019
21. Effect of AlGaN undershell on the cathodoluminescence properties of coaxial GaInN/GaN multiple-quantum-shells nanowires
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Tetsuya Tekeuchi, Kazuyoshi Iida, Isamu Akasaki, Satoshi Kamiyama, Motoaki Iwaya, Nanami Goto, Atsushi Suzuki, Weifang Lu, Dong-Pyo Han, and Naoki Sone
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Materials science ,business.industry ,Nanowire ,Cathodoluminescence ,02 engineering and technology ,Chemical vapor deposition ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Crystallographic defect ,0104 chemical sciences ,law.invention ,law ,Sapphire ,Optoelectronics ,General Materials Science ,Metalorganic vapour phase epitaxy ,0210 nano-technology ,business ,Light-emitting diode ,Diode - Abstract
Coaxial GaInN/GaN multiple-quantum-shells (MQSs) nanowires (NWs) were grown on an n-type GaN/sapphire template employing selective growth by metal-organic chemical vapour deposition (MOCVD). To improve the cathodoluminescence (CL) emission intensity, an AlGaN shell was grown underneath the MQS active structures. By controlling the growth temperature and duration, an impressive and up to 11-fold enhancement of CL intensity is achieved at the top area of the GaInN/GaN MQS NWs. The spatial distribution of Al composition in the AlGaN undershell was assessed as a function of position along the NW and analysed by energy-dispersive X-ray measurement and CL characterisation. By introducing an AlGaN shell underneath GaInN/GaN MQS, the diffusion of point defects from the n-core to MQS is effectively suppressed because of the lower formation energy of vacancies-complexes in AlGaN in comparison to GaN. Moreover, the spatial distribution of Al and In was attributed to the insufficient delivery of gas precursors to the bottom of the NWs and the anisotropy diffusion on the nonpolar m-planes. This investigation can shed light on the effect of the AlGaN undershell on improving the emission efficiency of NW-based white and micro-light-emitting diodes (LEDs).
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- 2019
22. Device fabrication for multiple quantum shell nanowires based laser diodes
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Naoki Sone, Isamu Akasaki, Masaki Ohya, Koichi Mizutani, Tetsuya Takeuchi, Satoshi Kamiyama, Koji Okuno, Weifang Lu, Yoshiya Miyamoto, Motoaki Iwaya, Renji Okuda, Kazuyoshi Iida, and Kazuma Ito
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Materials science ,business.industry ,Nanowire ,Cathodoluminescence ,Electroluminescence ,Laser ,law.invention ,law ,Etching (microfabrication) ,Tunnel junction ,Optoelectronics ,Light emission ,Metalorganic vapour phase epitaxy ,business - Abstract
GaInN/GaN multiple quantum shells (MQS) nanowires and p-GaN shells were embedded with n-GaN layers through tunnel junction (TJ) shells using metalorganic chemical vapor deposition (MOCVD) method. The MQS nanowires were selectively grown on n-GaN/sapphire or GaN substrates. The fabrication process of laser structures with different resonators of 600500, 750, 1000 μm, and cavity widths of 7, 12, and 17 μm were investigated with insulating layer on the sidewalls of the ridge. The structures of the fabricated devices were characterized by scanning electron microscope (SEM) and current-voltage-light output characteristics were evaluated. Two different methods for mirror formation, etching and cleavage, were developed for the laser devices. During the investigation, a superior mirror formation suffered from the difference in etching rate between GaInN and GaN, generating concaves in the MQS region. Bluegreen light emission was observed from the entire ridge surface of the MQS index-guided laser structures. A maximum current density of emission at 17.9 kA/cm2 has been confirmed in the devices. The electroluminescence and cathodoluminescence measurements demonstrated that the r-plane and c-plane at the top of the MQS are dominant at low current densities, and the m-plane emission becomes stronger as the current density increases.
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- 2021
23. Crystal growth of n-GaN on nanowire-based light emitter including multiple-quantum-shell and tunnel junction
- Author
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Naoki Sone, Satoshi Kamiyama, Weifang Lu, Yoshiya Miyamoto, Tetsuya Takeuchi, Renji Okuda, Koichi Mizutani, Isamu Akasaki, Koji Okuno, Kazuyoshi Iida, Kazuma Ito, Masaki Ohya, and Motoaki Iwaya
- Subjects
Morphology (linguistics) ,Materials science ,Electrical resistivity and conductivity ,Tunnel junction ,business.industry ,Nanowire ,Shell (structure) ,Optoelectronics ,Crystal growth ,Metalorganic vapour phase epitaxy ,business ,Layer (electronics) - Abstract
A tunnel junction and a n-GaN cap layer grown on the multi-quantum shells (MQS) /nanowires are introduced to decrease the resistivity and optical loss. The selective-area growth of the MQS/nanowire core-shell structures on the template was performed by metalorganic vapour phase epitaxy (MOVPE). Further, the MQS structure was covered with the tunnel junction and the n-GaN cap layer. Here, the growth conditions of the n-GaN cap layer were systemically investigated. The effect of p-GaN shape on the morphology of grown n-GaN cap layer was also assessed.
- Published
- 2021
24. Developments of GaN-based VCSELs with epitaxially grown DBRs
- Author
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Motoaki Iwaya, Isamu Akasaki, Satoshi Kamiyama, and Tetsuya Takeuchi
- Subjects
Materials science ,business.industry ,law ,Wafer curvature ,Optoelectronics ,Epitaxy ,business ,Laser ,law.invention ,Vertical-cavity surface-emitting laser - Abstract
We will show our two recent results related to GaN-based VCSELs with AlInN/GaN DBRs. One is in-situ wafer curvature control of AlInN/GaN DBRs. Curvature data successfully provides us the strain condition in the AlInN layers, which leads to a high reproducibility for fabricating the AlInN/GaN DBRs. The other is laser operation of the VCSELs with large current apertures. While our previous VCSELs with only 8-15 µm apertures showed RT CW operations, our recent VCSELs now show RT CW operations with 5-30 µm apertures. such larger apertures will eventually provide much higher LOPs in the future.
- Published
- 2021
25. High-concentration doping of donor and acceptor in fluorescent 4H-SiC by closed sublimation growth
- Author
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Satoshi Kamiyama, Kosuke Yanai, Motoaki Iwaya, Weifang Lu, Yoma Yamane, Isamu Akasaki, and Tetsuya Takeuchi
- Subjects
Materials science ,Photoluminescence ,chemistry ,Doping ,Analytical chemistry ,chemistry.chemical_element ,Quantum efficiency ,Sublimation (phase transition) ,Epitaxy ,Boron ,Acceptor ,Fluorescence - Abstract
As a potential material for white light source, high luminous efficiency of fluorescent SiC requires a concentration of higher than 1.0×1019 cm-3 for both the donor (Nd) and acceptor (Na) and a film thickness of about 120 μm. In this study, we focus on the epitaxial growth of fluorescent 4H-SiC by closed sublimation growth in terms of different growth temperatures to increase the doping concentration of boron and nitrogen. In addition, the effect of growth pressure on the surface morphology was investigated. The photoluminescence (PL) intensity of the sample grown at 1900 °C is higher than that grown at 1800 °C. This result indicates that a higher temperature of 1900 °C can increase the boron (B) and nitrogen (N) doping concentration. However, it was confirmed that the surface of the sample grown at 1900 °C was rough. The surface morphology was significantly improved with an increased pressure to 9000 Pa. Meanwhile, the growth time was extended to increase the film thickness, and the amount of BN, which is the doping source of B acceptor, was increased accordingly. As a result, strong PL emission intensity with high internal quantum efficiency (IQE) was successfully demonstrated in 4H-SiC sample, which is comparable to that in fluorescent 6H-SiC.
- Published
- 2021
26. Improvement of device performance in UV-B laser diodes
- Author
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Satoshi Kamiyama, Kazuki Yamada, Isamu Akasaki, Moe Shimokawa, Kosuke Sato, Hideto Miyake, Sayaka Ishizuka, Tetsuya Takeuchi, Motoaki Iwaya, Sho Iwayama, Shohei Teramura, Shunya Tanaka, Tomoya Omori, and Yuya Ogino
- Subjects
Materials science ,business.industry ,Doping ,Laser oscillation ,medicine.disease_cause ,Laser ,Polarization (waves) ,law.invention ,Semiconductor laser theory ,law ,medicine ,Optoelectronics ,business ,Realization (systems) ,Ultraviolet ,Diode - Abstract
Ultraviolet (UV) semiconductor lasers are widely used in medical and industrial applications, and their commercialization is strongly desired. Recently, laser oscillation by current injection in the UV-C and UV-B regions has been reported . From now on, it is necessary to demonstrate CW operation, which is indispensable for practical applications. In order to realize this, it is important to understand the internal loss and optical gain of the current devices. In this presentation, we report the details of our UV-B lasers. Specifically, we would like to discuss the results of the variable stripe length analysis of the internal loss of the obtained device and the performance of the UV-B lasers. The results show that the internal losses of the optimized UV-B laser are relatively low and good values are obtained. We also discuss the details of polarization doping, which is very effective in the realization of these UV lasers.
- Published
- 2021
27. Emission wavelength control of GaInN/GaN multi-quantum shells/nanowires grown by metalorganic vapor phase epitaxy
- Author
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Satoshi Kamiyama, Weifang Lu, Yoshiya Miyamoto, Motoaki Iwaya, Isamu Akasaki, Naoki Sone, Kazuma Ito, Renji Okuda, and Tetsuya Takeuchi
- Subjects
Materials science ,business.industry ,Nanowire ,Epitaxy ,Volumetric flow rate ,law.invention ,Crystal ,Wavelength ,law ,Optoelectronics ,Metalorganic vapour phase epitaxy ,business ,Quantum well ,Light-emitting diode - Abstract
The selective-monolithic growth of coaxial GaInN/GaN NWs was investigated by changing the TEG flow rate, barrier and well growth temperature during MQS growth. In incorporation increased with a higher TEG flow rate. However, In-rich flakes were formed the NWs resulting in the deterioration of crystal quality. Using a higher growth temperature of quantum barriers, abnormal growth at the top of NWs was eliminated. As a result, the CL emission intensity was enhanced. Furthermore, the occurrence of In desorption was suppressed by decreasing the growth temperature of quantum wells. Therefore, these results are promising for NW-based white LEDs.
- Published
- 2021
28. Identifying the cause of thermal droop in GaInN-based LEDs by carrier- and thermo-dynamics analysis
- Author
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Motoaki Iwaya, Dong-Soo Shin, Gyeong Won Lee, Jong-In Shim, Sangjin Min, Satoshi Kamiyama, Dong-Pyo Han, Tetsuya Takeuchi, and Isamu Akasaki
- Subjects
Materials science ,lcsh:Medicine ,02 engineering and technology ,01 natural sciences ,Article ,law.invention ,law ,0103 physical sciences ,Thermal ,Radiative transfer ,Lasers, LEDs and light sources ,Voltage droop ,lcsh:Science ,Electrical impedance ,Diode ,010302 applied physics ,Multidisciplinary ,business.industry ,lcsh:R ,Carrier lifetime ,021001 nanoscience & nanotechnology ,Complex dynamics ,Inorganic LEDs ,Optoelectronics ,lcsh:Q ,0210 nano-technology ,business ,Light-emitting diode - Abstract
This study aims to elucidate the carrier dynamics behind thermal droop in GaInN-based blue light-emitting diodes (LEDs) by separating multiple physical factors. To this end, first, we study the differential carrier lifetimes (DCLs) by measuring the impedance of a sample LED under given driving-current conditions over a very wide operating temperature range of 300 K–500 K. The measured DCLs are decoupled into radiative carrier lifetime (τR) and nonradiative carrier lifetime (τNR), via utilization of the experimental DCL data, and then very carefully investigated as a function of driving current over a wide range of operating temperatures. Next, to understand the measurement results of temperature-dependent τR and τNR characteristics, thermodynamic analysis is conducted, which enables to look deeply into the temperature-dependent behavior of the carriers. On the basis of the results, we reveal that thermal droop is originated by the complex dynamics of multiple closely interrelated physical factors instead of a single physical factor. In particular, we discuss the inherent cause of accelerated thermal droop with elevated temperature.
- Published
- 2020
29. Voltage-controlled anodic oxidation of porous fluorescent sic for effective surface passivation
- Author
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Dong-Pyo Han, Motoaki Iwaya, Satoshi Kamiyama, Haiyan Ou, Weifang Lu, Tetsuya Takeuchi, Kosuke Yanai, Isamu Akasaki, and Yoma Yamane
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Surface passivation ,Fabrication ,Photoluminescence ,Materials science ,Passivation ,Anodic oxidation ,General Chemical Engineering ,Fluorescent SiC ,02 engineering and technology ,01 natural sciences ,Article ,lcsh:Chemistry ,porous structures ,chemistry.chemical_compound ,Hydrofluoric acid ,0103 physical sciences ,General Materials Science ,fluorescent SiC ,Porosity ,surface passivation ,010302 applied physics ,anodic oxidation ,technology, industry, and agriculture ,equipment and supplies ,021001 nanoscience & nanotechnology ,Porous structures ,Fluorescence ,Blueshift ,Wavelength ,lcsh:QD1-999 ,chemistry ,Chemical engineering ,photoluminescence ,0210 nano-technology - Abstract
This study investigated the fabrication of porous fluorescent SiC using a constant voltage-controlled anodic oxidation process. The application of a high, constant voltage resulted in a spatial distinction between the porous structures formed inside the fluorescent SiC substrates, due to the different etching rates at the terrace and the large step bunches. Large, dendritic porous structures were formed as the etching process continued and the porous layer thickened. Under the conditions of low hydrofluoric acid (HF) concentration, the uniformity of the dendritic porous structures through the entire porous layer was considerably improved compared with the conditions of high HF concentration. The resulting large uniform structure offered a sizable surface area, and promoted the penetration of atomic layer-deposited (ALD) Al2O3 films (ALD&ndash, Al2O3). The emission intensity in the porous fluorescent SiC was confirmed via photoluminescence (PL) measurements to be significantly improved by a factor of 128 after ALD passivation. With surface passivation, there was a clear blueshift in the emission wavelength, owing to the effective suppression of the non-radiative recombination rate in the porous structures. Furthermore, the spatial uniformity of emitted light was examined via PL mapping using three different excitation lasers, which resulted in the observation of uniform and distinctive emissions in the fluorescent SiC bulk and porous areas.
- Published
- 2020
30. Growth Defects in InGaN‐Based Multiple‐Quantum‐Shell Nanowires with Si‐Doped GaN Cap Layers and Tunnel Junctions
- Author
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Koji Okuno, Koichi Mizutani, Kazuyoshi Iida, Masaki Ohya, Naoki Sone, Weifang Lu, Renji Okuda, Yoshiya Miyamoto, Kazuma Ito, Satoshi Kamiyama, Tetsuya Takeuchi, Motoaki Iwaya, and Isamu Akasaki
- Subjects
Condensed Matter Physics ,Electronic, Optical and Magnetic Materials - Published
- 2022
31. Improved passivation depth of porous fluorescent 6H-SiC with Si/C faces using atomic layer deposition
- Author
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Kosuke Yanai, Weifang Lu, Yoma Yamane, Keita Kodera, Yiyu Ou, Haiyan Ou, Satoshi Kamiyama, Tetsuya Takeuchi, Motoaki Iwaya, and Isamu Akasaki
- Subjects
Physics and Astronomy (miscellaneous) ,General Engineering ,General Physics and Astronomy - Abstract
We investigated the effects of different growth facets of 6H-SiC and different voltage waveforms on the porous structure and luminescence properties. The structure formed on the surface after anodic etching significantly changed because of the difference in the growth plane, whereas dendritic and columnar pores were observed inside the Si- and C-face samples. These large porous structures were shown to promote the penetration depth of the atomic-layer-deposited Al2O3 films, and a recorded passivation depth of 30 μm layer was confirmed in C-face porous SiC. From the results using a fluorescence microscope and photoluminescence spectra measurement, it was concluded that the pulsed-voltage etching was preferable for fabricating uniform porous structures compared with the constant-voltage etching. However, the enhancement of the luminescence intensity needs to be further improved to realize high luminescent efficiency in porous fluorescent SiC.
- Published
- 2022
32. MOVPE growth of Si-doped GaN cap layers embedding GaN nanowires with multiple-quantum shells
- Author
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Weifang Lu, Koichi Mizutani, Motoaki Iwaya, Satoshi Kamiyama, Tetsuya Takeuchi, Yoshiya Miyamoto, Renji Okuda, Koji Okuno, Kazuyoshi Iida, Isamu Akasaki, Naoki Sone, Kazuma Ito, and Masaki Ohya
- Subjects
Void (astronomy) ,Materials science ,business.industry ,Nanowire ,Gallium nitride ,Condensed Matter Physics ,Epitaxy ,Inorganic Chemistry ,chemistry.chemical_compound ,chemistry ,Tunnel junction ,Electrode ,Materials Chemistry ,Optoelectronics ,Metalorganic vapour phase epitaxy ,business ,Layer (electronics) - Abstract
In this study, the growth mechanisms of n-type gallium nitride (GaN) cap layers for embedding nanowire-based multi-quantum-shell (NW-MQS) with a tunnel junction were investigated using the metal–organic vapor-phase epitaxy method. Herein, instead of a p-type GaN layer, n-type GaN cap layers were applied as a contact layer to the anode electrode via a tunnel junction. NW-MQSs were prepared in a rectangular grid arrangement aligning with the m- and a-axis of GaN. The growth mode of the cap layers was controlled in three stages using different growth pressures and temperatures. The first cap layer was a faceted growth mode primarily comprising the r-plane, the second was a lateral growth mode that expanded the area of the c-plane, and the third was an acceleration of lateral growth. Finally, cap layers with a flat surface were realized. Void formation in the cap layers occurred only between the NW-MQSs along the m-axis of GaN. This observation can be attributed to the difference in the coalescence of the cap layers in the a- and m-axis directions. We believe that the NW-MQS structure with the cap layer that was optimized in this study can be used as a highly efficient optical device.
- Published
- 2022
33. AlGaN-based ultraviolet-B laser diode at 298 nm with threshold current density of 25 kA/cm2
- Author
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Sho Iwayama, Kosuke Sato, Satoshi Kamiyama, Kazuki Yamada, Tetsuya Takeuchi, Yuya Ogino, Motoaki Iwaya, Sayaka Ishizuka, Shunya Tanaka, Tomoya Omori, Hideto Miyake, Isamu Akasaki, Shinji Yasue, and Shohei Teramura
- Subjects
Threshold current ,Materials science ,020205 medical informatics ,Laser diode ,business.industry ,Ultraviolet b ,02 engineering and technology ,medicine.disease_cause ,Cladding (fiber optics) ,law.invention ,law ,0202 electrical engineering, electronic engineering, information engineering ,medicine ,Sapphire ,Optoelectronics ,business ,Current density ,Ultraviolet ,Voltage - Abstract
Ultraviolet-B laser diode at 298 nm was realized by developing the technologies; growth method of lattice-relaxed Al 0.6 Ga 0.4 N layer on AlN/sapphire template and III-group composition-graded p-AlGaN cladding layer. The threshold current, current density and voltage were 5.0 A, 25 kA/cm2, and 34.4 V, respectively.
- Published
- 2020
34. Crystal growth and optical property in core-shell structure consisting of GaN nanowire and GaInN/GaN multi-quantum shell (MQS) (Conference Presentation)
- Author
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Satoshi Kamiyama, Isamu Akasaki, Motoaki Iwaya, Lu Weifang, and Tetsuya Takeuchi
- Subjects
Materials science ,business.industry ,Shell (structure) ,Nanowire ,Structure (category theory) ,Crystal growth ,Laser ,law.invention ,Crystal ,Quality (physics) ,law ,Optoelectronics ,business ,Quantum - Abstract
The three-dimensional core-shell structure consisting of GaN nanowire and GaInN/GaN multi-quantum shell (MQS) is thought to be promising for high-performance light-emitting devices, because of its advantages such as non-polar surface orientation, dislocation-free and tolerance of misfit strain due to small size crystal. However, their crystal growth mechanism and its crystalline quality have not yet fully understood, and they are still under the basic considerations. The key issues in the nanowire/MQS structure are its crystalline quality and shape control, which are greatly dependent on the growth condition. Thus, in this paper, selective growth of nanowire/MQS and its optical properties are described.
- Published
- 2020
35. Characterization of nonpolar a-plane GaN epi-layers grown on high-density patterned r-plane sapphire substrates
- Author
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Hisayoshi Daicho, Tetsuya Takeuchi, Isamu Akasaki, Shunya Otsuki, Shogo Sugimori, Satoshi Kamiyama, Motoaki Iwaya, and Daiki Jinno
- Subjects
010302 applied physics ,Threading dislocations ,Materials science ,Morphology (linguistics) ,Plane (geometry) ,Analytical chemistry ,High density ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Characterization (materials science) ,Inorganic Chemistry ,0103 physical sciences ,Materials Chemistry ,Sapphire ,Nanometre ,0210 nano-technology - Abstract
To reduce the number of threading dislocations (TDs) in nonpolar a -plane GaN ( a -GaN) epi-layers grown on flat r -plane sapphire substrates ( r -FSS), we investigated the effects on the crystalline quality of the a -GaN epi-layers of high-density patterned r -plane sapphire substrates ( r -HPSS), the patterns of which were placed at intervals of several hundred nanometers. Two types of r -HPSS, the patterns of which had diameters and heights on the order of several hundred nanometers ( r -NHPSS) or several micrometers ( r -MHPSS), were prepared with conventional r -FSS. The effect of these r -HPSS on the a -GaN epi-layers was demonstrated by evaluating the surface morphology and the crystalline quality of the epi-layers. The surfaces of the a -GaN epi-layer grown on r -FSS and r -NHPSS were pit-free and mirror-like, whereas the surface of the a -GaN epi-layer grown on r -MHPSS was very rough due to the large, irregular GaN islands that grew on the patterns, mainly at the initial growth stage. The crystalline quality of the a -GaN epi-layer grown on r -NHPSS was better than that of the a -GaN epi-layer grown on r -FSS. We confirmed that there were fewer TDs in the a -GaN epi-layer grown on r -NHPSS than there were in the a -GaN epi-layer grown on r -FSS. The TDs propagating to the surface in a -GaN epi-layer grown on r -NHPSS were mainly generated on the flat sapphire regions between the patterns. Interestingly, it was also found that the TDs that propagated to the surface concentrated with a periodic pitch along the c -axis direction. The TD densities of a -GaN epi-layers grown on r -FSS and r -NHPSS were estimated to be approximately 5.0 × 10 10 and 1.5 × 10 9 cm −2 , respectively. This knowledge will contribute to the further development of a -GaN epi-layers for high-performance devices.
- Published
- 2018
36. Analysis of impurity doping in tunnel junction grown on core–shell structure composed of GaInN/GaN multiple-quantum-shells and GaN nanowire
- Author
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Naoki Sone, Satoshi Ishimura, Weifang Lu, Motoaki Iwaya, Koichi Mizutani, Satoshi Kamiyama, Isamu Akasaki, Yukimi Jinno, Jun Koyama, Dong-Pyo Han, Shiori Yamamura, Yoshiya Miyamoto, Satoru Nakajima, Daiki Jinno, Renji Okuda, Koji Okuno, Tetsuya Takeuchi, and Norihito Mayama
- Subjects
Core shell ,Materials science ,Physics and Astronomy (miscellaneous) ,Tunnel junction ,business.industry ,Multiple quantum ,General Engineering ,Nanowire ,General Physics and Astronomy ,Optoelectronics ,Impurity doping ,business - Abstract
This study aimed to investigate and analyze the impurity doping characteristics in tunnel junctions (TJs) grown on core–shell structures, comprising GaInN/GaN multiple-quantum-shells (MQSs) and GaN nanowires. To this end, the impurity, structural, and electrical properties of the samples were characterized by scanning electron microscopy, scanning transmission electron microscopy, atom probe tomography (APT), nanoscale secondary ion mass spectrometry (NanoSIMS), and electroluminescence of the device which was fabricated for a prototype laser device to demonstrate an electrical operation of the MQSs layer. From the experimental results of NanoSIMS and APT, we demonstrated that the Mg-related problems in the TJ, such as the diffusion to the n++-GaN layer from the p+-GaN layer and formation of clusters in p+-GaN, are critical. Consequently, they cause a high operating voltage and dot-like spot emission of the light-emitting device. Based on the analysis, we suggested remedies and strategies to further improve the TJs that work well.
- Published
- 2021
37. Characterization and optimization of sputtered AlN buffer layer on r-plane sapphire substrate to improve the crystalline quality of nonpolar a-plane GaN
- Author
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Shogo Sugimori, Tetsuya Takeuchi, Satoshi Kamiyama, Motoaki Iwaya, Daiki Jinno, Isamu Akasaki, Hisayoshi Daicho, and Shunya Otsuki
- Subjects
010302 applied physics ,Materials science ,Annealing (metallurgy) ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Epitaxy ,01 natural sciences ,Layer thickness ,Buffer (optical fiber) ,Inorganic Chemistry ,Crystallography ,0103 physical sciences ,Materials Chemistry ,Sapphire substrate ,Composite material ,0210 nano-technology - Abstract
Here we examined the use of AlN buffer layers of various thicknesses to improve the crystalline quality of nonpolar a -plane GaN ( a -GaN) grown on an r -plane sapphire substrate. Three types of AlN buffer layers were used: sputtered AlN buffer layers (sp-AlNs) with or without annealing, and epitaxially grown AlN buffer layers (ep-AlNs). Buffer layer thicknesses of 30, 90, and 180 nm were used. We found that the surface morphological transitions with increasing thickness were different between the sp-AlNs and the ep-AlNs, and that the sp-AlNs had poorer crystallographic orientations than did the ep-AlNs. Annealing caused marked changes to occur in the surface morphologies and crystallographic orientations of the sp-AlNs; however, the positive effect of annealing was limited because the in-plane crystallographic orientation degraded with increasing layer thickness. The optimal buffer layer was found to be the 30-nm-thick annealed sp-AlN, which was composed of uniformly arranged oval grains with better crystallographic orientation than the other sp-AlNs and annealed sp-AlNs. The crystalline quality of the a -GaN epilayer grown on 30-nm-thick annealed sp-AlN had a narrower X-ray rocking curve–full width at half maximum for both the on- and off-axis planes compared with that grown on any other AlN buffer layers.
- Published
- 2017
38. Development of ultra-high speed light emitters and detectors for visible light communication
- Author
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Tetsuya Takeuchim Motoaki Iwaya, Satoshi Kamiyama, and Isamu Akasaki
- Subjects
Ultra high speed ,Materials science ,business.industry ,Detector ,Visible light communication ,Optoelectronics ,business - Published
- 2018
39. Quasi-ballistic thermal conduction in 6H–SiC
- Author
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Hiroshi Amano, Isamu Akasaki, Daiki Tanaka, Nakib Haider Protik, T. Takeuchi, Shangkun Wang, Satoshi Kamiyama, Samuel Graham, Weifang Lu, Zhe Cheng, Motoaki Iwaya, and Jingjing Shi
- Subjects
Materials science ,Physics and Astronomy (miscellaneous) ,Phonon ,FOS: Physical sciences ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,Condensed Matter::Materials Science ,Thermal conductivity ,Condensed Matter::Superconductivity ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,Thermal ,General Materials Science ,Condensed Matter - Materials Science ,Condensed Matter - Mesoscale and Nanoscale Physics ,Phonon scattering ,Condensed matter physics ,Scattering ,Doping ,Wide-bandgap semiconductor ,Materials Science (cond-mat.mtrl-sci) ,021001 nanoscience & nanotechnology ,Thermal conduction ,0104 chemical sciences ,0210 nano-technology ,Energy (miscellaneous) - Abstract
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 scatterings and laws of diffusive thermal conduction fail, resulting in significant reduction in the effective thermal conductivity. This work reports, for the first time, 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.
- Published
- 2021
40. Influence of silane flow rate on the structural and optical properties of GaN nanowires with multiple-quantum-shells
- Author
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Isamu Akasaki, Koichi Mizutani, Satoshi Kamiyama, Kazuma Ito, Yoshiya Miyamoto, Kazuyoshi Iida, Koji Okuno, Weifang Lu, Naoki Sone, Motoaki Iwaya, Masaki Ohya, Tetsuya Takeuchi, Renji Okuda, and Dong-Pyo Han
- Subjects
010302 applied physics ,Materials science ,business.industry ,Scanning electron microscope ,Nanowire ,Cathodoluminescence ,02 engineering and technology ,Semiconductor device ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Silane ,Volumetric flow rate ,Inorganic Chemistry ,Core (optical fiber) ,chemistry.chemical_compound ,chemistry ,0103 physical sciences ,Scanning transmission electron microscopy ,Materials Chemistry ,Optoelectronics ,0210 nano-technology ,business - Abstract
In this study, we discuss the influence of SiH4 flow rates on the structural and optical properties of GaN nanowires (NWs) with multiple-quantum-shells (MQSs). To this end, we prepared two n-GaN core NW samples with different SiH4 flow rates. Subsequently, MQS active layers of the same structure were grown on each n-GaN core NW under identical growth conditions. The samples were characterized by scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray, and cathodoluminescence (CL) mapping. From the experimental results, we ascertained that a Si-rich layer was created between the sidewall of the NWs and MQSs, in which the number of Si atoms was mainly determined by the SiH4 flow rate. These Si atoms diffused into the MQSs during the growth, and significantly impacted the structural and optical properties, such as the shape and crystalline quality of the MQSs and NWs, and the CL intensity of the MQSs. On the basis of experimental results, we conclude that the SiH4 flow rate during the NW growth plays a critical role in the performance of an MQS-based optoelectronic semiconductor device.
- Published
- 2021
41. Effects of Mg dopant in Al-composition-graded Al x Ga1−x N (0.45 ≤ x) on vertical electrical conductivity of ultrawide bandgap AlGaN p–n junction
- Author
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Motoaki Iwaya, Kosuke Sato, Konrad Sakowski, Tetsuya Takeuchi, Stanislaw Krukowski, Yoshihiro Kangawa, Jacek Piechota, Satoshi Kamiyama, Pawel Kempisty, Kazuki Yamada, and Isamu Akasaki
- Subjects
Materials science ,Dopant ,Electrical resistivity and conductivity ,Band gap ,General Engineering ,medicine ,Analytical chemistry ,General Physics and Astronomy ,Composition (combinatorics) ,medicine.disease_cause ,p–n junction ,Ultraviolet - Published
- 2021
42. Space-charge effect on photogenerated-current and -voltage in III-nitride optoelectronic semiconductors
- Author
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Dong-Pyo Han, Tetsuya Takeuchi, Motoaki Iwaya, Satoshi Kamiyama, and Isamu Akasaki
- Subjects
Shockley diode ,Materials science ,business.industry ,Nitride ,Space charge ,Atomic and Molecular Physics, and Optics ,Electronic, Optical and Magnetic Materials ,law.invention ,symbols.namesake ,Semiconductor ,Stark effect ,law ,symbols ,Optoelectronics ,business ,Excitation ,Voltage ,Light-emitting diode - Abstract
In this study, we attempted to elucidate the carrier dynamics behind the abnormal characteristics of photogenerated current and voltage ( I Ph and V Ph ) at cryogenic temperature in III-nitride optoelectronic semiconductors by employing space-charge theory. To this end, we carefully investigated and analyzed excitation-power-dependent I – V (PDIV) curves operated by quasiresonant excitation of an AlGaInN-based p − i − n junction semiconductor at 300 K and 15 K. At 300 K, the curves exhibited typical characteristics and were well described by the conventional theory. However, the PDIV curves at 15 K could no longer be described by the conventional theory. To elucidate the mechanism behind this phenomenon, we proposed a model in which the space-charge effect (SCE) plays a key role. Based on this model, we proposed the modified Shockley diode equation, which can explain the PDIV characteristic at 15 K, including the SCE. We also discussed the SCE on the efficiency of devices.
- Published
- 2021
43. Identification of multi-color emission from coaxial GaInN/GaN multiple-quantum-shell nanowire LEDs.
- Author
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Kazuma Ito, Weifang Lu, Sae Katsuro, Renji Okuda, Nanami Nakayama, Naoki Sone, Koichi Mizutani, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, and Isamu Akasaki
- Published
- 2022
- Full Text
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44. Room temperature pulsed operation of nitride nanowire-based multi-quantum shell laser diodes by MOVPE
- Author
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Motoaki Iwaya, Koji Okuno, Koichi Mizutani, Satoshi Kamiyama, Yoshiya Miyamoto, Renji Okuda, Isamu Akasaki, Kazuyoshi Iida, Tetsuya Takeuchi, Weifang Lu, Naoki Sone, Kazuma Ito, and Masaki Ohya
- Subjects
Materials science ,business.industry ,General Engineering ,Nanowire ,Shell (structure) ,General Physics and Astronomy ,Nitride ,Laser ,law.invention ,law ,Optoelectronics ,Metalorganic vapour phase epitaxy ,business ,Quantum ,Diode - Published
- 2021
45. Analysis of carrier injection efficiency of AlGaN UV-B laser diodes based on the relationship between threshold current density and cavity length
- Author
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Isamu Akasaki, Motoaki Iwaya, Hideto Miyake, Sayaka Ishizuka, Satoshi Kamiyama, Kosuke Sato, Tetsuya Takeuchi, Kazuki Yamada, Sho Iwayama, Shohei Teramura, Shunya Tanaka, and Tomoya Omori
- Subjects
Threshold current ,Materials science ,Physics and Astronomy (miscellaneous) ,business.industry ,law ,General Engineering ,General Physics and Astronomy ,Optoelectronics ,business ,Laser ,Diode ,law.invention - Published
- 2021
46. GaN-based tunnel junctions and optoelectronic devices grown by metal-organic vapor-phase epitaxy
- Author
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Isamu Akasaki, Tetsuya Takeuchi, Satoshi Kamiyama, and Motoaki Iwaya
- Subjects
Materials science ,business.industry ,Condensed Matter Physics ,Epitaxy ,Organic vapor ,Electronic, Optical and Magnetic Materials ,Metal ,Tunnel junction ,Phase (matter) ,visual_art ,Materials Chemistry ,visual_art.visual_art_medium ,Optoelectronics ,Metalorganic vapour phase epitaxy ,Electrical and Electronic Engineering ,business - Abstract
This paper mainly describes the status and prospects of GaN-based tunnel junctions grown by metal-organic vapor-phase epitaxy. GaN-based tunnel junctions are expected to offer an alternative structure for hole injection in various optoelectronic devices, simultaneously providing additional features, such as electrical contacts between cascaded devices, current confinement, simple device fabrication processes, and novel controllability in band engineering. After reviewing the role of tunnel junctions and the history of the development of GaN-based tunnel junctions, the development details of GaInN, GaN, and AlGaN tunnel junctions are separately summarized, including those grown by molecular beam epitaxy. Various optoelectronic devices utilizing GaN-based tunnel junctions are reviewed from the viewpoint of device characteristics.
- Published
- 2021
47. Discrete AlN mole fraction of n/12 (n = 4–8) in Ga-rich zones functioning as electron pathways created in nonflat AlGaN layers grown on high-miscut sapphire substrates
- Author
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Ryuichi Sugie, Yosuke Nagasawa, Hideki Sako, Yoshio Honda, Isamu Akasaki, Kazunobu Kojima, Masamichi Ippommatsu, Shigefusa F. Chichibu, Hiroshi Amano, Ai Hashimoto, and Akira Hirano
- Subjects
Materials science ,Electron diffraction ,Scanning electron microscope ,Scanning transmission electron microscopy ,Sapphire ,Analytical chemistry ,General Physics and Astronomy ,Cathodoluminescence ,Electron ,Spectroscopy ,Mole fraction - Abstract
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.
- Published
- 2021
48. AlGaN-based UV-B laser diode with a wavelength of 290 nm on 1 μm periodic concavo–convex pattern AlN on a sapphire substrate
- Author
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Hideto Miyake, Tomoya Omori, Kosuke Sato, Satoshi Kamiyama, Tetsuya Takeuchi, Motoaki Iwaya, Sho Iwayama, Moe Shimokawa, Shunya Tanaka, Shohei Teramura, Kazuki Yamada, and Isamu Akasaki
- Subjects
Wavelength ,Materials science ,Laser diode ,law ,business.industry ,General Engineering ,General Physics and Astronomy ,Sapphire substrate ,Optoelectronics ,Metalorganic vapour phase epitaxy ,Dislocation ,business ,law.invention - Abstract
Room-temperature pulsed oscillation with a laser wavelength of 290 nm and a threshold current density of 35 kA cm−2 was achieved by fabricating a UV-B laser diode on a thick AlGaN film formed on a 1 μm periodic concavo–convex pattern AlN (PCCP-AlN) on a sapphire substrate. The advantage of this method using PCCP-AlN is that it promotes the nucleation of AlGaN crystals. Planarization of this growth nucleus with AlGaN reduces the threading dislocation density at the top of the AlGaN growth layer while suppressing the formation of giant micrometer-sized hillocks and V-shaped pits that appear irregularly.
- Published
- 2021
49. AlGaN-based UV-B laser diode with a high optical confinement factor
- Author
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Motoaki Iwaya, Satoshi Kamiyama, Shohei Teramura, Ayumu Yabutani, Sho Iwayama, Tomoya Omori, Reo Ogura, Yuya Ogino, Shunya Tanaka, Sayaka Ishizuka, Hideto Miyake, Moe Shimokawa, Kosuke Sato, Tetsuya Takeuchi, Isamu Akasaki, and Kazuki Yamada
- Subjects
010302 applied physics ,Materials science ,Threshold current ,Physics and Astronomy (miscellaneous) ,Laser diode ,business.industry ,02 engineering and technology ,021001 nanoscience & nanotechnology ,medicine.disease_cause ,Cladding (fiber optics) ,Laser ,01 natural sciences ,Waveguide (optics) ,law.invention ,law ,0103 physical sciences ,medicine ,Optoelectronics ,0210 nano-technology ,business ,Layer (electronics) ,Ultraviolet ,Diode - Abstract
To reduce the threshold current density (Jth) of ultraviolet (UV)-B AlGaN-based laser diodes, we investigated the critical parameters aiming to increase the injection efficiency ηi and the optical confinement factor Γ. Optimization of the thickness of the waveguide layer, the average Al content of the p-type AlGaN cladding layer, and the film thickness of the cladding layer demonstrated that the device characteristics can be improved. This optimization achieved a reduction in Jth to 13.3 kA cm−2 at a lasing wavelength of 300 nm, thus offering the lowest Jth value yet achieved for a UV-B laser diode.
- Published
- 2021
50. Crystal Growth and Optical Property of GaN Nanowire Cores and GaInN/GaN Multi-Quantum Shells Grown by Metalorganic Vapor Phase Epitaxy
- Author
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Isamu Akasaki, Weifang Lu, Motoaki Iwaya, Tetsuya Takeuchi, and Satoshi Kamiyama
- Subjects
Materials science ,020205 medical informatics ,business.industry ,Nanowire ,Shell (structure) ,Crystal growth ,02 engineering and technology ,Epitaxy ,Crystallographic defect ,Crystal ,0202 electrical engineering, electronic engineering, information engineering ,Optoelectronics ,Metalorganic vapour phase epitaxy ,business ,Quantum - Abstract
An n-GaN nanowire and a GaInN/GaN multi-quantum shell (MQS) were uniformly grown by the continuous flow mode MOVPE with the optimal growth condition. Although the extended defect-free crystal was successfully grown, the MQS may contain point defects acting as the nonradiative recombination centers. The AlGaN underlying shell was found to be effective point defect trap to improve the optical property of the MQS.
- Published
- 2019
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