93 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. 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
9. 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
10. 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
11. 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
12. Identifying the cause of thermal droop in GaInN-based LEDs by carrier- and thermo-dynamics analysis
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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
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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.
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- 2020
13. Voltage-controlled anodic oxidation of porous fluorescent sic for effective surface passivation
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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
14. Quasi-ballistic thermal conduction in 6H–SiC
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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
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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.
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- 2021
15. Space-charge effect on photogenerated-current and -voltage in III-nitride optoelectronic semiconductors
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Dong-Pyo Han, Tetsuya Takeuchi, Motoaki Iwaya, Satoshi Kamiyama, and Isamu Akasaki
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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.
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- 2021
16. Identification of multi-color emission from coaxial GaInN/GaN multiple-quantum-shell nanowire LEDs.
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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
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- 2022
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17. AlGaN-based UV-B laser diode with a wavelength of 290 nm on 1 μm periodic concavo–convex pattern AlN on a sapphire substrate
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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
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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.
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- 2021
18. Tuning the Resonant Frequency of a Surface Plasmon by Double-Metallic Ag/Au Nanoparticles for High-Efficiency Green Light-Emitting Diodes
- Author
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Kengo Yamamoto, Motoaki Iwaya, Satoshi Kamiyama, Ryoya Mano, Tetsuya Takeuchi, Isamu Akasaki, Dong-Pyo Han, and Seiji Ishimoto
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Materials science ,resonant frequency ,surface plasmon ,Nanoparticle ,02 engineering and technology ,Green-light ,01 natural sciences ,lcsh:Technology ,law.invention ,lcsh:Chemistry ,law ,0103 physical sciences ,General Materials Science ,Instrumentation ,lcsh:QH301-705.5 ,Diode ,010302 applied physics ,Fluid Flow and Transfer Processes ,business.industry ,lcsh:T ,Process Chemistry and Technology ,nanoparticle ,Surface plasmon ,General Engineering ,021001 nanoscience & nanotechnology ,lcsh:QC1-999 ,Computer Science Applications ,Wavelength ,lcsh:Biology (General) ,lcsh:QD1-999 ,lcsh:TA1-2040 ,Optoelectronics ,light-emitting diode ,internal quantum efficiency ,Quantum efficiency ,0210 nano-technology ,business ,lcsh:Engineering (General). Civil engineering (General) ,Layer (electronics) ,lcsh:Physics ,Light-emitting diode - 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.
- Published
- 2019
- Full Text
- View/download PDF
19. Development of Monolithically Grown Coaxial GaInN/GaN Multiple Quantum Shell Nanowires by MOCVD
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Satoshi Kamiyama, Renji Okuda, Weifang Lu, Isamu Akasaki, Motoaki Iwaya, Kazuma Ito, Tetsuya Tekeuchi, Naoki Sone, and Yoshiya Miyamoto
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In incorporation ,Materials science ,General Chemical Engineering ,Nanowire ,Cathodoluminescence ,02 engineering and technology ,Chemical vapor deposition ,01 natural sciences ,Article ,law.invention ,lcsh:Chemistry ,chemistry.chemical_compound ,law ,multi-color emission ,0103 physical sciences ,coaxial MQS nanowires ,General Materials Science ,Metalorganic vapour phase epitaxy ,Triethylgallium ,Quantum well ,010302 applied physics ,business.industry ,monolithic growth ,021001 nanoscience & nanotechnology ,lcsh:QD1-999 ,chemistry ,Optoelectronics ,Coaxial ,0210 nano-technology ,business ,Light-emitting diode - 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&ndash, 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).
- Published
- 2020
20. Effects of Mg and Si doping in the guide layers of AlGaN-based ultraviolet-B band lasers
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Satoshi Kamiyama, Shunya Tanaka, Motoaki Iwaya, Isamu Akasaki, Shinji Yasue, Kosuke Sato, Tetsuya Takeuchi, and Yuya Ogino
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010302 applied physics ,Materials science ,Photoluminescence ,business.industry ,Doping ,Ultraviolet b ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,medicine.disease_cause ,Laser ,01 natural sciences ,law.invention ,Inorganic Chemistry ,law ,Excited state ,0103 physical sciences ,Materials Chemistry ,medicine ,Optoelectronics ,Quantum efficiency ,0210 nano-technology ,business ,Ultraviolet ,Power density - Abstract
We investigated the effects of Si and Mg doping concentrations in the guide layers on the threshold power density of an AlGaN-based optically pumped ultraviolet-B band (UV-B) laser. For Si, a decrease in the threshold power density required for laser oscillation was confirmed at doping concentrations up to an order of 1017 cm−3. However, an increase was confirmed at high concentrations of approximately 1018 cm−3. For Mg doping, no decrease in the threshold power density of the optically pumped laser was confirmed even at low Mg concentrations, and it increased rapidly at 1018 cm−3. These results were characterized by weakly excited photoluminescence and variable stripe length, indicating that they were due to changes in optical gain with increasing or decreasing internal quantum efficiency and changes in internal loss. This suggests that Mg doping of the guide layer in UV-B lasers should be lower than 1018 cm−3 to realize low-threshold power density lasers. On the other hand, doping on an order of up to 1017 cm−3 for Si was effective at reducing the threshold power density. This indicates that the threshold power density increases if it is too high. These results could provide guidance for designing UV-B lasers in the future.
- Published
- 2020
21. Controlled synthesis of nonpolar GaInN/GaN multiple-quantum-shells on GaN nanowires by metal-organic chemical vapour deposition
- Author
-
Motoaki Iwaya, Mizuki Terazawa, Kazuyoshi Iida, Hedeki Murakami, Dong-Pyo Han, Satoshi Kamiyama, Tetsuya Tekeuchi, Naoki Sone, Isamu Akasaki, Nanami Goto, and Weifang Lu
- Subjects
Chemical substance ,Materials science ,business.industry ,Nanowire ,General Physics and Astronomy ,Cathodoluminescence ,02 engineering and technology ,Surfaces and Interfaces ,General Chemistry ,Chemical vapor deposition ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,law.invention ,law ,Scanning transmission electron microscopy ,Optoelectronics ,Metalorganic vapour phase epitaxy ,0210 nano-technology ,business ,Light-emitting diode ,Diode - Abstract
Nonpolar GaInN/GaN multiple-quantum shells (MQSs) on nanowires (NWs) were investigated for high-efficiency light-emitting diodes (LEDs). The growth conditions of NWs were systematically optimized via a continuous growth mode in metal-organic chemical vapour deposition (MOCVD). The In incorporation rate on the m-planes decreased as the growth temperature elaborated, whereas the crystalline quality is improved. The cathodoluminescence (CL) results revealed that longer growth time of the GaInN well can induce additional In-rich droplets and degrade the emission properties of MQSs. The CL emission intensity and the peak wavelength increased as the number of MQS pairs increased from one to three pairs, which was attributed to the increased In incorporation as the diameter enhanced. The linearly enhanced CL emission intensity with barrier thickness was ascribed to the increase of the electron-hole states from the GaN barrier to the wells, resulting in a larger recombination probability. The scanning transmission electron microscopy (STEM) results demonstrated that a thicker barrier shell can suppress the formation of In-rich droplets. Overall, the feasibility of obtaining high-quality m-plane coaxial GaInN/GaN MQSs structures are promising for NW-based white and micro LEDs.
- Published
- 2020
22. High Crystallinity and Highly Relaxed Al 0.60 Ga 0.40 N Films Using Growth Mode Control Fabricated on a Sputtered AlN Template with High‐Temperature Annealing
- Author
-
Isamu Akasaki, Yuta Kawase, Motoaki Iwaya, Tetsuya Takeuchi, Sho Iwayama, Satoshi Kamiyama, Yusuke Sakuragi, Hideto Miyake, and Shohei Teramura
- Subjects
Materials science ,Annealing (metallurgy) ,Surfaces and Interfaces ,Condensed Matter Physics ,Laser ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,law.invention ,Crystallinity ,law ,Materials Chemistry ,Mode control ,Electrical and Electronic Engineering ,Composite material - Published
- 2020
23. Analysis of Spontaneous Subpeak Emission from the Guide Layers of the Ultraviolet‐B Laser Diode Structure Containing Composition‐Graded p‐AlGaN Cladding Layers
- Author
-
Satoshi Kamiyama, Yuya Ogino, Isamu Akasaki, Shinji Yasue, Kosuke Sato, Tetsuya Takeuchi, and Motoaki Iwaya
- Subjects
Materials science ,Laser diode ,business.industry ,Ultraviolet b ,Surfaces and Interfaces ,Electroluminescence ,Condensed Matter Physics ,medicine.disease_cause ,Cladding (fiber optics) ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,law.invention ,law ,Materials Chemistry ,medicine ,Optoelectronics ,Electrical and Electronic Engineering ,business ,Ultraviolet - Published
- 2020
24. Improved Uniform Current Injection into Core‐Shell‐Type GaInN Nanowire Light‐Emitting Diodes by Optimizing Growth Condition and Indium‐Tin‐Oxide Deposition
- Author
-
Naoki Sone, Motoaki Iwaya, Hideki Murakami, Masaki Ohya, Mizuki Terazawa, Satoshi Kamiyama, Atsushi Suzuki, Kazuyoshi Iida, Dong-Pyo Han, Weifang Lu, Isamu Akasaki, Nanami Goto, and Tetsuya Takeuchi
- Subjects
Materials science ,business.industry ,Nanowire ,Surfaces and Interfaces ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Indium tin oxide ,law.invention ,Core shell ,law ,Materials Chemistry ,Optoelectronics ,Electrical and Electronic Engineering ,Current (fluid) ,business ,Deposition (chemistry) ,Light-emitting diode - Published
- 2019
25. Growth and Characterization of Core-Shell Structures Consisting of GaN Nanowire Core and GaInN/GaN Multi-Quantum Shell
- Author
-
Satoshi Kamiyama, Isamu Akasaki, Weifang Lu, Tetsuya Takeuchi, and Motoaki Iwaya
- Subjects
010302 applied physics ,Materials science ,business.industry ,Diffusion ,Doping ,Nanowire ,Shell (structure) ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Epitaxy ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,Characterization (materials science) ,Core (optical fiber) ,0103 physical sciences ,Optoelectronics ,0210 nano-technology ,business ,Intensity (heat transfer) - Abstract
The growth of dislocation-free and uniform GaN nanowires and high emission efficiency in a GaInN/GaN multi-quantum shell (MQS) are demonstrated. Simultaneous-supply-mode metal-organic vapor phase epitaxy and a high growth temperature are applied, and uniform GaN nanowires are obtained without a too high Si doping concentration. The GaInN/GaN MQS grown on the n-GaN nanowire has a thickness variation in the height direction, possibly owing to the interplane diffusion of Ga atoms. The MQS exhibits distributed CL emission in the case of a too high Si doping concentration in the n-GaN nanowires. For further improvement of the optical property in the MQS, a low-temperature-grown thin AlGaN shell, located just below the MQS active region, is introduced, and 6 times higher CL intensity is observed.
- Published
- 2019
26. Demonstration of electron beam laser excitation in the UV range using a GaN/AlGaN multiquantum well active layer
- Author
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Yuta Kawase, Takafumi Hayashi, Sho Iwayama, Tetsuya Takeuchi, Motoaki Iwaya, Noriaki Nagata, Satoshi Kamiyama, Takashi Senga, Isamu Akasaki, and Takahiro Matsumoto
- Subjects
Materials science ,Science ,Physics::Optics ,02 engineering and technology ,01 natural sciences ,Article ,law.invention ,Optical pumping ,law ,0103 physical sciences ,Power density ,010302 applied physics ,Multidisciplinary ,business.industry ,021001 nanoscience & nanotechnology ,Laser ,Active layer ,Wavelength ,Excited state ,Cathode ray ,Optoelectronics ,Medicine ,Physics::Accelerator Physics ,0210 nano-technology ,business ,Excitation - 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.
- Published
- 2017
27. 450 nm GaInN ridge stripe laser diodes with AlInN/AlGaN multiple cladding layers
- Author
-
Motoaki Iwaya, Isamu Akasaki, Satoshi Kamiyama, Kei Arakawa, Makoto Miyoshi, Kohei Miyoshi, Tetsuya Takeuchi, Ryosuke Iida, and Yuki Kato
- Subjects
Materials science ,Threshold current ,Physics and Astronomy (miscellaneous) ,Laser diode ,business.industry ,General Engineering ,General Physics and Astronomy ,Cladding (fiber optics) ,Laser ,law.invention ,law ,Optoelectronics ,business ,Visible laser ,Diode - Abstract
We investigated and improved optical waveguides along the vertical and horizontal directions in 450 nm GaInN laser diodes. As a result, we demonstrated a low threshold current density (1.15 kA cm−2) of a GaInN ridge stripe laser diode containing a 3-pair 40 nm Al0.82In0.18N/25 nm Al0.03Ga0.97N multiple bottom cladding layer at room temperature under pulsed condition. This threshold current density is smaller than our typical value with a 1 μm Al0.03Ga0.97N bottom cladding layer. AlInN/AlGaN multiple layers are useful as n-type cladding layers in visible laser diodes to achieve higher optical confinement factors while smooth surfaces were obtained.
- Published
- 2019
28. Ultraviolet-B band lasers fabricated on highly relaxed thick Al0.55Ga0.45N films grown on various types of AlN wafers
- Author
-
Tetsuya Takeuchi, Yuta Kawase, Satoshi Kamiyama, Motoaki Iwaya, Yusuke Sakuragi, Sho Iwayama, Hideto Miyake, Isamu Akasaki, Shinji Yasue, and Syunya Ikeda
- Subjects
Materials science ,Physics and Astronomy (miscellaneous) ,business.industry ,law ,General Engineering ,General Physics and Astronomy ,Optoelectronics ,Ultraviolet b ,Wafer ,business ,Laser ,law.invention - Abstract
In this paper, we investigated the dependence of threshold power density on the Al0.55Ga0.45N underlying layer film thickness in ultraviolet-B band (UV-B) lasers on various AlN wafers (four types). We also prepared and compared AlN templates for AlN freestanding substrates, AlN films fabricated by metalorganic vapor phase epitaxy, and annealed sputtered AlN templates at high temperature. The initial growth of AlGaN became three-dimensional by inserting a homoepitaxial Ga-doped AlN layer between the AlN template and Al0.55Ga0.45N, before it shifted to two-dimensional growth. It is possible to reduce the dislocation in Al0.55Ga0.45N using this mode. The dependence of AlGaN film thickness and that of the AlN template on samples with an inserted homoepitaxial Ga-doped AlN layer were studied. Compared with Al0.55Ga0.45N having a thickness of 5 μm, there was almost no noticeable difference between the dark spot density characterized by cathodoluminescence and the threshold power density in UV-B lasers for the AlN template. Besides, the characteristics were noticeably different for the film thickness of Al0.55Ga0.45N. The threshold power density in UV-B laser and dark spot density were reduced by increasing the film thickness. Through the optimization of the crystal growth condition, the threshold power density in UV-B laser and dark spot density were reduced to 36 kW cm−2 and 7.5 × 108 cm−2, respectively.
- Published
- 2019
29. Influence of trap level on an Al0.6Ga0.4N/Al0.5Ga0.5N metal—semiconductor—metal UV photodetector
- Author
-
Saki Ushida, Isamu Akasaki, Satoshi Kamiyama, Motoaki Iwaya, Akira Yoshikawa, and Tetsuya Takeuchi
- Subjects
010302 applied physics ,Photocurrent ,Materials science ,Physics and Astronomy (miscellaneous) ,business.industry ,General Engineering ,General Physics and Astronomy ,Photodetector ,01 natural sciences ,Arrhenius plot ,Photosensitivity ,Rise time ,0103 physical sciences ,Ultraviolet light ,Optoelectronics ,Irradiation ,business ,Intensity (heat transfer) - Abstract
The irradiation intensity and temperature characteristics of each response speed sample are measured to clarify the high photosensitivity mechanism in an Al0.6Ga0.4N/Al0.5Ga0.5N metal–semiconductor–metal photodetector. A tradeoff relation could be observed between the photosensitivity and response speed when the dependence of the saturated photocurrent on the irradiation intensity was investigated by changing the irradiation intensity from 1 nW cm−2 to 45 μW cm−2. The rise time of the photocurrent after exposure to ultraviolet light was measured to determine the trap level density that was estimated to be ca. 1012 cm−2. Further, the decay time constant was obtained by investigating the dependence of the photocurrent decay time on temperature (25 °C–180 °C). By analyzing the Arrhenius plot of the decay time constant versus inverse temperature, trap level depths of 0.23 and 0.67 eV were obtained. Thus, the high photosensitivity in the Al0.6Ga0.4N/Al0.5Ga0.5N photodetector can be attributed to the carriers trapped in deep-level traps.
- Published
- 2019
30. Electrical properties of relaxed p-GaN/p-AlGaN superlattices and their application in ultraviolet-B light-emitting devices
- Author
-
Kosuke Sato, Tetsuya Takeuchi, Motoaki Iwaya, Isamu Akasaki, Shinji Yasue, Yuya Ogino, and Satoshi Kamiyama
- Subjects
010302 applied physics ,Materials science ,Physics and Astronomy (miscellaneous) ,business.industry ,Superlattice ,Drop (liquid) ,General Engineering ,General Physics and Astronomy ,Ultraviolet b ,01 natural sciences ,law.invention ,law ,0103 physical sciences ,Relaxation (physics) ,Optoelectronics ,business ,Waveguide ,Diode - Abstract
The relaxation ratio of p-GaN/p-AlGaN superlattices was controlled by using different AlGaN underlying layers and the effects of relaxation on the electrical properties were investigated. High hole concentrations over 1 × 1018 cm−3 at room temperature and low activation energies below 120 meV were obtained regardless of the relaxation ratio. Using a p-GaN/p-Al0.5Ga0.5N superlattice, current injection at 1 A corresponding to 33.3 kA cm−2 was achieved without a significant drop in output power in an ultraviolet-B light-emitting diode including undoped AlGaN waveguide layers.
- Published
- 2019
31. GaN-based vertical-cavity surface-emitting lasers using n-type conductive AlInN/GaN bottom distributed Bragg reflectors with graded interfaces
- Author
-
Shotaro Yoshida, Takanobu Akagi, Junichiro Ogimoto, Yasuto Akatsuka, Motoaki Iwaya, Ryouta Fuwa, Isamu Akasaki, Satoshi Kamiyama, Tetsuya Takeuchi, Sho Iwayama, and Wataru Muranaga
- Subjects
010302 applied physics ,Resistive touchscreen ,Materials science ,Physics and Astronomy (miscellaneous) ,Equivalent series resistance ,business.industry ,Thermal resistance ,General Engineering ,General Physics and Astronomy ,Distributed Bragg reflector ,Laser ,01 natural sciences ,law.invention ,Vertical-cavity surface-emitting laser ,Thermal conductivity ,law ,0103 physical sciences ,Optoelectronics ,business ,Electrical conductor - Abstract
A 4.0λ-cavity GaN-based vertical-cavity surface-emitting laser (VCSEL) using an n-type conductive AlInN/GaN bottom distributed Bragg reflector (DBR) showed a light output power of 1.8 mW with a low differential resistance of 90 Ω. In order to obtain low resistive AlInN/GaN DBRs, 5 nm AlGaInN graded interfaces were introduced. A 40-pair AlInN/GaN DBR with the graded interfaces showed a peak reflectivity over 99.8% and a series resistance of 17 Ω. At the same time, the maximum light output power of a 1.5λ-cavity GaN-based VCSEL with the conductive DBR was only 0.03 mW due to a high thermal resistance of 2700 K W−1 caused by a high thermal resistivity of AlInN. We also conclude that a short cavity typically used in GaAs-based VCSELs is not appropriate in GaN-based VCSELs.
- Published
- 2019
32. Fabrication and Evaluation of porous SiC
- Author
-
Yoshimi Iwasa, Weifang Lu, Fumiharu Teramae, Satoshi Kamiyama, Haiyan Ou, Tetsuya Takeuchi, Motoaki Iwaya, and Isamu Akasaki
- Published
- 2016
33. Development of an Ultraviolet A1 Light Emitting Diode-based Device for Phototherapy
- Author
-
Satoshi Kamiyama, Isamu Akasaki, Takuya Furuhashi, Kan Torii, Akimichi Morita, Shunko Albano Inada, and Hiroshi Amano
- Subjects
Materials science ,Ultraviolet a1 ,business.industry ,Dermatology ,law.invention ,Full width at half maximum ,Wavelength ,law ,Rise time ,Ultraviolet light ,Optoelectronics ,Irradiation ,business ,Diode ,Light-emitting diode - Abstract
We developed a novel phototherapy device based on an ultraviolet light emitting diode (UV LED) with a peak wavelength of 365 nm and the full width at half maximum of 10 nm. The equipment comprised a 16 x 16 (50 cm x 50 cm) UV LED matrix. The system was designed to irradiate only the diseased part of the skin. To evaluate the characteristics of this device, we compared consumed power, irradiation intensity, uniformity of the irradiation intensity, rise time and stability of the irradiation intensity, and in vivo irradiation of mice between a conventional UVA1 (340-400 nm) phototherapy device and the UV LED device. The UVA1 LED device exhibited more desirable characteristics than the UVA1 lamp device, i.e., fewer thermal effects on in vitro and in vivo systems. Furthermore, to evaluate the efficacy of both light sources, cultured T cells were irradiated and the induction of apoptosis was analyzed. Both light sources efficiently induced apoptosis.
- Published
- 2012
34. Growth of High‐Quality AlN and AlGaN Films on Sputtered AlN/Sapphire Templates via High‐Temperature Annealing
- Author
-
Yuta Kawase, Sho Iwayama, Motoaki Iwaya, Lin Dong, Satoshi Kamiyama, Isamu Akasaki, Junya Hakamata, Hideto Miyake, and Tetsuya Takeuchi
- Subjects
010302 applied physics ,Materials science ,Annealing (metallurgy) ,business.industry ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,Template ,Sputtering ,0103 physical sciences ,Sapphire ,Optoelectronics ,Metalorganic vapour phase epitaxy ,0210 nano-technology ,business - Published
- 2018
35. A GaN-Based VCSEL with a Convex Structure for Optical Guiding
- Author
-
Motoaki Iwaya, Satoshi Kamiyama, Tetsuya Takeuchi, Junichiro Ogimoto, Takashi Furuta, Takanobu Akagi, Natsumi Hayashi, Isamu Akasaki, Kenjo Matsui, and Sho Iwayama
- Subjects
010302 applied physics ,Materials science ,business.industry ,02 engineering and technology ,Surfaces and Interfaces ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Vertical-cavity surface-emitting laser ,Convex structure ,0103 physical sciences ,Materials Chemistry ,Optoelectronics ,Electrical and Electronic Engineering ,0210 nano-technology ,business - Published
- 2018
36. Status and Prospects of GaN-Based Blue Lasers and Photo Detectors
- Author
-
Satoshi Kamiyama, Motoaki Iwaya, Isamu Akasaki, and Tetsuya Takeuchi
- Subjects
Materials science ,business.industry ,law ,Detector ,Optoelectronics ,business ,Laser ,law.invention - Published
- 2018
37. Influence of Well-Width Fluctuations on the Electronic Structure of GaN/AlxGa1-xN Multiquantum Wells with Graded Interfaces
- Author
-
Per Persson, Isamu Akasaki, S. Dimitrov, Bo Monemar, H. Haratizadeh, Evgenia Valcheva, and Hiroshi Amano
- Subjects
Lattice constant ,Photoluminescence ,Materials science ,Condensed matter physics ,Monolayer ,General Physics and Astronomy ,Chemical vapor deposition ,Electronic structure ,Chemical composition ,Spectral line - Abstract
Experimental and computation results based on chemical composition assessment of metal-organic chemical vapour deposition grown undoped GaN/AlxGa1-xN multiquantum well structures in the low composition limit of x = 0.07 and wide wells demonstrate composition fluctuations in the barrier layers which lead to large-scale nonuniformities and inequivalence of the different wells. As a consequence the experimental photoluminescence spectra at low temperature show a double peak structure indicative of well-width fluctuations by one lattice parameter (2 monolayers).
- Published
- 2007
38. High Efficiency UV Light Emitter Using High-Crystalline-Quality AlGaN
- Author
-
Isamu Akasaki, Hiroshi Amano, Motoaki Iwaya, and Satoshi Kamiyama
- Subjects
Dc current ,Light emitter ,Wavelength ,Optics ,Quality (physics) ,Materials science ,business.industry ,Optoelectronics ,Dislocation ,business ,Power (physics) ,Diode - Abstract
We demonstrated the UV-light-emitting diode on the low dislocation density AlGaN. This new UV-light-emitting diode shows peak wavelength of 363 nm and output power of 4.7 mW at 100 mA DC current injection.
- Published
- 2004
39. The Evolution of Blue Light-Emitting Devices and Nitride Semiconductors
- Author
-
Isamu Akasaki
- Subjects
Blue laser ,Materials science ,business.industry ,Saturation velocity ,Optoelectronics ,Crystal growth ,Electronics ,Electrical and Electronic Engineering ,Nitride ,Conductivity ,Nitride semiconductors ,business ,Diode - Abstract
Breakthroughs in crystal growth and conductivity control of nitride semiconductors during last two decades have led to such developments as high-brightness blue, green and white light-emitting diodes and long-lived violet laser diode and so forth. The high electron saturation velocity in GaN is also suitable for application in high-speed/high power electronic devices. All of these nitride-based devices are the most environmentally-friendly ones available. They are tough and should enable a tremendous saving in energy. Further progress in the area of crystal growth and device engineering is necessary for the development of new frontier devices based on nitride semiconductors.
- Published
- 2004
40. UV‐LED Using p‐Type GaN/AlN Supperlattice Cladding Layer
- Author
-
A. Miyazaki, S. Kamiyama, S. Takanami, Motoaki Iwaya, S. Terao, Isamu Akasaki, and Hiroshi Amano
- Subjects
Materials science ,Equivalent series resistance ,Composite material ,Cladding (fiber optics) ,Ternary alloy - Abstract
GaN/AlN supperlattice was used as the p-type cladding layer of UV-LED. I–V characteristics indicates that UV-LED having GaN/AlN supperlattice has much lower series resistance than the conventional UV-LED in which ternary alloy AlGaN was used as the cladding layer.
- Published
- 2002
41. Plasmon enhanced green GaN light-emitting diodes - Invited paper
- Author
-
Haiyan Ou, Ahmed Fadil, Daisuke Iida, Yuntian Chen, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, and Isamu Akasaki
- Abstract
High-efficiency garnium nitride (GaN) based blue light-emitting diode (LED) paves the way for solid statelighting to take the place of the conventional incandescent bulbs and fluorescent light tubes.Compared to the traditional light sources, solid state lighting is more efficient, more flexible in spectral design, more compact etc. TheIII-nitride (GaN, InNetc.) semiconductors are attracting a lot of research effort because the combination of both could emit light with wavelength range from UV to infrared. Basically one material platform could provide all the solutions to light sources.However huge amount of material development is needed before high efficiency devices are achieved. Among them,one effort area is the so called ‘greengap’, i.e. low efficiency for green color LEDs due to the large piezoelectric field in the quantum wells (QWs) when the In composition is high. From the material growth point of view, the efficiency of green LED is being improved by growing the GaInN material on non-polar or semi-polar surface of sapphire substrate. In parallel with the material growth effort, surface plasmons are implemented by taking use of the interactionbetween metals and active areas to increase the efficiency.In this paper, our work on using silver (Ag) nanoparticles (NPs ) to enhance the efficiency of the green LEDs is reviewed. Both random and periodic Ag nanoparticles are studied. The random Ag nanoparticles are formed by thermal annealing of thin films.Periodic Ag nanoparticles are formed through nanosphere lithography. For both cases, emission enhancement is demonstrated. For periodic Ag nanoparicles, aphotoluminescence enhancement of 2.7 is observed with a nanodisk diameter of 330 nm.It is found that an optimalpitch exists for a given particle size.For the random Ag nanoparticles,low temperature photoluminescence (LTPL) was measured and internal quantum efficiency (IQE) enhancement by thesurface plasmons was derived. Excitation power dependence of IQE enhancement is derived as well fromthe PL measurement under different excitation power densities.It was found that the strong PL enhancement was partly due to LSP-QW coupling, and partly due to excitation source enhancement fromthe Ag NPs, and separating these effects we noted an IQE improvement due to LSP-QW coupling at 530 nm emission from 19.4% to 44.1% using large sized Ag NPs at 756 W/cm2. It was also found that the IQE enhancement is strongly dependent on excitation power density, yielding highest enhancement factors atlow free carrier densities. Where an IQE enhancement by a factor of 2.3 was observed at 756 W/cm2, an enhancement factor of 8.1 was observedat 1 W/cm2.
- Published
- 2014
42. Near K-Edge Absorption Spectra of III-V Nitrides
- Author
-
Kazutoshi Fukui, Yoshinobu Aoyagi, Satoru Tanaka, Hiroshi Amano, Shigeo Yamaguchi, Isamu Akasaki, A. Yamamoto, R. Hirai, and Hideki Hirayama
- Subjects
Absorption spectroscopy ,Chemistry ,Linear polarization ,Analytical chemistry ,Synchrotron radiation ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,K-edge ,Absorption edge ,Physics::Chemical Physics ,Absorption (electromagnetic radiation) ,Ternary operation ,Wurtzite crystal structure - Abstract
Nitrogen and aluminum near K-edge absorption measurements of wurtzite AlN, GaN and InN, and their ternary compounds (AlGaN, InGaN and InAIN) at various molar fractions have been performed using synchrotron radiation. Using the linear polarization of synchrotron radiation, absorption measurements with different incident light angles were also performed. The spectral distribution of the nitrogen K absorption spectra clearly depends on both the incident light angles and the molar fractions of the samples. That of the aluminum K absorption spectra also shows the clear angle dependence, but it does not show the drastic molar dependence. Spectral shape comparisons among the various molar fractions, different incident angles and between the two ion sites are discussed. The numerical component analysis of the K absorption spectra is also presented.
- Published
- 2001
43. Spectroscopy in Polarized and Piezoelectric AlGaInN Heterostructures
- Author
-
T. Takeuchi, Christian Wetzel, Isamu Akasaki, and Hiroshi Amano
- Subjects
Materials science ,business.industry ,Energy level splitting ,Heterojunction ,Field strength ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Polarization (waves) ,Piezoelectricity ,Band offset ,Condensed Matter::Materials Science ,Dipole ,Optoelectronics ,General Materials Science ,business ,Quantum well - Abstract
Uniaxial wurtzite group-III nitride heterostructures are subject to large polarization effects with significant consequences for device physics in optoelectronic and transport device applications. A central aspect for the proper implementation is the experimental quantification of polarization charges and associated fields. In modulated reflection spectroscopy of thin films and heterostructures of AlGaInN we observe pronounced Franz-Keldysh oscillations that allow direct and accurate readings of the field strength induced by polarization dipoles at the heterointerfaces. In piezoelectric GaInN/GaN quantum wells this dipole is found to induce an asymmetry in barrier heights with a respective splitting of interband transitions. This splitting energy appears to reflect in the transitions of spontaneous and stimulated luminescence in the well. From these experiments the polarization dipole is identified as controllable type-II staggered band offset between adjacent barrier layers which can extend the flexibility in AlGaInN bandstructure design. The derived field values can serve as important input parameters in the further interpretation of the entire system.
- Published
- 2000
44. Nitride Semiconductor Surfaces. Characterization of Initial Growth Stage of GaInN Multilayered Structure by X-ray CTR Scattering and X-ray Reflectivity Method
- Author
-
Hiroshi Amano, Masao Tabuchi, Tetsuya Takeuchi, Yoshikazu Takeda, and Isamu Akasaki
- Subjects
X-ray reflectivity ,Optics ,Materials science ,business.industry ,Scattering ,X-ray ,Optoelectronics ,Stage (hydrology) ,business ,Nitride semiconductors ,Characterization (materials science) - Published
- 2000
45. Mass Transport, Faceting and Behavior of Dislocations in GaN
- Author
-
Isamu Akasaki, T. Kashima, Shugo Nitta, Yohei Yukawa, Hiroshi Amano, Michihiko Kariya, and Shigeo Yamaguchi
- Subjects
Threading dislocations ,Mass transport ,Materials science ,Condensed matter physics ,business.industry ,Microstructure ,Surface energy ,Faceting ,Crystallography ,Semiconductor ,Transmission electron microscopy ,Microscopy ,General Materials Science ,Thin film ,Anisotropy ,business - Abstract
The behavior of threading dislocations during mass transport of GaN was investigated in detail by transmission electron microscopy. Mass transport occurred at the surface. Therefore, growing species are supplied from the in-plane direction. The behavior of threading dislocations was found to be strongly affected by the mass transport process as well as the high crystallographic anisotropy of the surface energy of the facets particular to GaN.
- Published
- 2000
46. The Role of the Multi Buffer Layer Technique on the Structural Quality of GaN
- Author
-
J.H. Mazur, Hiroshi Amano, Motoaki Iwaya, W. Swider, Isamu Akasaki, Mourad Benamara, Zuzanna Liliental-Weber, and Jack Washburn
- Subjects
Materials science ,Fabrication ,business.industry ,Edge (geometry) ,Microstructure ,Buffer (optical fiber) ,Crystallography ,Quality (physics) ,Semiconductor ,Transmission electron microscopy ,Optoelectronics ,General Materials Science ,Dislocation ,business ,Layer (electronics) ,Burgers vector - Abstract
Successive growth of thick GaN layers separated by either LT-GaN or LT-AlN interlayers have been investigated by transmission electron microscopy techniques. One of the objectives of this growth method was to improve the quality of GaN layers by reducing the dislocation density at the intermediate buffer layers that act as barriers to dislocation propagation. While the use of LT-AlN results in the multiplication of dislocations in the subsequent GaN layers, the LT-GaN reduces dislocation density. Based upon Burgers vector analysis, the efficiency of the buffer layers for the propagation of the different type of dislocations is presented. LT-AlN layer favor the generation of edge dislocations, leading to a highly defective GaN layer. On the other hand, the use of LT-GaN as intermediate buffer layers appears as a promising method to obtain high quality GaN layer.
- Published
- 2000
47. Nitride Semiconductor Surfaces. Effect of Low-temperature Deposited Layer on the Growth of Group-III Nitrides on Sapphire
- Author
-
Isamu Akasaki and Hiroshi Amano
- Subjects
Incandescent light bulb ,Materials science ,business.industry ,Flame detection ,Transistor ,Optical storage ,Nitride ,law.invention ,law ,Sapphire ,Optoelectronics ,Electronics ,business ,Layer (electronics) - Abstract
Surface control by low-temperature deposited buffer layer enables the growth of high-quality GaN on a sapphire substrate the lattice mismatch of which is as large as 14%. With use of such highly mismatched systems, novel electronic devices such as new lighting source which replaces conventional light bulb, super-high-density optical storage systems, high-power and high-speed transistors and UV image sensors for remote sensing and flame detection will be realized. This paper describes the details of the mechanism of this surface control technology.
- Published
- 2000
48. High-Quality AlxGa1−xN Using Low Temperature-Interlayer and its Application to UV Detector
- Author
-
Motoaki Iwaya, Akira Hirano, T. Kashima, S. Terao, Isamu Akasaki, Nobuaki Hayashi, Hiroshi Amano, Cyril Pernot, and Theeradetch Detchprohm
- Subjects
Photocurrent ,Quality (physics) ,Semiconductor ,Materials science ,Uv detector ,business.industry ,Optoelectronics ,Photodetector ,General Materials Science ,Dislocation ,business ,Dark current - Abstract
Low-temperature (LT-) AlN interlayer reduces tensile stress during growth of AlxGa1−xN, while simultaneously acts as the dislocation filter, especially for dislocations of which Burger’s vector contains [0001] components. UV photodetectors using thus-grown high quality AlxGa1−xN layers were fabricated. The dark current bellow 50 fA at 10 V bias for 10 μm strip allowing a photocurrent to dark current ratio greater than one even at 40 nW/cm2 have been achieved.
- Published
- 2000
49. Photoluminescence Investigations of AlGaN on GaN Epitaxial Films
- Author
-
Bruno K. Meyer, G. Steude, A. Göldner, Isamu Akasaki, Axel Hoffmann, and Hiroshi Amano
- Subjects
Materials science ,Photoluminescence ,Condensed matter physics ,Band gap ,business.industry ,Exciton ,Binding energy ,Condensed Matter Physics ,Epitaxy ,Mole fraction ,Electronic, Optical and Magnetic Materials ,Effective mass (solid-state physics) ,Optoelectronics ,Luminescence ,business - Abstract
AlGaN on GaN epitaxial films with Al contents between 6% and 76% were investigated by stationary photoluminescence experiments which allows to determine the dependence of the energy gap on alloy composition. The observed increase of the luminescence linewidth as a function of the Al molar fraction can be explained by alloy broadening. The localization energy of the bound exciton increases considerably and reaches a value of 53 meV for 61% Al. It could imply that the donor binding energy would markedly deviate from its effective mass value, an unexpected result if the residual donor is Si.
- Published
- 1999
50. Electronic structure of the 0.88-eV luminescence center in electron-irradiated gallium nitride
- Author
-
Robert F. Davis, Irina Buyanova, N. V. Edwards, Hiroshi Amano, Weimin Chen, Michael D. Bremser, J. L. Lindström, Isamu Akasaki, Matthias Wagner, and Bo Monemar
- Subjects
chemistry.chemical_compound ,Materials science ,Photoluminescence ,chemistry ,Excited state ,Gallium nitride ,Electronic structure ,Atomic physics ,Ionization energy ,Ground state ,Spectroscopy ,Luminescence - Abstract
Photoluminescence (PL) spectroscopy is employed to determine the nature of a near-infrared PL emission with a no-phonon line at ∼0.88 eV, commonly present in electron-irradiated GaN. This PL emission is suggested to originate from an internal transition between a moderately shallow excited state (with an ionization energy ∼21 meV) and the deep ground state (with an ionization energy ∼900 meV) of a deep defect. The existence of a higher-lying second excited state related to the 0.88-eV PL center is also shown from temperature-dependent studies. A different electronic character of the wave functions related to the first and second excited states has been revealed by PL polarization measurements. Since the PL emission has been observed with comparable intensity in all electron-irradiated GaN samples independent of doping on the starting material, it is proposed that either native defects, or common residual contaminants or their complexes are involved. The substitutional ON donor (or related complex) is considered as the most probable candidate, based on the observed striking similarity in the local vibrational properties between the 0.88-eV PL centers and the substitutional OP donor in GaP.
- Published
- 1999
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