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158 results on '"Kushimoto, Maki"'

1. Experimental Observation of High Intrinsic Thermal Conductivity of AlN

Author

Cheng, Zhe, Koh, Yee Rui, Mamun, Abdullah, Shi, Jingjing, Bai, Tingyu, Huynh, Kenny, Yates, Luke, Liu, Zeyu, Li, Ruiyang, Lee, Eungkyu, Liao, Michael, Wang, Yekan, Yu, Hsuan Ming, Kushimoto, Maki, Luo, Tengfei, Goorsky, Mark S., Hopkins, Patrick E., Amano, Hiroshi, Khan, Asif, and Graham, Samuel

Subjects
Condensed Matter - Materials Science
Abstract

AlN is an ultra-wide bandgap semiconductor which has been developed for applications including power electronics and optoelectronics. Thermal management of these applications is the key for stable device performance and allowing for long lifetimes. AlN, with its potentially high thermal conductivity, can play an important role serving as a dielectric layer, growth substrate, and heat spreader to improve device performance. However, the intrinsic high thermal conductivity of bulk AlN predicted by theoretical calculations has not been experimentally observed because of the difficulty in producing materials with low vacancy and impurity levels, and other associated defect complexes in AlN which can decrease the thermal conductivity. This work reports the growth of thick AlN layers by MOCVD with an air-pocketed AlN layer and the first experimental observation of intrinsic thermal conductivity from 130 K to 480 K that matches density-function-theory calculations for single crystal AlN, producing some of the highest values ever measured. Detailed material characterizations confirm the high quality of these AlN samples with one or two orders of magnitude lower impurity concentrations than seen in commercially available bulk AlN. Measurements of these commercially available bulk AlN substrates from 80 K to 480 K demonstrated a lower thermal conductivity, as expected. A theoretical thermal model is built to interpret the measured temperature dependent thermal conductivity. Our work demonstrates that it is possible to obtain theoretically high values of thermal conductivity in AlN and such films may impact the thermal management and reliability of future electronic and optoelectronics devices.

Published
2019
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8. Impact of unintentionally formed compositionally graded layer on carrier injection efficiency in AlGaN-based deep-ultraviolet laser diodes.

Author

Zhang, Ziyi, Yoshikawa, Akira, Kushimoto, Maki, Aoto, Koji, Sasaoka, Chiaki, and Amano, Hiroshi

Subjects
DEBYE temperatures, SEMICONDUCTOR lasers, ACTIVE medium
Abstract

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

Published
2024
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12. Improvement of Algan Homoepitaxial Tunnel Junction Deep-UV Light-Emitting Diodes by Controlling the Growth of N-Type Algan and Polycrystalline Mgzno/Al Electrodes

Author

Nagata, Kengo, primary, Matsubara, Taichi, additional, Saito, Yoshiki, additional, Kataoka, Keita, additional, Narita, Tetsuo, additional, Horibuchi, Kayo, additional, Kushimoto, Maki, additional, Tomai, Shigekazu, additional, Katsumata, Satoshi, additional, Honda, Yoshio, additional, Takeuchi, Tetsuya, additional, and Amano, Hiroshi, additional

Published
2023
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13. Visualization of depletion layer in AlGaN homojunction p–n junction

Author

Nagata, Kengo, Anada, Satoshi, Saito, Yoshiki, Kushimoto, Maki, Honda, Yoshio, Takeuchi, Tetsuya, Yamamoto, Kazuo, Hirayama, Tsukasa, Amano, Hiroshi, Nagata, Kengo, Anada, Satoshi, Saito, Yoshiki, Kushimoto, Maki, Honda, Yoshio, Takeuchi, Tetsuya, Yamamoto, Kazuo, Hirayama, Tsukasa, and Amano, Hiroshi

Abstract

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

Published
2023

14. Structural design optimization of 279 nm wavelength AlGaN homojunction tunnel junction deep-UV light-emitting diode

Author

Nagata, Kengo, Anada, Satoshi, Miwa, Hiroshi, Matsui, Shinichi, Boyama, Shinya, Saito, Yoshiki, Kushimoto, Maki, Honda, Yoshio, Takeuchi, Tetsuya, Amano, Hiroshi, Nagata, Kengo, Anada, Satoshi, Miwa, Hiroshi, Matsui, Shinichi, Boyama, Shinya, Saito, Yoshiki, Kushimoto, Maki, Honda, Yoshio, Takeuchi, Tetsuya, and Amano, Hiroshi

Abstract

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

Published
2023

15. Sputtered polycrystalline MgZnO/Al reflective electrodes for enhanced light emission in AlGaN-based homojunction tunnel junction DUV-LED

Author

Matsubara, Taichi, Nagata, Kengo, Kushimoto, Maki, Tomai, Shigekazu, Katsumata, Satoshi, Honda, Yoshio, Amano, Hiroshi, Matsubara, Taichi, Nagata, Kengo, Kushimoto, Maki, Tomai, Shigekazu, Katsumata, Satoshi, Honda, Yoshio, and Amano, Hiroshi

Abstract

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

Published
2023

16. Demonstration of AlGaN-on-AlN p-n Diodes With Dopant-Free Distributed Polarization Doping

Author

Kumabe, Takeru, Yoshikawa, Akira, Kawasaki, Seiya, Kushimoto, Maki, Honda, Yoshio, Arai, Manabu, Suda, Jun, and Amano, Hiroshi

Abstract

Nearly ideal vertical AlxGa $_{{1}-{x}}\text{N}$ ( ${0.7} \leq {x} < {1.0}$ ) p-n diodes are fabricated on an aluminum nitride (AlN) substrate. Distributed polarization doping (DPD) was employed for both p-type and n-type layers of the p-n junction, instead of conventional impurity doping, to overcome the major bottleneck of AlN-based material: the control of conductivity. Capacitance-voltage measurements revealed that the net charge concentration agreed well with the DPD charge concentration expected from the device layer structure. The fabricated devices exhibited a low turn-on voltage of 6.5 V, a low differential specific on-resistance of 3 $\text{M}\Omega $ cm2, electroluminescence (maximum at 5.1 eV), and an ideality factor of 2 for a wide range of temperatures (room temperature—573 K). Moreover, the breakdown electric field was 7.3 MV cm−1, which was almost twice as high as the reported critical electric field of GaN at the same doping concentration. These results clearly demonstrate the usefulness of DPD in the fabrication of high-performance AlN-based power devices.

Published
2024
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19. Impact of heat treatment process on threshold current density in AlGaN-based deep-ultraviolet laser diodes on AlN substrate

Author

Kushimoto, Maki, Zhang, Ziyi, Sugiyama, Naoharu, Honda, Yoshio, Schowalter, Leo J., Sasaoka, Chiaki, Amano, Hiroshi, Kushimoto, Maki, Zhang, Ziyi, Sugiyama, Naoharu, Honda, Yoshio, Schowalter, Leo J., Sasaoka, Chiaki, and Amano, Hiroshi

Abstract

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

Published
2022

20. Reduction in operating voltage of AlGaN homojunction tunnel junction deep-UV light-emitting diodes by controlling impurity concentrations

Author

Nagata, Kengo, Makino, Hiroaki, Miwa, Hiroshi, Matsui, Shinichi, Boyama, Shinya, Saito, Yoshiki, Kushimoto, Maki, Honda, Yoshio, Takeuchi, Tetsuya, Amano, Hiroshi, Nagata, Kengo, Makino, Hiroaki, Miwa, Hiroshi, Matsui, Shinichi, Boyama, Shinya, Saito, Yoshiki, Kushimoto, Maki, Honda, Yoshio, Takeuchi, Tetsuya, and Amano, Hiroshi

Abstract

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

Published
2022

21. Local stress control to suppress dislocation generation for pseudomorphically grown AlGaN UV-C laser diodes

Author

Kushimoto, Maki, Zhang, Ziyi, Yoshikawa, Akira, Aoto, Koji, Honda, Yoshio, Sasaoka, Chiaki, Schowalter, Leo J., Amano, Hiroshi, Kushimoto, Maki, Zhang, Ziyi, Yoshikawa, Akira, Aoto, Koji, Honda, Yoshio, Sasaoka, Chiaki, Schowalter, Leo J., and Amano, Hiroshi

Abstract

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

Published
2022

22. Key temperature-dependent characteristics of AlGaN-based UV-C laser diode and demonstration of room-temperature continuous-wave lasing

Author

Zhang, Ziyi, Kushimoto, Maki, Yoshikawa, Akira, Aoto, Koji, Sasaoka, Chiaki, Schowalter, Leo J., Amano, Hiroshi, Zhang, Ziyi, Kushimoto, Maki, Yoshikawa, Akira, Aoto, Koji, Sasaoka, Chiaki, Schowalter, Leo J., and Amano, Hiroshi

Abstract

Although the pulsed operation of AlGaN-based laser diodes at UV-C wavelengths has been confirmed in previous studies, continuous oscillation without cooling is difficult because of the high operating voltage. In this study, the temperature dependence of key parameters was investigated and their impact on achieving continuous-wave lasing was discussed. A reduction in threshold voltage was achieved by tapering the sides of the laser diode mesa and reducing the lateral distance between the n- and p-electrodes. As a result, continuous-wave lasing at room temperature was demonstrated for the first time at a threshold current density of 4:2kA/cm2 and a threshold voltage of 8:7V.

Published
2022

35. Dielectric Ruduced Surface Field Effect on Vertical GaN-on-GaN Nanowire Schottky Barrier Diodes

Author

Liao, Yaqiang, primary, Chen, Tao, additional, Wang, Jia, additional, Ando, Yuto, additional, Yang, Xu, additional, Watanabe, Hirotaka, additional, Hirotani, Jun, additional, Kushimoto, Maki, additional, Deki, Manato, additional, Tanaka, Atsushi, additional, Nitta, Shugo, additional, Honda, Yoshio, additional, Chen, Kevin J., additional, and Amano, Hiroshi, additional

Published
2020
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37. Impact of high-temperature implantation of Mg ions into GaN

Author

Takahashi, Masahiro, primary, Tanaka, Atsushi, additional, Ando, Yuto, additional, Watanabe, Hirotaka, additional, Deki, Manato, additional, Kushimoto, Maki, additional, Nitta, Shugo, additional, Honda, Yoshio, additional, Shima, Kohei, additional, Kojima, Kazunobu, additional, Chichibu, Shigefusa F., additional, and Amano, Hiroshi, additional

Published
2020
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38. Experimental observation of high intrinsic thermal conductivity of AlN

Author

Cheng, Zhe, primary, Koh, Yee Rui, additional, Mamun, Abdullah, additional, Shi, Jingjing, additional, Bai, Tingyu, additional, Huynh, Kenny, additional, Yates, Luke, additional, Liu, Zeyu, additional, Li, Ruiyang, additional, Lee, Eungkyu, additional, Liao, Michael E., additional, Wang, Yekan, additional, Yu, Hsuan Ming, additional, Kushimoto, Maki, additional, Luo, Tengfei, additional, Goorsky, Mark S., additional, Hopkins, Patrick E., additional, Amano, Hiroshi, additional, Khan, Asif, additional, and Graham, Samuel, additional

Published
2020
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39. Suppression of Green Luminescence of Mg‐Ion‐Implanted GaN by Subsequent Implantation of Fluorine Ions at High Temperature

Author

Takahashi, Masahiro, primary, Tanaka, Atsushi, additional, Ando, Yuto, additional, Watanabe, Hirotaka, additional, Deki, Manato, additional, Kushimoto, Maki, additional, Nitta, Shugo, additional, Honda, Yoshio, additional, Shima, Kohei, additional, Kojima, Kazunobu, additional, Chichibu, Shigefusa F., additional, Chen, Kevin J., additional, and Amano, Hiroshi, additional

Published
2020
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44. Suppression of Green Luminescence of Mg-Ion-Implanted GaN by Subsequent Implantation of Fluorine Ions at High Temperature

Author

Takahashi, Masahiro, Tanaka, Atsushi, Ando, Yuto, Watanabe, Hirotaka, Deki, Manato, Kushimoto, Maki, Nitta, Shugo, Honda, Yoshio, Shima, Kohei, Kojima, Kazunobu, Chichibu, Shigefusa F., Chen, Jing, Amano, Hiroshi, Takahashi, Masahiro, Tanaka, Atsushi, Ando, Yuto, Watanabe, Hirotaka, Deki, Manato, Kushimoto, Maki, Nitta, Shugo, Honda, Yoshio, Shima, Kohei, Kojima, Kazunobu, Chichibu, Shigefusa F., Chen, Jing, and Amano, Hiroshi

Abstract

Herein, gallium nitride (GaN) samples implanted with magnesium (Mg) and fluorine (F) ions are investigated by photoluminescence (PL) measurements. In low-temperature PL measurements, the characteristic green luminescence (GL) band attributable to nitrogen vacancies (VN) is observed in Mg-ion-implanted GaN. As VN are likely to act as donors, suppressing their formation is essential to realizing p-type conductivity. The energy required for a F impurity to replace VN in GaN and eventually form F on a N site decreases when the Fermi level approaches the valence band maximum, and therefore F is employed as a subsequent implantation element to compensate for VN. The GL band peak disappears upon implanting Mg and F ions at a high temperature and adjusting the F concentration to an appropriate value. This result suggests that VN generated by Mg ion implantation can be suppressed using an element with a lower formation energy than that of VN. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Published
2019

45. V-shaped dislocations in a GaN epitaxial layer on GaN substrate

Author

Tanaka, Atsushi, Nagamatsu, Kentaro, Usami, Shigeyoshi, Kushimoto, Maki, Deki, Manato, Nitta, Shugo, Honda, Yoshio, Bockowski, Michal, Amano, Hiroshi, Tanaka, Atsushi, Nagamatsu, Kentaro, Usami, Shigeyoshi, Kushimoto, Maki, Deki, Manato, Nitta, Shugo, Honda, Yoshio, Bockowski, Michal, and Amano, Hiroshi

Abstract

In this study, V-shaped dislocations in a GaN epitaxial layer on a free-standing GaN substrate were observed. Our investigation further revealed that the V-shaped dislocations were newly generated at the interface in the epilayer rather than propagated from the GaN substrate. V-shaped dislocations consist of two straight parts. The straight parts of the V-shaped dislocations were separated from each other in the m-direction and tilted toward the step-flow direction of the GaN epitaxial layer. The V-shaped dislocations are continuous single dislocations having a Burgers vector component of 1a and an intrinsic stacking fault between their straight parts.

Published
2019

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