Papers - Yamada, Junpei

Improving the luminous efficiency of red nanocolumn μ-LEDs by reducing electrode size to ϕ2.2 μm

Katsumi Kishino, Ai Mizuno, Tatsuya Honda, Jumpei Yamada, Rie Togashi

Applied Physics Express (IOP Publishing)  17 ( 1 ) 014004 - 014004 2023.12

ISSN  1882-0778

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Abstract

A red InGaN-based nanocolumn micro μLED with an emission diameter of ϕ2.2 μm was demonstrated to achieve an on-wafer external quantum efficiency (EQE) of 2.1% at the peak wavelength of 615 nm. The LED was fabricated by repeating the electrode process on the same nanocolumn pattern area and reducing the emission diameter from ϕ80 to ϕ2.2 μm. The peak EQE, which was maximized at ∼25 A cm⁻², increased by decreasing the emission diameter from 1.2% to 2.1%. This behavior, which differs from that of InGaN-film LEDs, is characterized as a unit of independent nano-LEDs with passivated sidewalls of nanocolumn LEDs.

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InGaN/GaN superlattice underlayer for fabricating of red nanocolumn μ-LEDs with (10-11) plane InGaN/AlGaN MQWs

Jumpei Yamada, Ai Mizuno, Tatsuya Honda, Keigo Yoshida, Rie Togashi, Ichirou Nomura, Tomohiro Yamaguchi, Tohru Honda, Katsumi Kishino

Nanotechnology (IOP Publishing)  34 ( 43 ) 435201 - 435201 2023.08

Lead author,  ISSN  0957-4484

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Abstract

In this study, the growth behavior of Indium gallium nitride (InGaN)-based nanocolumn arrays was investigated, and red emission nanocolumn micro-light emitting diodes (μ-LEDs) were fabricated. The internal structure of the InGaN/GaN superlattice (SL) layer under the multiple-quantum-well (MQW) active layers was evaluated using scanning transmission electron microscopy (STEM) analysis. It was revealed that the InGaN crystal plane at the top of the nanocolumn changed from the c-plane, (1-102) plane, to the (10-11) plane as the number of SL pairs increased. A semipolar (10-11) plane was completely formed on top of the nanocolumn by growing InGaN/GaN SLs over 15–20 pairs, where the InGaN/GaN SL layers were uniformly piled up, maintaining the (10-11) plane. Therefore, when InGaN/AlGaN MQWs were grown on the (10-11) plane InGaN/GaN SL layer, the growth of the (10-11) plane semipolar InGaN active layers was observed in the high-angle annular dark field (HAADF)-STEM image. Moreover, the acute nanocolumn top of the (10-11) plane of the InGaN/GaN SL underlayer did not contribute to the formation of the c-plane InGaN core region. Red nanocolumn μ-LEDs with an φ12 μm emission window were fabricated using the (10-11) plane MQWs to obtain the external quantum efficiency of 1.01% at 51 A cm⁻². The process of nanocolumn μ-LEDs suitable for the smaller emission windows was provided, where the flat p-GaN contact layer contributed to forming a fine emission window of φ5 μm.

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Effect of crystallization of Ni catalyst on direct precipitation of multilayer graphene using W capping layer

Jumpei Yamada, Yuki Ueda, Takahiro Maruyama, Seiji Fujikawa, Takuo Sasaki, Masamitu Takahasi, Shigeya Naritsuka

Journal of Crystal Growth (Elsevier BV)  555   125969 - 125969 2021.02

Lead author,  ISSN  0022-0248

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In recent years, graphene growth technology has been greatly progressed, and it is possible to grow graphene with millimeter-size single crystals using metal-catalysis CVD. However, the transfer process is unavoidably which largely deteriorates the graphene. We have proposed direct precipitation method of graphene using a W capping layer. The method does not only require a transfer process, but also the introduction of wrinkles will be suppressed by optimizing the cooling rate at the graphene precipitation. The growth conditions such as thickness of amorphous carbon, annealing temperature, cooling rate and crystallinity of Ni catalyst were systematically changed to study the mechanism of the precipitation method using the W cap layer. By the optimization of the conditions, wrinkle-free multilayer graphene with D/G ratio of less than 0.1 were successfully obtained directly on a sapphire substrate.

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X-ray in situ observation of graphene precipitating directly on sapphire substrate with and without Ti capping layer

Shigeya Naritsuka, Jumpei Yamada, Yuki Ueda, Asato Nakashima, Tatsuya Kashio, Takahiro Maruyama, Seiji Fujikawa, Takuo Sasaki, Masamitu Takahasi

Journal of Crystal Growth (Elsevier BV)  549   125861 - 125861 2020.11

ISSN  0022-0248

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In situ X-ray diffraction measurement was performed to study the precipitation mechanism of graphene from Ni catalyst with and without Ti capping layer using X-ray beam from a synchrotron radiation facility. The graphene precipitated on the surface of the catalyst in the case without the Ti capping layer while it did at the interface between the catalyst and the sapphire substrate in the case with the Ti capping layer. Each process, such as graphene nucleation and precipitation was successfully monitored even though graphene precipitated under the metal catalyst. The detailed mechanisms for both cases were discussed using the experimental results. Consequently, the crystallization of the catalyst is found to effectively suppress the precipitation at low temperature, which is useful to improve the quality of the precipitated graphene. The adjustment of the amount of the carbons is another important factor to precisely control the precipitation because graphene also precipitates after the carbons saturate the catalyst.

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Precipitation of multilayer graphene directly on gallium nitride template using Tungsten capping layer

Jumpei Yamada, Yuki Ueda, Takahiro Maruyama, Shigeya Naritsuka

Journal of Crystal Growth (Elsevier BV)  534   125493 - 125493 2020.03

Lead author, Corresponding author,  ISSN  0022-0248

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The direct growth of graphene was investigated with precipitating graphene on a GaN template at various temperatures. In the method, a carbon source and catalyst were firstly deposited, and the sample was annealed to precipitate graphene. Tungsten capping layer was deposited on the surface to suppress the graphene precipitating to the sample surface. Consequently, the graphene was precipitated at the interface between the catalyst and the GaN template. After the removal of the catalyst, the graphene was successfully obtained on the GaN template. The Raman D/G ratio of the graphene decreased with increasing the annealing temperature. At 700 °C, fine graphene was obtained on the GaN template while maintaining the flatness and smoothness of the GaN surface. XRD and XPS measurements were also performed to investigate the precipitation of the graphene in detail.

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Crystal orientation effects of sapphire substrate on graphene direct growth by metal catalyst-free low-pressure CVD

Yuki Ueda, Jumpei Yamada, Taishi Ono, Takahiro Maruyama, Shigeya Naritsuka

Applied Physics Letters (AIP Publishing)  115 ( 1 ) 013103 - 013103 2019.07

ISSN  0003-6951

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Transfer-free fabrication of a graphene transparent electrode on a GaN-based light-emitting diode using the direct precipitation method

Jumpei Yamada, Shigeyoshi Usami, Yuki Ueda, Yoshio Honda, Hiroshi Amano, Takahiro Maruyama, Shigeya Naritsuka

Japanese Journal of Applied Physics (IOP Publishing)  58 ( 4 ) 040904 - 040904 2019.03

Lead author, Corresponding author,  ISSN  0021-4922

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Effect of growth pressure on graphene direct growth on r-plane and c-plane sapphires by low-pressure CVD

Yuki Ueda, Jumpei Yamada, Kyosuke Fujiwara, Daichi Yamamoto, Takahiro Maruyama, Shigeya Naritsuka

Japanese Journal of Applied Physics (IOP Publishing)  58 ( SA ) SAAE04 - SAAE04 2018.12

ISSN  0021-4922

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Direct growth of multilayer graphene by precipitation using W capping layer

Jumpei Yamada, Yuki Ueda, Takahiro Maruyama, Shigeya Naritsuka

Japanese Journal of Applied Physics (IOP Publishing)  55 ( 10 ) 100302 - 100302 2016.09

Lead author, Corresponding author,  ISSN  0021-4922

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Abstract

In this study, the direct growth of multilayer graphene from amorphous carbon on a sapphire (0001) substrate by precipitation using a nickel catalyst layer and a tungsten capping layer was examined. The findings revealed that a tungsten carbide layer was formed on top of the catalyst, and this suppressed the diffusion of carbon atoms upwards towards the surface. This caused the graphene layer to precipitate below the catalyst layer rather than above it. Under optimized growth conditions, Raman spectroscopy indicated that a high-quality graphene layer was formed with a low D/G peak intensity ratio of 0.10.

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Precipitation of high-quality multilayer-graphene using Al₂O₃ barrier and Au cap layers

Jumpei Yamada, Manabu Suzuki, Yuki Ueda, Takahiro Maruyama, Shigeya Naritsuka

MRS Proceedings (Springer Science and Business Media LLC)  1786   13 - 18 2015.08

Lead author, Corresponding author,  ISSN  0272-9172

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ABSTRACT

The mechanism for the precipitation of multilayer graphene was investigated with respect to the use of an Al₂O₃ barrier layer and Au capping layer. The Al₂O₃ barrier layer suppresses the dissolution of carbon into the catalyst, especially at low temperature, and assists a decrease in the density of graphene nuclei. On the other hand, the Au capping layer is beneficial to weaken the strong binding between the catalyst and the graphene carbon atoms, and enhances the surface migration of precipitated carbon adatoms. A combination of the Al₂O₃ barrier layer and Au capping layer is useful for the synthesis of high-quality graphene with large grains. On a sample with both layers annealed for 60 min, the area of 5-layer graphene islands is as large as 10 μm, and covers 60% of the entire surface. The Raman D/G band intensity ratio of 0.024 indicates the precipitated graphene is high quality.

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Selective growth of GaN by liquid phase electroepitaxy using aluminum oxide mask

Daisuke Kambayashi, Hiroyuki Takakura, Masafumi Tomita, Muneki Iwakawa, Yosuke Mizuno, Jumpei Yamada, Takahiro Maruyama, Shigeya Naritsuka

Japanese Journal of Applied Physics (IOP Publishing)  53 ( 11S ) 11RC06 - 11RC06 2014.10

Lead author, Corresponding author,  ISSN  0021-4922

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