Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

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

DOI： 10.35848/1347-4065/acb74c

Web of Science

Scopus

Publisher：Applied Physics Letters  

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

DOI： 10.1063/5.0120723

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Scopus

Language：English   Publisher：Optics Letters  

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

DOI： 10.1364/OL.456993

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Scopus

PubMed

Publisher：Journal of Applied Physics  

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

DOI： 10.1063/5.0085384

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Crystal Growth  

This study examines the effect of (0 0 0 −1) GaN substrate misorientation on the residual impurities and surface morphology of N-polar GaN grown by metalorganic vapor-phase epitaxy. Carbon, silicon, and oxygen concentrations decreased with increasing GaN substrate misorientation angle, with the lowest impurity concentration achieved for a misorientation angle of 2° toward the m-axis, with 6 × 10 15 cm −3 carbon, 6 × 10 15 cm −3 silicon, and 4 × 10 17 cm −3 oxygen atoms. The oxygen concentration was measured at a depth of 0.5 μm below the wafer surface, and the oxygen concentration decreased with increasing thickness. The incorporation of carbon and oxygen revealed a strong dependence on the misorientation angle. The step distance height of the steps parallel to the [1 1 −2 0] direction (or perpendicular to the [1 −1 0 0] m-direction) was confirmed to be a double-height layer step. This phenomenon indicated that m-direction steps are stable for N-polar growth in GaN. In cases of large misorientation toward the m-axis in of the GaN substrate it was difficult to control the misorientation perpendicular to the nominal direction leading to a-axis direction by wafer bowing at wafer manufacturing. Therefore, step-bunching was generated for each symmetric m-axis due to an increase in the compound's off-angle, thus causing the surface roughness to become large.

DOI： 10.1016/j.jcrysgro.2019.02.013

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nanomaterials  

Copper nitride particles have a low decomposition temperature, they absorb light, and are oxidation-resistant, making them potentially useful for the development of novel wiring inks for printing circuit boards by means of intense pulsed light (IPL) sintering at low-energy. Here, we compared the thermal decomposition and light absorption of copper materials, including copper nitride (Cu3N), copper(I) oxide (Cu2O), or copper(II) oxide (CuO). Among the copper compounds examined, copper nitride had the second highest light absorbency and lowest decomposition temperature; therefore, we concluded that copper nitride was the most suitable material for producing a wiring ink that is sintered by means of IPL irradiation. Wiring inks containing copper nitride were compared with those of wiring inks containing copper nitride, copper(I) oxide, or copper(II) oxide, and copper conversion rate and sheet resistance were also determined. Under low-energy irradiation (8.3 J cm−2), copper nitride was converted to copper at the highest rate among the copper materials, and provided a sheet resistance of 0.506 Ω·sq−1, indicating that copper nitride is indeed a candidate material for development as a wiring ink for low-energy intense pulsed light sintering-based printed circuit board production processes.

DOI： 10.3390/nano8080617

Web of Science

Scopus

PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physica Status Solidi (A) Applications and Materials Science  

In this study, GaN m-plane Schottky barrier diodes are fabricated by metalorganic vapor-phase epitaxy (MOVPE) on several off-angle gallium nitride (GaN) substrates, and the off-cut angle dependence of impurity incorporation is investigated. We show that the MOVPE layer on the substrate inclined 5° toward the [000–1] direction has extremely low impurity incorporation. These results provide important suggestions for the fabrication of m-plane power devices.

DOI： 10.1002/pssa.201700645

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1063/1.4922512

Authorship：Lead author   Language：English  

Authorship：Lead author   Language：English  

Authorship：Lead author   Language：English  

Authorship：Lead author   Language：English  

Language：English  

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1063/1.1616646

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Event date： 2022.6 - 2022.7

Language：English   Presentation type：Oral presentation (general)  

Country：United States  

Event date： 2022.6 - 2022.7

Language：English   Presentation type：Oral presentation (general)  

Country：United States  

Event date： 2022.4

Language：English   Presentation type：Oral presentation (general)  

Venue：JAPAN  

Event date： 2021.11 - 2021.1

Language：English   Presentation type：Poster presentation  

Venue：On-Line   Country：Japan  

Event date： 2021.8

Language：English   Presentation type：Poster presentation  

Venue：On-Line   Country：Korea, Republic of  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：English   Presentation type：Oral presentation (general)  

Venue：California Long beach Convention center, (USA)  

Language：English   Presentation type：Oral presentation (general)  

Venue：Baltimore Convention Center, Maryland, (USA)  

Language：English   Presentation type：Oral presentation (general)  

Venue：Jeju island, (Korea),  

Language：English   Presentation type：Oral presentation (general)  

Venue：Chiba (JAPAN)  

Language：English   Presentation type：Oral presentation (general)  

Venue：Turku (Finland)  

Language：English   Presentation type：Oral presentation (general)  

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Language：English   Presentation type：Oral presentation (general)  

Country：Taiwan, Province of China  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：青山学院大学相模原キャンパス  

Venue：武蔵工業大学世田谷キャンパス  

Venue：上智大学四谷キャンパス  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学札幌キャンパス  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：中部大学三浦幸平メモリアルホール  

Venue：同志社大学京田辺キャンパス