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

DOI： 10.1109/TED.2023.3345822

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DOI： 10.35848/1882-0786/ad120b

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DOI： 10.1117/12.3010733

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DOI： 10.18494/SAM4647

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DOI： 10.1117/12.3009947

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DOI： 10.1109/MWP62612.2024.10736216

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DOI： 10.1063/5.0177681

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DOI： 10.1063/5.0180062

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DOI： 10.1109/LED.2023.3285938

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

DOI： 10.1109/LED.2023.3274306

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DOI： 10.1063/5.0155363

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DOI： 10.1063/5.0149838

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

DOI： 10.3390/cryst13030524

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

DOI： 10.2322/tjsass.66.10

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CiNii Research

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

DOI： 10.1117/12.2657032

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DOI： 10.1038/s41598-022-25522-6

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PubMed

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DOI： 10.1063/5.0124512

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DOI： 10.1063/5.0088908

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DOI： 10.1038/s41598-022-12628-0

PubMed

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DOI： 10.1038/s41598-022-10610-4

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PubMed

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

DOI： 10.1063/5.0085384

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DOI： 10.1063/5.0083194

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DOI： 10.1109/LED.2021.3131057

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DOI： 10.1109/JEDS.2022.3208028

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DOI： 10.1109/TED.2021.3119528

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DOI： 10.1007/s00259-021-05460-7

PubMed

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

DOI： 10.1063/5.0077364

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2021.1.0_1347

CiNii Research

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

DOI： 10.1109/JEDS.2021.3081463

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

DOI： 10.35848/1882-0786/ab93a0

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DOI： 10.35848/1882-0786/ab9166

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

DOI： 10.1002/pssb.201900554

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2020.1.0_3111

CiNii Research

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

DOI： 10.1063/1.5125623

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2019.1.0_3122

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2018.1.0_1670

CiNii Research

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

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2017.1.0_1554

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2017.1.0_1136

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2016.2.0_2888

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2016.2.0_2867

CiNii Research

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

CiNii Research

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

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2014.2.0_1745

CiNii Research

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

DOI： 10.1016/j.tsf.2013.02.048

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

DOI： 10.1063/1.4825124

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

DOI： 10.7567/JJAP.52.08JC05

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

DOI： 10.7567/JJAP.52.08JK09

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

DOI： 10.7567/JJAP.52.08JB16

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

DOI： 10.7567/JJAP.52.08JE07

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

DOI： 10.7567/JJAP.52.08JE06

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

DOI： 10.7567/JJAP.52.08JB11

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

DOI： 10.7567/JJAP.52.08JB14

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

DOI： 10.7567/JJAP.52.08JB09

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Although plasma-assisted molecular beam epitaxy(PA-MBE) is a promising technique for GaN growth,the growth rate obtained by this technique is lower than that obtained by metal organic vapor phase epitaxy(MOVPE).In order to improve the growth rate of the technique,high density radical source(HDRS) was developed.By vacuum ultraviolet absorption spectroscopy(VUVAS) measurement,two orders of magnitude higher radical density was confirmed in a comparison of the HDRS and a conventional radical source(CRS).While faster growth rate of 1.4μm/h in GaN homoepitaxy was achieved,better crystalline quality of InxGa_<(1-x)>N(x=0.03〜0.16) epilayers with approximately 1.4μm thickness were also achieved by introducing the HDRS in PA-MBE.

DOI： 10.1299/jsmeiip.2013.5

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

We grew high-quality semipolar (1-101) GaN stripes on a Si substrate by selective area growth and regrew InGaN/GaN MQW on these stripes with MOVPE. The PL peak wavelength at 300 K and InGaN well thickness for the obtained samples were 435-590 nm and 2-9 nm, respectively. In PL measurements from the surface, we found that a shorter wavelength and thinner well sample have PL polarization perpendicular to the c axis. On the other hand, as the PL wavelength is longer and/or the well is thicker, the PL polarization shifts from perpendicular to parallel. In the PL measurement from the (11 -22) edge surface of the sample with shorter wavelength, we observed the narrowing of PL spectrum. In addition, the parallel component of PL polarization increased with increase in well thickness. Therefore, we can control the direction of polarization by managing well thickness and indium composition and apply to make a LD structure.

CiNii Research

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

DOI： 10.1002/pssb.201100445

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

DOI： 10.1088/0268-1242/27/2/024006

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Association for Crystal Growth  

(1-101) GaN and (11-22) GaN was grown by MOVPE on etched Si substrate. A characteristic of optical, electrical and impurity was evaluated on each face GaN. We also reported an Indium incorporation and a process of dislocation propagation on the (1-101) GaN in this paper.

DOI： 10.19009/jjacg.38.4_241

CiNii Research

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

DOI： 10.1143/JJAP.50.122101

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

DOI： 10.1143/APEX.4.092102

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

DOI： 10.1002/pssa.201000907

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

DOI： 10.1016/j.jcrysgro.2010.10.054

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

DOI： 10.1143/JJAP.50.01AD04

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

DOI： 10.1007/s11431-010-4182-2

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

DOI： 10.1143/APEX.4.012105

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

DOI： 10.1063/1.3666474

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

DOI： 10.1002/pssc.201000969

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：National Institute of Technology, Toyota College  

We have studied the residual strain and stress in semi-polar (1-101)GaN and conventional (0001)GaN grown on Si by means of Raman spectroscopy. By adopting the stress coefficients reported in the references, the residual strain of samples were deduced from the frequency shifts of the phonon modes in Raman spectra. As the results, it was found that the strain tensor of (1-101) GaN/Si was smaller than that of the (0001)GaN/Si in good agreement with the results obtained from the X-ray diffraction measurements.

DOI： 10.20692/toyotakosenkiyo.kj00005889042

CiNii Research

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

DOI： 10.3938/jkps.54.2363

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

DOI： 10.1016/j.jcrysgro.2009.01.032

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

DOI： 10.1016/j.jcrysgro.2009.01.064

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

DOI： 10.1088/0022-3727/42/4/045112

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CiNii Research

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

DOI： 10.1093/jmicro/dfm016

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PubMed

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

CiNii Research

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

DOI： 10.1093/jmicro/dfm013

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PubMed

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

DOI： 10.1016/j.jcrysgro.2006.10.229

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

DOI： 10.1143/JJAP.45.7655

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

DOI： 10.1002/pssb.200565115

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

DOI： 10.1143/JJAP.45.4015

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

Using a selective MOVPE method, (1-101)GaN was grown on a 7-degree off-axis (001)Si substrate. The top (1-101) surface of the sample was terminated with N, which is contrast to that the (0001) face is terminated with Ga. The doping of Si and C was attempted to control the type of conduction. A nominally undoped GaN/AlN heterostructure showed p-type conduction. The doping of Si changed the type of conduction from p-type to n-type. On the other hand, the hole concentration increased with the C doping level. These results proves that Si acts as a donor and C acts as an acceptor in (1-101)GaN. We might conclude that the p-type conduction is due to the incorporation of C on the top surface of the N face of the GaN.

CiNii Research

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

DOI： 10.3950/jibiinkoka.106.856

PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

A GaN/AlGaN double heterostructure was fabricated on the (1101) facets of a GaN triangular structure prepared by selective area growth by metalorganic vapor phase epitaxy. The photoluminescence (PL) and cathode luminescence (CL) spectra were investigated to characterize the structure. It was found that the PL peak wavelength due to the GaN quantum well is dependent on the well thickness. However, the Al composition of the AlGaN layer is dependent on the position on the (1101) facet. The anomalous gradient of the composition is attributed to the difference in the diffusion coefficients of Al and Ga on the surface.

DOI： 10.1143/jjap.40.1896

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

Selective area growth (SAG) of GaN on (111) Si substrate was studied using AlGaN as an intermediate layer. A hexagonal dot with a (0001) plane on the top and of a 5 µm or a submicron size was obtained using a patterned dot structure of silicon dioxide (SiO<SUB>2</SUB>) mask. The facet structure revealed that the <11\=20> axis of hexagonal GaN is parallel to the <110> axis of the Si substrate. The cathodoluminescence (CL) spectrum at 133 K exhibited a strong near-band-edge emission band for the submicron dots, which suggests excellent crystallinity. Epitaxial lateral overgrowth (ELO) of GaN on the Si substrate is demonstrated.

DOI： 10.1143/jjap.37.l966

CiNii Research

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

Results of a thermodynamic study of Sn doping and fabrication of a Sn-doped GaN freestanding layer with high structural quality by halide vapor phase epitaxy (HVPE) are described in this paper. Thermodynamic analysis revealed that SnCl2 and/or SnCl act as Sn precursors through the reaction between Sn metal and HCl gas. The equilibrium partial pressures of SnCl2 and SnCl increase with the input HCl partial pressure. To generate Sn precursors effectively, it is desirable that the reaction between Sn metal and HCl gas occurs in the inert gas ambient. On the basis of results of the thermodynamic study, the Sn-doped GaN freestanding layer with a Sn concentration of 5.7 × 1019 cm−3 is fabricated by removing the GaN seed substrate after HVPE growth. The Sn-doped GaN freestanding layer has high crystal quality, and the lattice constants along the c- and a-axes of the Sn-doped GaN freestanding layer are larger than those of the GaN seed substrate because of the high electron density and the size effect of Sn atoms.

DOI： 10.1016/j.jcrysgro.2024.127923

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Scopus

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

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τPLRT) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τPLRT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τPLRT in n-GaN samples are compared with those of p-GaN, in which τPLRT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

DOI： 10.1063/5.0201931

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

DOI： 10.1016/j.jcrysgro.2023.127529

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

DOI： 10.1016/j.jcrysgro.2023.127552

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

Lanthanide (Ln)-implanted gallium nitride (GaN) semiconductors have a variety of potential applications as light-emitting devices and quantum light sources, but their optical properties are not well understood. In this study, we investigate the room temperature optical properties of praseodymium (Pr) ion implanted single-crystal GaN and their changes upon ultrahigh pressure annealing (UHPA) up to 1480 °C. Photoluminescence (PL) spectra, luminescence transition lifetime, and excitation cross section of the implanted Pr ions are analyzed. In addition, the recovery of implantation-induced damage and the thermal diffusion of implanted Pr ions by UHPA are investigated by X-ray diffraction, Raman spectroscopy, and secondary ion mass spectrometry. The results show that the implantation damage is recovered by annealing at temperatures above 1200 °C, but Pr ions thermally diffuse to the surface as the annealing temperature increases. The annealing temperature at which a maximum PL intensity is obtained increases with increasing the implantation dose. However, the PL intensity decreases in all cases after annealing at 1480 °C, indicating that a quenching factor is dominant in this temperature range.

DOI： 10.1016/j.nimb.2023.165181

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Scopus

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

DOI： 10.1002/pssa.202200871

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

DOI： 10.1063/5.0146080

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

DOI： 10.3390/ma16052016

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PubMed

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

Abstract

We demonstrated nanoplatelet In _x Ga_1−xN 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

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/acb74c/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：A V S AMER INST PHYSICS  

Pulsed electron beams from a photocathode using an InGaN semiconductor have brought selectively scanning technology to scanning electron microscopes, where the electron beam irradiation intensity and area can be arbitrarily selected within the field of view in SEM images. The p-type InGaN semiconductor crystals grown in the metalorganic chemical vapor deposition equipment were used as the photocathode material for the electron beam source after the surface was activated to a negative electron affinity state in the electron gun under ultrahigh vacuum. The InGaN semiconductor photocathode produced a pulsed electron beam with a rise and fall time of 3 ns, consistent with the time structure of the irradiated pulsed laser used for the optical excitation of electrons. The InGaN photocathode-based electron gun achieved a total beam operation time of 1300 h at 15 mu A beam current with a downtime rate of 4% and a current stability of 0.033% after 23 cycles of surface activation and continuous beam operation. The InGaN photocathode-based electron gun has been installed in the conventional scanning electron microscope by replacing the original field emission gun. SEM imaging was performed by selective electron beaming, in which the scanning signal of the SEM system was synchronized with the laser for photocathode excitation to irradiate arbitrary regions in the SEM image at arbitrary intensity. The accuracy of the selection of regions in the SEM image by the selective electron beam was pixel by pixel at the TV scan speed (80 ns/pix, 25 frame/s) of the SEM.

DOI： 10.1116/6.0002111

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

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

Abstract

We evaluated Mg-diffusion into GaN from GaN/Mg mixture. The diffusion depth of Mg increased with diffusion temperature from 1100 °C to 1300 °C, whereas the Mg concentration remained constant at 2–3 × 10¹⁸cm⁻³ independent of temperature. The estimated activation energy for Mg diffusion was 2.8 eV, from which the substitutional diffusion mechanism was predicted. Mg-diffused GaN samples showed p-type conductivity with a maximum hole mobility of 27.7 cm² V⁻¹ s⁻¹, suggesting that substitutional diffusion contributes to Mg activation. This diffusion technique can be used to easily form p-type GaN and has potential as a p-type selective doping technique.

DOI： 10.35848/1882-0786/ac9c83

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac9c83/pdf

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

A high quantum efficiency (QE) can be obtained on negative electron affinity (NEA) surfaces. It is well-known that NEA surfaces can be formed on semiconductor materials such as GaAs by the alternating supply of cesium (Cs) and oxygen (O2), which is called the yo-yo method. While GaN and related compounds such as InGaN are expected to realize an NEA photocathode with a long lifetime, the surface reactions between GaAs and nitride semiconductors are completely different with respect to the increasing rate of QE induced by the supply of O2. In addition, the surface processes of photoemission from NEA nitride semiconductors have not yet been elucidated. In the present study, a higher QE was achieved in InGaN by simultaneously supplying Cs and O2 instead of using the conventional yo-yo method. The possible Cs adsorption states in relation to the photoemission are also discussed based on the QE tendencies and the temperature programmed desorption (TPD) spectra of the NEA surfaces formed under elevated temperature conditions. This study suggests that the Cs oxide species, which is one of the key compounds for imparting the NEA nature, decomposes at approximately 350 degrees C and that the InGaN-Cs2O2 structure is a possible candidate for NEA photocathodes.

DOI： 10.1016/j.apsusc.2022.153882

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

DOI： 10.1063/5.0122292

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

DOI： 10.3390/ma15176165

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.jcrysgro.2022.126749

Web of Science

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

Abstract

Energy-dispersive X-ray signals calibrated by Rutherford backscattering indicated the generation of Al_13/24Ga_11/24N in Ga-rich stripes in a nonflat Al_0.58Ga_0.42N layer. Also, the CL peak wavelengths of ∼259 and 272 nm also showed the generation of Al_15/24Ga_9/24N and Al_13/24Ga_11/24N in Al-rich zones and Ga-rich stripes, respectively. The wavelength of a strong CL peak at ∼246 nm, which was observed from the Al_0.7Ga_0.3N layer in our previous study, is also considered to correspond to the near-band-emission wavelengths of Al_17/24Ga_7/24N. In particular, the stronger reproducibility of metastable Al_15/24Ga_9/24N generation was confirmed, in agreement with the computed predictions by other research groups.

DOI： 10.35848/1882-0786/ac79a1

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac79a1/pdf

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

Herein, the operation of dopant‐free GaN‐based p‐n junctions formed by distributed polarization doping (DPD) is experimentally demonstrated and their space charge profiles and carrier transport properties are investigated. The device exhibits ideal space charge profiles explained by polarization effects and demonstrates the excellent controllability of DPD. In addition, it shows rectification and electroluminescence under forward‐biased conditions. The carrier transport properties could be explained by the conventional recombination/diffusion model used for impurity‐doped p‐n junctions. Repeatable breakdowns are also observed in all devices and the temperature‐dependent breakdown voltages reveal that the breakdowns are caused by avalanche multiplication, which is also the same as those reported in impurity‐doped GaN p‐n diodes. These results indicate that DPD is a promising doping technology for GaN‐based power devices overcoming any issues associated with conventional impurity doping.

DOI： 10.1002/pssr.202200127

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/pssr.202200127

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

DOI： 10.1088/1361-6463/ac5d03

Web of Science

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

Abstract

An AlGaN/GaN heterojunction bipolar transistor (HBT) with N-p-n configuration was fabricated by the “regrowth-free” method, resulting in a contamination-free emitter-base AlGaN/GaN heterojunction. The low-bias-power-based low-damage inductively coupled plasma–reactive ion etching was employed in this study for emitter mesa definition instead of the conventional selective-area-regrowth technique. The method successfully minimized the etching-induced damage in the p-GaN base layer and the contamination at the emitter-base AlGaN/GaN heterojunction. Consequently, the fabricated device exhibited a high current gain of 25, the highest current density of 15.0 kA cm⁻², and the lowest on-state voltage offset of 0.75 V ever reported for AlGaN/GaN HBTs.

DOI： 10.35848/1882-0786/ac6197

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac6197/pdf

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

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.

DOI： 10.35848/1882-0786/ac5acf

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Scopus

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

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.

DOI： 10.35848/1882-0786/ac60c7

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac60c7/pdf

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

<title>Abstract</title>
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.

DOI： 10.35848/1882-0786/ac53e2

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac53e2/pdf

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

We evaluated the beam current dependence of defect formation during Mg ion implantation into GaN at a high temperature of 1100 °C with two beam currents. Photoluminescence spectra suggest that low-beam-current implantation reduced the vacancy concentration and activated Mg to a greater extent. Moreover, scanning transmission electron microscopy analysis showed that low-beam-current implantation reduced the density of Mg segregation defects with inactive Mg and increased the density of intrinsic dislocation loops, suggesting decreases in the densities of Ga and N vacancies. The formation of these defects depended on beam current, which is an important parameter for defect suppression.

DOI： 10.35848/1882-0786/ac481b

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Scopus

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

Abstract

The presence of hexagonal-pyramid-shaped hillocks (HPHs) in AlGaN epitaxial films affects device characteristics; this effect is significant in DUV laser diodes (LDs) on AlN substrates, where the presence of HPHs under the p-electrode increases the threshold current density and inhibits the lasing. In this study, we investigated the difference between the lasing characteristics of LDs with and without HPHs. It was found that in the presence of HPHs, the threshold excitation power density increased and the slope efficiency decreased by optical excitation. To investigate the cause of these phenomena, we performed structural, optical, and electrical analyses of the HPHs. Various imaging techniques were used to directly capture the characteristics of the HPHs. As a result, we concluded that HPHs cause the degradation of LD characteristics due to a combination of structural, optical, and electrical factors.

DOI： 10.35848/1347-4065/ac3a1d

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ac3a1d/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/5.0076764

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

DOI： 10.1088/1361-6463/ac2065

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Other Link： https://iopscience.iop.org/article/10.1088/1361-6463/ac2065/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：A V S AMER INST PHYSICS  

In this study, we generated 25 multielectron beam (MEB) using an InGaN photocathode with a negative electron affinity state irradiating with 25 multilaser beam. The uniformity of the MEB and the total electron beam current were evaluated. A laser beam was split into 25 laser beams using a spatial light modulator. The coefficient of variation (CV) of laser power was 20%. The CV of quantum efficiency was 1.1%. The CV of electron beam current was 12%, and the total current was about 1.2 mu A. These results will enhance the development of the MEB-defect inspection using the InGaN photocathode.

DOI： 10.1116/6.0001272

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/5.0066139

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

In this study, vertical-type thick BGaN PIN diodes were successfully fabricated to improve the neutron capture rate of BGaN diodes by improving the BGaN epitaxial growth technique. In this technique, 5-μm-thick epitaxial growth was achieved using trimethylboron as a B metal-organic source, which suppressed the gas-phase reaction. The α-particle energy spectrum, the neutron pulse signal, and the residual energy of particles emitted from a neutron capture reaction were measured using the fabricated BGaN diodes by performing radiation detection measurements. The detected signal position in the neutron detection signal spectrum was similar to that of 2.3 MeV α-particles. These results indicate that vertical-type BGaN diodes can be used as effective neutron detectors.

DOI： 10.1063/5.0051053

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Scopus

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

We have investigated the possibility of applying lasers to slice GaN substrates. Using a sub-nanosecond laser with a wavelength of 532 nm, we succeeded in slicing GaN substrates. In the laser slicing method used in this study, there was almost no kerf loss, and the thickness of the layer damaged by laser slicing was about 40 µm. We demonstrated that a standard high quality homoepitaxial layer can be grown on the sliced surface after removing the damaged layer by polishing.

DOI： 10.1038/s41598-021-97159-w

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Scopus

PubMed

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

Gallium nitride (GaN)-based devices surpass the traditional silicon-based power devices in terms of higher breakdown voltage, faster-switching speed, higher thermal conductivity, and lower on-resistance. However, heteroepitaxial GaN growths like GaN on sapphire are not suitable for power devices due to the threading dislocation densities as high as 108/cm2. Recently, homoepitaxial GaN growth has become possible thanks to the native GaN substrates with dislocation densities in the order of 104/cm2 but the extremely high cost of the GaN substrates makes the homoepitaxy method unacceptable for industrial applications, and the slicing of wafers for reusing them is an effective solution for cost reduction. In this study, we will investigate a route for slicing the GaN single crystal substrate by controlling the laser pulse energy and changing the distance between each laser shot. The 2D and 3D crack propagations are observed by a multiphoton confocal microscope, and the cross section of samples is observed by a scanning electron microscope (SEM). The results showed that two types of radial and lateral cracking occurred depending on the pulse energy and shot pitch, and controlling them was of importance for attaining a smooth GaN substrate slicing. Cross-sectional SEM images showed that at suitable pulse energy and distance, crack propagation could be controlled with respect to the irradiation plane.

DOI： 10.1007/s00339-021-04808-y

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Scopus

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

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.

DOI： 10.35848/1882-0786/ac0fb6

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Scopus

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

The interface properties of GaN metal-insulator-semiconductor (MIS) structures with a gate electrode metal deposited by electron beam (EB) or resistive heating evaporation were investigated. Also, the impact of the interface properties on the channel mobility in GaN MIS field-effect transistors was investigated. It was confirmed that interface charges including both interface states and positive fixed charges were introduced to an Al2O3/GaN interface by the formation of a gate electrode by EB evaporation. Consequently, the introduced interface charges degraded the electron mobility in the MIS channel.

DOI： 10.35848/1882-0786/ac0ffa

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Scopus

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

DOI： 10.35848/1882-0786/ac154c

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac154c/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.jcrysgro.2021.126173

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

An optimized top-down approach was utilized to fabricate vertical GaN-on-GaN nanowire Schottky barrier diodes (NWSBDs) in this letter. The suppressed reverse leakage current and enhanced breakdown voltage (BV) of the vertical GaN NWSBDs are attributed to the reduced electric field at the interface of the Schottky junction achieved through the dielectric reduced surface field technique. As-fabricated NWSBD delivers a low turn-on voltage of 0.80 V, a near-unity ideality factor of 1.04, along with a soft BV of 480 V. The measured soft BV is comparable with the avalanche BV of the p-n diode with a similar net doping concentration in the drift region.

DOI： 10.35848/1347-4065/ac06b5

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

Inductively coupled plasma–reactive ion etching (ICP–RIE)-induced damage in heavily Mg-doped p-type GaN ([Mg] = 2 × 1019 cm−3) was investigated by low-temperature photoluminescence (PL) and depth-resolved cathodoluminescence (CL) spectroscopy. From PL measurements, we found broad yellow luminescence (YL) with a maximum at around 2.2–2.3 eV, whose origin was considered to be isolated nitrogen vacancies (VN), only in etched samples. The depth-resolved CL spectroscopy revealed that the etching-induced YL was distributed up to the electron-beam penetration depth of around 200 nm at a high ICP–RIE bias power (Pbias). Low-bias-power (low-Pbias) ICP–RIE suppressed the YL and its depth distribution to levels similar to those of an unetched sample, and a current–voltage characteristic comparable to that of an unetched sample was obtained for a sample etched with Pbias of 2.5 W.

DOI： 10.35848/1347-4065/abd538

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Scopus

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

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.

DOI： 10.35848/1882-0786/abf443

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/abf443/pdf

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

DOI： 10.1063/5.0042036

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

We report the first experimental demonstration of microwave oscillation in GaN impact ionization avalanche time transit (IMPATT) diodes at the X-band. The device used in this study is a single drift diode with a p+–n simple abrupt junction and vertical mesa termination. The reverse I–V characteristic of the diode shows low leakage current, clear avalanche breakdown, and high avalanche capability, as required for IMPATT operation. Microwave testing is performed in an X-band waveguide circuit with a reduced-height waveguide resonant cavity. Oscillations are observed at 9.52 GHz at a power of ∼56 mW.

DOI： 10.35848/1882-0786/abe3dc

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Scopus

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

Numerical simulations are a valuable tool for the design and optimization of crystal growth processes because experimental investigations are expensive and access to internal parameters is limited. These technical limitations are particularly large for ammonothermal growth of bulk GaN, an important semiconductor material. This review presents an overview of the literature on simulations targeting ammonothermal growth of GaN. Approaches for validation are also reviewed, and an overview of available methods and data is given. Fluid flow is likely in the transitional range between laminar and turbulent; however, the time-averaged flow patterns likely tend to be stable. Thermal boundary conditions both in experimental and numerical research deserve more detailed evaluation, especially when designing numerical or physical models of the ammonothermal growth system. A key source of uncertainty for calculations is fluid properties under the specific conditions. This originates from their importance not only in numerical simulations but also in designing similar physical model systems and in guiding the selection of the flow model. Due to the various sources of uncertainty, a closer integration of numerical modeling, physical modeling, and the use of measurements under ammonothermal process conditions appear to be necessary for developing numerical models of defined accuracy.

DOI： 10.3390/cryst11040356

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

Photo-electrochemical (PEC) etching is a promising technique for fabricating GaN microelectromechanical systems devices. In this study, we demonstrate the fabrication of GaN cantilevers by the bandgap-selective PEC etching of an InGaN superlattice sacrificial layer. By using an InGaN superlattice as a sacrifice layer, we found the PEC etching rate became higher than using a normal InGaN layer. As a result, the InGaN superlattice was completely etched and we fabricated GaN-based cantilevers whose resonance characteristics were measured. The Young’s modulus of GaN was determined from the resonance characteristics of GaN cantilevers to be the same as the highest value reported previously.

DOI： 10.35848/1882-0786/abe657

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Scopus

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

Thermal boundary conditions for numerical simulations of ammonothermal GaN crystal growth are investigated. A global heat transfer model that includes the furnace and its surroundings is presented, in which fluid flow and thermal field are treated as conjugate in order to fully account for convective heat transfer. The effects of laminar and turbulent flow are analyzed, as well as those of typically simultaneously present solids inside the autoclave (nutrient, baffle, and multiple seeds). This model uses heater powers as a boundary condition. Machine learning is applied to efficiently determine the power boundary conditions needed to obtain set temperatures at specified locations. Typical thermal losses are analyzed regarding their effects on the temperature distribution inside the autoclave and within the autoclave walls. This is of relevance because autoclave wall temperatures are a convenient choice for setting boundary conditions for simulations of reduced domain size. Based on the determined outer wall temperature distribution, a simplified model containing only the autoclave is also presented. The results are compared to those observed using heater-long fixed temperatures as boundary condition. Significant deviations are found especially in the upper zone of the autoclave due to the important role of heat losses through the autoclave head.

DOI： 10.3390/cryst11030254

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{MDPI} {AG}  

The development of pulsed electron sources is applied to electron microscopes or electron beam lithography and is effective in expanding the functions of such devices. The laser photocathode can generate short pulsed electrons with high emittance, and the emittance can be increased by changing the cathode substrate from a metal to compound semiconductor. Among the substrates, nitride-based semiconductors with a negative electron affinity (NEA) have good advantages in terms of vacuum environment and cathode lifetime. In the present study, we report the development of a photocathode electron gun that utilizes photoelectron emission from a NEA-InGaN substrate by pulsed laser excitation, and the purpose is to apply it to material nanofabrication and high-speed observation using a pulsed transmission electron microscope (TEM) equipped with it.

DOI： 10.3390/qubs5010005

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Scopus

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

The leakage current caused by the Si pileup at the regrowth interface of AlGaN/GaN high electron mobility transistors (HEMTs) is significantly suppressed by the semi-insulating Mg-doped GaN layer. Mg is unintentionally doped and can be originated from the graphite susceptor of metal organic vapor phase epitaxy. Before regrowth of the AlGaN/GaN heterostructure, the GaN template is treated with hydrochloric acid (HCl) and hydrogen peroxide/potassium hydroxide (H2O2/KOH) solution to promote the adsorption of Mg on the GaN surface. The Mg-doped GaN channel layer is highly resistive due to the passivation of hydrogen. The p-n junction formed by the weakly p-type Mg-doped GaN layer and the n-type Si pileup layer depletes the excess electrons at the regrowth interface. As a result, the off-state drain leakage current of the HEMT device can be decreased by two orders of magnitude at 40 V.

DOI： 10.1063/5.0034584

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OSA - The Optical Society  

We correct an error in the Eq. (1).

DOI： 10.1364/OME.420328

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Scopus

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

DOI： 10.1063/5.0028516

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

DOI： 10.35848/1882-0786/abcb49

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/abcb49/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Hexagonal boron nitride (hBN) and diamond are promising materials for next-generation electronics and optoelectronics. However, their combination is rarely reported. In this study, we for the first time demonstrate the success to direct growth of two-dimensional (2D) hBN crystal layers on diamond substrates by metalorganic vapor phase epitaxy. Compared with the disordered growth we found on diamond (100), atomic force microscopy, X-ray diffraction, and transmission electron microscopy results all support 2D hBN with highly oriented lattice formation on diamond (111). Also, the epitaxial relationship between hBN and diamond (111) substrate is revealed to be [0 0 0 1](hBN) // [1 1 1](diamond) and [1 0 (1) over bar 0](hBN) // [1 1 (2) over bar](diamond) The valence band offset at hBN/diamond (111) heterointerface determined by X-ray photoelectron spectroscopy is 1.4 +/- 0.2 eV, thus yielding a conduction band offset of 1.0 +/- 0.2 eV and type II staggered band alignment with a bandgap of 5.9 eV assumed for hBN. Furthermore, prior thermal cleaning of diamond in a pure H-2 atmosphere smoothens the surface for well-ordered layered hBN epitaxy, while thermal cleaning in a mixed H-2 and NH3 atmosphere etches the diamond surface, creating many small faceted pits that destroy the following epitaxy of hBN.

DOI： 10.1021/acsami.0c11883

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Wide bandgap semiconductors are increasingly important for bioimaging applications, as they can possess good biocompatibility and host a large range of fluorescent defects spanning the visible to infrared. Gallium nitride is one promising host for photostable fluorophores. In particular, neodymium (Nd)-doped gallium nitride (GaN) shows bright near-infrared fluorescence and narrow room temperature linewidth and is therefore a candidate material for fluorescent probes for bioimaging. To explore the conditions necessary to generate biomarkers based on Nd:GaN, this paper reports the room temperature photoluminescence (FL) properties of small ensembles of Nd ions implanted into the nanoscale regions of GaN epilayers. The minimum volume of Nd-implanted GaN that can be optically detected in this study is about 8x10(4) nm(3) and the minimum detected ensemble of Nd ions is about 4x10(3), although not all of implanted Nd ions activate as luminescence centers. We show from the PL excitation spectra that the strongest resonant excitation appears at 619 nm, attributed to the I-4(9/2) -> (4)G(5/2) ((4)G(7/)(2)) transition in the 4f-shell. We measure the luminescence lifetime to be several tens of microseconds. We also identify the presence of a different excitation mechanism from the resonant excitation when excited below 510 nm (above 2.43 eV). (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

DOI： 10.1364/OME.401765

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

DOI： 10.1016/j.nimb.2020.06.007

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

DOI： 10.1063/5.0010774

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1347-4065/aba0d5

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

DOI： 10.1039/d0tc01369b

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

DOI： 10.1063/5.0010664

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

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

DOI： 10.1002/pssa.201900955

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

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

DOI： 10.1016/j.jcrysgro.2020.125643

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

DOI： 10.35848/1347-4065/ab8b3d

Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ab8b3d/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Herein, a newly developed birefringence microscope is used to observe dislocations in gallium nitride (GaN) substrates. The comparison results of the observation method using this microscope with other observation methods, such as X-ray topography and Raman microscopy, confirms that dislocations in a GaN substrate can be detected with a birefringence microscope. In addition, the observation can be carried out as easily as with an optical microscope. It is also found that under certain observation conditions, the direction of the edge component of dislocations can be determined.

DOI： 10.1002/pssb.201900553

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

DOI： 10.1063/1.5145017

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

DOI： 10.1088/1361-6641/ab63f1

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2020.1.0_1516

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

The quantum efficiency (QE) of an InGaN photocathode as a function of InGaN layer thickness (240, 100, and 70 nm) was investigated. To activate the sample surface, Cs and O were deposited on the surface in an ultrahigh-vacuum chamber. The QE for different optical power densities was measured by irradiating excitation light from the front and back sides of each sample. The QEs for InGaN layer thickness of 240, 100, and 70 nm with back-side irradiation were 0.9, 9.8, and 7.5%, respectively. For the thicknesses of 70 and 100 nm, the QEs were higher for back-side irradiation than for front-side irradiation, whereas for the thickness of 240 nm, the QE was higher for front-side irradiation. The InGaN layer thickness dependence of the QEs for back- and front-side irradiations was calculated using a continuous equation considering processes such as excitation, diffusion, recombination, and escape of electrons from the surface of the photocathode. The tendency of the experimental results, where QE was maximum at 100-120 nm, corresponded to that of the calculated results.

DOI： 10.1016/j.mee.2020.111229

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The decomposition of trimethylgallium (Ga(CH3)(3)) was examined under H-2 and N-2 atmospheres using a high-resolution (about 0.002 u) time-of-flight mass spectrometry system, which enabled the separation of N-2 and ethene (C2H4) signals. Under H-2 atmosphere, the main decomposition product was methane (CH4) formed by the hydrogenolysis of Ga(CH3)(3). However, under N-2 atmosphere, the main product is ethane (C2H6) at temperature lower than 660 degrees C. Above 660 degrees C, the C2H6 further decomposes into CH4, acetylene (C2H2) and C2H4, and the main components in the gas phase are C2H2 and C2H4 above 1000 degrees C. Since these are effective carbon dopants, the molar ratio of H-2 to the Ga(CH3)(3) raw material in the carrier gas should be maintained above 300 to inhibit the formation of hydrocarbons in N-2. (C) 2020 The Japan Society of Applied Physics

DOI： 10.35848/1347-4065/ab6fb0

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

DOI： 10.1093/jmicro/dfz037

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We studied the nucleation and growth of hexagonal BN (h-BN) on AlN template on c-plane sapphire by metalorganic vapor phase epitaxy as functions of growth temperature, deposition time, and triethylboron (TEB) partial pressure. A lateral growth rate of about 25 nm min(-1) for h-BN nuclei was obtained by atomic force microscopy and a nucleation activation energy of 2.1 eV was extracted from the temperature dependence of the nucleation density. A large TEB flow rate strongly enhances the formation of h-BN nuclei. At a reduced TEB flow rate, we observed a significantly decreased nuclei density and a delay in nucleation due to TEB desorption. By fine tuning the growth parameters, single-crystalline multilayer h-BN was successfully formed on AlN surface, as confirmed by x-ray diffraction and transmission electron microscopy (TEM). The epitaxial relationship between h-BN and AlN was [0 0 0 1](h-BN) parallel to [0 0 0 1](AlN) and [1 0 -10](h-BN) parallel to [1 1 -20](AlN) from TEM and electron backscatter diffraction measurements. In addition, TEM showed that the initial h-BN layers are not parallel and tend to form half-domes. On those half-domes (cap-shaped-like) a 2D lateral growth sets on, resulting in a well-oriented 2D multilayer observed in TEM. Thus, the surface topography further develops to form a relatively flat surface without wrinkles and finally a typical hexagon-like wrinkled surface at thicker h-BN layers. Particularly, the small h-BN nuclei have dangling bonds at their periphery that can interact with the substrate, forming actual bonds with AlN. Hence the choice on the substrate is important, despite the basal planes of multilayer h-BN are bonded by a weak van der Waals force.

DOI： 10.1088/2053-1583/ab46e6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：A V S AMER INST PHYSICS  

In this paper, the authors describe the effectiveness of thermal annealing in vacuum for quantum efficiency (QE) recovery from Cs/O-activated GaN and GaAs photocathodes. The QE of Cs/O-activated GaN photocathodes at 3.4eV dropped from 1.0% to <0.001% upon exposure to nitrogen and then increased to 0.6% upon annealing. On the other hand, the QE of Cs/O-activated GaAs at 1.42eV did not increase after annealing. In addition, after Cs/O activation, the sample was exposed to normal laboratory air and installed in an X-ray photoemission spectroscopy system. Upon annealing at 330<degrees>C, three key results were confirmed as follows: (1) the work function decreased by 0.32eV, (2) the chemical states of Cs 4d and Ga 3d were unchanged, and (3) the intensities of O 1s and C 1s on the high-binding-energy side decreased. In conclusion, the experimental results indicate that the annealing recovers the QE of Cs/O-activated GaN photocathode.

DOI： 10.1116/1.5120417

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Association for Crystal Growth  

<p>  The microscopic structural and optical characteristics of AlGaN light-emitting diodes (LEDs) fabricated on the AlN templates with dense macrosteps are shown to clarify the origin of their high internal quantum efficiency of radiation (IQE). The cross-sectional transmission electron microscopy observations under the high-angle annular dark field scanning mode and microscopic energy dispersive X-ray spectroscopy revealed that the AlGaN cladding layer under the AlGaN quantum well (QW) layer had a microscopic compositional modulation, which originates from the macrosteps at the AlN template surface. Moreover, Ga-rich portions in the cladding layer behaved as current micropaths, and the micropaths are connected with the carrier localization structure formed in QWs. The in-plane cathodoluminescence (CL) spectroscopy showed a significant inhomogeneity of the CL characteristics. The gentle slopes at the sample surface showed brighter emissions with a lower peak photon energy, confirming the carrier localization. This carrier localization structure in the QWs combined with the current micropaths in the cladding layer can increase the IQE as well as external quantum efficiency of the AlGaN LEDs.</p>

DOI： 10.19009/jjacg.47-3-04

CiNii Research

Language：English   Publishing type：Research paper (international conference proceedings)  

In this work, gallium nitride (GaN) nanowire (NW) Schottky barrier diodes was fabricated using well-optimized top-down approach. As-fabricated $100 \times 800$-nm-diameter NWs SBD with high current density over 1kA/cm2 at a forward bias of 2.2V, a low differential specific ON-resistance of $0.15\mathrm{m} \Omega \cdot cm^{2}$ are demonstrated. By the virtue of dielectric Reduced Surface Field (RESURF) effect, the device also delivers a breakdown voltage of 515V, leading to a competitive Baliga's Figure of merit of 1.76 GW/cm2.

DOI： 10.1109/ispsd46842.2020.9170101

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Scopus

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

DOI： 10.1088/1361-6641/ab4d2c

Web of Science

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

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

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

DOI： 10.3390/s19235107

PubMed

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

DOI： 10.1038/s41598-019-52067-y

Web of Science

PubMed

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

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.

DOI： 10.1063/1.5114866

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Cambridge University Press (CUP)  

DOI： 10.1017/s1431927619000989

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

DOI： 10.7567/1347-4065/ab2657

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The decomposition of ammonia (NH3) in nitrogen (N-2) ambient was studied under non-equilibrium conditions similar to those in a metal organic vapor phase epitaxy (MOVPE) reactor during the epitaxial growth of group-III nitrides. The gas phase was sampled at different positions and analyzed using a time-of-flight mass spectrometry system with a high resolution (better than 0.002 u). Our results expand earlier findings. Even at the high temperature of 1200 degrees C, only 26% of NH3 decomposed in a clean metal-free reactor, whereas a higher ratio of NH3 decomposition was realized in the presence of stainless steel. The activation energy in the clean reactor was calculated to be 0.965 +/- 0.004 eV. These results demonstrate the capability of our setup and shed new light on the elucidation of the vapor phase growth mechanism of group III-nitrides by MOVPE.

DOI： 10.1016/j.jcrysgro.2019.03.025

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Mg diffusion is a common problem in GaN devices with p-n junctions. Although this impurity diffusion is reported to occur through threading dislocations (TDs), no direct evidence has yet been obtained. Therefore, we tried the direct observation of Mg diffusion by atom probe tomography (APT) analysis. The n-type drift layer of the fabricated p-n diode was exposed, and etch pits were formed on the drift layer to identify the TD position. The APT analysis around TDs was carried out by lifting out the drift layer around specific etch pits using a focused ion beam to include TDs. The relationship between the etch pit shape and the TD type was confirmed by cross-sectional scanning transmission electron microscopy observation. The APT analysis of two types of etch pits formed on the mixed dislocations was performed, and Mg diffusion was clearly observed through the mixed dislocations. In this work, we show direct evidence of Mg diffusion via mixed dislocations in GaN p-n diodes and its effect on reverse leakage current.

DOI： 10.1063/1.5097767

Web of Science

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

DOI： 10.7567/1347-4065/ab124e

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

DOI： 10.7567/1347-4065/ab1411

Web of Science

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

DOI： 10.7567/1347-4065/ab06b9

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A vertical p-n diode with a simple edge termination structure on a GaN free-standing substrate is demonstrated. The edge of this device is terminated simply by etching a drift layer deeply and vertically. A device simulation revealed that the electric field at the device edge was more relaxed and uniformly applied by etching the mesa deeper than the depletion region. The fabricated device showed low leakage current and avalanche capability, and its breakdown characteristics could be reproduced many times. By emission microscopy observation, we found that there was no leakage current at the side wall of the device and that avalanche breakdown occurred throughout the inside of the device. This indicates that the electric field crowding at the side wall of the device was completely suppressed and a uniform electric field distribution was obtained by this structure. (C) 2019 The Japan Society of Applied Physics

DOI： 10.7567/1347-4065/ab106c

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We fabricated p-n diodes under different growth pressures on free-standing GaN substrates of the same quality and observed a noteworthy difference in the reverse leakage current. A large reverse leakage current was generated by nanopipes, which were formed from screw dislocations in the homoepitaxial layer. There were two types of screw dislocation observed in this study. The first type already existed in the substrate and the other was newly generated in the epilayer by the coalescence of edge and mixed dislocations. An increase in the growth pressure suppressed the transformation of screw dislocations into nanopipes, which led to a reduction in the reverse leakage current. To reduce the leakage current further, it is necessary to apply growth conditions that do not transform screw dislocation into nanopipes and to use a free-standing substrate without threading dislocations, that become nanopipes. (C) 2019 The Japan Society of Applied Physics

DOI： 10.7567/1347-4065/ab1250

Web of Science

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

DOI： 10.7567/1347-4065/ab1252

Web of Science

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

DOI： 10.7567/1882-0786/ab21a9

Web of Science

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

DOI： 10.1016/j.jcrysgro.2019.02.058

Web of Science

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

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

Web of Science

Scopus

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

DOI： 10.7567/1347-4065/aafe70

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Wafer trays with different gaps (the distance between the top of the pocket and the bottom of the wafer) were used to grow InGaN/GaN multiple quantum wells (MQWs) in a horizontal metalorganic vapor phase epitaxy (MOVPE) reactor. The numerical reactor simulation revealed that at the similar surface temperature the gas phase temperature around the wafer increases due to an increased wafer tray temperature. This increased gas phase temperature is expected to increase the effective V/III ratio by enhanced NH3 decomposition. Photoluminescence (PL) intensities of long-wavelength MQWs increased at the same indium content by the enhanced gas phase temperature, while the emission became narrow. This is related to a smoother topography at higher gas phase temperatures.

DOI： 10.1016/j.jcrysgro.2018.12.007

Web of Science

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

DOI： 10.3390/ma12050689

Web of Science

PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2019.1.0_1474

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

DOI： 10.1016/j.jcrysgro.2018.12.028

Web of Science

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

DOI： 10.1016/j.jcrysgro.2018.11.013

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A simple structure with high breakdown voltage and a low leakage current of a vertical GaN p-n diode on a GaN free-standing substrate is demonstrated. We describe a vertical p-n diode with a simple edge termination that has a drift layer etched deeply and vertically. A device simulation revealed that the electric field was more relaxed at the device edge and applied uniformly in the entire device with increasing etching depth. We fabricated the simulated structure and succeeded in reducing the leakage current and improving the breakdown voltage. With this structure, a stable avalanche breakdown can be observed. (C) 2019 The Japan Society of Applied Physics

DOI： 10.7567/1882-0786/aafdb9

Web of Science

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

DOI： 10.1063/1.5063735

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Institute of Metals and Materials  

DOI： 10.2320/materia.58.103

CiNii Research

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2019 IEEE. It is known that threading dislocations degrade the performance of GaN-based electronic devices. Electronic structure of threading dislocations in GaN is not fully understood. Accordingly, we examine the electronic structures of threading dislocations in GaN using first principles calculations based on density functional theory (DFT) and to clarify the origin of the leakage current. We have comprehensively studied the relation between threading core structures and electronic property in GaN thin films. Our calculation models of threading dislocations are the edge dislocations with Burgers vectors of 1/3 [11-20] and the screw dislocations with Burgers vectors of [0001]. We examined various core types of the threading dislocations. We found that both edge dislocations and screw dislocations do not cause the leakage currents in n-type GaN based devices because no defect level appears near the conduction band bottom.

DOI： 10.1109/iciprm.2019.8819270

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：A V S AMER INST PHYSICS  

A III-V semiconductor with a few monolayers of alkali metals (e.g., Cs) forms a negative electron affinity (NEA) surface, for which the vacuum level lies below the conduction band minimum of the base semiconductor. The photocathodes that form an NEA surface (NEA photocathodes) have various advantages, such as low emittance, a large current, high spin polarization, and ultrashort pulsed operation. The NEA-InGaN photocathode, which is sensitive to blue light, has been studied as a material for the next-generation robust photocathode. However, the proper conditions for forming NEA surfaces remain unknown. The authors consider whether the suitable process for NEA surfaces can be understood by investigating the relationship between the electron emission and the adsorption state of alkali metals. In this study, the relationship between the electron emission and the adsorption state of Cs on the p-type InGaN (0001) was analyzed by the temperature programed desorption (TPD) method using a quadrupole mass spectrometer. From the results of the TPD measurements, it was shown that there were several adsorption states of Cs on InGaN. The quantum efficiency (QE), which indicates the ratio of emitted electrons to incident photons, increased while Cs desorption occurred. The authors divided the formation process of an NEA surface into several sections to investigate the adsorption states of Cs related to the electron emission and to discuss the reasons why the QE increased despite the desorbed Cs. From the results of the NEA activation in each section, it was shown that there were sections where the QE increased by reacting with O-2 after Cs supply stopped. There is a possibility that several layers reacting with O-2 and those not reacting with O-2 are formed by performing NEA activation until the QE saturates. From the results of the TPD measurements in each section, it was suggested that there was a Cs peak at above 700 degrees C when the TPD method was carried out immediately after confirming the electron emission. Therefore, the adsorption state of Cs that formed a peak at above 700 degrees C had a close relation to the electron emission. It is considered that the increase of the QE in the TPD was affected by adsorbed Cs compounds that reacted with O-2. Although the mechanism is not understood, it is known that the QE was increased by the reaction of Cs adsorbed compounds and O-2 in previous studies. It was suspected that layers that reacted with O-2 appeared from TPD and then the QE increased by reacting with O-2. Published by the AVS.

DOI： 10.1116/1.5048061

Web of Science

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

DOI： 10.7567/JJAP.57.105501

Web of Science

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

DOI： 10.1016/j.jcrysgro.2018.07.015

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2018.2.0_1571

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

DOI： 10.7567/JJAP.57.091001

Web of Science

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

DOI： 10.1002/pssr.201800124

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)  

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

DOI： 10.7567/JJAP.57.070302

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley-VCH Verlag  

The main chain structure of optically isotropic amorphous fluorine resin for encapsulating AlGaN-based deep-ultraviolet LEDs (λ &lt
300 nm) is studied. One type of amorphous fluorine resin has a five-membered ring with a single oxygen atom and the other has that one with two oxygen atoms. Reliability testing is performed using AlGaN-based LEDs of 262 and 289 nm. For the resin with two oxygen atoms in the ring, visible damage to the electrode and significantly increased leakage current are considered to been caused by the photolysis of the ring induced by irradiation with deep-ultraviolet light. In contrast, electrode damage and increased leakage are not observed in the case of the resin with the single-oxygen ring.

DOI： 10.1002/pssa.201700525

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley-VCH Verlag  

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)  

DOI： 10.7567/APEX.11.051002

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

A gallium nitride (GaN) epitaxial layer with a low density of threading dislocations is successfully grown on a silicon substrate by using in situ gas etching. Silicon nitride (SiNx) film is used as a mask, and ammonia is intermittently supplied in hydrogen ambient during the etching. After etching, high-density deep pits appeared on the surface of a GaN template layer and corresponded to the threading dislocations in the layer. In this novel method, before growing an additional GaN layer on the template GaN layer, a second SiNx layer is deposited after the etching process, and this layer prevents GaN nuclei from growing on the upper side-walls of the pits. By using this method, the density of threading dislocations of the GaN surface is reduced to 6.7x10(7)cm(-2). This method is cost effective, completing all the necessary processes in one growth run without taking samples out from a metalorganic chemical vapor deposition (MOCVD) reactor.

DOI： 10.1002/pssb.201700387

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Dislocations that cause a reverse leakage current in vertical p-n diodes on a GaN free-standing substrate were investigated. Under a high reverse bias, dot-like leakage spots were observed using an emission microscope. Subsequent cathodoluminescence (CL) observations revealed that the leakage spots coincided with part of the CL dark spots, indicating that some types of dislocation cause reverse leakage. When etch pits were formed on the dislocations by KOH etching, three sizes of etch pits were obtained (large, medium, and small). Among these etch pits, only the medium pits coincided with leakage spots. Additionally, transmission electron microscopy observations revealed that pure screw dislocations are present under the leakage spots. The results revealed that lc pure screw dislocations are related to the reverse leakage in vertical p-n diodes. Published by AIP Publishing.

DOI： 10.1063/1.5024704

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Hexagonal boron nitride (h-BN) was directly grown on sapphire substrate using alternating ammonia (NH3) and triethylboron (TEB) supply (pulsed mode) in metalorganic vapor phase epitaxy. The best condition is when just enough NH3 is supplied to fully convert the TEB within one cycle. Excess NH3 caused islands on h-BN film surface while a lack of NH3 does not form h-BN at all. The epitaxial relationship between grown h-BN layer and c-plane sapphire was confirmed to be [0001](h-BN)||[0001](sapphire) and [10-10](h-BN)||[11-20](sapphire). It is known that, compared to AlN, BN requires higher V/III ratios for good crystallinity, which due to severe gas-phase reactions is difficult to achieve using continuous supply. Thus using pulsed mode the FWHM of the symmetric (0002) diffraction was almost halved and the growth rate was several times faster. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2017.10.036

Web of Science

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

DOI： 10.1149/08612.0041ecst

Web of Science

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

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have developed a charge-to-time converting (QTC) leading-edge discriminator for plastic-scintillator signals, which is packaged in a NIM-standard double-width module. The pulse width of the discriminator output is approximately proportional to the charge integration of input signals up to 40-50 pC. The non-linearity is 20%, 10%, 4%, and 2% at maximum for the input charges below 7 pC, for 7-10 pC, for 10-20 pC, and above 20 pC, respectively, for the minimum input charges larger than 2 pC. The discriminator could be suitable in the measurement of the timing and charge of plastic-scintillator signals together with a multi-hit time-to-digital converter (TDC) module. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nima.2017.09.040

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Electrical characteristics of leakage current paths in vertical-type n-GaN Schottky barrier diodes (SBDs) on free-standing GaN substrates are investigated by using photon emission microscopy (PEM). The PEM mapping shows that the initial failure of the SBD devices at low voltages is due to the leakage current paths from polygonal pits in the GaN epilayers. It is observed that these polygonal pits originate from carbon impurity accumulation to the dislocations with a screw-type component by microstructure analysis. For the SBD without polygonal pits, no initial failure is observed and the first leakage appeals at the edge of electrodes as a result of electric field concentration. The mechanism of leakage at pits is explained in terms of trap assisted tunneling through fitting current-voltage characteristics.

DOI： 10.1063/1.4994627

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

In this study, GaN m-plane Schottky barrier diodes fabricated with a metalorganic vapor-phase epitaxy on a GaN substrate were investigated using emission microscope, photoluminescence, and cathodoluminescence. In addition, facet dependence of leakage current under reverse-biased condition was observed. We showed that the leakage-current distribution was caused by the facet dependence of the carrier concentration and oxygen concentration. These results can provide important suggestions for the fabrication of m-plane devices. (a) four-faceted hillocks on m-plane GaN MOVPE sample, facet dependence of (b) leakage current and (c) PL peak intensity of the m-plane GaN Schottky barrier diode.

DOI： 10.1002/pssa.201600829

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

We investigate the influence of crystal defects on p-type GaN grown by metalorganic vapor phase epitaxy. Sets of p-type GaN films were grown on sapphire substrates and on free-standing GaN (F-GaN) substrates simultaneously using various Et-Cp2Mg flow rates. Although there is a difference of two orders of magnitude between the threading dislocation densities of p-type GaN grown on sapphire and F-GaN substrates, there is no significant difference in hole concentration. However, there are problems with the surface morphology of p-type GaN grown on sapphire. The deterioration of the surface was caused by the difference in nanopipe density. The electrical properties of a p-n junction diode formed on sapphire with a high density of nanopipes were observed using emission microscopy under both forward- and reverse-bias conditions. Our results demonstrate that the nanopipes are electrically inactive, and that other types of threading dislocation have more influence on the current-voltage characteristics.

DOI： 10.1002/pssa.201600837

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

In this study, by using time-of-flight (TOF) high-resolution and high-sensitivity mass spectrometry measurements, the decomposition of trimethylgallium into dimethylgallium and monomethylgallium along with its adduct formation with NHx in a commercially available horizontal metalorganic vapor-phase epitaxy (MOVPE) reactor with a resistive heater was investigated and observed in more detail than previously reported. The results confirmed the use of the TOF monitoring system in analyzing the elementary reaction process in actual MOVPE systems. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssb.201600737

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The internal electric fields in III-polar (0001), N-polar (0001), and semipolar (1011) InGaN/GaN light-emitting diodes were investigated by electroreflectance (ER) spectroscopy. The ER spectra reflected the difference in the direction and strength of internal electric fields. Phase analyses of the ER signal revealed that only III-polar InGaN wells have the opposite direction of the internal electric field at zero bias voltage; this finding is in good agreement with the results of numerical analyses. Quantitative analyses of internal electric fields were conducted by the linewidth analyses of ER spectra. Our experimental results indicate that the absolute value of internal electric fields can be measured from ER spectra. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/APEX.10.082101

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Injecting current with a uniform carrier concentration is important for applications with three-dimensional architectures such as vertical power devices or displays. In III-nitride nanostructures, dopants not only incorporate differently depending on the surface orientation but can also seriously affect the kinetic equilibrium shapes of the nanorods. Herein, we report selective-area growth of doped GaN nanorods grown by pulsed-mode metalorganic chemical vapor deposition. Two dopants, Si and Mg, were employed as donor and acceptor atoms, respectively, for a mono-doping approach. Furthermore, a mixed flow of Si and Mg was supplied for a co-doping approach. We compared the morphological effects and growth rates of each doped GaN nanorod array. Then, we proposed appropriate growth mechanisms for the doped GaN nanorods on the basis of our structural characterizations. These results might extend the morphological functionality of GaN nanorods by including doping and may also provide an appropriate foundation for the design of nanostructure-based electronic or photonic devices. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssb.201600722

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

A-plane GaN was attempted to be grown on (11-20) 4H-SiC bulk substrate without using a traditional thick buffer layer. By inducing TMAl treatment before the GaN growth step and using both a low pressure and V/III ratio, the interlayer thickness of the a-plane GaN/SiC was reduced to 1.7 +/- 0.5 nm. The ultrathin interlayer was observed to be either AlN or AlGaN with a low Ga composition. This study is aimed to contribute to the understanding of GaN growth on the sidewalls of c-plane trench structure SiC. The ultrathin growth technique is also hoped to be applied to the fabrication of GaN-based vertical structure nonpolar optical or electrical devices.

DOI： 10.1016/j.jcrysgro.2017.01.031

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have investigated the effect of V/III ratio on the surface morphology, impurity concentration and electrical properties of m-plane (10 (1) over bar0) Gallium Nitride (GaN) homoepitaxial layers. Four-sided pyramidal hillocks are observed on the nominally on-axis m-plane GaN films. Hillocks sizes relatively increase by increasing the V/III ratio. All facets of pyramidal hillocks exhibit well-defined step-terrace features. Secondary ion mass spectrometry depth profiles reveal that carbon impurities decrease by increasing the V/III ratio while the lowest oxygen content is found at an optimized V/III ratio of 900. Vertical Schottky barrier diodes fabricated on the m-GaN samples were characterized. Low leakage current densities of the order of 10(-10) A/cm(2) at -5 V are obtained at the optimum V/III ratio. Oxygen impurities and screw-component dislocations around hillocks are found to have more detrimental impact on the leakage current mechanism.

DOI： 10.1016/j.jcrysgro.2016.12.012

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We successfully grew semipolar (10 (1) over bar3) and (10 (1) over bar5) GaN films on Si(001) substrates employing metal-organic chemical vapor deposition (MOCVD) by inserting a directionally sputtered AlN (DS-AlN) buffer layer. To improve the crystal quality of the orientation-controlled semipolar (10 (1) over bar3) and (10 (1) over bar5) GaN films, a two-step epitaxial lateral overgrowth (ELO) process was performed with a striped mask. According to low-temperature cathodoluminescence (LT-CL) characterization, the ELO results in a coalesced morphology and a low defect density of &lt; 2.72x10(8) cm(-2) for both semipolar (10&lt;(1)over bar&gt;3) and (10 (1) over bar5) GaN films. For comparing the properties of planar and ELO semipolar GaN, a rocking curve of x-ray diffraction (XRD) and low-temperature photoluminescence (LT-PL) spectra was measured. The crystal orientation of semipolar GaN films was confirmed using electron backscatter diffraction (EBSD).

DOI： 10.1016/j.jcrysgro.2016.11.116

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Effect of rapid thermal annealing (RTA) on crystal defects in a GaN layer grown on a (111) Si substrate was investigated by photoluminescence (PL) and transmission electron microscopy (TEM) analyses. The PL spectra suggested that the density of gallium vacancy is not changed by the heat treatment up to 700 degrees C. In the TEM specimen, we had dislocation half loops generated by off-axis propagation of the threading dislocation. We found that the half-loop of c-type dislocation shrinks/moves by a repetitive RTA at 600-700 degrees C. In contrast, we could find no remarkable changes in the a-type or a+c-type dislocations.

DOI： 10.1016/j.jcrysgro.2017.01.001

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Severe melt-back etching has forced the epitaxy of GaN on Si to use an AlN buffer layer for growing high-quality two-dimensional layers, despite its high resistivity. Herein, we report a metal-based pre-orienting layer (POL) for growing GaN nanostructures (NSs) to replace the traditional AlN buffer layer. Two metals, titanium (Ti) and hafnium (Hf), were evaluated as POLs. We succeeded in fabricating arrays of GaN NSs with highly preferred orientation using selective-area growth. The crystallographic phase of the POLs critically affected the evolved orientation of the crystals. Photoluminescence measurements revealed that GaN NSs with Hf-based POLs were of reasonably high quality. We believe that this result will facilitate broader III-V semiconductor applications using alternative substrates moving beyond conventional Si-based optoelectronics.

DOI： 10.1016/j.jcrysgro.2016.10.032

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

DOI： 10.7567/JJAP.56.061002

Web of Science

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

DOI： 10.1038/srep45345

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

High Al-composition AlGaN and AlN epilayers were grown directly on Si(111) substrate by a hydride vapor phase epitaxy (HVPE) method with a melted mixed source in a graphite boat set in a source zone with high temperatures of T = 700 and 800 degrees C, respectively. The presence of the Ga material in the mixed source of Ga and Al promoted the growth of AlN and AlGaN epilayers in the growth zone. When the temperature in the source zone was 800 degrees C, the crystalline quality of the AlN and AlGaN epilayers increased as the ratio of Ga to Al increased, and the optimum mix ratio of Ga to Al for the growth of AlN epilayers was approximately 0.35-0.42, obtained from a numerical fitting analysis of the X-ray diffraction (XRD) data for these epilayers. It appears that they can be grown directly by our melted-mixed-source HVPE method in a high-temperature source zone. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.56.01AD07

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We developed a vertical-type light-emitting diode (LED) in which the substrate is removed using a hydride vapor phase epitaxy (HVPE) apparatus consisting of a multi-graphite boat filled with a mixed source and a high-temperature (T approximate to 900 degrees C) RF heating coil outside the source zone. The new chip-growth process with a significant reduction in the number of production steps is completed in only four steps, namely, photolithography, epitaxial layer growth, sorting, and metallization. We analyze the emission mechanism of these lights from measurement results to validate the characteristics of the light emitted from these vertical-type blue LEDs and white LEDs (WLEDs) without substrates, and propose that this mixed-source HVPE method may be a promising production technique for LEDs. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.56.01AD03

Web of Science

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

Web of Science

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

DOI： 10.1109/iedm.2017.8268477

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The planar epitaxial growth of semipolar (10 (1) over bar3) GaN on a Si(001) substrate was performed on a directionally sputtered AlN buffer layer. Three types of interlayers, i.e., single AlN, double AlN, and a stack of AlN/GaN layers were grown by metalorganic chemical vapor deposition (MOCVD) to achieve high quality GaN films. The results for the stack of AlN/GaN layers provide highest crystal quality and optical properties for GaN. Comparing the top (Ga face) and bottom (N face) surfaces of grown semipolar (10 (1) over bar3) GaN confirms the defect density reduction that is due to the application of interlayers. Moreover, reduced inversion domain density on the bottom surface is attributed with the insertion of interlayers. Improving the quality of semipolar GaN on Si(001) substrates is expected to be useful for GaN/Si(001) integrated optoelectronics. (C) 2016 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2016.09.004

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The selective area growth (SAG) of GaN was implemented on patterned GaN/sapphire templates by hydride vapor phase epitaxy (HVPE) to fabricate regular arrays of Ga-polar GaN microrods. The control of growth parameters such as H-2/N-2 carrier gas ratio, growth temperature, and absolute NH3/HCl gas flow resulted in changes in the growth morphology. In particular, for an optimized mixed-carrier gas ratio of H-2 to N-2, we achieved vertically well-aligned microrods. The topmost regions of the GaN microrods were terminated with pyramidal facets, indicating typical Ga polarity. The optical properties of the grown microrods were characterized by cathodoluminescence (CL) at a low temperature. This revealed that the GaN microrods had high crystal quality since they exhibited suppressed yellow luminescence as well as strong band edge emission. (C) 2016 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2016.05.008

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

To replace mercury lamps with AlGaN-based deep-ultraviolet (DUV) LEDs, a simple and low-cost package with increased light extraction efficiency (LEE) is indispensable. Therefore, resin encapsulation is considered to be a key technology. However, the photochemical reactions induced by DUV light cause serious problems, and conventional resins cannot be used. In the former part of this study, a comparison of a silicone resin and fluorine polymers was carried out in terms of their suitability for encapsulation, and we concluded that only one of the fluorine polymers can be used for encapsulation. In the latter part, the endurance of encapsulation using the selected fluorine polymer was investigated, and we confirmed that the selected fluorine polymer can guarantee a lifetime of over 6,000 h at a wavelength of 265nm. Furthermore, a 3 x 4 array module of encapsulated dies on a simple AlN submount was fabricated, demonstrating the possibility of W/cm(2)-class lighting. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.082101

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Using an in situ laser absorption and scattering method, the surface roughness and incorporation of In in InGaN layers grown by metal organic vapor-phase epitaxy (MOVPE) were monitored. We observed that the laser light with energy higher than the GaN bandgap was fully absorbed in a GaN layer with a smooth film surface. On the other hand, we observed that the scattering laser light from the surface when the roughness of the InGaN surface increased owing to the formation of In droplets. Laser light with energy lower than the GaN bandgap was weakly absorbed by the GaN layer and was scattered at the back surface of the wafer. Furthermore, laser light intensity decreased during InGaN growth because of In incorporation. The threshold of trimethyl-In (TMIn) for the formation of In droplets as a function of growth temperature was determined using our in situ system. Moreover, we observed that the In droplets were removed by thermal or H-2 treatment. The results indicate that multiwavelength laser absorption and scattering enable the optimization of the growth conditions for In-rich InGaN. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.05FD03

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Previously, we reported a growth method for GaN on SiC by metalorganic vapor phase epitaxy. By using a preflow trimethylaluminum treatment, the poor wetting problem of gallium on the SiC surface was alleviated, resulting in a 1.2-mu m-thick crack-free GaN grown on an on-axis 6H-SiC(0001) substrate via an ultrathin AlGaN interlayer. In this study, the impact of the preflow trimethylaluminum treatment time is investigated to understand why a crack-free epilayer was realized. To demonstrate the electrical performance of devices formed by our technique, GaN/SiC vertical Schottky barrier diodes were fabricated and compared with GaN/AlN/SiC and GaN/GaN vertical Schottky barrier diodes. Compared with diodes including a high-resistance AlN interlayer, the series resistance of GaN/SiC Schottky barrier diodes incorporating the ultrathin interlayer with 5 s of TMAl treatment showed a marked reduction from 4.0 x 10(7) to 2.0 x 10(-1) Omega.cm(2). The ultrathin interlayer growth technique is expected to be applied in future GaN/SiC hybrid high-power and high-frequency devices. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.05FB06

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A thin p-type InGaN with a negative electron affinity (NEA) surface was used to measure the relaxation time of a surface charge limit (SCL) by irradiating rectangular laser beam pulses at changing time interval. The p-type InGaN film was grown by metal organic vapor phase epitaxy and the NEA activation was performed after the sample was heat cleaned. 13 nC per pulse with 10ms width was obtained from the InGaN photocathode. The current decreased exponentially from the beginning of the pulse. The initial current value after the laser irradiation decreased with the time interval. As a result, the SCL relaxation time was estimated through the InGaN photocathode measurements at 100 ms. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.05FH05

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Low-toxity high-In-content InGaN is an attractive option for short-distance communications through plastic optical fibers because its performance is only slightly affected by temperature. However, its fabrication on the c-plane is impaired by In droplets and V pits, which form at low-growth temperature. On the other hand, unlike the c-plane, (1 (1) over bar 01) InGaN relaxes with tilting. Therefore, in this study, we first grew a high-In-content InGaN single layer, and then we fabricated an InGaN tilting layer between (1 (1) over bar 01) InGaN-based multiple quantum wells (MQWs) and GaN stripes/(001) Si. The emission wavelength increased with the InGaN tilting layer's growth time because the strain was relaxed by misfit dislocations at the heterointerface. This layer also extended the emission peak of InGaN/GaN MQWs and increased the photoluminescence intensity with respect to that of a single-layered InGaN. Therefore, the InGaN tilting layer is effective for growing high-In-content (1 (1) over bar 01) InGaN MQWs. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.05FA10

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Even though a patterned sapphire substrate (PSS) has been used for the growth of a high-quality epilayer because of its many advantages, it has not been successfully used to grow an AlN epilayer for ultraviolet (UV) light-emitting diodes (LEDs) on a PSS up to now. We report the growth of a high-quality AlN epilayer on a PSS, as a substrate for the manufacture of UV LEDs, by hydride vapor phase epitaxy (HVPE). The X-ray diffraction (XRD) peaks for the AlN epilayer grown on the PSS indicate that crystalline AlN with a wurtzite structure was grown successfully on the PSS. Furthermore, HVPE combining both in situ HVPE technology and liquid-phase epitaxy (LPE) using a mixed source is proposed as a novel method for the growth of a flat AlN epilayer on a PSS. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.05FC02

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

In this paper, we discuss the influence of parameters such as type of carrier gas and NH3/HCl flow ratio on the growth of vertical GaN microstructures by selective-area growth (SAG) hydride vapor phase epitaxy (HVPE). On various strain-induced templates such as GaN/sapphire, GaN/Si, and AlN/Si, regular arrays of Ga-polar GaN microrods were properly achieved by adjusting the growth parameters. The photoluminescence and micro-Raman measurements reveal not only the crystal quality of the GaN microrods but also strain distribution. These results will give insight into the control of the morphology of GaN microrods in terms of the strain induced from templates in SAG-HVPE. The precisely controlled arrays of GaN microrods can be used for next-generation light-emitting diodes (LEDs) by realizing InGaN/GaN multi-quantum wells (MQWs) with a radial structure. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.05FF03

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We investigated the surface reconstruction of InN(0001) during raised-pressure metalorganic vapor-phase epitaxy using an ab initio-based approach. We observed that the reconstructed structure changes from In-rich surfaces such as In bilayer and monolayer surfaces to an ideal surface with increasing growth temperature. In addition, we investigated the effects of surface reconstruction on the growth process using a newly improved thermodynamic analysis method. Although no barrier is present in the growth reaction when the In-rich surfaces appear, the results suggest that the surface phase acts as a barrier in the growth reaction when the ideal surface appears. Furthermore, we discuss the growth conditions that enable high-temperature growth with a smooth reaction path. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.05FM01

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Recently, GaN, which has remarkable properties as a material for optical devices and high-power electron devices, has also attracted attention as a material for radiation detectors. We previously suggested the use of BGaN as a neutron detector material. However, the radiation detection characteristics of GaN itself are not yet adequately understood. For realizing a BGaN neutron detector, the understanding of the radiation detection characteristics of GaN, which is a base material of the neutron detector, is important. In this study, we evaluated the radiation detection characteristics of GaN. We performed I-V and energy spectrum measurements under alpha ray, gamma ray, and thermal neutron irradiations to characterize the radiation detection characteristics of a GaN diode. The obtained results indicate that GaN is an effective material for our proposed new BGaN-based neutron detector. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.05FJ02

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We report on the material and optical properties of core-shell InGaN layers grown on GaN nanorod arrays. The core-shell InGaN layers were well grown on polarization-reduced surfaces such as semipolar pyramids and nonpolar sidewalls. In addition, to compensate the biaxial strain between GaN and InGaN layers, we grew interlayers underneath a thick InGaN layer. Here, the interlayers were composed of multiple superlattice structures. We could observe that the indium composition of core-shell InGaN structures increased with the number of interlayers. This indicates that the absorption energy band of InGaN alloys can be better matched to the spectral irradiance of the solar spectrum in nature. We also implemented a simulation of Ga-polar and nonpolar InGaN-based solar cells based on the indium composition obtained from the experiments. The result showed that nonpolar InGaN solar cells had a much higher efficiency than Ga-polar InGaN solar cells with the same thickness of the absorption layer. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.05FG03

Web of Science

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

We report the selective-area growth of a gallium nitride (GaN)-nanorod-based InGaN/GaN multiple-quantum-well (MQW) core-shell structure embedded in a three-dimensional (3D) light-emitting diode (LED) grown by metalorganic chemical vapor deposition (MOCVD) and its optical analysis. High-resolution transmission electron microscopy (HR-TEM) observation revealed the high quality of the GaN nanorods and the position dependence of the structural properties of the InGaN/GaN MQWs on multiple facets. The excitation and temperature dependences of photoluminescence (PL) revealed the m-plane emission behaviors of the InGaN/GaN core-shell nanorods. The electroluminescence (EL) of the InGaN/GaN core-shell-nanorod-embedded 3D LED changed color from green to blue with increasing injection current. This phenomenon was mainly due to the energy gradient and deep localization of the indium in the selectively grown InGaN/GaN core-shell MQWs on the 3D architecture.

DOI： 10.1186/s11671-016-1441-6

Web of Science

PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Using a SiNx insertion layer to reduce dislocations, enhanced photovoltaic properties could be obtained in p-i-n InGaN/GaN heterojunction solar cell. To investigate the influence of the dislocations on the photovoltaic behaviors, a sample grown without SiNx insertion layer was identically prepared for comparison. From optical properties measurements, the reduction in the number of non-radiative centers and a stronger In localization effect was shown in the sample with SiNx insertion layer. However, the quantum confined stark effect was almost negligible in both the samples. Electrical properties measurement showed reduced saturation current and increased shunt resistance in the sample with SiNx insertion layer due to the reduced dislocation density. By comparing these results and using a numerical model, the influence of the dislocation density on the different photovoltaic properties such as open-circuit voltage and fill factor has been confirmed. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.030306

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

To understand the effect of piezoelectric fields on carrier dynamics, we numerically investigated a simple p-GaN/i-InxGa1-xN/n-GaN solar cell structure. A reliable simulation model was obtained by comparing the experimental and simulated results in advance. The same p-i-n InGaN structures were re-simulated with and without the piezoelectric field effect, as spontaneous polarization remained unchanged. The sample with the piezoelectric field effect showed higher short current density (J(sc)), a staircase-like feature in its I-V curve, and higher open circuit voltage (V-oc) with a lower fill factor (F.F.) and reduced conversion efficiency (C.E.) than the sample with no piezoelectric fields. In addition, with increasing In fraction (x), the V-oc value gradually increased while the J(sc) value significantly decreased, correspondingly leading to a reduction in C.E. and F.F. values of the structure with the piezoelectric field effect. To solve the current loss problem, we applied various piezoelectric field elimination techniques to the simulated structures.

DOI： 10.1088/0022-3727/49/2/025103

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Previously, we reported a growth method by metalorganic vapor phase epitaxy using a single two- dimensional growth step, resulting in 1.2-mu m crack-free GaN directly grown on 6H-SiC substrate. The introduction of Al-treatment prior to the standard GaN growth step resulted in improved surface wetting of gallium on the SiC substrate. Transmission electron microscope and energy dispersive spectrometer analysis of the epitaxial interface to the SiC determined that an ultra-thin AlGaN interlayer had formed measuring around 2-3 nm. We expect our growth technique can be applied to the fabrication of GaN/SiC high frequency and high power devices. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.010303

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

AlGaN-based LEDs are expected to be useful for sterilization, deodorization, photochemical applications such as UV curing and UV printing, medical applications such as phototherapy, and sensing. Today, it has become clear that efficient AlGaN-based LED dies are producible between 355 and 250 nm with an external quantum efficiency (EQE) of 3% on flat sapphire. These dies were realized on flat sapphire without using a special technique, i.e., reduction in threading dislocation density or light extraction enhancement techniques such as the use of a photonic crystal or a patterned sapphire substrate. Despite the limited light extraction efficiency of about 8% owing to light absorption at a thick p-GaN contact layer, high EQEs of approximately 6% has been reproducible between 300 and 280 nm without using special techniques. Moreover, an EQE of 3.9% has been shown at 271 nm, despite the smaller current injection efficiency (CIE). The high EQEs are thought to correspond to the high internal quantum efficiency (IQE), indicating a small room for improving IQE. Accordingly, resin encapsulation on a simple submount is strongly desired. Recently, we have succeeded in demonstrating fluorine resin encapsulation on a ceramic sheet (chip-on-board, COB) that is mass-producible. Furthermore, the molecular structure of a resin with a durability of more than 10,000 h is explained in this paper from the photochemical viewpoint. Thus, the key technologies of AlGaN-based DUV-LEDs having an EQE of 10% within a reasonable production cost have been established. The achieved efficiency makes AlGaN-based DUV-LEDs comparable to high-pressure mercury lamps.

DOI： 10.1117/12.2235398

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

To extend the availability of nanostructure-based optoelectronic applications, vertically elongated nanorods with precisely controlled morphology are required. For group III nitrides, pulsed-mode growth has recently been reported as an effective method for growing nanorod arrays with geometric precision. Here, we demonstrated the growth of arrays of highly elongated nanorods on Si substrates by metal-organic chemical vapor deposition using a pulsed-mode approach. Unlike the thick and high (or middle)-quality GaN templates normally used, nanorod growth was performed on an ultrathin and low-quality AlN/Si platform. Using kinetically controlled growth conditions and a patterning process, exceptionally long GaN nanorods were achieved with high geometric precision. The grown nanorods showed considerably improved optical and structural properties while remaining in uniform arrays. This approach can be used with a variety of materials to obtain nanorods with high quality, high uniformity, and high aspect ratio, and it can also serve as an effective fabrication method for InAlGaN-alloyed core/shell nanostructures for optoelectronic nano-devices with ultrahigh efficiency.

DOI： 10.1039/c5ce02056e

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

The optical properties of InGaN/GaN multiple quantum wells (MQWs) have been studied by means of photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy. The radiative and nonradiative recombination lifetimes were evaluated as a function of the excitation energy density. The radiative recombination lifetime decreased and subsequently reached a nearly constant value with increasing excitation energy density, which was attributed to screening of internal electric fields by photoexcited carriers. On the other hand, the nonradiative recombination lifetime increased and subsequently decreased with increasing excitation energy density. The initial increase in the nonradiative recombination lifetime could be attributed to saturation of nonradiative recombination centers with photoexcited carriers. The nonradiative recombination lifetime was found to decrease to a considerably weaker extent than that expected for the Auger recombination process of free carriers. This indicated that the decrease in the internal quantum efficiency (IQE) at high carrier densities could not be explained only by the Auger recombination process of free carriers. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssb.201451491

Web of Science

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

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

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We demonstrated lasing action and investigated the optical properties of (1 (1) over bar 01) multiquantum-well (MQW) stripe crystals on patterned (001) Si substrates. Longitudinal and higher order transverse modes were observed from a ridge waveguide structure. These results strongly suggest the possibility of fabricating (1 (1) over bar 01) InGaN MQW laser diodes on (001) Si. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.022702

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Highly ordered, position-controlled gallium nitride (GaN) nanowire based multiple-quantum-wells (MQWs) core-shell architecture arrays are synthesized by metalorganic chemical vapor deposition (MOCVD). We investigate the possibility of using GaN nanowire arrays as a basal template for the growth of InxGa1-xN/GaN MQWs. The MQWs on three different crystal facets (c(-), m(-), and semipolar-plane) of GaN nanowire exhibit dissimilar structural properties. The structural characteristics of InGaN/GaN core-shell arrays are inspected by cross-sectional high-resolution transmission electron microscopy (HR-TEM). We also investigate the optical properties of MQW core-shell structure nanoarrays. The luminescent characteristics of InGaN/GaN core-shell structure arrays are determined by photoluminescence (PL) and cathodoluminescence (CL) measurements. The monochromatic CL images clearly show the light emission behavior of InGaN/GaN MQW coaxial structure. Two distinguishable light emission peaks were observed in the GaN nanowire based core-shell structure. The characteristic of light emission mainly depends on the properties of MQWs, which are generated from different crystal facets of GaN. In addition, the light emission intensity shows different behaviors depending on the area of the GaN nanowire m-plane. The results of this study suggest that GaN nanowire arrays can be used as a good alternative basal template for next-generation light-emitting diodes (LEDs). (C) 2014 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.nanoen.2014.11.003

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Materials Research Society  

We developed a new GaN on SiC growth method by metalorganic vapour phase epitaxy (MOVPE) using of a single 2-dimension-growth step. Prior to epitaxy, to inhibit pre-reaction of Si-face SiC substrate with TMGa and NH&lt
inf&gt
3&lt
/inf&gt
, TMA1 was flowed without NH&lt
inf&gt
3&lt
/inf&gt
. 1.5 μm of undoped crack-firee GaN was grown on 6H-SiC (Si-face). Without buffer layer, the vertical resistance of GaN/SiC structure was found to be around 82.1Ω as determined by I-V characteristic. Further reduction in vertical resistance is expected by growth of n-GaN (1.5 μm)/SiC structure (300 μm). We also expect a SiC-based GaN heterostructure vertical FET will achieve high power and high switching speed performance.

DOI： 10.1557/opl.2015.102

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

A photocathode electron source using p-type GaN and p-type InGaN semiconductors with a negative electron affinity (NEA) surface has been studied for its ability to maintain an extended NEA state. The key technology of NEA photocathodes is the formation of electric dipoles by atoms on the surface, which makes it possible for photo excited electrons in the conduction band minimum to escape into the vacuum. This means that in order to keep the electron energy spread as small as possible, the excitation photon energy should be tuned to the band gap energy. However, the NEA surface is damaged by the adsorption of residual gas and the back-bombardment of ionized residual gas by photoelectrons. The p-type GaN and InGaN semiconductors were measured a lifetime of quantum yield of excitation energy corresponding to the band gap energy in comparison to the p-type GaAs as the conventional NEA photocathode. Lifetime of NEA-photocathodes using the GaN and InGaN were 21 times and 7.7 times longer respectively than that using the GaAs.

DOI： 10.1117/12.2076681

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

High-optical-quality GaN was grown on a (111) Si substrate by metal-organic vapor phase epitaxy using an AlInN buffer layer and an indium doped AlN nucleation layer. The photoluminescence spectra at room temperature showed a strong and narrow edge-emission peak and weak defect-related emission bands. We found four spectral peaks in the green and yellow luminescence bands at G(0): 514.5 nm (2.410 eV), G(1): 546.5nm (2.269 eV), Y-1: 553.5nm (2.240 eV), and Y-0: 584.5nm (2.121 eV), independent of the growth methods/conditions. The results suggest that the emission is associated with an intrinsic defect such as a Ga vacancy. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.11RC02

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

The photocathode using Negative Electron Affinity (NBA) surface on a semiconductor has been applied for science technologies such as an elemental particle experiment as an electron beam source. Yet, since NBA-surface is fragile, it is necessary to figure out how to make its life time longer in order to develop new technologies. Then, we realized that considering the band gap in semiconductor, the life time becomes longer as the negative affinity goes even smaller. Therefore, we produced the photocathodes using GaN and InGaN that have wider band gaps than that of GaAs. As a result, using GaN and InGaN, we gained 17 times and 7 times longer life time NEA-surface than that of GaAs, respectively. On the other hand, the result indicates that we can produce the NEA-surface with only cesium.

CiNii Books

Language：English   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人電子情報通信学会  

We realized p-GaN by high temperature Mg ion implantation with pre-sputter(PS) technique. Due to the new PS technique and high temperature implantation, ion implantation damage was greatly reduced. This soft implantation made the post anneal more effectively to recover the implantation damage and activate the implanted Mg ions. We believe this p-GaN by Mg ion implantation will contribute to GaN to enter high output power device application field.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

We realized p-GaN by high temperature Mg ion implantation with pre-sputter(PS) technique. Due to the new PS technique and high temperature implantation, ion implantation damage was greatly reduced. This soft implantation made the post anneal more effectively to recover the implantation damage and activate the implanted Mg ions. We believe this p-GaN by Mg ion implantation will contribute to GaN to enter high output power device application field.

CiNii Books

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

We realized p-GaN by high temperature Mg ion implantation with pre-sputter(PS) technique. Due to the new PS technique and high temperature implantation, ion implantation damage was greatly reduced. This soft implantation made the post anneal more effectively to recover the implantation damage and activate the implanted Mg ions. We believe this p-GaN by Mg ion implantation will contribute to GaN to enter high output power device application field.

CiNii Books

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The characteristics of nonpolar a-plane (11 (2) over bar0) GaN (a-GaN) grown using single and double nanopillar SiO2 masks were investigated. The two nanopillar SiO2 masks were directly fabricated on an r-plane sapphire substrate and a-GaN by the epitaxial lateral overgrowth (ELOG) technique. Through the use of the single and double nanopillar SiO2 masks, the crystalline quality and optical properties of a-GaN were markedly improved because of the nanoscale ELOG effect and a number of voids in the single and double nanopillar SiO2 mask areas in comparison with the planar sample. The submicron pit densities of the planar, single, and double nanopillar mask samples were similar to 2 x 10(9), similar to 7 x 10(8), and similar to 4 x 10(8) cm(-2), respectively. The internal quantum efficiency (IQE) values at room temperature of three-period InGaN/GaN multiple quantum wells (MQWs) grown using the planar, single, and double nanopillar masks were 45, 60, and 68% at a carrier concentration of 1.0 x 10(18) cm(-3), respectively. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.05FL01

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The effects of GaN quantum barriers with changing growth temperatures on the interfacial characteristics of GaN/InGaN single quantum well (SQW) grown on GaN templates by metalorganic vapour phase epitaxy were in situ investigated by X-ray crystal truncation rod (CTR) scattering and X-ray reflectivity measurements at growth temperature using a laboratory level X-ray diffractometer. Comparing the curve-fitting results of X-ray CTR scattering spectra obtained at growth temperature with that at room temperature, the InxGa1-xN with indium composition less than 0.11 was stabile of the indium distribution at the interface during the whole growth processes. By using several monolayers thickness GaN capping layer to protect the InGaN well layer within temperature-ramping process, the interfacial structure of the GaN/InGaN SQW was drastically improved on the basis of the curve-fitting results of X-ray CTR scattering spectra, and the narrow full width at half-maximum and strong luminous intensity were observed in room temperature photoluminescence spectra. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4867640

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Regularly distributed nanopyramid arrays with a highly uniform size were grown by selective-area growth. The nanopyramid size could be tuned by varying the growth time. Two emission peaks were observed from multiple quantum wells grown on the nanopyramids. The longer-wavelength peak originated from the apexes of pyramids with threefold symmetry. High-density stacking faults observed at these triangular apexes might be responsible for the higher indium incorporation. The formation of embedded voids inside the pyramids was explained by the different growth rates at the edge and center of the mask window caused by the low diffusion length. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.030306

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

A novel activation process for Mg-implanted GaN was demonstrated. As opposed to the conventional thermal annealing process, an H-2/NH3 alternating supply annealing process achieved better optical activation, stronger near-ullraviolel luminescence and weaker yellow luminescence in the phololuminescence spectroscopy. After this process, small hexagonal hillocks were observed on the surface, which indicated that crystal regrowth was induced by this process, consisting of decomposition of GaN by H-2 supplies and re-crystallization by NH3 supplies. It was revealed that the implanted Mg could easily be located at the activation site by means of crystal regrowth by this process. (C) 2013 Elsevier B.V. All rights reserved

DOI： 10.1016/j.jcrysgro.2013.07.011

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Nonpolar a-plane (11-20) GaN (a-GaN) layers with low overall defect density and high crystalline quality were grown on r-plane sapphire substrates using etched a-GaN. The a-GaN layer was etched by pulse NH3 interrupted etching. Subsequently, a 2-mu m-thick Si-doped a-GaN layer was regrown on the etched a-GaN layer. A fully coalescent n-type a-GaN layer with a low threading dislocation density (similar to 7.5 x 10(8) cm(-2)) and a low basal stacking fault density (similar to 1.8 x 10(5) cm(-1)) was obtained. Compared with a planar sample, the full width at half maximum of the (11-20) X-ray rocking curve was significantly decreased to 518 arcsec along the c-axis direction and 562 arcsec along the m-axis direction. (C) 2014 Optical Society of America

DOI： 10.1364/OE.22.003585

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PubMed

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DOI： 10.1002/pssc.201300437

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DOI： 10.1002/pssc.201300670

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

A GaN layer of 20 mu m thickness grown by the liquid-phase epitaxy on c-plane sapphire was used as a template for the growth of blue light-emitting diodes (LEDs) with emission peak wavelengths of about 450 nm. As the underlying layer of the active region, an InGaN/GaN superlattice or two pairs of 100 nm undoped GaN and 20 nm GaN: Si layers on an n-type GaN: Si layer was found to be effective for reducing the forward voltage. By optimizing the multiple-quantumwell structure, LEDs having a 2.5-nm-thick InGaN well and 5-nm-thick GaN barrier exhibited the highest internal quantum efficiency (IQE) at both low and high currents. This IQE was much higher than that of LEDs on a sapphire substrate. The IQE of LEDs using the liquid-phase GaN grown on sapphire substrate exceeded more than 75% at a forward current density of over 200 A/cm(2).

DOI： 10.1117/12.2038764

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

In this paper, we demonstrate a scalable process for the precise position-controlled selective growth of GaN nanowire arrays by metalorganic chemical vapor deposition (MOCVD) using a pulsed-mode growth technique. The location, orientation, length, and diameter of each GaN nanowire are controlled via pulsed-mode growth parameters such as growth temperature and precursor injection and interruption durations. The diameter and length of each GaN nanowire are in the ranges of more than 240 nm and 250-1250 nm, respectively, with different vertical-to-lateral aspect ratios that depend on the growth temperature. Also, it is found that a higher growth temperature helps increase the vertical growth rate and reduces the lateral growth rate of GaN nanowire arrays. Furthermore, in the case of longer TMGa injection duration, the Ga-rich region allows the higher lateral growth rate of GaN nanostructures, which leads to a transition in the morphology from nanowires to a thin film, while in the case of longer NH3 injection duration, the surface morphology changes from nanowires to pyramidal structures. In addition, the surface structure can also be controlled by varying the precursor interruption duration. Finally, we report and discuss a growth model for GaN nanowire arrays under pulsed-mode MOCVD growth.

DOI： 10.1039/c3ce42266f

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.1002/pssc.201300470

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人電子情報通信学会  

MOVPE法を用いてGaN及びAlGaNへのMgとCの同時ドーピングを行い、電気的、光学的特性を評価した。GaNへの同時ドーピングでは比較的低濃度のCドーピングにより正孔密度の増加が確認できたが、AlGaNにおいてはC_2H_2流量増加に伴い正孔密度は減少した。cathodeluminescence(CL)測定により、全ての試料で440nm付近の発光を確認した。GaNへのMgとCのδドーピングを行った試料に対して同様の測定を行った。MgとCの同時δドーピングは通常のMgドーピングよりも高い正孔密度を示した。

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人電子情報通信学会  

MOVPE法を用いてGaN及びAlGaNへのMgとCの同時ドーピングを行い、電気的、光学的特性を評価した。GaNへの同時ドーピングでは比較的低濃度のCドーピングにより正孔密度の増加が確認できたが、AlGaNにおいてはC_2H_2流量増加に伴い正孔密度は減少した。cathodeluminescence(CL)測定により、全ての試料で440nm付近の発光を確認した。GaNへのMgとCのδドーピングを行った試料に対して同様の測定を行った。MgとCの同時δドーピングは通常のMgドーピングよりも高い正孔密度を示した。

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人電子情報通信学会  

MOVPE法を用いてGaN及びAlGaNへのMgとCの同時ドーピングを行い、電気的、光学的特性を評価した。GaNへの同時ドーピングでは比較的低濃度のCドーピングにより正孔密度の増加が確認できたが、AlGaNにおいてはC_2H_2流量増加に伴い正孔密度は減少した。cathodeluminescence(CL)測定により、全ての試料で440nm付近の発光を確認した。GaNへのMgとCのδドーピングを行った試料に対して同様の測定を行った。MgとCの同時δドーピングは通常のMgドーピングよりも高い正孔密度を示した。

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人電子情報通信学会  

我々は、負の電子親和力を持つ半導体表面を用いた電子源(NEA半導体フォトカソード電子源)の高輝度、高耐久化を目指して、p-GaN半導体を用いた機能性表面の寿命に対する電子源の高輝度条件を検討した。超高真空中でp-GaN基板に対してNEA表面活性化を行った後、その量子効率の波長依存性及びその寿命の測定実験を行った。その結果から、p-GaN基板は、従来技術のp-GaAs基板に比べて20倍以上寿命が長く、より高耐久性能を持つことが分かった。また、表面機能が低下するに連れて、バンドギャップエネルギー以下の励起エネルギーでの量子効率が優先的に減少することが分かった。これらの結果より、p-GaN基板を用いたNEA半導体フォトカソード電子源は、高耐久である一方で、表面機能が劣化した状態がより電子ビーム源として単色化に適していることを明らかにした。

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人電子情報通信学会  

我々は、負の電子親和力を持つ半導体表面を用いた電子源(NEA半導体フォトカソード電子源)の高輝度、高耐久化を目指して、p-GaN半導体を用いた機能性表面の寿命に対する電子源の高輝度条件を検討した。超高真空中でp-GaN基板に対してNEA表面活性化を行った後、その量子効率の波長依存性及びその寿命の測定実験を行った。その結果から、p-GaN基板は、従来技術のp-GaAs基板に比べて20倍以上寿命が長く、より高耐久性能を持つことが分かった。また、表面機能が低下するに連れて、バンドギャップエネルギー以下の励起エネルギーでの量子効率が優先的に減少することが分かった。これらの結果より、p-GaN基板を用いたNEA半導体フォトカソード電子源は、高耐久である一方で、表面機能が劣化した状態がより電子ビーム源として単色化に適していることを明らかにした。

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人電子情報通信学会  

我々は、負の電子親和力を持つ半導体表面を用いた電子源(NEA半導体フォトカソード電子源)の高輝度、高耐久化を目指して、p-GaN半導体を用いた機能性表面の寿命に対する電子源の高輝度条件を検討した。超高真空中でp-GaN基板に対してNEA表面活性化を行った後、その量子効率の波長依存性及びその寿命の測定実験を行った。その結果から、p-GaN基板は、従来技術のp-GaAs基板に比べて20倍以上寿命が長く、より高耐久性能を持つことが分かった。また、表面機能が低下するに連れて、バンドギャップエネルギー以下の励起エネルギーでの量子効率が優先的に減少することが分かった。これらの結果より、p-GaN基板を用いたNEA半導体フォトカソード電子源は、高耐久である一方で、表面機能が劣化した状態がより電子ビーム源として単色化に適していることを明らかにした。

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

We report on the combined effects of a-plane GaN layers on a nanoscale patterned insulator on an r-plane sapphire substrate and epitaxial lateral overgrowth (ELOG) techniques. The fully coalescent a-plane GaN layer using nano- and microscale SiO<inf>2</inf>masks showed the formation of nano- and microscale voids on the masks, respectively. Atomic force microscopy (AFM) measurements revealed a surface roughness of 0.63 nm and a submicron pit density of {\sim}7.8 \times 10^{7} cm^-2. Photoluminescence (PL) intensity was enhanced by a factor of 9.0 in comparison with that of a planar sample. Omega full-width at half-maximum (FWHM) values of the (11\bar{2}0) X-ray rocking curve along the c- and m-axes were 553 and 788 arcsec, respectively. A plan-view cathodoluminescence (CL) mapping image showed high luminescence intensity on the SiO<inf>2</inf>masks.

DOI： 10.7567/JJAP.52.08JC04

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A thermal-etching process for fabricating nanoporous GaN templates in a metalorganic vapor phase epitaxy (MOVPE) reactor was proposed. Such nanoporous templates exhibited nearly complete strain relaxation and considerably increased photoluminescence (PL) intensity. The threading dislocation density (TDD) after overgrowth was reduced from 3 x 10(9) to 4 x 10(8) cm(-2) for the template with poor quality, whereas little improvement was observed for the high-quality template (TDD, 4 x 10(8) cm(-2)). Multiple quantum wells (MQWs) grown on a nanoporous template fabricated from the high-quality GaN template were used to assess the effect on LED performance. The PL intensity of the sample with a nanoporous structure showed a twofold increase in PL intensity compared with the control sample. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.52.08JB03

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DOI： 10.7567/JJAP.52.08JE10

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