Faculty Profiles - AMANO, Hiroshi

Papers - AMANO, Hiroshi

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Dynamic properties of excitons in ZnO/AlGaN/GaN hybrid nanostructures Reviewed

Mathias Forsberg, Carl Hemmingsson, Hiroshi Amano , Galia Pozina

Scientific Reports Vol. 5 page： 7889 2015.1

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

III-nitride semiconductor/organic polymer hybrid heterostructures combine advantages of epitaxially grown semiconductor quantum wells (QW) with inexpensive polymers having efficient luminescence in the visible region1. Such hybrid micro-structured light emitting diodes (LED) are promising for fabrication of low-cost and highly efficient microlight sources that can be used in full-color displays, imaging systems, miniature chemical and biological sensors. In typical polyfluorene/GaN-based LED hybrids, UV emission from a GaN heterostructure down converts to the organic polymer fluorescence in the visible region via a radiative energy transfer. Overlapping between the UV luminescence and the polyfluorene absorption is required for the operation
of these hybrids. Today, a novel class of hybrid structures is suggested, in which a non-radiative resonant energy transfer (NRET) from excitation generated in inorganic QWs to excitons in organic films can be utilized. Such LEDs might be considerably more efficient than their radiative energy transfer analogues. In addition to the necessity of a significant spectral overlap between the QW emission and the polymer absorption spectrum, these devices require that the two materials are placed in a close interaction distance of a few nm. The bottleneck is that the operation lifetime of organic/semiconductor hybrid LED structures is limited by degradation
of polyfluorenes. Using colloidal semiconductor nanocrystals (NCs) instead of polymers can significantly improve the lifetime of such devices. In addition to superior luminescence properties, relatively low cost and chemical stability, the spectral tunability can be achieved by changing the particle chemistry and size. The efficiency of non-radiative resonance energy transfer is typically determined using transient photoluminescence
(PL) measurements from the quenching of the QW exciton lifetime in the presence of acceptor material (i.e.
colloidal NCs or polyfluorene). It might be correct in assumption that NRET is the only additional recombination channel appearing in hybrids compared to the bareQWstructure. However, other factors can play also a significant role. For example, surface potential effects have to be considered when non-radiative resonant energy transfer is measured using dynamic properties of the QW excitons.
Thus, in this work we have studied and discussed the possibility of NRET in hybrid structures fabricated using ZnO NCs films coated on the top of the AlGaN/GaN QWs samples. ZnO NCs satisfies the requirement of
absorption overlapping with GaN emission (a room temperature band gap energy is 3.3 and 3.4 eV for ZnO and GaN, respectively). Dynamic properties ofQWexcitons in the hybrids and in the bareQWsamples are analyzed in dependence on the QWs cap layer thickness.

DOI： doi:10.1038/srep07889

Development of underfilling and encapsulation for deep-ultraviolet LEDs Reviewed

Kiho Yamada, Yuuta Furusawa, Shoko Nagai, Akira Hirano, Masamichi Ippommatsu, Ko Aosaki, Naoki Morishima, Hiroshi Amano and Isamu Akasaki

Applied Physics Express Vol. 8 ( 1 ) page： 012101 2015.1

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

The currently available resins for underfilling and encapsulation are limited to wavelengths longer than approximately 330 nm because almost all resins are decomposed by deep-ultraviolet light. Fluorine resins are possible materials for underfilling and encapsulation in deep-ultraviolet LEDs, and we have investigated methods of underfilling and encapsulation using such resins. Also, their durability at wavelengths of 265 and 285 nm has been studied from the photochemical viewpoint, and we propose a stable amorphous fluorine resin that is optically isotropic. Also, a trial of encapsulation using the resin has been made.

DOI： doi:10.7567/APEX.8.012101

Highly ordered catalyst-free InGaN/GaN core-shell architecture arrays with expanded active area region Reviewed

Jung, Byung Oh; Bae Si-Young; Kim, Sang Yun; Lee, Seunga; Lee, Jeong Yong; Lee, Dong-Seon; Kato, Yoshihiro; Honda, Yoshio ;Amano, Hiroshi

Nano Energy Vol. 11 page： 294-303 2015.1

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

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).

DOI： DOI: 10.1016/j.nanoen.2014.11.003

Continuous in situ X-ray reflectivity investigation on epitaxial growth of InGaN by metalorganic vapor phase epitaxy Reviewed

Guangxu Ju, Shingo Fuchi, Masao Tabuchi, Hiroshi Amano, Isamu Akasaki

Journal of Crystal Growth Vol. 407 page： 68-73 2014.12

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

Continuous in situ X-rayreflectivity(XRR)measurementswereusedtoinvestigatethegrowthprocessof
an In0.11Ga0.89N epilayeranditssinglequantumwellgrownonc-planeGaN/sapphiretemplatesusingan
in-house-designed metalorganicvaporphaseepitaxyinstalledinalaboratory-gradeX-raydiffract-
ometer.Thesurfacerougheningoftheepilayerasafunctionofgrowthtimewascalculatedfromthe
continuous in situ XRR curve.Thegrowthrate,criticalthickness hc(r) for surfaceroughening,and
roughening ratewereobtained.Theexperimentalcriticalthickness hc(r) of theIn0.11Ga0.89N epilayer
analyzed fromthecontinuous in situ XRR curvewas14.870.4 nm.Basedonthecalculatedtheoretical
critical thickness hc and theexperimental hc(r,2), Fischer's modelseemstobeappropriatefordescribing
the criticalthicknessoftheInGaN/GaN.

DOI： 10.1016/j.jcrysgro.2014.08.023

Photoemission lifetime of a negative electron affinity gallium nitride photocathode Reviewed

Tomohiro Nishitani, Masao Tabuchi, Hiroshi Amano, Takuya Maekawa, Makoto Kuwahara and Takashi Meguro

Journal of Vacuum Science and Technology Vol. B32 ( 6 ) page： 06F901 2014.11

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

A photocathode electron source using p-type GaN semiconductor 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 cesium and gallium atoms on the surface, which makes it possible for photoexcited 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 semiconductor was measured time evolution in quantum yield during NEA surface activation, and 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. In NEA surface activation process, the quantum yield of the GaN was more than 3 orders of magnitude higher than that of the GaAs by only cesium deposition. The exposure amount of cesium in the NEA surface activation of the GaAs was 1.5 times as that of the GaN, even though the quantum yield of the GaAs was the same value as the GaN. Lifetime of NEA-photocathodes using the GaN was 21 times longer than that using the GaAs. The decrease of quantum yield of the GaAs was well correlated in the form of the exponential decrease function with a decrease time of 4.4 h, while the decrease of quantum yield of the GaN was well correlated in the form of the exponential decrease function with two decrease times of 47 and 174 h.

Nature of yellow luminescence band in GaN grown on Si substrate Reviewed

Shogo Ito, Taihei Nakagita, Nobuhiko Sawaki, Hyung Soo Ahn, Masashi Irie, Toshiki Hikosaka, Yoshio Honda, Masahito Yamaguchi and Hiroshi Amano

Japanese Journal of Applied Physics Vol. 53 page： 11RC02/1-5 2014.9

Atom probe tomography study of Mg-doped GaN layers Reviewed

S Khromov, D Gregorius, R Schiller, J Lösch, M Wahl, M Kopnarski, H Amano, B Monemar, L Hultman and G Pozina

Nanotechnology Vol. 25 ( 27 ) 2014.6

Characterization of nonpolar a-plane InGaN/GaN multiple quantum well Reviewed

Ji-Su Son, Yoshio Honda, Masahito Yamaguchi and Hiroshi Amano

Japanese Journal of Applied Physics Vol. 53 page： 05FL01 2014.5

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

The characteristics of nonpolar a-plane (1120) 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 >2 ' e9, >7e8, and >4e8cm-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 1.0e18cm-3, respectively.

DOI： 10.7567/JJAP.53.05FL01

X-ray investigations of GaInN single quantum wells grown by atomic layer epitaxy and metalorganic vapor phase epitaxy Reviewed

Guangxu Ju, Yoshihiro Kato, Yoshio Honda, Masao Tabuchi, Yoshikazu Takeda, and Hiroshi Amano

physica status solidi (c) Vol. 11 ( 3-4 ) page： 393-396 2014.4

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

A set of GaN/GaInN single quantum wells (SQWs) were grown on c-plane GaN/sapphire templates using metal organic vapor phase epitaxy (MOVPE) and atomic layer epitaxy (ALE). The structural information on the interfaces and surfaces was obtained by fitting the experimental X-ray spectra and was compared between MOVPE and ALE mode. The X-ray crystal truncation rod (CTR) scattering measurement results showed that the samples grown by ALE had sharper interfaces than those grown by MOVPE. Both the X-ray reflectivity (XRR) curve-fitting results and atomic force microscope (AFM) results indicated that the surfaces of the ALE-grown SQWs were smoother than those of MOVPE-grown SQWs.

DOI： 10.1002/pssc.201300670

Enhancement of light output power on GaN based light emitting diodes using two direction stripe patterned sapphire substrate Reviewed

Okuno, Koji; Oshio, Takahide; Shibata, Naoki; Honda, Yoshio; Yamaguchi, Masahito; Amano, Hiroshi

Physica Status Solidi C Vol. 11 page： 722-725 2014.4

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

To achieve both the reduction of threading dislocation density (TDD) and the enhancement of light extraction efficiency (LEE) of light-emitting diodes (LEDs), a two-directional stripe-patterned sapphire substrate (TPSS) was fabricated. First, a stripe pattern parallel to the a-axis of Al2O3 was formed on an as-received c -sapphire wafer and then a second stripe pattern rotated around 3° from the m-axis of Al2O3 was formed on the first stripe PSS. The light output power of TPSS LEDs was around 1.25 times higher than that of LEDs on a flat sapphire substrate. This enhancement is caused by the increase of both the internal quantum efficiency (IQE) and the LEE. On the other hand, although the LED on TPSS with the second stripe just parallel to the m-axis of Al2O3 showed a similar enhancement ratio of the light output power, the current leakage of this LED increased with the concentration of threading dislocations on the surface of LEDs. Stripes rotated 3° from the m-axis of Al2O3 changed the growth mode and realized GaN films with a smooth surface without pit formation. Therefore, the misalignment of the stripe from the m-axis of Al2O3 will become a new design parameter for realizing high-efficiency reliable LEDs.

DOI： 10.1002/pssc.201300470

Epitaxial growth of GaN by REMOCVD in the downflow of a very high frequency(VHF) N2/H2 excited plasma Reviewed

Yi Lu, Hiroki Kondo,Kenji Ishikawa,Osamu Oda,KeigoTakeda,Makoto Sekine, Hiroshi Amano,Masaru Hori

Journal of Crystal Growth Vol. 391 page： 97-103 2014.4

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

We found that this REMOCVD allowed the growth of epitaxial GaN films of the wurtzite structure of (0001) orientation on sapphire substrates with a high growth rate of 0.42 μm/h at a low temperature of 800 °C. The present REMOCVD is a promising method for GaN growth at relatively low temperature and without using costly ammonia gas.

DOI： 10.1016/j.jcrysgro.2014.01.014

Growth of InGaN/GaN multiple quantum wells on size-controllable nanopyramid arrays Reviewed

Cao Miao, Yoshio Honda, Masahito Yamaguchi and Hiroshi Amano

Japanese Journal of Applied Physics Vol. 53 page： 0303060 2014.3

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

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.

DOI： 10.7567/JJAP.53.030306

In situ X-ray investigation of changing barrier growth temperatures on InGaN single quantum wells in metal-organic vapor phase epitaxy Reviewed

Guangxu Ju, Yoshio Honda, Masao Tabuchi, Yoshikazu Takeda, and Hiroshi Amano

Journal of Applied Physics Vol. 115 page： 094906 2014.3

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

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.

DOI： 10.1063/1.4867640

Multijunction GaInN-based solar cells using a tunnel junction Reviewed

Hironori Kurokawa, Mitsuru Kaga, Tomomi Goda, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, Isamu Akasaki and Hiroshi Amano

Applied Physics Express Vol. 7 ( 3 ) page： 034104 2014.3

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

We fabricated and characterized a two-junction GaInN-based solar cell using a tunnel junction fabricated by crystal growth. This solar cell has two active layers with a differing bandgap energy corresponding to blue or green light. We confirmed that the open-circuit voltage (VOC) in this solar cell was increased by the series connection using the tunnel junction. The short-circuit current density, VOC, fill factor, and energy conversion efficiency of this solar cell were 0.28 mA/cm2, 3.0 V, 0.5, and 0.41%, respectively, under an air mass filter of 1.5 G at 1-sun irradiation and room temperature.

DOI： 10.7567/APEX.7.034104

Novel activation process for Mg-implanted GaN Reviewed

Shin Hashimoto, Takao Nakamura, Yoshio Honda, Hiroshi Amano

Journal of Crystal Growth Vol. 388 page： 112-115 2014.2

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

A novel activation process for Mg-implanted GaN was demonstrated. As opposed to the conventional thermal annealing process, an H2/NH3 alternating supply annealing process achieved better optical activation, stronger near-ultraviolet luminescence and weaker yellow luminescence in the photoluminescence 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 H2 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.

DOI： doi:10.1016/j.jcrysgro.2013.07.011

Properties of the main Mg-related acceptors in GaN from optical and structural studies Reviewed

B. Monemar, P. P. Paskov, G. Pozina, C. Hemmingsson, J. P. Bergman, S. Khromov, V. N. Izyumskaya, V. Avrutin, X. Li, H. Morkoç, H. Amano, M. Iwaya, and I. Akasaki

Journal of Applied Physics Vol. 115 page： 053507 2014.2

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

The luminescent properties of Mg-doped GaN have recently received particular attention, e.g., in the
light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be
the main optical signature of the substitutional MgGa acceptor, thus, having a rather large binding
energy and a strong phonon coupling in optical transitions. We present new experimental data on
homoepitaxial Mg-doped layers, which together with the previous collection of data give an
improved experimental picture of the various luminescence features in Mg-doped GaN.

DOI： 10.1063/1.4862928

Growth of low-defect-density nonpolar a-plane GaN on r-plane sapphire using pulse NH3 interrupted etching Reviewed

Ji-Su Son, Yoshio Honda, and Hiroshi Amano

Optics Express Vol. 22 ( 3 ) page： 3585-3592 2014.2

Morphology development of GaN nanowires using a pulsed-mode MOCVD growth technique Reviewed

Byung Oh Jung, Si-Young Bae, Yoshihiro Kato, Masataka Imura, Dong-Seon Lee, Yoshio Honda and Hiroshi Amano

CrystEngComm Vol. 16 page： 2273-2282 2014.1

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

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.

DOI： 10.1039/c3ce42266f

Characteristics of a-plane GaN films grown on optimized silicon-dioxide-patterned r-plane sapphire substrates Reviewed

Ji-Su Son, Y. Honda, M. Yamaguchi, H. Amano, K. H. Baik, Y. G. Seo and S. M. Hwang

Thin Solid Films Vol. 546 ( 11 ) page： 108-113 2013.11

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

We report on the characteristics of a-plane GaN films directly grown on optimized silicon-dioxide-patterned r-plane sapphire substrates. Various shapes and sizes of silicon dioxide patterns were considered with the aim of achieving fully coalescent a-plane GaN films with a smooth surface and high crystalline quality. The omega full widths at half maximum of the X-ray rocking curve values of optimized a-plane GaN films with regular hexagonal patterns of 1 micron window width and 6 micron mask width were measured to be 597 arcsec along the c-axis direction and 457 arcsec along the m-axis direction. Atomic force microscopy images revealed a significant reduction in the density of submicron pits in the mask region.

Effects of exciton localization on internal quantum efficiency of InGaN nanowires Reviewed

Hideaki Murotani, Yoichi Yamada, Takuya Tabata, Yoshio Honda, Masahito Yamaguchi and Hiroshi Amano

Journal of Applied Physics Vol. 114 page： 153506 2013.10

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Papers - AMANO, Hiroshi