Faculty Profiles - AMANO, Hiroshi

Papers - AMANO, Hiroshi

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High-Energy Computed Tomography as a Prospective Tool for In Situ Monitoring of Mass Transfer Processes inside High-Pressure Reactors-A Case Study on Ammonothermal Bulk Crystal Growth of Nitrides including GaN Reviewed Open Access

Schimmel, S; Salamon, M; Tomida, D; Neumeier, S; Ishiguro, T; Honda, Y; Chichibu, SF; Amano, H

MATERIALS Vol. 15 ( 17 ) 2022.9

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

For the fundamental understanding and the technological development of the ammonothermal method for the synthesis and crystal growth of nitrides, an in situ monitoring technique for tracking mass transport of the nitride throughout the entire autoclave volume is desirable. The feasibility of using high-energy computed tomography for this purpose was therefore evaluated using ex situ measurements. Acceleration voltages of 600 kV were estimated to yield suitable transparency in a lab-scale ammonothermal setup for GaN crystal growth designed for up to 300 MPa operating pressure. The total scan duration was estimated to be in the order of 20 to 40 min, which was sufficient given the comparatively slow crystal growth speed in ammonothermal growth. Even shorter scan durations or, alternatively, lower acceleration voltages for improved contrast or reduced X-ray shielding requirements, were estimated to be feasible in the case of ammonoacidic growth, as the lower pressure requirements for this process variant allow for thinned autoclave walls in an adapted setup designed for improved X-ray transparency. Promising nickel-base and cobalt-base alloys for applications in ammonothermal reactors with reduced X-ray absorption in relation to the maximum operating pressure were identified. The applicability for the validation of numerical simulations of the growth process of GaN, in addition to the applicability of the technique to further nitride materials, as well as larger reactors and bulk crystals, were evaluated.

DOI： 10.3390/ma15176165

Improvement of performance of N-Polar AlGaN/AlN Heterojunction FET

Inahara Daisuke, Matuda Shunsuke, Matsumura Wataru, Okuno Ryo, Hanasaku Koki, Kowaki Taketo, Miyamoto Minagi, Kanezaki Ren, Saito Shunsuke, Fujii Kai, Kurai Satoshi, Okada Narihito, Yao Yongzhao, Ishikawa Yukari, Tanaka Atsushi, Nitta Shugo, Honda Yoshio, Amano Hiroshi, Yamada Yoichi

JSAP Annual Meetings Extended Abstracts Vol. 2022.2 ( 0 ) page： 2869 - 2869 2022.8

Understanding indium incorporation of InGaN grown on polar, semi-polar, and non-polar orientation by metal-organic vapor phase epitaxy Reviewed

Hu, N; Avit, G; Pristovsek, M; Honda, Y; Amano, H

APPLIED PHYSICS LETTERS Vol. 121 ( 8 ) 2022.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics Letters

We studied indium incorporation into InGaN/GaN quantum wells grown by metal-organic vapor phase epitaxy by systematically varying of gallium and indium precursor flows on (0001), (10 1 ¯3), (11 2 ¯2), and (10 1 ¯0) orientations. The layer thickness and indium composition obtained from x-ray diffraction analysis were correlated with a model based on indium and gallium incorporation efficiencies. In the model, the indium incorporation efficiency is reproduced by the Langmuir surface coverage of gallium, indicating that indium atoms close to gallium atoms can be incorporated preferably.

DOI： 10.1063/5.0088908

Surface kinetics in halide vapor phase epitaxial growth of GaN layers on GaN (0001) freestanding substrates Reviewed Open Access

Ohnishi, K; Fujimoto, N; Nitta, S; Watanabe, H; Honda, Y; Amano, H

JOURNAL OF CRYSTAL GROWTH Vol. 592 2022.8

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

The effects of halide vapor phase epitaxial (HVPE) growth conditions such as input V/III ratio and substrate off-cut angle on the surface morphology of n-type GaN layers grown on GaN (0001) freestanding substrates were investigated to develop a model for growing smooth surfaces. The spiral hillock density increased with increasing input V/III ratio and/or decreasing off-cut angle. The critical off-cut angle between the spiral growth and the step-flow growth depended on the vapor supersaturation calculated by thermodynamic analysis. To understand the transition of the growth mode, we proposed a Burton–Cabrera–Frank-theory-based model considering the effect of spiral growth, which was utilized to explain the obtained experimental results. The developed growth model can be effective for predicting the HVPE growth mode between the spiral growth and the step-flow growth.

DOI： 10.1016/j.jcrysgro.2022.126749

Improved performance of deep ultraviolet AlGaN-based light-emitting diode by reducing contact resistance of Al-based reflector Reviewed

Sim K.B., Jin J.Y., Kim S.K., Ko Y.J., Hwang G.W., Seong T.Y., Amano H.

Journal of Alloys and Compounds Vol. 910 2022.7

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Journal of Alloys and Compounds

In this study, chlorine (Cl) treatment was carried out on p-AlGaN to enhance the performance of ultraviolet-C light emitting diodes (UVC LEDs) by modifying ITO work function and hence reducing the contact resistance of ITO/Al reflector. The Cl-treated UVC LEDs exhibit the forward voltage of 6.88 V at 20 mA, whereas the reference samples show 7.50 V. The light output power and relative wall plug efficiency (WPE) of the Cl-treated UVC LEDs are enhanced by 17.1% at 500 mW and 19.5% at 100 mA, respectively, as compared to the reference. Additionally, the Cl-treated LEDs also display reduction in both the leakage current and ideality factor. Further, the photoluminescence (PL) intensity of AlGaN micro-disks is also enhanced by the Cl-treatment. X-ray photoemission spectroscopy (XPS) results indicate the formation of Cl-ITO at the ITO/p-AlGaN interface and the passivation of the surface states of AlGaN by Cl radicals. Based on the XPS results, a possible mechanism for the improved performance of Cl-treated UVC AlGaN-based LEDs is described and discussed.

DOI： 10.1016/j.jallcom.2022.164895

Weak metastability of Al<i> <sub>x</sub> </i>Ga<sub>1-<i>x</i> </sub>N (<i>x</i>=13/24, 15/24, 17/24) shown by analyzing AlGaN grown on AlN with dense macrosteps Reviewed

Hirano, A; Nagasawa, Y; Ippommatsu, M; Sako, H; Hashimoto, A; Sugie, R; Honda, Y; Amano, H; Kojima, K; Chichibu, SF

APPLIED PHYSICS EXPRESS Vol. 15 ( 7 ) 2022.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics Express

Energy-dispersive X-ray signals calibrated by Rutherford backscattering indicated the generation of Al13/24Ga11/24N in Ga-rich stripes in a nonflat Al0.58Ga0.42N layer. Also, the CL peak wavelengths of ∼259 and 272 nm also showed the generation of Al15/24Ga9/24N and Al13/24Ga11/24N in Al-rich zones and Ga-rich stripes, respectively. The wavelength of a strong CL peak at ∼246 nm, which was observed from the Al0.7Ga0.3N layer in our previous study, is also considered to correspond to the near-band-emission wavelengths of Al17/24Ga7/24N. In particular, the stronger reproducibility of metastable Al15/24Ga9/24N generation was confirmed, in agreement with the computed predictions by other research groups.

DOI： 10.35848/1882-0786/ac79a1

Space-Charge Profiles and Carrier Transport Properties in Dopant-Free GaN-Based p-n Junction Formed by Distributed Polarization Doping Reviewed

Kumabe, T; Kawasaki, S; Watanabe, H; Nitta, S; Honda, Y; Amano, H

PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS Vol. 16 ( 7 ) 2022.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Physica Status Solidi - Rapid Research Letters

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

Dual-peak electroluminescence spectra generated from Al <i><sub>n</sub></i> <sub>/12</sub>Ga<sub>1-<i>n</i>/12</sub>N (<i>n</i>=2, 3, 4) for AlGaN-based LEDs with nonflat quantum wells Reviewed

Nagasawa, Y; Kojima, K; Hirano, A; Ippommatsu, M; Honda, Y; Amano, H; Chichibu, SF

JOURNAL OF PHYSICS D-APPLIED PHYSICS Vol. 55 ( 25 ) 2022.6

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

The metastability of Al n/12Ga1-n/12N (n = 2, 3, and 4) was investigated by the statistical analysis of electroluminescence (EL) spectra having dual peaks with a peak-to-peak distance (pp) of >10 nm generated from nonflat Al x Ga1-x N (x ∼0.2) quantum wells (QWs) fabricated on c(0001) sapphire substrates with a miscut of 1.0° towards the m[1 1ˉ 00] axis. To explain the origins of the dual-peak EL (DPEL) spectra, which are often observed for AlGaN-QWs with Ga content of greater than 0.7, a nonflat QW model incorporating two metastable compositions, Al(n-1)/12Ga1-(n-1)/12N and Al n/12Ga1-n/12N (n: integer), is proposed. By the statistical analysis of peak wavelengths in DPEL spectra and the verification of EL spectral shapes, two series of featured EL peak wavelengths with intervals of 2-3 nm were obtained from five out of six LED wafers. The two series of featured EL peak wavelengths were assigned by comparison with the calculated EL wavelengths. Then, Al2/12Ga10/12N and Al3/12Ga9/12N were determined to be the origins of peaks with the longer and shorter wavelengths in the DPEL, respectively, in addition to the metastable Al n/12Ga1-n/12N (n= 4-9) compositions observed in our previous studies. When DPEL (pp> 10 nm) appeared, the difference in QW thickness between Al2/12Ga10/12N and Al3/12Ga9/12N tended to be larger than one monolayer (ML), indicating a significant amount of Ga or GaN mass transport. Furthermore, the Al2/12Ga10/12N and Al3/12Ga9/12N QWs are considered to have thicknesses of m ML (m: consecutive integers), suggesting the 1 ML configuration of Al and Ga atoms on the c(0001) plane. In addition, the DPEL obtained from nonflat Al x Ga1-x N (x ∼0.25) QWs by another research group was shown to be related to two metastable Al n/12Ga1-n/12N (n = 3, 4), similarly to our one exceptional LED wafer, which also agreed with the model proposed in this work.

DOI： 10.1088/1361-6463/ac5d03

Author Correction: Laser slice thinning of GaN-on-GaN high electron mobility transistors. Open Access

Tanaka A, Sugiura R, Kawaguchi D, Wani Y, Watanabe H, Sena H, Ando Y, Honda Y, Igasaki Y, Wakejima A, Ando Y, Amano H

Scientific reports Vol. 12 ( 1 ) page： 8175 2022.5

Interplay of sidewall damage and light extraction efficiency of micro-LEDs Reviewed

Park, JH; Pristovsek, M; Cai, WT; Cheong, H; Kumabe, T; Lee, DS; Seong, TY; Amano, H

OPTICS LETTERS Vol. 47 ( 9 ) page： 2250 - 2253 2022.5

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

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

DOI： 10.1364/OL.456993

<p>The effect of dry etching condition on the performance of blue micro light-emitting diodes with reduced quantum confined Stark effect epitaxial layer</p> Reviewed

Park Jeong-Hwan, Cai Wentao, Cheong Heajeong, Ushida Yasuhisa, Lee Da-Hoon, Ando Yuto, Furusawa Yuta, Honda Yoshio, Lee Dong-Seon, Seong Tae-Yeon, Amano Hiroshi

JOURNAL OF APPLIED PHYSICS Vol. 131 ( 15 ) 2022.4

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

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

DOI： 10.1063/5.0085384

"Regrowth-free" fabrication of high-current-gain AlGaN/GaN heterojunction bipolar transistor with N-p-n configuration Reviewed Open Access

Kumabe, T; Watanabe, H; Ando, Y; Tanaka, A; Nitta, S; Honda, Y; Amano, H

APPLIED PHYSICS EXPRESS Vol. 15 ( 4 ) 2022.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics Express

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

Mechanism and enhancement of anti-parasitic-reaction catalytic activity of tungsten-carbide-coated graphite components for the growth of bulk GaN crystals Reviewed Open Access

Nakamura, D; Iida, K; Horibuchi, K; Aoki, Y; Takahashi, N; Mori, Y; Moriyama, M; Nitta, S; Amano, H

APPLIED PHYSICS EXPRESS Vol. 15 ( 4 ) 2022.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics Express

The working mechanism of the anti-parasitic-reaction (APR) catalyst of tungsten carbide (WC) coating on graphite in hydride vapor phase epitaxy GaN growth were examined. During NH3 annealing, the surface of WC is reduced as well as nitrided. The W2N topmost layer was found to work as an APR-active catalyst to suppress the formation of GaN polycrystals during high-rate HVPE-GaN growth, while the regions covered with thick pyrolytic graphite residues were catalytically inert. The formation of an additional W2C top layer on the WC underlayer was demonstrated to exhibit superior APR activity, i.e. complete suppression of GaN polycrystal formation.

DOI： 10.35848/1882-0786/ac5ba4

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

Matsubara, T; Nagatat, K; Kushimoto, M; Tomai, S; Katsumata, S; Honda, Y; Amano, H

APPLIED PHYSICS EXPRESS Vol. 15 ( 4 ) 2022.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics Express

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

Structural design optimization of 279 nm wavelength AlGaN homojunction tunnel junction deep-UV light-emitting diode Reviewed Open Access

Nagata K., Anada S., Miwa H., Matsui S., Boyama S., Saito Y., Kushimoto M., Honda Y., Takeuchi T., Amano H.

Applied Physics Express Vol. 15 ( 4 ) 2022.4

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Applied Physics Express

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

Improved device performance of vertical GaN-on-GaN nanorod Schottky barrier diodes with wet-etching process Reviewed Open Access

Liao Y., Chen T., Wang J., Cai W., Ando Y., Yang X., Watanabe H., Tanaka A., Nitta S., Honda Y., Chen K.J., Amano H.

Applied Physics Letters Vol. 120 ( 12 ) 2022.3

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Applied Physics Letters

In this work, a deliberate etching-based top-down approach is proposed to fabricate the GaN nanorod (NR) Schottky barrier diode (SBD). As a key step during the fabrication, the impact of the wet-etching process on device performance is systematically studied. By virtue of the reduced surface states at the sidewall, the performance of NR SBD with the wet-etching process is substantially improved, delivering a forward turn-on voltage of 0.65 V, a current density of ∼10 kA/cm2 at 3 V, an ideality factor of 1.03, an ON/OFF current ratio of ∼1010, and no severe current collapse, along with a reverse breakdown voltage of 772 V.

DOI： 10.1063/5.0083194

Inhomogeneous Barrier Height Characteristics of <i>n</i>-Type AlInP for Red AlGaInP-Based Light-Emitting Diodes Reviewed

Cha, JS; Lee, DH; Sim, KB; Lee, TJ; Seong, TY; Amano, H

ECS JOURNAL OF SOLID STATE SCIENCE AND TECHNOLOGY Vol. 11 ( 3 ) 2022.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ECS Journal of Solid State Science and Technology

For micro-light-emitting diode (LED)-based display applications, such as virtual reality and augmented reality, high-performance Ohmic contacts (namely, the improvement of current injection efficiency) is vital to the realization of high-efficiency micro-LEDs. The surface Fermi level pinning characteristics could be comprehended in terms of the relation between work function of metals (M) and Schottky barrier height (SBH, B). In this study, we have investigated the surface Fermi level pinning characteristics of (001) n-AlInP surfaces by employing Schottky diodes with different metals. With an increase in the temperature, B increases linearly and ideality factors (n) decreases. This behavior is related to the barrier height inhomogeneity. Inhomogeneity-modelbased B is evaluated to be in the range of 0.86 1.30 eV, which is dependent on the metal work functions and are similar to those measured from capacitance-voltage relation. Further, The S-parameter, the relation between B and M (d B/d M), is 0.36. This is indicative of the partial pinning of the surface Fermi level at the surface states placed at 0.95 eV below the conduction band. Furthermore, it is also shown that (NH4)2S-passivation results in an increases the mean SBH and the S-parameter (e.g., 0.52).

DOI： 10.1149/2162-8777/ac5d66

Visualization of depletion layer in AlGaN homojunction p-n junction Reviewed Open Access

Nagata, K; Anada, S; Saito, Y; Kushimoto, M; Honda, Y; Takeuchi, T; Yamamoto, K; Hirayama, T; Amano, H

APPLIED PHYSICS EXPRESS Vol. 15 ( 3 ) 2022.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics Express

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

The photoemission characteristic of InGaN in NEA activation under heating

Kashima Masahiro, Itokawa Yuya, Kanai Toshiya, Sato Daiki, Koizumi Atsushi, Iijima Hokuto, Nishitani Tomohiro, Honda Yoshio, Amano Hiroshi, Meguro Takashi

JSAP Annual Meetings Extended Abstracts Vol. 2022.1 ( 0 ) page： 1343 - 1343 2022.2

Effect of beam current on defect formation by high-temperature implantation of Mg ions into GaN Reviewed Open Access

Itoh, Y; Watanabe, H; Ando, Y; Kano, E; Deki, M; Nitta, S; Honda, Y; Tanaka, A; Ikarashi, N; Amano, H

APPLIED PHYSICS EXPRESS Vol. 15 ( 2 ) 2022.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics Express

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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