Faculty Profiles - AMANO, Hiroshi

Papers - AMANO, Hiroshi

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Laser slice thinning of GaN-on-GaN high electron mobility transistors

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： 7363 2022.5

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

As a newly developed technique to slice GaN substrates, which are currently very expensive, with less loss, we previously reported a laser slicing technique in this journal. In the previous report, from the perspective of GaN substrate processing, we could only show that the GaN substrate could be sliced by a laser and that the sliced GaN substrate could be reused. In this study, we newly investigated the applicability of this method as a device fabrication process. We demonstrated the thinning of GaN-on-GaN high-electron-mobility transistors (HEMTs) using a laser slicing technique. Even when the HEMTs were thinned by laser slicing to a thickness of 50 mm after completing the fabrication process, no significant fracture was observed in these devices, and no adverse effects of laser-induced damage were observed on electrical characteristics. This means that the laser slicing process can be applied even after device fabrication. It can also be used as a completely new semiconductor process for fabricating thin devices with thicknesses on the order of 10 mm, while significantly reducing the consumption of GaN substrates.

DOI： 10.1038/s41598-022-10610-4

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

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>

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

Continuous-wave lasing of AlGaN-based ultraviolet laser diode at 274.8 nm by current injection

Zhang, ZY; Kushimoto, M; Yoshikawa, A; Aoto, K; Schowalter, LJ; Sasaoka, C; Amano, H

APPLIED PHYSICS EXPRESS Vol. 15 ( 4 ) 2022.4

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

We demonstrated continuous-wave lasing of an AlGaN-based ultraviolet laser diode, fabricated on a single-crystal AlN substrate when operating at 5 °C. The threshold current density and device series resistance were reduced by improvements to the epitaxial structure and electrode arrangement. A peak wavelength of 274.8 nm was observed for lasing at a drive current over 110 mA, which corresponded to a threshold current density of 3.7 kA cm-2. The operating voltage at the threshold current was as low as 9.6 V.

DOI： 10.35848/1882-0786/ac6198

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

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

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

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

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

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

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

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

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

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

Defect characterization of { 10 1 ¯ 3 } GaN by electron microscopy

Kusch G., Frentrup M., Hu N., Amano H., Oliver R.A., Pristovsek M.

Journal of Applied Physics Vol. 131 ( 3 ) 2022.1

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

Advances in obtaining untwinned (10 1 ¯3)-oriented semi-polar GaN enable a new crystal orientation for the growth of green and red LED structures. We present a scanning electron microscopy study that combines the structural characterization of electron channeling contrast imaging with the optical characterization of cathodoluminescence hyperspectral imaging on a (10 1 ¯ 3) GaN layer. An extensive defect analysis revealed that the dominant defects consist of basal plane stacking faults (BSFs), prismatic stacking faults, partial dislocations, and threading dislocations. With a defect density of about an order of magnitude lower than in comparable. The optical properties of the defects have been characterized from 10 to 320 K, showing BSF luminescence at room temperature indicating a reduced density of non-radiative recombination centers in the as-grown samples compared to established semi- and non-polar orientations. Our findings suggest that growth along (10 1 ¯3) has the potential for higher radiative efficiency than established semi-polar orientations.

DOI： 10.1063/5.0077084

Perspective on thermal conductance across heterogeneously integrated interfaces for wide and ultrawide bandgap electronics

Cheng, Z; Graham, S; Amano, H; Cahill, DG

APPLIED PHYSICS LETTERS Vol. 120 ( 3 ) 2022.1

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

Heterogeneous integration is important to create multi-functionality in future electronic devices. However, few thermal studies of the interfaces formed in these integrated devices have been reported before. Recently, integrated interfaces by surface-Activated bonding were found to have high thermal boundary conductance, which provides a solution for heat dissipation of GaN and β-Ga2O3-based power electronics. Here, we review the recent progress on the interfacial thermal transport across heterogeneously integrated interfaces, including transferred van der Waals force bonded interfaces, surface-Activated bonded interfaces, plasma bonded interfaces, and hydrophilic bonded interfaces. This Perspective specifically focuses on applications of thermal management strategies of electronics, especially power electronics. Finally, the challenges, such as high-Throughput thermal measurements of buried interfaces, thermal property-structure relations of interfaces bonded under different conditions, theoretical understanding of interfacial thermal transport, and device demonstrations, are pointed out.

DOI： 10.1063/5.0077039

Threshold increase and lasing inhibition due to hexagonal-pyramid-shaped hillocks in AlGaN-based DUV laser diodes on single-crystal AlN substrate

Kushimoto, M; Zhang, ZY; Honda, Y; Schowalter, LJ; Sasaoka, C; Amano, H

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 61 ( 1 ) 2022.1

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

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

Ohmic Contact to <i>p</i>-Type GaN Enabled by Post-Growth Diffusion of Magnesium

Wang, J; Lu, S; Cai, WT; Kumabe, T; Ando, Y; Liao, YQ; Honda, Y; Xie, YH; Amano, H

IEEE ELECTRON DEVICE LETTERS Vol. 43 ( 1 ) page： 150 - 153 2022.1

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

We demonstrated the formation of excellent Ohmic contact to p-type GaN (including the plasma etching-damaged p-type GaN which otherwise exhibited undetectable current within ±5 V) by the post-growth diffusion of magnesium. The specific contact resistivity on the order of 10-4Omega cm2 (extracted at V=0 V) was achieved on the plasma-damaged p-GaN with linear current-voltage characteristics by the transfer length method (TLM) measurement. The improvement in current by a factor of over 109 was also obtained on the plasma-damaged p-n junction diode after the same Mg-treatment. These experimental results indicate a great potential of post-growth diffusion of Mg to overcome the bottleneck of forming a good Ohmic contact to p-GaN.

DOI： 10.1109/LED.2021.3131057

Frontier electronics in memory of Professor Isamu Akasaki

Amano, H

GALLIUM NITRIDE MATERIALS AND DEVICES XVII Vol. 12001 2022

Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of SPIE - The International Society for Optical Engineering

ISAMU AKASAKI, crystal grower, a pioneer of GaN-based blue light-emitting diodes (LEDs), and a Nobel Laureate in Physics, passed away because of pneumonia on April 1, 2021 at the age of 92. According to the Nobel Foundation, the LED lamp holds considerable promise for improving the quality of life of over 1.5 billion people worldwide who lack access to electricity grids. Owing to its low power requirements, it can be powered by cheap local solar energy. In this article, his pursuit of blue LEDs for 20 years is described.

DOI： 10.1117/12.2619005

An Accurate Approach to Develop Small Signal Circuit Models for AlGaN/GaN HEMTs Using Rational Functions and Dependent Current Sources

Jadhav, A; Ozawa, T; Baratov, A; Asubar, JT; Kuzuhara, M; Wakejima, A; Yamashita, S; Deki, M; Nitta, S; Honda, Y; Amano, H; Roy, S; Sarkar, B

IEEE JOURNAL OF THE ELECTRON DEVICES SOCIETY Vol. 10 page： 797 - 807 2022

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of the Electron Devices Society

In this paper, a technique to develop small signal circuit (SSC) models of AlGaN/GaN high electron mobility transistors (HEMTs) using dependent current sources is presented. In this technique, experimentally measured broadband Y-parameters of AlGaN/GaN HEMTs are mathematically modeled as a sum of pole-residue terms. By representing each pole-residue term as a dependent current source, it is possible to develop an accurate SSC models for HEMTs which otherwise may not be possible using passive resistive-inductive-capacitive elements. The accuracy of the proposed SSC model is validated against the conventional SSC model using a 2nd, 3rd and 4th order rational function representation of the admittance branches of AlGaN/GaN HEMTs. Therefore, the proposed SSC model turns out to be highly robust in nature and can take care of any form of the transfer functions of the admittance branches between the gate, drain, and source terminal of an AlGaN/GaN HEMT.

DOI： 10.1109/JEDS.2022.3208028

Promising Results of National Project by Japanese Ministry of the Environment to Develop GaN on GaN Power Devices and Prove their Usefulness in Real Systems

Otoki Y., Shibata M., Mishima T., Ohta H., Mori Y., Imanishi M., Tamura S., Kidera K., Takino J., Okayama Y., Watanabe K., Okamoto N., Honda Y., Yamamoto M., Shiozaki K., Amano H.

2022 International Conference on Compound Semiconductor Manufacturing Technology, CS MANTECH 2022 page： 237 - 242 2022

Language：English Publishing type：Research paper (scientific journal) Publisher：2022 International Conference on Compound Semiconductor Manufacturing Technology, CS MANTECH 2022

Exciting high performance power electronic devices have been widely demonstrated and manufactured using GaN epitaxial layers, but the majority of these devices have been fabricated on lattice mismatched substrates, including SiC, Si, or sapphire. Unfortunately, using lattice mismatched substrates inevitably introduces high concentrations of point defects and dislocations into the GaN epilayers, and these defects degrade the electrical performance of the fabricated GaN devices. Also, the mismatch of lattice constant and coefficient of thermal expansion cause strain and wafer bowing in the GaN epi, which further degrade the quality material for device fabrication. Using lattice matched GaN substrates provides solutions to these problems. In 2014, the Ministry of the Environment launched a national project to develop the required technology and to prove the superiority of GaN on GaN devices in real systems, with more than 10 partnerships from academia and industry. The project included work in several areas, including GaN substrate growth, GaN epitaxy, material characterization, and fabrication of devices and ICs for application in various systems. Large area GaN substrates have been grown with low defect densities, which has enabled fabrication of new types of vertical and horizontal GaN devices. The GaN devices have been used in servers, solar cell power conditioners, microwave ICs, distribution transformers, electric vehicle power converters. The performance improvements were compared with conventional approaches in each case. An “ALL GaN vehicle” has also been demonstrated, in which GaN devices are used in all power components. In this talk, we will present these results which show the great potential of GaN on GaN devices in the industry.

Development of Desktop Air Curtain System to Block Aerosols in Exhaled Breath

TAKAMURE Kotaro, SAKAMOTO Yasuaki, YAGI Tetsuya, IWATANI Yasumasa, AMANO Hiroshi, UCHIYAMA Tomomi

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2022 ( 0 ) page： S055-10 2022

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

<p>A desktop-type air curtain device capable of being installed on a desk to protect healthcare workers from infectious diseases was developed. Pseudo-exhaled air containing aerosol particles emitted from a mannequin was blown toward the air curtain generated by the air curtain device. The aerosol blocking effect of the air curtain device was investigated using particle image velocimetry measurements. Air curtain flow was maintained inside the gate of the air curtain device. The aerosol particles approaching the air curtain device were observed to abruptly bend towards the suction port without passing through the gate, signifying that the aerosol particles were blocked by the air curtain flow.</p>

DOI： 10.1299/jsmemecj.2022.s055-10

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