Faculty Profiles - AMANO, Hiroshi

Papers - AMANO, Hiroshi

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GaInN-based solar cells using GaInN/GaInN superlattices Reviewed

Fujii, Takahiro; Kuwahara, Yousuke; Iida, Daisuke; Fujiyama, Yasuhara; Morita, Yoshiki; Sugiyama, Toru; Isobe, Yasuhiro; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; et al

Physica Status Solidi C Vol. 8 ( 7-8 ) page： 2463-2465 2011.7

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

We report on the fabrication of GaInN-based solar cells using GaInN/GaInN superlattices as active layers and also as underlying layers beneath the active layers. We obtained pit-free surfaces, even with a high InN molar fraction, using the superlattices. As a result, the max. external and internal quantum efficiencies reached 60%, and 88%, resp. The open-circuit voltage of the soalr cells was 1.77 V, the short-circuit c.d. was 3.08 mA/cm2, and the fill factor was 70.3%. A conversion efficiency of 2.46% was achieved at room temp. under simulared 1.5 sun × AM1.5G illumination using a solar simulator.

Achieving high-growth-rate in GaN homoepitaxy using high-density nitrogen radical source Reviewed

Kawai, Yohjiro; Chen, Shang; Honda, Yoshio; Yamaguchi, Masahito; Amano, Hiroshi; Kondo, Hiroki; Hirmatsu, Mineo; Kano, Hiroyuki; Yamakawa, Koji; Den, Shoji; et al

Physica Status Solidi C Vol. 8 ( 7-8 ) page： 2089-2091 2011.7

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

The key issue in GaN growth by radio-frequency plasma-assisted mol. beam epitaxy is the low growth rate compared with that obtained using an ammonia source. To reduce the processing time and to improve the cryst. quality of the epilayer, a high-d. radical source (HDRS) with high stability has been developed. The growth rate of the GaN epilayer was improved using the HDRS rather than a conventional radical source. During the growth, a sharp streak pattern obtained by RHEED was maintained. An atomically smooth surface was confirmed by at. force microscopy observation.

AlGaN/GaInN/GaN heterostructure field-effect transistor Reviewed

Ikki, Hiromichi; Isobe, Yasuhiro; Iida, Daisuke; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; Amano, Hiroshi; Bandoh, Akira; Udagawa, Takashi

Physica Status Solidi A Vol. 208 ( 7 ) page： 1614-1616 2011.7

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

We report on the elec. properties of AlGaN/GaInN heterostructures fabricated with various InN molar fractions from 0 to 0.60 in GaInN on a GaN template. The sheet carrier d. of the AlGaN/GaInN heterostructure monotonically increased with increasing InN molar fraction in GaInN, reaching 5.0 × 1013 cm-2 at an InN molar fraction of 0.60. The Al0.30Ga0.70N/Ga0.40In0.60N heterostructure exhibited static field-effect transistor (FET) characteristics.

Development of high efficiency 255-355 nm AlGaN-based light-emitting diodes Reviewed

Pernot, Cyril; Fukahori, Shinya; Inazu, Tetsuhiko; Fujita, Takehiko; Kim, Myunghee; Nagasawa, Yosuke; Hirano, Akira; Ippommatsu, Masamichi; Iwaya, Motoaki; Kamiyama, Satoshi, Amano Hiroshi; et al

Physica Status Solidi A: Vol. 208 ( 7 ) page： 1594-1596 2011.7

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

We report on the fabrication and characterization of high efficiency UV light emitting diodes (LEDs) with emission wavelength ranging from 255 to 355 nm. Epi-layers of UV LEDs were grown on AlGaN templates with sapphire substrates. Flip-chip configuration without removing sapphire is used for characterization of the UV LEDs. External quantum efficiencies (EQEs) over 3% were obtained for all the investigated wavelengths with max. value reaching 5.1% for 280 nm LED. Under RT DC operation at a current of 500 mA, output powers of 38, 77, and 64 mW were measured for 257, 280, and 354 nm, resp. By using enhanced light extn. technologies, such as, moth-eye structure on the back side of the sapphire substrate, we expect to improve these values by up to 50%.

Drain bias stress and memory effects in AlGaN/GaN heterostructure field-effect transistors with p-GaN gate Reviewed

Sugiyama, Takayuki; Honda, Yoshio; Yamaguchi, Masahito; Amano, Hiroshi; Oshimura, Yoshinori; Iida, Daisuke; Iwaya, Motoaki; Akasaki, Isamu

Physica Status Solidi C Vol. 8 ( 7-8 ) page： 2424-2426 2011.7

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

We measured the drain bias stress effects in normally off mode AlGaN/GaN (JHFETs) with a p-GaN gate, whose AlGaN barrier was exposed to air by dry etching. A large current collapse and a memory effect decreased the drain current (IDS). IDS of an unpassivated sample became the off leakage level of normally off mode JHFETs at a drain stress bias of 15 V. On the other hand, IDS in SiN-passivated JHFETs decreased at a slow rate upon applying drain stress bias. The decrease in IDS in SiN-passivated JHFETs was almost the same as that in asgrown HFETs. These drain stress and memory effects were perfectly deletable by light exposure, and were rewritable similarly in an EPROM.

Nonpolar a-plane AlGaN/GaN heterostructure field-effect transistors grown on freestanding GaN substrate Reviewed

Isobe, Yasuhiro; Ikki, Hiromichi; Sakakibara, Tatsuyuki; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; Sugiyama, Takayuki; Amano, Hiroshi; Imade, Mamoru; et al

Applied Physics Express Vol. ４ ( ６ ) page： 064102/1-064102/3 2011.6

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

We fabricated and characterized nonpolar a-plane AlGaN/GaN heterostructure field-effect transistors (HFETs) grown on an a-plane freestanding GaN substrate. By optimizing the growth conditions, the unintentionally doped oxygen concn. was much reduced in the a-plane GaN buffer layer. As a result, the low leakage current in the buffer layer was realized without doping of deep acceptors, such as Fe and C, by which an impurity-contamination-free channel layer can be successfully grown.

Partial strain relaxation by stacking fault generation in InGaN multiple quantum wells grown on (1-101) semipolar GaN Reviewed

Wu, Z. H.; Tanikawa, T.; Murase, T.; Fang, Y.-Y.; Chen, C. Q.; Honda, Y.; Yamaguchi, M.; Amano, H.; Sawaki, N.

Applied Physics Letters Vol. ９８ ( ５ ) page： 051902/1-051902/3 2011.5

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

The authors have studied the structural properties and relaxation phenomenon of InGaN multiple quantum wells (QWs) on (1.hivin.101) semipolar GaN templates grown on patterned (001) Si substrates by selective area growth technique. Studies by TEM and x-ray diffraction reciprocal space mapping reveal that QWs emitting light at 540 nm experience significant strain relaxation along the in-plane [1.hivin.10.hivin.2] direction by the generation of an array of basal stacking faults (BSF). The generation of BSFs in 540 nm QWs could be an important factor limiting its luminescence efficiency.

Growth of AlGaN/GaN heterostructure on vicinal m-plane free-standing GaN substrates prepared by the Na flux method Reviewed

Isobe, Yasuhiro; Iida, Daisuke; Sakakibara, Tatsuyuki; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; Amano, Hiroshi; Imade, Mamoru; Kitaoka, Yasuo; et al

Physica Status Solidi A Vol. 208 ( 5 ) page： 1191-1194 2011.5

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

We fabricated and characterized AlGaN/GaN heterostructure growth by MOVPE on vicinal m-plane free-standing GaN substrates prepd. by the Na flux method. The miscut angle in the LPE-GaN substrate has a great influence on the surface morphol. and cryst. quality of epitaxial GaN and AlGaN films. In particular, AlGaN/GaN on a 4° miscut-angle LPE-GaN substrate shows large step bunching, small-miscut-angle LPE GaN substrate is essential for fabricating on FET structure with a flat surface.

Internal quantum efficiency of whole-composition-range AlGaN multiquantum wells Reviewed

Ban, Kazuhito; Yamamoto, Jun-ichi; Takeda, Kenichiro; Ide, Kimiyasu; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; Amano, Hiroshi

Applied Physics Express Vol. 4 ( 5 ) page： 052101/1-052101/3 2011.5

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

We analyze the internal quantum efficiency (IQE) of whole-compn.-range AlGaN multiquantum wells (MQWs) on AlGaN with various dislocation densities (DDs) by excitation-d.-dependent photoluminescence measurement. IQEs of deep UV/UV (DUV/UV) MQWs are strongly dependent on the DD. IQE with an excess carrier d. of 1 × 1018 cm-3 changes from 4 to 64% when the DD changes from 6 × 109 to 2 × 108 cm-2. This trend is almost the same for DUV/UV MQWs with emission wavelength ranging from 230 to 350 nm. Thus, the redn. of the DD is very important for the realization of a high-IQE DUV/UV active layer.

Microstructural analysis of thick AlGaN epilayers using Mg-doped AlN underlying layer Reviewed

Nonaka, K.; Asai, T.; Ban, K.; Yamamoto, J.; Iwaya, M.; Takeuchi, T.; Kamiyama, S.; Akasaki, I.; Amano, H.; Wu, Z. H.

Physica Status Solidi C Vol. 8 ( 5 ) page： 1467-1470 2011.5

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

This paper reports the microstructural anal. of 20-μm-thick Al0.5Ga0.5N with improved cryst. quality owing to the use of a Mg-doped AlN underlying layer. The threading dislocation d. in 20-μm-thick Al0.5Ga0.5N on Mg-doped AlN was 8.6 × 108 cm-2, which is about one-fifth lower than that of Al0.5Ga0.5N on an undoped AlN underlying layer. The microstructural anal. was carried out to clarify dislocation behaviors in the Al0.5Ga0.5N layer on the Mg-doped AlN underlying layer.

Evidence for moving of threading dislocations during the VPE growth in GaN thin layers Reviewed

Kuwano, Noriyuki; Miyake, Hideto; Hiramatsu, Kazumasa; Amano, Hiroshi; Akasaki, Isamu

Physica Status Solidi C Vol. 8 ( 5 ) page： 1487-1490. 2011.5

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

Cross-sectional transmission electron microscope (TEM) observation was performed in detail to analyze the morphol. of threading dislocations (TDs) in GaN thin layers with various thicknesses. The GaN layers were overgrown on an Al0.28Ga0.72N layer by the metal-org. vapor-phase epitaxy (MOVPE) method. In a GaN layer about 50 nm in thickness, TDs running up in the AlGaN layer pass into the GaN layer and most of them reach the top surface without bending. In thicker GaN layers, on the other hand, many of TDs form a hairpin-configuration on or above the interface of GaN and AlGaN to be annihilated. This difference in morphol. of TDs indicates that the TDs have moved down inside the GaN layer. Since the formation of hairpins is attributed to a stress-relief, there should be an extra half-plane between the paired TDs. Therefore, the movement of TDs should be of "climb motion". Another example of possible TD movement inside a GaN layer is also described. It is emphasized that the possibility of TD-movements inside the thin film crystal during the growth should be taken into account in anal. of thin-layer growth through the behavior of TDs.

Effect of lateral vapor phase diffusion during the selective growth of InGaN/GaN MQW on semipolar and nonpolar GaN stripes Reviewed

Tanikawa, Tomoyuki; Honda, Yoshio; Yamaguchi, Masahito; Amano, Hiroshi

Physica Status Solidi A Vol. 208 ( 5 ) page： 1175-1178 2011.5

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

We investigated the InGaN/GaN multiple quantum well (MQW) thickness and cathode luminescence (CL) distribution on nonpolar (11-20)GaN, semipolar (11-22)GaN, and (1-101)GaN microstripes grown by selective metal-org. vapor phase epitaxy (MOVPE) on patterned Si substrates. All samples exhibited ridge growth. To clarify the effect of vapor phase diffusion, the two-dimensional diffusion equation was solved. The results were in good agreement with the thickness distribution on the (11-20)GaN stripe when we assumed D/k0 to be 1 μm. However, the results were not in agreement for the semipolar case. On the semipolar face, the surface migration length might be greater, thus increasing the thickness nonuniformity. CL anal. indicated a uniform indium compn. on the (11-20) and (11-22) faces. On the (1-101)GaN stripe, the indium compn. decreased near the (0001) edge. Excess Ga is considered to affect the compositional nonuniformity on the (1-101) facet.

Freestanding highly crystalline single crystal AlN substrates grown by a novel closed sublimation method Reviewed

Yamakawa, Masayasu; Murata, Kazuki; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; Amano, Hiroshi; Azuma, Masanobu

Applied Physics Express Vol. 4 ( 4 ) page： 045503/1-045503/3 2011.4

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

We fabricated thick freestanding AlN films by a novel close-spaced sublimation method. The spacing between a sintered AlN polycrystal and a SiC substrate is 1 mm. A Ta ring was used to control the spacing between the AlN polycrystal and the SiC substrate. In addn., a special AlN adhesive was also used to fill in the gap between the AlN polycrystal, the Ta ring, and the SiC substrate. By a combination of these techniques, an AlN growth rate as high as 600 μm/h was achieved. A freestanding AlN layer was obtained by the sublimation of the SiC substrate during the AlN growth.

Spontaneous formation of highly regular superlattice structure in InGaN epilayers grown by molecular beam epitaxy Reviewed

Wu, Z. H.; Kawai, Y.; Fang, Y.-Y.; Chen, C. Q.; Kondo, H.; Hori, M.; Honda, Y.; Yamaguchi, M.; Amano, H.

Applied Physics Letters Vol. 98 ( 14 ) page： 141905/1-141905/3 2011.3

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

Structural properties of thick InGaN layers grown on GaN by plasma-assisted mol. beam epitaxy using two growth rates of 1.0 and 3.6 Å/s have been investigated. A highly regular superlattice (SL) structure formed spontaneously in the film grown at 3.6 Å/s but not in the film grown at 1.0 Å/s. The faster grown film also exhibited superior structural quality, which could be due to the surface roughness suppression caused by kinetic limitation, and the inhibition of the Frank-Read dislocation generation mechanism within the spontaneously formed SL structure.

Demonstration of diamond field effect transistors by AlN/diamond heterostructure Reviewed

Imura, Masataka; Hayakawa, Ryoma; Watanabe, Ei-Ichiro; Liao, Mei-Yong; Koide, Yasuo; Amano, Hiroshi

Physica Status Solidi RRL: Vol. 5 ( 3 ) page： 125-127. 2011.3

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

This is the first report on an AlN/diamond heterojunction field effect transistor (HFET). The AlN epilayer is grown on oxygen-terminated (111) diamond substrates using metalorg. vapor phase epitaxy at a temp. as high as 1240 °C. The transistor and gate capacitance-voltage characteristics indicate that the HFET behaves as a p-channel FET with a normally-on depletion mode. The HFET channel is located at the AlN/diamond interface, and holes are accumulated in diamond close to the interface. The development of the AlN/diamond HFET creates a new possibility for diamond-based power electronics.

Role of nonradiative recombination centers and extended defects in nonpolar GaN on light emission efficiency Reviewed

Polyakov, A. Y.; Smirnov, N. B.; Govorkov, A. V.; Amano, H.; Pearton, S. J.; Lee, I.-H.; Sun, Q.; Han, J.; Karpov, S. Yu.

Applied Physics Letters Vol. 98 ( 7 ) page： 072104/1-072104/3 2011.2

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

The correlation of integrated microcathodoluminescence efficiency with cryst. quality and deep trap d. of nonpolar GaN films grown by metal org. CVD on semi-insulating 6H-m-SiC or r-sapphire is analyzed. The results suggest a strong influence of nonradiative recombination centers whose concn. decreases with decreased d. of extended defects. Electron traps with energy levels at Ec-0.6 eV and which pin the Fermi level in films with high defect d. are the most likely candidates for the decrease in light emission efficiency in nonpolar GaN.

Internal quantum efficiency and internal loss of ultraviolet laser diodes on the low dislocation density AlGaN underlying layer Reviewed

Takeda, Kenichiro; Nagata, Kengo; Ichikawa, Tomoki; Nonaka, Kentaro; Ogiso, Yuji; Oshimura, Yoshinori; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; et al

Physica Status Solidi C Vol. 8 ( 2 ) page： 464-466 2011.2

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

We fabricated and evaluated GaN/AlGaN multi-quantum wells (MQWs) and UV laser diodes (UV LDs) on high and low dislocation d. underlying layers by epitaxial lateral overgrowth (ELO) method. We analyzed the internal quantum efficiency (IQE) vs. carrier concn. characteristics quant. by excitation intensity dependent photoluminescence method. The IQE of the MQWs on the ELO AlGaN is 75% when the carrier d. is 1 × 1019 cm-3. We demonstrated the UV LD on the ELO AlGaN. However, the UV LD on flat AlGaN did not operate. Also, we investigated the internal loss (α1) and the IQE multiplied the injection efficiency by changing the reflectivity of the facets. The results showed that the internal loss is 6 cm-1, and the IQE multiplied by the injection efficiency is 18%

GaInN-based solar cells using strained-layer GaInN/GaInN superlattice active layer on a freestanding GaN substrate Reviewed

Kuwahara, Yousuke; Fujii, Takahiro; Sugiyama, Toru; Iida, Daisuke; Isobe, Yasuhiro; Fujiyama, Yasuharu; Morita, Yoshiki; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi, Amano Hiroshi; et al

Applied Physics Express Vol. 4 ( 2 ) page： 021001/1-021001/3 2011.2

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

High-performance GaInN-based solar cells with high open-circuit voltage, high short-circuit c.d., and good fill factor have been obtained using a combination of two different GaInN superlattice structures. The GaInN barrier thicknesses (3 and 0.6 nm) in both superlattice structures were optimized, resulting in a thick GaInN-based active layer with a low pit d. in the device. The conversion efficiency is 2.5% under a solar simulator of air-mass 1.5G and an irradn. intensity of 155 mW/cm2.

High-temperature operation of normally off-mode AlGaN/GaN heterostructure field-effect transistors with p-GaN gate Reviewed

Sugiyama, Takayuki; Amano, Hiroshi; Iida, Daisuke; Iwaya, Motoaki; Kamiyama, Satoshi; Akasaki, Isamu

Japanese Journal of Applied Physics Vol. 50 ( 1 ) page： 01AD03/1-01AD03/3 2011.1

MBE-VLS growth of catalyst-free III-V axial heterostructure nanowires on (1 1 1)Si substrates Reviewed

Paek, Jihyun; Yamaguchi, Masahito; Amano, Hiroshi

Journal of Crystal Growth Vol. 323 ( 1 ) page： 315-318 2011.1

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

For application in novel opto-electronic nanowire devices, Ga-assisted GaAs/InxGa1-xAs axial heterostructure nanowires grown on a (1 1 1)Si substrate at a high In/Ga flux ratio were investigated. The increased diam. of the InxGa1-xAs region of the nanowire was obsd. from a SEM image. The In compn. of 0.01-0.02 of the InxGa1-xAs was shown by EDX point anal. The In concn. of 0.62 of an In-Ga alloy droplet was estd. from the diam. ratio of the InxGa1-xAs/GaAs region. From these results, it was considered that the excess In was collected in the droplet, resulting in an increased nanowire diam. The increased diam. of the InxGa1-xAs region was also discussed together with the results of thermodn. calcn.

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Papers - AMANO, Hiroshi