Faculty Profiles - AMANO, Hiroshi

Papers - AMANO, Hiroshi

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Improvement of Light Extraction Efficiency for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Reviewed

Tetsuhiko Inazu, Shinya Fukahori, Cyril Pernot, Myung Hee Kim, Takehiko Fujita, Yosuke Nagasawa, Akira Hirano, Masamichi Ippommatsu, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, Masahito Yamaguchi, Yoshio Honda, Hiroshi Amano, and Isamu Akasaki

Japanese Journal of Applied Physics Vol. 50 page： 122101 2011.12

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

AlGaN-based deep ultraviolet light-emitting diodes (LEDs) with aluminum reflective electrodes deposited to cover both p- and n-mesh contact electrodes have been fabricated. A 1.55-fold increase in light extraction efficiency has been demonstrated. Despite their reduced contact area, the LEDs exhibited only a slight increase in forward voltage of 0.45 V at 20 mA. Also, their 50% lifetime was estimated to be about 10,000 h at 20 mA DC at room temperature by extrapolation. Owing to the reflective electrodes, a 288 nm LED with external quantum efficiency as high as 5.4% was achieved. The light output power was 4.6 mW at 20 mA.

DOI： 10.1143/JJAP.50.122101

Dependence of Resonance Energy Transfer on Exciton Dimensionality Reviewed

Jan Junis Rindermann, Galia Pozina, Bo Monemar, Lars Hultman, Hiroshi Amano, and Pavlos G. Lagoudakis

Physical Review Letters Vol. 107 page： 236805 2011.11

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

We investigate the dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality. We exploit the excitonic potential disorder in a single quantum well to tune the balance between localized and free excitons by scaling the Boltzmann distribution of excitons through temperature. Theoretical calculations predict the experimentally observed temperature dependence of resonance energy transfer and allow us to quantify the contribution of localized and free excitons. We show that free excitons can undergo resonance energy transfer with an order of magnitude higher rate compared to localized excitons, emphasizing the potential of hybrid optoelectronic devices utilizing resonance energy transfer as a means to overcome charge transfer related limitations.

Fabrication of Nonpolar a-Plane Nitride-Based Solar Cell on r-Plane Sapphire Substrate Reviewed

Tatsuro Nakao, Takahiro Fujii, Toru Sugiyama, Shota Yamamoto, Daisuke Iida, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, Isamu Akasaki, and Hiroshi Amano

Applied Physics Express Vol. ４ page： 101001 2011.10

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

We fabricated and characterized a nonpolar a-plane nitride-based solar cell on an r-plane sapphire substrate. The maximum external quantum efficiency of the solar cell reached 62% at a wavelength of approximately 400 nm. The open-circuit voltage, the short-circuit current density, and the fill factor of the solar cell were 0.9 V, 4.8 mA/cm2, and 57%, respectively. A conversion efficiency of 1.6% was obtained from the solar cell under a solar simulator of air mass 1.5 G and an irradiation intensity of 155 mW/cm2 at room temperature. ©2011 The Japan Society of Applied Physics

DOI： 10.1143/APEX.4.101001

AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates Reviewed

Myunghee Kim, Takehiko Fujita, Shinya Fukahori, Tetsuhiko Inazu, Cyril Pernot, Yosuke Nagasawa, Akira Hirano, Masamichi Ippommatsu, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, Masahito Yamaguchi, Yoshio Honda, Hiroshi Amano, and Isamu Akasaki

Applie Physics Express Vol. 4 page： 092102 2011.9

AlGaN/GaN Heterostructure Field-Effect Transistors on Fe-Doped GaN Substrates with High Breakdown Voltage Reviewed

Yoshinori Oshimura, Takayuki Sugiyama, Kenichiro Takeda, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, Isamu Akasaki and Hiroshi Amano

Japanese Journal of Applied Physics Vol. 50 page： 084102 2011.8

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

lGaN/GaN heterostructure field-effect transistors (HFETs) were fabricated by metalorganic vapor phase epitaxy (MOPVE) on c-plane Fe-doped GaN (GaN:Fe) substrates with different growth conditions of GaN buffer layers. The GaN buffer layers were grown at a V/III ratio of 272 at 300 Torr to realize highly resistive GaN buffer layers with automatic carbon doping. HFETs with carbon-doped GaN buffer layers showed the best characteristics among all the samples in this study. The carbon concentration was 5×1016 cm-3. In the sample with a gate-drain length of 50 µm, the off-state breakdown voltage was 2 kV or higher and the on-resistance was 18.5 mΩ·cm2. We found that the carbon-doped GaN buffer layers are necessary for high breakdown voltages even using semi-insulating GaN:Fe substrates.

S. Khromov, C. G. Hemmingsson, H. Amano, B. Monemar, L. Hultman, and G. Pozina

Pysical Review Vol. B84 page： 075324 2011.8

Reduction in threshold current density of 355 nm UV laser diodes Reviewed

Nagata, Kengo; Takeda, Kenichiro; Nonaka, Kentaro; Ichikawa, Tomoki; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; Amano, Hiroshi; Yoshida, Harumasa; et al

Physica Status Solidi C Vol. ８ ( ５ ) page： 1564-1568 2011.8

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

We have investigated the characteristics of 355 nm UV laser diodes. Two laser diode structures were prepd. on a grooved AlGaN template as well as on a flat AlGaN template. The threading dislocation densities on the grooved and flat AlGaN were 2.5 × 108 cm-2 and 6.5 × 109 cm-2, resp. The threshold c.d. of the laser diode on the grooved AlGaN was 6 kA/cm2, while no lasing was obsd. from the laser diode on the flat AlGaN. We also estd. the internal quantum efficiency as well as the injection efficiency in the UV laser diode structures by considering internal quantum efficiency of both optical and elec. excitation. At a carrier d. of 1.2 × 1019 cm-3, the internal quantum efficiency of the laser diode structure on the grooved AlGaN was about 50%. This is twice that of the laser diode on the flat AlGaN. At the same time, the internal quantum efficiency estd. from the optical excitation also reached about 50%, even at a carrier d. of 3 × 1018 cm-3. This implies that the injection efficiency in our UV laser diode structure was only 25%. Improving the injection efficiency should be effective for significantly reducing the threshold c.d.

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.

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.

Injection efficiency in AlGaN-based UV laser diodes Reviewed

Nagata, Kengo; Takeda, Kenichiro; Oshimura, Yoshinori; Takehara, Kosuke; Aoshima, Hiroki; Ito, Shun; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; et al

Physica Status Solidi C Vol. 8 ( 7-8 ) page： 2384-2386 2011.7

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

We evaluated AlGaN-based 355 nm UV laser diodes prepd. under two different Mg activation conditions. The annealing processes for Mg activation in p-layers were carried out under N2 or O2 ambient to investigate the effect of the ambient gas on the laser characteristics. The threshold current densities and operating voltages of the UV laser diodes were improved by annealing under O2 compared with those under N2. We then estd. the injection efficiencies of the laser diode structures by considering the internal quantum efficiencies of optical excitation and elec. excitation. The internal quantum efficiency of the elec. excited spontaneous emission from the laser structure annealed under O2 reached 50% at a carrier d. of 7.0 × 1018 cm-3, while the structure annealed under N2 required a 1.8-fold higher carrier d. of 1.2 × 1019 cm-3 to reach the same internal quantum efficiency. In addn., the internal quantum efficiency estd. from optical excitation reached 50% even at a carrier d. of 3.0 × 1018 cm-3. This implies that the injection efficiencies in the UV laser diode structures annealed under N2 and O2 were 25% and 45%, resp. Mg activation by O2 annealing is effective for increasing the injection efficiency.

Low leakage current in AlGaN/GaN HFETs with preflow of Mg source before growth of u-GaN buffer layer Reviewed

Oshimura, Yoshinori; Sugiyama, Takayuki; Takeda, Kenichiro; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; Amano, Hiroshi

Physica Status Solidi A Vol. 208 ( 7 ) page： 1607-1610 2011.7

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

AlGaN/GaN heterostructure field-effect transistors were grown by metalorg. vapor phase epitaxy on Fe-doped GaN substrates with a preflow of the Mg source. We realized a low drain leakage current, 1.05 μA/mm, at VDS = 20 V and VGS = -5 V with LGD = 3 μm. This leakage current was one order of magnitude lower than that obtained without a preflow of the Mg source. The breakdown voltage was over 250 V with LGD = 10 μm. The on resistance was estd. to be 3.6 mΩ cm2. No significant redistribution or memory effect of Mg was obsd. by secondary ion mass spectroscopy measurement.

Optical properties of (1-101) InGaN/GaN MQW stripe laser structure on Si substrate Reviewed

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

Physica Status Solidi C Vol. 8 ( 7-8 ) page： 2160-2162 2011.7

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

A smooth laser structure on (1-101) GaN microstripes was successfully fabricated on a patterned (100) 8°-off Si substrate. The sample has a high internal quantum efficiency (IQE) compared with blue LEDs on a sapphire substrate. The high-d. excitation PL spectrum obsd. from the cross section of the sample shows marked narrowing, and the integral PL intensity also shows a steep increase. These results indicate the onset of stimulated emission from an InGaN-based multi quantum well (MQW) active layer on (1-101) GaN (100) Si. To the best of our knowledge, this is the first report of the observation of a stimulated emission from a (1-101) MQW on a Si substrate. We succeeded in fabricating and verifying the high-quality laser structure on semipolar (1-101) GaN on a Si substrate.

Optimization of initial MOVPE growth of non-polar m- and a-plane GAN on Na flux grown LPE-GaN substrates 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 C Vol. 8 ( 7-8 ) page： 2095-2097 2011.7

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

We optimized the initial GaN growths of nonpolar m- and a-plane GaN grown on liq. phase epitaxy (LPE-) GaN substrates. Such nonpolar LPE-GaN substrates were produced by cutting slices from c-plane LPE-GaN bulk single crystals grown by the Na-flux method. We investigated the qualities of epitaxially grown GaN layers on the LPE-GaN substrates, with different polishing methods, which are chem. mech. polishing and plasma dry etch polishing. We found that the cryst. quality of epitaxial GaN on m-plane GaN is very sensitive to the surface polishing even the minute unevenness in GaN substrate. Our expts. also indicated that a low initial growth rate was necessary to obtain high-cryst.-quality epitaxial m-plane GaN. In contrast, high-cryst.-quality a-plane GaN was obtained even with a high initial growth rate, indicating that the cryst. quality of a-plane GaN is not sensitive to surface roughness.

Selective MOVPE growth of InGaN/GaN MQW on microfacet GaN stripes Reviewed

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

Physica Status Solidi C Vol. 8 ( 7-8 ) page： 2038-2040 2011.7

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

We demonstrated the effects of growth conditions such as growth pressure and growth temp. during the selective growth of InGaN/GaN multiple quantum wells (MQWs) on semipolar GaN microstripes on Si substrates. Grown layers exhibited the ridge growth, which indicates the thickness fluctuation, owing to the source supply from the apexes of the GaN microstructure. Ridge growth was suppressed by increasing growth pressure and decreasing growth temp. Cathode luminescence (CL) anal. showed uniform luminescence. As a result, high pressure and low temp. are suitable for obtaining uniform heterostructures in the selective growth of InGaN/GaN MQWs on GaN microstripes.

Transparent electrode for UV light-emitting-diodes Reviewed

Takehara, Kosuke; Takeda, Kenichiro; Nagata, Kengo; Sakurai, Hisashi; Ito, Shun; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; Amano, Hiroshi

Physica Status Solidi C Vol. 8 ( 7-8 ) page： 2375-2377 2011.7

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

We have investigated the transparent electrodes using indium tin oxide (ITO) for UV light-emitting diodes (LEDs). The carrier concn. of ITO was increased from 1.1 × 1018 to 1.5 × 1021 cm-3 after annealing under N2 ambient at 600 °C. The fundamental absorption edge was also shifted to a shorter wavelength owing to the increase in carrier concn., through the so-called the Burstein-Moss effect. The annealed ITO forms an ohmic contact with p-GaN with a specific contact resistance of 1.2 × 10-3 Ωcm2. The light output power of a 350 nm LED with the annealed ITO contact is 1.2 times higher than that with a conventional Ni/Au contact.

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.

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.

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Papers - AMANO, Hiroshi