Faculty Profiles - AMANO, Hiroshi

Papers - AMANO, Hiroshi

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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.

Realization of nitride-based solar cell on freestanding GaN substrate Reviewed

Kuwahara, Yosuke; Fujii, Takahiro; Fujiyama, Yasuharu; Sugiyama, Tohru; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu; Amano, Hiroshi

Applied Physics Express Vol. 3 ( 11 ) page： 111001/1-111001/3 2010.11

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

A p-i-n solar cell composed of an undoped GaInN layer sandwiched between n- and p-type GaN layers was grown on a freestanding c-plane GaN substrate and a c-plane sapphire substrate covered with a low-temp.-deposited buffer layer. The open-circuit voltage is 2.23 V, the fill factor is 61%, the short-circuit c.d. is 1.59 mA/cm2, and the conversion efficiency of the solar cell on the GaN substrate is 1.41% using a solar simulator (1.5 suns). Compared with the solar cell characteristics of the device grown on the sapphire substrate, pit d. markedly decreases. As a result, shunt resistance increases, suppressing the open-circuit voltage drop.

Strain relaxation mechanisms in AlGaN epitaxy on AlN templates Reviewed

Wu, Zhihao; Nonaka, Kentaro; Kawal, Yohjiro; Asai, Toshiaki; Ponce, Fernando A.; Chen, Changqing; Iwaya, Motoaki; Kamiyama, Satoshi; Amano, Hiroshi; Akasaki, Isamu

Applied Physics Express Vol. 3 ( 11 ) page： 111003/1-111003/3 2010.11

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

Two strain relaxation processes have been obsd. in AGaN layers grown on thick AlN templates. In Process I, a type threading dislocations (TDs) with b = 1/3.ltbbrac.11.hivin.20.rtbbrac. from the AlN underlayer are inclined away from the [0001] axis toward the .ltbbrac.1.hivin.100.rtbbrac. directions when they enter the AlGaN film, forming dislocation line projection perpendicular to the Burger vector. In Process II, a + c-type TDs from the AlN underlayer with Burgers vector of b = 1/3.ltbbrac.11.hivin.23.rtbbrac. glide on {0.hivin.111} planes when they enter the AlGaN film to generate interfacial misfit dislocations lying along the .ltbbrac..hivin.2110.rtbbrac. directions at the AlGaN/AlN interface.

Temperature dependence of normally off mode AlGaN/GaN heterostructure field-effect transistors with p-GaN gate Reviewed

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

Physica Status Solidi C: Vol. 7 ( 10 ) page： 2419-2422 2010.10

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

We demonstrated high-temp. operation of normally off-mode junction-heterostructure-field-effect transistors (JHFETs) with a p-GaN gate that shows a very small shift of the threshold voltage against ambient temp. Distinct normally off-mode operation with a max. drain current of 93.2 mA/mm at 300 °C was realized. Therefore, normally off-mode GaN-based JFETs are greatly superior to Si-based devices as high-temp. switching devices.

GaInN/GaN p-i-n light-emitting solar cells Reviewed

Fujiyama, Y.; Kuwahara, Y.; Iwaya, M.; Kamiyama, S.; Amano, H.; Akasaki, I.

Physica Status Solidi C Vol. 7 ( 10 ) page： 2382-2385. 2010.10

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

GaInN/GaN p-i-n double-heterojunction structures were grown by metal-org. vapor phase epitaxy on c-plane sapphire substrates, and light-emitting solar cells with different GaInN active layer thicknesses were fabricated. The thickness of the GaInN active layer was varied from 100 to 400 nm, while the thickness of both n-type and p-type GaN layers was kept const. A semitransparent ohmic contact to p-type GaN was formed by electron-beam evapn. of Ni/Au (5 nm/5 nm). The film thickness of p-GaN was 50 nm. The external quantum efficiency (EQE) of the device with a 400 nm GaInN layer exceeded 60% at a wavelength of approx. 380 nm. The transparency of the Ni/Au electrode was 68%. Therefore, the internal quantum efficiency of this device exceeded 95%. Note that this device emitted green light when a forward voltage was applied. The electroluminescence peak wavelength was approx. 525 nm, which was much longer than the EQE peak wavelength. The origin of this large emission shift is discussed.

Analysis of polar direction of AlN grown on (0001) sapphire and 6H-SiC substrates by high-temperature metal-organic vapor phase epitaxy using coaxial impact collision ion scattering spectroscopy Reviewed

Imura, Masataka; Ohnishi, Tsuyoshi; Sumiya, Masatomo; Liao, Meiyong; Koide, Yasuo; Amano, Hiroshi; Lippmaa, Mikk

Physica Status Solidi C Vol. 7 ( 10 ) page： 2365-2367 2010.10

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

High-cryst. quality AlN layers with an atomically flat surface are grown on (0001) sapphire and 6H-SiC substrates by metal-org. vapor phase epitaxy at 1400 °C without low-temp. buffer layer. The polar direction of AlN layers is investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS). For AlN growth on sapphire substrate, trimethylaluminum (TMAI) is supplied for 10 s before introducing ammonia (NH3). For AlN growth on SiC substrate, TMAl and NH3 are supplied at the same time. The CAICISS spectrum of AlN layers is analyzed by measuring the dependence of the Al atoms signal intensity on the angle of ion beam incidence considering the shadowing and focusing effects for detg. the polar direction of AlN layers. The CAICISS spectra clearly indicate the polar direction of AlN layers and both AlN layers are found to be had the Al-polarity.

Atomic layer epitaxy of AlGaN Reviewed

Nagamatsu, Kentaro; Iida, Daisuke; Takeda, Kenichiro; Nagata, Kensuke; Asai, Toshiaki; Iwaya, Motoaki; Kamiyama, Satoshi; Amano, Hiroshi; Akasaki, Isamu

Physica Status Solidi C: Vol. 7 ( 10 ) page： 2368-2370 2010.10

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

Atomic layer epitaxy of AlGaN with an av. growth rate of as high as 4.5 mm/h has been achieved by the high-speed switching-valves (HSSVs) technique in a raised-pressure metalorg. vapor phase epitaxy system. Regarding the duration of group III metalorgs. and NH3 gases input, 0.1 s for each injection time is found to be sufficient to realize AlGaN growth of one monolayer per cycle over a 2-in. wafer with hydrogen and nitrogen carrier gases at a raised pressure. Low-temp. growth of high-quality AlGaN has been achieved using HSSVs.

Threshold voltage control using SiNx in normally off AlGaN/GaN HFET with p-GaN gate Reviewed

Sugiyama, T.; Iida, D.; Iwaya, M.; Kamiyama, S.; Amano, H.; Akasaki, I.

Physica Status Solidi C Vol. 7 ( 7-8 ) page： 1980-1982 2010.7

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

The threshold voltage (Vth) of normally off-mode AlGaN/GaN junction heterostructure field-effect transistors with a p-type GaN gate can be successfully controlled by inserting a SiNx insulator between the p-GaN and a Ni/Au electrode. The Vth can be controlled from +1 V to above +8 V. Moreover, the gate leakage current of transistors decreases and their gate voltage at which gate current steeply increases becomes higher. The mechanism of the threshold voltage change is analyzed by the equiv. circuit model.

Mg-related acceptors in GaN Reviewed

Monemar, B.; Paskov, P. P.; Pozina, G.; Hemmingsson, C.; Bergman, J. P.; Amano, H.; Akasaki, I.; Figge, S.; Hommel, D.; et al.

Physica Status Solidi C Vol. 7 ( 7-8 ) page： 1850-1852 2010.7

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

Photoluminescence spectra of c-plane Mg doped GaN samples grown by MOVPE on bulk GaN templates reveal previously unknown properties, like the presence of several Mg-related acceptors. The use of unstrained samples allows a study of both bound exciton (BE) and donor-acceptor pair (DAP) spectra. Two main acceptors A1 and A2 are obsd. strongly in BE spectra as well as in DAP spectra, they have similar binding energies, i.e. about 220 meV. The common assignment of the deeper blue PL emission at 2.8-3.0 eV to a deep donor-shallow acceptor transition is questioned, and discussed in connection with the compensation problem in p-GaN. It seems like the Fermi level in p-GaN is controlled by a set of Mg-related acceptors at energies 0.2-0.6 eV from the valence band top.

Nitride-based light-emitting solar cell Reviewed

Kuwahara, Y.; Fujiyama Y; Iwaya, M.; Kamiyama, S.; Amano, H.; Akasaki, I

Physica Status Solidi C Vol. ７ ( 7-8 ) page： 1807-1809 2010.7

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

We report on the characteristics of a group-III-nitride-based solar cell. A GaInN/GaN double heterojunction p-i-n solar cell composed of a GaInN active layer sandwiched between n-type and p-type GaN layers was grown by metal-org. vapor phase epitaxy through an undoped GaN layer on (0001) sapphire substrate. The InN molar fraction of the GaInN active layer was detd. to be 11% from a secondary ion mass spectrometry profile. The max. external quantum efficiency reached 65%, and the internal quantum efficiency was 95% or more. This device emitted light after forward current injection. The PL peak almost corresponded to the absorption edge. However, the electroluminescence (EL) peak was at the wavelength much longer than the absorption edge. The open-circuit voltage is 1.51 V, the fill factor is 52%, and the short-circuit c.d. is 1.6 mA/cm2.

Study of two-dimensional electron gas in AlGaN channel HEMTs with high crystalline quality Reviewed

Hashimoto, Shin; Akita, Katsushi; Tanabe, Tatsuya; Nakahata, Hideaki; Takeda, Kenichiro; Amano, Hiroshi

Physica Status Solidi C Vol. 7 ( 7-8 ) page： 1938-1940 2010.7

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

Epitaxial structures of AlGaN channel high electron mobility transistors (HEMTs) were grown on sapphire and AlN substrates. Redn. in the full width at half max. of X-ray rocking curve for (10-12) peak of the AlGaN channel layer owing to the redn. of threading dislocation densities resulted in a sharp decrease in the sheet resistance of 2-dimensional electron gas (2DEG). In the case of AlGaN channel HEMTs, it was found that improvement of the cryst. quality of AlGaN channel layers is essential to the redn. of the sheet resistance of 2DEG. The use of AlN substrates resulted in improved cryst. quality of the AlGaN layer and lower 2DEG resistance, suggesting the high potential of AlN substrates for AlGaN channel HEMTs.

Internal quantum efficiency of GaN/AlGaN-based multi quantum wells on different dislocation densities underlying layers Reviewed

Takeda, Kenichiro; Mori, Fumiaki; Ogiso, Yuji; Ichikawa, Tomoaki; Nonaka, Kentaro; Iwaya, Motoaki; Kamiyama, Satoshi; Amano, Hiroshi; Akasaki, Isamu

Physica Status Solidi C Vol. 7 ( 7-8 ) page： 1916-1918 2010.7

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

Internal quantum efficiency (IQE) of GaN/AlGaN multi quantum wells on the low dislocation d. Al0.25Ga0.75N grown by epitaxial lateral overgrowth (ELO) and facet controlled epitaxial lateral overgrowth were investigated by excitation intensity dependent photoluminescence measurement. The threading dislocation d. decreased from 4 × 109cm-2 to 2 × 108 cm-2 by using ELO method, then the IQE was much improved from 5% to 40% when the carrier d. was 1 × 1018 cm-3.

AlGaN/GaN HFETs on Fe-doped GaN substrates Reviewed

Oshimura, Yoshinori; Takeda, Kenichiro; Sugiyama, Takayuki; Iwaya, Motoaki; Kamiyama, Satoshi; Amano, Hiroshi; Akasaki, Isamu; Bandoh, Akira; Udagawa, Takashi

Physica Status Solidi C Vol. ７ ( ７－８ ) page： 1974-1976 2010.7

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

AlGaN/GaN HFETs with different undoped GaN thicknesses were grown on Fe-doped freestanding GaN substrates by conventional MOVPE. To realize a high drain current, thick undoped GaN is found to be necessary. SIMS measurement shows that Fe is redistributed into the epilayer, by which the scattering center is generated at the channel when the thickness of the undoped GaN is insufficient. We also obsd. a similar Fe profile in the GaN/sapphire template placed on the side of the Fe-doped GaN substrate during growth. Therefore, Fe in the Fe-doped GaN substrate is redistributed not only through a solid but also through vapor.

Growth and characterization of GaN grown on moth-eye patterned sapphire substrates Reviewed

Ishihara, Akihiro; Kawai, Ryousuke; Kitano, Thukasa; Suzuki, Atushi; Kondo, Toshiyuki; Iwaya, Motoaki; Amano, Hiroshi; Kamiyama, Satoshi; Akasaki, Isamu

Physica Status Solidi C Vol. 7 ( 7-8 ) page： 2056-2058 2010.7

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

To realize high-efficiency light-emitting diodes (LEDs), it is necessary to increase light extn. efficiency. Therefore, the introduction of a moth-eye structure, which consists of periodic cones with a pitch of optical wavelength magnitude, into a sapphire surface before the epitaxial growth of nitride films is very promising for reducing the reflectivity of light resulting in high light extn. efficiency. 450 nm GaInN/GaN LEDs were fabricated on conventional and moth-eye substrates by metal org. vapor phase epitaxy (MOVPE). The intensity of room-temp. photoluminescence emitted from the LED with the moth-eye-patterned sapphire substrate was 1.6 times higher than that emitted from the LED without the moth-eye substrate. Under a current injection of 50 mA, the output power of the moth-eye LED is 3.6 times higher than that of the conventional LED.
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Growth of GaInN by raised-pressure metalorganic vapor phase epitaxy Reviewed

Iida, Daisuke; Nagata, Kensuke; Makino, Takafumi; Iwaya, Motoaki; Kamiyama, Satoshi; Amano, Hiroshi; Akasaki, Isamu; Bandoh, Akira; Udagawa, Takashi

Applied Physics Express Vol. 3 ( 7 ) page： 075601/1-075601/2 2010.7

Growth of low-dislocation-density AlGaN using Mg-doped AlN underlying layer Reviewed

Asai, T.; Nonaka, K.; Ban, K.; Nagata, K.; Nagamatsu, K.; Iwaya, M.; Kamiyama, S.; Amano, H.; Akasaki, I.

Physica Status Solidi C Vol. 7 ( 7-8 ) page： 2101-2103 2010.7

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

We report on a new technol. for growing low-dislocation-d. AlGaN in which a Mg-doped AlN underlying layer is used. By growing AlxGa1-xN on AlN:Mg with AlN molar fractions x of 0.3, 0.5 and 0.7, the d. of misfit dislocations is much reduced compared with that in the case of growing AlGaN on undoped AlN. In addn., the surface becomes atomically flat.

Improved efficiency of 255-280 nm AlGaN-based light-emitting diodes Reviewed

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

Applied Physics Express Vol. 3 ( 6 ) page： 061004/1-061004/3 2010.6

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

We report on the fabrication and characterization of AlGaN-based deep UV light-emitting diodes (LEDs) with the emission wavelength ranging from 255-280 nm depending on the Al compn. of the active region. The LEDs were flip-chip bonded and achieved external quantum efficiencies of over 3% for all investigated wavelengths. Under cw operation, an output power of more than 1 mW at 10 mA was demonstrated. A moth-eye structure was fabricated on the back side of the Al2O3 substrate, and on-wafer output power measurement indicated a 1.5-fold improvement of light extn.

Compensation effect of Mg-doped a- and c-plane GaN films grown by metalorganic vapor phase epitaxy Reviewed

Iida, Daisuke; Tamura, Kenta; Iwaya, Motoaki; Kamiyama, Satoshi; Amano, Hiroshi; Akasaki, Isamu

Journal of Crystal Growth Vol. 312 ( 21 ) page： 3131-3135. 2010.6

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

The elec. and optical properties of Mg-doped a- and c-plane GaN films grown by MOVPE were systematically investigated. The photoluminescence spectra of Mg-doped a- and c-plane GaN films exhibit strong emissions related to deep donors when Mg doping concns. are above 1 × 1020 cm-3 and 5 × 1019 cm-3, resp. The elec. properties also indicate the existence of compensating donors because the hole concn. decreases at such high Mg doping concns. In addn., we estd. the ND/NA compensation ratio of a- and c-plane GaN by variable-temp. Hall effect measurement. The obtained results indicate that the compensation effect of the Mg-doped a-plane GaN films is lower than that of the Mg-doped c-plane GaN films.

Misfit strain relaxation in m-plane epitaxy of InGaN on ZnO Reviewed

Wu, Z. H.; Sun, K. W.; Wei, Q. Y.; Fischer, A. M.; Ponce, F. A.; Kawai, Y.; Iwaya, M.; Kamiyama, S.; Amano, H.; Akasaki, I.

Applied Physics Letters Vol. 96 ( ７ ) page： 071909/1-071909/3 2010.3

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

The microstructure of m-plane InGaN epilayers grown on m-plane ZnO has been found to depend significantly on indium content in the range from 0.07 to 0.17, where anisotropic lattice mismatch between InGaN and ZnO results in decreasingly tensile and increasingly compressive stress along the a and c lattice axes. For indium content below 0.10, periodic arrays of misfit dislocations with a Burgers vector of 1/3[11-20] are obsd. parallel to the [0001] direction. For indium content above 0.12, generation of basal-plane stacking faults relieve the compressive stress along the 0001 direction. These characteristic mechanisms of strain relaxation should provide new approaches to engineer thick InGaN layers with reduced lattice misfit strain.

Microstructure of AlN with two-domain structure on (001) diamond substrate grown by metal-organic vapor phase epitaxy Reviewed

Imura, Masataka, Nakajima, Kiyomi, Liao, Meiyong, Koide, Yasuo, Amano, Hiroshi

Diamond and Related Materials Vol. 19 ( 0 ) page： 131-133 2010.2

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

We investigate the microstructures of domain boundaries in an AlN layer grown on a (001) diamond substrate by metal-org. vapor phase epitaxy. The AlN layer has a two-domain structure with crystal orientation along either <112-0> AlN [named by AlNI domain] or <101-0> AlN [named by AlNII domain] parallel to [110] direction of diamond. The AlNI and AlNII domains are not atomically bonded at two-domain boundary from initial to final step of growth, while an edge-type dislocation is generated at single-domain boundary (SDB). In addn., an inversion AlNI domain [named by AlNI*] is randomly-ordered at the initial stage of the coalescence between the AlNI domains. The AlNI* is easily terminated with increasing the thickness of AlNI domain. The inversion domain boundary changes to the edge-type dislocation at the SDB with further growth, which reduces the defect d. in the AlNI domains.

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Papers - AMANO, Hiroshi