Yasuyuki Kobayashi

Nippon Telegraph and Telephone, Edo, Tōkyō, Japan

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Publications (22)85.39 Total impact

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    ABSTRACT: We have successfully released an InGaN/GaN light-emitting diode (LED) from a sapphire growth substrate and transferred it to a piece of commercially available adhesive tape using a mechanical transfer method called ``MeTRe'' (Mechanical Transfer using a Release layer). By this method, a 3-nm-thick hexagonal BN (h-BN) layer inserted between the sapphire substrate and the GaN-based layer acts as both a buffer layer for the growth of a high-quality GaN-based layer and a release layer in the transfer process. A very thin (<0.1 mm) vertical LED prototype wrapped with two pieces of adhesive tape emitted violet-blue light.
    Applied Physics Express 07/2012; 5(7):2102-. · 2.73 Impact Factor
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    ABSTRACT: An ultrathin (one monolayer thick) InN single quantum well (SQW) formed on a step-free GaN surface shows very sharp violet PL emission. The size (16 μm in diameter) is large enough for state-of-the-art nanotechnology to handle. Longer wavelength emissions, such as green and red, are expected by increasing the thickness of the SQW through the utilization of the quantum size effect.
    Advanced Materials 06/2012; 24(31):4296-300. · 14.83 Impact Factor
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    ABSTRACT: Nitride semiconductors are the materials of choice for a variety of device applications, notably optoelectronics and high-frequency/high-power electronics. One important practical goal is to realize such devices on large, flexible and affordable substrates, on which direct growth of nitride semiconductors of sufficient quality is problematic. Several techniques--such as laser lift-off--have been investigated to enable the transfer of nitride devices from one substrate to another, but existing methods still have some important disadvantages. Here we demonstrate that hexagonal boron nitride (h-BN) can form a release layer that enables the mechanical transfer of gallium nitride (GaN)-based device structures onto foreign substrates. The h-BN layer serves two purposes: it acts as a buffer layer for the growth of high-quality GaN-based semiconductors, and provides a shear plane that makes it straightforward to release the resulting devices. We illustrate the potential versatility of this approach by using h-BN-buffered sapphire substrates to grow an AlGaN/GaN heterostructure with electron mobility of 1,100 cm(2) V(-1) s(-1), an InGaN/GaN multiple-quantum-well structure, and a multiple-quantum-well light-emitting diode. These device structures, ranging in area from five millimetres square to two centimetres square, are then mechanically released from the sapphire substrates and successfully transferred onto other substrates.
    Nature 01/2012; 484(7393):223-7. · 38.60 Impact Factor
  • Tetsuya Akasaka, Yasuyuki Kobayashi, Makoto Kasu
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    ABSTRACT: Nucleus and spiral growth mechanisms of GaN were experimentally studied by varying the degree of supersaturation, σ , in selective-area metal organic vapor phase epitaxy. The spiral growth rate of GaN increased proportionally to σ<sup>2</sup> in the σ range from 0.0632 to 0.230. The nucleus growth rate of GaN was much smaller than the spiral one in the σ range. The nucleation rate was almost zero at σ lower than 0.130, suddenly increased at higher σ values, and reached ∼10<sup>7</sup> cm <sup>-2</sup>  s <sup>-1</sup> at σ of 0.230. These results are consistent with a theoretical analysis [W. K. Burton, N. Cabrera, and F. C. Frank, Philos. Trans. R. Soc. London, Ser. A 243, 299 (1951)].
    Applied Physics Letters 11/2010; · 3.52 Impact Factor
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    Tetsuya Akasaka, Yasuyuki Kobayashi, Makoto Kasu
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    ABSTRACT: We used selective-area metalorganic vapor phase epitaxy to study Frank--van der Merwe growth mechanisms of GaN. Step-free GaN surfaces with the diameter of 15--50 μm were fabricated within selective areas free of screw-type dislocations. The growth rate was independent of the area, indi cating multi-nucleation growth. The nucleation rate was in a range of 105--107 cm-2 s-1 and the average two-dimensional nucleus density was 5× 106 cm-2. Selective areas having screw-type dislocations resulted in double growth spirals consisting of monolayer steps. The degree of supersatu ration near the growing surface calculated from the interstep distance was independent of the area.
    Applied Physics Express 06/2010; 3(7):5602. · 2.73 Impact Factor
  • Yasuyuki KOBAYASHI, Chiun-Lung TSAI, Tetsuya AKASAKA
    Hyomen Kagaku 01/2010; 31(2):99-105.
  • Yasuyuki Kobayashi, Chiun-Lung Tsai, Tetsuya Akasaka
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    ABSTRACT: The optical band gap of hexagonal boron nitride (h-BN) epitaxial film is investigated by means of transmittance and reflectance measurements. We grew the h-BN epitaxial film on c-plane sapphire substrate by metalorganic vaper phase epitaxy. X-ray diffraction and reflection high-energy electron diffraction reveal that the h-BN epitaxial film has a smooth surface, and the epitaxial relationship is found to be (0001)h-BN|| (0001)sapphire with an in-plane orientation relationship of [110]h-BN|| [100]sapphire. The photon energy dependence of the squared absorption coefficient is linear, indicating a direct band gap in the h-BN, and the optical band gap is determined to be 5.9 eV. The single-phase h-BN epitaxial film provides a more intrinsic value of the optical band gap than the values estimated for polycrystalline or pyrolytic BN. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2010; 7:1906-1908.
  • Tetsuya Akasaka, Yasuyuki Kobayashi, Makoto Kasu
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    ABSTRACT: Selective-area metalorganic vapor phase epitaxy of GaN has been investigated using the optimized growth conditions for the layer (Frank--van der Merwe) growth and GaN-template substrates with low dislocation density. The surface of a GaN hexagon with 16-mum diameter has a single wide terrace over almost the whole area (step-free surface), when there are no screw-type dislocations in the finite area. Step-free GaN hexagons grew in the two-dimensional nucleus growth mode and had approximately an eight times lower growth rate than that of a GaN film grown in the step-flow mode under the growth conditions used in this study.
    Applied Physics Express - APPL PHYS EXPRESS. 01/2009; 2(9).
  • Tetsuya Akasaka, Yasuyuki Kobayashi, Makoto Kasu
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    ABSTRACT: Nonpolar Al1-xGaxN (0
    Applied Physics Letters 01/2008; 93. · 3.52 Impact Factor
  • Tetsuya Akasaka, Yasuyuki Kobayashi, Toshiki Makimoto
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    ABSTRACT: Nonpolar AlBN (110) and (100) films were grown using flow-rate modulation epitaxy. The transmission electron diffraction and lattice image reflect the wurtzite crystal structure of an AlBN (110) film. The boron compositions in AlBN (110) and (100) films (B ∼ 2%), estimated by x-ray diffraction assuming the wurtzite structure, agree well with the compositions measured by secondary ion mass spectroscopy, indicating that boron atoms are incorporated exactly into the wurtzite lattice sites. The (110) face is more promising than the (100) one for the growth of nonpolar AlBN because it has fewer dangling bonds of nitrogen on the surface.
    Applied Physics Letters 07/2007; 91(4):041914-041914-3. · 3.52 Impact Factor
  • Tetsuya Akasaka, Yasuyuki Kobayashi, Toshiki Makimoto
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    ABSTRACT: Nonpolar AlN (110) and (100) films were grown on SiC substrates by flow-rate modulation epitaxy (FME), wherein trimethylaluminum and NH3 were alternately supplied. FME provides both AlN (110) and (100) films with good crystallinity and smooth surfaces, whereas AlN (100) films obtained by conventional metal-organic chemical vapor deposition exhibit poor crystallinity and rough surfaces with deep trenches consisting of (000) and (101) N-face microfacets. FME effectively eliminates these trenches, because the microfacets are unstable and have faster growth rates because of the enhanced migration of Al atoms in the absence of excess N surface coverage under the Al-rich condition.
    Applied Physics Letters 03/2007; 90(12):121919-121919-3. · 3.52 Impact Factor
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    ABSTRACT: The growth of thin boron nitride (BN) films on graphitized 6H-SiC substrates was investigated in an attempt to reduce the large lattice mismatch between 6H-SiC and BN, which would improve the three-dimensional ordering in BN thin films grown by metalorganic vapor phase epitaxy (MOVPE). BN thin films were grown by low-pressure (300 Torr) MOVPE using triethylboron and ammonia on graphitized 6H-SiC substrates with surfaces displaying (1× 1) reconstruction as determined by low energy electron diffraction (LEED). The (1× 1) surfaces were formed by annealing at 1500 °C in ultrahigh vacuum with a base pressure of 10-10 Torr. The LEED patterns showed that the surfaces were covered with single-crystal graphite several monolayers thick. X-ray diffraction revealed that the c-axis lattice constant of the BN was 6.72 Å, which is close to the 6.66 Å of bulk hexagonal BN. In contrast, BN films grown on non-graphitized 6H-SiC substrates by MOVPE under the same conditions were mostly amorphous. Use of a graphitized 6H-SiC substrate covered with graphite several monolayers thick improves the degree of three-dimensional ordering in BN thin films grown by MOVPE.
    Japanese Journal of Applied Physics 01/2007; 46:2554-2557. · 1.07 Impact Factor
  • Tetsuya Akasaka, Yasuyuki Kobayashi, Toshiki Makimoto
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    ABSTRACT: We discuss the growth mechanism of GaN films and report very high two-dimensional electron gas (2DEG) mobility in AlGaN/AlN/GaN heterostructures fabricated on sapphire using BGaN micro-islands as novel buffers by metalorganic vapor phase epitaxy. The three-dimensional growth of BGaN (formation of BGaN micro-islands) occurs due to the phase separation of BGaN. However, the surface of the overgrown GaN on the BGaN micro-islands becomes smooth and continuous through the epitaxial lateral overgrowth process. The threading dislocations (TDs) in GaN consist mainly of pure edge-type ones and are effectively annihilated using single and double layers of BGaN micro-islands from 2×1010 to 2×109 and 2×108 cm−2, respectively. An n-type GaN film shallowly doped with Si exhibits an electron concentration and high Hall mobility of 3.0×1016 cm−3 and 669 cm2/Vs at room temperature (RT). Very high Hall 2DEG mobility in an Al0.10Ga0.90N/AlN/GaN heterostructure is obtained: 1910 and 20,600 cm2/Vs at RT and 77 K, respectively. The sheet carrier density had almost constant values of 6.9−5.7×1012 cm−2 in the temperature range from 77 to 500 K, indicating that the parallel conduction due to the residual electrons in the GaN underlying layer was negligible.
    Journal of Crystal Growth. 01/2007;
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    ABSTRACT: A series of InGaN quantum wells (QWs) emitting blue-green, blue, violet, or ultraviolet light was grown on InGaN underlying layers (ULs). The potential fluctuation in these InGaN QWs was carefully measured using time-resolved photoluminescence, taking several steps to reduce the quantum confinement Stark effect. The potential fluctuation of InGaN QWs on InGaN ULs was smaller than that on conventional GaN ULs with the identical emission wavelength. A violet-light-emitting diode using an InGaN UL had the electroluminescence intensity approximately five times higher than the one using a conventional GaN UL under the low injection-current conditions, indicating that an InGaN UL effectively eliminates the nonradiative recombination centers in the InGaN QWs.
    Applied Physics Letters 09/2006; 89(10):101110-101110-3. · 3.52 Impact Factor
  • Yasuyuki Kobayashi, Toshiki Makimoto
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    ABSTRACT: Boron nitride (BN) layers on 6H-SiC substrate were grown by metalorganic vapor phase epitaxy (MOVPE) using triethylboron (TEB) and ammonia (NH3). The growth rate of the BN decreased as the NH3 flow rate increased, indicating that a strong parasitic reaction occurred between TEB and NH3. Flow-rate modulation epitaxy (FME), which is based on alternating the gas supply, was applied to the BN growth for the first time and it was found that the parasitic reactions could be effectively reduced. The structural properties of BN grown by FME were also investigated by X-ray diffraction (XRD) and transmission electron microscopy. In contrast with amorphous BN layers grown by MOVPE, the BN structure grown by FME was turbostratic with a weakly preferred orientation to the c-axis.
    Japanese Journal of Applied Physics 01/2006; 45:3519-3521. · 1.07 Impact Factor
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    ABSTRACT: We report discrete photoluminescence (PL) spectra with narrow linewidths from spatially localized excitons obtained in InGaN quantum wells (QWs) that are of a similar structure to those used in conventional optical devices. A micro-PL measurement combined with submicrometer-scale fabrication techniques allows us to create a small number of excitons in a laser spot and to detect the PL from single spatially localized excitons. A sharp PL line (linewidth: 0.34 meV limited by the resolution) is clearly obtained in a 0.2 mum mesa-shaped QW, which originates from a single localized exciton. We show that the technique is a more powerful method of examining excitonic effects than previously reported methods.
    Japanese Journal of Applied Physics 01/2005; 44. · 1.07 Impact Factor
  • Yasuyuki Kobayashi, Tetsuya Akasaka, Naoki Kobayashi
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    ABSTRACT: The thermal stability of low-temperature (LT) GaN and AlN buffer layers during the annealing process in metalorganic vapor phase epitaxy (MOVPE) was monitored in situ by shallow-angle reflectance using p-polarized ultra-violet light (325 nm). A LT-GaN or LT-AlN buffer layer was grown on c-plane sapphire substrates at 600 and 700°C, respectively, by low pressure (76 Torr) MOVPE. These LT-buffer layers were annealed in H2 or N2 carrier gases and their thermal stability was examined. During annealing at 1020°C under NH3 supply with H2 carrier gas, the reflectivity dipped, indicating the desorption of the LT-GaN. In contrast, the reflectivity remained constant during annealing in N2 carrier gas, indicating the LT-GaN layer was stable in N2. The LT-AlN buffer layer was stable even in H2 carrier gas. These results indicate that N2 carrier gas stabilizes the LT-GaN buffer layer during annealing over 1000°C.
    Japanese Journal of Applied Physics 09/1998; 37:L1208-L1210. · 1.07 Impact Factor
  • Yasuyuki Kobayashi, Naoki Kobayashi
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    ABSTRACT: The surface stoichiometry of GaN grown by metal-organic vapor-phase epitaxy (MOVPE) on (0001) sapphire substrate at temperatures up to about 1000°C in N2 and H2 carrier gases was monitored in situ by surface photoabsorption (SPA). In the N2 carrier gas with NH3 supply, a stable N-rich surface was formed at temperatures up to 1030°C. In contrast, the surface in H2 carrier gas was N-rich at temperatures below 850–900°C. Above these temperatures the surface became Ga-rich. These results indicate that GaN MOVPE growth at temperatures around 1000°C proceeds under N-rich and Ga-rich surface stoichiometry in N2 and H2, respectively. The N desorption rate in N2 was lower than the rate in H2, indicating that the N2 carrier gas suppresses the N desorption from the GaN MOVPE surface compared with H2.
    Journal of Crystal Growth 06/1998; 189:301-304. · 1.55 Impact Factor
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    ABSTRACT: Selective metalorganic vapor-phase epitaxy (MOVPE) of GaN and AlxGa1−xN was investigated as a function of growth conditions such as substrate temperature and partial pressure of NH3. GaN hexagonal microprisms (HMPs) with vertical facets were grown when the surface coverage of atomic nitrogen was rather low, while GaN hexagonal micro-pyramids with facets were grown when the surface coverage was higher. This is because the growth rate of a GaN surface is more sensitive to the nitrogen surface coverage than a surface. The GaN HMPs have atomically smooth top surfaces because the growth mode is a balance between adsorption and desorption of film forming precursors and also because of the finite-area effect. HMPs and stripes of Al0.05Ga0.95N with smooth vertical facets were also fabricated. Stimulated emission was observed at room temperature from photopumped GaN HMPs 50 μm in diameter. The longitudinal modes having a 0.33 nm separation indicate an inscribed hexagonal optical path in the GaN HMP.
    Journal of Crystal Growth 06/1998; · 1.55 Impact Factor
  • Yasuyuki Kobayashi, Tetsuya Akasaka, Naoki Kobayashi
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    ABSTRACT: The surface flatness of GaN grown by metalorganic vapor phase epitaxy (MOVPE) on c-plane sapphire substrates was monitored in situ by shallow-angle reflectance using p-polarized ultra-violet light (325 nm). During low-temperature GaN (LT-GaN) buffer growth at 600°C, an oscillation due to an optical interference was observed, indicating the thickness can be controlled in situ to a few nm accuracy. During GaN growth at 1050°C on LT-GaN, clear oscillations and no decrease of reflectivity indicate that the growth proceeds smoothly within a roughness of 10 nm. When both the surface roughness and hexagon density on the surface increased, the oscillation was quickly damped and the reflectivity decreased monotonously. The decrease in reflectivity is proportional to the product of the surface roughness and hexagon density. These results indicate that this method is a powerful tool for monitoring in situ the surface flatness and also for optimizing growth parameters, which are interrelated in GaN MOVPE.
    Journal of Crystal Growth 01/1998; · 1.55 Impact Factor