Yasuyuki Kobayashi

Hirosaki University, Khirosaki, Aomori, Japan

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Publications (33)

  • Masanobu Hiroki · Kazuhide Kumakura · Yasuyuki Kobayashi · [...] · Hideki Yamamoto
    [Show abstract] [Hide abstract] ABSTRACT: We fabricated AlGaN/GaN high electron mobility transistors (HEMTs) on h-BN/sapphire substrates and transferred them from the host substrates to copper plates using h-BN as a release layer. In current–voltage characteristics, the saturation drain current decreased by about 30% under a high-bias condition before release by self-heating effect. In contrast, after transfer, the current decrement was as small as 8% owing to improved heat dissipation: the device temperature increased to 50 °C in the as-prepared HEMT, but only by several degrees in the transferred HEMT. An effective way to improve AlGaN/GaN HEMT performance by a suppression of self-heating effect has been demonstrated.
    Article · Jan 2015 · Applied Physics Letters
  • Masanobu Hiroki · Kazuhide Kumakura · Yasuyuki Kobayashi · [...] · Toshiki Makimoto
    [Show abstract] [Hide abstract] ABSTRACT: We demonstrated the transfer of GaN-devices from sapphire substrates to foreign materials using an h-BN release layer. In LEDs transferred onto indium sheets, no discernible degradation of the light emission performance occurred. Remarkably, the self-heating effect was suppressed in HEMTs transferred to copper plates because of improved heat dissipation.
    Conference Paper · Jul 2014
  • Tetsuya Akasaka · Yasuyuki Kobayashi · Makoto Kasu · Hideki Yamamoto
    [Show abstract] [Hide abstract] ABSTRACT: GaN was grown in spiral growth mode by metalorganic vapor phase epitaxy in selective areas having screw-type dislocations. Relationships between the growth rate and supersaturation provide a novel way to estimate the evaporation energy of GaN, which turns out to be carrier gas dependent: 4.3 +/- 0.9 eV for N-2 and 2.1 +/- 0.4 eV for H-2. The latter is significantly smaller, probably due to enhanced etching by H-2. Suppression of excessive nucleation by etching in H-2 may be responsible for the formation of step-free GaN surfaces at low temperatures in selective areas free from screw-type dislocations. (C) 2013 The Japan Society of Applied Physics
    Article · Oct 2013 · Applied Physics Express
  • Tetsuya Akasaka · Hideki Gotoh · Yasuyuki Kobayashi · Hideki Yamamoto
    [Show abstract] [Hide abstract] 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.
    Article · Aug 2012 · Advanced Materials
  • Toshiki Makimoto · Kazuhide Kumakura · Yasuyuki Kobayashi · [...] · Hideki Yamamoto
    [Show abstract] [Hide abstract] 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.
    Article · Jul 2012 · Applied Physics Express
  • Yasuyuki Kobayashi · Kazuhide Kumakura · Tetsuya Akasaka · Toshiki Makimoto
    [Show abstract] [Hide abstract] 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.
    Article · Apr 2012 · Nature
  • Tetsuya Akasaka · Yasuyuki Kobayashi · Makoto Kasu
    [Show abstract] [Hide abstract] 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)].
    Article · Nov 2010 · Applied Physics Letters
  • Yasuyuki Kobayashi · Chiun-Lung Tsai · Tetsuya Akasaka
    [Show abstract] [Hide abstract] 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)
    Article · Jul 2010 · physica status solidi (c)
  • Tetsuya Akasaka · Yasuyuki Kobayashi · Makoto Kasu
    [Show abstract] [Hide abstract] 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.
    Article · Jun 2010 · Applied Physics Express
  • Yasuyuki KOBAYASHI · Chiun-Lung TSAI · Tetsuya AKASAKA
    Article · Jan 2010 · Hyomen Kagaku
  • Tetsuya Akasaka · Yasuyuki Kobayashi · Makoto Kasu
    [Show abstract] [Hide abstract] 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.
    Article · Sep 2009 · Applied Physics Express
  • Tetsuya Akasaka · Yasuyuki Kobayashi · Makoto Kasu
    [Show abstract] [Hide abstract] ABSTRACT: Nonpolar Al(1-x)Ga(x)N (0 <= x <= 0.196) (11 (2) over bar0) films were grown on 4H-SiC (11 (2) over bar0) substrates by metal organic vapor phase epitaxy. Al(1-x)Ga(x)N (0 <= x <= 0.057) films grew almost pseudomorphically on the substrates due to balanced in-plane stresses along [0001] and [1 (1) over bar 00], while Al(1-x)Ga(x)N (0.057 < x) films were strained along [0001] but partially relaxed along [1 (1) over bar 00] due to the absence of the balance. The crystal tilts of the films toward [0001] decreased monotonically with increasing Ga composition due to the correspondence between the (0001) plane distances of the films and the (0002) plane distance of substrates and due to a decrease in the in-plane strain along [0001]. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2995994]
    Article · Oct 2008 · Applied Physics Letters
  • Tetsuya Akasaka · Yasuyuki Kobayashi · Toshiki Makimoto
    [Show abstract] [Hide abstract] 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.
    Article · Jul 2007 · Applied Physics Letters
  • Yasuyuki Kobayashi · Hiroki Hibino · Tomohiro Nakamura · [...] · Nobuo Matsumoto
    [Show abstract] [Hide abstract] 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.
    Article · Apr 2007 · Japanese Journal of Applied Physics
  • Tetsuya Akasaka · Yasuyuki Kobayashi · Toshiki Makimoto
    [Show abstract] [Hide abstract] 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.
    Article · Mar 2007 · Applied Physics Letters
  • Tetsuya Akasaka · Yasuyuki Kobayashi · Toshiki Makimoto
    [Show abstract] [Hide abstract] 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.
    Article · Jan 2007 · Journal of Crystal Growth
  • Tetsuya Akasaka · Hideki Gotoh · Yasuyuki Kobayashi · [...] · Toshiki Makimoto
    [Show abstract] [Hide abstract] 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.
    Article · Sep 2006 · Applied Physics Letters
  • Yasuyuki Kobayashi · Toshiki Makimoto
    [Show abstract] [Hide abstract] 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.
    Article · Apr 2006 · Japanese Journal of Applied Physics
  • Tetsuya Akasaka · Yasuyuki Kobayashi · Toshiki Makimoto
    Conference Paper · Jan 2006
  • Yasuyuki Kobayashi · Hiroki Hibino · Tomohiro Nakamura · [...] · Nobuo Matsumoto
    Conference Paper · Jan 2006

Publication Stats

391 Citations


  • 2014-2015
    • Hirosaki University
      Khirosaki, Aomori, Japan
  • 2008-2013
    • NTT DATA Corporation
      Edo, Tōkyō, Japan
  • 2002
    • Universität Stuttgart
      • Institute of Physics
      Stuttgart, Baden-Württemberg, Germany