High-Power Continuous-Wave Operation of BeZnCdSe Single-Quantum-Well Green Laser Diodes

2012-05-18T09:00:00+09:00

Authors: Sumiko Fujisaki, Jun-ichi Kasai, Ryouichi Akimoto, Shigehisa Tanaka, Shinji Tsuji, Toshifumi Hasama, and Hiroshi Ishikawa
The laser performance of BeZnCdSe single-quantum-well green laser diodes (LDs) was improved. In particular, high-power lasing over 50 mW was achieved. The high-resolution lasing spectrum indicates a peak wavelength of 536 nm. The far-field patterns (FFPs) of the 5-µm-wide mesa LDs show unimodal characteristics, and the perpendicular and horizontal beam-divergence angles for full width at half maximum are 21 and 2.2°, respectively. The wall-plug efficiency (WPE) of the 2-µm-wide mesa LDs is 3.6% at an optical output power of 50 mW. The internal loss (α_{i}) and internal differential quantum efficiency (η_{i}) of the 10-µm-wide-mesa LDs were estimated to be 3.5 cm^{-1} and 0.22, respectively.©2012 The Japan Society of Applied Physics

Current-Induced Domain Wall Motion in Perpendicularly Magnetized Co/Ni Nanowire under In-Plane Magnetic Fields

2012-05-17T09:00:00+09:00

Authors: Yoko Yoshimura, Tomohiro Koyama, Daichi Chiba, Yoshinobu Nakatani, Shunsuke Fukami, Michihiko Yamanouchi, Hideo Ohno, and Teruo Ono
We have investigated current-induced domain wall (DW) motion in a perpendicularly magnetized Co/Ni nanowire under in-plane (hard-axis) and perpendicular (easy-axis) external magnetic fields. The DW velocity was found to be almost independent of them in the range of ±50 Oe. The result shows that reliable device operation against an external magnetic field disturbance can be achieved using the present system.©2012 The Japan Society of Applied Physics

X-ray Microbeam Three-Dimensional Topography Imaging and Strain Analysis of Basal-Plane Dislocations and Threading Edge Dislocations in 4H-SiC

Ryohei Tanuma, Daisuke Mori, Isaho Kamata, and Hidekazu Tsuchida — 2012-05-16T09:00:00+09:00

Authors: Ryohei Tanuma, Daisuke Mori, Isaho Kamata, and Hidekazu Tsuchida
This paper demonstrates the X-ray microbeam three-dimensional (3D) topography of basal-plane dislocations (BPDs) and threading edge dislocations (TEDs) in 4H-SiC. Stereographic images showing the propagation of BPDs from a substrate to an epilayer and the conversion of BPDs into TEDs near the epilayer/substrate interface are successfully obtained. The narrowing of BPD images is observed just before the BPD-TED conversion points. The images of effective misorientations Δω provide a spatial resolution of 1–2 µm for a TED, and the range of Δω corresponds to strains on the order of ±10^{-5}. We also discuss the image-formation mechanism in 3D topography.©2012 The Japan Society of Applied Physics

A Self-Aligned InGaAs Quantum-Well Metal–Oxide–Semiconductor Field-Effect Transistor Fabricated through a Lift-Off-Free Front-End Process

Jianqiang Lin, Tae-Woo Kim, Dimitri A. Antoniadis, and Jesús A. del Alamo — 2012-05-16T09:00:00+09:00

Authors: Jianqiang Lin, Tae-Woo Kim, Dimitri A. Antoniadis, and Jesús A. del Alamo
We present a novel n-type InGaAs quantum-well metal–oxide–semiconductor field-effect transistor (QW-MOSFET) fabricated by a self-aligned gate-last process and investigate relevant Si-like manufacturing issues in future III–V MOSFETs. The device structure features a composite InP/Al_{2}O_{3} gate barrier with a capacitance equivalent thickness (CET) of 3 nm and non alloyed Mo ohmic contacts. We have found that RIE introduces significant damage to the intrinsic device resulting in poor current drive and subthreshold swing. The effect is largely removed through a thermal annealing step. Thermally annealed QW-MOSFETs exhibit a subthreshold swing of 95 mV/dec, indicative of excellent interfacial characteristics. The peak mobility of the MOSFET is 2780 cm^{2} V^{-1} s^{-1}.©2012 The Japan Society of Applied Physics

Diffusion and Stability of Hydrogen in Mg-Doped GaN: A Density Functional Study

Ji-Sang Park and Kee Joo Chang — 2012-05-16T09:00:00+09:00

Authors: Ji-Sang Park and Kee Joo Chang
Using hybrid functional calculations, we study the diffusion and thermal stability of hydrogen in Mg-doped GaN. Compared with the generalized gradient approximation, we obtain a higher activation barrier for dissociating a Mg–H complex, which is attributed to the increase in the binding energy of Mg–H. Kinetic Monte Carlo simulations yield the annealing temperature of around 800 °C for activating Mg acceptors, close to the measured values. The results provide an insight to understanding the annealing effect such that the annealing temperature generally increases with the Mg–H concentration, and the retrapping of H is partly responsible for the low doping efficiencies at high Mg concentrations.©2012 The Japan Society of Applied Physics

21.7 kV 4H-SiC PiN Diode with a Space-Modulated Junction Termination Extension

Hiroki Niwa, Jun Suda, and Tsunenobu Kimoto — 2012-05-15T09:00:00+09:00

Authors: Hiroki Niwa, Jun Suda, and Tsunenobu Kimoto
Ultrahigh-voltage 4H-SiC mesa PiN diodes are fabricated and characterized. An original space-modulated two-zone junction termination extension (SM-two-zone JTE) has realized a laterally tapered profile of the JTE dose, which enlarged the tolerance to the deviation of effective JTE dose compared with a conventional two-zone JTE. We demonstrate a SiC PiN diode with a breakdown voltage of 21.7 kV (81% of the ideal breakdown voltage calculated from the epilayer structure), which is the highest breakdown voltage among any semiconductor devices ever reported.©2012 The Japan Society of Applied Physics

Fast Two-Dimensional Ferritin Crystal Formation Realized by Mutant Ferritin and Poly(ethylene glycol) Modified SiO_{2} Substrate

Rikako Tsukamoto, Makoto Igarashi, Seiji Samukawa, and Ichiro Yamashita — 2012-05-15T09:00:00+09:00

Authors: Rikako Tsukamoto, Makoto Igarashi, Seiji Samukawa, and Ichiro Yamashita
The adsorption behavior of mutant ferritins, which have 24 carbonaceous material binding peptides, onto a polymer-coated SiO_{2} substrate was studied. A SiO_{2} surface was modified by poly(L-lysine) (PLL) and poly(ethylene glycol) (PEG) and the spin-coating technique was used. The state of the ferritin array on PLL-coated SiO_{2} was random adsorption, however, many adjacent domains of hexagonal close-packed ferritin arrays about 100–200 nm in diameter were formed on PEG-coated SiO_{2}. PEG and the protein portion of ferritin were removed by heat treatment. The fabricated dispersed hexagonal close packed nanodot arrays on SiO_{2} substrates can be applied to semiconductor devices.©2012 The Japan Society of Applied Physics

Strain-Balanced InGaAs/GaAsP Superlattice Solar Cell with Enhanced Short-Circuit Current and a Minimal Drop in Open-Circuit Voltage

Yun Peng Wang, Kentaroh Watanabe, Yu Wen, Masakazu Sugiyama, and Yoshiaki Nakano — 2012-05-11T09:00:00+09:00

Authors: Yun Peng Wang, Kentaroh Watanabe, Yu Wen, Masakazu Sugiyama, and Yoshiaki Nakano
A strain-balanced InGaAs/GaAsP superlattice (SL) was inserted in the intrinsic region of a p–i–n GaAs to adjust the performance of a photovoltaic device. The SL cell exhibited an excellent enhancement in short-circuit current (I_{sc}) of 3.0 mA/cm^{2}, while maintaining a minimum drop of open-circuit voltage (V_{oc}) of 0.03 V compared with those of a control GaAs p–i–n cell. For the device fabrication, the implementation of a strain-balanced epitaxy by using an interface managing (IM) technique is a primary breakthrough to obtain SL with a high crystal quality. The formation of superlattice may enable a hybrid carrier extraction mechanism, combining tunneling and thermal hopping, which may enhance the carrier transportation within the SL region.©2012 The Japan Society of Applied Physics

Preparation of a Freestanding AlN Substrate from a Thick AlN Layer Grown by Hydride Vapor Phase Epitaxy on a Bulk AlN Substrate Prepared by Physical Vapor Transport

2012-05-11T09:00:00+09:00

Authors: Yoshinao Kumagai, Yuki Kubota, Toru Nagashima, Toru Kinoshita, Rafael Dalmau, Raoul Schlesser, Baxter Moody, Jinqiao Xie, Hisashi Murakami, Akinori Koukitu, and Zlatko Sitar
The structural and optical quality of a freestanding AlN substrate prepared from a thick AlN layer grown by hydride vapor phase epitaxy (HVPE) on a bulk (0001)AlN substrate prepared by physical vapor transport (PVT) were investigated. The prepared HVPE-AlN substrate was crack- and stress-free. High-resolution X-ray diffraction ω-rocking curves of symmetric (0002) and skew-symmetric (10-11) reflections had small full widths at half maximum (FWHMs) of 31 and 32 arcsec, respectively. Deep-ultraviolet optical transparency of the HVPE-AlN substrate was higher than that of the PVT-AlN substrate, which was related to lower concentrations of C, O impurities, and Al vacancy.©2012 The Japan Society of Applied Physics

Head-On Inverse Compton Scattering X-rays with Energy beyond 10 keV from Laser-Accelerated Quasi-Monoenergetic Electron Bunches

2012-05-11T09:00:00+09:00

Authors: Yoshitaka Mori, Hajime Kuwabara, Katsuhiro Ishii, Ryohei Hanayama, Toshiyuki Kawashima, and Yoneyoshi Kitagawa
Inverse Compton X-rays from laser-accelerated multiple electron bunches are observed. A Ti:sapphire laser (pulse energy: 500 mJ; pulse width: 150 fs) beam is divided into two beams. The main beam is focused onto an edge of a helium gas jet to accelerate electrons to energies of 14 and 23 MeV, which inversely scattered the head-on colliding secondary laser beam into 6 and 12 keV X-rays; this agrees well with that calculated from the electron spectra obtained. This demonstrates a first on-axis inverse Compton scattering X-ray energy detection beyond 10 keV induced by laser-accelerated electrons.©2012 The Japan Society of Applied Physics

Thermoelectric Properties of Mineral Tetrahedrites Cu_{10}Tr_{2}Sb_{4}S_{13} with Low Thermal Conductivity

Koichiro Suekuni, Kojiro Tsuruta, Tomoki Ariga, and Mikio Koyano — 2012-05-10T09:00:00+09:00

Authors: Koichiro Suekuni, Kojiro Tsuruta, Tomoki Ariga, and Mikio Koyano
We have investigated thermoelectric properties of synthesized mineral Cu_{10}Tr_{2}Sb_{4}S_{13} (Tr = Mn, Fe, Co, Ni, Cu, and Zn) tetrahedrites, which have a cubic and complex crystal structure. The mother phase Tr = Cu shows metal–semiconductor transition and anomalous hysteresis. Through various Tr substitutions, the thermopower was increased and thermal conductivity was decreased. Results show that Tr = Ni had the largest dimensionless figure of merit ZT of 0.15 at 340 K. The main advantage for the large ZT is the quite low lattice thermal conductivity. Because of the large ZT and the environmentally friendly components, tetrahedrites are anticipated as a good thermoelectric material.©2012 The Japan Society of Applied Physics

Tailoring Monodomain in Blue Phase Liquid Crystal by Surface Pinning Effect

2012-05-10T09:00:00+09:00

Authors: Prasenjit Nayek, Heon Jeong, Hye Ryung Park, Shin-Woong Kang, Seung Hee Lee, Heung Shik Park, Hyuck Jin Lee, and Hee Seop Kim
Surface pinning effect of the optically isotropic, blue phase liquid crystal (BPLC) has been studied. Polycrystalline, platelet, multi-domain topological defects without surface treatment have been transformed to uniform monodomain in rubbed surfaces of the cell. The operating voltage was reduced by 27%, owing to a dramatic increase in the Kerr constant. Hysteresis was reduced by 63%, whereas the transmission efficiency gets doubled. The surface anchoring effect on the blue phase plays a significant role in the elastic free energy, coherence length and topological defects. BPLC displays with a low operating voltage, and reduced hysteresis can be realized with surface treatment.©2012 The Japan Society of Applied Physics

Excimer Emission from Pulsed Tandem Microhollow Cathode Discharges in Xenon

2012-05-10T09:00:00+09:00

Authors: Byung-Joon Lee, Hasibur Rahaman, Sang Hoon Nam, Marcus Iberler, Christian Teske, Joachim Jacoby, and Klaus Frank
This paper describes an extension of a basic single microhollow cathode discharge (MHCD) to a tandem MHCD, i.e., two discharges in series from an anode–cathode–anode configuration. When a high-voltage pulse is superimposed with a direct current (DC) tandem MHCD, an intense excimer emission along the discharge axis in a high pressure xenon gas is generated which is two orders of magnitude higher than that of the DC tandem MHCD. In addition, the emission intensity increases to almost twice by increasing cathode thickness from 250 to 1000 µm. The emission is further enhanced by increasing the gas pressure from 400 to 800 mbar.©2012 The Japan Society of Applied Physics

AlN/GaN Short-Period Superlattice Coherently Grown on 6H-SiC(0001) Substrates by Molecular Beam Epitaxy

2012-05-09T09:00:00+09:00

Authors: Ryosuke Kikuchi, Hironori Okumura, Mitsuaki Kaneko, Tsunenobu Kimoto, and Jun Suda
We demonstrate the coherent growth of AlN/GaN short-period superlattice (SPSL) on 6H-SiC(0001) substrates by molecular beam epitaxy. A high-quality 5-nm-thick AlN layer was grown on SiC as a template layer, followed by the growth of AlN (12 BL)/GaN (2 BL) SPSL, which consists of 40 periods (total thickness: 140 nm). The SPSL was coherently grown on SiC, and its threading dislocation density (TDD) was as low as 8×10^{8} cm^{-2}. The SPSL, which had 3-BL-thick GaN layers, was relaxed, and the TDD increased to 8×10^{10} cm^{-2}.©2012 The Japan Society of Applied Physics

Room-Temperature Electroluminescence from Ge Quantum Dots Embedded in Photonic Crystal Microcavities

2012-05-09T09:00:00+09:00

Authors: Toshiki Tsuboi, Xuejun Xu, Jinsong Xia, Noritaka Usami, Takuya Maruizumi, and Yasuhiro Shiraki
Room-temperature electroluminescence (EL) was successfully observed from current-injected light-emitting diodes formed on photonic crystal (PhC) microcavities with Ge self-assembled quantum dots (QDs). The Ge QDs were grown on silicon-on-insulator (SOI) substrate by solid-source molecular beam epitaxy (SS-MBE), and two-dimensional PhC microcavity was used to enhance the light emission efficiency. A vertical PIN diode was fabricated on the microcavity for carriers injection. At an injected current larger than 50 µA, EL peaks corresponding to the cavity modes were clearly seen in the wavelength range of 1.2–1.4 µm.©2012 The Japan Society of Applied Physics

Resonant Tunneling Transport in a GaN/AlN Multiple-Quantum-Well Structure

2012-05-09T09:00:00+09:00

Authors: Salam Sakr, Yulia Kotsar, Maria Tchernycheva, Elias Warde, Nathalie Isac, Eva Monroy, and François H. Julien
In this work, we have investigated the vertical electron transport through a seven-period GaN/AlN multiple-quantum-well structure. The devices show asymmetric current–voltage characteristics displaying negative differential resistance at room temperature. These features persist for multiple scans and are reproducible for both upward and downward sweeping voltages. We interpret the negative differential resistance as a consequence of the resonant tunneling between the fundamental and excited states of adjacent quantum wells. The experimental results are in good agreement with the predictions of an electron transport simulation.©2012 The Japan Society of Applied Physics

Spin Accumulation and Spin Lifetime in p-Type Germanium at Room Temperature

2012-05-09T09:00:00+09:00

Authors: Satoshi Iba, Hidekazu Saito, Aurelie Spiesser, Suguru Watanabe, Ron Jansen, Shinji Yuasa, and Koji Ando
The electrical creation and detection of spin accumulation in p-type Ge were successfully demonstrated at room temperature by spin-polarized tunneling in epitaxial Fe/MgO contacts on Ge with a hole concentration of 8×10^{18} cm^{-3}. In Hanle measurements, the spin accumulation produces a spin signal of about 40 µV per mA of tunnel current. The extracted spin lifetime of holes is 13 ps, which is much longer than the momentum relaxation time. The corresponding spin-diffusion length is 80 nm, suggesting that communication of spin information in p-type Ge is possible over the typical channel length of a field-effect transistor.©2012 The Japan Society of Applied Physics

Optical Deposition of Graphene Saturable Absorber Integrated in a Fiber Laser Using a Slot Collimator for Passive Mode-Locking

Zhi-Chao Luo, Wen-Jun Cao, Ai-Ping Luo, and Wen-Cheng Xu — 2012-05-09T09:00:00+09:00

Authors: Zhi-Chao Luo, Wen-Jun Cao, Ai-Ping Luo, and Wen-Cheng Xu
A compact and efficient intracavity optical deposition of graphene saturable absorber (SA) for a passively mode-locked fiber laser is proposed and demonstrated. The setup for optical deposition is integrated in a fiber ring laser through a slot collimator. By dripping the dimethylformamide (DMF) solution of graphene into the gap of the slot collimator between two fiber ferrules, graphene can be well deposited onto the fiber end to form an SA. The transmission loss of graphene SA is measured. As for application, the fiber ring laser is passively mode-locked with the prepared graphene SA and generates a 1 ps, 13.6 MHz soliton pulse train.©2012 The Japan Society of Applied Physics

Schottky Source/Drain Ge Metal–Oxide–Semiconductor Field-Effect Transistors with Directly Contacted TiN/Ge and HfGe/Ge Structures

2012-05-02T09:00:00+09:00

Authors: Keisuke Yamamoto, Takeshi Yamanaka, Kenji Harada, Takahiro Sada, Keita Sakamoto, Syuta Kojima, Haigui Yang, Dong Wang, and Hiroshi Nakashima
Metal/Ge contacts with high Schottky barrier heights on p- and n-Ge substrates could be formed using direct sputter depositions from TiN and Hf, respectively. The barrier heights were 0.53 and 0.60 eV for the p- and n-Ge substrates, respectively, which are available to source/drains (S/Ds) in Schottky n- and p-channel metal–oxide–semiconductor field-effect transistors (MOSFETs). We fabricated the n- and p-MOSFETs and demonstrated their operation. In particular, the n-MOSFET operation is the first demonstration of S/D using directly contacted metal/Ge. This work opens a method for embodying Schottky S/D Ge complementary MOS devices.©2012 The Japan Society of Applied Physics

Micro-Photoluminescence Study on the Influence of Oxidation on Stacking Faults in 4H-SiC Epilayers

Hikaru Yamagata, Shuhei Yagi, Yasuto Hijikata, and Hiroyuki Yaguchi — 2012-05-02T09:00:00+09:00

Authors: Hikaru Yamagata, Shuhei Yagi, Yasuto Hijikata, and Hiroyuki Yaguchi
We have investigated the influence of thermal oxidation on stacking faults (SFs) in 4H-SiC epilayers by performing micro-photoluminescence measurements for 4H-SiC substrates before and after thermal oxidation. We found that SF (emission wavelength: ∼425.5 nm) thought to be a single Shockley stacking fault was expanded by thermal oxidation. In addition, as a result of comparison between before and after Ar annealing, the SF was not extended after Ar annealing. We also found that only the SFs extended by laser irradiation is eliminated by oxidation.©2012 The Japan Society of Applied Physics

Real-Time Observation of Temperature-Dependent Strain in Poly(3-hexylthiophene) Crystals in a Mixed Donor and Acceptor Thin Film

Hyun Hwi Lee and Hyo Jung Kim — 2012-05-02T09:00:00+09:00

Authors: Hyun Hwi Lee and Hyo Jung Kim
We observed the temperature-dependent real-time lattice parameters and strain exhibited by poly(3-hexylthiophene) crystals in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) films during thermal annealing in the absence and presence of an Al layer. The strain and relaxed lattice parameters were quantitatively determined. The as-prepared P3HT:PCBM films displayed tensile strain, regardless of the presence of an Al layer. In the absence of an Al layer, the internal strain relaxed at an annealing temperature of 180 °C. In the presence of an Al layer, the strain relaxed, then transitioned to a compressive strain above an annealing temperature of 120 °C.©2012 The Japan Society of Applied Physics

Ultranarrow Silicon Inverse Taper Waveguide Fabricated with Double-Patterning Photolithography for Low-Loss Spot-Size Converter

2012-05-02T09:00:00+09:00

Authors: Ryohei Takei, Emiko Omoda, Masao Suzuki, Shoko Manako, Toshihiro Kamei, Masahiko Mori, and Youichi Sakakibara
Using a double-patterning process of i-line photolithography that twice performs a pair of photoresist patterning and dry etching processes, we were able to form an ultranarrow silicon inverse taper waveguide with a tip end width that was much narrower than the resolution limit of photolithography. We fabricated a spot-size converter (SSC) consisting of a 50-µm-long silicon taper waveguide with gradually decreasing width from 400 to 50 nm and a polyimide second core. The insertion loss of the SSC was 0.55 dB for the transverse electric-like mode, which was the lowest value for an SSC fabricated using photolithography.©2012 The Japan Society of Applied Physics

Efficient Ultraviolet Second-Harmonic Generation from a Walk-Off-Compensating β-BaB_{2}O_{4} Device with a New Structure Fabricated by Room-Temperature Bonding

2012-05-01T09:00:00+09:00

Authors: Kenjiro Hara, Shinnosuke Matsumoto, Tomomi Onda, Wataru Nagashima, and Ichiro Shoji
We have fabricated a second-harmonic-generation device with a new walk-off-compensating structure of β-BaB_{2}O_{4} (BBO) by room-temperature bonding. The 5-mm-long device, which consists of four 1-mm-thick plates and additional input and output plates of 0.5 mm thickness, generated nearly twice the UV second-harmonic power compared with a bulk BBO crystal. Moreover, we found that the device can improve the beam shape of the output UV light, which is otherwise degraded in a bulk crystal due to the walk-off effect.©2012 The Japan Society of Applied Physics

Fabrication of Near-Field Plasmonic Tip by Photoreduction for Strong Enhancement in Tip-Enhanced Raman Spectroscopy

Takayuki Umakoshi, Taka-aki Yano, Yuika Saito, and Prabhat Verma — 2012-04-27T09:00:00+09:00

Authors: Takayuki Umakoshi, Taka-aki Yano, Yuika Saito, and Prabhat Verma
Tip-enhanced Raman spectroscopy (TERS) offers one of the best techniques for optical analysis and imaging of samples at nanoscale. The most important point in TERS experiments is to obtain a high signal enhancement through a metallic nanotip. Compared with fully metallized tips, the tips that have only one metallic nanoparticle at the apex show better enhancement. Here, we demonstrate a new and simple way to fabricate metallic nanoparticles selectively at the tip apex through photoreduction. By controlling the nanoparticle size, the plasmon resonance of the tip can be tuned. Finally, we demonstrate that such tips give better enhancement in TERS.©2012 The Japan Society of Applied Physics

Signature of Coherent Transport in Epitaxial Spinel-Based Magnetic Tunnel Junctions Probed by Shot Noise Measurement

2012-04-23T09:00:00+09:00

Authors: Takahiro Tanaka, Tomonori Arakawa, Kensaku Chida, Yoshitaka Nishihara, Daichi Chiba, Kensuke Kobayashi, Teruo Ono, Hiroaki Sukegawa, Shinya Kasai, and Seiji Mitani
We measured the shot noise in fully epitaxial Fe/MgAl_{2}O_{X}/Fe-based magnetic tunneling junctions (MTJs). While the Fano factor to characterize the shot noise is very close to unity in the antiparallel configuration, it is reduced to 0.98 in the parallel configuration. This observation shows the sub-Poissonian process of electron tunneling in the parallel configuration, indicating the coherent tunneling through the spinel-based tunneling barrier of the MTJs.©2012 The Japan Society of Applied Physics

Afterglow Suppression by Codoping with Bi in CsI:Tl Crystal Scintillator

2012-04-20T09:00:00+09:00

Authors: Daisuke Totsuka, Takayuki Yanagida, Yutaka Fujimoto, Yuui Yokota, Federico Moretti, Anna Vedda, and Akira Yoshikawa
Single crystals of CsI, CsI:Tl, and CsI:Tl,Bi were grown by the vertical Bridgman method. We studied the effect of Bi^{3+} codoping on the CsI:Tl X-ray-induced afterglow. Thermally stimulated luminescence measurements were carried out in order to investigate the role of traps. The results showed a remarkable reduction of afterglow, of about an order of magnitude, and of trap concentration by Bi codoping. These improvements could be related to a possible role of Bi in the compensation of intrinsic nonstoichiometry defects.©2012 The Japan Society of Applied Physics

White Lighting Upconversion in Tm^{3+}/Ho^{3+}/Yb^{3+} Co-Doped CaWO_{4}

Jun Ho Chung, Sang Yeop Lee, Kwang Bo Shim, and Jeong Ho Ryu — 2012-04-20T09:00:00+09:00

Authors: Jun Ho Chung, Sang Yeop Lee, Kwang Bo Shim, and Jeong Ho Ryu
Controllable white upconversion (UC) luminescence was obtained from Tm^{3+}/Ho^{3+}/Yb^{3+}-codoped CaWO_{4}. Under the excitation of a 980 nm single wavelength laser diode, the Tm^{3+}/Ho^{3+}/Yb^{3+}-codoped CaWO_{4} exhibited bright white UC luminescence composed of blue emission from Tm^{3+} and green and red emissions from Ho^{3+} visible to the naked eye. The intensity ratios of green, red, and blue UC emissions varied with Tm^{3+}/Ho^{3+} concentrations, which can control white UC emission ranging from the cool to the warm region. Various white UC colors can be easily changed by adjusting the Tm^{3+}/Ho^{3+} concentrations in the CaWO_{4} matrix.©2012 The Japan Society of Applied Physics

A New Planar Single-Channel Shim Coil Using Multiple Circular Currents for Magnetic Resonance Imaging

Daiki Tamada, Katsumi Kose, and Tomoyuki Haishi — 2012-04-20T09:00:00+09:00

Authors: Daiki Tamada, Katsumi Kose, and Tomoyuki Haishi
We propose a new planar single-channel shim coil for magnetic resonance imaging (MRI) permanent magnets. The coil design is based on the superposition of multiple circular currents and the stream function method. The designed shim coil was implemented for a permanent magnet with 1.0 T and a 90 mm gap. When the shim coil current was optimized, the magnetic field inhomogeneity decreased from 240 to 97 ppm (peak-to-peak) in the central cylindrical area (54.6 mm diameter, 60.0 mm height), demonstrating that the single-channel shim coil proposed here is a useful device for permanent narrow-gap magnets with complicated magnetic field distribution.©2012 The Japan Society of Applied Physics

Effect of Hydrogen Reduction on Characteristics of Cu Thin-Films Deposited by RF-Driven Ar/H_{2} Atmospheric Pressure Plasma Jet

2012-04-19T09:00:00+09:00

Authors: Hisashi Nakahiro, Peng Zhao, Akihisa Ogino, Wei Zheng, Yuedong Meng, and Masaaki Nagatsu
The effect of hydrogen reduction on the characteristics of Cu films deposited using a 13.56 MHz RF Ar/H_{2} atmospheric pressure plasma jet with a Cu thin wire set inside a quartz tube was studied. By adding a small amount of H_{2} gas into Ar carrier gas, it was found that intense emissions of N_{2} molecule second positive systems, OH and O emissions were significantly suppressed, while Cu and Hα lines were intensified. It was confirmed by an X-ray photoelectron spectrometry that a high purity Cu film was synthesized via hydrogen reduction reaction, preventing the oxidization of Cu film.©2012 The Japan Society of Applied Physics

Evolution of Field Electron Emission Pattern from Multilayered Graphene Induced by Structural Change of Edge

Kazuya Nakakubo, Koji Asaka, Hitoshi Nakahara, and Yahachi Saito — 2012-04-18T09:00:00+09:00

Authors: Kazuya Nakakubo, Koji Asaka, Hitoshi Nakahara, and Yahachi Saito
Structures and field emission (FE) properties of exfoliated multilayered graphene have been investigated by in situ transmission electron microscopy (TEM) and field emission microscopy (FEM). TEM revealed that edges of multilayered graphene, which were open before FE measurement, became closed after FE under high emission current, say over a few tens µA. Corresponding to this change in the edge structure, FEM images evolved from a striped pattern for an open edge to a dim one for a closed edge. The structural change under the high current condition is presumably induced by Joule heating of the graphene tip.©2012 The Japan Society of Applied Physics

Highly Uniform Thin-Film Transistors Printed on Flexible Plastic Films with Morphology-Controlled Carbon Nanotube Network Channels

Hideaki Numata, Kazuki Ihara, Takeshi Saito, Hiroyuki Endoh, and Fumiyuki Nihey — 2012-04-18T09:00:00+09:00

Authors: Hideaki Numata, Kazuki Ihara, Takeshi Saito, Hiroyuki Endoh, and Fumiyuki Nihey
Carbon nanotube (CNT) transistor arrays were fabricated on plastic films by printing. All the device elements were directly patterned by maskless printing without any additional patterning process, and minimum materials were used. During fabrication, the morphology of the CNT random network was controlled by an adsorption mechanism on the surface to be printed, which resulted in excellent and uniform electrical properties. The field-effect mobility was further improved by post-treatment to modify the morphology of the CNT network. These results are promising for realizing printed electronics integrated with CNT transistors.©2012 The Japan Society of Applied Physics

Dependence of Magnetic Anisotropy in Co_{20}Fe_{60}B_{20} Free Layers on Capping Layers in MgO-Based Magnetic Tunnel Junctions with In-Plane Easy Axis

2012-04-17T09:00:00+09:00

Authors: Hiroyuki Yamamoto, Jun Hayakawa, Katsuya Miura, Kenchi Ito, Hideyuki Matsuoka, Shoji Ikeda, and Hideo Ohno
Perpendicular magnetic anisotropy (PMA) in Co_{20}Fe_{60}B_{20} films depending on the adjacent layers of Ta, Ru, and MgO was studied for reduction of switching current density J_{c0} in in-plane magnetic tunnel junctions (MTJs). A MgO layer reduces the out-of-plane saturation field (H_{s}) of in-plane easy axis Co_{20}Fe_{60}B_{20} films by inducing a strong PMA at the Co_{20}Fe_{60}B_{20}/MgO interface. The PMA is not affected much by the crystallinity in Co_{20}Fe_{60}B_{20} films with different capping layers. MTJ with a MgO capping layer shows a lower J_{c0} than that with a Ru capping layer, in accordance with the reduction of H_{s}.©2012 The Japan Society of Applied Physics

Threading Dislocation Blocking in Metamorphic InGaAs/GaAs for Growing High-Quality In_{0.5}Ga_{0.5}As and In_{0.3}Ga_{0.7}As on GaAs Substrate by Using Metal Organic Chemical Vapor Deposition

2012-04-17T09:00:00+09:00

Authors: Hong-Quan Nguyen, Edward Yi Chang, Hung-Wei Yu, Hai-Dang Trinh, Chang-Fu Dee, Yuen-Yee Wong, Ching-Hsiang Hsu, Binh-Tinh Tran, and Chen-Chen Chung
High quality In_{0.3}Ga_{0.7}As and In_{0.51}Ga_{0.49}As epilayers have been successfully grown on the GaAs substrate by MOCVD. A cross-sectional study by transmission electron microscopy showed that the threading dislocations (TDs) have been successfully contained and limited within the buffer layers designed to stop the elongation of TDs into the In_{0.3}Ga_{0.7}As and In_{0.51}Ga_{0.49}As epilayers. A TD density of 1×10^{6} cm^{-2} in a fully relaxed In_{0.51}Ga_{0.49}As epilayer was achieved. The measurement of lifetimes of n- and p-type In_{0.51}Ga_{0.49}As has been done by using time-resolved photoluminescence. A great reduction in the number of recombination centers in the InGaAs epilayer has been shown.©2012 The Japan Society of Applied Physics

Defect Reduction via Selective Lateral Epitaxy of GaN on an Innovative Masked Structure with Serpentine Channels

2012-04-16T09:00:00+09:00

Authors: Lei Li, Justin P. C. Liu, Lei Liu, Ding Li, Lei Wang, Chenghao Wan, Weihua Chen, Zhijian Yang, Yahong Xie, Xiaodong Hu, and Guoyi Zhang
We demonstrated an innovative lateral epitaxy method to grow c-plane GaN film using serpentine masked structures, which simplified the entire fabrication process with only one single epitaxial growth step and could efficiently block the threading dislocations. The microstructural and optical properties of GaN indicated that the crystalline quality was effectively improved. Unlike the conventional epitaxial lateral overgrowth (ELOG) or the double ELOG method, the presented serpentine masked structure needs no regrowth process for obtaining low-defect-density GaN materials, and is promising for growing high-performance III–nitride-based devices including laser diodes (LDs), power transistors, and light-emitting diodes (LEDs).©2012 The Japan Society of Applied Physics

Vertically Aligned InP Nanowires Grown via the Self-Assisted Vapor–Liquid–Solid Mode

Guoqiang Zhang, Kouta Tateno, Hideki Gotoh, and Tetsuomi Sogawa — 2012-04-16T09:00:00+09:00

Authors: Guoqiang Zhang, Kouta Tateno, Hideki Gotoh, and Tetsuomi Sogawa
We have demonstrated the growth of vertical InP nanowires via the self-assisted vapor–liquid–solid mode on an InP(111) substrate. Single nanowires exhibit a highly uniform diameter along the axial direction despite their 15 µm length. We show direct evidence of the self-assisted growth mode by performing a compositional analysis of the NW tip. We demonstrated that the In particle at the NW tip could be removed by modifying the V/III source material ratio during growth. Single InP nanowires exhibit a distinct TO phonon peak and show the luminescence of the excitonic emission at 4 K.©2012 The Japan Society of Applied Physics

Positive Giant Surface Potential of Tris(8-hydroxyquinolinolato) Aluminum (Alq_{3}) Film Evaporated onto Backside of Alq_{3} Film Showing Negative Giant Surface Potential

Youichi Okabayashi, Eisuke Ito, Takashi Isoshima, and Masahiko Hara — 2012-04-16T09:00:00+09:00

Authors: Youichi Okabayashi, Eisuke Ito, Takashi Isoshima, and Masahiko Hara
To examine the origin of spontaneous noncentrosymmetric molecular dipole orientation of an evaporated film of tris(8-hydroxyquinolinolato) aluminum (Alq_{3}), overlayer deposition of Alq_{3} on the backside of a peeled Alq_{3} film was investigated. Although the surface potential (SP) of the backside of the Alq_{3} film was negative, the SP showed a positive shift by overlayer deposition of Alq_{3}. This means that the spontaneous dipole orientation of Alq_{3} molecules in the overlayer is opposite to that in the underlayer. A molecular geometric effect due to a bulky molecular shape is proposed as the origin of the noncentrosymmetry.©2012 The Japan Society of Applied Physics

As-Grown Superconducting SmFeAs(O,F) Thin Films by Molecular Beam Epitaxy

Shinya Ueda, Soichiro Takeda, Shiro Takano, and Michio Naito — 2012-04-13T09:00:00+09:00

Authors: Shinya Ueda, Soichiro Takeda, Shiro Takano, and Michio Naito
As-grown superconducting thin films of SmFeAs(O,F) with T_{c} higher than 50 K were successfully grown by molecular beam epitaxy. In the growth of the films, we codeposited Sm, SmF_{3}, Fe, and As in oxygen atmosphere. The most crucial factor for obtaining superconducting films has turned out to be the substrate choice. We have found that CaF_{2}-buffered LaAlO_{3} is much more suitable for the growth of SmFeAs(O,F) films than bare LaAlO_{3} or CaF_{2} and yielded nearly single-phased SmFeAs(O,F) films with T_{c}^{on} (T_{c}^{end}) ∼ 52 K (50 K).©2012 The Japan Society of Applied Physics

Performance Analysis of Junctionless Transistors Based on Monte Carlo Simulation

Jaeil Choi, Katsuyuki Nagai, Shunsuke Koba, Hideaki Tsuchiya, and Matsuto Ogawa — 2012-04-13T09:00:00+09:00

Authors: Jaeil Choi, Katsuyuki Nagai, Shunsuke Koba, Hideaki Tsuchiya, and Matsuto Ogawa
A junctionless (JL) transistor has no pn junctions and has a number of advantages to fabricate ultrashort-channel metal–oxide–semiconductor field-effect transistors. In this paper, we study the electron transport in JL transistors based on a Monte Carlo simulation. We demonstrate that high channel doping will not degrade the drive current seriously, because ionized impurities scatter electrons mostly forward, and thus there is less chance for scattered electrons to return back to the source. We also find that smaller parasitic resistance in the source of a JL transistor also contributes to achieve high drive current.©2012 The Japan Society of Applied Physics

High-Rate Growth of High-Crystallinity LiCoO_{2} Epitaxial Thin Films by Pulsed Laser Deposition

Tsuyoshi Ohnishi and Kazunori Takada — 2012-04-13T09:00:00+09:00

Authors: Tsuyoshi Ohnishi and Kazunori Takada
Pulsed laser deposition is widely used to form complex oxide thin films due to the relatively small deviation in composition between the target and the film. The deviation, although small, is not completely prevented: it highly depends on the ablation conditions. Furthermore, gas pressure affects it in the case of lithium compounds, because lithium is even lighter than oxygen. In other words, the difference in composition between the target and the film is controllable by adjusting these parameters. In this study, we succeeded in the high-rate epitaxial growth of stoichiometric LiCoO_{2} films from a lithium-enriched target through composition control.©2012 The Japan Society of Applied Physics

Cs^{+} Trapping in Size-Controlled Nanospaces of Hexacyanoferrates

Ayako Omura and Yutaka Moritomo — 2012-04-13T09:00:00+09:00

Authors: Ayako Omura and Yutaka Moritomo
We demonstrated that divalent manganese/zinc ions (Mn^{II}/Zn^{II}) and ferricyan ion [Fe^{III}(CN)_{6}]^{3-} cooperatively trap the cesium ion in aqueous solution within the self-fabricated cubic nanospace with high efficiency. In the low-Cs^{+}-concentration region (≤100 ppm), addition of 5 mmol/L [Fe(CN)_{6}]^{3+} and 5 mmol/L Mn^{II}/Zn^{II} significantly reduces the Cs^{+} concentration 0.003/0.008 ppm. In the higher-concentration region, the Cs trapping capacity (T≡M_{Cs}/M_{host}, where M_{Cs} and M_{host} are the masses of the trapped cesium and host, respectively) reaches T = 420/410 mg/g above 6 mmol/L. The size matching between the nanospace (5.30/5.20 Å for the Mn/Zn compounds, respectively) and the ionic radius (r_{Cs^{+}} = 1.74 Å) of the guest Cs^{+} is responsible for the high trapping efficiency.©2012 The Japan Society of Applied Physics

Highly Stable Vortex State in Sub-100 nm Nanomagnets

Xinghua Wang, Indra Purnama, Murapaka Chandra Sekhar, and Wen Siang Lew — 2012-04-12T09:00:00+09:00

Authors: Xinghua Wang, Indra Purnama, Murapaka Chandra Sekhar, and Wen Siang Lew
We report on the magnetization reversal process in sub-100 nm Ni_{80}Fe_{20} asymmetric ring. The switching mechanism involves a stable vortex state due to the strong anisotropy imposed on the narrow arm of the ring. Experimental demonstration shows that such vortex configuration does not annihilate until a large reversal field of 1200 Oe. The asymmetry of the structure promotes a unique reversal process, which gives us control over the chirality of the vortex configuration. Micromagnetic simulations reveal that the highly stable vortex configuration is sustainable in the asymmetric ring structures with a diameter as small as 30 nm.©2012 The Japan Society of Applied Physics

Anisotropic Thermoelectric Properties of MnSi_{γ} Film Prepared on R-Sapphire

Komei Takeda, Yuta Kikuchi, Kei Hayashi, Yuzuru Miyazaki, and Tsuyoshi Kajitani — 2012-04-12T09:00:00+09:00

Authors: Komei Takeda, Yuta Kikuchi, Kei Hayashi, Yuzuru Miyazaki, and Tsuyoshi Kajitani
We attempted to obtain an epitaxial MnSi_{γ} (γ∼1.7) film on R-sapphire, i.e., Sapphire(1-102), substrate by pulsed laser deposition. We prepared MnSi_{γ} films by changing the substrate temperature gradient. It was found that the MnSi_{γ} film, whose temperature gradient in a substrate is parallel to Sapphire[11-20], could be grown epitaxially on the substrate. The epitaxial relationship was MnSi_{γ}(1000)[0010] ∥ Sapphire(1-102)[11-20]. The thermoelectric properties of the epitaxial MnSi_{γ} film were different in the a- and c-axes, reflecting the anisotropic MnSi_{γ} crystal structure. The anisotropic thermoelectric properties are discussed in terms of the electronic structure.©2012 The Japan Society of Applied Physics

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Threading Dislocation Blocking in Metamorphic InGaAs/GaAs for Growing High-Quality In_{0.5}Ga_{0.5}As and In_{0.3}Ga_{0.7}As on GaAs Substrate by Using Metal Organic Chemical Vapor Deposition

Defect Reduction via Selective Lateral Epitaxy of GaN on an Innovative Masked Structure with Serpentine Channels

Vertically Aligned InP Nanowires Grown via the Self-Assisted Vapor–Liquid–Solid Mode

Positive Giant Surface Potential of Tris(8-hydroxyquinolinolato) Aluminum (Alq_{3}) Film Evaporated onto Backside of Alq_{3} Film Showing Negative Giant Surface Potential

As-Grown Superconducting SmFeAs(O,F) Thin Films by Molecular Beam Epitaxy

Performance Analysis of Junctionless Transistors Based on Monte Carlo Simulation

High-Rate Growth of High-Crystallinity LiCoO_{2} Epitaxial Thin Films by Pulsed Laser Deposition

Cs^{+} Trapping in Size-Controlled Nanospaces of Hexacyanoferrates

Highly Stable Vortex State in Sub-100 nm Nanomagnets

Anisotropic Thermoelectric Properties of MnSi_{γ} Film Prepared on R-Sapphire