bluuebunny/crossref_metadata_2025 huggingface dataset:
Current Biased Resistive Switching in ZnO Whiskers
Authors: Mohan Rajneesh , Kim Sang-Jae
Published: April 2011

In this paper, the resistive switching effects in ZnO whiskers are reported. The resistive switching properties were studied by current controlled current–voltage (I–V) sweeps. On increasing the biasing current value, the resistance of the ZnO whiskers switches from high resistance state (HRS) to low resistance state (LRS). Between the HRS and LRS, anomalous resistance fluctuations were observed during the resistive transition. These resistive switching effects were studied for ZnO whiskers of different diameters. It was observed that resistive switching depends on the diameter of the ZnO whisker. As the diameter of a ZnO whisker decreases, the resistance switching increases. The mechanism of the observed resistive switching is also proposed.
Level Broadening Effect in Electron Tunneling through Double Quantum Dots with Different g Factors
Authors: Huang Shiu-Ming , Tokura Yasuhiro , Akimoto Hikota , Kono Kimitoshi , Lin Juhn-Jong , Tarucha Seigo , Ono Keiji
Published: April 2011

The spin bottleneck effect was first observed in vertical double quantum dots with different g factors in high magnetic fields. We further investigate the spin-dependent resonance tunneling through the same quantum dot system in low magnetic fields. There are no resonance tunneling peak lines, even though one of the Zeeman levels is aligned, because the mismatch of the other Zeeman sublevels blocks the resonance tunneling. However, the level broadening effect partially releases the spin-dependent blockade. As a compromise between two effects, one resonance tunneling peak line splits into two peak lines and forms a kink structure. The split of the two current peak lines is half of Zeeman energy difference between two dots.
Source Engineering for Tunnel Field-Effect Transistor: Elevated Source with Vertical Silicon–Germanium/Germanium Heterostructure
Authors: Han Genquan , Guo Pengfei , Yang Yue , Fan Lu , Yee Ye Sheng , Zhan Chunlei , Yeo Yee-Chia
Published: April 2011

In this work, we demonstrate by simulation and experiment that the performance of a p+ Si0.5Ge0.5 source tunnel field-effect transistor (TFET) can be improved by inserting an undoped Ge layer between source and channel. The Ge layer suppresses diffusion of boron into the Si channel and it also forms a Si0.5Ge0.5/Ge/Si hole quantum well, leading to an abrupt boron profile and a high hole concentration at the source edge. At the Ge/Si heterojunction, the presence of compressive strain in the Ge layer increases the valence band offset, while the tensile strain in the Si channel increases the conduction band offset, which effectively reduces the tunnel barrier and enhances the tunnel probability. Compared with a control device without the Ge layer, TFETs with a Si0.5Ge0.5/Ge source show a higher on-state current I
ON and improved threshold voltage V
TH and subthreshold characteristics.
Power Efficiency Improvement of White Phosphorescent Organic Light-Emitting Diode with Thin Double-Emitting Layers and Hole-Trapping Mechanism
Authors: Juang Fuh-Shyang , Hong Lin-Ann , Wang Shun-Hsi , Tsai Yu-Sheng , Gao Ming-Hong , Chi Yun , Shieh Han-Ping , Hsu Jen-Sung
Published: April 2011

This study is carried out to discuss how to reduce the driving voltage of blue phosphorescent organic light-emitting diodes (PHOLEDs) by using a thin double-emission layer. A hole transport-type host (TCTA) is inserted between the hole transport layer (TAPC) and the emitting layer (EML), constituting a buffer layer between them with the aim of improving charge carrier balance. Furthermore, in this study, we also utilize the interface between double light-emitting layers of devices by codoping them with a red phosphorescent dopant [Os(bpftz)2(PPh2Me)2]. An Os complex with a high-lying highest occupied molecular orbital (HOMO) energy level (trapping holes) is codoped at the interface between emitting layers and an exciton-formation zone is expanded to obtain a white PHOLED with high efficiency. From the results, the optimal structure of the white device exhibits a yield of 35 cd A-1, a power efficiency of 22 lm W-1, and CIE coordinates of (0.33,0.38) at a luminance of 1000 cd m-2. Furthermore, the power efficiency can be improved to 30 lm W-1by attaching the outcoupling enhancement film.
Current-Density Dependence of Transient Properties in Green Phosphorescent Organic Light-Emitting Diodes
Authors: Kajii Hirotake , Takahota Noriyoshi , Wang Yadong , Ohmori Yutaka
Published: April 2011

The current-density dependence of the transient characteristics of a green phosphorescent organic light-emitting diode (OLED) with tris(2-phenylpyridine)iridium(III) [Ir(ppy)3] as the emissive layer and the quenching process, which contributes to roll-off in a device, were investigated. A maximum quantum efficiency of 11% and luminance of 45,000 cd/m2 were achieved for a green OLED. The behaviors of the transient characteristics and efficiency are analyzed using the triplet–triplet annihilation model. The device exhibited a gradual decrease in quantum current efficiency owing to the triplet–triplet annihilation at a high current density. At a higher current density, the reduced rise and decay times are due to high-density triplet excitons related to the enhanced triplet–triplet annihilation and the increase of the nonradiative process. The cut-off frequency of the device is mainly limited by the phosphorescent recombination lifetime.
Highly Efficient White Organic Light-Emitting Diodes with a p–i–n Tandem Structure
Authors: Yokoyama Meiso , Su Shui-Hsiang , Hou Cheng-Chieh , Wu Chung-Ta , Kung Chun-Hao
Published: April 2011

In this study, we demonstrate the fabrication of highly efficient white organic light-emitting diodes (WOLEDs) with a p–i–n tandem structure using lithium (Li)-doped tris(8-hydroxyquinoline) aluminum (Alq3)/molybdenum oxide (MoO
x
)-doped 4,4',4''-tris[2-naphthyl(phenyl)amino] triphenylamine (2-TNATA) as an effective interconnecting layer (ICL). The tandem device exhibited a luminance of 3800 cd/m2, a luminous efficiency of 18.8 cd/A, a power efficiency of 5.48 lm/W, an external quantum efficiency of 6.5%, and the Commission Internationale d'Eclairage (CIE) coordinates of (x=0.312, y=0.396) at 20 mA/cm2. The electroluminescence color of this p–i–n tandem device nearly did not change significantly with driving voltage variation and viewing angle. The various interfaces of ICL in such a tandem device were studied using a photovoltaic setup and were used to elucidate the mechanisms of the tandem devices.
Maskless Patterning of Vapor-Deposited Photosensitive Film and its Application to Organic Light-Emitting Diodes
Authors: Muroyama Masakazu , Saito Wataru , Yokokura Seiji , Tanaka Kuniaki , Usui Hiroaki
Published: April 2011

Photosensitive thin films were prepared by the codeposition of a polymerizable monomer and a photoinitiator, which were patterned by UV irradiation followed by development in an organic solvent. This technique was applied for the preparation of a phosphorescent layer of an organic light emitting diode (OLED) by combining a vinylcarbazole monomer, a dopant of a vinyl derivative of a iridium complex, and a photoinitiator of a benzophenone derivative. An OLED with multiple emitting elements was prepared by repeating this patterning process. It was confirmed that the patterning does not cause harmful damage to the device characteristics. Moreover, the device characteristics can be improved after the photo patterning because the photopolymerization stabilizes the deposited films. This method is advantageous as a new technique for patterning polymer films without using a shadow mask for vapor deposition or a photoresist.
Influences of Intensity of Electric Field on Properties of Poly(vinylidene fluoride–tetrafluoroethylene) Thin Films during Annealing Process
Authors: Jeong Jong-Hyeon , Terashima Daiki , Kimura Chiharu , Aoki Hidemitsu
Published: April 2011

In the annealing of a poly(vinylidene fluoride–tetrafluoroethylene) thin film, the electrical properties of the film was improved by application of a certain electric field. The aim of this study was to investigate changes in characteristics following the application of an electric field. For an electric field lower than a critical intensity, the influence can be slight. For an electric field higher than a critical intensity, on the other hand, the recombination of monomers can be affected. On the basis of these predictions, we have attempted to measure the critical intensity of an electric field and investigate the influences of an electric field application by X-ray photoelectron spectroscopy (XPS) in this study. Moreover, in order to control the contamination by impurities, we have carried out the annealing process using a vacuum chamber.
Function of Interfacial Dipole Monolayer in Organic Field Effect Transistors
Authors: Ou-Yang Wei , Weis Martin , Lee Keanchuan , Manaka Takaaki , Iwamoto Mitsumasa
Published: April 2011

The effect of interfacial dipole monolayer on carrier injection property of pentacene OFET was studied to understand the origin of contact resistance, which was evaluated from a modified transmission line model. The results showed the contact resistance of pentacene OFET with the dipole monolayer is much higher. This large discrepancy was found due to a small potential drop difference at the interface generated by the dipole monolayer. In addition, the strong local electric field confirmed the reason for the large negative threshold voltage shift. These studies show the importance of local electric field at pentacene–SiO2 interface generated by the dipole monolayer.
Evaluation of Carrier Density in Organic Field-Effect Transistor by Charge Modulated Spectroscopy
Authors: Manaka Takaaki , Kawashima Satoshi , Iwamoto Mitsumasa
Published: April 2011

We demonstrated that the carrier density in the channel of pentacene field-effect transistor (FET) is allowed to be evaluated on the basis of optical measurement, charge modulated spectroscopy (CMS). Signal modulation corresponds well with the electrical properties; device exhibited only p-type characteristics in ambient atmosphere and only negative gate voltage induces the reflectance modulation, indicating the suppression of n-type operation. Reflectance modulation that increases linearly with negative gate voltage was reproduced by the multilayer optical analysis taking into account the channel formation. Based on such analysis, carrier density in the channel was evaluated from a magnitude of modulation at specific wavelength.
Investigation of the Voltage Establishment and Relaxation Processes in a Double-Layer Device by Time-Resolved Optical Second-Harmonic Generation
Authors: Zhang Le , Taguchi Dai , Li Jun , Manaka Takaaki , Iwamoto Mitsumasa
Published: April 2011

By using time-resolved electric-field-induced optical second-harmonic generation (TR-EFISHG) measurements, we studied the voltage establishment in an indium tin oxide (ITO)/polyimide/pentacene/Au double-layer device in terms of Maxwell–Wagner type charging and discharging. Results showed that the voltage establishment and relaxation in the pentacene layer depend on the polarity of the applied voltage, suggesting different carrier behaviors of electrons and holes in the pentacene layer. The interfacial relaxation time (τMW) also showed a different voltage dependence on the carrier species. A computer simulation using the relaxation time ratio T
r (= τRC/τMW) well accounts for the results.
Direct Observation of Carrier Behavior Leading to Electroluminescence in Tetracene Field-Effect Transistor
Authors: Ohshima Yuki , Satou Hideki , Hirako Nobuaki , Kohn Hideki , Manaka Takaaki , Iwamoto Mitsumasa
Published: April 2011

By using electric-field-induced second-harmonic generation (EFISHG) measurements, we directly probed carrier motion in tetracene field-effect transistors (FETs), followed by electroluminescence (EL). Under a pulse voltage application, positive charges were injected from the source electrode and accumulated on the gate insulator. After the voltage was removed, positive charges remained and formed a space charge field. As a result, electrons were pulled into the FET to recombine with the remaining holes for radiating EL. The EFISHG measurement provides a way to directly probe the electric field transient that contributes to the EL mechanism of light-emitting transistors.
Printed Electrode for All-Printed Polymer Diode
Authors: Yoshida Manabu , Suemori Kouji , Uemura Sei , Hoshino Satoshi , Takada Noriyuki , Kodzasa Takehito , Kamata Toshihide
Published: April 2011

We developed a novel pressure-annealing method for fabricating printed low-work-function metal patterns and printed metal alloy patterns. The pressure-annealed metal electrodes were used as bottom electrodes of printed polymer diodes. As a result, all-printed polymer diodes having various rectification properties were successfully produced.
Oriented Polyfluorene Films Dye-Doped for Whitening of Polarized Electroluminescent Devices
Authors: Heck Claire , Mizokuro Toshiko , Misaki Masahiro , Azumi Reiko , Tanigaki Nobutaka
Published: April 2011

Whitening of the emission of electroluminescent (EL) devices was performed by doping α-sexithiophene (6T), which is an orange emitting dye, into blue emitting oriented poly(9,9-dioctylfluorene) (PFO) polymer films. These oriented PFO films were produced by means of the friction transfer method and the doping of 6T was performed by means of the vapor transportation method which is a solvent-free process. The excellent orientation of PFO films and of the doped 6T led to very good polarization of the light emitted by these EL devices.
Effects of Annealing on Polymer Solar Cells with High Polythiophene–Fullerene Concentrations
Authors: Ho Chiu-Sheng , Huang E-Ling , Hsu Wei-Chou , Lee Ching-Sung , Lai Ying-Nan , Lai Wen-Hsuan
Published: April 2011

The effects of annealing at high temperatures on polymer solar cells consisting of conjugated poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) composites with high concentrations of 3 and 5 wt % are reported. As compared with as-casted samples, the optical intensity in absorption spectra for the 3 wt % film was improved after annealing at temperatures of 110–150 °C, and the device made of 5 wt % blend film after annealing at 150 °C demonstrated nearly sevenfold improvements in both short-circuit current density (J
sc) and power conversion efficiency (PCE).
Silver-Nanoparticle-Assisted Photocurrent Generation in Polythiophene–Fullerene Thin Films
Authors: You Jing , Arakawa Taichi , Munaoka Takatoshi , Akiyama Tsuyoshi , Takahashi Yukina , Yamada Sunao
Published: April 2011

We have investigated the effect of silver nanoparticles (AgPs) on the photocurrent generation of a polyphiophene-fullerene photovoltaic film. Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM) were used for the electron donor and acceptor, respectively. First, AgPs were electrostatically deposited upon the surface of an indium tin oxide (ITO) electrode via a polycation. Then, a film of P3HT or a mixture of P3HT and PCBM was prepared by spin coating. The thickness of the film was evaluated by atomic force microscopy. Absorption and fluorescence spectral measurements were carried out to investigate the effects of AgPs. Photocurrent spectra were also measured, and the effects of AgPs on photocurrent enhancement were verified.
Ultraviolet-Patternable Polymer Insulator for Organic Thin-Film Transistors on Flexible Substrates
Authors: Wu Chung-Ming , Su Shui-Hsiang , Wang Hong-Tai , Yokoyama Meiso , Fu Shen-Li
Published: April 2011

In this work, we describe the fabrication of pentacene-based organic thin-film transistors (OTFTs) on a flexible substrate using a UV-patternable polymer material, mr-UVCur06, as the gate insulator. The device structure is poly(ethylene terephthalate) (PET)/indium–tin oxide (ITO)/mr-UVCur06/pentacene/Au (source/drain). In addition to its solution-processable capability, mr-UVCur06 is directly patternable by UV light in a low-temperature process. The OTFT has an on–off ratio that approaches 105, and its pattern resolution can reach 5 µm. Additionally, UV/ozone post-treatment of the patterned mr-UVCur06 can illuminate the organic contaminants from its surface and significantly improve the performance of OTFTs. Moreover, the effect of UV/ozone post-treatment on the polymer dielectric is confirmed using a Fourier transform infrared (FT-IR) spectrometer. Owing to its highly desired characteristics such as photopatternability and low-temperature process, mr-UVCur06 is feasible for low-cost, large-area flexible device applications.
Planar Multielectrode Array Coupled Complementary Metal Oxide Semiconductor Image Sensor for In vitro Electrophysiology
Authors: Nakajima Arata , Noda Toshihiko , Sasagawa Kiyotaka , Tokuda Takashi , Ishikawa Yasuyuki , Shiosaka Sadao , Ohta Jun
Published: April 2011

A multielectrode array coupled complementary metal oxide semiconductor (CMOS) image sensor (MARC sensor) was developed for an in vitro electrophysiology experiment. The sensor chip was fabricated by a standard CMOS process, and has a 64 microelectrode array and a 180×180 photodiode pixel array. The size of an on-chip microelectrode is 60×60 µm2 and the pixel size is 7.5×7.5 µm2. In addition, Pt black was fabricated on microelectrodes to obtain stimulation electrodes. The imaging function was demonstrated by the anatomical observation of a mouse hippocampal slice, and the electrical recording function was also demonstrated by monitoring the extracellular field potentials of the brain slice using on-chip microelectrodes. The MARC sensor is compatible with existing in vitro multichannel recording systems. This is the first report supporting our concept that microelectrode arrays and large-sized optical systems can be integrated onto single large-scale integration (LSI) architecture.
Detection of Antigen–Antibody Reaction Using Si Ring Optical Resonators Functionalized with an Immobilized Antibody-Binding Protein
Authors: Fukuyama Masataka , Nishida Mizue , Abe Yosuke , Amemiya Yoshiteru , Ikeda Takeshi , Kuroda Akio , Yokoyama Shin
Published: April 2011

We propose the integrated biosensor chip using Si ring resonators, where different receptor is immobilized on each sensor. Signal detection is carried out by the matrix of light-input and detection waveguides, which are respectively connected to laser diodes and photodetectors. The Si rings are arranged at the cross points. The unique point of our work is to use the silicon-binding protein (designated Si-tag), which binds to SiO2surface, as an anchoring molecule to immobilize bioreceptor on the Si rings in an oriented manner. In the integrated biosensor chip, many kinds of Si-tag-receptor fusions are required for high-throughput detection of analyte. In this paper, the Si ring biosensors were functionalized with various antibodies using the Si-tagged protein A as an intermediate binder, and the label-free detection of antigen have been achieved. We have developed the rapid functionalization method of Si-ring resonators with antibodies using Si-tagged protein A. Since various kinds of antibody can be used as receptors for biosensing, this method promises to realize the integrated biosensors for high-throughput analyte detection.
Differential Setup of Light-Addressable Potentiometric Sensor with an Enzyme Reactor in a Flow Channel
Authors: Miyamoto Ko-ichiro , Yoshida Midori , Sakai Taito , Matsuzaka Atsushi , Wagner Torsten , Kanoh Shin'ichiro , Yoshinobu Tatsuo , Schöning Michael J.
Published: April 2011

The light-addressable potentiometric sensor (LAPS) was combined with an enzyme reactor in a fluidic channel. The fluidic channel was mounted on the sensor plate and the enzyme reactor was connected to the fluidic channel. The enzyme reactor was filled with glass beads as enzyme carrier modified by urease which catalyzed production of ammonia depending on the concentration of urea. Double-channel LAPS measurement was performed at the both side of upper and lower stream of the enzyme reactor which realized a differential measurement and eliminated the drift component of the measurement.
Planar Microfluidic System Based on Electrophoresis for Detection of 130-nm Magnetic Labels for Biosensing
Authors: Takamura Tsukasa , Morimoto Yoshitaka , Sandhu Adarsh
Published: April 2011

Superparamagnetic beads (SPBs) used as magnetic labels offer potential for the realization of high sensitivity and low cost biosensors for point of care treatment (POCT). For better biomolecular affinity and higher sensitivity, it is desirable to use sub-200-nm-diameter SPBs comparable in size to actual biomolecules. However, the detection of small concentrations of such SPBs by magnetoresistive devices is extremely challenging due to small magnetic response of SPBs. As a solution to these limitations, we describe a simple detecting procedure where the capture of micro-SPBs by immobilized nano-target SPBs due to self-assembly induced by an external magnetic field, which was monitored under an optical microscope. Here we describe biosensing system based on self-assembly of micro-SPBs by nanoSPBs targets using a system without external pumps, thereby enabling greater miniaturization and portability.
Positional Control of Crystal Grains in Silicon Thin Film Utilizing Cage-Shaped Protein
Authors: Tojo Yosuke , Miura Atsushi , Yamashita Ichiro , Uraoka Yukiharu
Published: April 2011

We have proposed a new crystallization method for silicon thin films utilizing a cage-shaped protein (ferritin), called “bio-nano crystallization”, which combines semiconductor processing technology and biotechnology. We utilized nickel nanoparticle-accommodated ferritins as metal catalysts, and succeeded in performing the crystallization. When the ferritin was adsorbed randomly onto the film, crystal nuclei were formed at random places, thus grain position was randomly distributed. In this study, we performed the positional controlled deposition of ferritin by electrostatic interaction for location control of crystal grains. Positively charged areas were formed on negatively charged SiO2 using 3-amino-propyltriethoxysilane (APTES) as the electrostatic pattern. As a result, we could optimize Ni ferritin concentration to make a large adsorption difference between APTES and amorphous silicon. Therefore, nickel nanoparticles adsorption areas were controlled using APTES patterns. Furthermore, the location control of crystallized areas was achieved by optimizing the concentration of Ni ferritin and the APTES pattern.
High-Speed Magnetoresistive Random-Access Memory Random Number Generator Using Error-Correcting Code
Authors: Tanamoto Tetsufumi , Shimomura Naoharu , Ikegawa Sumio , Matsumoto Mari , Fujita Shinobu , Yoda Hiroaki
Published: April 2011

A high-speed random number generator (RNG) circuit based on magnetoresistive random-access memory (MRAM) using an error-correcting code (ECC) post processing circuit is presented. ECC post processing increases the quality of randomness by increasing the entropy of random number. We experimentally show that a small error-correcting capability circuit is sufficient for this post processing. It is shown that the ECC post processing circuit powerfully improves the quality of randomness with minimum overhead, ending up with high-speed random number generation. We also show that coupling with a linear feedback shift resistor is effective for improving randomness.
Magnetic Anisotropy of GaMnAs Film and Its Application in Multi-valued Memory Devices
Authors: Lee Sanghoon , Yoo Taehee , Lee Hakjoon , Khym Sungwon , Liu Xinyu , Furdyna Jacek K.
Published: April 2011

The magnetic anisotropy of GaMnAs ferromagnetic semiconductor film was investigated by magneto-transport measurements. Values of cubic and uniaxial anisotropies were obtained by analyzing the angular dependences of Hall effects based on magnetic free energy. The presence of strong cubic anisotropy gave four magnetic easy axes in the film's plane, resulting in two-step switching behavior in field scans of the planar Hall effect. Asymmetric loops containing four resistance plateaus were observed in minor scans of the planar Hall effect owing to the formation of stable multi-domain structures during magnetization reversal. Four-valued memory function based on the four observed Hall resistance states was demonstrated by using appropriate sequences of magnetic field pulses.
Study on Device Parameters of Carbon Nanotube Field Electron Transistors to Realize Steep Subthreshold Slope of Less than 60 mV/Decade
Authors: Algul Berrin Pinar , Kodera Tetsuo , Oda Shunri , Uchida Ken
Published: April 2011

The gate-induced band-to-band tunneling in carbon nanotube field effect transistors (CNFETs) is studied by solving the Poissson and carrier transport equations self-consistently. The transmission coefficient through the bandgap has been calculated using the Wentzel–Kramers–Brillouin (WKB) approximation. The device parameters of CNFETs with uniformly doped source/drain (S/D) regions have been investigated to find the parameter window to observe subthreshold slope (SS) of less than 60 mV/dec. It is demonstrated that the band-to-band tunneling (BTBT) current can be significantly enhanced by reducing the thickness of inter-layer oxide (t
int) between the substrate and carbon nanotube (CNT). With a thin t
int of 10 nm (SiO2) and optimized S/D doping concentrations, a steep SS of less than 60 mV/dec can be achieved.
Metal-Catalyst-Free Growth of Silica Nanowires and Carbon Nanotubes Using Ge Nanostructures
Authors: Uchino Takashi , Hutchison John L. , Ayre Greg N. , Smith David C. , Groot Kees de , Ashburn Peter
Published: April 2011

The use of Ge nanostructures is investigated for the metal-catalyst-free growth of silica nanowires and carbon nanotubes (CNTs). Silica nanowires with diameters of 10–50 nm and lengths of ≤1 µm were grown from SiGe islands, Ge dots, and Ge nanoparticles. High-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) reveal that the nanowires grow from oxide nanoparticles on the sample surface. We propose that the growth mechanism is thermal diffusion of oxide through the GeO2 nanostructures. CNTs with diameters 0.6–2.5 nm and lengths of less than a few µm were similarly grown by chemical vapor deposition from different types of Ge nanostructures. Raman measurements show the presence of radial breathing mode peaks and the absence of the disorder induced D-band, indicating single walled CNTs with a low defect density. HRTEM images reveal that the CNTs also grow from oxide nanoparticles, comprising a mixture of GeO2 and SiO2.
Observation of Band Gap in Epitaxial Bilayer Graphene Field Effect Transistors
Authors: Tanabe Shinichi , Sekine Yoshiaki , Kageshima Hiroyuki , Nagase Masao , Hibino Hiroki
Published: April 2011

Bilayer graphene was grown on the Si-face of SiC by thermal decomposition. Its electronic properties were investigated in top-gate Hall bar devices. By controlling the carrier density using gate voltage, we were able to access the charge neutrality point. The conductance at the charge neutrality point showed a strong temperature dependence, and its temperature dependence was well fitted with thermal activation and variable-range hopping mechanisms. The electrical detection of a band gap opening in bilayer graphene grown on SiC is a promising step toward the realization of graphene-based electronics using epitaxial graphene.
New Observations and Impacts of Diameter-Dependent Annealing Effects in Silicon Nanowire Transistors
Authors: Wang Runsheng , Yu Tao , Huang Ru , Ding Wei , Wang Yangyuan
Published: April 2011

In this paper, an abnormal diameter-dependent annealing (DDA) effect in silicon nanowire transistors (SNWTs) is observed and its impacts on device performance are investigated. It is found that the implanted dopants diffuse faster in thin nanowires than those in thick nanowires during the rapid annealing process, which results in underestimating the length of designed source/drain (S/D) extension (SDE) region in SNWTs. The impacts of DDA on SNWTs are studied in terms of S/D series resistance (R
SD), tradeoff between parasitic capacitance and resistance, and process parameter dependence. The random dopant fluctuations (RDF) in nanowire SDE regions (SDE-RDF) are also discussed. The results indicate that SDE-RDF induced R
SD variation in SNWTs is enhanced by DDA effects, which aggravates the driving current variations with the downscaling of SNWT diameter.
Single-Electron Transport through Semiconducting Nanowires: A Comparison between Silicon and Germanium
Authors: Shin Sung-Kwon , Huang Shaoyun , Fukata Naoki , Ishibashi Koji
Published: April 2011

Single-electron transistors (SETs) have been fabricated with n-type monocrystalline silicon nanowires (SiNWs) and germanium nanowires (GeNWs) separately. Comparisons of the single-electron transport characteristics between them have been studied at low temperatures. Coulomb oscillations of both devices were found with almost equidistant peak spacing and largely varied peak heights, while charge-stability diagrams showed almost identical diamond-shaped dimensions. Noticeably, for the GeNW-SET, the spacing between neighboring Coulomb-oscillation peaks alternately changed in some gate-voltage (V
g) regions, indicating the even–odd effect. On the contrary, the SiNW-SET did not show such effect in the entire measured V
g region. The comparison indicates that the quantum effect is much more prominent in the GeNW-SET than in the SiNW-SET.
High Field-Emission Stability of Offset-Thin-Film Transistor-Controlled Al-Doped Zinc Oxide Nanowires
Authors: Yang Po-Yu , Wang Jyh-Liang , Tsai Wei-Chih , Wang Shui-Jinn , Lin Jia-Chuan , Lee I-Che , Chang Chia-Tsung , Cheng Huang-Chung
Published: April 2011

Aluminum-doped zinc oxide (AZO) nanowire (NW) arrays incorporating an offset thin-film transistor (offset-TFT) have been proposed to achieve high field-emission (FE) stability. The AZO NW field emission arrays (FEAs) were hydrothermally grown at a low temperature of 85 °C. The uncontrolled AZO NW FEAs demonstrated superior FE characteristics (i.e., turn-on field of ∼2.17 V/µm and threshold field of ∼3.43 V/µm) compared with those of the conventional CNT FEAs grown at a temperature below 600 °C. However, uncontrolled AZO NW FEAs show a larger current fluctuation of 15.6%. Therefore, the offset-TFTs were used to control the AZO NW FEAs. Consequently, the fluctuation of AZO NW FEAs could be significantly reduced to less than 2%. This novel field emission device exhibits good emission stability, low-voltage controllability, low-temperature processing, and structural simplicity, making it promising for applications in flat panel displays.
Microstructure of Polycrystalline Silicon Films Formed through Explosive Crystallization Induced by Flash Lamp Annealing
Authors: Ohdaira Keisuke , Ishii Shohei , Tomura Naohito , Matsumura Hideki
Published: April 2011

We perform transmission electron microscopy investigation of the microstructures of polycrystalline silicon (poly-Si) films formed through explosive crystallization (EC) induced by flash lamp annealing (FLA) of precursor amorphous silicon (a-Si) films. Two characteristic regions, formed periodically as a result of EC, show different microstructures: one consists of randomly oriented, densely packed fine grains of approximately 10 nm in size, whereas the other has relatively large (>100 nm), stretched grains, probably formed through liquid-phase epitaxy onto solid-phase-nucleated grains. Little a-Si tissue surrounding grains can be observed in the lattice images of flash-lamp-crystallized poly-Si films, which would be favorable for the rapid transport of photocarriers.
Modeling of Reduced Surface Field Laterally Diffused Metal Oxide Semiconductor for Accurate Prediction of Junction Condition on Device Characteristics
Authors: Saito Takashi , Tanaka Akihiro , Hayashi Takuro , Kikuchihara Hideyuki , Kanamoto Toshiki , Masuda Hiroo , Miyake Masataka , Amakawa Shuhei , Mattausch Hans J. , Miura-Mattausch Michiko
Published: April 2011

In this paper, an accurate model of resistive drift region in laterally diffused metal oxide semiconductor (LDMOS) is presented. We have improved the dynamically varying depletion width model at the drift/substrate junction, by considering both the drain and substrate bias effects. The both bias condition causes the resistance modification of the high resistive drift region for the reduced-surface-field structure (RESURF). In addition, new expansion-effect model is developed by considering the substrate bias effect. The developed models have been implemented into Hiroshima University starc insulated gate field effect transistor model for high voltage (HiSIM-HV) and it is proved that the models reproduce measured direct current (DC) characteristics for the wide variety of the substrate bias variation.
Effects of Surface and Crystalline Defects on Reverse Characteristics of 4H-SiC Junction Barrier Schottky Diodes
Authors: Katsuno Takashi , Watanabe Yukihiko , Fujiwara Hirokazu , Konishi Masaki , Yamamoto Takeo , Endo Takeshi
Published: April 2011

To clarify the relationship between the dispersed reverse characteristics of 4H-SiC junction barrier Schottky (JBS) diodes and defects, we investigated the sensitivity of the reverse characteristics to surface and crystalline defects in 4H-SiC epitaxial layers. Strong correlations were obtained between the reverse characteristics of 4H-SiC JBS diodes and surface defects. Micropipes or particles reduced blocking voltage and carrot defects increased leakage current. Furthermore, the leakage current of 4H-SiC JBS diodes depends on the etch pit density of threading dislocations (TDs). Etch pits formed from TDs increased leakage current by about 10-9 A in samples without surface defects. In addition, hexagonal etch pits formed from unusual crystalline defects were observed; they also increased leakage current.
First Principles Calculations of Defect Formation in In-Free Photovoltaic Semiconductors Cu2ZnSnS4 and Cu2ZnSnSe4
Authors: Maeda Tsuyoshi , Nakamura Satoshi , Wada Takahiro
Published: April 2011

To quantitatively evaluate the formation energies of Cu, Zn, Sn, and S vacancies in kesterite-type Cu2ZnSnS4 (CZTS), first-principles pseudopotential calculations using plane-wave basis functions were performed. The formation energies of neutral Cu, Zn, Sn, and S vacancies were calculated as a function of the atomic chemical potentials of constituent elements. We compared the vacancy formation in the In-free photovoltaic semiconductor CZTS with those of Cu2ZnSnSe4 (CZTSe) and CuInSe2 (CIS). The obtained results were as follows. (1) Under the Cu-poor and Zn-rich condition, the formation energy of the Cu vacancy was generally smaller than those of the Zn, Sn and S vacancies in CZTS, as is the case for CZTSe. (2) The formation energies of Cu, Zn, and Sn vacancies in CZTS were larger than those in CZTSe. On the other hand, the formation energy of the S vacancy is smaller than that of the Se vacancy in CZTSe. (3) Under the Cu-poor and Zn-rich condition, the formation energies of the Cu vacancy in CZTS and CZTSe are much larger than that in CIS. These results indicate that in kesterite-type CZTS and CZTSe, the Cu vacancy is easily formed under Cu-poor, Zn-rich, and S(Se)-rich condition, but it is more difficult than that in CIS.
Numerical Analysis of a Solar Cell with Tensile-Strained Ge as a Novel Narrow-Band-Gap Absorber
Authors: Hoshina Yutaka , Shimizu Masayuki , Yamada Akira , Konagai Makoto
Published: April 2011

A narrow-band-gap (0.6–0.7 eV) bottom cell absorber material is one of the most important and relatively undeveloped components for future five- or six-junction solar cells. Tensile-strained Ge is a promising material for a novel bottom cell absorber since it has a high absorption coefficient and an “adjustable” lattice constant. In this study, we numerically demonstrate the possibility of tensile-strained Ge as a bottom cell material for multijunction solar cells. The design examples of lattice-matched five-junction cells using tensile-strained Ge as bottom cells are also presented. It is shown that sub-µm tensile-strained Ge can produce same efficiency as that of a 100–300 µm bulk unstrained Ge cell. Using tensile-strained Ge as a bottom cell gives a higher efficiency and solves the lattice mismatch problem compared with traditional bottom cell materials such as a Ge substrate and a high In-composition InxGa1-xAs.
Application of Sputtered ZnO1-xSx Buffer Layers for Cu(In,Ga)Se2 Solar Cells
Authors: Okamoto Akira , Minemoto Takashi , Takakura Hideyuki
Published: April 2011

A CdS buffer layer is commonly used in high efficiency Cu(In,Ga)Se2 (CIGS) solar cells. The CdS layer is typically prepared by chemical bath deposition. From the viewpoint of the environmental impact of Cd and the need for in-line processing for mass production, there is a need to find an alternative to wet-chemically deposited CdS. In this study, ZnO1-x
S
x
thin films were prepared by radio frequency co-sputtering using ZnO and ZnS targets, which should allow for high controllability of the compositional ratios of O and S. The ZnO1-x
S
x
thin film with an S content x of 0.18 showed a band gap energy (E
g) of 2.9 eV. The performance of a CIGS solar cell with a ZnO0.82S0.18 buffer layer was compared with that with a CdS buffer layer which had an E
g of 2.5 eV. The CIGS solar cell with the ZnO0.82S0.18 buffer layer yielded an efficiency approaching that of the cell with the CdS buffer layer. This is due to a higher short-circuit current density due to both a suitable conduction band offset of the ZnO1-x
S
x
/CIGS interface and a decrease in absorption loss compared to the CIGS solar cell with the CdS buffer layer. These results indicate that, similar to CdS, ZnO1-x
S
x
can be very useful as a buffer layer in CIGS solar cells.
Improvement of Film Quality in CuInSe2 Thin Films Fabricated by a Non-Vacuum, Nanoparticle-Based Approach
Authors: Zhang Yiwen , Ito Manabu , Tamura Tomoaki , Yamada Akira , Konagai Makoto
Published: April 2011

To improve the quality of CuInSe2 (CIS) thin films fabricated by a non-vacuum, nanoparticle-based approach, in this study, two categories of nanoparticles, Cu (InGa) Se2 (CIGS) nanoparticles and copper selenide (Cu–Se) with indium selenide (In–Se) nanoparticles are investigated. It is found that the Cu–Se with In–Se nanoparticles show a higher crystallization velocity than CIGS nanoparticles. The films obtained from Cu–Se with In–Se nanoparticles exhibit higher crystallinity with a larger grain size. Thiourea is applied as a sintering additive during the selenization process. It is clarified that the addition of thiourea is very effective for grain growth and the fabrication of a dense CIS layer. The cell performance is measured under Air Mass 1.5 irradiation. The efficiency of the solar cell, fabricated using Cu–Se, In–Se nanoparticles with thiourea, is 2.15%, higher than that of the solar cell fabricated using CIGS nanoparticles, which is 0.28%.
Simulation of Temperature Characteristics of InGaP/InGaAs/Ge Triple-Junction Solar Cell under Concentrated Light
Authors: Sakurada Yuya , Ota Yasuyuki , Nishioka Kensuke
Published: April 2011

Using an equivalent circuit model, the temperature characteristics of an InGaP/InGaAs/Ge triple-junction solar cell under concentrated light conditions were analyzed in detail. The current–voltage (I–V) characteristics of the single-junction solar cells (InGaP, InGaAs, and Ge solar cells) were measured at various temperatures. From the dark I–V characteristics of each single-junction solar cell, the diode parameters and temperature exponents were extracted. The extracted diode parameters and temperature exponents were applied to the equivalent circuit model for the triple-junction solar cell, and the solar-cell performance was calculated. There was good agreement between the measured and calculated I–V characteristics of the triple-junction solar cell at various temperatures under concentrated light conditions.
Shallow Carrier Trap Levels in GaAsN Investigated by Photoluminescence
Authors: Inagaki Makoto , Suzuki Hidetoshi , Suzuki Akio , Mutaguchi Kazumasa , Fukuyama Atsuhiko , Kojima Nobuaki , Ohshita Yoshio , Yamagichi Masafumi
Published: April 2011

Shallow carrier trap levels in GaAs1-xNx(0.0010≤x≤0.0038) were investigated by photoluminescence (PL) and photoreflectance (PR) ranging from 4.2 to 300 K. The band gap energies of the GaAsN were clearly determined in the whole temperature range by the PR fitting analysis. It is clarified by peak decomposing that there were three emission peaks in the near-band-edge PL spectra of GaAsN. One of them was originated from band-to-band transition. The energies of two emission peaks were located at approximately 6 and 17 meV below the band edge. The existence of these peaks is evidence of carrier localization at the near-band-edge. The intensity ratio of the peak at the low energy side to other peaks increases with increasing N composition. This behavior is similar to the degradation of electrical properties.
In the Virtual Extension
Authors: CACM Staff
Published: April 2011

To ensure the timely publication of articles,
Communications
created the Virtual Extension (VE) to expand the page limitations of the print edition by bringing readers the same high-quality articles in an online-only format. VE articles undergo the same rigorous review process as those in the print edition and are accepted for publication on merit. The following synopses are from articles now available in their entirety to ACM members via the Digital Library.
Deus ex machina
Authors: Krakovsky Marina
Published: April 2011

Computational metaphysics is helping philosophers answer age-old questions, such as whether God exists.
Web science meets network science
Authors: Wright Alex
Published: April 2011

A pair of divergent scientific communities discusses their similarities and differences, and search for common ground.
Reaching learners beyond our hallowed halls
Authors: Dorn Brian
Published: April 2011

Rethinking the design of computer science courses and broadening the definition of computing education both on and off campus.
Bell labs and centralized innovation
Authors: Wu Tim
Published: April 2011

Replaying the long-term costs of monopolized innovation.
The importance of reviewing the code
Authors: Añel Juan A.
Published: April 2011

Highlighting the significance of the often overlooked underlying software used to produce research results.
Mobile application development
Authors: Charland Andre , Leroux Brian
Published: April 2011

Web apps are cheaper to develop and deploy than native apps, but can they match the native user experience?
Privacy-preserving network forensics
Authors: Afanasyev Mikhail , Kohno Tadayoshi , Ma Justin , Murphy Nick , Savage Stefan , Snoeren Alex C. , Voelker Geoffrey M.
Published: April 2011

Privacy-preserving attribution of IP packets can help balance forensics with an individual's right to privacy.
Self-similarity-based image denoising
Authors: Buades Antoni , Coll Bartomeu , Morel Jean-Michel
Published: April 2011

The search for efficient image denoising methods is still a valid challenge at the crossing of functional analysis and statistics. In spite of the sophistication of the recently proposed methods, most algorithms have not yet attained a desirable level of applicability. All show an outstanding performance when the image model corresponds to the algorithm assumptions but fail in general and create artifacts or remove image fine structures. The main focus of this paper is, first, to define a general mathematical and experimental methodology to compare and classify classical image denoising algorithms and, second, to describe the nonlocal means (NL-means) algorithm introduced in 2005 and its more recent extensions. The mathematical analysis is based on the analysis of the "method noise," defined as the difference between a digital image and its denoised version. NL-means, which uses image self-similarities, is proven to be asymptotically optimal under a generic statistical image model. The denoising performance of all considered methods are compared in four ways: mathematical, asymptotic order of magnitude of the method noise under regularity assumptions; perceptual-mathematical, the algorithms artifacts and their explanation as a violation of the image model; perceptual-mathematical, analysis of algorithms when applied to noise samples; quantitative experimental, by tables of
L
2
distances of the denoised version to the original image.
Challenges and business models for mobile location-based services and advertising
Authors: Dhar Subhankar , Varshney Upkar
Published: April 2011

Mobile advertising will become more pervasive and profitable, but not before addressing key technical and business challenges.
Is open source security a myth?
Authors: Schryen Guido
Published: April 2011

What does vulnerability and patch data say?
Invisible work in standard bibliometric evaluation of computer science
Authors: Wainer Jacques , Goldenstein Siome , Billa Cleo
Published: April 2011

Most of a computer scientist's production can go uncounted if a standard bibliographic service is used.
Diet-induced obesity leads to the development of leptin resistance in vagal afferent neurons
Authors: de Lartigue Guillaume , Barbier de la Serre Claire , Espero Elvis , Lee Jennifer , Raybould Helen E.
Published: April 2011

Ingestion of high-fat, high-calorie diets is associated with hyperphagia, increased body fat, and obesity. The mechanisms responsible are currently unclear; however, altered leptin signaling may be an important factor. Vagal afferent neurons (VAN) integrate signals from the gut in response to ingestion of nutrients and express leptin receptors. Therefore, we tested the hypothesis that leptin resistance occurs in VAN in response to a high-fat diet. Sprague-Dawley rats, which exhibit a bimodal distribution of body weight gain, were used after ingestion of a high-fat diet for 8 wk. Body weight, food intake, and plasma leptin levels were measured. Leptin signaling was determined by immunohistochemical localization of phosphorylated STAT3 (pSTAT3) in cultured VAN and by quantifaction of pSTAT3 protein levels by Western blot analysis in nodose ganglia and arcuate nucleus in vivo. To determine the mechanism of leptin resistance in nodose ganglia, cultured VAN were stimulated with leptin alone or with lipopolysaccharide (LPS) and SOCS-3 expression measured. SOCS-3 protein levels in VAN were measured by Western blot following leptin administration in vivo. Leptin resulted in appearance of pSTAT3 in VAN of low-fat-fed rats and rats resistant to diet-induced obesity but not diet-induced obese (DIO) rats. However, leptin signaling was normal in arcuate neurons. SOCS-3 expression was increased in VAN of DIO rats. In cultured VAN, LPS increased SOCS-3 expression and inhibited leptin-induced pSTAT3 in vivo. We conclude that VAN of diet-induced obese rats become leptin resistant; LPS and SOCS-3 may play a role in the development of leptin resistance.