Showing papers in "Japanese Journal of Applied Physics in 2003"

Base-Metal Electrode-Multilayer Ceramic Capacitors: Past, Present and Future Perspectives

Abstract: Multilayer ceramic capacitor (MLCC) production and sales figures are the highest among fine-ceramic products developed in the past 30 years. The total worldwide production and sales reached 550 billion pieces and 6 billion dollars, respectively in 2000. In the course of progress, the development of base-metal electrode (BME) technology played an important role in expanding the application area. In this review, the recent progress in MLCCs with BME nickel (Ni) electrodes is reviewed from the viewpoint of nonreducible dielectric materials. Using intermediate-ionic-size rare-earth ion (Dy2O3, Ho2O3, Er2O3, Y2O3) doped BaTiO3 (ABO3)-based dielectrics, highly reliable Ni-MLCCs with a very thin layer below 2 µm in thickness have been developed. The effect of site occupancy of rare-earth ions in BaTiO3 on the electrical properties and microstructure of nonreducible dielectrics is studied systematically. It appears that intermediate-ionic-size rare-earth ions occupy both A- and B-sites in the BaTiO3 lattice and effectively control the donor/acceptor dopant ratio and microstructural evolution. The relationship between the electrical properties and the microstructure of Ni-MLCCs is also presented.

Demonstration of X-Ray Talbot Interferometry

Abstract: First Talbot interferometry in the hard X-ray region was demonstrated using a pair of transmission gratings made by forming gold stripes on glass plates. By aligning the gratings on the optical axis of X-rays with a separation that caused the Talbot effect by the first grating, moire fringes were produced inclining one grating slightly against the other around the optical axis. A phase object placed in front of the first grating was detected by moire-fringe bending. Using the technique of phase-shifting interferometry, the differential phase corresponding to the phase object could also be measured. This result suggests that X-ray Talbot interferometry is a novel and simple method for phase-sensitive X-ray radiography.

Large Piezoelectric Constant and High Curie Temperature of Lead-Free Piezoelectric Ceramic Ternary System Based on Bismuth Sodium Titanate-Bismuth Potassium Titanate-Barium Titanate near the Morphotropic Phase Boundary

Abstract: A lead-free piezoelectric ceramic ternary system based on bismuth sodium titanate, (Bi1/2Na1/2)TiO3 (BNT) - bismuth potassium titanate (Bi1/2K1/2)TiO3 (BKT) - barium titanate BaTiO3 (BT) near the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases has been investigated. In the case of a(Bi1/2Na1/2)TiO3–bBaTiO3–c(Bi1/2K1/2)TiO3 [BNBK(100a/100b/100c)] solid solution ceramics, the highest piezoelectric constant d33=191 pC/N, Curie temperature, Tc=301°C, electromechanical coupling factor, k33=0.56 and dielectric constant, e33T/e0=1141 are observed for the BNBK(85.2/2.8/12) composition which has a tetragonal phase near the MPB. The d33 value is the highest so far reported for all lead-free piezoelectric ceramics with Tc>300°C. The BNT-BKT-BT ternary ceramics system sintered at 1200°C for 2 h in air has a pure perovskite structure and a high density more than 95% of the theoretical density.

RF-Molecular Beam Epitaxy Growth and Properties of InN and Related Alloys

Abstract: The fundamental band gap of InN has been thought to be about 1.9 eV for a long time. Recent developments of metalorganic vapor phase epitaxy (MOVPE) and RF-molecular beam epitaxy (RF-MBE) growth technologies have made it possible to obtain high-quality InN films. A lot of experimental results have been presented very recently, suggesting that the true band-gap energy of InN should be less than 1.0 eV. In this paper, we review the results of the detailed study of RF-MBE growth conditions for obtaining high-quality InN films. The full widths at half maximum (FWHMs) of ω-mode X-ray diffraction (XRD), ω–2θ mode XRD and E2 (high-frequency)-phonon-mode peaks in the Raman scattering spectrum of the grown layer were 236.7 arcsec, 28.9 arcsec and 3.7 cm-1, respectively. The carrier concentration and room temperature electron mobility were 4.9×1018 cm-3 and 1130 cm2/Vs, respectively. Photoluminescence and optical absorption measurements of these high-quality InN films have clearly demonstrated that the fundamental band gap of InN is about 0.8 eV. Studies on the growth and characterization of InGaN alloys over the entire alloy composition further supported that the fundamental band gap of InN is about 0.8 eV.

High Mobility Thin Film Transistors with Transparent ZnO Channels.

Abstract: We have fabricated high performance ZnO thin film transistors (TFTs) using CaHfOx buffer layer between ZnO channel and amorphous silicon?nitride gate insulator. The TFT structure, dimensions, and materials set are identical to those of the commercial amorphous silicon (a-Si) TFTs in active matrix liquid crystal display, except for the channel and buffer layers replacing a-Si. The field effect mobility can be as high as 7 cm2?V-1?s-1 for devices with maximum process temperature of 300?C. The process temperature can be reduced to 150?C without much degrading the performance, showing the possibility of the use of polymer substrate.

Preparation of Freestanding GaN Wafers by Hydride Vapor Phase Epitaxy with Void-Assisted Separation

Abstract: We have developed a novel technique for preparing large-scale freestanding GaN wafers. Hydride vapor phase epitaxy (HVPE) growth of thick GaN layer was performed on a GaN template with a thin TiN film on the top. After the cooling process of the HVPE growth, the thick GaN layer was easily separated from the template by the assistance of many voids generated around the TiN film. As a result, a freestanding GaN wafer was obtained. The wafer obtained had a diameter of 45 mm, and a mirror-like surface. The-full-width-at-half-maximum (FWHM) of (0002) and (1010) peaks in the X-ray rocking curve profile were 60 and 92 arcsec, respectively. The dislocation density was evaluated at 5×106 cm-3 by etch pit density measurement.

Large Tunneling Magnetoresistance at Room Temperature Using a Heusler Alloy with the B2 Structure

Abstract: A Co2Cr0.6Fe0.4Al Heusler alloy film exhibited a B2 structure, which was deposited using a magnetron sputtering system on a thermally oxidized Si substrate at room temperature without any buffer layers. The film exhibited the magnetic moment of 2.04µB per formula unit, nearly the integer number of Bohr magnetons, suggesting a localized nature of ferromagnetism similar to that of many Heusler compounds, which is a necessary condition for half metallicity. A spin- valve-type tunneling junction with a Co2(Cr, Fe)Al Heusler alloy film was fabricated using metal masks, which consists of Co2Cr0.6Fe0.4Al(10 nm)/AlOx (1.8 nm)/CoFe (3 nm)/NiFe (5 nm)/IrMn (15 nm)/Ta (5 nm), deposited on a thermally oxidized Si substrate without a buffer layer. The junction demonstrated large tunneling magnetoresistances of 16% at room temperature and 26.5% at 5 K.

Preparation of nm-Sized Barium Titanate Fine Particles and Their Powder Dielectric Properties

Abstract: Barium titanate (BaTiO3) crystallites with various particle sizes from 17 to 100 nm were prepared by the 2-step thermal decomposition method of barium titanyl oxalate (BaTiO(C2O4)24H2O). The crystal structure of these BaTiO3 particles was assigned to cubic m-3m by a X-ray diffraction (XRD) measurement while it was assigned to tetragonal 4mm by a Raman scattering measurement. Investigation of impurity in these particles using both TG-DTA and FT-IR measurements revealed that no impurity was detected in the BaTiO3 lattice while hydroxyl and carbonate groups were detected only on the surface. The dielectric constants of these powders were measured using suspensions by a modified powder dielectric measurement method. As a result, the dielectric constant of BaTiO3 particles with a size of around 70 nm exhibited a maximum of over 15,000. This study revealed that BaTiO3 particles with a size around 70 nm were the most desirable for capacitor application.

Red-enhanced white-light-emitting diode using a new red phosphor

Abstract: We fabricated a high-color-rendering, red-enhanced white-light-emitting diode (LED) using a new red phosphor and a short-wavelength YAG phosphor. When the new white-LED was operated at a forward-bias current of 20 mA at room temperature (RT), color temperature (Tcp), the general color rendering index (Ra) and luminous efficiency were 4670 K, 87.7 and 25.5 lm/W, respectively. Most of the color-rendering indexes (CRIs) of the new white-LED were larger than those of current white-LEDs, in which only YAG is used. In particular, the CRI-No.9 value, which shows the color reproduction in the red region, is improved from -2.5 to 62.6.

Separation of Gas Molecule Using Tetra-n-butyl Ammonium Bromide Semi-Clathrate Hydrate Crystals

Abstract: Tetra-n-butyl ammonium bromide (TBAB) forms a semi-clathrate hydrate crystal with water molecules even under atmospheric pressure. We found that TBAB hydrate could encage methane molecules in mixtures of methane and propane or methane and ethane. Our preliminary result of single-crystal analysis using X-ray shows that TBAB hydrate has empty cages, all of which are small, dodecahedral cages. Therefore, TBAB hydrate crystals can be used to separate small gas molecules which fit in these dodecahedral cages. We concluded that these empty unisized cages of TBAB hydrate crystals function as a sieve for gas molecules.

Photoluminescent and Structural Properties of Precipitated ZnO Fine Particles

Abstract: The correlation between photoluminescence (PL) and structural properties of ZnO crystals has been investigated using fine particles prepared by a precipitation method and subsequent heat treatment. The ZnO particles heat-treated at temperatures less than 600°C exhibited red emission centered at 1.9 eV for a weak light excitation similar to that observed in ZnO thin films sputter-grown under thermally non-equilibrium conditions. Heat treatment at high temperatures reduced the red PL while it enhanced the green PL around 2.45 eV. Raman scattering and X-ray diffraction measurements showed that the ZnO particles obtained at lower temperatures have a greater lattice disorder accompanied by a specific lattice expansion along the c-axis of the ZnO crystal which would introduce defect states in the band gap. In addition, electrical conductivity measurements suggested that the lattice disorder is associated with interstitial Zn. These results indicate that the lattice disorder along the c-axis of the ZnO crystal, i.e., the interstitial Zn is most likely responsible for the red PL observed for both ZnO particles and thin films. On the other hand, recombination centers such as singly charged oxygen vacancies contributing to the green emission may be too low in concentration to affect the structural quality.

Temperature Dependence of GaAs1-xBix Band Gap Studied by Photoreflectance Spectroscopy

Abstract: We performed photoreflectance (PR) spectroscopy in order to investigate the fundamental band gap of GaAs1-xBix alloys. The temperature dependence of the band gap energy was evaluated by analyzing of the Franz–Keldysh oscillation in the PR spectra. With increasing Bi content, the band gap energy of GaAs1-xBix alloy is reduced, which shows a large optical bowing. The temperature coefficient of the band gap decreases appreciably in alloys with increasing Bi content. For x=0.026, the temperature coefficient near room temperature is -0.15 meV/K which is 1/3 of the value for GaAs.

Langmuir-Blodgett Films of Single-Wall Carbon Nanotubes: Layer-by-layer Deposition and In-plane Orientation of Tubes

Abstract: The Langmuir–Blodgett technique has been applied to build optically homogeneous thin films of chemically solubilized single-wall carbon nanotubes (s-SWNTs) which possess good surface spreading properties at the air/water interface. Deposition can be performed in a layer-by-layer fashion up to 100 or more layers either by horizontal lifting or vertical dipping, allowing to readily control the film thickness. Their visible to near-infrared absorption spectra showing the characteristic features of semiconducting and metallic SWNTs prove the intactness of their one-dimensional electronic states during the preparation process. Polarized absorption spectroscopy and atomic force microscope (AFM) observation demonstrate that the tubes are oriented in the direction of the trough barrier (horizontal lifting) or in the dipping direction (vertical dipping). These are attributed to compression-induced or flow-induced orientation, respectively, the latter found to be much stronger than the former. The realization of homogeneous thin films of SWNTs with a controllable thickness and tube orientation should be an important basis for the future development of their scientific understanding and technological applications.

High Birefringence Isothiocyanato Tolane Liquid Crystals

Abstract: The phase transition temperatures, birefringence and visco-elastic coefficient of several high birefringence isothiocyanato tolane compounds were evaluated. The polarizability of these compounds was calculated by the Austin Model 1 (AM1) and Modified Neglect of Diatomic Overlap (MNDO) methods. Using these compounds, we have formulated a eutectic mixture exhibiting a wide nematic range and high figure of merit.

Memory Effect and Redistribution of Mg into Sequentially Regrown GaN Layer by Metalorganic Chemical Vapor Deposition

Abstract: Mg redistribution into a subsequently regrown GaN epilayer by metalorganic chemical vapor deposition (MOCVD) is studied. Dopant profiles from secondary ion mass spectrometry (SIMS) on n–p–n GaN samples have been analyzed. The regrowth study in a Mg-free reactor reveals that a Mg-rich film is present on MOCVD as-grown GaN:Mg base layers and can be removed by an acid etch, and that a slow Mg decay into the sequentially regrown GaN results from this Mg-rich surface film. We believe the commonly seen Mg memory effect in MOCVD causes the accumulation of Mg on the surface. From a MOCVD regrowth on n–p–n GaN grown by molecular beam epitaxy (MBE), the Mg diffusion constant is calculated to be about 3 ×10-15 cm2/s at 1160°C for Mg concentrations between 5 ×1017 cm-3 and 1 ×1019 cm-3. The roles of memory effect, surface segregation, and diffusion associated with Mg are addressed.

Electrostatically Levitated Ring-Shaped Rotational-Gyro/Accelerometer

Abstract: This paper reports an electrostatically levitated inertia measurement system which is based on the principle of a rotational gyro. The device has several advantages: the levitation of the rotor in a vacuum eliminates mechanical friction resulting in high sensitivity; the position control for the levitation allows accelerations to be sensed in the tri-axis; and the fabrication of the device by a micromachining technique has the cost advantages afforded by miniaturization. Latest measurements yield a noise floor of the gyro and that of the accelerometer as low as 0.15 deg/h1/2 and 30 µG/Hz1/2, respectively. This performance is achieved by a new sensor design. To further improve of the previous device, a ring-shaped structure is designed and fabricated by deep reactive ion etching using inductively coupled plasma. The rotor levitation is performed with capacitive detection and electrostatic actuation. Multiaxis closed-loop control is realized by differential capacitance sensing and frequency multiplying. The rotation of the micro gyro is based on the principle of a planar variable capacitance motor.

Molecular Beam Epitaxy of High Magnesium Content Single-Phase Wurzite MgxZn1-xO Alloys (x≃ 0.5) and Their Application to Solar-Blind Region Photodetectors

Abstract: A series of single-phase wurzite MgxZn1-xO alloys from x=0 to 0.5 were successfully obtained by molecular beam epitaxial growth on sapphire substrates, resulting in artificial tuning of band gap energy Eg from 3.3 to 4.5 eV which covers UV-A, UV-B, and solar-blind spectral regions. The problem of phase separation ever reported in the growth of MgxZn1-xO with higher Mg content, e.g., x>0.4 and Eg>3.9 eV, has been overcome by using a ZnO buffer layer. The Mg0.5Zn0.5O layers have been applied to a planar geometry Schottky type metal-semiconductor-metal photodetector, exhibiting the photoresponse for the wavelength shorter than 270 nm which correspond to the solar-blind region.

Nano-Sized Hollow Bump Array Generated by Single Femtosecond Laser Pulse

Abstract: The generation of a nano-sized hollow bump array of gold thin film by uniformly spaced melting and inflation of the film induced by a single shot of four interfering fs laser beams is reported. The shape changed from a bump, to a bead on bump, to a standing bead on a hole, and finally to a hole, with the increase in fs laser fluence. The bump height, diameter, and shape as functions of laser fluence are shown. Moreover, the basic shape of the bump changed upon changing the number of interfering beams.

Prospects of Doped Sb-Te Phase-Change Materials for High-Speed Recording

Abstract: Recording parameters such as crystallization rate, archival life stability and media noise have been studied for phase-change optical discs based on doped Sb–Te materials It has been found that the crystallization rate can be tuned easily by varying the Sb/Te ratio or the dopant atom; higher crystallization rates are obtained for higher Sb/Te ratios and for In- and Ga-doped Sb–Te However, it has been found that measures that enhance the crystallization rate often also influence other material properties, such as archival life stability and media noise For example, a higher Sb/Te ratio leads to a lower archival life and higher media noise As a consequence, the demands on doped Sb–Te materials to combine sufficiently high crystallization rate, good archival life stability and low media noise become increasingly difficult to satisfy for high recording speeds

Metastable GaAsBi alloy grown by molecular beam epitaxy

Abstract: GaAs1-xBix has been grown at a substrate temperature (Tsub) between 350 and 410°C by molecular beam epitaxy. The relationship between GaBi molar fraction (x) evaluated by Rutherford backscattering spectroscopy and the lattice constant showed good linearity. To achieve Bi incorporation into the epilayer, As flux was adjusted in a limited range on the brink of As shortage on the growing surface. The Bi incorporation was saturated at a large Bi flux, probably due to a low miscibility of Bi with GaAs. The value of x increased up to 4.5% with decreasing Tsub to 350°C.

Direct Synthesis of fct-FePt Nanoparticles by Chemical Route

Abstract: Direct synthesis of fct-FePt nanoparticles was achieved by reducing platinum and iron acetylacetonates in tetraethylene glycol (TEG) at 300°C. The particle diameter was between 5 and 10 nm. The X-ray diffraction profile of the as-prepared FePt particles exhibited the superlattice reflections (001) and (110), which signified the tetragonality. The Hk of 31 kOe measured at room temperature (RT) from hysteresis loss analysis confirmed the presence of FePt particles with fractionally ordered fct structure which posses substantially high anisotropy. However, the RT coercivity was only 370 Oe due to strong magnetostatic interaction of the particles.

Discontinuous Shift of Lasing Wavelength with Temperature in Cholesteric Liquid Crystal

Abstract: The laser action in a dye-doped cholesteric liquid crystal has been investigated as a function of temperature. The discontinuous shift of the lasing wavelength has been observed. It has also been found that the shift of the stop band in the cholesteric liquid crystal is discontinuous when the size of the measured area for the spectrum is small. These results have been interpreted to originate from the disappearance of every half period of the cholesteric liquid crystal helix in the unwinding process as the temperature decreases.

Mechanical Stability of Externally Deformed Indium–Tin–Oxide Films on Polymer Substrates

Abstract: In this study, we investigated the basic mechanical properties of indium–tin–oxide (ITO) films on polymer substrates which are exposed to externally inducing bending force. By using the Storney formula including the triple layer structure and bulge tests for measuring the conductive changes of patterned ITO islands as a function of bending curvature, the mechanical stability of ITO films on polymer substrates was intensively investigated. Numerical analyses and experimental results show that externally induced mechanical stress in the films depends on substrate material and its thickness, respectively. Therefore, an organic buffer layer was employed to improve the mechanical stability of ITO films, and then, the effects of the buffer layer were quantified in terms of conductivity–strain variations. As a result, it is found that a buffer layer is also a critical factor for determining the magnitude of mechanical stress and that the layer with Young's modulus lower than the specified value can contribute to relieving the mechanical stress of the films.

Carbon Nanofilm with a New Structure and Property

Abstract: We have prepared a carbon film of nanometer thickness, which is called here a carbon nanofilm (CNF), starting from the oxidation of graphite. The structure and thickness of the CNF are determined by high-resolution transmission electron microscopy and electron diffraction. The structure is of a new type (S.G.: P3), in which carbon six-membered-ring planes are stacked with the sequence of ...AA.... According to electron energy loss spectroscopy, a substantial amount of oxygen is detected but the molar ratio of oxygen to carbon is possibly decreased to less than 0.1. The CNF changes from an insulator to a semiconductor when reduced on heating at 250°C.

Synthesis of ZnO nanorods by nanoparticle assisted pulsed-laser deposition

Abstract: Nano-structured ZnO thin films were synthesized by the nanoparticle assisted pulsed-laser deposition in an oxygen background gas. Crystallized and c-axis oriented ZnO nanorods with a size of approximately 300 nm in average diameter and 6 µm in length were grown on sapphire substrates heated at approximately 700°C by the pulsed-laser deposition technique without any catalyst. Strong photoluminescence near the band-gap was observed from nanorods under excitation at 308 nm. The Rayleigh scattering diagnostics of the plume was also conducted, revealing that the nanorods grew from ZnO nanoparticles which formed in the plume and were transported onto the substrate.

Investigation of Gate-Induced Drain Leakage(GIDL) Current in Thin Body Devices: Single-Gate Ultra-Thin Body, Symmetrical Double-Gate, and Asymmetrical Double-Gate MOSFETs

Abstract: Gate-induced drain leakage (GIDL) current is investigated in single-gate (SG) ultra-thin body field effect transistor (FET), symmetrical double-gate (DG) FinFET, and asymmetrical DG metal oxide semiconductor field effect transistor (MOSFET) devices. Measured reductions in GIDL current for SG and DG thin-body devices are reported for the first time. The thin-body devices exhibit much lower GIDL current than bulk-Si MOSFETs, and the GIDL is found to decrease with decreasing body thickness. These results can be explained by the reduction in transverse electric field at the surface of the drain and the increase in transverse effective mass with decreasing body thickness.

Laser irradiated growth of protein crystal

Abstract: We succeeded in the first ever generation of protein crystals by laser irradiation. We call this process Laser Irradiated Growth Technique (LIGHT). Effective crystallization was confirmed by applying an intense femtosecond laser. The crystallization period was dramatically shortened by LIGHT. In addition, protein crystals were obtained by LIGHT from normally uncrystallized conditions. These results indicate that intense femtosecond laser irradiation generates crystal nuclei; protein crystals can then be grown from the nuclei that act as seeds in a supersaturated solution. The nuclei formation is possible primarily due to nonlinear nucleation processes of an intense femtosecond laser with a peak intensity of over a gigawatt (GW).

Ferroelectric and Piezoelectric Properties of KNbO3 Ceramics Containing Small Amounts of LaFeO3

Abstract: Dense KNbO3 ceramics have been successfully synthesized by pressure-less sintering under optimized heat-treatment conditions using a small amount of La2O3 and Fe2O3 additives KNbO3 forms (K1-xLax)(Nb1-xFex)O3 solid solutions and changes in the crystal system, depending on the additive content, from orthorhombic to tetragonal at x of 0020, and from tetragonal to cubic at x of 0200 or higher When only 0002 mol of La2O3 and Fe2O3 (x=0002) was added into KNbO3, the highest value (988%) of the theoretical density was obtained This specimen showed orthorhombic symmetry with a high Curie temperature of 420°C, and demonstrated a well-saturated ferroelectric hysteresis loop with large remanent polarization (Pr) of 18 µC/cm2, which is comparable to the value reported for pure KNbO3 ceramics fabricated by hot pressing Furthermore, the x=0002 specimen showed a planar electromechanical coupling ratio (kp) of 017 and piezoelectric d33 constant of 98 pC/N, regardless of the unsaturated poling state

Ferroelectric NaNbO3 Ceramics Fabricated by Spark Plasma Sintering

Abstract: High-density ceramic samples of NaNbO3 (NN) have been successfully fabricated by spark plasma sintering (SPS), which is a process that uses microscopic electrical discharge between particles under pressure. The obtained NN ceramic shows a hysteresis loop characteristic of ferroelectrics. The coercive electric field, Ec, and remanent polarization, Pr, are determined to be 12 kV/cm and 0.42 µC/cm2, respectively, for an unpoled sample and 16 kV/cm and 27 µC/cm2, respectively, for a poled sample. The NN ceramic also shows piezoelectricity with a planar electromechanical coupling factor, Kp, of about 20%. The ferroelectricity and piezoelectricity in the NN ceramic are stable at high temperatures. The piezoelectric characteristic does not change after annealing at 300°C but it disappears after annealing at 350°C.

Homoepitaxial Growth of High-Quality Zn-Polar ZnO Films by Plasma-Assisted Molecular Beam Epitaxy

Abstract: High-quality ZnO films have been grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy. With increasing O/Zn ratio from the stoichiometric to the O-rich flux condition, the growth mode and the surface morphology changed from three-dimensional growth with a rough surface to two-dimensional growth with a smooth surface. The minimum linewidth from the (1010) ω-rocking curve was 100 arcsec, and the n = 2 state of A-exciton was clearly observed in the photoluminescence at 4.2 K. Due to the reduction in the edge-type threading dislocation density, the residual carrier concentration in these homoepitaxial ZnO films was as low as 2.2×1016 cm-3, which is one order of magnitude lower than that previously reported for heteroepitaxial ZnO films.