Showing papers in "Japanese Journal of Applied Physics in 2005"

TiO2 Photocatalysis: A Historical Overview and Future Prospects

Abstract: Photocatalysis has recently become a common word and various products using photocatalytic functions have been commercialized. Among many candidates for photocatalysts, TiO2 is almost the only material suitable for industrial use at present and also probably in the future. This is because TiO2 has the most efficient photoactivity, the highest stability and the lowest cost. More significantly, it has been used as a white pigment from ancient times, and thus, its safety to humans and the environment is guaranteed by history. There are two types of photochemical reaction proceeding on a TiO2 surface when irradiated with ultraviolet light. One includes the photo-induced redox reactions of adsorbed substances, and the other is the photo-induced hydrophilic conversion of TiO2 itself. The former type has been known since the early part of the 20th century, but the latter was found only at the end of the century. The combination of these two functions has opened up various novel applications of TiO2, particularly in the field of building materials. Here, we review the progress of the scientific research on TiO2 photocatalysis as well as its industrial applications, and describe future prospects of this field mainly based on the present authors' work.

Recent Advances in X-ray Phase Imaging

Abstract: Since the middle of the 1990s, X-ray phase imaging including phase tomography has been attracting increasing attention. The advantage of X-ray phase imaging is that an extremely high sensitivity is achieved for weak-absorbing materials, such as biological soft tissues, which generate a poor contrast by conventional methods. Medical and biological imaging is the main target of X-ray phase imaging, and several trials using synchrotron radiation sources and laboratory sources have been made. Measuring and controlling the X-ray phase are also significant for X-ray microscopy with a high spatial resolution, and innovative techniques are attracting intense interest. The progress of X-ray phase imaging is supported by the developments in X-ray sources such as third-generation synchrotron radiation sources, optical elements, and image detectors. This article describes the advantages of using X-ray phase information and reviews various techniques studied for X-ray phase imaging.

III-Nitride UV Devices

Abstract: The need for efficient, compact and robust solid-state UV optical sources and sensors had stimulated the development of optical devices based on III–nitride material system. Rapid progress in material growth, device fabrication and packaging enabled demonstration of high efficiency visible-blind and solar-blind photodetectors, deep-UV light-emitting diodes with emission from 400 to 250 nm, and UV laser diodes with operation wavelengths ranging from 340 to 350 nm. Applications of these UV optical devices include flame sensing; fluorescence-based biochemical sensing; covert communications; air, water and food purification and disinfection; and biomedical instrumentation. This paper provides a review of recent advances in the development of UV optical devices. Performance of state-of-the-art devices as well as future prospects and challenges are discussed.

Blue Light-Emitting Diode Based on ZnO

Abstract: A near-band-edge bluish electroluminescence (EL) band centered at around 440 nm was observed from ZnO p–i–n homojunction diodes through a semi-transparent electrode deposited on the p-type ZnO top layer. The EL peak energy coincided with the photoluminescence peak energy of an equivalent p-type ZnO layer, indicating that the electron injection from the n-type layer to the p-type layer dominates the current, giving rise to the radiative recombination in the p-type layer. The imbalance in charge injection is considered to originate from the lower majority carrier concentration in the p-type layer, which is one or two orders of magnitude lower than that in the n-type one. The current-voltage characteristics showed the presence of series resistance of several hundreds ohms, corresponding to the current spread resistance within the bottom n-type ZnO. The employment of conducting ZnO substrates may solve the latter problem.

Si Nano-Photodiode with a Surface Plasmon Antenna

Abstract: Nano-photodiodes with a subwavelength active area using the optical near-field enhanced by surface plasmon resonance are proposed. We fabricated a Si Schottky photodiode that consists of an active area of 300 nm in diameter and a surface plasmon antenna to generate the carrier within the active area efficiently. The fabricated photodiode shows an increase of the photocurrent by several tenfold compared to that without a surface plasmon antenna. This result suggests an enhanced photogeneration of carriers in a semiconductor via surface plasmon resonance. Such a Si nano-photodiode is a potential high-speed optical signal detector because the opto-electronic conversion process occurs within a subwavelength scale.

Strongly Enhanced Phosphor Efficiency in GaInN White Light-Emitting Diodes Using Remote Phosphor Configuration and Diffuse Reflector Cup

Abstract: Enhancement of phosphor efficiency is reported for GaInN-based white light-emitting diodes (LEDs) employing a large separation between the primary LED emitter and the wavelength converter, and a diffuse reflector cup. Ray-tracing simulations show that extraction efficiency of wavelength-converted light is enhanced by 75%. The experimental improvement in phosphor efficiency of blue-pumped yellow phosphor is 15.4% compared with conventional phosphor-based white LEDs. The improvement is attributed to reduced re-absorption of wavelength-converted light by the LED chip.

Dependence of Giant Tunnel Magnetoresistance of Sputtered CoFeB/MgO/CoFeB Magnetic Tunnel Junctions on MgO Barrier Thickness and Annealing Temperature

Abstract: We investigated the dependence of giant tunnel magnetoresistance (TMR) on the thickness of an MgO barrier and on the annealing temperature of sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions deposited on SiO2/Si wafers. The resistance-area product exponentially increases with MgO thickness, indicating that the quality of MgO barriers is high in the investigated thickness range of 1.15–2.4 nm. High-resolution transmission electron microscope images show that annealing at 375°C results in the formation of crystalline CoFeB/MgO/CoFeB structures, even though CoFeB electrodes are amorphous in the as-sputtered state. The TMR ratio increases with annealing temperature and is as high as 260% at room temperature and 403% at 5 K.

Raman Scattering Study of Piezoelectric (Na0.5K0.5)NbO3-LiNbO3 Ceramics

Abstract: A Raman scattering study of (Na0.5K0.5)NbO3 (NKN)-LiNbO3 (LN) lead-free piezoceramics has been carried out on nominal compositions of (1-x)NKN-xLN (0 ≤x ≤0.70). The Raman spectra demonstrated a variety of changes with x, mainly classified as lattice translations involving motions of the alkaline cations and internal modes of NbO6 octahedra. At 0.05 ≤x ≤0.07, the broadening of the scattering peaks corresponding to the internal modes of the NbO6 octahedra occurred preferably, which is consistent with the evidence for a morphotropic phase boundary (MPB). Furthermore, an abrupt shift was observed in the peak position of the symmetric stretching mode v1 to a higher frequency, resulting from the distortion of O-Nb-O angles caused by incorporating small Li ions into the perovskite units. This is considered to be a prologue for the structural transformation in the NKN-LN solid solution from orthorhombic to tetragonal symmetry. At compositions in which x increases above the MPB, the translational mode of the Li+ cation emerged clearly, and the overall scattering pattern gradually changed to complex patterns caused by the formation of a tungsten-bronze K3Li2Nb5O15 (KLN) secondary phase and an increase in the number of distorted LiNbO3-type crystal units.

Piezoelectric Windmill: A Novel Solution to Remote Sensing

Abstract: This study demonstrates a technology, "Piezoelectric Windmill", for generating the electrical power from wind energy. The electric power-generation from wind energy is based on piezoelectric effect and utilizes the bimorph actuators. Piezoelectric Windmill consists of piezoelectric actuators arranged along the circumference of the mill in the cantilever form. Using the camshaft gear mechanism an oscillating torque is generated through the flowing wind and applied on the actuators. A working prototype was fabricated utilizing 12 bimorphs (60 ×20 ×0.5 mm3) having a preload of 23.5 gm. Under a nominal torque level corresponding to normal wind flow and oscillating frequency of 6 Hz, a power of 10.2 mW was successfully measured across a load of 4.6 kΩ after rectification. Combined with the wireless transmission, this technology provides a practical solution to the remote powering of sensors and communication devices.

High Curie Temperature and Nano-Scale Spinodal Decomposition Phase in Dilute Magnetic Semiconductors

Abstract: We show that spinoadal decomposition phase in dilute magnetic semiconductors (DMS) offers the possibility to have high Curie temperatures (TC) even if the magnetic exchange interaction is short ranged. The spinodal decomposition is simulated by applying the Monte Carlo method to the Ising model with realistic (ab initio) chemical pair interactions between magnetic impurities in DMS. Curie temperatures are estimated by the random phase approximation with taking disorder into account. It is found that the spinodal decomposition phase inherently occurs in DMS due to strong attractive interactions between impurities. This phase decomposition supports magnetic network over the dimension of the crystal resulting in a high-TC phase.

Current-Driven Magnetization Switching in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

Abstract: Current-driven magnetization switching in low-resistance Co40Fe40B20/MgO/Co40Fe40B20 magnetic tunnel junctions (MTJs) is reported. The critical-current densities Jc required for current-driven switching in samples annealed at 270 and 300°C are found to be as low as 7.8×105 and 8.8×105 A/cm2 with accompanying tunnel magnetoresistance (TMR) ratios of 49 and 73%, respectively. Further annealing of the samples at 350°C increases TMR ratio to 160%, while accompanying Jc increases to 2.5×106 A/cm2. We attribute the low Jc to the high spin-polarization of tunnel current and small MsV product of the CoFeB single free layer, where Ms is the saturation magnetization and V the volume of the free layer.

Characterization of Planar Semipolar Gallium Nitride Films on Spinel Substrates

Abstract: Specular, planar semipolar gallium nitride films were grown by hydride vapor phase epitaxy. Planar films of (1013) and (1122) GaN have been grown on (1010) m-plane sapphire. The in-plane epitaxial relationship for (1013) GaN was [3032]GaN ∥[1210]sapphire and [1210]GaN ∥[0001]sapphire. The in-plane epitaxial relationship for (1122) GaN was [1121]GaN ∥[0001]sapphire and [1100]GaN ∥[1210]sapphire. The (1013) films were determined to have N-face sense polarity and a threading dislocation density of 9×108 cm-2. The (1122) films have Ga-face sense polarity and have a threading dislocation density of 2×1010 cm-2. The basal plane stacking fault density was 2 ×105 cm-1 for both orientations. The RMS roughness of the films was under 10 nm for a 5 ×5 µm2 area.

Demonstration of Nonpolar m-Plane InGaN/GaN Light-Emitting Diodes on Free-Standing m-Plane GaN Substrates

Abstract: We report the fabrication of nonpolar m-plane InGaN/GaN multiple-quantum well light-emitting diodes (LEDs) on free-standing m-plane GaN substrates. On-wafer continuous wave output power of 240 µW was measured at 20 mA for a 300×300 µm2 device, and output power as high as 2.95 mW was measured at 300 mA. There was no sign of saturation of the output power at high drive currents. An emission peak at 450 nm was obtained on electroluminescence measurements with high drive currents. The current-voltage characteristics of these LEDs showed rectifying behavior with a turn-on voltage of 3–4 V.

Electronic Properties of Amorphous and Crystalline Ge2Sb2Te5 Films

Abstract: Optical and electrical properties of sputtered Ge2Sb2Te5 films in amorphous and crystalline states have been studied. The optical band-gaps of amorphous, cubic (NaCl-type), and hexagonal Ge2Sb2Te5 are 0.74, 0.5, and 0.5 eV, respectively. Electrically, the amorphous and cubic states behave as semiconductors with activation energies of 0.45 and 0.14 eV, while the hexagonal state is metallic. The resistivity decreases slightly at the melting point of ~600°C. All the states show p-type thermoelectric power, in which the amorphous and the cubic state have the activation energies of 0.3 and 0.14 eV. Carrier parameters and electronic densities-of-states are estimated and considered.

Huge Spin-Polarization of L21-Ordered Co2MnSi Epitaxial Heusler Alloy Film

Abstract: Magnetic tunnel junctions (MTJs) with a stacking structure of epitaxial Co2MnSi/Al–O barrier/poly-crystalline Co75Fe25 were fabricated using an ultrahigh vacuum sputtering system. The epitaxial Co2MnSi bottom electrode exhibited highly ordered L21 structure and very smooth surface morphology. Observed magnetoresistance (MR) ratios of 70% at room temperature (RT) and 159% at 2 K are the highest values to date for MTJs using a Heusler alloy electrode. A high spin-polarization of 0.89 at 2 K for Co2MnSi obtained from Julliere's model coincided with the half-metallic band structure that was predicted by theoretical calculations.

Ferroelectric and Piezoelectric Properties of (Bi1/2K1/2)TiO3 Ceramics

Abstract: The dielectric, ferroelectric and piezoelectric properties of bismuth potassium titanate, (Bi1/2K1/2)TiO3 (BKT), ceramics were studied. Single-phase BKT ceramics with a high relative density of 97% were obtained by the hot pressing (HP) method. The resistivities of BKT ceramics hot-pressed at 1060 and 1080°C (hereafter abbreviated as BKT-HP1060°C and BKT-HP1080°C) were fairly high being of the order of 1013 Ωcm at room temperature (RT). The Curie temperature Tc of BKT-HP1060°C was 437°C, which is relatively higher than those of other lead-free piezoelectric materials. In this study, the ferroelectric properties of BKT ceramics were successfully obtained with fully saturated hysteresis loops. The remanent polarization Pr and coercive field Ec of BKT-HP1080°C were 22.2 µC/cm2 and 52.5 kV/cm, respectively. D–E hysteresis loops for these ceramics were observed even at 260°C. The electromechanical coupling factor k33 and piezoelectric constant d33 of BKT-HP1080°C were 0.28 and 69.8 pC/N, respectively. The second-phase transition temperature T2 of 340°C was determined from the temperature dependence of piezoelectric and dielectric measurements.

Interface Properties of Metal–Oxide–Semiconductor Structures on 4H-SiC{0001} and (1120) Formed by N2O Oxidation

Abstract: 4H-SiC(0001), (0001), and (1120) have been directly oxidized by N2O at 1300°C, and metal–oxide–semiconductor (MOS) interfaces have been characterized. The interface state density has been significantly reduced by N2O oxidation on any face, compared to conventional wet O2 oxidation at 1150°C. Planar n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) fabricated on 4H-SiC(0001), (0001) and (1120) faces have shown effective channel mobilities of 26, 43, and 78 cm2/Vs, respectively. Secondary ion mass spectrometry analyses have revealed a clear pileup of nitrogen atoms near the MOS interface. The thickness of the interfacial transition layer can be decreased by N2O oxidation. The crystal face dependence of the interface structure is discussed. A simple consideration of chemistry indicates that NO, generated from the decomposition of N2O, may be a more efficient oxidant of carbon than O2.

Electronic and Excitonic Structures of Inorganic–Organic Perovskite-Type Quantum-Well Crystal (C4H9NH3)2PbBr4

Abstract: The electronic and excitonic structures of an inorganic–organic perovskite-type quantum-well crystal (C4H9NH3)2PbBr4 have been investigated by optical absorption, photoluminescence, electroabsorption, two-photon absorption, and magnetoabsorption spectroscopies. Excitons in (C4H9NH3)2PbBr4 are of the Wannier-type, and ns (n≥2) excitons form an ideal two-dimensional Wannier exciton system. The binding energy, longitudinal–transverse splitting energy, and exchange energy of 1s excitons have been determined to be 480, 70 and 31 meV, respectively. These high values originate from both a strong two-dimensional confinement and the image charge effect. These values are larger than those in (C6H13NH3)2PbI4, owing to the smaller dielectric constant of the well layer in (C4H9NH3)2PbBr4 than that in (C6H13NH3)2PbI4. The seemingly unusual electric-field dependence of excitons resonance is also reasonably understood by taking the image charge effect into account.

DNA Analysis Chip Based on Field-Effect Transistors

Abstract: We have been developing a genetic field-effect transistor (FET) based on the potentiometric detection of hybridization and intercalation on the Si3N4 gate insulator. In this study, we demonstrated the detection of charge density change as a result of hybridization and intercalation using genetic FETs. Since the electrical output signal is obtained with the genetic FET without any labeling reagent, as compared with the conventional fluorescence-based DNA chips, the genetic FET platform is suitable for a simple and inexpensive system for genetic analysis in clinical diagnostics.

Electrical Properties of Carbon Nanotube Bundles for Future Via Interconnects

Abstract: We have developed carbon nanotube (CNT) vias consisting of about 1000 tubes using thermal chemical vapor deposition (CVD) at a growth temperature of 450°C with cobalt catalysts, titanium carbide ohmic contacts, and tantalum barrier layers on copper wiring. The lowest resistance obtained was about 5 Ω/via. The total resistance of the CNT via was three orders of magnitude lower than that of one CNT, indicating that the current flows in parallel through about 1000 tubes. No degradation was observed for 100 hours at via current densities of 2×106 A/cm2, which is favorably compared with Cu vias.

Welding of Transparent Materials Using Femtosecond Laser Pulses

Abstract: We report on laser welding between transparent materials without the insertion of intermediate layers such as glue by use of near-infrared femtosecond laser pulses. When femtosecond laser pulses are focused at the interface of transparent materials, the material around the focal point is melted and resolidified because of the temperature increases due to the localized nonlinear absorption of optical pulse energy. We experimentally succeeded in laser welding between two pieces of silica glass without the insertion of an intermediate layer by femtosecond laser pulses. This technique has a possible application to the joining of semiconductors such as silicon crystals.

Effect of Li Substitution on the Piezoelectric Properties of Potassium Sodium Niobate Ceramics

Abstract: (K,Na,Li)NbO3 (KNN) ceramics with 0.38 mol % K5.4Cu1.3Ta10O29 (KCT) have been prepared by a solid state reaction and their electrical properties were examined. The Curie temperature of (K,Na,Li)NbO3 increased with increasing Li substitution in both the Na and K sites. The 6 mol % Li-substituted sample showed a tetragonal phase at room temperature. Furthermore, a 1–2 mol % Li substitution for Na or K in (K0.5Na0.5)NbO3 improved several of its electrical properties. Most importantly, (K0.49Li0.01Na0.5)NbO3 attained high kp and Qm values of 0.43 and 2000, respectively. The value of the field-induced strain was also increased by substituting Li into (K0.5Na0.5)NbO3. The piezoelectric constant d33 of (K0.5Li0.02Na0.48)NbO3 was approximately 200 pm/V, which was calculated from the slope of the field-induced strain curve at 30–40 kV/cm under a unipolar driving field.

Brilliant Blue Observation from a Morpho-Butterfly-Scale Quasi-Structure

Abstract: The Morpho-butterfly wing reflects interfered brilliant blue, which originates from nanostructures on its scales, for any incidence angle of white light. We have, for the first time, fabricated a Morpho-butterfly-scale quasi-structure using focused-ion-beam chemical-vapor-deposition (FIB-CVD) and observed brilliant blue reflection from this quasi-structure with an optical microscope. We measured the reflection from real Morpho-butterfly scales and from the quasi-structure with a photonic multi-channel spectral analyzer system. The reflection spectra of the quasi-structure were very similar to those of Morpho-butterfly scales.

Evaluation of spin-transfer switching in CoFeB/MgO/CoFeB magnetic tunnel junctions

Abstract: Current-induced magnetization switching was demonstrated on Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junctions (MTJs), which exhibited giant tunnel magnetoresistance ratios of about 100%. Switching current density at a pulse duration of 100 ms was about 6×106 A/cm2 at room temperature. The switching current density was reduced to one-third of the smallest value for the MgO-based MTJs reported to date. Dependence of the switching current on pulse duration and on the external magnetic field was discussed based on a theoretical model incorporating thermally activated spin-transfer switching. The spin-transfer switching in the MgO-based MTJs realizes low writing power consumption and a high read-out signal in high-density magnetoresistive random access memory.

Ta-doped Anatase TiO2 Epitaxial Film as Transparent Conducting Oxide

Abstract: We present electrical transport and optical properties of Ta-doped TiO2 epitaxial thin films with varying Ta concentration grown by the pulsed laser deposition method. The Ti0.95Ta0.05O2 film exhibited a resistivity of 2.5×10-4 Ω cm at room temperature, and an internal transmittance of 95% in the visible light region. These values are comparable to those of a widely used transparent conducting oxide (TCO), indium tin oxide. Furthermore, this new material falls into a new category of TCOs that utilizes d electrons.

Hall Effect of Quasi-Hole Gas in Organic Single-Crystal Transistors

Abstract: Hall effect is detected in organic field-effect transistors, using appropriately shaped rubrene (C42H28) single crystals. It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The presence of the normal Hall effect means that the electromagnetic character of the surface charge is not of hopping carriers but resembles that of a two-dimensional hole-gas system.

Nanoparticles with Multiple Perfluorocarbons for Controllable Ultrasonically Induced Phase Shifting

Abstract: In studying the feasibility of developing tissue-targeted contrast media that can be administered as liquids and vaporized by an external stimulus such as ultrasound, we have investigated the properties of emulsions of nanometer-size particles containing perfluoropentane and 2H,3H-perfluoropentane. We found that the ultrasound intensity required to induce echographically significant vaporization can be controlled by changing the ratio of 2H,3H-perfluoropentane to perfluoropentane and that the intensity threshold increases as this ratio increases. Significant azeotropic phenomena were not observed when the perfluorocarbon mixtures were heated, which indicates that mechanisms other than azotropy are involved in the threshold change. The vaporization of 2H,3H-perfluoropentane may have been due not only to ultrasound energy but also to the energy deposited by ultrasonically induced bubbles of perfluoropentane. Our results might lead to a phase-shift contrast agent with controllable ultrasound energy for phase shifting.

Optical Band Gap of Barium Titanate Nanoparticles Prepared by RF-plasma Chemical Vapor Deposition

Abstract: The size dependence of the band gap for barium titanate nanoparticles prepared by RF-plasma chemical vapor deposition was investigated by measuring the diffuse reflectance spectra. The band gap of the indirect transition gradually increased, as the particle sizes decreased to less than 11.5 nm. Consequently, the band gap for particles of 6.7 nm in size was measured to be 3.47 eV, which was approximately 0.25 eV larger than that of BaTiO3 bulk. This phenomenon was discussed on the basis of quantum confinement model.

Development of Piezoelectric Thin Film Resonator and Its Impact on Future Wireless Communication Systems

Abstract: The bulk acoustic wave filter composed of piezoelectric thin film resonators has many features superior to those of other small filters such as a surface acoustic wave (SAW) filter and a ceramic filter. As it has no fine structure in its electrode design, it has a high Q factor that leads to low-loss and sharp-cut off characteristics and a high power durability particularly in the high-frequency range. Furthermore, it has the potentiality of integrated devices on a Si substrate. In this paper, we review the recent developments of piezoelectric thin film resonator filters in the world, including our development for mobile communication applications. After describing the feature and history of the piezoelectric thin film resonator filters, our technologies are introduced in focusing on the resonator structures, the piezoelectric thin film and electrode film materials, the cavity structures, the filter structure and its design rules and characteristics, comparing with SAW filters. The competition and coexistence between the piezoelectric thin film resonator filters and the SAW filters are also described. In this paper, we describe the development of a piezoelectric thin film resonator from the standpoint of researchers who have a long experience of SAW filter development.

Hydrogen Gas Generation by Splitting Aqueous Water Using n-Type GaN Photoelectrode with Anodic Oxidation

Abstract: Hydrogen gas generation from a counterelectrode was clearly observed for the first time using light-illuminated n-type GaN as a working photoelectrode in an electrolyte. The application of extra bias to a working electrode was required to obtain a sufficient volume of generated gas. The reactions at the GaN photoelectrode were both GaN decomposition and water oxidization, simultaneously.