Showing papers by "Tokyo University of Science published in 1997"

Luminescences from localized states in InGaN epilayers

Abstract: Optical spectra of the bulk three-dimensional InGaN alloys were measured using the commercially available light-emitting diode devices and their wafers. The emission from undoped InxGa1−xN(x<0.1) was assigned to the recombination of excitons localized at the potential minima originating from the large compositional fluctuation. The emission from heavily impurity-doped InGaN was also pointed out related to the localized states.

Biaxial strain dependence of exciton resonance energies in wurtzite GaN

Abstract: We have systematically studied the strain dependence of the free-exciton resonance energies in wurtzite GaN by photoreflectance measurements using well-characterized samples. The experimental data have been analyzed using the appropriate Hamiltonian for the valence bands in wurtzite GaN and determined the values of the crystal field splitting, the spin–orbit splitting, the shear deformation potential constants, and the energy gap in the unstrained crystal. Discussions are given on the strain dependence of the energy gaps, of the effective masses, and of the binding energies for the free-exciton ground states as well as on the valence-band parameters.

Lead-Free Piezoelectric Ceramics of (Bi1/2Na1/2)TiO3–1/2(Bi2O3·Sc2O3) System

Abstract: Bismuth sodium titanate, (Bi1/2Na1/2)TiO3 (BNT)-based solid solution, (1- x)(Bi1/2Na1/2)TiO3– x1/2(Bi2O3Sc2O3) [BNST-100 x], was studied in order to determine its dielectric, ferroelectric and piezoelectric properties as a new group of lead-free piezoelectric ceramics. Measurements of dielectric and piezoelectric properties show that the BNST-100 x ceramics of composition near BNT composition have relatively low free permittivity, e33 T/e0 (=431) and high electromechanical coupling factor, k33 (=0.42). These ceramics are good candidates for use as lead-free piezoelectric ceramics.

Brillouin scattering study of gallium nitride: elastic stiffness constants

Abstract: High-resolution Brillouin scattering measurements on a high-quality wurtzite gallium nitride (GaN) single crystal were carried out and elastic stiffness constants were determined. A comparison is given with the results of a recently reported model for calculation of the elastic constants of III-V semiconductors based on the modified version of Keyes's relations. A good agreement is found between the experimental and theoretical elastic constants for GaN.

Photocatalytic decomposition of water into h2 and o2 over novel photocatalyst k3ta3si2o13 with pillared structure consisting of three tao6 chains

Abstract: The novel catalysts of native and NiO-loaded K3Ta3Si2O13 with pillared structure consisting of three linear TaO6 chains showed luminescence and high photocatalytic activities for the decomposition of distilled water into H2 and O2 in stoichiometric amounts under UV irradiation.

A low-voltage, low-power CMOS fifth-order elliptic GM-C filter for baseband mobile, wireless communication

Abstract: The design and performance of a rail-to-rail low-voltage CMOS fifth-order elliptic low-pass GM-C filter for baseband mobile communication are presented. The operational transconductance amplifier (OTA) used in this filter is a low-voltage rail-to-rail voltage-to-current converter (V-I converter). In this V-I converter, an N-type V-I converter cell is connected in parallel with its counterpart, a P-type V-I converter cell, to achieve common-mode (CM) rail-to-rail operation. Two maximum-current selecting circuits and an output current subtraction circuit are utilized to generate constant-g/sub m/ output currents for this OTA. This fifth-order elliptic low-pass GM-C filter operates at a supply voltage of 3 V and has a cutoff frequency of 280 to 405 kHz. It provides up to 700 mV/sub pp/ output with 1% total harmonic distortion (THD), dissipates 2.48 mW at V/sub cm/=1.5 V, and occupies 1.62 mm/sup 2/ in a 1.2-/spl mu/m CMOS technology.

Exciton Spectra of Cubic and Hexagonal GaN Epitaxial Films

Abstract: Fundamental exciton structures in both cubic (c) and hexagonal (h) polytypes of GaN epitaxial films were compared. The photoreflectance (PR) spectra of both polytypes exhibited excitonic transitions up to room temperature (RT). The lowest exciton resonance energy of c-GaN (3.267 eV at 10 K) was confirmed to be smaller by 0.21 eV than that of h-GaN. The low-temperature photoluminescence (PL) spectra of both polytypes were dominated by excitonic emissions; h-GaN exhibited well-resolved bound and free exciton peaks and c-GaN exhibited a broadened excitonic emission, which was dominated by bound and free excitons below and above 80 K, respectively. The PL spectra at RT of h-GaN exhibited predominant A and B free exciton emissions. The E2 phonon replicas of excitons were found in the PL spectra of h-GaN, indicating the coupling of excitons with nonpolar phonons through the deformation potential.

Urbach-Martienssen tails in a wurtzite GaN epilayer

Abstract: Effects of an interaction between the electronic and lattice systems on the optical spectra of a wurtzite GaN epilayer were investigated. The exponentially increasing absorption tail was well explained as an Urbach–Martienssen tail, giving the characteristic phonon energy of 30 meV. The result indicates that few longitudinal optical phonons contribute to the exciton-phonon coupling even at room temperature.

Visible and Ultraviolet Photoluminescence from Cu-III-VI2 Chalcopyrite Semiconductors Grown by Metalorganic Vapor Phase Epitaxy.

Abstract: We optimized the overall process of heteroepitaxial growth of wide-gap Cu–III–VI2 chalcopyrite semiconductors by using the low-pressure metalorganic vapor phase epitaxy technique, and obtained very high quality epilayers of them. All end-point compounds, namely CuGaSe2, CuGaS2, CuAlSe2 and CuAlS2, exhibited predominant free and bound excitonic photoluminescence (PL) peaks at low temperature. The color of the emission varied from red to ultraviolet. The room temperature (RT) PL spectra exhibited predominant near-band-edge emission except for that for CuAlSe2. A noticeable excitonic feature was found in the PL spectra for CuAlS2 and CuGaS2 even at RT. Time-resolved and excitation intensity-dependent PL measurements revealed the existence of free-to-bound (FB) and donor-acceptor pair recombination emission centers in some undoped and impurity-doped compounds and alloys. The ionization energy of the recombination centers increased with increasing band gap energy of the matrix, reflecting an increase in the hole effective mass. The potential of heteroepitaxial layers of chalcopyrite semiconductors as new light-emitting materials was demonstrated.

Synthesis of Heterotelechelic Poly(ethylene glycol) Macromonomers. Preparation of Poly(ethylene glycol) Possessing a Methacryloyl Group at One End and a Formyl Group at the Other End

Abstract: Poly(ethylene glycol) (PEG) having a polymerizable methacryloyl group at one end and an aldehyde group at the other end was quantitatively synthesized. Potassium 3,3-diethoxy-1-propanoxide initiated the anionic polymerization of ethylene oxide to form an acetal-ended PEG having potassium alkolate at the ω chain end. By adding methacrylic anhydride to the reaction mixture, the ω end was quantitatively converted to the methacryloxy group. When the PEG with an acetal group at one end and a methacryloyl group at the other end was treated with 90% acetic acid solution, the hydrolysis of the acetal end group proceeded to form a PEG macromonomer possessing an aldehyde end group. The end groups were characterized by 1H NMR and MALDI-TOF-MS spectroscopies. Copolymerization of the aldehyde-PEG macromonomer with methacrylate monomers quantitatively gave the PEG graft polymer with an aldehyde group at the free end of the graft chain.

Optical properties of tensile-strained wurtzite GaN epitaxial layers

Abstract: Photo-modulated electroreflectance and photoluminescence spectra of wurtzite GaN epilayers grown on a 6H-SiC (0001)Si or a Si (111) substrate were measured as a function of temperature. The valence band structure of a biaxially tensile-strained (−0.15% in c axis) epilayer was drastically changed compared to that in unstrained and compressively strained ones; the optical transition between the conduction and the uppermost valence band exhibiting the minimum transition energy was polarized parallel to the c axis, which is characteristic of the spin split-off valence band in unstrained crystal.

Enantioselective Borohydride Reduction of N-Diphenylphosphinyl Imines Using Optically Active Cobalt(II) Complex Catalysts

Abstract: The enantioselective borohydride reduction using optically active cobalt(II) complex catalysts was successfully applied to various aryl N-diphenylphosphinyl imines, and the corresponding reduced products were obtained in good yields with high enantiomeric excesses (up to 99% ee). The optically active primary amines were obtained by the successive hydrolysis under mild conditions.

60 GHz flip-chip assembled MIC design considering chip-substrate effect

Abstract: In this paper, 60-GHz microwave integrated circuits (MIC's) with flip chip assembled 0.1-/spl mu/m-gate GaAs pseudomorphic high electron-mobility transistors (p-HEMT's) are demonstrated. To clarify the millimeter-wave characteristics of the flip-chip assembled structure, the parameters for the assembly equivalent circuit are examined using three-dimensional (3-D) electromagnetic-field analysis. The analytical results indicate that the optimum height of the bump is 30 /spl mu/m to minimize degradation of the millimeter-wave characteristics. A 60-GHz-band MIC two-stage amplifier and 30/60-GHz frequency doubler designed using the results of the field analysis have been fabricated. The amplifier has maximum gain of 12.8 dB at 58.6 GHz, P/sub 1dB/ of 12.9 dBm has been obtained at 60 GHz. A 30/60-GHz frequency doubler has maximum conversion gain of 0.4 dB and fundamental frequency suppression of -23.0 dB at the input frequency of 30.4 GHz. Good agreement between the measured and the simulated results demonstrates the potential of the structure and design method.

Photocatalytic Activities of Na2W4O13 with Layered Structure.

Abstract: Na2W4O13 with a layered structure possessed an energy gap between those of bulky WO3 and polyacid [Si(W3O10)4]4− photocatalysts and was active for photocatalytic hydrogen and oxygen evolution reactions in the presence of sacrificial reagents.

Effects of annealing on the structure and switching characteristics of VDF/TrFE copolymers

Abstract: Structure-property relationship has been investigated in concern with the ferroelectric switching characteristics of a VDF(78)/TrFE(22) copolymer. Unoriented and oriented film samples prepared by melt-quenching and melt-extrusion, respectively, were subjected to annealing and resulting structural changes were characterized by SEM, X-ray, density and DSC measurements. It was found that annealing just above the Curie point (120–130°C) caused a marked increase in crystallinity, whereas annealing in the paraelectric state (130–145°C) induced a crystal growth into thick lamellae. Such structural changes resulted in the ferroelectric switching characteristics with larger remanent polarization and shorter switching time. The unoriented sample yielded a smaller remanent polarization than the oriented sample by a factor of π/4, which substantiated that polarization reversal occurred as a result of rotation of molecules rotation about their chain axis. The melt-extruded and well-annealed sample consisted o...

Adsorbed serum protein mediated adhesion and growth behavior of bovine aortic endothelial cells on polyamine graft copolymer surfaces

Abstract: Polyamine-brushed substrata for cell culture were designed by solvent casting of polystyrene-graft-polyamine copolymer (SA) on hydrophobically modified glass surface, and adhesion, spreading, and proliferation of bovine aortic endothelial cells (BAEC) on these substrata were evaluated. Adhesion and spreading of BAEC increased with increasing polyamine content in the copolymer. Close correlation was found between cellular spreading and subsequent cell growth; the surface inducing better spreading of adhered cells showed higher endothelial cell growth. Adhesion and spreading of BAEC were significantly influenced by fetal calf serum (FCS)-pretreatment of the SA copolymer surfaces, being increased with increasing polyamine content, whereas on bovine serum albumin (BSA)-preadsorbed surfaces, BAEC adhesion was considerably prevented and eventually no cell spreading was observed. Then, adsorption of cell-adhesion proteins, fibronectin (FN), and vitronectin (VN), out of FCS onto SA copolymer surfaces were evaluat...

Room-temperature near-band-edge photoluminescence from CuInSe2 heteroepitaxial layers grown by metalorganic vapor phase epitaxy

Abstract: High purity CuInSe2 heteroepitaxial layers were successfully grown by low-pressure metalorganic vapor phase epitaxy. A certain amount of excitonic absorption was found in the optical absorption spectra even at room temperature (RT). A predominant near-band-edge photoluminescence peak was observed at RT for the first time from a (001) oriented epilayer grown on a GaAs(001) substrate. The epilayers were grown in order to carry out a systematic investigation of intrinsic defects and intentional dopants in the matrix.

Steady-state creep rate of a composite: Two-dimensional analysis

Abstract: A new method is proposed for calculating the steady-state creep rate of a composite, when the inclusion geometry and the matrix creep law are known. The method is demonstrated in a simple two-dimensional problem. During steady-state creep, an increment in plastic strain in the matrix causes a jump in displacement on the interface between the matrix and an individual elastic inclusion. The jump is counterbalanced by that due to diffusion and sliding on the interface. The rate of diffusion is determined by a normal force distributed on the interface and that of sliding by a tangential force. From these forces, the average stress in the inclusions is calculated; it is proportional to the steadystate creep rate. From the condition that an external stress is a volumetric average of the stress in the inclusions and that in the matrix, the constitutive equation of stationary creep of the composite is formulated.

Automatic feature extraction of facial organs and contour

Abstract: This paper deals with algorithms for extracting contour lines of eyebrow, eye, nose, mouth, and face contour. First we extract the position of both irises using a correlation technique. Then for the eyebrow, edge-image data are used for determining the position of the eyebrow and the contour of the eyebrow is obtained from binary image data around the eye area. For eyes, the edge between the eyeball and skin is determined easily since the edge is clearly differentiated in the brightness distribution of the image data. For the nose, the edge clearly appears in the image data and then the correlation can determine the nose contour. For the mouth, the position of mouth is determined from image-edge information and brightness distribution and then the correlation can determine the most outer corner points of the mouth. We employ an image energy function to be minimized for obtaining the face contour and a dynamic programming like algorithm is used to obtain the minimum. An average human face contour used as a template can give a good extraction of the face contour.

A Structure-Specific Endonuclease from Cauliflower (Brassica Oleracea var. Botrytis) Inflorescence

Abstract: A protein with structure-specific endonuclease activity has been purified to near homogeneity from cauliflower ( Brassica oleracea var. botrytis) inflorescence through five successive column chromatographies. The protein is a single polypeptide with a molecular mass of 40 kDa. Using three different branched DNA structures (flap, pseudo-Y and stem-loop) we found that the enzyme, a cauliflower structure-specific endonuclease, cleaved the single-stranded tail in the 5'-flap and 5'-pseudo-Y structures, whereas it could not incise the 3'-flap and 3'-pseudo-Y structures. The incision points occur around the single strand-duplex junction in these DNA substrates and the enzyme leaves 5'-PO4 and 3'-OH termini on DNA. The protein also endonucleolytically cleaves on the 3'-side of the single-stranded region at the junction of unpaired and duplex DNA in the stem-loop structure. The structure-specific endonuclease activity is stimulated by Mg2+ and by Mn2+, but not by Ca2+. Like mammalian FEN-1, the protein has weak 5'-->3' double-stranded DNA-specific exonuclease activity. These results indicate that the cauliflower protein is a plant structure-specific endonuclease like mammalian FEN-1 or may be the plant alternative.

Locally Modulated Structures of Fluorite-Related Y2O3–Nb2O5 Solid Solutions

Abstract: Modulated structures formed in defect fluorite-type (Y2O3)1 − x(Nb2O5)x (0.21 ≤ x ≤ 0.27) solid solutions have been investigated by electron diffraction. [1{\bar 1}0] diffraction patterns showed a pair of diffuse spots in the [11{\bar 2}] direction at both sides of the ½½½ reciprocal-lattice point. The distance between the diffuse spots decreased from 0.064 to 0.045 A−1 with increasing x from 0.21 to 0.27. High-resolution electron microscopy images revealed the existence of small domains with an antiphase structure. The two-dimensional antiphase structure projected on the (1{\bar 1}0) plane is described by compositional waves of Y3+/Nb5+ and O2−/vacancy and displacive waves of Y3+ + Nb5+ and O2− along the [11{\bar 2}] direction. The composition at cation sites is expressed by a sinusoidal wave with occupation changes between Y and 0.58Y + 0.42Nb. The occupational probability of O at anion sites is also expressed by a sinusoidal wave with occupation changes between 0.806 and 0.904. Amplitudes of the displacive waves for anions and cations were determined to be about 0.3 and 0.02 A, respectively.

Asymptotic behavior in time of solutions to the derivative nonlinear Schrödinger equation revisited

Abstract: We continue to study the asymptotic behavior in time of solutions to the derivative nonlinear Schrodinger equation $ i u_t + u_{x x} + ia(|u|^2u)_x = 0, \quad (t,x) \in \mathbf{R}\times \mathbf{R},$ $ u(0,x) = u_0 (x), \quad x\in \mathbf{R},\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad$(DNLS) where $a \in \mathbf{R}$. We prove that if $ ||u_0||_{ H^{1,\gamma}} + ||u_0||_{ H^{1+\gamma,0}}$ is sufficiently small with $\gamma > 1/2$, then the solution of (DNLS) satisfies the time decay estimate $ ||u(t)||_{L^\infty} + ||u_x(t)||_{L^\infty}\le C(1+|t|)^{-1/2}, $ where $H^{m,s}= \{f\in \mathcal{S}'; ||f||_{m,s}= ||(1+|x|^2)^{s/2}(1-\partial_x^2)^{m/2}f||_{L^2} < \infty\}$, $m,s\in \mathbf{R}$. In the previous paper [4,Theorem 1.1] we showed the same result under the condition that $\gamma \ge 2$. Furthermore we show the asymptotic behavior in time of solutions involving the previous result [4,Theorem 1.2].

Catalytic Stereoselective Synthesis of α-D-Galactopyranosides from 2,3,4,6-Tetra-O-benzyl-D-galactopyranose and Several Alcoholic Nucleophiles

Abstract: The glycosylation reaction of several alcoholic nucleophiles with 2,3,4,6-tetra-O-benzyl-D-galactopyranose (1-hydroxy sugar) is successfully carried out by the combined use of Sn(OTf)2-Me3SiCl catalyst system and lithium perchlorate, an effective additive. Several α-D-galactopyranosides are obtained in good yields with high stereoselectivities.

Gain spectra in cw InGaN/GaN MQW laser diodes

Abstract: A systematic study on the optical gain of continuous wave InGaN/GaN multiple quantum well laser diode wafers has been achieved by means of the variable excitation-stripe length (VEL) method. It will be demonstrated that mechanisms producing optical gain may vary according to the degree of fluctuation of InGaN composition in the lateral plane.

Enantioselective addition of dialkylzincs to N-diphenylphosphinylimines using polymer-supported N,N-dialkylnorephedrines as chiral ligands

Abstract: Chiral N,N-dialkylnorephedrines supported on polystyrene resin catalyse the enantioselective addition of dialkylzincs to N-diphenylphosphinylimine, affording optically active N-diphenylphosphinylamines with high enantiomeric excess. The influence on the enantioselectivity of the carbon-chain spacer between the chiral function and polymer resin has been examined. Reaction temperature- and solvent-dependency on the chiral polymer-catalysed reaction has also been examined.