Showing papers in "Japanese Journal of Applied Physics in 1999"

Dielectric and Piezoelectric Properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3 Systems

Abstract: The structural, dielectric and piezoelectric properties of [Bi0.5(Na1-XKX)0.5]TiO3 ceramics were studied for the compositional range, X=0–1.0. The samples were prepared by a conventional sintering technique. From X-ray diffraction results, it was found that the morphotropic phase boundary (MPB) between (Bi0.5Na0.5)TiO3 (rhombohedral) and (Bi0.5K0.5)TiO3 (tetragonal) exists in the range of X=0.16–0.20. Polarization-Electric field hysteresis loops show that the remanent polarization at X=0.20 (BNTK20) is larger than that at X=0.0 (BNT). Peaks of electromechanical coupling factor, dielectric constant, piezoelectric constant and elastic compliance were obtained at X=0.16–0.20 of the MPB region. Dielectric and piezoelectric properties are enhanced in the MPB region for [Bi0.5(Na1-XKX)0.5]TiO3, similar to that observed for Pb(Zr, Ti)O3.

Characteristics of InGaN-Based UV/Blue/Green/Amber/Red Light-Emitting Diodes

Abstract: Highly efficient light-emitting diodes (LEDs) emitting ultraviolet (UV), blue, green, amber and red light have been obtained through the use of InGaN active layers instead of GaN active layers. Red LEDs with an emission wavelength of 675 nm, whose emission energy was almost equal to the band-gap energy of InN, were fabricated. The dependence of the emission wavelength of the red LED on the current (blue shift) is dominated by both the band-filling effect of the localized energy states and the screening effect of the piezoelectric field. In the red LEDs, a phase separation of the InGaN layer was clearly observed in the emission spectra, in which blue and red emission peaks appeared. In terms of the temperature dependence of the LEDs, InGaN LEDs are superior to the conventional red and amber LEDs due to a large band offset between the active and cladding layers. The localized energy states caused by In composition fluctuation in the InGaN active layer contribute to the high efficiency of the InGaN-based emitting devices, in spite of the large number of threading dislocations and a large effect of the piezoelectric field. The blue and green InGaN-based LEDs had the highest external quantum efficiencies of 18% and 20% at low currents of 0.6 mA and 0.1 mA, respectively.

p-Type Electrical Conduction in ZnO Thin Films by Ga and N Codoping.

Abstract: We report the realization of p-type behavior in ZnO thin films, which are prepared by codoping method using Ga (donor) and N (acceptor) as the dopants. Especially, using active N formed by passing N2O gas through an electron cyclotron resonance (ECR) plasma source is quite effective for the acceptor doping. We have observed a room temperature resistivity of 2 Ωcm and a hole concentration of 4×1019 cm-3. These values are enough high for practical applications in various oxide electronic devices.

Solution Using a Codoping Method to Unipolarity for the Fabrication of p-Type ZnO

Abstract: We have investigated the electronic structures of n- or p-type doped ZnO based on ab initio electronic band structure calculations. We find unipolarity in ZnO; n-type doping using Al, Ga or In species decreases the Madelung energy while p-type doping using N species increases the Madelung energy, in addition to causing substantial localization of the N states. Codoping using reactive codopants, Al, Ga or In, enhances the incorporation of N acceptors in p-type codoped ZnO. We find the delocalized states of N for p-type ZnO codoped with N and Al (Ga).

High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center

Abstract: Multilayer organic light-emitting devices with phosphorescent guest emitter, tris(2-phenylpyridine) iridium doped in a host 4,4'-N,N'-dicarbazol-biphenyl layer were prepared. The device with the 6.5 wt% guest emitter exhibited external quantum efficiency and power luminous efficiency of 13.7% and 38.3 lm/W, respectively at the luminance of 105 cd/m2 driven at the voltage of 4.0 V and current density of 0.215 mA/cm2. The half decay lifetime under continuous constant-current driving for the initial luminance of 500 cd/m2 was 170 h.

Enhanced Piezoelectric Property of Barium Titanate Single Crystals with Engineered Domain Configurations

Abstract: Piezoelectric properties of barium titanate single crystals were investigated at room temperature as a function of crystallographic orientation. When a unipolar electric field was applied along [001], its strain vs electric-field curve showed a large hysteresis, and finally barium titanate crystal became to single-domain state with piezoelectric constant d33 of 125 pC/N over 20 kV/cm. On the other hand, electric-field exposure below 6 kV/cm along [111] resulted in a high d33 of 203 pC/N and a hysteresis-free strain vs electric-field behavior, which suggested the formation of an engineered domain configuration in a tetragonal barium titanate crystal. Moreover, when an electric field over 6 kV/cm was applied along [111], two discontinuous changes were observed in its strain vs electric-field curve. In situ domain observation and Raman measurement under an electric field suggested an electric-field-induced phase transition from tetragonal to monoclinic at around 10 kV/cm, and that from monoclinic to rhombohedral at around 30 kV/cm. Moreover, in a monoclinic barium titanate crystal, electric-field exposure along [111] resulted in the formation of another new engineered domain configuration with d33 of 295 pC/N.

Large Free-Standing GaN Substrates by Hydride Vapor Phase Epitaxy and Laser-Induced Liftoff

Abstract: Free-standing GaN, nearly equal in area to the original 2 inch wafer, was produced from 250–300 µm thick GaN films grown on sapphire by hydride vapor phase epitaxy (HVPE). The thick films were separated from the growth substrate by laser-induced liftoff, using a pulsed laser to thermally decompose a thin layer of GaN at the film-substrate interface. Sequentially scanned pulses were employed and the liftoff was performed at elevated temperature (>600°C) to relieve postgrowth bowing. After liftoff, the bow is only slight or absent in the resulting free GaN.

Microstructure and Electrical Properties of Lead Zirconate Titanate (Pb(Zr52/Ti48)O3) Thick Films Deposited by Aerosol Deposition Method

Abstract: Lead zirconate titanate (PZT) films with a thickness of more than 10 µm were prepared by the aerosol deposition method and their microstructure and chemical composition were investigated by transmission electron microscopy (TEM) and energy dispersive X-ray spectra (EDX) analysis. A damage layer was observed at the interface between PZT and the Si substrate during the deposition. The microstructure of the as-deposited film at room temperature consisted of randomly oriented small crystallites with sizes of less than 40 nm and large crystallites of 100 nm to 300 nm size, which were observed in the primary powder. The Pb/Ti/Zr ratio along the film stacking direction and around the grain boundaries was almost the same as that observed inside the crystallites and the primary powder with a morphotropic phase boundary composition of (Pb(Zr0.52Ti0.48)O3). The marked improvement of the electrical properties observed in the deposited films after annealing was mainly due to the crystal growth of small crystallites.

Photonic Crystal Using Anodic Porous Alumina

Abstract: Two-dimensional (2D) photonic crystals in the visible wavelength region were fabricated using anodic porous alumina with a highly ordered hole array configuration. The transmission properties of an ordered triangular array of an air cylinders with high aspect ratio in alumina matrix showed a stop band in the spectrum which corresponds to the band gap in the 2D photonic crystals.

Piezoelectric Properties of Bismuth Layer-Structured Ferroelectric Ceramics with a Preferred Orientation Processed by the Reactive Templated Grain Growth Method

Abstract: Dense CaBi4Ti4O15 (CBT) and Na0.475Ca0.05Bi4.475Ti4O15 (NCBT) ceramics with a highly preferred {001} orientation were prepared by the reactive templated grain growth (RTGG) method. Plate-like Bi4Ti3O12 (BIT) particles were synthesized by a molten salt technique and used as the reactive template. The template particles were mixed with other oxide and carbonate powders and aligned by tape-casting. During the sintering, oriented CBT and NCBT were formed in situ topotaxially on the oriented BIT particles, and textured CBT and NCBT ceramics were eventually fabricated by the templated grain growth and densification. The Lotgering {001} orientation degree of the textured ceramics exceeded 90% for secondary-laminate sintered specimens. Textured CBT and NCBT ceramics poled in the perpendicular direction to the preferred axis exhibited electromechanical coupling coefficient (k33) and piezoelectric coefficients (d33 and g33) three times higher than the values for nontextured ceramics with the same composition.

A Blue Organic Light Emitting Diode

Abstract: A blue organic light emitting diode (OLED) which has a structure like an SH-B type diode has been developed. The blue OLED consists of a hole-injection layer (m-MTDATA), a hole-transporting emissive layer, a hole-blocking layer and an electron-injection layer (Alq3) formed on an ITO anode by vacuum vapor deposition. α-NPD was used for the hole-transporting emissive layer, which has an emission peak at around 455 nm. For the hole-blocking layer, we found that 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (Bathocuproine) is a very effective material. As Bathocuproine has a good hole-blocking ability, the recombination area is in the α-NPD layer. The electroluminescence peak from the new blue OLED is at around 455 nm. The color coordinate in CIE chromaticity is (0.15, 0.16). The blue OLED has a potential of over 10,000 cd/m2 at 9.5 V under DC operation with an AlLi cathode. When this OLED is driven under a 1/100 duty ratio, the peak luminance is over 54,000 cd/m2. The luminous efficiency was 1.1 lm/W at 150 cd/m2. The blue OLED device is also a good green device without the Bathocuproine layer. The green OLED shows electroluminescence from the Alq3 layer with luminance of 40,000 cd/m2 at 10 V under DC operation, and the color coordinate in CIE chromaticity is (0.33, 0.55).

GaN on Si Substrate with AlGaN/AlN Intermediate Layer

Abstract: A single crystal GaN thin film was successfully grown on a Si (111) substrate by means of atmospheric pressure metalorganic chemical vapor deposition. Though there is a large difference in thermal expansion coefficients between GaN and Si, an intermediate layer consisting of AlN and AlGaN improved the quality of GaN on Si and reduced meltback etching during growth. Pits and cracks were not observed on the substrate and a mirror-like surface was obtained. The full-width at half maximum (FWHM) of the double-crystal X-ray rocking curve for GaN(0004) was 600 arcsec. Photoluminescence measurement at room temperature for a Si-doped film revealed a sharp band-edge emission with a FWHM of 62.5 meV, which is the narrowest value reported to date.

Plasma Absorption Probe for Measuring Electron Density in an Environment Soiled with Processing Plasmas

Abstract: In this paper, we describe a novel and simple technique for measuring electron density using a plasma absorption probe (PAP). PAP enables us to measure the local absolute electron density even when the probe surface is soiled with processing plasmas. The technique relies on the resonant absorption of surface waves (SWs) excited in a "cavity" at the probe head. The PAP consists of a small antenna connected with a coaxial cable and is enclosed in a tube (dielectric constant e) inserted in a plasma (electron plasma frequency ωp). A network analyzer feeds a rf signal to the antenna and displays the frequency dependence of the power absorption. A series of resonant absorptions are observed at frequencies slightly above the SW resonance frequency, ωSW = ωp/(1+e)1/2, which allows us to determine the electron density. The measured electron densities are in good agreement with the data obtained by the plasma oscillation method.

Combinatorial Laser Molecular Beam Epitaxy (MBE) Growth of Mg Zn O Alloy for Band Gap Engineering

Abstract: We have developed a new combinatorial synthesis system integrating a combinatorial shadow mask into a laser MBE (molecular beam epitaxy) chamber. This combinatorial Laser MBE system can be used for fabricating a number of crystalline films with different compositions on a substrate under programmed temperature and pressure conditions. The method was applied to alloying and band gap engineering of ZnO by positional substitution of Mg into ZnO thin films. The superiority of the combinatorial methodology to conventional one-by-one synthesis is evident from the high linearity of the c-axis length and band gap dependence on Mg content.

Ultraviolet InGaN and GaN Single-Quantum-Well-Structure Light-Emitting Diodes Grown on Epitaxially Laterally Overgrown GaN Substrates

Abstract: Ultraviolet (UV) InGaN and GaN single-quantum-well-structure light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. When the emission wavelength of UV InGaN LEDs was shorter than 380 nm, the external quantum efficiency (EQE) of the LED on ELOG was much higher than that on sapphire only under high-current operation. At low-current operation, both LEDs had the same EQE. When the active layer was GaN, EQE of the LED on sapphire was much lower than that on ELOG even under low-and high-current operations, due to the lack of localized energy states formed by alloy composition fluctuations. When the emission wavelengths were in the blue and green regions, EQE was almost the same between LEDs on both ELOG and sapphire due to a large number of deep localized energy states formed by large alloy composition fluctuations. The localized energy states are responsible for the high efficiency of InGaN-based LEDs in spite of a large number of dislocations.

Multidimensional Director Modeling Using the Q Tensor Representation in a Liquid Crystal Cell and Its Application to the π Cell with Patterned Electrodes

Abstract: To estimate numerically multidimensional director configurations in a liquid crystal cell, it is important to use the Q tensor representation of the strain free energy because it solves the problem of the difference between the directors, n and -n, in the Frank-Oseen free energy representation. In this paper, we discuss the numerical methods for calculating the multidimensional director configurations, using Berreman's Q tensor representation. Numerical issues discussed include the relaxation method for the director calculation, the liquid crystal (LC)/glass interface problem, the boundary conditions for the electric potential, and the possible ways to obtain faster convergence. We compare the calculated results obtained from the Frank-Oseen and Q tensor representations. By considering a π cell with patterned electrodes, we show the consistency of the model used with experimental observations. The calculated data explain well the position shift of the defects that appear in the test π cell.

Violet InGaN/GaN/AlGaN-Based Laser Diodes Operable at 50°C with a Fundamental Transverse Mode

Abstract: A violet InGaN multi-quantum-well (MQW)/GaN/AlGaN separate-confinement-heterostructure laser diode (LD) was grown on epitaxially laterally overgrown GaN on sapphire. The threshold current density was 3.9 kA/cm2. The LDs with cleaved mirror facets showed an output power as high as 30 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode in the near-field patterns was observed at an output power up to 30 mW. The lifetime of the LDs at a constant output power of 5 mW was more than 1,000 h under CW operation at an ambient temperature of 50°C. The estimated lifetime was approximately 3,000 h under these high-power and high-temperature operating conditions.

Dynamic range of optical reflectometry with spectral interferometry

Abstract: I present an optical reflectometry (OR) technique with spectral interferometry (SIOR) that realizes high dynamic range compared with a conventional OR system using the delayed heterodyne technique (DHOR), and report on the application of this system to noninvasive in vivo measurements of the structure of the skin and the nail of an index finger. The theoretically derived dynamic range of SIOR is m/4-times superior to that of DHOR, where m is the number of independent image pixels. A dynamic range of 105 db was experimentally realized, which is comparable to the theoretically expected dynamic range of 112 db.

A Detailed Investigation of the Polarization-Dependent Surface-Relief-Grating Formation Process on Azo Polymer Films

Abstract: We have carried out two-beam interference experiments to form surface-relief gratings (SRGs) on azo polymer films under different polarization combinations. The mechanism of SRG formation on azobenzene chromophore functionalized polymer films has been shown to depend on both the spatial variation of the intensity as well as the polarization state of the incident field pattern. The treatment presented in this paper clearly defines the intensity and polarization distribution across the two-beam interference region for different polarization configurations of the interfering beams. The efficient photoisomerization process due to the spatial variation of the optical field renders the material anisotropic and the surface mobile. The simultaneous presence of a component of electric field gradient in the grating vector direction results in large-modulation-depth SRGs. We have carried out this investigation in one class of prototypical amorphous epoxy-based azo polymer thin films. Our results confirm that efficient SRG formation on azo functionalized polymer film requires photoinduced anisotropy and a component of optical field gradient in the direction of the grating vector.

Device Physics of Ferroelectric Thin-Film Memories

Abstract: Band structure match-ups are given theoretically from X-ray photoemission spectroscopy (XPS) for the electrode interfaces between platinum electrodes and the ferroelectric thin-film materials commonly used for random access memories (DRAMs and nonvolatile FRAMs): strontium bismuth tantalate (SBT), barium strontium titanate (BST), and lead zirconate titanate (PZT). The results all agree with experimentally measured Schottky barrier heights. The electronegativity constant or S-factor (derivative of Schottky barrier height with respect to electron affinity) is found to be approximately 0.7 for these materials, not the purely ionic value of 1.0. The reduction of a factor of a million in the effective Richardson coefficient is explained. And the paradox of avalanche breakdown but decreasing breakdown fields with increasing temperature is reconciled.

Quantum-Confined Stark Effect in an AlGaN/GaN/AlGaN Single Quantum Well Structure

Abstract: Excitonic absorption was observed in a transmittance spectrum of AlGaN/GaN/AlGaN single quantum well structure with a well width of 5 nm at room temperature. The total internal electric field strength in the well was about 0.73 MV/cm, which was estimated from the absorption peak position based on a simple calculation, neglecting excitons. The observation is clearly due to the quantum-confined Stark effect. While excitonic absorption was clearly observed even in such a high internal field, no light emission was detected under a He-Cd laser excitation at temperatures ranging from room temperature to T = 10 K. Light emission accompanied by a blue shift of the emission peak and an increase of emission intensity was observed under higher excitation power density. The obvious conclusion in the present case is that the presence of a high internal electric field in the well is a disadvantage for light emission.

A New Red-Emitting Phosphor, SrTiO3:Pr3+, for Low-Voltage Electron Excitation.

Abstract: A new red phosphor, SrTiO3:Pr3+, was developed for vacuum fluorescent displays (VFDs) and field emission displays (FEDs). It was discovered that the addition of Al is essential to achieve a high luminous efficiency of low-voltage cathodoluminescence and photoluminescence. The mechanism underlying the improvement of the luminous efficiency by the addition of Al is discussed. With the addition of 17 mol% Al, luminance of 800 cd/m2 was obtained at the anode voltage of 50 V. Powder X-ray diffraction and Auger analysis showed that the Al ions react with the Sr ions to form SrAl12O19.

Thickness Dependence of Ferroelectricity in Heteroepitaxial BaTiO3 Thin Film Capacitors

Abstract: Heteroepitaxial BaTiO3 films of various thicknesses ranging from 12 nm to 79 nm were prepared on SrRuO3/SrTiO3 substrates by radio-frequency magnetron sputtering employing a two-step deposition technique, and the crystallographic and ferroelectric properties of the heteroepitaxial films were evaluated A ferroelectric hysteresis loop was clearly observed in the heteroepitaxial BaTiO3 films even when the thickness was reduced to 12 nm, probably due to improved crystallinity at the interface between the ferroelectric film and the electrodes through optimization of the preparation technique The observation of hysteresis at the temperature of 200°C was explained in terms of modification of the Curie temperature from the inherent 130°C to far above 200°C by lattice misfit strain A possible relationship between the artificially raised Curie temperature and the limited thickness dependence of ferroelectricity was discussed

Novel Interatomic Potential Energy Function for Si, O Mixed Systems

Abstract: A novel interatomic potential energy function is proposed for condensed systems composed of silicon and oxygen atoms, from SiO2 to Si crystal. The potential function is an extension of the Stillinger-Weber potential, which was originally designed for pure Si systems. All parameters in the potential function were determined based on ab initio molecular orbital calculations of small clusters. Without any adjustment to empirical data, the order of stability of five silica polymorphs is correctly reproduced. This potential realizes a large-scale modeling of SiO2/Si interface structures on average workstation computers.

Detection of Magnetic Nanoparticles with Superconducting Quantum Interference Device (SQUID) Magnetometer and Application to Immunoassays

Abstract: A system is developed to magnetically measure biological antigen-antibody reactions with a superconducting quantum interference device (SQUID) magnetometer. In this system, antibodies are labeled with magnetic nanoparticles of γ-Fe2O3, and the antigen-antibody reactions are measured by detecting the magnetic field from the magnetic nanoparticles. A setup of the system is described, and the sensitivity of the system is studied in terms of detectable weight of nanoparticles. Magnetic particles as small as 600 pg can be detected at present. An experiment is also conducted to measure antigen-antibody reaction with the present system. It is shown that the sensitivity of the present system is better than that of the conventional optical method. A one order of magnitude improvement of sensitivity will be realized by the sophistication of the present system.

New Recording Method Combining Thermo-Magnetic Writing and Flux Detection

Abstract: We have developed a new perpendicular thermo-magnetic recording method coupled with magnetic flux detection. The resolution is substantially improved by flux detection. Compared to the latest magneto-optical disk drives, the carrier-to-noise ratio of a reproduced signal is 5 dB higher, and the recording density reaches 8.3 Gbit/inch2. Moreover, this recording method has a good affinity to near-field optics, and it is an effective technique for attaining a higher density beyond the super-paramagnetic limitation in longitudinal magnetic recording.

Three-Dimensional Microdrilling of Glass by Multiphoton Process and Chemical Etching

Abstract: We demonstrate the three-dimensional microdrilling of glass by the multiphoton process with nonresonant femtosecond laser pulses and by the subsequent chemical etching. We use photomachinable glass which is sensitive to cw UV light of a wavelength shorter than 320 nm. After the focused irradiation of nonresonant femtosecond laser pulses at 400 nm and subsequent heat treatment, crystallites of Li2OSiO2, which are more soluble in dilute hydrofluoric acid than matrix glass, precipitate in the focused area of the laser within the glass sample. After etching the crystallites, three-dimensional holes are formed in the glass sample. We produce straight and Y-branched holes in the glass sample. This technique can be applied to the fields of microoptics, microelectronics and microchemicals.

Electrical and Structural Properties of Tin-Doped Indium Oxide Films Deposited by DC Sputtering at Room Temperature

Abstract: Tin-doped indium oxide (ITO) films were deposited on soda-lime glass plates without substrate heating by dc magnetron sputtering. Crystallinity and electrical properties of the films were investigated by X-ray diffraction and Hall-effect measurements, which showed clear dependence on target–substrate distance (T–S) and on total gas pressure (Ptot) during deposition. Degradation in crystallinity was observed at relatively high or low Ptot, where the upper or lower Ptot level for depositing films with high crystallinity was increased with decreasing T–S. Based on a hard sphere collision model, the crystallinity of the films was considered to be strongly affected both by the kinetic energy of sputtered In (or Sn) particles and by the bombardment of high energy particles arriving at the growing film surface. The former could enhance the crystallinity, whereas the latter degraded both the crystallinity and conductivity. Such degradation in electrical properties was mainly due to a decrease in carrier density.

Universal Theory of Si Oxidation Rate and Importance of Interfacial Si Emission

Abstract: The essential role that Si atoms emitted from the interface play in determining the silicon-oxidation rate is theoretically pointed out, and a universal theory for the oxide growth rate taking account of the interfacial Si-atom emission is developed. Our theory can explain the oxide growth rate for the whole range of the oxide thickness without any empirical modifications, while the rate for an oxide thickness of less than 10 nm in dry oxidation cannot be explained with the Deal-Grove theory.

Quantum-Dot Cellular Automata:Line and Majority Logic Gate

Abstract: An introduction to the operation of quantum-dot cellular automata is presented, along with recent experimental results. Quantum-dot cellular automata (QCA) is a transistorless computation paradigm that addresses the issues of device density and interconnection. The basic building blocks of the QCA architecture, such as AND, OR, and NOT are presented. The experimental devices presented are QCA cells where the dots are metal islands, coupled by capacitors and tunnel junctions. A line of three two-dot cells is presented, which demonstrates that there are no metastable states in a QCA line. The final experiment presented is a QCA majority gate, a programmable AND/OR logic gate.