Electrical properties at p-ZnSe/metal interfaces

In order to prepare low resistance ohmic contacts to p-ZnSn by the “deposition and annealing (DA)” technique which has been extensively used for GaAs and Si-based devices, formation of a heavily doped layer by the p-ZnSe/metal reaction is required. For p-ZnSe/Ni contacts, Ni and Se reacted preferentially at the ZnSe/Ni interface upon annealing at temperatures higher than 250°C. However, capacitance-voltage measurements showed that the net acceptor concentration (N_A-N_D) close to the p-ZnSe/Ni interface was reduced upon the Ni/ZnSe reaction, resulting in high contact resistance. For p-ZnSe/Au contacts, neither Au/ZnSe reaction nor reduction of the acceptor concentration were observed after annealing at temperatures lower than 300°C. This indicates that although the metal/p-ZnSe reaction is mandatory to prepare a heavily doped layer, the reaction induced an increase in the compensation donors in the p-ZnSe substrate. In order to increase the acceptor concentration in the vicinity of the p-ZnSe/metal interface through diffusion from the contact materials, Li or O which was reported to play the role of an acceptor in ZnSe was deposited with a contact metal and annealed at elevated temperatures. Ni or Ag was selected as the contact metal, because these metals were expected to enhance Li or O doping by reacting with ZnSe. However, the current density-voltage characteristics of the Li(N)/Ni and Ag(O) contacts exhibited rectifying behavior, and the contact resistances increased with increasing annealing temperature. The present results indicated that, even though the acceptor concentration in the p-ZnSe substrate increased by diffusion of the dopants from the contact elements, an increment of the compensation donors was larger than that of the acceptors. The present experiments indicated that preparation of low resistance ohmic contacts by forming a heavily doped intermediate layer between p-ZnSe and metal is extremely difficult by the DA technique.     

Schottky barrier heights of contact metals to p-type ZnSe

Schottky barrier heights (SBHs) of a variety of metals (In, Cd, Nb, Ti, W, Cu, Ag, Au, Ni, Pt, and Se) contacting to p-ZnSe grown by a molecular beam epitaxy method were determined by analyzing capacitance-voltage (C-V) and/or current density-voltage (J-V) curves. The SBH values of the Au and Ni contacts were determined from intersections of straight lines of the C⁻²-V curves to be 1.23 and 1.13 eV, respectively. The J-V calculations provided a large SBH value of 1.2 ± 0.1 eV for a variety of metals, indicating that the Fermi-level could be pinned at the contact interface. Reduction of the SBH values to a level lower than 0.4 eV and/or increase of doping concentrations to a level higher than 10²⁰ cm⁻³ are essential to obtain an ohmic contact with contact resistivity of around 10⁻³ Ω·cm².     

Analysis of course errors on CVOR antennas (including effects ofmutual coupling between elements)

A VHF omnidirectional radio range (VOR) is a navigation aid radio beacon facility, which provides aircraft with azimuth information relative to the VOR station in question as the origin. In Japan, two types of VOR-the conventional type (referred to as a CVOR) and the Doppler type (referred to as a DVOR)-are currently in use. An element known as the Alford loop antenna (ALA), which changes the loading reactance, is used for the VOR because the horizontally polarized wave and nondirectivity in the horizontal plane are preferred. A VOR antenna consists of a carrier antenna and a sideband antenna-an aircraft receives separate signals from these two antennas and compares them to obtain azimuth information. The mutual coupling between the elements forming the carrier and the sideband antenna affects the directivity of the single elements, resulting in errors in azimuth information. With the mutual coupling between the antenna elements being taken into consideration, a quantitative calculation was made by using the moment method-the results of the calculation made it clear that a loading reactance value of -320 Ω is better to make not mutually coupled elements nondirectional while a loading reactance value of -600 Ω is optimum to minimize the azimuth error of a CVOR     

Polarization-controlled single-mode VCSEL

Relative intensity noise (RIN) in a vertical-cavity surface-emitting laser (VCSEL) was greatly reduced through the use of polarization control to eliminate competition between two orthogonal polarization states by ensuring there was only one polarization state. Polarization was stable with optical feedback of up to 10%. Polarization control was achieved by inducing a small loss anisotropy in fundamental transversal mode VCSEL's. Anisotropic post structures, such as a rectangular post, an oblique post, or a zigzag-sidewall post, were found to be effective in creating loss anisotropy without serious degradation of other VCSEL characteristics such as light-output power or beam profile     

Transmission line with distributed erbium-doped fiber amplifier

We show the advantages of distributed erbium-doped fiber amplifiers (EDFAs) over lumped EDFAs. Using the distributed EDFA for 50%-100% of the transmission line lessens the signal degradation compared to the conventional transmission line. We also show the feasibility of the gain-flattened d-EDFA transmission line. An experiment shows that the gain peak is shifted and flattened at low pump powers     

Driving voltage reduction in a two-phase CCD by suppression ofpotential pockets in inter-electrode gaps

This study reports an optimum design for a two-phase charge-coupled device (CCD) and limitations on its driving voltage reduction. The two-phase CCD to be used as a horizontal-CCD (H-CCD) in a CCD image sensor requires low-voltage and high-speed operation. Reducing the driving voltage, however, may induce potential pockets in the channel under the inter-electrode gaps which results in a fatal decrease in charge-transfer efficiency. In this case it is necessary to optimize the CCD design to be free of pocket generation. For this requirement, we conducted two-dimensional (2-D) device simulations for the two-phase CCD, whose potential barriers are formed by boron ion-implantation. Our simulations indicated that the edge position of the potential barrier region and the dose of boron-ion implantation would be important parameters for controlling the size of potential pockets. At an optimum edge position and a boron dose, the minimum driving voltage appears to be reducible to 1.1 V. Characteristics of potential pockets and methods of their suppression are also discussed     

A Tuning Circuit with Two Half-Wave Distributed Junctions for All-NbN SIS Mixers

A novel broadband tuning circuit composed of two low-current-density half-wave NbN/MgO/NbN tunnel junctions connected by a half-wave NbN/MgO/NbN microstrip line has been successfully tested in a quasi-optical mixer at frequencies above 700 GHz. The circuit had a designed center frequency of 870 GHz, was integrated in a center-fed twin-slot antenna, and was fed via a quarter-wave impedance transformer. Heterodyne measurments showed double-side-band receiver noise temperatures equivalent to 6-9 quanta from 675 to 810 GHz for a mixer with a current density of 6.7 kA/cm². The RF bandwidth was broader than that of a conventional mixer using a full-wave junction with the same current density.     

Effect of a hepatocyte growth factor/heparin-immobilized collagen system on albumin synthesis and spheroid formation by hepatocytes

A hepatocyte growth factor (HGF)/heparin-immobilized collagen system was used as a synthetic extracellular matrix for hepatocyte culture. The albumin synthesis, nucleus numbers and morphology of the hepatocytes were determined separately to evaluate the hepatocyte number and hepatocyte-specific function under this system. The benefits of the HGF/heparin-immobilized collagen system for hepatocyte culture were confirmed by three types of culture methods in vitro, namely 2D film cultures, 2D gel cultures and 3D gel cultures. In 2D collagen film cultures, hepatocytes exhibited the highest albumin synthesis (1.42 μg/well/day) in HGF/heparin-immobilized collagen films at 7 days of culture. Heparin inhibited hepatocyte adhesion while HGF promoted hepatocyte migration, and spheroid formation was easily detected in HGF/heparin-immobilized collagen films. In 2D collagen gel cultures, albumin synthesis of around 15 μg/well/day was detected and maintained for more than 18 days on HGF/heparin-immobilized collagen gels. Similar findings were obtained in 3D HGF/heparin-immobilized collagen gel cultures, which exhibited albumin synthesis of up to 30 μg/well/day. The albumin synthesis by hepatocytes was two-fold higher in 3D gel cultures compared with 2D gel cultures, and was maintained for over 2 weeks compared with 2D film cultures using the HGF/heparin-immobilized collagen system. Taken together, the HGF/heparin-immobilized collagen system was effective for albumin synthesis by hepatocytes in both 2D film cultures and 3D gel cultures, and therefore shows good potential for tissue engineering use.     

Significance of the detection of staphylococcal enterotoxin A gene in low fat milk which caused a serious outbreak of food poisoning

In June 2000, there was a large-scale outbreak of food poisoning after consumption of Snow Brand low fat milk. In the evening of a day the incident made public, some cartons of low fat milk were brought to our laboratory for examination. Next day, we detected only staphylococcal enterotoxin (SE) A gene among SE (A-E) genes by PCR in left-over milk samples or samples from the same lots that patients had consumed. We presumed that the outbreak was caused by the intake of SEA. We subsequently confirmed the presence of SEA in these samples. To investigate the existence of SE (A-E) genes in milk, we examined 100 samples of commercial low fat milk and milk by PCR, but none of the genes was detected. We estimated the detection limit of SEA gene in low fat milk by PCR. Four strains of SEA-producing Staphylococcus aureus cultures were serially diluted in low fat milk. The SEA gene was detected at levels of 5.5 x 10(2) to 1.6 x 10(4) cfu/mL of S. aureus. These amounts of S. aureus are higher than the values in raw milk reported previously. Therefore we consider that SE genes in low fat milk should usually be undetectable by our PCR. This study shows that quick detection of SE genes by PCR is very helpful to analyze outbreaks, especially if no significant bacterium can be cultured.     

Effect of fluid motion on radiation-induced polymerization of ethylene in a tubular reactor

The influence of the turbulence of reactant on the radiation-induced polymerization of ethylene in 40 mole-% Freon-114 (C₂Cl₂F₄) was studied using a tubular reactor at 400 kg/cm² and 25°C with a dose rate of 1.3 × 10⁵ rad/hr. At constant linear velocity and tube diameter, the polymer concentration was shown to increase linearly with the reactor tube length. This indicates that the polymerization is in a stationary state. By changing the linear velocity from 3.5 to 42.7 cm/sec and the tube diameter from 5 to 14 mm, the space time yield and the molecular weight of polymer were found to vary between 0.21 and 0.46 mole ethylene/1.-hr and from 5.0×10³ to 10.5×10³, respectively. The space time yield and molecular weight decreased sharply to about one half those in the static polymerization with increasing fluid turbulence and then slowly increased in the highly turbulent state. Similar effects were observed in a tank reactor when the stirring speed was changed.