Hiroya Andoh

Raman Scattering Study of InGaN Grown by Metalorganic Vapor Phase Epitaxy on (0001) Sapphire Substrates

We have studied the asymmetric broadening of the Raman spectra of InxGa1-xN grown on sapphire substrates with the aid of the spatial correlation model. The asymmetric broadening of the E2 phonon mode is enhanced in the region of immiscibility by increasing the indium molar fraction. The correlation length, which corresponds to the decay length of the E2 phonon mode, was estimated for the first time. It was on the order of 6–10 nm, which is on the same order of magnitude as the size of the columnar structure suggested by transmission electron microscope analyses.

Numerical Matrix Analysis for Performances of Wideband 100GHz Branch-Line Couplers

The wideband waveguide 3-dB branch-line coupler suitable for use in SSB heterodyne receivers in radio astronomy is designed and its performances are reported. The coupler is designed by a numerical analytical method by using matrices based on the circuit theory and has good performances in the range of frequencies from 84GHz to 116GHz.The numerical analytical method presented in this manuscript enables quick calculations for performances of RF components and possesses advantages over electromagnetic simulation software in common use.

Ultrashort Pulse Generators Using Resonant Tunneling Diodes and Their Integration with Antennas on Ceramic Substrates

Ultrashort pulse generations were demonstrated on two types of resonant tunneling diode (RTD) pulse generators. The circuits were fabricated with InGaAs/AlAs RTDs on an InP substrate. One of the circuits was a simple resistor-RTD series circuit, and the other was a circuit consisting of an RTD pair. The full widths at half maximum (FWHMs) of the pulses were about 10 ps. Good symmetric opposite polarity peaks were obtained for the resistor-RTD pair circuit, while the peaks for the resistor-RTD series circuit were asymmetric. Moreover, the simple resistor-RTD-type pulse generator was integrated with antennas on ceramic substrates. A sending and receiving experiment showed that a 70 GHz wave packet was transmitted between antennas.

Emission Wavelength Dependence of Internal Quantum Efficiency in InGaN Nanowires

The internal quantum efficiency (IQE) of InGaN nanowires with different emission wavelength of 485, 515, 555, and 580 nm has been studied by means of photoluminescence (PL) spectroscopy. It was found from the analysis of IQE as a function of excitation power density that the IQE was unchanged at about 100% under weak excitation conditions at low temperature. This indicated that the effects of nonradiative recombination processes were negligibly small at low temperature. Moreover, the IQE increased from 5 to 12% with increasing emission wavelength from 485 to 580 nm. Since the clear correlation between the IQE and the PL blue shift due to band filling effects of localized states was observed, the increase in the IQE reflected the increase in the effect of exciton localization with increasing indium composition.

Numerical Analysis of Influence of Surface Barrier on Current-Voltage Characteristics for Narrow Superconducting Lines

The variation of critical current density with reduced line width and the temperature dependence of voltage for superconducting narrow lines are calculated theoretically. Critical currents of superconducting narrow lines increase progressively as the lines become narrower. The surface barriers for superconducting narrow lines are discussed in order to understand the progressive increase in critical currents of superconducting narrow lines. Furthermore, the thermally activated flux lines jumping over the surface barrier are discussed in order to understand the temperature dependence of voltage for superconducting narrow lines. The results of calculation using the surface barriers agree well with experimental results.

Atomic Sites of S on (NH4)2Sx-Treated GaAs(100) Surface

Locations of S atoms on (NH4)2Sx -treated GaAs(100) surfaces annealed at 200°C and 400°C have been studied on atomic scale by X-ray photoemission spectroscopy (XPS), scanning tunneling microscopy (STM) and medium-energy ion scattering (MEIS) using blocking. XPS spectra showed that the growth of native oxide layers on GaAs surfaces was suppressed after (NH4)2Sx treatment and that most S atoms were bonded to only As or Ga atoms depending on the annealing temperature. STM observations revealed that S atoms on a GaAs(100) surface were reconstructed in a periodic structure of 1×1 in the case of annealing at 200°C, while 2×1 reconstruction was observed after annealing at 400°C. The distance between the S adsorbed layer and the uppermost layer of GaAs(100) could be estimated from the angle-resolved MEIS blocking spectra.

Vortex Flow Transistors with Nano-Structures

We have demonstrated superconducting vortex flow transistors based on nano-bridge arrays made of YBa2Cu3O7-δ thin films. The transistor showed the periodical modulation of critical current and that of the voltage for an external magnetic field as well as a parallel array of Josephson junctions. The maximum modulation depth of voltage and the flux-to-voltage transfer function are 0.5mV and 2.6mV/Φ0 in a two-bridge array, respectively. The switching time of the transistor was estimated to be 5ps, though the device operation was dominated by Abrikosov vortex motion.

Alternating-current transport losses of melt-cast processed Bi-2212 bulk superconductor bars

Using a melt-casting method, we have fabricated two pieces of Bi-2212 bulk superconductor bar with square and rectangular cross-sections, and we have investigated the alternating-current (ac) transport self-field losses at 77 K. Despite the main contribution of hysteresis loss of the superconductor, there is some difference in the loss behaviour between these two samples. To elucidate the origin, we make numerical calculations on the ac transport self-field losses as a function of current amplitude I0 below the critical current Ic. At a fixed I0, the calculated values using the uniform Jc distribution and the actual cross-sectional geometry are much higher than the experimental data for the sample with a square cross-section 7.5 × 7.5 mm2, while there is good agreement between the calculation and the experiment for the sample with a rectangular cross-section 4.5 × 13.6 mm2. The discrepancy appearing in the sample with a square cross-section is ascribed to the actual Jc distribution, which is confirmed by critical current measurements when scraping off the sample. The local Jc value decreases significantly in going from the surface to the interior of the sample. This suppresses the extension of the flux-penetration region to the interior under ac current transmission and lowers the loss generation compared with the calculated results obtained by the uniform Jc distribution.

Superconducting Properties of Melt-Cast Processed Bi2Sr2CaCu2Oy Compounds

A significant enhancement of the superconducting properties in Bi2Sr2CaCu2Oy compounds has been achieved by using melt-casting process (MCP). The weak links have been largely eliminated in the MCP samples. The samples show sharp drops associated with the onset of intragranular superconductivity. The value of Tc increases for SrSO4 addition up to approximately 10wt%. The zero resistance is observed at 90.4K for the 10wt% SrSO4 added sample annealed at 800°C in air. The SrSO4 admixture improves the superconducting properties of the intergranular regions.

New vortex flow transistors made of YBa/sub 2/Cu/sub 3/O/sub y/ thin films

We have investigated the performances of several types of vortex flow transistors including nanobridge vortex flow transistors (NBVFTs) based on a parallel array of nanobridges, planar Josephson vortex flow transistors (planar JVFTs) based on a parallel array of grain boundary Josephson junctions, and new JVFTs with a stacked structure (stacked JVFTs). Considering the integration and the reduction of the L/R time constant, the areas of the transistors were restricted to less than 350 /spl mu/m/sup 2/. A NBVFT showed a flux-to-voltage transfer function of 2.6 mV//spl Phi//sub 0/, which was one order of magnitude larger than that of the other transistors. In contrast, the NBVFTs showed a very small current gain due to a large kinetic inductance of a nanobridge, while the NBVFT had the smallest area among the three. A planar JVFT with asymmetric geometry was easy to fabricate and showed a current gain of 2.2 at 4.2 K. However, the planar JVFT requires a large area, leading to a long response time other than the internal delay time. A stacked JVFT also showed a current gain of 2.5 at 4.2 K. A layered structure yielded a strong coupling between the body of the JVFT and the control line. Due to this strong coupling, the response time of the stacked JVFT was considerably improved compared to that of the planar JVFT.