Akihiro Moto

Metalorganic Vapor Phase Epitaxial Growth of GaNAs Using Tertiarybutylarsine (TBA) and Dimethylhydrazine (DMHy).

GaNAs alloys were successfully grown on GaAs substrates by low-pressure metalorganic vapor phase epitaxy (MOVPE) with all organometallic sources of triethylgallium (TEG), tertiarybutylarsine (TBA), and dimethylhydrazine (DMHy). For nitrogen, the desorption coefficient of 30 kcal/mol was derived from the nitrogen incorporation dependence on growth temperature. Since the nitrogen concentration above 3% was easily achieved by our growth technique, the combination of TBA-DMHy as V precursors is a candidate for the growth of other III-V alloys containing nitrogen. We observed a decrease in PL intensity with enhancing nitrogen incorporation into solids. In order to recover from degradation of optical properties, rapid thermal annealing (RTA) was demonstrated and found to be effective. Therefore MOVPE using TBA-DMHy combined with postgrowth annealing is expected to obtain GaNAs alloys with high nitrogen concentration as well as excellent optical properties.

Highly reliable AlGaInAs buried heterostructure lasers for uncooled 10 Gb/s direct modulation

High reliability (estimated median lifetime of 240,000 hours) of 1.3 /spl mu/m AlGaInAs buried heterostructure lasers has been demonstrated by more than 10,000 hours accelerated aging tests. Distributed-feedback lasers have successfully operated at 10 Gb/s at 95/spl deg/C.

ESR and Raman studies on Mn-substituted Y-Ba-Cu-O

ESR and Raman spectra were studied for YBa2(Cu1-xMnx)3O7-y. Among Raman active phonon modes, only the 440 cm-1 mode was found to depend on the Mn content. The two-magnon peak is shifted to the lower frequency side by the addition of Mn. These facts indicate that Mn ions preferentially occupy the site of Cu in the CuO2 plane. The ESR signal originating from antiferromagnetically interacting Mn ions in phase-separated clusters was obtained in all the Mn-doped samples. Both the Mn content dependences of the ESR intensity and the lattice constant show that the content of Mn ions in the CuO2 plane is saturated with an increase in the Mn content above x>0.05.

Structural Analysis on Tc Variation of Bi2Sr2-xCa1+xCu2O8+y

The effects of heat treatment for Bi2Sr2-xCa1+xCu2O8+y (x=0.25, 0.5, 0.75, 1.0) were studied by X-ray diffraction and Raman scattering measurements. The curve of Tc vs normal state conductivity ρ-1 is found to attain a maximum through heat treatment and a compositional change, suggesting the presence of an optimum hole concentration, as is observed in the La- and Y-based superconductors. In most cases, Tc decreases with an increase in ρ-1, except for the sample with x=1.0. Furthermore, we found no peak shift of the 466 cm-1 Raman mode induced by the heat treatment and the compositional change, which are expected to vary the Bi-Cu distance.

Preparation of Ba2YCu3Ox Superconducting Films by Laser Evaporation and Rapid Laser Annealing

A Ba2YCu3Ox superconducting film was prepared by laser evaporation and rapid laser annealing followed by 450°C furnace annealing in flowing O2. This is the first report on the preparation of a superconducting film by laser annealing. This technique supplies the first step in the preparation of superconducting films on various kinds of substrates without a prolonged high-temperature treatment.

5×5 cm2 GaAs and GaInAs Solar Cells with High Conversion Efficiency

We fabricated GaAs and GaInAs solar cells with a large cell area of 5×5 cm2 as the first step toward their practical use. The 5×5 cm2 cells showed high conversion efficiencies equivalent to those of 1×1 cm2 cells owing to both a good uniformity of epitaxial film characteristics in a 3-inch wafer and the reduction of the series resistance of grid electrodes. Moreover, a higher conversion efficiency of 26.0% under 1-sun air-mass 1.5 global conditions was achieved for the 5×5 cm2 GaAs cell by optimizing the antireflection coating. This is the highest efficiency among GaAs cells ever reported. The 5×5 cm2 GaInAs cell showed an efficiency of 4.2% as the bottom cell under a GaAs top cell. Therefore, a conversion efficiency of more than 30% can be expected for the 5×5 cm2 GaAs/GaInAs mechanically stacked tandem cell.

Preparation of Ba-Y-Cu-O Superconducting Films by Laser Ablation with and without Laser Irradiation on Growing Surface

Superconducting Ba-Y-Cu-O films were prepared by the excimer laser ablation method. Films with a good superconducting property were prepared on various substrates with substrate temperatures Ts of 650°C~800°C without a high-temperature annealing. In particular the film (1.7 µm thick) prepared on crystalline Si without a buffer-layer exhibits zero resistance at 82 K. To reduce Ts, we examine the effect of the laser irradiation onto the growing film surface. We found that excimer laser irradiation onto the growing film surface improves the superconducting properties of the films prepared at a lower Ts.

A Low Power Quad 25.78-Gbit/s 2.5 V Laser Diode Driver Using Shunt-Driving in 0.18 µm SiGe-BiCMOS

We report on circuit design and measurement results of the newly developed quad 25.78-Gbit/s laser diode driver, fabricated by 0.18 μm SiGe-BiCMOS (ft/fmax =200/200 GHz). Power dissipation of the laser diode driver is only 40 mW/lane and 190 mW/lane including a laser bias current when the DFB laser is driven under 2.5 V supply. This IC is co-packaged with laser diodes in small size transmitter optical subassembly. We confirm excellent optical waveform, which is compliant with 100GBASE-LR4 optical eye specifications in IEEE802.ba.

ESR and X-Ray Diffraction Studies on Ba–Y–Cu–O Superconductors

ESR, X-ray diffraction and resistivity measurements were carried out for as-sintered Ba2Y1Cu3Ox, quenched Ba2Y1Cu3Ox and as-sintered Ba2Y1Cu3OxFy. The ESR spin density is found to be a good measure for the amount of an impurity phase Ba1Y2Cu1O5. A decrease in the sintering temperature reduces the amount of the impurity phase. A transition from a superconducting orthorhombic to a nonsuperconducting tetragonal structure induces no large change in the ESR signal. It is also found that an as-sintered Ba2Y1Cu3OxFy superconductor shows a strong ESR signal originating mainly from the impurity phase.

Mechanically Stacked GaAs/GaInAsP Dual-Junction Solar Cell with High Conversion Efficiency of More than 31%

We successfully fabricated high-performance GaAs and GaInAsP (band gap = 0.95 eV) single-junction solar cells with an area of 1×1 cm2. The conversion efficiencies of the GaAs and GaInAsP cells were 25.0 and 19.3%, respectively, under 1-sun air-mass 1.5 global (AM1.5G) conditions. The GaInAsP cell as the bottom cell under the mechanically stacked GaAs top cell also showed a high efficiency of 6.1%, and a total efficiency of 31.1% was achieved for the GaAs/GaInAsP tandem cell. This is the highest efficiency obtained under 1-sun AM1.5G conditions among the dual-junction cells ever reported.