We performed modal analysis for 40-stacked InAs/GaAs quantum dot semiconductor optical amplifiers (QDSOAs) as a function of the waveguide width using an equivalent refractive index technique. QDSOAs with 5- and 11-μm-waveguide widths show multi-mode operations. The theoretical simulation reproduced well the experimental electroluminescence spectrum and unveiled that the output signals comprise several transverse modes. Besides, we confirmed a waveguide width less than 1.28 μm is essential to realize single-mode QDSOAs. The modal gain spectra were analyzed by using the Hakki-Paoli method. Multi peaks arisen from the multi-mode operation were also observed in the gain spectrum, suggesting precise control of the transverse mode is important for a practical realization of the single-mode device.
We systematically studied two-step photocurrent generation as functions of the excitation intensities for the inter-band and inter-subband transitions in an InAs/GaAs/Al0.3Ga0.7As dot-in-well (DWELL) intermediate-band solar cell.The two-step photoexcitation current shows saturation as the inter-band excitation intensity becomes strong, and we found that the inter-band excitation intensity showing the current saturation strongly depends on the inter-subband excitation intensity. To interpret the current-saturation behavior, we proposed a model and carried out theoretical simulation. Simulated results excellently reproduce the experimental observations. It has been clarified that the photocurrent saturation is caused by filling the intermediate states with electrons.Furthermore, the recombination lifetime in DWELL was pointed out to be extremely long.Our results suggest that this carrier lifetime is an important key to realize strong enhancement of two-step photoexcitation.
Low-temperature-grown (LTG) InxGa1-xAs with thicknesses of 2.0 μm on InP substrates were grown by molecular beam epitaxy at a substrate temperature of 200-240 ℃; they were characterized by high-resolution X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), and X-ray reciprocal space mapping (RSM) measurements. While XRD peaks and RBS channeling dip curves were observed for LTG In0.45Ga0.55As samples grown at 220 ℃, there were no peaks and dip curves for LTG In0.44Ga0.56As samples grown at 200 ℃ in XRD and RBS channeling measurements, respectively. These indicated deterioration of crystalline quality at a growth temperature between 200 and 220 ℃. However, the RBS suggested that the In atoms were located in interstitial sites along the  direction and that the ratio of the interstitial In atoms was estimated to be almost 40% of all In atoms even in the LTG In0.45Ga0.55As grown at 220 ℃. X-ray RSM revealed that the LTG In0.45Ga0.55As layer was compressively strained, although its lattice constant parallel to the substrate surface was smaller than that of the InP substrate, implying that defect structures are present in the layer. Moreover, the results of XRD measurement and RBS implied transformation of excess As into As precipitates by thermal annealing in both these LTG In0.45Ga0.55As and In0.44Ga0.56As samples.
Wide-bandgap semiconductors such as SiC have been studied eagerly as a next generation power devices because of its superior physical property. Though very high carrier density is main reason for superior characteristics of that, there are some problems to realize the predicted performance. One is heat radiation limited by package performance. The other is SCSOA (Short Circuit Safe Operating Area) limited by heat capacity of power device itself. Besides, parasitic inductance of power circuitry affects switching losses and switching speed. High current density operation is the key role to realize the predicted performance, and with that purpose, semiconductor device technology, package technology, and circuit technology must be optimized as the total engineering.
The mist chemical vapor deposition (mist CVD) method, which uses ultrasonically atomized solutions as sources, is an environmental friendly and cost-effective technology for the growth of compound semiconductors. This growth process is realized under atmospheric pressure and allows us to use many kinds of salts, complexes, and compounds with low toxicity for sources. Using the oxidizability of water including the source, most of the previous reports of the mist-CVD method are on oxide materials. In this study, we fabricated zinc sulfide (ZnS) films by mist-CVD method using thiourea-based water solutions as sources. Investigating the growth of ZnS by mist-CVD under various growth conditions and experimental setups, we proposed zinc-chloride complexes are necessary for the growth of ZnS and vaporized mist sources act as precursors.
Bubble Concrete, introduced in the present paper, is a composite material made from concrete mixed with high strength hollow bubbles, which leads to lightweight and remains its strength and stiffness higher enough for practical application. In the present paper, the authors introduce the basic principle of the Bubble Concrete, and report the results of compression tests of concrete mixed with steel spheres, and analyze its failure mechanism by researching the deformation features of the steel spheres. The authors point out that the density of the Bubble Concrete may be reduced to 1.9-2.2g/cm3, the strengths may be degraded but the Young’s modulus declined slightly, and splitting and stretching failures due to the lateral expansion of the steel spheres are observed.
The effect of crystalline grain size on the macroscopic non-uniform deformation of polycrystalline materials was evaluated by FEM simulation based on the rate-form second-order homogenization method. The conventional crystalline plasticity theory was applied to represent the scale-independent deformation behavior of microscopic crystalline structure. Numerical simulations of macroscopically uniform bending and uniaxial tension of curved gage section specimen were performed to give the non-uniform deformation on the macroscopic region. Polycrystalline microstructures with different grain sizes were given to all Gauss integration points on macrostructure. In the bending simulation, the effect of microstructure size on the relationship between macroscopic strain gradient and its work conjugate higher-order stress was evaluated. Larger energy was required for the bending of larger microstructure model, and a similar characteristic was confirmed in the simulation of full scale model. Furthermore, higher deformation localization in the macrostructure was observed when a larger tensile deformation was given to the curved gage section specimen with smaller crystal grain. This size effect on the non-uniform deformation of the polycrystalline materials was caused by non-uniform deformation in microstructure, which was strongly characterized by the macroscopic strain gradient and the size of the microstructure.
The effect of Al content on the fracture-behavior transition from brittle to ductile fracture due to strain-rate changes in various Fe-Si-Al (Si + Al = 4 wt%) alloys was studied. The fracture-behavior transition was investigated using tensile tests at a wide range of strain rates ranging from 10-3 to 103 s-1. For Fe-Si-Al alloys with low Al content, the tensile sample fractured in a brittle manner at high-strain rates. On the other hand, at low-strain rates, nominal stress-plastic strain curves represented both uniform and local elongations, and the specimen fractured in a ductile manner. The fracture-behavior transition due to strain-rate changes was observed in Fe-Si-Al alloys. The strain rate at which the fracture-behavior transition occurred in Fe-Si-Al alloys increased with increasing Al content, and the brittle fracture of Fe-Si (high Fe content) alloys could be successfully suppressed in the presence of Al.
This study deals with the influence of Ti addition on softening behavior of the martensite structure and precipitation behavior for 2 1/4Cr-1Mo-V steel. The fine MX type (Mo, V) C and (Mo, V, Ti ) C precipitate with heat treatment at from 600 to 700 deg. C after cooling from 1350 deg. C. These fine carbides coarse with the solute and precipitation of V, then the composition of carbides change to V-rich. (Mo, V, Ti) C in Ti-bearing steel keeps fine size even after long time heat treatment at high temperature. It is difficult to understand the retardation of coarsening of (Mo, V, Ti) C by Ostwald ripening theory. We therefore propose a hypothesis that (Mo, V, Ti) C is consequently maintained at fine size without coarsening by Ostwald ripening theory because the fine (Mo, V, Ti) C is practically insoluble due to very low solubility of Ti at heat treat temperature.
Carbon fiber reinforced plastics (CFRP) are widely used as structural components and are regarded as the major candidates for reducing the structural weight. Especially, application of CFRP to the cryogenic liquid fuel tanks is one of the most yearnings for achieving the drastic weight reduction. To apply CFRP for the fuel tank instead of the aluminium alloy, behaviors of CFRP such as fuel leakage or gas permeation under the cryogenic temperature should be cleared. The purpose of this study is to improve the nitrogen gas permeation properties of Plain-Woven CFRP. To improve the permeation properties of CFRP (V-CFRP), the nanoclay filler, which is montmorillonite is compounded (M-CFRP). The leakage pathway of nitrogen gas through the laminates is also revealed in order to identify the major factor of leakage. The leak rate of CFRP after cryothermal fatigue tests was increased as cyclic number increases. This is because that the interfacial debonding and micro cracks were initiated in CFRP due to cryothermal cycling. The increase of leak rate due to crack initiation was significantly higher than that due to the nitrogen diffusion through the matrix. The micro crack density and crack length of M-CFRP were improved compared to V-CFRP. This is because that the fracture toughness of epoxy resin improved by filling the montmorillonite. Therefore, the leak rate of M-CFRP was decreased, and the permeation properties can be improved by filling montmorillonite. The velocity of diffusion of M-CFRP until reaching steady leakage was lower than that of V-CFRP. It should be said that the montmorillonite, which is the inorganic material plays the role of the obstacle for nitrogen molecule.