This paper reviews the evolution of corundum-structured gallium oxide (α-Ga2O3) semiconductors from crystal growth of single-crystalline films to potential device applications. In spite of thermodynamically metastable phase, high-quality α-Ga2O3 can be grown on sapphire substrates by the use of mist chemical vapor deposition (CVD), or mist epitaxy, allowing low-cost and high performance power devices. N-type conductivity control is achieved by Sn doping with the carrier concentration from 1017 to 1019 cm-3. Marked progress in performance of Schottky barrier diodes (SBDs) has been brought by the development of device structures. The most up-to-date results show on resistance and breakdown voltage of 0.1 mΩ・cm2 and 531 V (SBD1) or 0.4 mΩ・cm2 and 855 V (SBD2), respectively, and the record-low on resistance was demonstrated at the high breakdown voltage. These results encourage the future development of low cost and high performance power devices with α-Ga2O3.
Recently, BaSnO3 is attracting great attention as one of the promising oxide semiconductors with large bandgap (3.1 eV) and high mobility. The lattice constant and the band gap of this material can be tuned by changing the A site ion to Sr and Ca that should be an advantage if it is stacked to other oxide films with perovskite structure. In this paper, the growth and the characterization of (Ba,La)SnO3 thin films on (111) SrTiO3 substrate by pulsed laser deposition method are described. The effect of the O2 gas pressure on the stoichiometry, crystallinity and the deposition rate of film are investigated. Eventually (111) (Ba,La)SnO3 epitaxial film with the electron concentration of 2.2×1019 cm-3 and the hall mobility of 22.5 cm2V-1s-1 was successfully obtained.
Thin film solar cells using perovskite materials are very intensively studied since the perovskite materials such as CH3NH3PbI3 crystal have been found to show sensitizing effects on a TiO2 electron transport layer. This class of solar cell has made tremendous progress during the last few years, leading to a recently certified record power conversion efficiency (PCE) of 21.02%. In the perovskite /crystalline-Si (c-Si) tandem solar cell, furthermore, the theoretical value of PCE is expected as large as 35 %. Toward this application, the perovskite solar cell must be highly transparent at near-infrared wavelengths so that sufficient light is transmitted to the narrow-bandgap bottom cell. In this study, we fabricated the organic-lead halide perovskite solar cells comprising a transparent sputtered indium tin oxide (ITO) top electrode. We observed the PCE of 1.5% in transparent perovskite solar cells with a thin molybdenum oxide buffer layer and ITO electrode. Moreover, we obtained the PCE of 2% even in solar cells with ITO electrode sputtered directly on the organic charge transport layer. The photovoltaic property could be confirmed under the light irradiation from the ITO top electrode side.
We studied effects of the internal electric field on the two-step photocurrent generation in quantum dot superlattice (QDSL) solar cells. We calculated the quantum efficiency of intersubbad photoexcited carriers in QDSL as a function of the internal electric field. In our calculation, we proposed a model of a QDSL structure in which electrons created by the interband transition are excited by subbandgap photons corresponding to the intersubband transition. We found that extra photocurrent caused by the two-step photoexcitation shows the maximum at a reverse biased electric field, whereas current generated by only the interband photoexcitation increases monotonically with increasing the electric field. The internal electric field of the solar cell can separate photocreated electron and hole in the SL miniband, and electron lifetime is extended, which improve the intersubband transition strength, and, therefore, the two-step photocurrent increases. Thus, the calculated result unveils that there is a trade-off relation between carrier separation in the SL miniband and electric-field induced carrier escape from QDSL. These results clarify that long electron lifetime extended by carrier separation is a key maximizing the two-step photocurrent generation in a QDSL solar cell.
We improved the carrier mobility of the pentacene thin film transistors (TFT), which were fabricated with polysilsesquioxane (PSQ) gate dielectric layers, from 0.082 to 0.31 cm2V-1s-1 by treating the PSQ surface with ultra-violet irradiation (UV)/O3 and 1,1,1,3,3,3-hexamethyldisilazane (HMDS). It was found that the PSQ layers were flattened by the UV/O3 treatment, and the PSQ surface became hydrophilic at the same time because the organic functional groups on the PSQ surface were changed to hydroxyl groups. The grains of the pentacene films deposited on the UV/O3-treated PSQ surfaces were found to be as large as a few microns. However, the carrier mobility of the pentacene TFTs was not so much improved as expected from the largely grown pentacene grains probably because the hydroxyl groups scattered the charged carriers. In addition, the off-current of the pentacene TFTs increased by 4 orders of magnitude. It is thus considered that the hydroxyl groups also worked as hopping sites for the increased off-current which flew without the gate voltage. On the other hand, the carrier mobility of the pentacene TFTs fabricated with the PSQ dielectric layers of which surfaces were treated with UV/O3 and HMDS became ~4 times larger than that without any surface treatment of the PSQ layers, and also the off-current decreased by 3 orders of magnitude because the hydroxyl groups were changed with silyl groups by the HMDS treatment.
The internal stress in crystalline thermoplastics, polyphenylene sulphide (PPS), reinforced by carbon fibers of 30 mass% was measured by the diffraction method using synchrotron with energy of 12.3 keV. The stress in the matrix was determined by the sin2ψ method with side-inclination optics of transmitted X-ray diffractions. Using skin-layer strips cut parallel, perpendicular and 45° to the molding direction of the injection molded plates, the matrix stress was measured under the uniaxial applied stress. The matrix stresses in the fiber direction, σ1m and perpendicular to the fiber direction, σ2m, and shear stress τ12m were expressed as the functions of the applied (macro) stresses, σ1A , σ2A , τ12A as follows: σ1m = α11σ1A + α12σ2A , σ2m = α21σ1A + α22σ2A , 12m = α55τ12A and the stress-partitioning coefficients, α11, α12, α21, α22, α55, were determined. The coefficients determined by the transmission method are fairly close to the reported values determined by the reflection method. The experimental values were at least qualitatively agreed with the prediction derived based on micromechanics. The quantitative difference between experiment and prediction is mainly due to the neglect of the distribution of fiber orientations in the micromechanics prediction. Tensile residual stresses were measured in the matrix in the transmission method and were larger in the fiber direction than in the parallel direction. These residual stresses were caused by the mismatch of the thermal expansion coefficient between matrix and fibers.
It is necessary to control weld residual stress which has negative influence on fracture strengths. In structural steel welds, complex residual stress fields are formed due to phase transformation that occur according to the thermal cycles. Therefore, it is important to evaluate the stress generating process during phase transformation in welding. In this study, in-situ evaluation of phase transformation and transitional stress simultaneously during welding is discussed. In the test using SM490A, after cooling process, stress evaluated by this system showed good agreement with that evaluated by lab X-ray. During austenite to ferrite transformation in weld metal, tensile stress occurred in austenite and compressive stress occurred in ferrite. These behavior are agree with internal stresses behavior depend on phase transformation strain and the difference of thermal expansion rate between ferrite and austenite. Moreover, stress concentration was occurred in ferrite phase immediately after the start of phase transformation. Also, stress concentration was occurred in austenite phase just before the end of phase transformation. Thus in-situ observation phase transformation and stress simultaneously during welding and phase transformation process can be done in weld metal.
In this study, Giga-cycle fatigue properties for A5083P-O aluminum alloy were investigated by ultrasonic fatigue testing at 20 kHz, rotating bending fatigue testing at 100 Hz and electromagnetic resonance type fatigue testing at 100 Hz. Not only the ultrasonic fatigue testing but also rotating bending fatigue testing were conducted up to 1010 cycles to validate the ultrasonic fatigue testing. The ultrasonic fatigue test results showed negligible difference from those of conventional fatigue testing results. In addition, the slope of S-N curve becomes smaller in 108 ~ 1010 cycles and fatigue limit like behavior was discovered. Furthermore, successive surface observation discovered the phenomena that fatigue crack stopped propagating during 109 cycles fatigue test. It is clarified that non-propagating microscopic fatigue crack develops fatigue limit of A5083P-O aluminum alloy.
Rotating bending loading and axial loading fatigue tests were conducted to investigate the effect of zirconia shot peening on fatigue properties of Ti-6Al-4V ELI alloy. Specimen was treated in three different shot peening conditions. As a result of the rotating bending fatigue tests, the fatigue lives of shot peened specimen were longer life than non-peened specimen. Fish eyes were observed in the fracture surface of the most shot peened specimen. And there was the facet of α-Ti or the fine granular region near the crack initiation site of the fish eye. And then, fracture morphology of shot peened specimens is different by peening conditions. From the result of axial loading fatigue tests, it becomes obvious that the facet of α-Ti was formed by action of tensile stress over 500MPa without compressive stress and that the fine granular region was formed by action of compressive stress. These results suggested that low level tensile residual stress existed at subsurface near the surface of the SP treated specimen, and the residual stress distribution decided on internal crack origin type between the facet of α-Ti or the fine granular region.