We have studied van der Waals epitaxy of indium nitride (InN) on graphitic substrates using radio frequency plasma assisted molecular beam epitaxy (RF-MBE) and droplet elimination by radical ion beam (DERI) method. InN nanocrystals smaller than ~ 100 nm were densely grown on the single layer graphene supported on SiO2/Si while larger hexagonal shape nanocrystals larger than 500 nm were obtained on the thick graphite. Our result suggested that both defects on the graphitic substrate and flatness plays important role to limit the crystal size, such that these parameters act on the In droplet coverage at initial growth stage. With inserting aluminum nitride (AlN) buffer layer, the coalescent of these crystals can be improved and highly oriented wurtzite structure becomes dominant. These findings give the new insights for the improvement of the crystal growth of InN thin film.
Corundum-structured gallium oxide (α-Ga2O3) on sapphire substrates is advantageous in views of band gap engineering and low-cost, but heterogeneous phases and rotation domains tend to be included owing to the heteroepitaxy. The insertion of α-(AlxGa1-x)2O3 alloy buffer layers was effective for their elimination. This paper showed the elimination of heterogeneous phases (especially the β-Ga2O3 phase) and rotational domains, which have been often seen in α-Ga2O3 thin films on c-plane sapphire substrates with the use of α-(AlxGa1-x)2O3 buffer layers. These results are attributed to the stress reduction in α-Ga2O3 grown on the underlying layer. This was done without modifying the growth conditions, that is, without narrowing the windows of growth conditions. This result also suggests the important role of any buffer layers on sapphire substrates in order for further improvements of crystal quality of α-Ga2O3, leading to evolution of a variety of Ga2O3-based devices on low-cost and large-area on sapphire substrates.
We fabricated pentacene thin-film transistors (TFTs) with ultra-violet (UV) light cured polysilsesquioxane (PSQ) gate dielectric layers using different photo-initiators to reduce UV-curing time. When PSQ layers were cured using IrgacureTM 184 as a photo-initiator, it took 60 minutes to cure them completely. However, the curing time was reduced to be 10 minutes when IrgacureTM 907 was used with a sensitizer because the UV light was absorbed more efficiently. It was also demonstrated that the hole mobility of the pentacene TFTs was not affected by changing the photo-initiator from IrgacureTM 184 to IrgacureTM 907 with a sensitizer.
Mg-doping-concentration dependence in p-GaN Schottky contacts was characterized by current-voltage (I-V), photoresponse (PR), and photocapacitance (PHCAP) measurements. Mg-doped p-GaN films were grown on sapphire using metalorganic chemical vapor deposition. The Mg concentration was varied from 1.3 to 20×1018 cm-3. After buffered hydrofluoric acid treatment, 100-nm-thick Ni films were deposited by electron-beam evaporation to form Schottky contacts. In the I-V characteristics, a memory effect was observed, and large Schottky-barrier-height (qϕB) (over 2 eV) was obtained for the samples with low-Mg-doping concentration. As the Mg doping concentration increased, the diodes became leaky, and the apparent qϕB decreased. For all the samples, large PR signals were detected, and qϕB values were determined to be as high as around 2.1-2.4 eV independently of the Mg doping level. We found that PR measurements have an advantage to characterize heavily-doped p-GaN contacts. In addition, because the threshold energy in the PHCAP results was consistent with the qϕB value, it can be considered that acceptor-type defects induced the Fermi level pinning.
III-nitride semiconductors with wide-bandgap energy are promising materials for optoelectronic devices. Particularly, blue and green light-emitting diodes (LEDs) based on InGaN/GaN multi quantum wells are commercially available at present. To realize a highly-efficient red LED, we have focused on Eu-doped GaN (GaN:Eu) and demonstrated GaN:Eu-based LED (maximum external quantum efficiency of 9.2%) with a high output power of 1.25 mW at 20 mA. As a next challenge of GaN:Eu-based optical devices, we have paid attention to fabricate laser diodes (LDs). In this paper, we focus on two-dimensional photonic crystal (2D-PhC) nanocavities with high-Q-factors and extremely small modal volumes as a prominent candidate towards laser oscillation. Based on finite-difference time-domain (FDTD) simulations, photonic band structures of 2D-PhC with hexagonal air-holes, which are suitable for wurtzite crystals, and possibility of laser oscillation of GaN:Eu are investigated. We find that appropriate lattice constants and radii of the air-holes form sufficiently-wide photonic bandgap like the case of circular air-holes conventionally used for cubic crystals. Then, we choose the line-defect cavity (LN cavity), widely used in 2D-PhC LDs, as nanocavity structures, and the Q-factors and material gain thresholds (gth), which are at least required material gain for GaN:Eu lasing, are calculated. We clarify that long cavities show high Q-factors and low gth. For a L6 cavity, the gth is calculated to be 12 cm-1, which is lower than experimentally estimated optical gain of GaN:Eu at room temperature (19 cm-1). This result indicates that appropriate 2D-PhC cavity-design allows laser oscillation of GaN:Eu.
We prepared Yb-doped yttrium aluminum oxide thin film on c-sapphire and fused-quartz substrates by a RF-magnetron sputtering technique for laser cooling application. The sputtered film on c-sapphire shows the high transparency, which is preferable for laser cooling application. The sputtering target of Yb-doped yttrium aluminum perovskite [(Yb:Y)AP] results in the eutectic phase of Yb-doped yttrium aluminum garnet [(Yb:Y)AG)] and Yb-doped yttrium aluminum monoclinic [(Yb:Y)AM)]. A PL spectrum of (Yb:Y)AG+(Yb:Y)AM/c-sapphire shows several strong, sharp peaks originated from the intra-orbital f-f transitions of Yb ions due to the weak reabsorption in the sputtered film. The ideal cooling efficiency of 1.5% in (Yb:Y)AG+(Yb:Y)AM/c-sapphire is comparable to that of a (Yb:Y)AG ceramics. These results suggest that the high laser cooling power is achievable in the sputtered film with high Yb-doping concentration.
It is required to establish a technique for repairing a cracked structural component as well as for evaluating precisely crack initiation life in a main component of land-based gas turbine. It is known that the weld overlay technique is one of repairing processes and is widely applied to aged components in thermal power plants. However, the aged combustor in gas turbine has been replaced with a new one. Expensive alloys such as cobalt based and nickel-based super alloys have been used, which lead to increase of maintenance cost. This gave us motivation to develop an alternative repairing process, fiber laser process, to repair a fatigue crack initiated at the edge of stress concentrated part in thin-walled structures. In this study, residual stress is induced by irradiating a laser ahead of fatigue crack tip, and the delaying effect of residual stress on crack propagation rate is then evaluated. SUS 304 plate specimen with a hole was prepared and fatigue loading was enforced to induce natural cracks at the edge of the hole. Then, the area ahead of crack tip was irradiated by fiber laser and fatigue crack propagation behavior was examined. It was clarified that fatigue crack propagation rate was delayed to approach the laser irradiated area. It was confirmed that tensile residual stress was formed in the laser irradiated area by a hole drilling method. As a result, it was proven that fiber laser can be used as the advanced repairing process to repair a fatigue crack.
Surface treatment of commercially available carbon fibers is not suitable for thermoplastic matrix because they are assumed to be used mainly with epoxy matrix. The interfacial shear strength between carbon fiber and PA12 is lower than that between carbon fiber and epoxy resin. For the practical use of fiber reinforced thermoplastic, it is necessary to develop a surface treatment method on carbon fibers for thermoplastic matrix. It has been reported that the mechanical properties of carbon fiber and PA66 was improved by applying polyamide resin on the surface of carbon fiber. It was reported that PA6 synthesis on carbon fibers improves the adhesion between carbon fibers and PA12, and increases the tensile strength of composites, however this method requires many procedures for chemical reactions. Therefore, it is necessary to develop a simple surface treatment method for carbon fibers. Copolyamide emulsion is a water-based emulsion mainly composed of copolyamide resin and it is a suitable surface treatment agent for carbon fibers with its excellence in adhesion, chemical and heat resistance. In this study, wettability test and single fiber pull-out test were conducted to clarify the effect of copolyamide surface treatment of carbon fiber on the interfacial properties of carbon fiber and PA12. The carbon fiber coated with copolyamide emulsion showed better wettability and higher interfacial shear strength with PA12 than that of unsized carbon fiber.
In this paper, we present a shape identification of a cavity in a structure based on the finite element and the adjoint variable methods using the hammering testing data. The governing equation is employed the equation of motion in the three dimensions. A formulation of the problem based on the adjoint variable method is performed to find the optimal cavity shape so as to minimize the residual between the computed displacement and the observed displacement. The random tunneling algorithm is introduced to obtain the optimal cavity shape. Some numerical experiments are carried out to investigate the effects of several hammering force on the accuracy of the identified cavity shape.
The processing of the wastewater containing high concentrations of hexavalent chromium, which has been commonly used as a powerful oxidant in many industrial production processes, is not able to be implemented easily in developing countries. To develop a simple processing method for wastewater containing high-concentration of hexavalent chromium that can be easily implemented at low cost in developing countries, we examined the processing of artificial wastewater containing high-concentration hexavalent chromium (dichromate ion) using an inexpensive treatment agents. In 100 mL of artificial wastewater containing potassium chromate (hexavalent chromium ion: 1000 mg / L), hexavalent chromium was completely removed after processing in the order of calcium hydroxide (1000 mg) and ferrous chloride (500 or 1000 mg). The XRD spectra of the precipitates obtained after processing artificial wastewater containing potassium dichromate in the order of calcium hydroxide and ferrous chloride showed productions of Fe6Cl12-X(OH)12＋X and CaCO3. It was suggested that dichromate ions in artificial wastewater containing hexavalent chromium ion changed to chromate ions under strong basicity by adding calcium hydroxide, and then might react with Fe6Cl2-X(OH)12＋X to generate the precipitates containing iron-chromate complex (Fe6Cl6-X (OH) 6+X (CrO4) 3) and iron chromate (FeCrO4).