Current Research Projects 2014 (Japanese)
Monolithically integrated semiconductor optical devices and circuits are expected to play key roles in advanced optical communication, optical information processing, and optical sensing applications, since they could provide complicated functions and higher performances that cannot be obtained with discrete devices. We are developing and fabricating monolithically integrated photonic circuits with new functions by utilizing compound semiconductors as good materials for active devices, supplemented by silicon as a material with superior manufacturability. In this particular fiscal year, we are fabricating and characterizing phased-array optical switch circuits on silicon platforms. We also study a group of photonic integrated circuits controling polarization states, such as a photo-detector circuit integrated with polarization converters, a polarization control circuit with Al-based quantum wells, and a polarization analyzer circuit, as well as a group of circuits manipulating light in free space, such as in-plane/surface-normal 45 degree mirrors and a beam scanner circuit based on an optical phased array.
We are investigating next generation semiconductor lasers that should contribute to optical communication, optical information processing, and optical sensing applications. We are also studying semiconductor micro/nano light emitters with low power consumption for optical interconnect applications between or inside racks and chips. In this fiscal year, we are analyzing and fabricating compact photonic crystal lasers and micro capsule-shaped metalic cavity lasers. Design and fabrication of metal-clad micro cavities coupled with InP optical waveguides are carried out as well. In parallel, we are investigating self-seeded light sources utilizing reflective semiconductor optical amplifiers for wavelength-division-multiplex access systems.
We are studying and developing photovoltaic cells with ultra-high efficiencies toward 50%, based on compound semiconductor quantum micro structures grown by metal organic vapor phase epitaxy (MOVPE). More specifically, high efficiency cells of mutiple junction tantem and intermediate band types are being investigated. In this fiscal year, development of InGaAs/GaAsP super-lattice middle cells, analyses of carrier transport and light-concentration effects in quantum well cells, and fabrication of quantum wire-on-well structures by MOVPE are being conducted.
We are studying novel high efficiency and low cost structures of solar cells utilizing epitaxial lift-off and wafer bonding technologies, highly-efficient solar power modules, and their applications to power generation systems. The subject also includes the light management in ultra-high efficiency solar cells as well as cell characterization technologies. In this fiscal year, we mainly investigate fabrication of high-efficiency thin film quantum well cells, surface activation bonding, and detailed characterization of quantum well cells under concentrated sunlight. Fabrication of perovskite solar cells is also tried.
We study III-V and III-nitride compound semiconductor crystal growth technologies on heterogenious materials such as silicon, sapphire, and aluminum nitride (AlN) by metal-organic vapor phase epitaxy (MOVPE), and their applications to optical devices. In this fiscal year, we investigate dislocation reduction in GaN by a novel seed layer and nucleation control, high-quality GaN growth on silicon by in-situ wafer curvature analysis, InGaAs growth on silicon and its application to infrared image sensors and micro-disk light emitting diodes, and monolithic white light emitting diodes using growth on AlN.
In order to solve the problem of renewable energy being unevenly distributed in time and space, we are studying large-scale low-cost storage technologies for renewable energy. More specifically, we investigate energy storage by efficient hydrogen generation in electrochemical water splitting directly driven by the highly-efficient solar cells, as well as fuel and hydrocarbon raw material generation through direct reduction of carbon dioxide by the photovoltage. In this fiscal year, electrode application of nitride semiconductors and electrochemical reduction of CO2 in water are studied.
Under the presidential endowed chair of "sustainable global energy system based on sunlight energy (global solar plus initiative, GS+I)", we work towards eternally sustainable global energy system that should replace current fossil fuel system by harvesting and storing the sunlight energy in large scale in low latitude desert areas where sunlight energy resource is abundant, by utilizing the energy locally for the time being, and by transporting, circulating, and utilizing it all over the world in the future.