Browsing by Author "Higo, Akio"

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Effect of ALD-Al2O3 Passivated Silicon Quantum Dot Superlattices on p/i/n(+) Solar Cells
(2017-07-01)
Rahman, Mohammad Maksudur
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Tsai, Yi-Chia
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Lee, Ming-Yi
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Higo, Akio
;
Li, Yiming
The photovoltaic (PV) nature of the silicon (Si) quantum dot super lattice (QDSL) is studied with an atomic-layer-deposited aluminum oxide film (ALD-Al2O3) and a conventional sputtered-grown amorphous silicon carbide film (a-SiC). The QDSL structures act as an intermediate layer in a p/i/n(+) Si solar cell. The QDSL consists of 4-nm Si on 2-nm SiC nanodisks (NDs) arrayed in an ALD-Al2O3 and a-SiC passivation matrix. Formation of Si-NDs was confirmed by bright field scanning transmission electron microscope. A significant PV response in generating a high photocurrent density J(sc) of 30.15mA/cm(2), open circuit voltage V-oc of 0.50 V, fill factor FF of 0.61, and efficiency. of 9.12% was observed in ALD-Al2O3/QDSL solar cell with respect to a-SiC/QDSL solar cell with J(sc) of 26.94 mA/cm(2), V-oc of 0.50 V, FF of 0.47, and eta of 6.42%. A wide range of photo-carrier transports by the ALD-Al2O3/QDSL structure is possible in the external quantum efficiency spectra with respect to a-SiC/QDSL solar cell. The enhanced PV performance of the QD solar cells was clarified in terms of simulating the absorption contributions for all possible transitions in the nanostructure with different passivation films.
Fabrication of InGaN/GaN Nanodisk Structure by using Bio-template and Neutral Beam Etching Process
(2015)
Lai, Yi-Chun
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Higo, Akio
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Lee, Chang Yong
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Thomas, Cedric
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Tanikawa, Tomoyuki
Quantum dot optoelectronic devices are very attractive for their low power consumption, temperature stability, and high-speed modulation. We developed a defect-less, top-down fabrication process for sub-12-nm diameter InGaN quantum nanodisks (NDs) embedded in GaN barrier by using a combination of a bio-template and neutral beam etching. We have achieved high density of 2.6 x 10(11) cm(-2) with 6-nm in diameter and 15-nm-high nanopillars.
Impact of silicon quantum dot super lattice and quantum well structure as intermediate layer on p-i-n silicon solar cells
(2016-06)
Rahman, Mohammad Maksudur
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Lee, Ming-Yi
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Tsai, Yi-Chia
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Higo, Akio
;
Sekhar, Halubai
The photovoltaic effect of the silicon (Si)/silicon carbide (SiC) quantum dot super lattice (QDSL) and multi-quantum well (QW) strucutres is presented based on numerical simulation and experimental studies. The QDSL and QW structures act as an intermediate layer in a p-i-n Si solar cell. The QDSL consists of a stack of four 4-nm Si nano disks and 2-nm SiC barrier layers embedded in a SiC matrix fabricated with a top-down etching process. The Si nano disks were observed with bright field-scanning transmission electron microscopy. The simulation results based on the 3D finite element method confirmed that the quantum effect on the band structure for the QDSL and QW structures was different and had different effects on solar cell operation. The effect of vertical wave-function coupling to form a miniband in the QDSL was observed based on the solar-cell performance, showing a dramatic photovoltaic response in generating a high photocurrent density J(sc) of 29.24mA/cm(2), open circuit voltage V-oc of 0.51V, fill factor FF of 0.74, and efficiency eta of 11.07% with respect to a i-QW solar cell with J(sc) of 25.27mA/cm(2), V-oc of 0.49V, FF of 0.69, and eta of 8.61% and an i-Si solar cell with J(sc) of 27.63mA/cm(2), V-oc of 0.55V, FF of 0.61, and eta of 10.00%. A wide range of photo-carrier transports by the QD arrays in the QDSL solar cell is possible in the internal quantum efficiency spectra with respect to the internal quantum efficiency of the i-QW solar cell. Copyright (C) 2015 John Wiley & Sons, Ltd.
Nanometer scale fabrication and optical response of InGaN/GaN quantum disks
(2016-10-21)
Lai, Yi-Chun
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Higo, Akio
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Kiba, Takayuki
;
Thomas, Cedric
;
Chen, Shula
In this work, we demonstrate homogeneously distributed In0.3Ga0.7N/GaN quantum disks (QDs), with an average diameter below 10 nm and a high density of 2.1 x 10(11) cm(-2), embedded in 20 nm tall nanopillars. The scalable top-down fabrication process involves the use of self-assembled ferritin bio-templates as the etch mask, spin coated on top of a strained In0.3Ga0.7N/GaN single quantum well (SQW) structure, followed by a neutral beam etch (NBE) method. The small dimensions of the iron cores inside ferritin and nearly damage-free process enabled by the NBE jointly contribute to the observation of photoluminescence (PL) from strain-relaxed In0.3Ga0.7N/GaN QDs at 6 K. The large blueshift of the peak wavelength by over 70 nm manifests a strong reduction of the quantum-confined Stark effect (QCSE) within the QD structure, which also agrees well with the theoretical prediction using a 3D Schrodinger equation solver. The current results hence pave the way towards the realization of large-scale III-N quantum structures using the combination of bio-templates and NBE, which is vital for the development of next-generation lighting and communication devices.
Type-II Ge/Si Quantum Dot superlattice for Intermediate-band Solar Cell Applications
(2013)
Hu, Weiguo
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Fauzi, Mohd Erman
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Igarashi, Makoto
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Higo, Akio
;
Lee, Ming-Yi
A Ge/Si type-II quantum dot (QD) has been developed for use in all-Si intermediate-band solar cell (IBSC) applications. A top-down process is used to fabricate the ultra-high-quality QD superlattice. A newly developed 3D finite element method was used to solve several key design problems in achieving a practical structure. Theoretical calculations revealed that a heavy hole state can act as an ideal intermediate band when the interdot space ranges from 0.5 to 4 nm. An IBSC based on this superlattice dramatically enhanced conversion efficiency for concentration applications. For one-sun applications, H-passivizing Si and/or regrowthing amorphous SiC have a great potential to improve the conversion efficiency.