Naoya Miyashita
| Department of Engineering Science | Associate Professor |
| Cluster III (Fundamental Science and Engineering) | Associate Professor |
Researcher Information
Research Keyword
Field Of Study
Career
- Apr. 2022 - Present
The University of Electro-Communications, Graduate School of Informatics and Engineering Department of Engineering Science, Associate Professor - May 2019 - Mar. 2022
The University of Tokyo, Research Center for Advanced Science and Technology (RCAST), Project Lecturer - Apr. 2013 - Apr. 2019
The University of Tokyo, Research Center for Advanced Science and Technology (RCAST), Project Assistant Professor - Apr. 2010 - Mar. 2013
The University of Tokyo, Research Center for Advanced Science and Technology (RCAST), Project Researcher
Research Activity Information
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Paper
- Abnormal photoluminescence properties of InAs/InAsSb in-plane ultrahigh-density quantum dots
Sim Jui Oon; Takumi Ohyama; Naoya Miyashita; Koichi Yamaguchi
Japanese Journal of Applied Physics, IOP Publishing, 63, 8, 085501-085501, 01 Aug. 2024, Peer-reviwed, Abstract
InAs/InAsSb ultrahigh-density quantum dots (UHD QDs) with a density of were fabricated using MBE, and their photoluminescence (PL) properties were characterized. Through temperature dependences of PL energy, intensity and decay time of UHD QDs, it was found that in-plane strong coupling between adjacent QDs of excited and ground states due to homogeneous broadening of QD levels progressed above 60 K and above 100 K, respectively. The findings regarding the strong coupling at ground states were consistent with the temperature dependence of PL minimum energy outlined in appendix. In addition, abnormal phenomenon in PL full width at half maximum was attributed to the in-plane miniband formation resulting from strong coupling.
Scientific journal - Resonant tunneling injection of electrons through double stacked GaAs/InAs quantum dots with nanohole electrode
Yuji Nakazato; Naoya Miyashita; Koichi Yamaguchi
Japanese Journal of Applied Physics, IOP Publishing, 24 Oct. 2023, Peer-reviwed, Abstract
Resonant tunneling diodes containing closely double-stacked InAs quantum dots (QDs) were grown on GaAs substrates by molecular beam epitaxy. After growing a thin GaAs capping layer on the double-stacked InAs QDs, nanoholes were selectively formed just above the larger second QDs by thermal annealing. The Au thin film was deposited directly on top surface of the larger second QDs through the nanoholes. The second QDs contacted with Au film served as conducting dots, which can locally inject electrons into the underlying first QDs. In current versus voltage (I-V) measurements, (dI/dV) peaks were clearly observed in the forward bias voltage region. It was due to the tunneling current through a non-doped GaAs thin layer between double-stacked QDs and n-GaAs conduction band. The (dI/dV) peaks shifted toward the lower forward voltage region with increasing temperature. It was explained by the temperature dependence of the electron energy distribution in the GaAs conduction band.
Scientific journal - High-density and high-uniformity InAs quantum nanowires on Si(111) substrates
Ryusuke Nakagawa; Rikuta Watanabe; Naoya Miyashita; Koichi Yamaguchi
Journal of Applied Physics, AIP Publishing, 134, 15, 18 Oct. 2023, Peer-reviwed, InAs nanowires (NWs) were grown on SiOx pinholes formed on Si(111) substrates by molecular beam epitaxy. Influences of electron-beam (EB) irradiation on the SiOx layer on the pinhole formation and the subsequent InAs NW growth were studied. As the EB irradiation dose increased, the pinhole density in the SiOx layer decreased. From atomic force microscopy, transmission electron microscopy, and x-ray photoelectron spectroscopy results, it was found that the pinhole etching of the SiOx layer by Ga droplets was suppressed by carbon adsorption due to the EB irradiation. By forming high-density pinholes on the SiOx layer without the EB irradiation, high-density InAs NWs with 1–2 × 1010 cm−2 were grown successfully, and the uniformity in the NW diameter improved. The standard deviation of the NW diameter was 1.8 nm (8.8%) for high-density NWs. In addition, the NW diameter decreased with decreasing EB dose, and the NW diameter was controlled by adjusting the diameter of Ga droplets forming the pinholes. As the NW diameter decreased, photoluminescence spectra of the NWs shifted to higher energies than the bandgap energy of the wurtzite InAs bulk. From these results, we successfully fabricated high-density and high-uniformity InAs NWs with quantum size effects on EB-unirradiated SiOx/Si(111).
Scientific journal - Inverse design of intermediate band solar cell via a joint drift-diffusion simulator and deep reinforcement learning scheme
Kodai Shiba; Naoya Miyashita; Yoshitaka Okada; Tomah Sogabe
Japanese Journal of Applied Physics, May 2023, Peer-reviwed
Scientific journal, English - High-efficiency InAs/GaAs quantum dot intermediate band solar cell achieved through current constraint engineering
Tomah Sogabe; Yasushi Shoji; Naoya Miyashita; Daniel J. Farrell; Kodai Shiba; Hwen-Fen Hong; Yoshitaka Okada
Next Materials, Apr. 2023, Peer-reviwed
Scientific journal, English - Ultrafast inverse design of quantum dot optical spectra via a joint TD-DFT learning scheme and deep reinforcement learning
Hibiki Yoshida; Katsuyoshi Sakamoto; Naoya Miyashita; Koichi Yamaguchi; Qing Shen; Yoshitaka Okada; Tomah Sogabe
AIP Advances, AIP Publishing, 12, 11, 115316-115316, Nov. 2022, Peer-reviwed, Here, we report a case study on inverse design of quantum dot optical spectra using a deep reinforcement learning algorithm for the desired target optical property of semiconductor Cd xSe yTe x− y quantum dots. Machine learning models were trained to predict the optical absorption and emission spectra by using the training dataset by time dependent density functional theory simulation. We show that the trained deep deterministic policy gradient inverse design agent can infer the molecular structure with an accuracy of less than 1 Å at a fixed computational time of milliseconds and up to 100–1000 times faster than the conventional heuristic particle swam optimization method. Most of the effective inverse design problems based on the surrogate machine learning and reinforcement learning model have been focused on the field of nano-photonics. Few attempts have been made in the field of quantum optical system in a similar manner. For the first time, our results, to our knowledge, provide concrete evidence that for computationally challenging tasks, a well-trained deep reinforcement learning agent can replace the existing quantum simulation and heuristics optimization tool, enabling fast and scalable simulations of the optical property of nanometer sized semiconductor quantum dots.
Scientific journal, English - Improvement of III-V dilute nitride thin films for solar cell application: Effect of antimony doping
N. AHSAN; N. MIYASHITA; K. M. Yu; W. WALUKIEWICZ; Y. OKADA
International Journal of the Society of Materials Engineering for Resources, The Society of Materials Engineering for Resources of Japan, 25, 2, 157-167, 31 Oct. 2022, Peer-reviwed
Scientific journal - Dependence of the radiative lifetime on the type-II band offset in GaAsxSb1−x/GaAs quantum dots including effects of photoexcited carriers
Yusuke Oteki; Yasushi Shoji; Naoya Miyashita; Yoshitaka Okada
Journal of Applied Physics, AIP Publishing, 132, 13, 134402-134402, 07 Oct. 2022, Peer-reviwed, In quantum dot (QD) heterostructures that have a type-II band alignment, either the electron or the hole is confined inside the QD. Due to smaller electron–hole overlap in such structures, relatively long radiative lifetimes can be realized, which is beneficial for devices such as intermediate-band solar cells. The use of GaAsxSb1− x/GaAs QDs allows us to control the energy level of the confined state by changing the type-II conduction-band offset (CBO) without the need of changing the QD size. However, the dependence of the radiative lifetime τr on the CBO needs to be considered to achieve optimum device performance. In this work, GaAsxSb1− x/GaAs QDs were grown by molecular beam epitaxy. The amount of deposition was controlled to obtain QDs with approximately the same size even for different values of As composition x, and the carrier lifetime was determined by time-resolved photoluminescence measurements. Since the CBO becomes smaller for larger values of x, a simple model would predict a larger electron–hole overlap for larger x values, and thus, the lifetime should decrease monotonically. However, the experimentally obtained lifetime does not decrease monotonically, which has interesting implications for applications. We explain the observed trend by the effect of photoexcited carriers; a triangular potential well is formed around the QDs in the case of high excitation densities, and thus, electrons are localized near the QDs. We also calculated τr considering the effect of photoexcited carriers to confirm our model, and a similar tendency was obtained.
Scientific journal - Demonstration of in-plane miniband formation in InAs/InAsSb ultrahigh-density quantum dots by analysis of temperature dependence of photoluminescence
Sho Tatsugi; Naoya Miyashita; Tomah Sogabe; Koichi Yamaguchi
Japanese Journal of Applied Physics, IOP Publishing, 61, 10, 102009-102009, Oct. 2022, Peer-reviwed, Abstract
Self-assembled InAs/InAsSb ultrahigh-density quantum dots (UHD QDs) were grown on GaAs(001) substrate by molecular beam epitaxy. The QD density was 1 × 1012 cm−2, and an average separation distance between adjacent QD edges was less than 3 nm. The temperature dependence of photoluminescence (PL) minimum energy of UHD QDs was measured and was divided clearly into three temperature regions: (1) fitting to upper Varshni shift at 15–80 K, (2) decreasing from upper Varshni shift from 80–180 K and (3) re-fitting to lower Varshni shift at 180–290 K. The energy difference between upper and lower Varshni curves increased with increasing QD density. This anomalous temperature dependence of PL minimum energy was demonstrated by a simulation model of miniband formation due to electronically strong coupling of adjacent QDs, including the temperature dependence of the homogeneous broadening in the QD.
Scientific journal - Enhanced current generation in quantum-dot intermediate band solar cells through optimizing the position of quantum dot layers
Yusuke Oteki; Naoya Miyashita; Maxime Giteau; Kodai Shiba; Tomah Sogabe; Yoshitaka Okada
Optical Materials: X, Elsevier BV, 16, 100207-100207, Oct. 2022, Peer-reviwed
Scientific journal - Co-deposition of MoS2 films by reactive sputtering and formation of tree-like structures
Myeongok Kim; Maxime Giteau; Nazmul Ahsan; Naoya Miyashita; Logu Thirumalaisamy; Chen Chen; Joan M Redwing; Yoshitaka Okada
Nanotechnology, IOP Publishing, 33, 34, 345708-345708, 20 Aug. 2022, Peer-reviwed, Abstract
Transition metal dichalcogenides are versatile layered materials with potential applications ranging from optoelectronic devices to water splitting. Top-down fabrication methods such as exfoliation are not practical for a large-scale production of high-quality devices: a bottom-up approach such as sputtering, a low-temperature deposition method, is more suitable. However, due to its anisotropic nature, the growth mechanism of molybdenum disulfide (MoS2) via sputtering is complex and remains to be investigated in detail. In this paper, we study the growth of MoS2 films co-deposited by using a sulfur (S) hot-lip cell and a molybdenum (Mo) sputtering target via reactive sputtering. The impact of S partial pressure on the structure and morphology of MoS2 films was systematically characterized, and it was observed that the growth is dominated by vertically-oriented sheets with horizontal branches, resulting in a tree-like structure. The growth front of the structures is ascribed to the anisotropic incorporation of adatoms with regards to the orientation of MoS2.
Scientific journal - Self-formation of InAs/InGaAsSb type-II superlattice structures on InP substrates by MBE and their application to mid-infrared LEDs
Kou Uno; Naoto Iijima; Naoya Miyashita; Koichi Yamaguchi
AIP Advances, AIP Publishing, 12, 8, 085301-085301, 01 Aug. 2022, Peer-reviwed, InAs/InGaAsSb type-II superlattice structures (SLSs) were spontaneously formed by the molecular beam epitaxy of InAs/GaAs0.86Sb0.14 SLSs on InP substrates. The strain due to lattice mismatch between InAs and GaAs0.86Sb0.14 induced two exchange reactions of In–Ga and As–Sb at both InAs/GaAs0.86Sb0.14 heterointerfaces, resulting in the strain relaxation and the self-formation of InAs/InGaAsSb type-II SLS. By energy dispersive x-ray spectroscopy analysis, the mixed crystal composition of the InGaAsSb layer was determined to be approximately In0.8Ga0.2As0.9Sb0.1. Electroluminescence spectra of LEDs, including the self-formed InAs/In0.8Ga0.2As0.9Sb0.1 2.5-periodic SLS, showed double peaks of 2.6 and 3.2 µm at 15 K. The luminescence spectrum was based on two transition mechanisms of type-I transition in InAs and type-II transition between InAs and InGaAsSb. The experimental results of luminescence spectra were supported by theoretical calculations. The 3.3 µm emission was maintained above 220 K.
Scientific journal - Spectral Splitting Solar Cells Constructed with InGaP/GaAs Two-Junction Subcells and Infrared PbS Quantum Dot/ZnO Nanowire Subcells
Haibin Wang; Shoichiro Nakao; Naoya Miyashita; Yusuke Oteki; Maxime Giteau; Yoshitaka Okada; Tatsuya Takamoto; Hidenori Saito; Shinichi Magaino; Katsuhiko Takagi; Tetsuya Hasegawa; Takaya Kubo; Takumi Kinoshita; Jotaro Nakazaki; Hiroshi Segawa
ACS Energy Letters, American Chemical Society (ACS), 2477-2485, 08 Jul. 2022, Peer-reviwed
Scientific journal - Collection of photocarriers in intermediate band solar cells: experiments and equivalent circuit analysis
Nazmul Ahsan; Naoya Miyashita; Kin Man Yu; Wladek Walukiewicz; Yoshitaka Okada
Journal of Photonics for Energy, SPIE-Intl Soc Optical Eng, 12, 03, 01 Jul. 2022, Peer-reviwed
Scientific journal - Diffusion photocurrent influenced by intraband excitation in an intermediate band solar cell with type-I band alignment
Yaxing Zhu; Shigeo Asahi; Naoya Miyashita; Yoshitaka Okada; Takashi Kita
Japanese Journal of Applied Physics, IOP Publishing, 61, 7, 074002-074002, 01 Jul. 2022, Peer-reviwed, Abstract
We elucidate a photocarrier collection mechanism in intermediate band solar cells (IBSCs) with InAs-quantum dots (QDs)-in-an-Al0.3Ga0.7As/GaAs-quantum well structures. When the Al0.3Ga0.7As barrier is excited, the device electrical output can be varied by additional infrared light for the electron intraband optical transition in QDs. The photocurrent in IBSC with a single QDs-in-a-well structure shows a monotonic increase with the intraband-excitation density. Conversely, IBSC with a multilayered QDs-in-a-well structure exhibits a photocurrent reduction when electrons in QDs are optically pumped out. The simultaneously measured photoluminescence spectra proved that the polarity of QD states changes depending on the intraband-excitation density. We discuss the drift and diffusion current components and point out that the hole diffusion current is significantly influenced by carriers inside the confinement structure. Under strong intraband excitations, we consider an increased hole diffusion current occurs by blocking hole-capture in the quantum structures. This causes unexpected photocurrent reduction in the multilayered device.
Scientific journal - Resonant absorption for multilayer quantum well and quantum dot solar cells
Maxime Giteau; Yusuke Oteki; Kento Kitahara; Naoya Miyashita; Ryo Tamaki; Yoshitaka Okada
Journal of Photonics for Energy, 12, 2, Apr. 2022, Peer-reviwed, Epitaxially grown quantum well and quantum dot solar cells suffer from weak light absorption, strongly limiting their performance. Light trapping based on optical resonances is particularly relevant for such devices to increase light absorption and thereby current generation. Compared to homogeneous media, the position of the quantum layers within the device is an additional parameter that can strongly influence resonant absorption. However, this effect has so far received little attention from the photovoltaic community. We develop a theoretical framework to evaluate and optimize resonant light absorption in a thin slab with multiple quantum layers. Using numerical simulations, we show that the position of the layers can make the difference between strong absorption enhancement and completely suppressed absorption, and that an optimal position leads to a resonant absorption enhancement two times larger than average. We confirm these results experimentally by measuring the absorption enhancement from photoluminescence spectra in InAs/GaAs quantum dot samples. Overall, this work provides an additional degree of freedom to substantially improve absorption, encouraging the development of quantum wells and quantum dots-based devices such as intermediate-band solar cells.
Scientific journal - Erratum: Two-photon photocurrent spectra of InAs quantum dot-in-well intermediated-band solar cells at room temperature (Journal of Applied Physics (2021) 130 (124505) DOI: 10.1063/5.0060569)
Yaxing Zhu; Shigeo Asahi; Naoya Miyashita; Yoshitaka Okada; Takashi Kita
Journal of Applied Physics, American Institute of Physics Inc., 130, 17, 07 Nov. 2021, Peer-reviwed, This article was originally published online on 29 September 2021 with author Takashi Kita incorrectly spelled. The name is correct as it appears above. All online versions of this article were corrected on 5 October 2021
the article is correct as it appears in the printed version of the journal. AIP Publishing apologizes for this error.
Scientific journal, English - Two-photon photocurrent spectra of InAs quantum dot-in-well intermediated-band solar cells at room temperature
Yaxing Zhu; Shigeo Asahi; Naoya Miyashita; Yoshitaka Okada; Takashi Kita
Journal of Applied Physics, AIP Publishing, 130, 12, 124505-124505, 28 Sep. 2021, Peer-reviwed
Scientific journal - Two-step excitation induced photovoltaic properties in an InAs quantum dot-in-well intermediate-band solar cell
Yaxing Zhu; Shigeo Asahi; Kohei Watanabe; Naoya Miyashita; Yoshitaka Okada; Takashi Kita
Journal of Applied Physics, AIP Publishing, 129, 7, 074503-074503, 21 Feb. 2021, Peer-reviwed
Scientific journal - Theoretical and experimental assessment of thinned germanium substrates for III–V multijunction solar cells
Iván Lombardero; Mario Ochoa; Naoya Miyashita; Yoshitaka Okada; Carlos Algora
Progress in Photovoltaics: Research and Applications, Wiley, 28, 11, 1097-1106, Nov. 2020, Peer-reviwed
Scientific journal - Fabrication and optical characterization of ultrathin III-V transferred heterostructures for hot-carrier absorbers
Maxime Giteau; Kentaroh Watanabe; Naoya Miyashita; Hassanet Sodabanlu; Fabien Atlan; Daniel Suchet; Stéphane Collin; Jean-François Guillemoles; Yoshitaka Okada
Physics, Simulation, and Photonic Engineering of Photovoltaic Devices IX, SPIE, 03 Mar. 2020, Peer-reviwed
International conference proceedings - Anneal mediated deep-level dynamics in GaInNAsSb dilute nitrides lattice-matched to GaAs
Naoya Miyashita; Yilun He; Nazmul Ahsan; Yoshitaka Okada
Lead, Journal of Applied Physics, 126, 14, 143104, 14 Oct. 2019, Peer-reviwed, © 2019 Author(s). This study focuses on the relationship among defects, annealing temperatures, and solar cell properties in the molecular beam epitaxy-grown GaInNAsSb solar cells. Samples with an n-i-p double heterostructure were annealed at various temperatures from 650 to 850 °C, and the dynamics of deep-level defects were analyzed. A significant improvement was obtained between 750 and 800 °C due to suppression of both the traps and the nonradiative recombination centers. They were distinguished using the DC bias dependence of admittance spectra and were reconfirmed from the electroluminescence and quantum efficiency spectroscopy studies.
Scientific journal, English - Inverted Lattice-Matched Triple Junction Solar Cells with 1.0 eV GaInNAsSb Subcell by MOCVD/MBE Hybrid Growth
Naoya Miyashita; Yilun He; Takaaki Agui; Hiroyuki Juso; Tatsuya Takamoto; Yoshitaka Okada
Lead, IEEE Journal of Photovoltaics, 9, 3, 666-672, May 2019, Peer-reviwed, © 2011-2012 IEEE. In this paper, lattice-matched GaInP/GaAs/GaInNAsSb 3-junction (3J) solar cells were fabricated using a hybrid growth technique consisting of metalorganic chemical vapor deposition (MOCVD) for the upper two subcells and molecular beam epitaxy (MBE) for the bottom cell. In the normal upright configuration, the MBE-grown GaInNAsSb solar cells often suffer from unintentional hydrogen incorporation due to the pyrolysis of MOCVD precursors during the growth of other subcells, which can lead to a degradation of the cell performance of GaInNAsSb subcell. In contrast, we studied the inverted growth sequence in order to avoid hydrogen incorporation. Then, we applied post-growth annealing to improve the carrier collection efficiency of GaInNAsSb bottom cell. The annealing effectively improved the carrier collection and increased the quantum efficiency of GaInNAsSb bottom cell in the present inverted 3J structure, whereas no significant disadvantages for the top and middle cells were observed. As a result, we achieved external quantum efficiency beyond 90% in 1.0 eV GaInNAsSb bottom cells with thicknesses of 1.5 and 2.0 μm, and successfully demonstrated high enough photocurrents, fulfilling the current matching condition in the 3J solar cells.
Scientific journal, English - Epitaxial Lift-Off of Ultrathin Heterostructures for Hot-Carrier Solar Cell Applications
Maxime Giteau; Kentaroh Watanabe; Hassanet Sodabanlu; Naoya Miyashita; Masakazu Sugiyama; Andrea Cattoni; Stephane Collin; Jean-Francois Guillemoles; Yoshitaka Okada
2019 COMPOUND SEMICONDUCTOR WEEK (CSW), IEEE, 2019
International conference proceedings, English - GaSb/GaAs quantum nanostructures for intermediate band solar cell under high sunlight concentration
Yusuke Oteki; Yasushi Shoji; Naoya Miyashita; Yilun He; Yoshitaka Okada
2019 COMPOUND SEMICONDUCTOR WEEK (CSW), IEEE, 2019, Peer-reviwed
Scientific journal, English - Light absorption enhancement in ultra-thin layers for hot-carrier solar cells: first developments towards the experimental demonstration of an enhanced hot-carrier effect with light trapping
Maxime Giteau; Kentaroh Watanabe; Naoya Miyashita; Hassanet Sodabanlu; Julie Goffard; Amaury Delamarre; Daniel Suchet; Ryo Tamaki; Zacharie Jehl; Laurent Lombez; Masakazu Sugiyama; Andrea Cattoni; St?phane Collin; Jean-Fran?ois Guillemoles; Yoshitaka Okada
Proc. SPIE, 10913, 10913D, 2019, Peer-reviwed, © 2019 SPIE. Hot-carrier solar cells (HCSC) can potentially overcome the Shockley-Queisser limit, by having carriers at a higher temperature than the lattice. To this end, the carriers need to thermalize slower than power is generated by absorbing photons. In thin films, a hot-carrier distribution can only be achieved with very high incident power, by saturating the thermalization channels. Ultra-thin absorbers have a smaller thermalization rate, due to fewer channels. However, they typically absorb only a limited amount of light, which prevents them from reaching high efficiencies. Light trapping is an excellent way to increase significantly the amount of light absorbed in an ultra-thin material. Yet, studies on the coupling between light trapping and hot carriers are still lacking, due to the complexity of the whole system. We analyze numerically and experimentally how light trapping can enable high-efficiency HCSC. This manuscript presents the progress towards the experimental demonstration of the enhancement of the hot-carrier effect with light trapping. 280 nm-thick devices have successfully been reported on a gold mirror using epitaxial lift-off (ELO) and gold-gold bonding. These devices have been characterized by photoluminescence spectroscopy. Hot carriers with a temperature 37 K above lattice temperature were measured, in accordance with theoretical predictions. We are now working towards the ELO of absorbers 10 times thinner, on which we will implement light trapping to increase the carrier temperature.
International conference proceedings, English - Material challenges for solar cells in the twenty-first century: directions in emerging technologies
Samy Almosni; Amaury Delamarre; Zacharie Jehl; Daniel Suchet; Ludmila Cojocaru; Maxime Giteau; Benoit Behaghel; Anatole Julian; Camille Ibrahim; Léa Tatry; Haibin Wang; Takaya Kubo; Satoshi Uchida; Hiroshi Segawa; Naoya Miyashita; Ryo Tamaki; Yasushi Shoji; Katsuhisa Yoshida; Nazmul Ahsan; Kentaro Watanabe; Tomoyuki Inoue; Masakazu Sugiyama; Yoshiaki Nakano; Tomofumi Hamamura; Thierry Toupance; Céline Olivier; Sylvain Chambon; Laurence Vignau; Camille Geffroy; Eric Cloutet; Georges Hadziioannou; Nicolas Cavassilas; Pierre Rale; Andrea Cattoni; Stéphane Collin; François Gibelli; Myriam Paire; Laurent Lombez; Damien Aureau; Muriel Bouttemy; Arnaud Etcheberry; Yoshitaka Okada; Jean-François Guillemoles
Science and Technology of Advanced Materials, Taylor and Francis Ltd., 19, 1, 336-369, 31 Dec. 2018, Peer-reviwed, Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan–French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.
Scientific journal, English - Incorporation of hydrogen into MBE-grown dilute nitride GaInNAsSb layers in a MOCVD growth ambient
Naoya Miyashita; Yilun He; Nazmul Ahsan; Takaaki Agui; Hiroyuki Juso; Tatsuya Takamoto; Yoshitaka Okada
Lead, Solar Energy Materials and Solar Cells, Elsevier B.V., 185, 359-363, 01 Oct. 2018, Peer-reviwed, In this work, lattice-matched 3-junction and 4-junction solar cells including the dilute nitride subcells were fabricated by a hybrid growth technique of metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). It was found that photocarrier extraction in the MBE grown dilute nitride GaInNAsSb subcells degraded after the overgrowth of the upper subcells by MOCVD. Then, we focused on the effect of annealing for a set of MBE grown GaInNAsSb single junction solar cells annealed in the MOCVD reactor under an arsine flow ambient. It was found that degradation of the carrier extraction due to the MOCVD growth ambient is in a correlation with the selective hydrogen incorporation into the GaInNAsSb layers. We demonstrated that the subsequent annealing in nitrogen ambient promoted the outdiffusion of hydrogen which led to a recovery of the GaInNAsSb solar cell performances.
Scientific journal, English - Enhancement of photocurrent in epitaxial lift-off thin-film GaInNAsSb solar cells due to light-confinement structure
Naoya Miyashita; Benoît Behaghel; Jean-François Guillemoles; Yoshitaka Okada
Lead, Applied Physics Express, Japan Society of Applied Physics, 11, 7, 72301, 01 Jul. 2018, Peer-reviwed, This work focuses on the characterization of GaInNAsSb solar cells whose substrates are removed via the epitaxial lift-off (ELO) technique. As a result of the substrate removal, increases in the photocurrent and the interference feature were clearly observed. This is clear evidence of the light-confinement effect, whereby some of the unabsorbed photons at the rear metal contact were reflected back towards the front side of the ELO thin-film cell. We successfully demonstrated that the ELO technique can be applied for the GaInNAsSb cell, and the light management should add flexibility in designing the cell structures.
Scientific journal, English - Characterization of InGaAs/GaAsN multiple quantum well with flat conduction band for improving carrier transport in multijuction solar cell
W. Yanwachirakul; N. Miyashita; H. Sodabanlu; K. Watanabe; Y. Okada; M. Sugiyama; Y. Nakano
Proceedings of SPIE - The International Society for Optical Engineering, SPIE, 10527, 10527E, 2018, Peer-reviwed, In0.227GaAs/GaAsN0.011 was introduced as a 1.2-eV multiple quantum well (MQW) with a flat conduction band (FCB) in which a conduction band edge of GaAsN was adjusted to be equal to that of InGaAs. This MQW was established as a candidate material for a middle absorption layer of three-junction solar cell since electron confinement was eliminated and a short electron lifetime in GaNAs was compensated by InGaAs layer. The band alignment of MQW was characterized by the power-dependent photoluminescence (PL) measurement under low temperature. According to the band-Anti crossing model, the FCB is possibly constructed since a small amount of incorporated N can drastically reduce the energy of the conduction band edge of GaAsN. The PL results demonstrated that In0.227GaAs/GaAsN MQW was a type-I structure when N content was below 1.1%, and became a type-II structure when N content was above 1.1%. The type-II MQW was characterized by the observation of blueshift of PL peak when increasing excitation power. This blueshift is a result of band-bending effect due to the accumulation of excited carriers at the interface between two materials, which is unique for the type-II MQW. In addition, it was observed that the activation energy estimated from the Arrhenius plot provided a minimum value in the structure with 1.1%N
the lowest activation energy indicated the weakest confinement energy of carriers in the structure. These results approved that a transition from type-I to type-II occurred when N content surpassed 1.1%, and our designed In0.227GaAs/GaAsN0.011 MQW was potentially the FCB structure.
International conference proceedings, English - Thin-film multiple-quantum-well solar cells fabricated by epitaxial lift-off process
Tatsuya Nakata; Kentaroh Watanabe; Naoya Miyashita; Hassanet Sodabanlu; Maxime Giteau; Yoshiaki Nakano; Yoshitaka Okada; Masakazu Sugiyama
Japanese Journal of Applied Physics, 57, 8, 08RF03, 2018, Peer-reviwed, © 2018 The Japan Society of Applied Physics. Thin-film solar cells fabricated using the epitaxial lift-off (ELO) process enable considerable cost reduction as well as light-trapping. A polyimide film was used as a support substrate for thin film devices to address the poor temperature tolerance of poly(ethylene terephthalate) films. Samples were stuck to polyimide films by an Au–Au bonding process. It was found that the insertion of a wetting layer enhanced the adhesion between a polyimide film and an Au layer. The achieved bonding was strong enough to carry out the ELO process, and no degradation was observed in cell performance. Thin-film solar cells with strain-balanced multiple quantum wells were also fabricated using the ELO process to benefit from the light-trapping effect. Quantum efficiency enhancement was observed in the long-wavelength range of 850–980 nm compared to the non-ELO-processed devices. The photo-absorption enhancement was due to the Fabry–Perot resonance between the surface and the rear Au electrodes.
Scientific journal, English - N-H-related deep-level defects in dilute nitride semiconductor GaInNAs for four-junction solar cells
Yilun He; Naoya Miyashita; Yoshitaka Okada
Japanese Journal of Applied Physics, 57, 08RD11, 2018, Peer-reviwed
Scientific journal, English - Design of free-barrier InGaAs/GaNAs multiple quantum well solar cells with 1.2 eV energy gap
Warakorn Yanwachirakul; Naoya Miyashita; Hassanet Sodabanlu; Kentaroh Watanabe; Masakazu Sugiyama; Yoshitaka Okada; Yoshiaki Nakano
JAPANESE JOURNAL OF APPLIED PHYSICS, IOP PUBLISHING LTD, 56, 8, 08MA04, Aug. 2017, Peer-reviwed, InGaAs and GaNAs were selected as components of a multiple quantum well (MQW) with a free-barrier conduction band (FB-CB) in which the quantum confinement for electrons was eliminated. Since the calculation demonstrated that the energy gap of a strain-balanced FB-CB InGaAs/GaNAs MQW could be decreased to 1.2 eV with lattice matching to Ge, this structure was expected as a potential absorber of the middle cell of a three-junction solar cell based on the Ge bottom cell. Additionally, the InGaAs/GaNAs MQW could mitigate detrimental impacts of the short lifetime of GaNAs because of the preferential existence of holes in InGaAs, and it can realize more efficient carrier transport than bulk GaInNAs. The time-resolved photoluminescence (TRPL) results demonstrated that the InGaAs/GaNAs MQW cell provided a significantly longer lifetime than the GaInNAs thin-film cell. The open-circuit voltage of the InGaAs/GaNAs MQW cell was superior to that of the GaInNAs thin-film cell. (C) 2017 The Japan Society of Applied Physics
Scientific journal, English - Effect of Si doping and sunlight concentration on the performance of InAs/GaAs quantum dot solar cells
Shunya Naito; Katsuhisa Yoshida; Naoya Miyashita; Ryo Tamaki; Takuya Hoshii; Yoshitaka Okada
JOURNAL OF PHOTONICS FOR ENERGY, SPIE-SOC PHOTO-OPTICAL INSTRUMENTATION ENGINEERS, 7, 2, 25505, Apr. 2017, Peer-reviwed, For intermediate band solar cells, the control of the carrier filling ratio in intermediate band is important to achieve high efficiency. We have investigated the effect of carrier doping of InAs/GaAs quantum dots (QDs) with Si and sunlight concentration on the quantum dots solar cell (QDSC) characteristics. The prefilling by Si doping of InAs/GaAs QDs was performed using two methods: modulation or delta-doping and direct doping. A gradual recovery in the open-circuit voltage with increasing Si doping concentration was observed, and it suggested a decrease of recombination via Si-doped QD states. Under high-concentrated sunlight illumination, QD states were additionally filled with photocarriers, and the open-circuit voltage increased nonlinearly with concentration ratio in both the nondoped and Si-doped QDSCs. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License.
Scientific journal, English - Improvement of 1.0eV GaInNAsSb solar cell performance upon optimal annealing
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Lead, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, WILEY-V C H VERLAG GMBH, 214, 3, 1600586, Mar. 2017, Peer-reviwed, This work studies the impact of optimal postannealing on the 1.0eV GaInNAsSb solar cells for application to high-efficiency multijunction solar cells. Compared to our past report in which reasonably good solar cell performance was achieved for devices annealed at 850 degrees C, further optimization was carried out in order to lower the temperature. Both the light current-voltage and internal quantum efficiency (IQE) characteristics reveal that significant improvements of the device qualities can be obtained with the lower annealing temperatures. For the optimal anneal condition, >90% of the IQEs was achieved with the 3.0m thick GaInNAsSb solar cell devices.
Scientific journal, English - Multiband modification of III-V dilute nitrides for IBSC application
Nazmul Ahsan; Naoya Miyashita; Kin Man Yu; Wladek Walukiewicz; Yoshitaka Okada
PHYSICS, SIMULATION, AND PHOTONIC ENGINEERING OF PHOTOVOLTAIC DEVICES VI, SPIE-INT SOC OPTICAL ENGINEERING, 10099, 100990I, 2017, Peer-reviwed, The subband features E- and E+ for the conduction band of III-V dilute nitride alloys make them promising for intermediate band solar cell application. However, presence of bandgap states can limit the two-step photon absorption activity, a necessary requirement for IBSC functionality. A model analysis is performed to characterize the density of states. The sub-band tails states are characterized using a temperature-dependent map of photo-modulated reflectance spectroscopy for GaNAs thin films grown on GaAs substrates using molecular beam epitaxy. The effect of indium and antimony incorporation on the subband features were investigated. Marked improvements in the thin films were observed both for the lower (E-) and the upper (E+) conduction bands (CB) when In was introduced with marginal enhancement by Sb. These improvements are associated with suppression of tail states below both the E- and E+ bands. Sb rather mainly plays a surfactant role improving the abruptness of the GaNAs/GaAs hetero-interface.
International conference proceedings, English - Modeling and characterization of double resonant tunneling diodes for application as energy selective contacts in hot carrier solar cells
Zacharie Jehl; Daniel Suchet; Anatole Julian; Cyril Bernard; Naoya Miyashita; Francois Gibelli; Yoshitaka Okada; Jean-Francois Guillemoles
PHYSICS, SIMULATION, AND PHOTONIC ENGINEERING OF PHOTOVOLTAIC DEVICES VI, SPIE-INT SOC OPTICAL ENGINEERING, 10099, 100990N, 2017, Peer-reviwed, Double resonant tunneling barriers are considered for an application as energy selective contacts in hot carrier solar cells. Experimental symmetric and asymmetric double resonant tunneling barriers are realized by molecular beam epitaxy and characterized by temperature dependent current-voltage measurements. The negative differential resistance signal is enhanced for asymmetric heterostructures, and remains unchanged between low-and room-temperatures. Within Tsu-Esaki description of the tunnel current, this observation can be explained by the voltage dependence of the tunnel transmission amplitude, which presents a resonance under finite bias for asymmetric structures. This effect is notably discussed with respect to series resistance. Different parameters related to the electronic transmission of the structure and the influence of these parameters on the current voltage characteristic are investigated, bringing insights on critical processes to optimize in double resonant tunneling barriers applied to hot carrier solar cells.
International conference proceedings, English - Characterization of Cr Doped CuGaS2 Thin Films Synthesized By Chemical Spray Pyrolysis
Nazmul Ahsan; Sivaperuman Kalainathan; Naoya Miyashita; Takuya Hoshii; Yoshitaka Okada
Mechanics, Materials Science and Engineering, 9, 380-387, 2017, Peer-reviwed
Scientific journal, English - Insights on energy selective contacts for thermal energy harvesting using double resonant tunneling contacts and numerical modeling
A. Julian; Z. Jehl; N. Miyashita; Y. Okada; J. -F. Guillemoles
SUPERLATTICES AND MICROSTRUCTURES, ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, 100, 749-756, Dec. 2016, Peer-reviwed, Energy selective electrical contacts have been proposed as a way to approach ultimate efficiencies both for thermoelectric and photovoltaic devices as they allow a reduction of the entropy production during the energy conversion process. A self-consistent numerical model based on the transfer matrix approach in the effective mass and envelope function approximation has been developed to calculate the electronic properties of double resonant tunneling barriers used as energy selective contacts in hot carrier solar cells. It is found that the application of an external electric bias significantly degrades the electronic transmission of the structure, and thus the tunneling current in the current-voltage characteristic. This is due to a symmetry breaking which can be offset using finely tuned asymmetric double resonant tunneling barriers, leading to a full recovery of the tunneling current in our model. Moreover, we model the heterostructure using electrons temperature in the emitter higher than that of the lattice, providing insights on the interpretation of experimental devices functioning in hot carrier conditions, especially regarding the previously reported shift of the resonance peak (negative differential resistance), which we interpret as related to a shift in the hot electron distribution while the maximum remains at the conduction band edge of the emitter. Finally, experimental results are presented using asymmetric structure showing significantly improved resonant properties at room temperature with very sharp negative differential resistance. (C) 2016 Elsevier Ltd. All rights reserved.
Scientific journal, English - Growth of ErAs nanodots by molecular beam epitaxy for application to tunneling junctions in multijunction solar cells
Chao-Yu Hung; Tomah Sogabe; Naoya Miyashita; Yoshitaka Okada
JAPANESE JOURNAL OF APPLIED PHYSICS, IOP PUBLISHING LTD, 55, 2, 21201, Feb. 2016, Peer-reviwed, ErAs nanodots (NDs) grown on GaAs(001) substrates by using molecular beam epitaxy (MBE) were investigated. Atomic force microscope images indicate that the size of ErAs NDs increases with deposition time and growth temperature. A calibration was performed to determine the deposition rate of ErAs in order that the size of NDs can be accurately controlled and hence optimized. Local current flow images and surface profiles around ErAs NDs were simultaneously measured to clarify the local conductivity distribution corresponding to a real space profile. Furthermore, we also fabricated and characterized an ErAs-ND-embedded GaAs tunnel junction (TJ), which resulted in a voltage drop of 30 mV for 15 A/cm(2) operation current equivalent to 1000 suns concentration, which is less than one-third of that of a conventional heavily doped tunnel junction. (C) 2016 The Japan Society of Applied Physics
Scientific journal, English - Improvement of 1.0 eV GaInNAsSb solar cell performance upon annealing
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Lead, 2016 COMPOUND SEMICONDUCTOR WEEK (CSW) INCLUDES 28TH INTERNATIONAL CONFERENCE ON INDIUM PHOSPHIDE & RELATED MATERIALS (IPRM) & 43RD INTERNATIONAL SYMPOSIUM ON COMPOUND SEMICONDUCTORS (ISCS), IEEE, 2016, Peer-reviwed
International conference proceedings, English - Generation and collection of photocarriers in dilute nitride GaInNAsSb solar cells
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Lead, PROGRESS IN PHOTOVOLTAICS, WILEY-BLACKWELL, 24, 1, 28-37, Jan. 2016, Peer-reviwed, An improved photocurrent production is demonstrated in dilute nitride GaInNAsSb solar cells grown by molecular beam epitaxy. The photovoltaic properties were investigated by increasing the thickness of GaInNAsSb layers from 1.0 to 3.0 mu m. The amount of photon absorption increased with increasing thickness. Yet an incomplete photocarrier collection was observed in the 2.0- and 3.0-mu m-thick GaInNAsSb devices. This feature is due to a partial lack of electric field in the GaInNAsSb region. In order for complete carrier collection aided by the electric field, the background-doping level should be as low as similar to 10(14) cm(-3) in a thick undoped-GaInNAsSb absorption layer. Here, the effectiveness of annealing on improving the field-assisted carrier collection is shown. This is ascribed to a decrease of the background doping in the GaInNAsSb layer. In a properly designed device for a 1.0-eV GaInNAsSb cell, it is demonstrated that the external quantum efficiency can reach as high as 90% at wavelengths longer than the GaAs bandgap. Copyright (C) 2015 John Wiley & Sons, Ltd.
Scientific journal, English - Evaluation of concentrator photovoltaic properties of GaInNAsSb solar cells for multijunction solar cell applications
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Lead, JAPANESE JOURNAL OF APPLIED PHYSICS, IOP PUBLISHING LTD, 54, 8, 08KE06, Aug. 2015, Peer-reviwed, In this work, we focus on the growth and characterization of undoped GaInNAsSb layers in solar cells and also study their concentrator photovoltaic properties. Concentrated light current-voltage characteristics of three GaInNAsSb-based solar cells with various thicknesses of GaInNAsSb layers (t(i)) from 1.0 to 3.0 mu m were evaluated. The photogenerated carriers from the GaInNAsSb layers are well collected even for the t(i) = 2.0 and 3.0 mu m devices. The comparison of the solar cell parameters under high concentration conditions showed no significant difference among the devices. This behavior is consistent with the results of series resistance analyses. This suggests that the bulk resistance due to the GaInNAsSb thickness is a minor factor for the series resistance in the present devices. (C) 2015 The Japan Society of Applied Physics
Scientific journal, English - Electron Barrier Engineering in a Thin-Film Intermediate-Band Solar Cell
Nazmul Ahsan; Naoya Miyashita; Kin Man Yu; Wladek Walukiewicz; Yoshitaka Okada
IEEE JOURNAL OF PHOTOVOLTAICS, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 5, 3, 878-884, May 2015, Peer-reviwed, Improved open-circuit voltages have been achieved in dilute nitride thin-film intermediate-band solar cells by optimizing intermediate-band barrier layers. The blocking properties of the AlGaAs electron barrier are found to critically depend on the barrier-doping level. Open-circuit voltages V-OC improved when electron-doping levels in the AlGaAs barriers were reduced. This is ascribed to the increased association of V-OC with the larger gap defined by the valence and conduction bands due to the improved electrical isolation of the intermediate band.
Scientific journal, English - Effect of antimony on the deep-level traps in GaInNAsSb thin films
Muhammad Monirul Islam; Naoya Miyashita; Nazmul Ahsan; Takeaki Sakurai; Katsuhiro Akimoto; Yoshitaka Okada
APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 105, 11, 112103, Sep. 2014, Peer-reviwed, Admittance spectroscopy has been performed to investigate the effect of antimony ( Sb) on GaInNAs material in relation to the deep-level defects in this material. Two electron traps, E1 and E2 at an energy level 0.12 and 0.41 eV below the conduction band (E-C), respectively, were found in undoped GaInNAs. Bias-voltage dependent admittance confirmed that E1 is an interface-type defect being spatially localized at the GaInNAs/GaAs interface, while E2 is a bulk-type defect located around mid-gap of GaInNAs layer. Introduction of Sb improved the material quality which was evident from the reduction of both the interface and bulk-type defects. (c) 2014 AIP Publishing LLC.
Scientific journal, English - Fabrication and characterization of GaAs tunnel diode and ErAs nanoparticles enhanced GaAs tunnel diode for multijunction solar cell
Tomah Sogabe; Yasushi Shoji; Mitsuyoshi Ohba; Naito Shunya; Naoya Miyashita; Chao-Yu Hung; Akio Ogura; Yoshitaka Okada
Journal of Management and Organization, Cambridge University Press, 1602, 3, 09 May 2014, Peer-reviwed, We report here the fabrication and characterization of GaAs tunnel diode (TD) and ErAs nanoparticles (Nps) enhanced GaAs TD. Four GaAs TDs with different contact area were fabricated by using MOCVD. We found extremely high peak current density of ~250A/cm2 for the TD with r=0.25mm contact area. Moreover a hysteresis loop was appeared during sweeping up and sweeping down the external voltage. A 'vector load line model' was proposed to explain the origin of the shape of the hysteresis loop and the onset of the bistability occurred at the intersect of the loadline and the current-voltage (I-V) curve of TD. Meanwhile, we have grown ErAs Nps on GaAs(100) surface by using MBE and succeeded in overgrowth of GaAs after ErAs deposition. GaAs(p+)/ErAs(Nps)/GaAs(n+) TDs were fabricated and characterized. We found the GaAs sample containing 70s deposition of ErAs showed the best TD behavior. No TD behavior was observed for the sample without addition of ErAs Nps, clearly indicating the strong tunneling enhancement effect from ErAs Nps.
Scientific journal, English - Fabrication and characterization of GaAs tunnel diode and ErAs nanoparticles enhanced GaAs tunnel diode for multijunction solar cell
Tomah Sogabe; Yasushi Shoji; Mitsuyoshi Ohba; Naito Shunya; Naoya Miyashita; Chao-Yu Hung; Akio Ogura; Yoshitaka Okada
Materials Research Society Symposium Proceedings, Materials Research Society, 1602, January, 1-7, 2014, Peer-reviwed, We report here the fabrication and characterization of GaAs tunnel diode (TD) and ErAs nanoparticles (Nps) enhanced GaAs TD. Four GaAs TDs with different contact area were fabricated by using MOCVD. We found extremely high peak current density of ∼250A/cm<
sup>
2<
/sup>
for the TD with r=0.25mm contact area. Moreover a hysteresis loop was appeared during sweeping up and sweeping down the external voltage. A 'vector load line model' was proposed to explain the origin of the shape of the hysteresis loop and the onset of the bistability occurred at the intersect of the loadline and the current-voltage (I-V) curve of TD. Meanwhile, we have grown ErAs Nps on GaAs(100) surface by using MBE and succeeded in overgrowth of GaAs after ErAs deposition. GaAs(p+)/ErAs(Nps)/GaAs(n+) TDs were fabricated and characterized. We found the GaAs sample containing 70s deposition of ErAs showed the best TD behavior. No TD behavior was observed for the sample without addition of ErAs Nps, clearly indicating the strong tunneling enhancement effect from ErAs Nps.
International conference proceedings, English - Photocapacitance study of MBE grown GaInNAsSb thin film solar cells
Muhammad Monirul Islam; Naoya Miyashita; Nazmul Ahsan; Takeaki Sakurai; Katsuhiro Akimoto; Yoshitaka Okada
JOURNAL OF CRYSTAL GROWTH, ELSEVIER SCIENCE BV, 378, 57-60, Sep. 2013, Peer-reviwed, GaInNAsSb based solar cell structure has been grown using atomic hydrogen assisted molecular beam epitaxy (H-MBE). Transient photocapacitance (TPC) spectroscopy was applied to investigate the optically active defects in the Si-doped GaInNAsSb layer (n-GaInNAsSb) of the structure. In addition to the interband transitions, a sub-band-gap optical transition was also determined using TPC spectroscopy. This sub-band-gap transition corresponds to an electron trap (OE1) at 0.78 eV below the conduction band (E-C) edge of n-GaInNAsSb material. Thermally active defects were probed by the electrical measurement using admittance spectroscopy. Thus, a defect profile in the entire band gap of the n-GaInNAsSb film was obtained. (c) 2013 Elsevier B.V. All rights reserved.
Scientific journal, English - Antimony mediated growth of high-density InAs quantum dots for photovoltaic cells
F. K. Tutu; J. Wu; P. Lam; M. Tang; N. Miyashita; Y. Okada; J. Wilson; R. Allison; H. Liu
APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 103, 4, 43901, Jul. 2013, Peer-reviwed, We report enhanced solar cell performance using high-density InAs quantum dots. The high-density quantum dot was grown by antimony mediated molecular beam epitaxy. In-plane quantum dot density over 1 x 10(11) cm(-2) was achieved by applying a few monolayers of antimony on the GaAs surface prior to quantum dot growth. The formation of defective large clusters was reduced by optimization of the growth temperature and InAs coverage. Comparing with a standard quantum dot solar cell without the incorporation of antimony, the high-density quantum dot solar cell demonstrates a distinct improvement in short-circuit current from 7.4mA/cm(2) to 8.3mA/cm(2). (C) 2013 AIP Publishing LLC.
Scientific journal, English - Effect of Sb on GaNAs Intermediate Band Solar Cells
Nazmul Ahsan; Naoya Miyashita; Muhammad Monirul Islam; Kin Man Yu; Wladek Walukiewicz; Yoshitaka Okada
IEEE JOURNAL OF PHOTOVOLTAICS, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 3, 2, 730-736, Apr. 2013, Peer-reviwed, We present a comparative study on the material properties and two-photon excitation (TPE) experiments that involve three bands between a GaNAs and a GaNAsSb absorber designed for intermediate band solar cells. The absorber layers were sandwiched between p-AlGaAs emitter layers and n-AlGaAs IB barrier layers. This permits production of above the bandgap electron-hole pairs by TPE involving two subband photons with the intermediate band as the stepping stone. A recovery in the carrier population in the intermediate band of the GaNAsSb absorber was realized due to an improved material quality. An enhancement in the photocurrent production due to TPE and an associated improvement in the open-circuit voltage were observed.
Scientific journal, English - Effect of antimony on uniform incorporation of nitrogen atoms in GaInNAs films for solar cell application
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Lead, SOLAR ENERGY MATERIALS AND SOLAR CELLS, ELSEVIER SCIENCE BV, 111, 127-132, Apr. 2013, Peer-reviwed, In this work, we have investigated the effect of antimony (Sb) on the uniform incorporation of nitrogen atoms in GaInNAs alloy grown with different levels of Sb fluxes. The photoluminescence (PL) intensity and full-width at half maximum (FWHM) are both improved for a narrow range of Sb flux between 1 x 10(-8) and 5 x 10(-8) Torr, in which Sb plays the role of surfactant. On the other hand, higher level of Sb flux deteriorates the PL characteristics most likely due to Sb-related defects. Furthermore in temperature dependent PL measurements, drastic peak energy shifts were observed in all samples, which indicate a strong carrier localization. Although GaInNAs sample showed a large energy shift of 53 meV, supply of Sb decreased the localization energies to 13-22 meV. These results show that optimized amount of Sb, maintaining a high growth temperature of 520 degrees C, enhances the homogeneity of potential energy in conduction band of GaInNAs alloy, since the carrier localization is led by inhomogeneous N incorporation. (C) 2013 Elsevier B.V. All rights reserved.
Scientific journal, English - InAs/GaAs quantum dot solar cell with an AlAs cap layer
F. K. Tutu; P. Lam; J. Wu; N. Miyashita; Y. Okada; K. -H. Lee; N. J. Ekins-Daukes; J. Wilson; H. Liu
APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 102, 16, 163907, Apr. 2013, Peer-reviwed, We report the effects of the deposition of an AlAs cap layer (CL) over InAs quantum dots (QDs) on the performance of QD solar cells (QDSCs). The growth of AlAs CL over InAs QDs led to the elimination of the wetting layer absorption and hence the enhancement of the open-current voltage, V-oc, of a 20-layer InAs/GaAs QDSC from 0.69 V to 0.79 V. Despite a slight reduction in short-circuit current, J(sc), for the QDSC with AlAs CL, the enhancement of the V-oc is enough to ensure that its efficiency is higher than the QDSC without AlAs CL. (C) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4803459]
Scientific journal, English - Defect study of molecular beam epitaxy grown undoped GaInNAsSb thin film using junction-capacitance spectroscopy
Muhammad Monirul Islam; Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Applied Physics Letters, 102, 7, 74104, 18 Feb. 2013, Peer-reviwed, Defects in undoped GaInNAsSb thin film (i-GaInNAsSb) were investigated by junction-capacitance technique using admittance and transient photocapacitance (TPC) spectroscopy. An electron trap D2 was identified at 0.34 eV below the conduction band (EC) of i-GaInNAsSb using admittance spectroscopy. Optical transition of valance band (EV) electrons to a localized state OH1 (EV + 0.75 eV) was manifested in negative TPC signal. Combined activation energy of OH1 and D2 defect corresponds to the band-gap of i-GaInNAsSb, suggesting that OH1/D2 acts as an efficient recombination center. TPC signal at ∼1.59 eV above EV was attributed to the nitrogen-induced localized state in GaInNAsSb. © 2013 American Institute of Physics.
Scientific journal, English - Composition control of quinary GaInNAsSb alloy grown by molecular beam epitaxy
Naoya Miyashita; Nazmul Ahsan; Muhammad Monirul Islam; Yoshitaka Okada
Lead, PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 10, NO 11, WILEY-V C H VERLAG GMBH, 10, 11, 1369-1372, 2013, Peer-reviwed, In order to precisely control the composition of quinary GaInNAsSb alloy, we investigated the incorporation behavior of constituent atoms during atomic hydrogen-assisted molecular beam epitaxial growth. The nitrogen (N) composition, in comparison of GaNAs and GaNAsSb, increased by the supply of antimony (Sb). However, addition of indium (In) decreases the N composition during Sb mediated growth of GaInNAsSb, which enables obtaining the same N composition when an adequate In composition is chosen. It was revealed that Sb incorporation was increased when (i) In composition decreased, (ii) Sb flux increased, (iii) growth temperature decreased, and (iv) growth rate increased. These results are thought to be related to the effect of competitive role among strain, coverage, desorption, and segregation. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
International conference proceedings, English - Effect of Sb on GaNAs Intermediate Band Solar Cells
Nazmul Ahsan; Naoya Miyashita; Muhammad M. Islam; Kin Man Yu; Wladek Walukiewicz; Yoshitaka Okada
2012 IEEE 38TH PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC), VOL 2, IEEE, 3, 730, 2013, Peer-reviwed, We present a comparative study on the material properties and two photon excitation (TPE) experiments involving three bands between a GaNAs and a GaNAsSb absorber designed for intermediate band solar cells. The absorber layers were sandwiched between p-AIGaAs emitter layers and n-AIGaAs IB barrier layers. This permits production of above the bandgap electron-hole pairs by TPE involving two subband photons with the intermediate band as the stepping stone. A recovery in the carrier population in the intermediate band of the GaNAsSb absorber was realized due to an improved material quality. An enhancement in the photocurrent production due to TPE, and an associated improvement in the open circuit voltage were observed.
International conference proceedings, English - Identification of defect types in moderately Si-doped GaInNAsSb layer in p-GaAs/n- GaInNAsSb/n-GaAs solar cell structure using admittance spectroscopy
Muhammad Monirul Islam; Naoya Miyashita; Nazmul Ahsan; Takeaki Sakurai; Katsuhiro Akimoto; Yoshitaka Okada
JOURNAL OF APPLIED PHYSICS, AMER INST PHYSICS, 112, 11, 114910, Dec. 2012, Peer-reviwed, Bias dependence of the admittance spectroscopy of GaInNAsSb based solar cell structure has been performed to identify and characterize the type of defects, for example interface and/or bulk type defects in a moderately Si doped GaInNAsSb (n-GaInNAsSb) layer in the structure. From the zero bias admittance spectrum, three peaks namely E1, E2, and E3 corresponding to the localized level at 0.03 eV, 0.07 eV, and 0.16 eV below the conduction band edge (E-C) of n-GaInNAsSb material, respectively, were found. Constant position of E2 and E3 peak in the admittance spectra in response to the various applied DC reverse bias suggests that E2 and E3 are related to the bulk type defects being spatially homogeneous throughout the bulk of the n-GaInNAsSb film. However, bias dependence admittance of the E1 peak along with the capacitance - voltage (C-V) measurement as well as characteristic feature in the temperature dependent junction capacitance value strongly suggests that E1 peak might be originated due to the free carrier relaxation in the n-GaInNAsSb layer in lower temperature. Conduction mechanism in the freeze-out regime has been discussed. Analysis of the admittance peak, E1 together with the characteristic features in the frequency dependence of the conduction in freeze out regime suggest that conduction properties of the n-GaInNAsSb material in the freeze-out condition is governed by Mott's variable range hopping mechanism. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768716]
Scientific journal, English - High electron mobility in Ga(In)NAs films grown by molecular beam epitaxy
Naoya Miyashita; Nazmul Ahsan; Makoto Inagaki; Muhammad Monirul Islam; Masafumi Yamaguchi; Yoshitaka Okada
Lead, APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 101, 22, 222112, Nov. 2012, Peer-reviwed, We report the highest mobility values above 2000 cm(2)/Vs in Si doped GaNAs film grown by molecular beam epitaxy. To understand the feature of the origin which limits the electron mobility in GaNAs, temperature dependences of mobility were measured for high mobility GaNAs and referential low mobility GaInNAs. Temperature dependent mobility for high mobility GaNAs is similar to the GaAs case, while that for low mobility GaInNAs shows large decrease in lower temperature region. The electron mobility of high quality GaNAs can be explained by intrinsic limiting factor of random alloy scattering and extrinsic factor of ionized impurity scattering. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768949]
Scientific journal, English - Extremely long carrier lifetime at intermediate states in wall-inserted type II quantum dot absorbers
Daisuke Sato; Junya Ota; Kazutaka Nishikawa; Yasuhiko Takeda; Naoya Miyashita; Yoshitaka Okada
JOURNAL OF APPLIED PHYSICS, AMER INST PHYSICS, 112, 9, 94305, Nov. 2012, Peer-reviwed, To realize highly efficient intermediate-band solar cells (IB-SCs), a long lifetime of photo-generated carriers in the IB is essential. We propose a new concept for this purpose based on IB absorbers using quantum-dots (QDs). By inserting potential walls between QDs and barriers that form a type II band alignment, electrons in the IB and holes in the valence band are farther separated compared to those in a conventional type II QD material, leading to significant reduction of radiative recombination. We designed a concrete structure using InAs QDs, GaAs1-xSbx barriers, and GaAs walls to find the suitable GaAs wall thickness and Sb content being 2 nm and x = 0.18, respectively, and demonstrated a lifetime of electrons excited to the IB as long as 220 ns. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764030]
Scientific journal, English - Two-photon excitation in an intermediate band solar cell structure
Nazmul Ahsan; Naoya Miyashita; Muhammad Monirul Islam; Kin Man Yu; Wladek Walukiewicz; Yoshitaka Okada
APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 100, 17, 172111, Apr. 2012, Peer-reviwed, We present evidence for the production of photocurrent due to two-photon excitation in an intermediate band solar cell structure. The structure consists of an n-GaNAs intermediate band layer sandwiched between a p-AlGaAs emitter and an n-AlGaAs barrier layer with suitable doping level to block electron escaping from the intermediate band to the bottom n-GaAs substrate. Multi-band transitions observed in two-photon excitation experiments are explained using photo-modulated reflectance spectrum, and further support for intermediate band solar cell operation of this structure is given by current-voltage measurements. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709405]
Scientific journal, English - Extremely long carrier lifetime over 200 ns in GaAs wall-inserted type II InAs quantum dots
Kazutaka Nishikawa; Yasuhiko Takeda; Tomoyoshi Motohiro; Daisuke Sato; Junya Ota; Naoya Miyashita; Yoshitaka Okada
APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 100, 11, 113105, Mar. 2012, Peer-reviwed, To realize highly efficient intermediate-band (IB) solar cells, long lifetime of photo-generated carriers in the TB is essential. For this purpose, we propose a concept for IB absorbers using GaAs wall-inserted type II InAs quantum-dots (QDs), in which electrons at the IB of the InAs QDs and holes in the valence band of the GaAsSb layers are farther separated compared to those in conventional type II QDs. We fabricated InAs/GaAs/GaAs0.82Sb0.18 type II QDs and performed time-resolved photoluminescence spectroscopy. The obtained lifetime was as long as 220 us for electrons at the IB. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3694284]
Scientific journal, English - Over 100 ns intrinsic radiative recombination lifetime in type II InAs/GaAs1-xSbx quantum dots
Kazutaka Nishikawa; Yasuhiko Takeda; Ken-ichi Yamanaka; Tomoyoshi Motohiro; Daisuke Sato; Junya Ota; Naoya Miyashita; Yoshitaka Okada
JOURNAL OF APPLIED PHYSICS, AMER INST PHYSICS, 111, 4, 44325, Feb. 2012, Peer-reviwed, We report very long intrinsic radiative recombination lifetime tau(rad) in type II InAs quantum dots embedded in GaAs1-xSbx. The dependence of photoluminescence (PL) decay time tau(PL) on both the Sb composition (x = 0-0.18) and excitation intensity (38-460 mW/cm(2)) was systematically investigated by time-resolved PL measurements with a time-correlated single-photon counting method. All PL decay curves exhibited non-exponential profiles, and tau(PL) was strongly dependent on the excitation intensity. These properties were well explained by solving rate equations of carrier density with neglecting nonradiative process, in which tau(rad) is inversely proportional to carrier density. The 18% Sb sample exhibited a tau(PL) of over 100 ns under weak excitation, which is longer than twice the previously reported values. We evaluated the value of tau(rad) in InAs/GaAs1-xSbx QDs relative to that in type I InAs/GaAs QDs based on an effective mass approximation and found that the observed extremely long tau(PL) corresponds to tau(rad). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3688864]
Scientific journal, English - Transmission electron microscopy study of GaInNAs(Sb) thin films grown by atomic hydrogen-assisted molecular beam epitaxy
R. Oshima; J. Y. Huang; N. Miyashita; K. Matsubara; Y. Okada; F. A. Ponce
APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 99, 19, 191907, Nov. 2011, Peer-reviwed, The quaternary GaInNAs is a promising material system for use in next generation multijunction photovoltaic devices. We have investigated the effect of introducing antimony on the growth by using transmission electron microscopy and energy dispersive x-ray (EDX) spectroscopy. Two-dimensional growth was observed in GaInNAs films with striation features associated with compositional fluctuation and nanometer scale elemental segregation on the growth front. On the contrary, GaInNAsSb films exhibit uniform contrast throughout. EDX profile indicates uniform compositional distribution, as antimony atoms suppress the surface mobilites of adatoms resulting in a lower probability to generate the favored bonds, such as Ga-N and In-As. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3660232]
Scientific journal, English - Improvement of GaInNAsSb films fabricated by atomic hydrogen-assisted molecular beam epitaxy
Naoya Miyashita; Shuhei Ichikawa; Yoshitaka Okada
Lead, JOURNAL OF CRYSTAL GROWTH, ELSEVIER SCIENCE BV, 311, 12, 3249-3251, Jun. 2009, Peer-reviwed, The effect of incorporation of antimony in GaInNAs films grown by atomic hydrogen-assisted molecular beam epitaxy (MBE) has been investigated. We show that the rate of incorporation of N and In forming GaInNAs do not depend on the Sb beam flux. However, the incorporation of Sb is strongly dependent on the Sb/As(2) flux ratio. Introducing a small amount of Sb (< similar to 1%) significantly improves the photoluminescence (PL) emission efficiency of GaInNAs, but Sb concentration of > 1% rapidly degrades the PL intensity, though a large redshift can still be achieved. Therefore, there is an optimum amount of Sb for the growth of low-strained GaInNAs films to improve the overall optical quality. (C) 2009 Elsevier B.V. All rights reserved.
Scientific journal, English - Carrier mobility characteristics in GaInNAs dilute nitride films grown by atomic hydrogen-assisted molecular beam epitaxy
Naoya Miyashita; Yukiko Shimizu; Yoshitaka Okada
Lead, JOURNAL OF APPLIED PHYSICS, AMER INST PHYSICS, 102, 4, 44904, Aug. 2007, Peer-reviwed, We have investigated the electrical properties of GaInNAs dilute nitride films grown by atomic hydrogen-assisted molecular beam epitaxy. We found that although the hole mobilities in Be-doped p-GaInNAs films exhibit a temperature dependence nearly identical to that for the homoepitaxial p-GaAs films, the electron mobilities in Si-doped n-GaInNAs films are strongly affected by the introduction of nitrogen into Ga(In)As. Further, the degree of scattering by the ionized impurity-like centers generated by N atoms decreased with increasing Si doping, while neutral impurity-like scattering became more dominant with increasing Si doping. These results suggest that the decrease of electron mobility and carrier concentration in Si-doped n-GaInNAs films is strongly correlated with the presence of N and Si atoms. (C) 2007 American Institute of Physics.
Scientific journal, English - Effect of growth temperature on the properties of Ga(In)NAs thin films by atomic hydrogen-assisted RF-MBE
Yukiko Shimizu; Naoya Miyashita; Yusuke Mura; Akira Uedono; Yoshitaka Okada
JOURNAL OF CRYSTAL GROWTH, ELSEVIER SCIENCE BV, 301, 579-582, Apr. 2007, Peer-reviwed, The effect of growth temperature on the crystal quality of Ga1-yInyNxAs1-x thin films by atomic hydrogen-assisted molecular beam epitaxy (H-MBE), and the characteristics of GaAs/Ga1-yInyNxAs1-x heterojunction solar cells were investigated. A high growth temperature of similar to 520 degrees C resulted in both an improved crystal quality and solar cell performance. The PL peak intensity measured for GaN0.003As0.997 film was three times higher than that for the sample grown at a more conventional growth temperature of 480 degrees C. Furthermore, the highest electron mobility of 250cm(2)/V s was obtained for the sample grown at 520 degrees C. An unoptimized p-GaAs/ i-n-GaInNAs heterojunction solar cell grown by H-MBE showed a maximum quantum efficiency of 50% and good diode factor of 1.4, respectively. (c) 2006 Elsevier B.V. All rights reserved.
Scientific journal, English - Optimization of Ga(In)NAs thin film growth by atomic hydrogen-assisted molecular beam epitaxy
Shimizu Yukiko; Miyashita Naoya; Okada Yoshitaka
PROGRESS IN SEMICONDUCTOR MATERIALS V-NOVEL MATERIALS AND ELECTRONIC AND OPTOELECTRONIC APPLICATIONS, 891, 509-+, 2006, Peer-reviwed
Scientific journal, English
MISC
- Light trapping and absorption enhancement in thin-film quantum dot solar cells
Kitahara Kento; Oteki Yusuke; Miyashita Naoya; Okada Yoshitaka
The Japan Photovoltaic Society, 24 Jun. 2022, Proceedings of the Annual Meeting of the Japan Photovoltaic Society, 2, 29-29, Japanese, 2436-6498 - Optimizing the positions of quantum dot layers to increase the light absorption in quantum dot solar cells with light trapping structure
Yusuke Oteki; Maxime Giteau; Kei Fukushima; Kento Kitahara; Naoya Miyashita; Ryo Tamaki; Yoshitaka Okada
We have investigated the effect of light trapping in quantum dot intermediate band solar cells, including the Fabry-Perot and guided diffracted mode. Using numerical simulations, we show that optimizing the positions of quantum dot layers can lead optical absorption enhancement. Further, we experimentally demonstrate that placing the InAs/GaAs quantum dot layers at the intensity peaks of the resonance results in an absorption enhancement. This effect can be used to improve device performance of solar cells as well as, photodetectors., IEEE, 2022, 6TH IEEE ELECTRON DEVICES TECHNOLOGY AND MANUFACTURING CONFERENCE (EDTM 2022), 44-46, English, WOS:000852566800013 - Optimizing the quantum dot layer position for increasing absorption of intermediate band solar cells
Oteki Y.; Giteau M.; Fukushima K.; Kitahara K.; Miyashita N.; Tamaki R.; Okada Y.
The Japan Photovoltaic Society, 12 Oct. 2021, Proceedings of the Annual Meeting of the Japan Photovoltaic Society, 1, 116-116, Japanese, 2436-6498 - Epitaxial Lifted-Off Thin Film GaInP/GaAs/GaInNAsSb Lattice-Matched Triple Junction Solar Cells
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada; Rao Tatavarti; Andree Wibowo; Noren Pan
2019, Proc. The 46th IEEE Photovoltaic Specialists Conference, English, Introduction international proceedings - High-Effciency InAs-InGaAs Quantum Dash Solar Cells Developed Through Current Constraint Engineering
Yoshitaka Okada; Naoya Miyashita; Yusuke Oteki; Yasushi Shoji
In principle, quantum-dot intermediate band solar cell (QD-IBSC) operates at a higher current density than a III-V multijunction solar cell (MJSC). Due to this inherent property, heat management becomes important when it is operated under a high concentrated illumination. In this work, we propose one way to circumvent this issue, where a wide bandgap cell is placed on top of QD-IBSC, which acts as a current constraint cell, but instead leads to a higher V-OC. We demonstrate a 32.1% efficiency under 225 suns with a bonded InGaP/ GaAs widegap cell // InAs-InGaAs QDSC configuration., IEEE, 2019, Proc. The 46th IEEE Photovoltaic Specialists Conference, 765-767, English, Introduction international proceedings, 0160-8371, WOS:000542034900157 - Thinned Germanium Substrates for III-V Multijunction Solar Cells
Iv?n Lombardero; Naoya Miyashita; Mario Ochoa; Yoshitaka Okada; Carlos Alogra
2019, Proc. The 46th IEEE Photovoltaic Specialists Conference, English, Introduction international proceedings - Growth of InGaAs: N delta-doped superlattices for multi-junction solar cells
Umeda, Shumpei; Yagi, Shuhei; Miyashita, Naoya; Okada, Yoshitaka; Yaguchi, Hiroyuki
InGaAs:N delta-doped superlattices (SLs) with a band gap of 1 eV were fabricated by molecular beam epitaxy as a candidate for the subcell material of 4-junction lattice-matched tandem solar cells. The N delta-doping was carried out by suppling N source during the growth interruption and the SL structures were obtained by alternate formation of a N delta-doped layer and an In0.06Ga0.94As spacer layer. It was revealed that excess N supply induces interstitial N incorporation and surface roughness. By adjusting the growth conditions and SL structural parameters, coherent growth of SLs with the band gap of 1 eV was successfully achieved., IEEE, 2018, 2018 IEEE 7TH WORLD CONFERENCE ON PHOTOVOLTAIC ENERGY CONVERSION (WCPEC) (A JOINT CONFERENCE OF 45TH IEEE PVSC, 28TH PVSEC & 34TH EU PVSEC), 1861-1864, English, Introduction international proceedings, 2159-2330, 2159-2349, WOS:000469200401194 - Hot Carrier Extraction Using Energy Selective Contacts and Feedback on the Remaining Distribution
Zacharie Jehl; Daniel Suchet; Naoya Miyashita; Benoit Behagel; Maxime Giteau; Amaury Delamarre; Bernice Mae; F. Yu Jeco; Jean-Francois Guillemoles; Yoshitaka Okada
2018, Proc. The 7th World Conference on Photovoltaic Energy Conversion, pp.1814-1816, English, Introduction international proceedings - Carrier Collection Improvement In InGaAs/GaAsN Multiple Quantum Well Solar Cell With Flat Conduction Band
Warakorn Yanwachirakul; Naoya Miyashita; Hassanet Sodabanlu; Kentaroh Watanabe; Masakazu Sugiyama; Yoshitaka Okada; Yoshiaki Nakano
© 2018 IEEE. InGaAs/GaAsN multiple quantum well (MQW) with flat conduction band (FCB) was introduced to improve electron transport in a dilute N absorber. This novel structure is expected for the middle cell in a three-junction solar cell on Ge. The I-V characteristic and carrier collection efficiency (CCE) of the cells were measured under AM 1.5G illumination with 665-nm long-pass filter corresponding to the absorption of InGaP top cell. The results showed that CCE of the FCB-MQW cell is better than that of the InGaAsN bulk cell with a similar short-circuit current. By increasing the MQW period to 60, the light absorption was enhanced, and the photocurrent exceeded a current-matching value for the three-junction cell on Ge., 2018, Proc. The 7th World Conference on Photovoltaic Energy Conversion, pp.1874-1877, English, Introduction international proceedings, 85059906214 - Analysis And Control Of Deep-Level Defects In Dilute Nitride Semiconductor GaInNAsSb
Yilun He; Naoya Miyashita; Yoshitaka Okada
2018, Proc. The 7th World Conference on Photovoltaic Energy Conversion, pp.896-900, English, Introduction international proceedings - 基板再利用に向けたELO後基板の清浄化工程と再成長の検討
宮下直也; 八木修平; 渡辺健太郎; 木村大希; SODABANLU Hassanet; 中田達也; 杉山正和; 杉山正和; 岡田至崇
Aug. 2017, 応用物理学会秋季学術講演会講演予稿集(CD-ROM), 78th, ROMBUNNO.6p‐PA5‐23, Japanese, 201702288830548138 - 1eV帯InGaAs:Nδドープ超格子の作製
梅田峻平; 八木修平; 宮下直也; 岡田至崇; 矢口裕之
01 Mar. 2017, 応用物理学会春季学術講演会講演予稿集(CD-ROM), 64th, ROMBUNNO.14p‐B6‐9, Japanese, 201702242750399704 - エピタキシャルリフトオフにより分離したGaAs基板上の堆積物分析
中田達也; 渡辺健太郎; SODABANLU H; 木村大希; 宮下直也; 杉山正和; 岡田至崇; 岡田至崇; 中野義昭; 中野義昭
01 Mar. 2017, 応用物理学会春季学術講演会講演予稿集(CD-ROM), 64th, ROMBUNNO.14p‐B6‐13, Japanese, 201702230203994233 - Multiband Formation in Cr doped CuGaS2 Thin Films Synthesized by Chemical Spray Pyrolysis
Nazmul Ahsan; Sivaperuman Kalainathan; Naoya Miyashita; Takuya Hoshii; Yoshitaka Okada
2017, Proc. 44th IEEE Photovoltaics Specialists Conference, pp.2334-2337, English, Introduction international proceedings - Analysis of Deposited Residues and Its Cleaning Process on GaAs Substrate after Epitaxial Lift-Off
Tatsuya Nakata; Kentaroh Watanabe; Hassanet Sodabanlu; Daiki Kimura; Naoya Miyashita; Yoshitaka Okada; Yoshiaki Nakano; Masakazu Sugiyama
The selective wet etching of the AlAs release layer with a HF solution was investigated for epitaxial lift-off (ELO) process. By measuring the lateral etch depth, the etch rate was estimated. A stress by the deposited Au electrode on the thin device layer enhanced the etch rate.For substrate reuse, the exposed substrate surface after the etching was observed by scanning electron microscopy (SEM). Two kinds of morphologic depositions were typically observed in this study. Streaky aligned depositions were found just after the ELO process. During long-term storage of the substrate, some parts of the depositions were gradually re-formed into micrometer-size crystals. These micro-crystals were revealed to be As203 upon the compositional characterization by energy dispersive X-ray spectrometry, and we found the annealing process removes the micro-crystals., IEEE, 2017, Proc. 44th IEEE Photovoltaics Specialists Conference, pp.854-857, English, Introduction international proceedings, 0160-8371, WOS:000455636000208 - Laser Beam Induced Current (LBIC) Mapping of InGaP/GaAs/Ge Triple Junction Solar Cells with Luminescence Coupling
Bernice Mae Yu Jeco; Tomah Sogabe; Akio Ogura; Naoya Miyashita; Ryo Tamaki; Yoshitaka Okada
In this study, we introduced the laser beam induced current (LBIC) mapping method of InGaP/GaAs/Ge triple junction solar cells considering the luminescence coupling effect. Results have shown qualitative agreement between the acquired simulation and measurement data. Moreover, the current generated in the limiting subcell has been observed to be a mixture of luminescence coupling and measurement artifacts in excess of its dark current. Using the LBIC mapping method can provide spatial luminescence coupling current distribution in the subcell, which can serve as a guide on how to design multijunction solar cell that optimizes this effect., IEEE, 2016, 2016 IEEE 43RD PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC), 1229-1234, English, Introduction international proceedings, 0160-8371, WOS:000399818701051 - Designing III-V dilute nitride alloys for IBSC application
Nazmul Ahsan; Naoya Miyashita; Kin Man Yu; Wladek Walukiewicz; Yoshitaka Okada
The location of multiple bands in a dilute nitride GaNAs intermediate band solar cell (IBSC) can be tuned by the level of N amount in the GaAs host. It is observed that the open circuit voltage improves with increasing level of the N amount in our MBE grown IBSCs in which GaNAs layers are sandwiched between AlGaAs layers. This is ascribed to the improved blockage of the carrier leakage of the IB electrons over the AlGaAs backside IB barrier due to increased separation between the IB and conduction bands., IEEE, 2016, 2016 IEEE 43RD PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC), 2761-2764, English, Introduction international proceedings, 0160-8371, WOS:000399818702180 - Siドープ量子ドット太陽電池の集光特性評価
内藤駿弥; 宮下直也; 下村北斗; 下村北斗; 玉置亮; 星井拓也; 岡田至崇
26 Feb. 2015, 応用物理学会春季学術講演会講演予稿集(CD-ROM), 62nd, ROMBUNNO.13A-A26-5, Japanese, 201502288802603447 - Characterization of 1.0 eV GaInNAsSb solar cells for multi-junction applications and the effect of annealing
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
2015, Proc. 25nd European Photovoltaic Solar Energy Conference, pp.1461-1465, English, Introduction international proceedings - Effect of light concentration on Si doped quantum dot solar cells
Shunya Naitoh; Naoya Miyashita; Kasidit Toprasertpong; Ryo Tamaki; Takuya Hoshii; Masakazu Sugiyama; Yoshitaka Okada
2015, Proc. 25nd European Photovoltaic Solar Energy Conference, pp.245-248, English, Introduction international proceedings - Concentrating Photovoltaic Properties of GaInNAsSb/Ge Dual Junction Tandem Solar Cell
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
In this work, the GaInNAsSb/Ge 2-junction (2J) tandem solar cell was fabricated for application to the lattice-matched 4J GaInP/GaAs/GaInNAsSb/Ge structure. We focused on the concentrating photovoltaic properties of GaInNAsSb based solar cells. In the present 2J cell, output current is limited by the Ge subcell. Voc was the sum of each subcell. Under concentrated light, it was found that a Voc increment with concentrated ratio for GaInNAsSb cell is higher than those for the conventional solar cells, such as GaAs and Ge. For 4J tandem solar cell under 1000 suns concentration, additional similar to 0.7 V of Voc can be expected by addition of GaInNAsSb subcell to the conventional GaInP/GaAs/Ge structure., IEEE, 2014, 2014 IEEE 40TH PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), 520-523, English, Introduction international proceedings, WOS:000366638900119 - Local conductivity characteristics of individual ErAs island for solar cell tunnel junction application
Chao-Yu Hung; Tomah Sogabe; Naoya Miyashita; Yasushi Shoji; Shunya Naitoh; Yoshitaka Okada
Conductive atomic force microscopy has been used to study both the topography and electronic properties of 30nm scale ErAs islands grown by molecular beam epitaxy (MBE) on n-type GaAs substrate. A calibration method was realized to determine the growth rate of the ErAs so that we can control the size of the ErAs islands. Current flow images and surface profile around ErAs islands were simultaneously measured for local conductive distribution corresponding to real space profile, where the correlations are revealed. The conductance on ErAs islands is found to be larger than that on the GaAs buffer layer from I-V characteristics via voltage sweeping from negative to positive voltage. The I-V curve and resistance of the tunnel junction with ErAs were evaluated. It exhibits that less voltage loss can be achieved even higher than 1000 suns., IEEE, 2014, 2014 IEEE 40TH PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC), 228-232, English, Introduction international proceedings, WOS:000366638900052 - Device Structure Engineering of GaInNAsSb/GaAs Heterojunction Solar Cells
Naoya Miyashita; Muhammad Monirul Islam; Nazmul Ahsan; Yoshitaka Okada
The device structures of GaInNAsSb/GaAs based heterojunction solar cells were studied. We fabricated a single-heterostructure (SH) and two double-heterostructures (DH) to investigate the carrier collection at the GaInNAsSb/GaAs hetero interface. The quantum efficiency at long wavelength region (lambda > 870 nm) which is contributed from the GaInNAsSb layer shows higher values for DH structures than SH sample. On the other hand, the open circuit voltage of SH sample is higher than those of DH samples. This can be ascribed to an increased recombination current at the potential barrier (i/n interface) existing in DH structures., IEEE, 2013, 2013 IEEE 39TH PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC), 2109-2112, English, Introduction international proceedings, 0160-8371, WOS:000340054100474 - Study on The Device Structure of GaInNAs(Sb) Based Solar Cells for Use in 4-junction Tandem Solar Cells
Naoya Miyashita; Nazmul Ahsan; Muhammad Monirul Islam; Yoshitaka Okada
We have evaluated n-GaAs/i-GaInNAsSb/p-GaAs (nip) and p-GaAs/i-GaInNAsSb/n-GaAs (pin) solar cells grown by RF-molecular beam epitaxy. The absorption edges of these samples were at around 1180 nm. The currents generated in GaInNAs(Sb) layers, which were calculated from AM 1.5 spectrum and QE spectra at lambda > 870 nm region, were 4.6 (nip) and 5.1 (pin) mA/cm(2), respectively. Illuminated current voltage curves show that the open circuit voltages (V-oc) and FF values were 0.42 V and 0.71, respectively, for both structures. From these measurements, since almost the same PV properties are achieved for both samples, we found no distinct disadvantage to adopt nip structure substituted for pin GaInNAsSb solar cell, although there seems to be some difficulties in carrier collection near by the deep i-GaInNAsSb/p-GaAs interface in nip sample. We observed an enhancement in the filtered current density when GaInNAsSb absorber thickness is increased in nip structure without any degradation of other photovoltaic parameters. In order to further improve filtered current density, increase of GaInNAs(Sb) absorber thickness is necessary., IEEE, 2012, 2012 38TH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC), 954-956, English, Introduction international proceedings, WOS:000309917801054 - Bias dependent admittance measurement of GaInNAsSb-based solar cell structure
Muhammad Monirul Islam; Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Bias dependence of the admittance spectroscopy of GalnNAs(Sb) based solar cell structure has been performed to identify and characterize the type of defects, for example interface and/or bulk defects. Three defect levels El, E2, and E3 were found at 0.03 eV, 0.07 eV, and 0.16 eV below the conduction band edge, respectively. Bias dependent admittance measurement suggests that E1 peak arises from the free carrier relaxation and the defect level E2 and E3 are bulk type defects., IEEE, 2012, 2012 38TH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC), 2543-2547, English, Introduction international proceedings, WOS:000309917802188 - Evaluation of GaInNAs(Sb) solar cells for use in next generation III-V tandem solar cells
Naoya Miyashita; Nazmul Ahsan; M. Monirul Islam; Yoshitaka Okada
We have evaluated p-GaAs/i-n-GaInNAs(Sb) and p-GaAs/i-GaInNAs(Sb)/n-GaAs solar cells grown by RF-molecular beam epitaxy. The absorption edges of these samples were found within 1170-1200 nm. The currents generated in GaInNAs(Sb) layers, which were calculated from AM 1.5 spectrum and QE spectra at λ >
870 nm region, were 6.1-7.3 mA/cm 2, and the open circuit voltages (V oc) were 0.37-0.41 V. Next we investigated current generation maps by using 405 nm, 785 nm and 1064 nm lasers. The spatial current generation in the top GaAs layers showed uniform distribution which was scanned by 405 nm and 785 nm laser. However, using 1064 nm laser revealed spatially uneven current distribution. This means that carriers generated in GaInNAs(Sb) layers are not collected sufficiently. We believe that this nonuniform carrier collection has connection to the cell parameters, such as defects, dislocations, and/or device structures, and degrades V oc in GaInNAs(Sb) solar cells. © 2011 IEEE., 2011, Conference Record of the IEEE Photovoltaic Specialists Conference, 000526-000529, English, Introduction international proceedings, 0160-8371, 84861045652 - Effect of antimony on electron trap density in GaInNAsSb solar cell grown by atomic hydrogen-assisted molecular beam epitaxy
Naoya Miyashita; Haruhiro Oigawa; Yoshitaka Okada
2010, Proc. 25th European Photovoltaic Solar Energy Conference / 5th IEEE World Conference on Photovoltaic Energy Conversion, pp.946-949, English, Introduction international proceedings - ANTIMONY ENHANCED HOMOGENEOUS NITROGEN INCORPORATION INTO GaInNAs FILMS GROWN BY ATOMIC HYDROGEN-ASSISTED MOLECULAR BEAM EPITAXY
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
In this work, we investigated the effect of Sb on N-induced localized states in GaInNAs films. From 77 K photoluminescence (PL) spectra for GaInNAs(Sb) films grown with different Sb fluxes, single band-to-band emissions observed in all samples. The emission wavelength redshifts with increasing Sb flux, which corresponds to an increase in Sb composition. The PL intensity and full-width at half maximum (FWHM) are both improved for a narrow range of Sb flux of 1 - 5x10(-8) Torr. On the other, higher Sb flux deteriorates the PL characteristics. It can be thought that any higher Sb flux induces some kind of Sb-related defects such as Sb-Ga antisites. Further in temperature dependent PL measurements, energy shifts, so-called "S-shaped" curves, were observed in all samples, which indicate a strong carrier localization. Although GaInNAs sample showed a large energy shift of 53 meV, irradiation of Sb decreases the localization energies to 13 - 22 meV. These results show that Sb can enhance the homogeneity of GaInNAs alloy, since the carrier localization is led by inhomogeneous N incorporation. The internal quantum efficiency characteristics for GaInNAsSb solar cell also improved by introducing an optimum amount of Sb and a redshift of fundamental absorption edge into 1 eV range was achieved., IEEE, 2010, 35TH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE, 2084-2088, English, Introduction international proceedings, 0160-8371, WOS:000287579502073 - Effect of indium composition on GaInNAsSb solar cells grown by atomic hydrogen-assisted molecular beam epitaxy
Naoya Miyashita; Shuhei Ichikawa; Yoshitaka Okada
We focus to develop a technique to accurately control the compositions of GaInNAsSb films as a function of In molar fraction grown by atomic hydrogen-assisted RF molecular beam epitaxy (H-MBE). Secondary ion mass spectroscopy analysis shows that the N composition decreases monotonically with increasing In composition from 0 to 17%. The Sb composition also decreases with increasing In composition. The PL intensity of GaInNAsSb films increases by a factor of 2.3 - 2.5 for small In composition of 5 - 13% and the full-width at half-maximum (FWHM) is also improved. However, any higher In compositions degrade the PL intensity and FWHM is broadened. The internal quantum efficiency characteristics for GaInxNyAsSb solar cells (x = 0, 0.045, 0.1, and y = 0.015 similar to 0.016) are improved by the introduction of In, and redshift of the absorption edge by 60 similar to 80 nm is obtained. The filtered current density > 870 nm is increased by adding In to GaNAsSb from 4.9 (In = 0%) to 6.2 mA/cm(2) (In = 10%), respectively., IEEE, 2009, 2009 34TH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE, VOLS 1-3, 1126-+, English, Introduction international proceedings, 0160-8371, WOS:000280345900245 - Effect of Increasing Nitrogen Composition on the Performance of GaAs/GaInNAs Heterojunction Solar Cells
Naoya Miyashita; Yukiko Shimizu; Yoshitaka Okada
2007, Proc. 22nd European Photovoltaic Solar Energy Conference and Exhibition, pp.414-417, English, Introduction international proceedings - Fabrication of GaInNAs-based solar cells for application to multi-junction tandem solar cells
Naoya Miyashita; Yukiko Shimizu; Naoto Kobayashi; Yoshitaka Okada; Masafumi Yamaguchi
We have investigated the characteristics of p-GaAs/i-n-Ga0.97In0.03N0.01As0.99 heterojunction solar cells with different intrinsic layer thickness. The solar cells studied in this study were fabricated by atomic hydrogen-assisted RF-MBE on GaAs(001) substrates. With an optimized Player thickness of 600nm, maximum quantum efficiency of > 80% has been obtained, and the hole diffusion length in n-Ga0.97In0.03N0.01As0.99 film was similar to 160nm. Then we fabricated our first homojunction GaInNAs solar cells, and short-circuit current density of 14.2 mA/cm(2) has been achieved., IEEE, 2006, CONFERENCE RECORD OF THE 2006 IEEE 4TH WORLD CONFERENCE ON PHOTOVOLTAIC ENERGY CONVERSION, VOLS 1 AND 2, 869-872, English, Introduction international proceedings, WOS:000241251600222 - Optimization of growth of GaInNAs dilute nitrides for multi-junction solar cell applications
Yukiko Shimizu; Naoya Miyashita; Yusuke Mura; Akira Uedono; Yoshitaka Okada
We have investigated the effect of growth temperature on the final crystal quality of GaInNAs solar cells in atomic hydrogen-assisted molecular beam epitaxy (H-MBE). A higher growth temperature of similar to 500 degrees C has been found to be the optimum and resulted in both improved crystal quality and solar cell performance. The photoluminescence (PL) peak intensity was one order of magnitude stronger and the electron mobility of 250 cm(2)/Vs was 30 % higher for GaInNAs films grown at T-sub = 520 degrees C than at the more conventional growth temperature of 480 degrees C. We have also investigated the vacancy-type defects in Ga(In)NAs films and found that the defect concentrations were as low as a LEC-GaAs substrate. Our unoptimized p-GaAs/i-n-GaInNAs heterojunction solar cell showed a maximum quantum efficiency of similar to 50 % and a low diode factor of 1.4., IEEE, 2006, CONFERENCE RECORD OF THE 2006 IEEE 4TH WORLD CONFERENCE ON PHOTOVOLTAIC ENERGY CONVERSION, VOLS 1 AND 2, 811-814, English, Introduction international proceedings, WOS:000241251600207 - Fabrication of GaAs/GaInNAs Heterojunction Solar Cells Applicable To High-Efficiency Multi-junction Tandem Structures
KOBAYASHI Naoto; MIYASHITA Naoya; SHIMIZU Yukiko; OKADA Yoshitaka; YAMAGUCHI Masafumi
13 Sep. 2005, Extended abstracts of the ... Conference on Solid State Devices and Materials, 2005, 352-353, English, 10022541889, AA10777858
Books and other publications
Lectures, oral presentations, etc.
- ELO薄膜タンデム太陽電池の発光結合特性評価
伊坪壮太; 稲葉大陸; 宮下直也; 山口浩一
Poster presentation, 第85回応⽤物理学会秋季学術講演会
19 Sep. 2024
16 Sep. 2024- 20 Sep. 2024 - Spalling工程による薄膜タンデム太陽電池の作製と評価
宮下直也; 庄司靖; 菅谷武芳; 曽我部東馬; 山口浩一; 岡田至崇
Oral presentation, 第85回応用物理学会秋季学術講演会
17 Sep. 2024
16 Sep. 2024- 20 Sep. 2024 - 2端子型ELO薄膜タンデム太陽電池のサブセルI-V解析
宮下直也; 稲葉大陸; 伊坪壮太; 曽我部東馬; 岡田至崇; 山口浩一
Poster presentation, 第21回「次世代の太陽光発電システム」シンポジウム（第4回⽇本太陽光発電学会学術講演会）
11 Jul. 2024
11 Jul. 2024- 12 Jul. 2024 - バイアス光照射による薄膜タンデム太陽電池の各サブセルのI-V測定
稲葉 大陸; 伊坪 壮太; 宮下 直也; 曽我部 東馬; 岡田 至崇; 山口 浩一
Poster presentation, 第71回応用物理学会春季学術講演会
24 Mar. 2024 - InAs/InAsSb面内超高密度量子ドット層の光伝導特性
大山 琢未; 荒尾 光哉; 宮下 直也; 山口 浩一
Poster presentation, 第71回応用物理学会春季学術講演会
24 Mar. 2024 - Si(111)基板上への高密度InAsSb/InAsコアシェルナノワイヤーの成長
渡部 陸太; 中川 竜輔; 宮下 直也; 山口 浩一
Poster presentation, 第71回応用物理学会春季学術講演会
24 Mar. 2024 - Dilute nitride based MQWs with strain compensation for 1.0 eV subcells
Naoya Miyashita; Yusuke Oteki; Nazmul Ahsan; Tomah Sogabe; Koichi Yamaguchi; Yoshitaka Okada
Oral presentation, English, The 33rd International Photovoltaic Science and Engineering Conference (PVSEC-33)
Nov. 2022 - Optimizing the positions of quantum dot layers to increase the light absorption in quantum dot solar cells with light trapping structure
Yusuke Oteki; Maxime Giteau; Kei Fukushima; Kento Kitahara; Naoya Miyashita; Ryo Tamaki; Yoshitaka Okada
6th IEEE Electron Devices Technology and Manufacturing Conference, EDTM 2022, We have investigated the effect of light trapping in quantum dot intermediate band solar cells, including the Fabry-Perot and guided diffracted mode. Using numerical simulations, we show that optimizing the positions of quantum dot layers can lead optical absorption enhancement. Further, we experimentally demonstrate that placing the InAs/GaAs quantum dot layers at the intensity peaks of the resonance results in an absorption enhancement. This effect can be used to improve device performance of solar cells as well as, photodetectors.
2022
2022 2022 - Enhancement of the light absorption rate of quantum dot solar cells by Fabry-Pérot resonance
Oteki Yusuke; Giteau Maxime; Fukushima Kei; Kei Kitahara; Naoya Miyashita; Ryo Tamaki; Yoshitaka Okada
Japanese, JSAP Annual Meetings Extended Abstracts, The Japan Society of Applied Physics
26 Feb. 2021
26 Feb. 2021- 26 Feb. 2021 - エピタキシャルリフトオフ法を用いた薄膜太陽電池の作製
宮下 直也; 岡田 至崇
Invited oral presentation, 第81回応用物理学会秋季学術講演会, Invited
Sep. 2020 - エピタキシャルリフトオフ法を用いたエピ薄膜分離と基板再生技術開発
宮下 直也; 岡田 至崇
Invited oral presentation, 第67回応用物理学会春季学術講演会, Invited
Mar. 2020 - Extending carrier lifetime by controlling the composition of GaAsxSb1-x/GaAs quantum dots
Oteki Yusuke; Shoji Yasushi; Miyashita Naoya; Okada Yoshitaka
Japanese, JSAP Annual Meetings Extended Abstracts, The Japan Society of Applied Physics
28 Feb. 2020
28 Feb. 2020- 28 Feb. 2020 - Growth rate dependency of GaAsSb / GaAs Quantum Dots
Oteki Yusuke; Shoji Yasushi; Miyashita Naoya; Okada Yoshitaka
Japanese, JSAP Annual Meetings Extended Abstracts, The Japan Society of Applied Physics
04 Sep. 2019
04 Sep. 2019- 04 Sep. 2019 - Strain Compensated Dilute Nitride MQWs As New 1 eV Solar Cell Absorber
Naoya Miyashita; Yoshitaka Okada
Oral presentation, English, SemiconNano 2019, Kobe, International conference
Sep. 2019 - Study of GaInP/GaAs/GaInNAsSb 3J solar cells: Effect of hydrogen incorporation to GaInNAsSb in MOCVD/MBE hybrid growth
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Oral presentation, English, Asia Pacific Society for Materials Research (APSMR) Annual Meeting 2019, Invited, Sapporo, International conference
Jul. 2019 - Epitaxial Lifted-Off Thin Film GaInP/GaAs/GaInNAsSb Lattice-Matched Triple Junction Solar Cells
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada; Rao Tatavarti; Andree Wibowo; Noren Pan
Oral presentation, English, The 46th IEEE Photovoltaic Specialists Conference (PVSC-46), Chicago, International conference
Jun. 2019 - Effect of Annealing on The Bottom Cell in GaInP/GaAs/GaInNAsSb Triple Junction Solar Cells by MBE/MOCVD Hybrid Growth
Naoya Miyashita; Yilun He; Nazmul Ahsan; Yoshitaka Okada
2019 Compound Semiconductor Week (CSW), IEEE, http://orcid.org/0000-0003-1311-1589
May 2019
May 2019 May 2019 - GaSb/GaAs quantum nanostructure solar cells under high sunlight concentration
Oteki Yusuke; Shoji Yasushi; Miyashita Naoya; He Yilun; Okada Yoshitaka
Japanese, JSAP Annual Meetings Extended Abstracts, The Japan Society of Applied Physics
25 Feb. 2019
25 Feb. 2019- 25 Feb. 2019 - GaSb/GaAs quantum nanostructures for intermediate band solar cell under high sunlight concentration
Yusuke Oteki; Yasushi Shoji; Naoya Miyashita; Yilun He; Yoshitaka Okada
English, 2019 COMPOUND SEMICONDUCTOR WEEK (CSW), IEEE
2019
2019 2019 - Epitaxial Lift-Off of Ultrathin Heterostructures for Hot-Carrier Solar Cell Applications
Maxime Giteau; Kentaroh Watanabe; Hassanet Sodabanlu; Naoya Miyashita; Masakazu Sugiyama; Andrea Cattoni; Stephane Collin; Jean-Francois Guillemoles; Yoshitaka Okada
English, 2019 COMPOUND SEMICONDUCTOR WEEK (CSW), IEEE
2019
2019 2019 - ELO-released thin film solar cells and application to multi-junction devices
Naoya Miyashita; Yilun He; Nazmul Ahsan; Beno?t Behaghel; Jean-Fran?ois Guillemoles; Yoshitaka Okada
Oral presentation, English, 7th LIA NextPV International Workshop, Bordeaux, International conference
Dec. 2018 - Inverted growth of lattice-matched multijunction solar cells with 1.0 eV GaInNAsSb subcell
Naoya Miayshita; Yilun He; Takaaki Agui; Hiroyuki Juso; Tatsuya Takamoto; Yoshitaka Okada
Oral presentation, English, The 7th World Conference on Photovoltaic Energy Conversion (WCPEC-7), Waikoloa, International conference
Jun. 2018 - Inverted and non-inverted grown lattice-matched multijunction solar cells with 1.0eV GaInNAsSb subcell
Naoya Miyashita; Yoshitaka Okada
Oral presentation, English, 6th LIA NextPV International Workshop, Tokyo, International conference
Dec. 2017 - Characterization of Inverted Grown Lattice-matching Multijunction Solar Cells with 1.0eV Dilute Nitride Subcell
Naoya Miyashita; Takaaki Agui; Hiroyuki Juso; Tatsuya Takamoto; Yoshitaka Okada
Oral presentation, English, Photovoltaic Science and Engineering Conference (PVSEC-27), Shiga, International conference
Nov. 2017 - Development of dilute nitrides and multijunction solar cells
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Oral presentation, English, 2nd International Conference on Advances in Materials Science and Technology, Invited, Vellore, International conference
Oct. 2017 - Effect of Hydrogen Annealing for Dilute Nitride GaInNAsSb Solar Cells
Naoya Miyashita; Yilun He; Yoshitaka Okada
Oral presentation, English, Compound Semiconductor Week (CSW) 2017, Berlin, International conference
May 2017 - Enhancement of photocurrent in thin film dilute nitride cells separated by epitaxial lift-off technique
Naoya Miyashita; Yoshitaka Okada
Oral presentation, The 63rd JSAP spring meeting (English session symposium: "Photovoltaic 4.0"）, Yokohama, Domestic conference
Mar. 2017 - Characterization of Dilute Nitride Based Photovoltaics for Multi-junction Solar Cell Applications
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Oral presentation, English, International Conference on Modern Materials Research (ICMMR2016), Invited, Uthangarai, International conference
Dec. 2016 - Improvement of Dilute Nitride GaInNAsSb Thin Films and Photovoltaic Characteristics
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Oral presentation, English, International Conference on Materials Processing and Applications (ICMPA2016), Invited, Vellore, International conference
Dec. 2016 - Enhancement of Photocurrent in Epitaxial Lift-Off Thin Film GaInNAsSb Solar Cells By The Light Confinement Structure
Naoya Miyashita; Beno?t Behaghel; Jean-Fran?ois Guillemoles; Yoshitaka Okada
Oral presentation, English, Photovoltaic Science and Engineering Conference (PVSEC-26), Singapore, International conference
Oct. 2016 - Improvement of 1.0 eV GaInNAsSb solar cell performance upon annealing
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Oral presentation, English, Compound Semiconductor Week (CSW) 2016, Toyama, International conference
Jun. 2016 - Enhancement of Carrier Collection in GaInNAs:Sb Solar Cells
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Oral presentation, English, EMN Prague Meeting, Invited, Prague, International conference
Jun. 2016 - Improved Carrier Collection in Thick GaInNAsSb Solar Cells
N. Miyashita; N. Ahsan; Y. Okada
Oral presentation, English, The 6th World Conference on Photovoltaic Energy Conversion (WCPEC-6), Kyoto, International conference
Nov. 2014 - 多接合太陽電池用GaInNAs(Sb)薄膜のMBE成長と高品質化
宮下直也; 岡田至崇
Oral presentation, Japanese, 第44回結晶成長国内会議, Invited, 東京, Domestic conference
Nov. 2014 - Effect of Electric Field in GaInNAsSb based Solar Cells
Naoya Miyashita; Nazmul Ahsan; Muhammad Monirul Islam; Yoshitaka Okada
Oral presentation, English, 2013 JSAP-MRS Joint Symposia, Kyoto, International conference
Sep. 2013 - Photovoltaic Characteristics of GaInNAsSb Solar Cells under Concentrated Light Source
N. Miyashita; K. Watanabe; Y. Okada
Oral presentation, English, 9th Conference on Concentrator Photovoltaics (CPV-9), Miyazaki, International conference
Apr. 2013 - Fabrication of nip and pin structure GaInNAsSb solar cells
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Oral presentation, English, 2012 Collaborative Conference on Crystal Growth, Invited, Orland, International conference
Dec. 2012 - High Electron Mobility in Ga(In)NAs Films Grown by MBE
Naoya Miyashita; Nazmul Ahsan; Makoto Inagaki; Muhammad Monirul Islam; Masafumi Yamaguchi; Yoshitaka Okada
Oral presentation, English, The 17th International Conference on Molecular Beam Epitaxy, Nara, International conference
Sep. 2012 - Antimony Surfactant Mediated Improvement of the Properties of GaInNAsSb Films for Multi-junction Tandem Solar Cells
Naoya Miyashita; Nazmul Ahsan; Kin Man Yu; Wladek Walukiewicz; Yoshitaka Okada
Oral presentation, English, 2011 MRS spring meeting, San Francisco, International conference
Apr. 2011 - Reduction of nitrogen-induced localized states in GaInNAs films grown by antimony-assisted MBE
Naoya Miyashita; Nazmul Ahsan; Yoshitaka Okada
Oral presentation, English, Renewable Energy 2010 International Conference and Exhibition, Yokohama, International conference
Jun. 2010 - Characteristics of GaInNAsSb Solar Cells Grown by Atomic Hydrogen-Assisted Molecular Beam Epitaxy
Naoya Miyashita; Shuhei Ichikawa; Yoshitaka Okada
Oral presentation, English, 18th International Photovoltaic Science and Engineering Conference, Kolkata, International conference
Jan. 2009
Courses
- Fundamental of Engineering Science
2024 - Present
The University of Electro-Communications - Fundamentals of VLSI Fabrication
2024 - Present
The University of Electro-Communications - Electronic Engineering Laboratory II ;XRD
2022 - Present
The University of Electro-Communications - Environmental Engineering
2022 - Present
The University of Electro-Communications - Introduction to Info-Power and Energy
2024
Tokyo University of Foreign Studies - Advanced Lectures on Optoelectronics
2020
Saitama University
Research Themes
- 移動体用太陽電池の研究開発（超高効率モジュール技術開発）
杉山正和; 岡田至崇; 宮下直也
NEDO 太陽光発電主力電源化推進技術開発／太陽光発電の新市場創造技術開発, Coinvestigator not use grants, 格子不整合系太陽電池の剥離技術開発
Jul. 2020 - Mar. 2025 - N.A.
宮下 直也
Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C), The University of Tokyo, Grant-in-Aid for Scientific Research (C), Principal investigator, 本研究はマルチバンド材料であるGaInNAsに着目し中間バンドを介した2段階励起レート向上と再緩和抑制に向け検討を進めている。GaInNAsはInとNの添加量を制御することによってGaAs基板上への格子整合が可能な材料として知られているが、N組成の増加に伴い導入される結晶欠陥の抑制が課題となる。 本年度は、GaAs基板上に成長可能な歪み補償型GaInNAs/GaNAs系多重量子井戸構造を用いた太陽電池を作製し、成膜条件、構造の検討を実施した。 高分解X線回折測定により良好な周期構造とヘテロ界面の形成を確認した。また、GaAs基板に対して格子整合を維持されており、設計通りの歪み補償効果が実現できていることを確認した。また、多重量子井戸層の成長温度の最適化を行い、発電特性の改善を得た。さらにポストアニールを実施し、周期構造、光吸収端エネルギー、発電特性への変化を調べ、最適条件の検討を進めた。また、多重量子井戸構造の改善に向け、ヘテロ界面へのGaAsスペーサー層の挿入を検討した。成膜時、およびポストアニールに伴うIn等の構成元素の相互拡散の影響について調べ、構造の最適化を進めた。以上の最適化検討を行い、標準条件下において多重量子井戸層の量子効率の向上を得た。また、活性層における2段階励起特性評価のため、評価システムの設計および白色光源の導入を行った。, 20K05682
Apr. 2020 - Mar. 2024 - 希釈窒化物半導体による近赤外域多重量子井戸構造の歪み制御技術開発
宮下直也
池谷科学技術振興財団, 単年度研究助成, 東京大学, Principal investigator, 0311063-A
Apr. 2019 - Mar. 2020 - 超高効率・低コストIII-V化合物太陽電池モジュールの研究開発（超高効率セルおよび低コスト化技術開発）
岡田至崇; 杉山正和; 宮下直也
NEDO 高性能・高信頼性太陽光発電の発電コスト低減技術開発／革新的新構造太陽電池の研究開発, Coinvestigator not use grants
Jul. 2015 - Mar. 2020 - 原子状水素を用いた希釈窒化物混晶への水素修飾とN-H結合の制御
宮下 直也
日本学術振興会, 科学研究費助成事業, Principal investigator
Apr. 2016 - Mar. 2018 - 高効率集光型太陽電池セル、モジュール及びシステムの開発（日EU共同開発）
山口真史; 岡田至崇; 宮下直也
NEDO 革新的太陽光発電技術研究開発（革新型太陽電池国際研究拠点設備事業）, Coinvestigator not use grants
2010 - 2014 - ポストシリコン超高効率太陽電池の研究開発
中野義昭; 宮野健次郎; 瀬川浩司; 岡田至崇; 宮下直也
NEDO 革新的太陽光発電技術研究開発（革新型太陽電池国際研究拠点設備事業）, Coinvestigator not use grants
2008 - 2014
