T. Inoue

Kobe University, Kōbe-shi, Hyogo-ken, Japan

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Publications (15)9.01 Total impact

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    ABSTRACT: Two-photon transition modelling is developed to study quantum mechanism and simulate device operation in intermediate band solar cell (IBSC). The interband-intranband transition, the detailed balance and carrier transport are coupled with each other. IB formation mechanism is studied within one-band envelope-function framework (Kronig-Penney type). Well-designed GaInAs/InP superlattice structures have been proved to separate IB from valence band (VB) and conduction band (CB), which is the precondition of IBSC operations. Further, we calculate two-photon absorption spectra and firstly combine quantum transitions into recent drift-diffusion and detail balanced model. With this model, we have studied a novel IBSC consist of In0.53Ga0.47As/InP superlattices (SLs). Our results show the interband-intraband transition determines the conversion efficiency. With well-designed quantum structure, the efficiency in 1.2 μm thick SLs is 46.13% under the maximum concentration. However, as the well or barrier thickness increases to 10 nm, the absorption peak of the intraband transition gradually redshifts and narrows, so the efficiency correspondingly decreases to below 40%.
    Conference Record of the IEEE Photovoltaic Specialists Conference 01/2011;
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    ABSTRACT: We propose an all-optical switch based on selfassembled InAs/GaAs quantum dots (QDs) within a vertical cavity. Two essential aspects of this novel device have been investigated, which includes the QD/cavity nonlinearity with appropriately designed mirrors and the intersubband carrier dynamics inside QDs. Vertical-reflection-type switches have been fabricated with an asymmetric cavity that consists of 12 periods of GaAs/Al<sub>0.8</sub>Ga<sub>0.2</sub>As for the front mirror and 25 periods for the back mirror. All-optical switching via the QD excited states has been achieved with a time constant down to 23 ps, wavelength tunability over 30 nm, and ultralow power consumption less than 1 fJ/μm<sup>2</sup> . These results demonstrate that QDs within a vertical cavity have great advantages to realize low-powerconsumption polarization-insensitive micrometer-sized switching devices for the future optical communication and signal processing systems.
    IEEE Journal of Quantum Electronics 12/2010; · 2.11 Impact Factor
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    W. G. Hu, T. Inoue, O. Kojima, T. Kita
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    ABSTRACT: The effects of absorption coefficients were incorporated in a detailed balance model to analyze the intermediate-band (IB) configuration in quantum dot (QD) solar cells. Our results show that the optimum IB level, EIB, depends on the ratio of two subbandgap absorption coefficient constants, αIC0/αVI0. Efficiency contour plots have been calculated to determine the optimum values of EIB and αIC0/αVI0. In many cases, a large αIC0 results in high conversion efficiency, especially for thin QD solar cells. Optimizing QD shape and size is a promising method to increase αIC0. Increasing the QD total thickness partially addresses the urgent demand for a large αIC0.
    Applied Physics Letters 11/2010; 97(19):193106-193106-3. · 3.79 Impact Factor
  • W. G. Hu, T. Inoue, O. Kojima, T. Kita
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    ABSTRACT: The net generation balance equation is incorporated into the Poisson equation and the continuty equation to deduce the IB quasi-Fermi EfIB and filling factor f. The QD state density is not enough high to pin the IB quasi-Fermi level EfIB at the position of the IB level EIB so that EfIB depends on the QD position and the operation voltage. In the case of the quasi-uniform absorption (the weak absorption and the same absorption coefficient constants of sub-bandgap photons), a rough approximation is that EfIB approximately parallels to EIB, and clamps in some equilibrium position. In addition, in the case of the nonuniform absorption, the proper ratio αIC0/αVI0 can effectively enhance the conversion efficiency. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 11/2010; 8(2):622 - 624.
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    ABSTRACT: In this paper, we report a control of the polarization property in quantum dot semiconductor optical amplifiers (QD-SOAs) using vertically-stacked, electronically-coupled InAs/GaAs QDs grown by molecular beam epitaxy. By optimizing the number of stacked layers and intermediate GaAs thickness, the 9-stacked QDs demonstrated the polarization-insensitive operation within 1.2 dB in the 1.3-mm optical communication band. Our results demonstrate that the electronically-coupled QDs are useful to realize the polarization-insensitive QD-SOAs.
    Journal of Physics Conference Series 09/2010; 245(1):012076.
  • W.G. Hu, T. Inoue, O. Kojima, T. Kita
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    ABSTRACT: This paper describes a net generation balance model to study the qasi-Fermi level split and the intermediate band (IB) filling. Our simulations reveal that the QD state density is not enough high to pin the IB quasi-Fermi level (Ef<sub>IB</sub>) at the position of the IB level (E<sub>IB</sub>), and the filling factor (f) is determined by the absorption-recombination process of sub-bandgap photons. In the quasi-uniform absorption (equivalent absorption coefficients), a rough approximation is that Ef<sub>IB</sub> parallel with E<sub>IB</sub>, and is clamped in the equilibrium position. In the nonuniform absorption case (inequivalent absorption coefficients), our simulations suggest that optimizing α<sub>IC0</sub>/α<sub>VI0</sub> ratio can effectively enhance the conversion efficiency.
    Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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    ABSTRACT: We investigated the effect of direct doping of quantum dots (QDs) with Si on the performance of QD solar cells (QDSCs). In order to control the Fermi level of intermediate band (IB) region, 25 layers of stacked InAs/GaNAs QDs were directly doped with Si impurity during the self-assembling stage of growth. A QDSC with Si doping shows an improved quantum efficiency (QE) in shorter wavelength region, which is from p-GaAs emitter layer. Further, the fact that applied external bias does not affect QE spectrum as well as photocurrent in QDSC with Si direct doping suggests that carrier collection has been enhanced in QD region as a result of reduction of recombination.
    Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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    ABSTRACT: An all-optical switching device has been proposed by using self-assembled InAs/GaAs quantum dots (QDs) within a vertical cavity structure for ultrafast optical communications. This device has several desirable properties, such as the ultra-low power consumption, the micrometre size, and the polarization insensitive operation. Due to the threedimensional confined carrier state and the broad size distribution of self-assembled InAs/GaAs QDs, it is crucial to enhance the interaction between QDs and the cavity with appropriately designed 1D periodic structure. Significant QD/cavity nonlinearity is theoretically observed by increasing the GaAs/AlAs pair number of the bottom mirror. By this consideration, we have fabricated vertical-reflection type QD switches with 12 periods of GaAs/Al0.8Ga0.2As for the top mirror and 25 periods for the bottom mirror to give an asymmetric vertical cavity. Optical switching via the QD excited state exhibits a fast switching process with a time constant down to 23 ps, confirming that the fast intersubband relaxation of carriers inside QDs is an effective means to speed up the switching process. A technique by changing the light incident angle realizes wavelength tunability over 30 nm for the QD/cavity switch.
    Proc SPIE 02/2010;
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    ABSTRACT: We have investigated at the first time an all-optical switch using self-assembled InAs/GaAs quantum dots (QDs) within a vertical cavity structure. The optical nonlinearity of the QD switch has been optimized by an asymmetric cavity to achieve the maximum differential reflectivity. Optical switching via QD excited states exhibits a fast decay with a time constant down to 23 ps and a wavelength tunability over 30 nm. By compared to the theoretical design, the absorption strength of QD layers within the cavity has been determined.
    Conference Proceedings - International Conference on Indium Phosphide and Related Materials 01/2010;
  • T Kudo, T Inoue, T Kita, O Wada
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    ABSTRACT: Self-assembling process of InAs/GaAs quantum dots (QDs) has been investigated by analyzing reflection high-energy electron diffraction (RHEED) images. During the island formation, the chevron diffraction of the RHEED shows dramatic changes depending on the As pressure. The self-assembling process has been found to consist of four steps. Initially islands are preferentially covered by high-index facets, which transform into low-index surfaces as the growth proceeds, and then the islands are covered by stable low-index facets. In this growth step, the island size becomes uniform, because of the self-limited growth. In the next step, we found indium flow back from the islands into the wetting layer, which causes shrinkage of the island size as well as formation of giant islands. This indium flow back can be controlled by As pressure.
    Indium Phosphide & Related Materials, 2007. IPRM '07. IEEE 19th International Conference on; 06/2007
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    ABSTRACT: We succeeded in observing atomic scale images of undamaged single InAs quantum dots (QDs) embedded in a GaAs matrix using a combined use of high resolution transmission electron microscope (HRTEM) and focused ion beam (FIB) system for the first time. The QD can be viewed from multi directions, and a conclusive and comprehensive interpretation of the size and shape anisotropy has been achieved. Asymmetry of the structural properties has been confirmed between the [110] and [-110] crystallographic directions. The embedded QD is elongated along the [-110] axis. The strain-field pattern is also asymmetric according to the shape anisotropy Our results will enable the investigation of exact structural anisotropy and their influence on the atom like properties of QDs.
    Indium Phosphide & Related Materials, 2007. IPRM '07. IEEE 19th International Conference on; 06/2007
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    ABSTRACT: We have studied emission-wavelength extension of nitrided InAs/GaAs quantum dots (QDs) with different sizes. Nitrided QDs have been shown to suppress In segregation during the capping layer growth. The emission wavelength reaches 1.3 μm at room temperature. Effects of the strain on the structural and optical properties of QDs have been investigated by photoluminescence spectroscopy and transmission electron microscopy. The nitrogen-incorporation process into the InAs QDs depends on the QD size at the moment of nitrogen irradiation. The large QDs are covered by a thin nitrided layer, while the small QDs tend to form nitrogen-containing alloys.
    Journal of Crystal Growth 01/2007; · 1.55 Impact Factor
  • N. Shimizu, T. Inoue, T. Kita, O. Wada
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    ABSTRACT: We studied a technique of atomically controlled nitridation doping on GaAs(001) using (3×3) nitrogen (N)-stabilized reconstruction. Ordering of the N-stabilized surface has been found to depend on the (2×4)-reconstructed structures of GaAs(001). Nitridation transforms the (2×4) surfaces into the (3×3) by way of a new intermediate (3×4) surface. From these results, we proposed a model of the nitridation process on the GaAs(001) surface. Furthermore, layer-by-layer growth of a GaAs-capping layer has been confirmed on the nitrided surface. The atomically doped N-related isoelectronic centers show strong emission lines of excitons bound to N pairs ordered along [110].
    Journal of Crystal Growth 01/2007; 301:34-37. · 1.55 Impact Factor
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    ABSTRACT: Nitrided InAs quantum dots (QDs) have been shown to suppress In-segregation in QDs and achieve emission at 1.3 μm. Effects of strain on structural and optical properties of QDs have been demonstrated through transmission electron microscope and photoluminescence analyses.
    Indium Phosphide and Related Materials Conference Proceedings, 2006 International Conference on; 06/2006
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    ABSTRACT: Emission wavelength extension into >1.3 μm region by atomic-layer nitridation of InAs QDs has been analyzed using AFM and TEM. We found that the nitridation increases the QD size and aspect ratio and modifies the strain distribution.
    Indium Phosphide and Related Materials, 2005. International Conference on; 06/2005