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ABSTRACT: The growth mechanism of GaAs nanowires (NWs) grown on polycrystalline silicon (poly-Si) thin films using selective-area metalorganic vapor-phase epitaxy was investigated. Wire structures were selectively grown in the mask openings on a poly-Si substrate. The appearance ratio of wire structures strongly depended on the growth conditions and deposition temperature of the poly-Si substrate. Evaluation of the grown shapes and growth characteristics revealed that GaAs NWs grown on a poly-Si substrate have the same growth mechanism as conventional GaAs NWs grown on a single-crystalline GaAs or Si substrate. Experiments showed that the wire structure yield can be improved by increasing the Si grain size and/or increasing the Si deposition temperature. The growth model proposed for understanding NW growth on poly-Si is based on the mask opening size, the Si grain size, and the growth conditions. The ability to control the growth mode is promising for the formation of NWs with complex structures on poly-Si thin layers.
Nanotechnology 03/2013; 24(11):115304. · 3.98 Impact Factor
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ABSTRACT: We present a bidirectional growth mode of InP nanowires grown by selective-area metalorganic vapor-phase epitaxy (SA-MOVPE). We studied the effect of the supply ratio of DEZn ([DEZn]) on InP grown structure morphology and crystal structures during the SA-MOVPE. Two growth regimes were observed in the investigated range of the [DEZn] on an InP(111)B substrate. At low [DEZn], grown structures formed tripod structures featuring three nanowires branched toward the [111]A directions. At high [DEZn], we obtained hexagonal pillar-type structures vertically grown on the (111)B substrate. These results show that the growth direction changes from [111]A to [111]B as [DEZn] is increased. We propose a growth mechanism based on the correlation between the incident facet of rotational twins and the shapes of the grown structures. Our results bring us one step closer to controlling the direction of nanowires on a Si substrate that has a nonpolar nature. They can also be applied to the development of InP nanowire devices.
Nano Letters 08/2012; 12(9):4770-4. · 13.20 Impact Factor
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ABSTRACT: Silicon transistors are expected to have new gate architectures, channel materials and switching mechanisms in ten years' time. The trend in transistor scaling has already led to a change in gate structure from two dimensions to three, used in fin field-effect transistors, to avoid problems inherent in miniaturization such as high off-state leakage current and the short-channel effect. At present, planar and fin architectures using III-V materials, specifically InGaAs, are being explored as alternative fast channels on silicon because of their high electron mobility and high-quality interface with gate dielectrics. The idea of surrounding-gate transistors, in which the gate is wrapped around a nanowire channel to provide the best possible electrostatic gate control, using InGaAs channels on silicon, however, has been less well investigated because of difficulties in integrating free-standing InGaAs nanostructures on silicon. Here we report the position-controlled growth of vertical InGaAs nanowires on silicon without any buffering technique and demonstrate surrounding-gate transistors using InGaAs nanowires and InGaAs/InP/InAlAs/InGaAs core-multishell nanowires as channels. Surrounding-gate transistors using core-multishell nanowire channels with a six-sided, high-electron-mobility transistor structure greatly enhance the on-state current and transconductance while keeping good gate controllability. These devices provide a route to making vertically oriented transistors for the next generation of field-effect transistors and may be useful as building blocks for wireless networks on silicon platforms.
Nature 08/2012; 488(7410):189-92. · 36.28 Impact Factor
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ABSTRACT: Semiconductor nanowire arrays integrated vertically on graphene films offer significant advantages for many sophisticated device applications. We report on van der Waals (VDW) epitaxy of InAs nanowires vertically aligned on graphene substrates using metal-organic chemical vapor deposition. The strong correlation between the growth direction of InAs nanowires and surface roughness of graphene substrates was investigated using various graphene films with different numbers of stacked layers. Notably, vertically well-aligned InAs nanowire arrays were obtained easily on single-layer graphene substrates with sufficiently strong VDW attraction. This study presents a considerable advance toward the VDW heteroepitaxy of inorganic nanostructures on chemical vapor-deposited large-area graphenes. More importantly, this work demonstrates the thinnest epitaxial substrate material that yields vertical nanowire arrays by the VDW epitaxy method.
Nano Letters 02/2012; 12(3):1431-6. · 13.20 Impact Factor
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ABSTRACT: The optical properties of In0.8Ga0.2As self-assembled quantum dots (SAQDs) grown on GaAs wire structures formed by utilizing SiO2-patterned exact and 5°-off (001) GaAs substrates have been studied with micro-photoluminescence (μ-PL). Single PL peak was occurred for In0.8Ga0.2As SAQDs grown on SiO2-patterned exact (001) GaAs, whereas double PL peaks were showed for SAQDs grown on 5°-off (001) GaAs substrates as the width of the opening windows increased. The power-dependent μ-PL spectra show that the first and second peaks of these double peaks were originated from the well-defined ground and excited state, respectively. These results demonstrated that In0.8Ga0.2As SAQDs selectively grown by utilizing SiO2-patterned 5°-off (001) GaAs substrates have well-defined zero-dimensional quantum states.
Nanoscale Research Letters 02/2012; 7:104. · 2.73 Impact Factor
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ABSTRACT: We demonstrate position-controlled III-V semiconductor nanowires (NWs) by using selective-area metal-organic vapor phase epitaxy and their application to solar cells. Efficiency of 4.23% is achieved for InP core-shell NW solar cells. We form a 'flexible NW array' without a substrate, which has the advantage of saving natural resources over conventional thin film photovoltaic devices. Four junction NW solar cells with over 50% efficiency are proposed and discussed.
AMBIO A Journal of the Human Environment 01/2012; 41 Suppl 2:119-24. · 2.03 Impact Factor
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Nano Letters 12/2011; · 13.20 Impact Factor
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ABSTRACT: We review the position-controlled growth of III-V nanowires (NWs) by selective-area metal-organic vapor-phase epitaxy (SA-MOVPE). This epitaxial technique enables the positioning of the vertical NWs on (111) oriented surfaces with lithographic techniques. Core-shell structures have also been achieved by controlling the growth mode during SA-MOVPE. The core-shell III-V NW-based devices such as light-emitting diodes, photovoltaic cells, and vertical surrounding-gate transistors are discussed in this article. Nanometer-scale growth also enabled the integration of III-V NWs on Si regardless of lattice mismatches. These demonstrated achievements should have broad applications in laser diodes, photodiodes, and high-electron mobility transistors with functionality on Si not made possible with conventional Si-CMOS techniques.
Journal of Materials Research. 09/2011; 26(17):2127 - 2141.
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ABSTRACT: Indium phosphide (InP) nanowires, which have crystal phase mixing and transition from zinc blende (ZB) to wurtzite (WZ), are grown in intermediate growth conditions between ZB and WZ by using selective-area metalorganic vapor phase epitaxy (SA-MOVPE). The shape of InP nanowires is tapered unlike ZB or WZ nanowires. A growth model has been developed for the tapered nanowires, which is simply described as the relationship between tapered angle and the ratio of ZB and WZ segments. In addition, the peak energy shift in photoluminescence measurement was attributed to the quantum confinement effect of the quantum well of the ZB region located in the polytypic structure of ZB and WZ in nanowires.
Nano Letters 09/2011; 11(10):4314-8. · 13.20 Impact Factor
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ABSTRACT: We report on unconventional, noncovalent heteroepitaxy of vertical indium arsenide (InAs) nanowires on thin graphitic films in terms of van der Waals (VDW) interactions. Nearly coherent in-plane lattice matching (misfit of 0.49%) between InAs and the graphitic surface plays a critical role in the epitaxial formation of vertical InAs nanowires on graphitic substrates. Otherwise, gallium arsenide (misfit of -6.22%) is grown to be island morphologies. Cross-sectional transmission electron microscopy analyses show that 1-2 monomolecular layer ledges or kinks facilitate heterogeneous nucleation of InAs on nonwetting graphitic surfaces, forming the nuclei and promoting the subsequent nanowire growth with strong VDW interactions at the heterojunction. We further demonstrate the controlled VDW epitaxy method for high-yield and uniform InAs nanowire arrays on honeycomb carbon surface utilizing substrate surface etching and patterning techniques. Our work opens a new platform for the III-arsenide/graphene hybrid junction electronics and optoelectronics.
ACS Nano 08/2011; 5(9):7576-84. · 10.77 Impact Factor
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ABSTRACT: We report on fabrication of tunnel field-effect transistor with III-V nanowire (NW)/Si heterojunction and surrounding-gate structure. The device fabricated by selective-area growth of an n<sup>+</sup> -InAs /undoped-InAs axial NW on a p<sup>+</sup> -Si (111) substrate showed switching behavior with an average subthreshold slope (SS) of 104 mV/dec under reverse bias condition. The switching behavior appeared under small supply voltage ( V <sub> ds </sub>=50 mV ) . Transmission electron microscopy revealed misfit dislocation formed at the interface degraded the SS and ON-state current. Coherent growth without misfit dislocations would promise realization of steep-slope transistor with a SS of <60 mV / dec .
Applied Physics Letters 03/2011; · 3.84 Impact Factor
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ABSTRACT: We report on integration of GaAs nanowire-based light-emitting-diodes (NW-LEDs) on Si substrate by selective-area metalorganic vapor phase epitaxy. The vertically aligned GaAs/AlGaAs core-multishell nanowires with radial p-n junction and NW-LED array were directly fabricated on Si. The threshold current for electroluminescence (EL) was 0.5 mA (current density was approximately 0.4 A/cm(2)), and the EL intensity superlinearly increased with increasing current injections indicating superluminescence behavior. The technology described in this letter could help open new possibilities for monolithic- and on-chip integration of III-V NWs on Si.
Nano Letters 04/2010; 10(5):1639-44. · 13.20 Impact Factor
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ABSTRACT: We study the catalyst-free growth of InP nanowires using selective-area metalorganic vapor phase epitaxy (SA-MOVPE) and show that they undergo transition of crystal structures depending on the growth conditions. InP nanowires were grown on InP substrates where the mask for the template of the growth was defined. The nanowires were grown only in the opening region of the mask. It was found that uniform array of InP nanowires with hexagonal cross section and with negligible tapering were grown under two distinctive growth conditions. The nanowires grown in two different growth conditions were found to exhibit different crystal structures. It was also found that the orientation and size of hexagon were different, suggesting that the difference of the growth behavior. A model for the transition of crystal structure is presented based on the atomic arrangements and termination of InP surfaces. Photoluminescence measurement revealed that the transition took place for nanowires with diameters up to 1 microm.
Nano Letters 04/2010; 10(5):1699-703. · 13.20 Impact Factor
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iiWAS'2010 - The 12th International Conference on Information Integration and Web-based Applications and Services, 8-10 November 2010, Paris, France; 01/2010
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ABSTRACT: We report on the fabrication of the nanowires with InGaAs/GaAs heterostructures on the GaAs(111)B substrate using selective-area metal organic vapor phase epitaxy. Fabry-Pérot microcavity modes were observed in the nanowires with perfect end facets dispersed onto the silicon substrate and not observed in the free-standing nanowires. We find that the calculated group refractive indices only considering the material dispersion do not agree with the experimentally determined values although this method was used by some researchers. The calculated group refractive indices considering both the material dispersion and the waveguide dispersion agree with the experimentally determined values well. We also find that Fabry-Pérot microcavity modes are not observable in the nanowires with the width less than about 180 nm, which is mainly caused by their poor reflectivity at the end facets due to their weak confinement to the optical field.
Optics Express 06/2009; 17(11):9337-46. · 3.59 Impact Factor
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ABSTRACT: We report on the fabrication of the nanowires with InGaAs/GaAs heterostructures on the GaAs(111)B substrate using selective-area metal organic vapor phase epitaxy. Fabry-Pérot microcavity modes were observed in the nanowires with perfect end facets dispersed onto the silicon substrate and not observed in the free-standing nanowires. We find that the calculated group refractive indices only considering the material dispersion do not agree with the experimentally determined values although this method was used by some researchers. The calculated group refractive indices considering both the material dispersion and the waveguide dispersion agree with the experimentally determined values well. We also find that Fabry-Pérot microcavity modes are not observable in the nanowires with the width less than about 180 nm, which is mainly caused by their poor reflectivity at the end facets due to their weak confinement to the optical field.
Optics Express. 05/2009; 17:9337--9346.
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ABSTRACT: We report on selective-area growth of vertically aligned GaAs nanowires on Si(111) substrate. Modification of the initial Si(111) surface by pretreatment under an AsH(3) atmosphere and low-temperature growth of GaAs were important for controlling the growth orientations of the GaAs nanowire on the Si(111) surface. We also found that the size of openings strongly affected the growth morphology of GaAs nanowires on Si(111). Small diameter openings reduced the antiphase defects and improved the optical properties in the GaAs nanowires. Moreover, we realized coherent growth without misfit dislocation at the GaAs/Si interface. Finally, we demonstrated fabrication of a GaAs/AlGaAs core-shell nanowire array on a Si surface and revealed that the luminescence intensity was markedly enhanced by passivation effects. These results are promising for future III-V nanowire-based optoelectronic integration on Si platforms.
Nanotechnology 05/2009; 20(14):145302. · 3.98 Impact Factor
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ABSTRACT: Highly uniform GaAs/GaAsP coaxial nanowires were prepared via selective-area metal organic vapor phase epitaxy. Photoluminescence spectra from a single nanowire indicate that the obtained heterostructures can produce near-infrared (NIR) lasing under pulsed light excitation. The end facets of a single nanowire form a natural mirror surface to create an axial cavity, which realizes resonance and give stimulated emission. This study is a considerable advance toward the realization of nanowire-based NIR light sources.
Nano Letters 01/2009; 9(1):112-6. · 13.20 Impact Factor
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ABSTRACT: We report on control of growth directions of InAs nanowires on Si substrate. We achieved to integrate vertical InAs nanowires on Si by modifying initial Si(111) surface in selective-area metal-organic vapor phase epitaxy with flow-rate modulation mode at low temperature. Cross-sectional transmission electron microscope and Raman scattering showed that misfit dislocation with local strains were accommodated in the interface.
Nano Letters 10/2008; 8(10):3475-80. · 13.20 Impact Factor
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ABSTRACT: We report on the photoluminescence (PL) and time-resolved PL (TR-PL) spectroscopy of wurtzite InP/InAs/InP core-multishell nanowires. Multiple peaks appeared in the PL spectra due to monolayer variation of InAs layers and each PL peak showed a blueshift with a cube-root dependence on the excitation power. This blueshift results from the band-bending in the type-II band configuration. Fast decay caused by the relaxation of the band-bending was observed at higher energy sides of the PL peaks in the TR-PL measurement. The blueshift calculated by a triangular quantum well approximation in type-II configuration was in good agreement with that observed in the experiment. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 09/2008; 6(1):205 - 208.
