1
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Chen J, Woods BD, Yu P, Hocevar M, Car D, Plissard SR, Bakkers EPAM, Stanescu TD, Frolov SM. Erratum: Ubiquitous Non-Majorana Zero-Bias Conductance Peaks in Nanowire Devices [Phys. Rev. Lett. 123, 107703 (2019)]. Phys Rev Lett 2024; 132:099901. [PMID: 38489658 DOI: 10.1103/physrevlett.132.099901] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/22/2024] [Indexed: 03/17/2024]
Abstract
This corrects the article DOI: 10.1103/PhysRevLett.123.107703.
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2
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Li A, Hauge HIT, Verheijen MA, Bakkers EPAM, Tucker RT, Vincent L, Renard C. Hexagonal silicon-germanium nanowire branches with tunable composition. Nanotechnology 2022; 34:015601. [PMID: 36126589 DOI: 10.1088/1361-6528/ac9317] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/19/2022] [Accepted: 09/20/2022] [Indexed: 06/15/2023]
Abstract
Hexagonal SiGe-2H has been recently shown to have a direct bandgap, and holds the promise to be compatible with silicon technology. Hexagonal Si and Ge have been grown on an epitaxial lattice matched template consisting of wurtzite GaP and GaAs, respectively. Here, we present the growth of hexagonal Si and SiGe nanowire branches grown from a wurtzite stem by the vapor-liquid-solid growth mode, which is substantiated byin situtransmission electron microscopy. We show that the composition can be tuned through the whole range of stoichiometry from Si to Ge, and the possibility to realize Si and SiGe heterostructures in these branches.
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Affiliation(s)
- A Li
- Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - H I T Hauge
- Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - M A Verheijen
- Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
- Eurofins Materials Science, High Tech Campus 11, 5656 AE Eindhoven, The Netherlands
| | - E P A M Bakkers
- Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - R T Tucker
- Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
- Department of Electrical & Computer Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada
| | - L Vincent
- Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, F-91120, Palaiseau, France
| | - C Renard
- Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, F-91120, Palaiseau, France
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3
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Pendharkar M, Zhang B, Wu H, Zarassi A, Zhang P, Dempsey CP, Lee JS, Harrington SD, Badawy G, Gazibegovic S, Op Het Veld RLM, Rossi M, Jung J, Chen AH, Verheijen MA, Hocevar M, Bakkers EPAM, Palmstrøm CJ, Frolov SM. Parity-preserving and magnetic field-resilient superconductivity in InSb nanowires with Sn shells. Science 2021; 372:508-511. [PMID: 33858990 DOI: 10.1126/science.aba5211] [Citation(s) in RCA: 21] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2019] [Accepted: 03/13/2021] [Indexed: 11/02/2022]
Abstract
Improving materials used to make qubits is crucial to further progress in quantum information processing. Of particular interest are semiconductor-superconductor heterostructures that are expected to form the basis of topological quantum computing. We grew semiconductor indium antimonide nanowires that were coated with shells of tin of uniform thickness. No interdiffusion was observed at the interface between Sn and InSb. Tunnel junctions were prepared by in situ shadowing. Despite the lack of lattice matching between Sn and InSb, a 15-nanometer-thick shell of tin was found to induce a hard superconducting gap, with superconductivity persisting in magnetic field up to 4 teslas. A small island of Sn-InSb exhibits the two-electron charging effect. These findings suggest a less restrictive approach to fabricating superconducting and topological quantum circuits.
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Affiliation(s)
- M Pendharkar
- Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA
| | - B Zhang
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA
| | - H Wu
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA
| | - A Zarassi
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA
| | - P Zhang
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA
| | - C P Dempsey
- Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA
| | - J S Lee
- California NanoSystems Institute, University of California, Santa Barbara, CA 93106, USA
| | - S D Harrington
- Materials Department, University of California, Santa Barbara, CA 93106, USA
| | - G Badawy
- Eindhoven University of Technology, 5600 MB Eindhoven, Netherlands
| | - S Gazibegovic
- Eindhoven University of Technology, 5600 MB Eindhoven, Netherlands
| | | | - M Rossi
- Eindhoven University of Technology, 5600 MB Eindhoven, Netherlands
| | - J Jung
- Eindhoven University of Technology, 5600 MB Eindhoven, Netherlands
| | - A-H Chen
- Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France
| | - M A Verheijen
- Eindhoven University of Technology, 5600 MB Eindhoven, Netherlands
| | - M Hocevar
- Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France
| | - E P A M Bakkers
- Eindhoven University of Technology, 5600 MB Eindhoven, Netherlands
| | - C J Palmstrøm
- Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA.,California NanoSystems Institute, University of California, Santa Barbara, CA 93106, USA.,Materials Department, University of California, Santa Barbara, CA 93106, USA
| | - S M Frolov
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA.
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4
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Jiang Y, de Jong EJ, van de Sande V, Gazibegovic S, Badawy G, Bakkers EPAM, Frolov SM. Hysteretic magnetoresistance in nanowire devices due to stray fields induced by micromagnets. Nanotechnology 2021; 32:095001. [PMID: 33142271 DOI: 10.1088/1361-6528/abc70f] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
We study hysteretic magnetoresistance in InSb nanowires due to stray magnetic fields from CoFe micromagnets. Devices without any ferromagnetic components show that the magnetoresistance of InSb nanowires commonly exhibits either a local maximum or local minimum at zero magnetic field. Switching of microstrip magnetizations then results in positive or negative hysteretic dependence as conductance maxima or minima shift with respect to the global external field. Stray fields are found to be in the range of tens of millitesla, comparable to the scale over which the nanowire magnetoresistance develops. We observe that the stray field signal is similar to that obtained in devices with ferromagnetic contacts (spin valves). We perform micromagnetic simulations which are in reasonable agreement with the experiment. The use of locally varying magnetic fields may bring new ideas for Majorana circuits in which nanowire networks require control over field orientation at the nanoscale.
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Affiliation(s)
- Y Jiang
- University of Pittsburgh, Pittsburgh, PA 15260, United States of America
| | - E J de Jong
- Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
| | - V van de Sande
- Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
| | - S Gazibegovic
- Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
| | - G Badawy
- Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
| | - E P A M Bakkers
- Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
| | - S M Frolov
- University of Pittsburgh, Pittsburgh, PA 15260, United States of America
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5
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Chen J, Woods BD, Yu P, Hocevar M, Car D, Plissard SR, Bakkers EPAM, Stanescu TD, Frolov SM. Ubiquitous Non-Majorana Zero-Bias Conductance Peaks in Nanowire Devices. Phys Rev Lett 2019; 123:107703. [PMID: 31573319 DOI: 10.1103/physrevlett.123.107703] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/08/2019] [Indexed: 06/10/2023]
Abstract
We perform tunneling measurements on indium antimonide nanowire-superconductor hybrid devices fabricated for the studies of Majorana bound states. At finite magnetic field, resonances that strongly resemble Majorana bound states, including zero-bias pinning, become common to the point of ubiquity. Since Majorana bound states are predicted in only a limited parameter range in nanowire devices, we seek an alternative explanation for the observed zero-bias peaks. With the help of a self-consistent Poission-Schrödinger multiband model developed in parallel, we identify several families of trivial subgap states that overlap and interact, giving rise to a crowded spectrum near zero energy and zero-bias conductance peaks in experiments. These findings advance the search for Majorana bound states through improved understanding of broader phenomena found in superconductor-semiconductor systems.
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Affiliation(s)
- J Chen
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
- Department of Electrical and Computer Engineering and Peterson Institute of NanoScience and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA
| | - B D Woods
- Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA
| | - P Yu
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
| | - M Hocevar
- Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France
| | - D Car
- Eindhoven University of Technology, 5600 MB, Eindhoven, Netherlands
| | - S R Plissard
- LAAS CNRS, Université de Toulouse, 31031 Toulouse, France
| | - E P A M Bakkers
- Eindhoven University of Technology, 5600 MB, Eindhoven, Netherlands
| | - T D Stanescu
- Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA
| | - S M Frolov
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
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6
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Badawy G, Gazibegovic S, Borsoi F, Heedt S, Wang CA, Koelling S, Verheijen MA, Kouwenhoven LP, Bakkers EPAM. High Mobility Stemless InSb Nanowires. Nano Lett 2019; 19:3575-3582. [PMID: 31094527 DOI: 10.1021/acs.nanolett.9b00545] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
High aspect-ratio InSb nanowires (NWs) of high chemical purity are sought for implementing advanced quantum devices. The growth of InSb NWs is challenging, generally requiring a stem of a foreign material for nucleation. Such a stem tends to limit the length of InSb NWs and its material becomes incorporated in the InSb segment. Here, we report on the growth of chemically pure InSb NWs tens of microns long. Using a selective-area mask in combination with gold as a catalyst allows complete omission of the stem, thus demonstrating that InSb NWs can grow directly from the substrate. The introduction of the selective-area mask gives rise to novel growth kinetics, demonstrating high growth rates and complete suppression of layer deposition on the mask for Sb-rich conditions. The crystal quality and chemical purity of these NWs is reflected in the significant enhancement of low-temperature electron mobility, yielding an average of 4.4 × 104 cm2/(V s), compared to previously studied InSb NWs grown on stems.
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Affiliation(s)
- Ghada Badawy
- Department of Applied Physics , Eindhoven University of Technology , 5600 MB Eindhoven , The Netherlands
| | - Sasa Gazibegovic
- Department of Applied Physics , Eindhoven University of Technology , 5600 MB Eindhoven , The Netherlands
- QuTech and Kavli Institute of NanoScience , Delft University of Technology , 2600 GA Delft , The Netherlands
| | - Francesco Borsoi
- QuTech and Kavli Institute of NanoScience , Delft University of Technology , 2600 GA Delft , The Netherlands
| | - Sebastian Heedt
- QuTech and Kavli Institute of NanoScience , Delft University of Technology , 2600 GA Delft , The Netherlands
| | - Chien-An Wang
- QuTech and Kavli Institute of NanoScience , Delft University of Technology , 2600 GA Delft , The Netherlands
| | - Sebastian Koelling
- QuTech and Kavli Institute of NanoScience , Delft University of Technology , 2600 GA Delft , The Netherlands
| | - Marcel A Verheijen
- Department of Applied Physics , Eindhoven University of Technology , 5600 MB Eindhoven , The Netherlands
- Eurofins Material Science Netherlands B.V. , High Tech Campus 11, 5656 AE Eindhoven , The Netherlands
| | - Leo P Kouwenhoven
- QuTech and Kavli Institute of NanoScience , Delft University of Technology , 2600 GA Delft , The Netherlands
- Microsoft Quantum Lab Delft , 2600 GA Delft , The Netherlands
| | - E P A M Bakkers
- Department of Applied Physics , Eindhoven University of Technology , 5600 MB Eindhoven , The Netherlands
- QuTech and Kavli Institute of NanoScience , Delft University of Technology , 2600 GA Delft , The Netherlands
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7
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Su Z, Zarassi A, Hsu JF, San-Jose P, Prada E, Aguado R, Lee EJH, Gazibegovic S, Op Het Veld RLM, Car D, Plissard SR, Hocevar M, Pendharkar M, Lee JS, Logan JA, Palmstrøm CJ, Bakkers EPAM, Frolov SM. Mirage Andreev Spectra Generated by Mesoscopic Leads in Nanowire Quantum Dots. Phys Rev Lett 2018; 121:127705. [PMID: 30296125 DOI: 10.1103/physrevlett.121.127705] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/10/2018] [Indexed: 06/08/2023]
Abstract
We study transport mediated by Andreev bound states formed in InSb nanowire quantum dots. Two kinds of superconducting source and drain contacts are used: epitaxial Al/InSb devices exhibit a doubling of tunneling resonances, while, in NbTiN/InSb devices, Andreev spectra of the dot appear to be replicated multiple times at increasing source-drain bias voltages. In both devices, a mirage of a crowded spectrum is created. To describe the observations a model is developed that combines the effects of a soft induced gap and of additional Andreev bound states both in the quantum dot and in the finite regions of the nanowire adjacent to the quantum dot. Understanding of Andreev spectroscopy is important for the correct interpretation of Majorana experiments done on the same structures.
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Affiliation(s)
- Z Su
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
| | - A Zarassi
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
| | - J-F Hsu
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
| | - P San-Jose
- Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Cantoblanco, 28049 Madrid, Spain
| | - E Prada
- Departamento de Fisica de la Materia Condensada, Condensed Matter Physics Center (IFIMAC) and Instituto Nicolas Cabrera, Universidad Autonoma de Madrid, E-28049 Madrid, Spain
| | - R Aguado
- Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Cantoblanco, 28049 Madrid, Spain
| | - E J H Lee
- Departamento de Fisica de la Materia Condensada, Condensed Matter Physics Center (IFIMAC) and Instituto Nicolas Cabrera, Universidad Autonoma de Madrid, E-28049 Madrid, Spain
| | - S Gazibegovic
- Eindhoven University of Technology, 5600 MB, Eindhoven, Netherlands
| | | | - D Car
- Eindhoven University of Technology, 5600 MB, Eindhoven, Netherlands
| | - S R Plissard
- LAAS CNRS, Université de Toulouse, 31031 Toulouse, France
| | - M Hocevar
- Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France
| | - M Pendharkar
- Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, California 93106, USA
| | - J S Lee
- California NanoSystems Institute, University of California Santa Barbara, Santa Barbara, California 93106, USA
| | - J A Logan
- Materials Department, University of California, Santa Barbara, Santa Barbara, California 93106, USA
| | - C J Palmstrøm
- Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, California 93106, USA
- California NanoSystems Institute, University of California Santa Barbara, Santa Barbara, California 93106, USA
- Materials Department, University of California, Santa Barbara, Santa Barbara, California 93106, USA
| | - E P A M Bakkers
- Eindhoven University of Technology, 5600 MB, Eindhoven, Netherlands
| | - S M Frolov
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
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8
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Assali S, Lähnemann J, Vu TTT, Jöns KD, Gagliano L, Verheijen MA, Akopian N, Bakkers EPAM, Haverkort JEM. Crystal Phase Quantum Well Emission with Digital Control. Nano Lett 2017; 17:6062-6068. [PMID: 28892396 PMCID: PMC5642001 DOI: 10.1021/acs.nanolett.7b02489] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/13/2017] [Revised: 09/06/2017] [Indexed: 05/31/2023]
Abstract
One of the major challenges in the growth of quantum well and quantum dot heterostructures is the realization of atomically sharp interfaces. Nanowires provide a new opportunity to engineer the band structure as they facilitate the controlled switching of the crystal structure between the zinc-blende (ZB) and wurtzite (WZ) phases. Such a crystal phase switching results in the formation of crystal phase quantum wells (CPQWs) and quantum dots (CPQDs). For GaP CPQWs, the inherent electric fields due to the discontinuity of the spontaneous polarization at the WZ/ZB junctions lead to the confinement of both types of charge carriers at the opposite interfaces of the WZ/ZB/WZ structure. This confinement leads to a novel type of transition across a ZB flat plate barrier. Here, we show digital tuning of the visible emission of WZ/ZB/WZ CPQWs in a GaP nanowire by changing the thickness of the ZB barrier. The energy spacing between the sharp emission lines is uniform and is defined by the addition of single ZB monolayers. The controlled growth of identical quantum wells with atomically flat interfaces at predefined positions featuring digitally tunable discrete emission energies may provide a new route to further advance entangled photons in solid state quantum systems.
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Affiliation(s)
- S. Assali
- Department
of Applied Physics, Eindhoven University
of Technology, 5600 MB, Eindhoven, The Netherlands
| | - J. Lähnemann
- Paul-Drude-Institut
für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany
| | - T. T. T. Vu
- Department
of Applied Physics, Eindhoven University
of Technology, 5600 MB, Eindhoven, The Netherlands
| | - K. D. Jöns
- Kavli
Institute of Nanoscience, Delft University
of Technology, 2600 GA, Delft, The Netherlands
| | - L. Gagliano
- Department
of Applied Physics, Eindhoven University
of Technology, 5600 MB, Eindhoven, The Netherlands
| | - M. A. Verheijen
- Department
of Applied Physics, Eindhoven University
of Technology, 5600 MB, Eindhoven, The Netherlands
- Philips
Innovation Services Eindhoven, High Tech Campus 11, 5656 AE, Eindhoven, The
Netherlands
| | - N. Akopian
- Department
of Applied Physics, Eindhoven University
of Technology, 5600 MB, Eindhoven, The Netherlands
- Kavli
Institute of Nanoscience, Delft University
of Technology, 2600 GA, Delft, The Netherlands
| | - E. P. A. M. Bakkers
- Department
of Applied Physics, Eindhoven University
of Technology, 5600 MB, Eindhoven, The Netherlands
- Kavli
Institute of Nanoscience, Delft University
of Technology, 2600 GA, Delft, The Netherlands
| | - J. E. M. Haverkort
- Department
of Applied Physics, Eindhoven University
of Technology, 5600 MB, Eindhoven, The Netherlands
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9
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Black LE, Cavalli A, Verheijen MA, Haverkort JEM, Bakkers EPAM, Kessels WMM. Effective Surface Passivation of InP Nanowires by Atomic-Layer-Deposited Al 2O 3 with PO x Interlayer. Nano Lett 2017; 17:6287-6294. [PMID: 28885032 PMCID: PMC5642000 DOI: 10.1021/acs.nanolett.7b02972] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/06/2023]
Abstract
III/V semiconductor nanostructures have significant potential in device applications, but effective surface passivation is critical due to their large surface-to-volume ratio. For InP such passivation has proven particularly difficult, with substantial depassivation generally observed following dielectric deposition on InP surfaces. We present a novel approach based on passivation with a phosphorus-rich interfacial oxide deposited using a low-temperature process, which is critical to avoid P-desorption. For this purpose we have chosen a POx layer deposited in a plasma-assisted atomic layer deposition (ALD) system at room temperature. Since POx is known to be hygroscopic and therefore unstable in atmosphere, we encapsulate this layer with a thin ALD Al2O3 capping layer to form a POx/Al2O3 stack. This passivation scheme is capable of improving the photoluminescence (PL) efficiency of our state-of-the-art wurtzite (WZ) InP nanowires by a factor of ∼20 at low excitation. If we apply the rate equation analysis advocated by some authors, we derive a PL internal quantum efficiency (IQE) of 75% for our passivated wires at high excitation. Our results indicate that it is more reliable to calculate the IQE as the ratio of the integrated PL intensity at room temperature to that at 10 K. By this means we derive an IQE of 27% for the passivated wires at high excitation (>10 kW cm-2), which constitutes an unprecedented level of performance for undoped InP nanowires. This conclusion is supported by time-resolved PL decay lifetimes, which are also shown to be significantly higher than previously reported for similar wires. The passivation scheme displays excellent long-term stability (>7 months) and is additionally shown to substantially improve the thermal stability of InP surfaces (>300 °C), significantly expanding the temperature window for device processing. Such effective surface passivation is a key enabling technology for InP nanowire devices such as nanolasers and solar cells.
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Affiliation(s)
- L. E. Black
- Eindhoven
University of Technology, Postbus 513, 5600 MB Eindhoven, The Netherlands
- E-mail:
| | - A. Cavalli
- Eindhoven
University of Technology, Postbus 513, 5600 MB Eindhoven, The Netherlands
| | - M. A. Verheijen
- Eindhoven
University of Technology, Postbus 513, 5600 MB Eindhoven, The Netherlands
- Philips
Innovation Laboratories, High Tech Campus 11, 5656 AE Eindhoven, The Netherlands
| | - J. E. M. Haverkort
- Eindhoven
University of Technology, Postbus 513, 5600 MB Eindhoven, The Netherlands
| | - E. P. A. M. Bakkers
- Eindhoven
University of Technology, Postbus 513, 5600 MB Eindhoven, The Netherlands
| | - W. M. M. Kessels
- Eindhoven
University of Technology, Postbus 513, 5600 MB Eindhoven, The Netherlands
- E-mail:
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10
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Conesa-Boj S, Li A, Koelling S, Brauns M, Ridderbos J, Nguyen TT, Verheijen MA, Koenraad PM, Zwanenburg FA, Bakkers EPAM. Boosting Hole Mobility in Coherently Strained [110]-Oriented Ge-Si Core-Shell Nanowires. Nano Lett 2017; 17:2259-2264. [PMID: 28231017 PMCID: PMC5391496 DOI: 10.1021/acs.nanolett.6b04891] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/23/2016] [Revised: 02/07/2017] [Indexed: 05/28/2023]
Abstract
The ability of core-shell nanowires to overcome existing limitations of heterostructures is one of the key ingredients for the design of next generation devices. This requires a detailed understanding of the mechanism for strain relaxation in these systems in order to eliminate strain-induced defect formation and thus to boost important electronic properties such as carrier mobility. Here we demonstrate how the hole mobility of [110]-oriented Ge-Si core-shell nanowires can be substantially enhanced thanks to the realization of large band offset and coherent strain in the system, reaching values as high as 4200 cm2/(Vs) at 4 K and 1600 cm2/(Vs) at room temperature for high hole densities of 1019 cm-3. We present a direct correlation of (i) mobility, (ii) crystal direction, (iii) diameter, and (iv) coherent strain, all of which are extracted in our work for individual nanowires. Our results imply [110]-oriented Ge-Si core-shell nanowires as a promising candidate for future electronic and quantum transport devices.
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Affiliation(s)
- S Conesa-Boj
- Kavli Institute of Nanoscience, Delft University of Technology , Lorentzweg 1, 2628 CJ Delft, The Netherlands
- Department of Applied Physics, TU Eindhoven , Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - A Li
- Kavli Institute of Nanoscience, Delft University of Technology , Lorentzweg 1, 2628 CJ Delft, The Netherlands
- Department of Applied Physics, TU Eindhoven , Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - S Koelling
- Department of Applied Physics, TU Eindhoven , Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - M Brauns
- NanoElectronics Group, MESA Institute for Nanotechnology, University of Twente , P.O. Box 217, 7500 AE Enschede, The Netherlands
| | - J Ridderbos
- NanoElectronics Group, MESA Institute for Nanotechnology, University of Twente , P.O. Box 217, 7500 AE Enschede, The Netherlands
| | - T T Nguyen
- NanoElectronics Group, MESA Institute for Nanotechnology, University of Twente , P.O. Box 217, 7500 AE Enschede, The Netherlands
| | - M A Verheijen
- Kavli Institute of Nanoscience, Delft University of Technology , Lorentzweg 1, 2628 CJ Delft, The Netherlands
- Philips Innovation Services Eindhoven , High Tech Campus 11, 5656 AE Eindhoven, The Netherlands
| | - P M Koenraad
- Department of Applied Physics, TU Eindhoven , Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - F A Zwanenburg
- NanoElectronics Group, MESA Institute for Nanotechnology, University of Twente , P.O. Box 217, 7500 AE Enschede, The Netherlands
| | - E P A M Bakkers
- Kavli Institute of Nanoscience, Delft University of Technology , Lorentzweg 1, 2628 CJ Delft, The Netherlands
- Department of Applied Physics, TU Eindhoven , Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
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11
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Assali S, Dijkstra A, Li A, Koelling S, Verheijen MA, Gagliano L, von den Driesch N, Buca D, Koenraad PM, Haverkort JEM, Bakkers EPAM. Growth and Optical Properties of Direct Band Gap Ge/Ge 0.87Sn 0.13 Core/Shell Nanowire Arrays. Nano Lett 2017; 17:1538-1544. [PMID: 28165747 DOI: 10.1021/acs.nanolett.6b04627] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Group IV semiconductor optoelectronic devices are now possible by using strain-free direct band gap GeSn alloys grown on a Ge/Si virtual substrate with Sn contents above 9%. Here, we demonstrate the growth of Ge/GeSn core/shell nanowire arrays with Sn incorporation up to 13% and without the formation of Sn clusters. The nanowire geometry promotes strain relaxation in the Ge0.87Sn0.13 shell and limits the formation of structural defects. This results in room-temperature photoluminescence centered at 0.465 eV and enhanced absorption above 98%. Therefore, direct band gap GeSn grown in a nanowire geometry holds promise as a low-cost and high-efficiency material for photodetectors operating in the short-wave infrared and thermal imaging devices.
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Affiliation(s)
- S Assali
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
| | - A Dijkstra
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
| | - A Li
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
- Kavli Institute of Nanoscience, Delft University of Technology , 2600 GA Delft, The Netherlands
- Beijing Key Lab of Microstructure and Property of Advanced Materials, Beijing University of Technology , Pingleyuan 100, Beijing 100024, P. R. China
| | - S Koelling
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
| | - M A Verheijen
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
- Philips Innovation Laboratories Eindhoven , High Tech Campus 11, 5656AE Eindhoven, The Netherlands
| | - L Gagliano
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
| | - N von den Driesch
- Peter Gruenberg Institute 9 (PGI 9) and JARA-Fundamentals of Future Information Technologies , Forschungszentrum Juelich, 52428 Juelich, Germany
| | - D Buca
- Peter Gruenberg Institute 9 (PGI 9) and JARA-Fundamentals of Future Information Technologies , Forschungszentrum Juelich, 52428 Juelich, Germany
| | - P M Koenraad
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
| | - J E M Haverkort
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
| | - E P A M Bakkers
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
- Kavli Institute of Nanoscience, Delft University of Technology , 2600 GA Delft, The Netherlands
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12
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Koelling S, Li A, Cavalli A, Assali S, Car D, Gazibegovic S, Bakkers EPAM, Koenraad PM. Atom-by-Atom Analysis of Semiconductor Nanowires with Parts Per Million Sensitivity. Nano Lett 2017; 17:599-605. [PMID: 28002677 DOI: 10.1021/acs.nanolett.6b03109] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
The functionality of semiconductor devices is determined by the incorporation of dopants at concentrations down to the parts per million (ppm) level and below. Optimization of intentional and unintentional impurity doping relies on methods to detect and map the level of impurities. Detecting such low concentrations of impurities in nanostructures is however challenging to date as on the one hand methods used for macroscopic samples cannot be applied due to the inherent small volumes or faceted surfaces and on the other hand conventional microscopic analysis techniques are not sufficiently sensitive. Here, we show that we can detect and map impurities at the ppm level in semiconductor nanowires using atom probe tomography. We develop a method applicable to a wide variety of nanowires relevant for electronic and optical devices. We expect that it will contribute significantly to the further optimization of the synthesis of nanowires, nanostructures and devices based on these structures.
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Affiliation(s)
- S Koelling
- Photonics and Semiconductor Nanophysics, Eindhoven University of Technology , Eindhoven, 5600 MB, The Netherlands
| | - A Li
- Photonics and Semiconductor Nanophysics, Eindhoven University of Technology , Eindhoven, 5600 MB, The Netherlands
- Key Lab of Microstructure and Property of Advanced Materials, Beijing University of Technology , Beijing, 100024, China
| | - A Cavalli
- Photonics and Semiconductor Nanophysics, Eindhoven University of Technology , Eindhoven, 5600 MB, The Netherlands
| | - S Assali
- Photonics and Semiconductor Nanophysics, Eindhoven University of Technology , Eindhoven, 5600 MB, The Netherlands
| | - D Car
- Photonics and Semiconductor Nanophysics, Eindhoven University of Technology , Eindhoven, 5600 MB, The Netherlands
- Quantum Transport Group, Kavli Institute , Delft, 2628 CJ, The Netherlands
| | - S Gazibegovic
- Photonics and Semiconductor Nanophysics, Eindhoven University of Technology , Eindhoven, 5600 MB, The Netherlands
- Quantum Transport Group, Kavli Institute , Delft, 2628 CJ, The Netherlands
| | - E P A M Bakkers
- Photonics and Semiconductor Nanophysics, Eindhoven University of Technology , Eindhoven, 5600 MB, The Netherlands
- Quantum Transport Group, Kavli Institute , Delft, 2628 CJ, The Netherlands
| | - P M Koenraad
- Photonics and Semiconductor Nanophysics, Eindhoven University of Technology , Eindhoven, 5600 MB, The Netherlands
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13
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Gagliano L, Belabbes A, Albani M, Assali S, Verheijen MA, Miglio L, Bechstedt F, Haverkort JEM, Bakkers EPAM. Pseudodirect to Direct Compositional Crossover in Wurtzite GaP/In xGa 1-xP Core-Shell Nanowires. Nano Lett 2016; 16:7930-7936. [PMID: 27960532 DOI: 10.1021/acs.nanolett.6b04242] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Thanks to their uniqueness, nanowires allow the realization of novel semiconductor crystal structures with yet unexplored properties, which can be key to overcome current technological limits. Here we develop the growth of wurtzite GaP/InxGa1-xP core-shell nanowires with tunable indium concentration and optical emission in the visible region from 590 nm (2.1 eV) to 760 nm (1.6 eV). We demonstrate a pseudodirect (Γ8c-Γ9v) to direct (Γ7c-Γ9v) transition crossover through experimental and theoretical approach. Time resolved and temperature dependent photoluminescence measurements were used, which led to the observation of a steep change in carrier lifetime and temperature dependence by respectively one and 3 orders of magnitude in the range 0.28 ± 0.04 ≤ x ≤ 0.41 ± 0.04. Our work reveals the electronic properties of wurtzite InxGa1-xP.
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Affiliation(s)
- L Gagliano
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
| | - A Belabbes
- Physical Science and Engineering Division (PSE), King Abdullah University of Science and Technology (KAUST) , Thuwal 23955-6900, Saudi Arabia
- Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universitat , Max-Wien-Platz 1, D-07743 Jena, Germany
| | - M Albani
- L-NESS and Department of Materials Science, University of Milano Bicocca , 20125, Milano, Italy
| | - S Assali
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
| | - M A Verheijen
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
- Philips Innovation Laboratories Eindhoven , High Tech Campus 11, 5656AE Eindhoven, The Netherlands
| | - L Miglio
- L-NESS and Department of Materials Science, University of Milano Bicocca , 20125, Milano, Italy
| | - F Bechstedt
- Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universitat , Max-Wien-Platz 1, D-07743 Jena, Germany
| | - J E M Haverkort
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
| | - E P A M Bakkers
- Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands
- Kavli Institute of Nanoscience, Delft University of Technology , 2600 GA Delft, The Netherlands
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14
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Cui Y, van Dam D, Mann SA, van Hoof NJJ, van Veldhoven PJ, Garnett EC, Bakkers EPAM, Haverkort JEM. Boosting Solar Cell Photovoltage via Nanophotonic Engineering. Nano Lett 2016; 16:6467-6471. [PMID: 27607337 DOI: 10.1021/acs.nanolett.6b02971] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/20/2023]
Abstract
Approaching the theoretically limiting open circuit voltage (Voc) of solar cells is crucial to optimize their photovoltaic performance. Here, we demonstrate experimentally that nanostructured layers can achieve a fundamentally larger Fermi level splitting, and thus a larger Voc, than planar layers. By etching tapered nanowires from planar indium phosphide (InP), we directly compare planar and nanophotonic geometries with the exact same material quality. We show that the external radiative efficiency of the nanostructured layer at 1 sun is increased by a factor 14 compared to the planar layer, leading to a 70 mV enhancement in Voc. The higher voltage arises from both the enhanced outcoupling of photons, which promotes radiative recombination, and the lower active material volume, which reduces bulk recombination. These effects are generic and promise to enhance the efficiency of current record planar solar cells made from other materials as well.
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Affiliation(s)
- Y Cui
- Applied Physics, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands
| | - D van Dam
- Applied Physics, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands
| | - S A Mann
- Center for Nanophotonics, FOM Institute AMOLF , 1098 XG Amsterdam, The Netherlands
| | - N J J van Hoof
- Applied Physics, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands
| | - P J van Veldhoven
- Applied Physics, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands
| | - E C Garnett
- Center for Nanophotonics, FOM Institute AMOLF , 1098 XG Amsterdam, The Netherlands
| | - E P A M Bakkers
- Applied Physics, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands
- Kavli Institute of Nanoscience, Delft University of Technology , 2600 GA Delft, The Netherlands
| | - J E M Haverkort
- Applied Physics, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands
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15
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Greil J, Assali S, Isono Y, Belabbes A, Bechstedt F, Valega Mackenzie FO, Silov AY, Bakkers EPAM, Haverkort JEM. Optical Properties of Strained Wurtzite Gallium Phosphide Nanowires. Nano Lett 2016; 16:3703-3709. [PMID: 27175743 PMCID: PMC4901362 DOI: 10.1021/acs.nanolett.6b01038] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/10/2016] [Revised: 04/29/2016] [Indexed: 06/01/2023]
Abstract
Wurtzite gallium phosphide (WZ GaP) has been predicted to exhibit a direct bandgap in the green spectral range. Optical transitions, however, are only weakly allowed by the symmetry of the bands. While efficient luminescence has been experimentally shown, the nature of the transitions is not yet clear. Here we apply tensile strain up to 6% and investigate the evolution of the photoluminescence (PL) spectrum of WZ GaP nanowires (NWs). The pressure and polarization dependence of the emission together with a theoretical analysis of strain effects is employed to establish the nature and symmetry of the transitions. We identify the emission lines to be related to localized states with significant admixture of Γ7c symmetry and not exclusively related to the Γ8c conduction band minimum (CBM). The results emphasize the importance of strongly bound state-related emission in the pseudodirect semiconductor WZ GaP and contribute significantly to the understanding of the optoelectronic properties of this novel material.
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Affiliation(s)
- J. Greil
- Department of Applied
Physics, Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
| | - S. Assali
- Department of Applied
Physics, Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
| | - Y. Isono
- Department
of Mechanical Engineering, Kobe University, Kobe 657-8501, Japan
| | - A. Belabbes
- Institut für Festkörpertheorie
und -optik, Friedrich-Schiller-Universität, Max-Wien-Platz 1, 07743 Jena, Germany
- King Abdullah University of Science & Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
| | - F. Bechstedt
- Institut für Festkörpertheorie
und -optik, Friedrich-Schiller-Universität, Max-Wien-Platz 1, 07743 Jena, Germany
| | | | - A. Yu. Silov
- Department of Applied
Physics, Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
| | - E. P. A. M. Bakkers
- Department of Applied
Physics, Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
- Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA, Delft, The Netherlands
| | - J. E. M. Haverkort
- Department of Applied
Physics, Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
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16
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Assali S, Gagliano L, Oliveira DS, Verheijen MA, Plissard SR, Feiner LF, Bakkers EPAM. Exploring Crystal Phase Switching in GaP Nanowires. Nano Lett 2015; 15:8062-8069. [PMID: 26539748 DOI: 10.1021/acs.nanolett.5b03484] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
The growth of wurtzite/zincblende (WZ and ZB, respectively) superstructures opens new avenues for band structure engineering and holds the promise of digitally controlling the energy spectrum of quantum confined systems. Here, we study growth kinetics of pure and thus defect-free WZ/ZB homostructures in GaP nanowires with the aim to obtain monolayer control of the ZB and WZ segment lengths. We find that the Ga concentration and the supersaturation in the catalyst particle are the key parameters determining growth kinetics. These parameters can be tuned by the gallium partial pressure and the temperature. The formation of WZ and ZB can be understood with a model based on nucleation either at the triple phase line for the WZ phase or in the center of the solid-liquid interface for the ZB phase. Furthermore, the observed delay/offset time needed to induce WZ and ZB growth after growth of the other phase can be explained within this framework.
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Affiliation(s)
- S Assali
- Department of Applied Physics Eindhoven, University of Technology , 5600 MB Eindhoven, The Netherlands
| | - L Gagliano
- Department of Applied Physics Eindhoven, University of Technology , 5600 MB Eindhoven, The Netherlands
| | - D S Oliveira
- Instituto de Física "Gleb Wataghin", Universidade Estadual de Campinas, UNICAMP , 13083-859 Campinas, São Paulo, Brazil
| | - M A Verheijen
- Department of Applied Physics Eindhoven, University of Technology , 5600 MB Eindhoven, The Netherlands
- Philips Innovation Services Eindhoven , High Tech Campus 11, 5656AE Eindhoven, The Netherlands
| | - S R Plissard
- CNRS-Laboratoire d'Analyse et d'Architecture des Systèmes (LAAS), Université de Toulouse , 7 avenue du colonel Roche, F-31400 Toulouse, France
| | - L F Feiner
- Department of Applied Physics Eindhoven, University of Technology , 5600 MB Eindhoven, The Netherlands
| | - E P A M Bakkers
- Department of Applied Physics Eindhoven, University of Technology , 5600 MB Eindhoven, The Netherlands
- Kavli Institute of Nanoscience, Delft University of Technology , 2600 GA Delft, The Netherlands
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17
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Swinkels MY, van Delft MR, Oliveira DS, Cavalli A, Zardo I, van der Heijden RW, Bakkers EPAM. Diameter dependence of the thermal conductivity of InAs nanowires. Nanotechnology 2015; 26:385401. [PMID: 26329133 DOI: 10.1088/0957-4484/26/38/385401] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/17/2023]
Abstract
The diameter dependence of the thermal conductivity of InAs nanowires in the range of 40-1500 nm has been measured. We demonstrate a reduction in thermal conductivity of 80% for 40 nm nanowires, opening the way for further design strategies for nanoscaled thermoelectric materials. Furthermore, we investigate the effect of thermal contact in the most common measurement method for nanoscale thermal conductivity. Our study allows for the determination of the thermal contact using existing measurement setups. The thermal contact resistance is found to be comparable to the wire thermal resistance for wires with a diameter of 90 nm and higher.
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Affiliation(s)
- M Y Swinkels
- Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
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18
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de Lange G, van Heck B, Bruno A, van Woerkom DJ, Geresdi A, Plissard SR, Bakkers EPAM, Akhmerov AR, DiCarlo L. Realization of Microwave Quantum Circuits Using Hybrid Superconducting-Semiconducting Nanowire Josephson Elements. Phys Rev Lett 2015; 115:127002. [PMID: 26431010 DOI: 10.1103/physrevlett.115.127002] [Citation(s) in RCA: 53] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/30/2015] [Indexed: 06/05/2023]
Abstract
We report the realization of quantum microwave circuits using hybrid superconductor-semiconductor Josephson elements comprised of InAs nanowires contacted by NbTiN. Capacitively shunted single elements behave as transmon circuits with electrically tunable transition frequencies. Two-element circuits also exhibit transmonlike behavior near zero applied flux but behave as flux qubits at half the flux quantum, where nonsinusoidal current-phase relations in the elements produce a double-well Josephson potential. These hybrid Josephson elements are promising for applications requiring microwave superconducting circuits operating in a magnetic field.
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Affiliation(s)
- G de Lange
- QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
| | - B van Heck
- Instituut-Lorentz, Leiden University, 2300 RA Leiden, The Netherlands
| | - A Bruno
- QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
| | - D J van Woerkom
- QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
| | - A Geresdi
- QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
| | - S R Plissard
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
| | - E P A M Bakkers
- QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
| | - A R Akhmerov
- QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
| | - L DiCarlo
- QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
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19
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Conesa-Boj S, Hauge HIT, Verheijen MA, Assali S, Li A, Bakkers EPAM, Fontcuberta i Morral A. Cracking the Si Shell Growth in Hexagonal GaP-Si Core-Shell Nanowires. Nano Lett 2015; 15:2974-2979. [PMID: 25922878 DOI: 10.1021/nl504813e] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Semiconductor nanowires have increased the palette of possible heterostructures thanks to their more effective strain relaxation. Among these, core-shell heterostructures are much more sensitive to strain than axial ones. It is now accepted that the formation of misfit dislocations depends both on the lattice mismatch and relative dimensions of the core and the shell. Here, we show for the first time the existence of a new kind of defect in core-shell nanowires: cracks. These defects do not originate from a lattice mismatch (we demonstrate their appearance in an essentially zero-mismatch system) but from the thermal history during the growth of the nanowires. Crack defects lead to the development of secondary defects, such as type-I1 stacking faults and Frank-type dislocations. These results provide crucial information with important implications for the optimized synthesis of nanowire-based core-shell heterostructures.
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Affiliation(s)
- S Conesa-Boj
- †École Polytechnique Fédérale de Lausanne (EPFL), Laboratoire des Matériaux Semiconducteurs (LMSC), 1015 Lausanne, Switzerland
| | - H I T Hauge
- ‡Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - M A Verheijen
- ‡Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
- §Philips Innovation Services Eindhoven, High Tech Campus 11, 5656 AE Eindhoven, The Netherlands
| | - S Assali
- ‡Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - A Li
- ‡Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
| | - E P A M Bakkers
- ‡Department of Applied Physics, TU Eindhoven, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
- ∥Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
| | - A Fontcuberta i Morral
- †École Polytechnique Fédérale de Lausanne (EPFL), Laboratoire des Matériaux Semiconducteurs (LMSC), 1015 Lausanne, Switzerland
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20
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Assali S, Zardo I, Plissard S, Kriegner D, Verheijen MA, Bauer G, Meijerink A, Belabbes A, Bechstedt F, Haverkort JEM, Bakkers EPAM. Direct band gap wurtzite gallium phosphide nanowires. Nano Lett 2013; 13:1559-63. [PMID: 23464761 PMCID: PMC3624814 DOI: 10.1021/nl304723c] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/21/2012] [Revised: 02/20/2013] [Indexed: 05/27/2023]
Abstract
The main challenge for light-emitting diodes is to increase the efficiency in the green part of the spectrum. Gallium phosphide (GaP) with the normal cubic crystal structure has an indirect band gap, which severely limits the green emission efficiency. Band structure calculations have predicted a direct band gap for wurtzite GaP. Here, we report the fabrication of GaP nanowires with pure hexagonal crystal structure and demonstrate the direct nature of the band gap. We observe strong photoluminescence at a wavelength of 594 nm with short lifetime, typical for a direct band gap. Furthermore, by incorporation of aluminum or arsenic in the GaP nanowires, the emitted wavelength is tuned across an important range of the visible light spectrum (555-690 nm). This approach of crystal structure engineering enables new pathways to tailor materials properties enhancing the functionality.
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Affiliation(s)
- S. Assali
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven,
The Netherlands
| | - I. Zardo
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven,
The Netherlands
| | - S. Plissard
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven,
The Netherlands
| | - D. Kriegner
- Institute
of Semiconductor and
Solid State Physics, Johannes Kepler University Linz, Altenbergerstrasse 69, A-4040 Linz, Austria
| | - M. A. Verheijen
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven,
The Netherlands
- Philips Innovation
Services Eindhoven, High Tech Campus 11, 5656AE Eindhoven,
The Netherlands
| | - G. Bauer
- Institute
of Semiconductor and
Solid State Physics, Johannes Kepler University Linz, Altenbergerstrasse 69, A-4040 Linz, Austria
| | - A. Meijerink
- Debye Institute, Utrecht University, Princetonplein 1, 3500TA Utrecht,
The Netherlands
| | - A. Belabbes
- Institut
für Festkörpertheorie
und −optik, Friedrich Schiller Universität, 07743 Jena, Germany
| | - F. Bechstedt
- Institut
für Festkörpertheorie
und −optik, Friedrich Schiller Universität, 07743 Jena, Germany
| | - J. E. M. Haverkort
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven,
The Netherlands
| | - E. P. A. M. Bakkers
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven,
The Netherlands
- Kavli
Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The
Netherlands
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21
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Pribiag VS, Nadj-Perge S, Frolov SM, van den Berg JWG, van Weperen I, Plissard SR, Bakkers EPAM, Kouwenhoven LP. Electrical control of single hole spins in nanowire quantum dots. Nat Nanotechnol 2013; 8:170-174. [PMID: 23416794 DOI: 10.1038/nnano.2013.5] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/06/2012] [Accepted: 01/09/2013] [Indexed: 06/01/2023]
Abstract
The development of viable quantum computation devices will require the ability to preserve the coherence of quantum bits (qubits). Single electron spins in semiconductor quantum dots are a versatile platform for quantum information processing, but controlling decoherence remains a considerable challenge. Hole spins in III-V semiconductors have unique properties, such as a strong spin-orbit interaction and weak coupling to nuclear spins, and therefore, have the potential for enhanced spin control and longer coherence times. A weaker hyperfine interaction has previously been reported in self-assembled quantum dots using quantum optics techniques, but the development of hole-spin-based electronic devices in conventional III-V heterostructures has been limited by fabrication challenges. Here, we show that gate-tunable hole quantum dots can be formed in InSb nanowires and used to demonstrate Pauli spin blockade and electrical control of single hole spins. The devices are fully tunable between hole and electron quantum dots, which allows the hyperfine interaction strengths, g-factors and spin blockade anisotropies to be compared directly in the two regimes.
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Affiliation(s)
- V S Pribiag
- Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands.
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van den Berg JWG, Nadj-Perge S, Pribiag VS, Plissard SR, Bakkers EPAM, Frolov SM, Kouwenhoven LP. Fast spin-orbit qubit in an indium antimonide nanowire. Phys Rev Lett 2013; 110:066806. [PMID: 23432291 DOI: 10.1103/physrevlett.110.066806] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/13/2012] [Indexed: 05/22/2023]
Abstract
Because of the strong spin-orbit interaction in indium antimonide, orbital motion and spin are no longer separated. This enables fast manipulation of qubit states by means of microwave electric fields. We report Rabi oscillation frequencies exceeding 100 MHz for spin-orbit qubits in InSb nanowires. Individual qubits can be selectively addressed due to intrinsic differences in their g factors. Based on Ramsey fringe measurements, we extract a coherence time T(2)(*)=8±1 ns at a driving frequency of 18.65 GHz. Applying a Hahn echo sequence extends this coherence time to 34 ns.
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Affiliation(s)
- J W G van den Berg
- Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
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23
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Standing AJ, Assali S, Haverkort JEM, Bakkers EPAM. High yield transfer of ordered nanowire arrays into transparent flexible polymer films. Nanotechnology 2012; 23:495305. [PMID: 23154816 DOI: 10.1088/0957-4484/23/49/495305] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
The factors affecting transfer of nanowire arrays from their substrates into flexible PDMS films have been systematically investigated. Experiments were carried out on gallium phosphide nanowires with a standard length of 10 μm with varying pitch (0.2-1.5 μm). The important factors were found to be penetration of the PDMS within the nanowire arrays and the strength/rigidity of the PDMS film. The PDMS penetration between wires in the arrays is affected by both the viscosity of the PDMS solution and the presence of air pockets trapped within nanowire arrays, particularly at small pitches. Dilution with hexane and curing in a vacuum desiccator solve the wire penetration problem, and an increase in cure/base ratio increases the rigidity and strength of the PDMS. The procedures for preparation and deposition of the PDMS solution are optimized and a high yield, up to 95%, of wire transfer across a range of nanowire pitches has been obtained.
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Affiliation(s)
- A J Standing
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands.
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24
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Frolov SM, Danon J, Nadj-Perge S, Zuo K, van Tilburg JWW, Pribiag VS, van den Berg JWG, Bakkers EPAM, Kouwenhoven LP. Suppression of Zeeman gradients by nuclear polarization in double quantum dots. Phys Rev Lett 2012; 109:236805. [PMID: 23368241 DOI: 10.1103/physrevlett.109.236805] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/07/2012] [Indexed: 06/01/2023]
Abstract
We use electric dipole spin resonance to measure dynamic nuclear polarization in InAs nanowire quantum dots. The resonance shifts in frequency when the system transitions between metastable high and low current states, indicating the presence of nuclear polarization. We propose that the low and the high current states correspond to different total Zeeman energy gradients between the two quantum dots. In the low current state, dynamic nuclear polarization efficiently compensates the Zeeman gradient due to the g-factor mismatch, resulting in a suppressed total Zeeman gradient. We present a theoretical model of electron-nuclear feedback that demonstrates a fixed point in nuclear polarization for nearly equal Zeeman splittings in the two dots and predicts a narrowed hyperfine gradient distribution.
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Affiliation(s)
- S M Frolov
- Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
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25
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Nadj-Perge S, Pribiag VS, van den Berg JWG, Zuo K, Plissard SR, Bakkers EPAM, Frolov SM, Kouwenhoven LP. Spectroscopy of spin-orbit quantum bits in indium antimonide nanowires. Phys Rev Lett 2012; 108:166801. [PMID: 22680747 DOI: 10.1103/physrevlett.108.166801] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/09/2012] [Indexed: 06/01/2023]
Abstract
A double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire. The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlet-triplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlet-triplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.
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Affiliation(s)
- S Nadj-Perge
- Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
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26
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Mourik V, Zuo K, Frolov SM, Plissard SR, Bakkers EPAM, Kouwenhoven LP. Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices. Science 2012; 336:1003-7. [PMID: 22499805 DOI: 10.1126/science.1222360] [Citation(s) in RCA: 902] [Impact Index Per Article: 75.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Majorana fermions are particles identical to their own antiparticles. They have been theoretically predicted to exist in topological superconductors. Here, we report electrical measurements on indium antimonide nanowires contacted with one normal (gold) and one superconducting (niobium titanium nitride) electrode. Gate voltages vary electron density and define a tunnel barrier between normal and superconducting contacts. In the presence of magnetic fields on the order of 100 millitesla, we observe bound, midgap states at zero bias voltage. These bound states remain fixed to zero bias, even when magnetic fields and gate voltages are changed over considerable ranges. Our observations support the hypothesis of Majorana fermions in nanowires coupled to superconductors.
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Affiliation(s)
- V Mourik
- Kavli Institute of Nanoscience, Delft University of Technology, Delft, Netherlands
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27
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Nadj-Perge S, Frolov SM, Bakkers EPAM, Kouwenhoven LP. Spin–orbit qubit in a semiconductor nanowire. Nature 2010; 468:1084-7. [DOI: 10.1038/nature09682] [Citation(s) in RCA: 532] [Impact Index Per Article: 38.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2010] [Accepted: 11/10/2010] [Indexed: 11/09/2022]
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28
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Stiegler JM, Huber AJ, Diedenhofen SL, Rivas JG, Algra RE, Bakkers EPAM, Hillenbrand R. Nanoscale free-carrier profiling of individual semiconductor nanowires by infrared near-field nanoscopy. Nano Lett 2010; 10:1387-1392. [PMID: 20302289 DOI: 10.1021/nl100145d] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
We report quantitative, noninvasive and nanoscale-resolved mapping of the free-carrier distribution in InP nanowires with doping modulation along the axial and radial directions, by employing infrared near-field nanoscopy. Owing to the technique's capability of subsurface probing, we provide direct experimental evidence that dopants in interior nanowire shells effectively contribute to the local free-carrier concentration. The high sensitivity of s-SNOM also allows us to directly visualize nanoscale variations in the free-carrier concentration of wires as thin as 20 nm, which we attribute to local growth defects. Our results open interesting avenues for studying local conductivity in complex nanowire heterostructures, which could be further enhanced by near-field infrared nanotomography.
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Affiliation(s)
- J M Stiegler
- CIC nanoGUNE Consolider, 20018 Donostia-San Sebastian Spain
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29
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Zwanenburg FA, van der Mast DW, Heersche HB, Kouwenhoven LP, Bakkers EPAM. Electric field control of magnetoresistance in InP nanowires with ferromagnetic contacts. Nano Lett 2009; 9:2704-2709. [PMID: 19537736 DOI: 10.1021/nl901184m] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
We demonstrate electric field control of sign and magnitude of the magnetoresistance in InP nanowires with ferromagnetic contacts. The sign change in the magnetoresistance is directly correlated with a sign change in the transconductance. Additionally, the magnetoresistance is shown to persist at such a high bias that Coulomb blockade has been lifted. We also observe the magnetoresistance when one of the ferromagnets is replaced by a nonmagnetic metal. We conclude that it must be induced by a single ferromagnetic contact, and that spin transport can be ruled out as the origin. Our results emphasize the importance of a systematic investigation of spin-valve devices in order to discriminate between ambiguous interpretations.
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Affiliation(s)
- F A Zwanenburg
- Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
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30
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Muskens OL, Treffers J, Forcales M, Borgström MT, Bakkers EPAM, Rivas JG. Modification of the photoluminescence anisotropy of semiconductor nanowires by coupling to surface plasmon polaritons. Opt Lett 2007; 32:2097-9. [PMID: 17671548 DOI: 10.1364/ol.32.002097] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
Abstract
We demonstrate efficient modification of the polarized light emission from single semiconductor nanowires by coupling this emission to surface plasmon polaritons on a metal grating. The polarization anisotropy of the emitted photoluminescence from single nanowires is compared for wires deposited on silica, a flat gold film, and a shallow gold grating. By varying the orientation of the nanowire with respect to the grating grooves, the large intrinsic polarization anisotropy can be either suppressed or enhanced. This modification is interpreted by the appearance of an additional emission channel induced by surface plasmon polaritons and their conversion to p-polarized radiation at the grating.
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Affiliation(s)
- O L Muskens
- FOM Institute for Atomic and Molecular Physics AMOLF, c/o Philips Research Laboratories, High Tech Campus 4, 5656 AE, Eindhoven, The Netherlands
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31
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Li HY, Wunnicke O, Borgström MT, Immink WGG, van Weert MHM, Verheijen MA, Bakkers EPAM. Remote p-doping of InAs nanowires. Nano Lett 2007; 7:1144-8. [PMID: 17425372 DOI: 10.1021/nl0627487] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/14/2023]
Abstract
We report on remote p-type doping of InAs nanowires by a p-doped InP shell grown epitaxially on the core nanowire. This approach addresses the challenge of obtaining quantitative control of doping levels in nanowires grown by the vapor-liquid-solid (VLS) mechanism. Remote doping of III-V nanowires is demonstrated here with the InAs/InP system. It is especially challenging to make p-type InAs wires because of Fermi level pinning around 0.1 eV above the conduction band. We demonstrate that shielding with a p-doped InP shell compensates for the built-in potential and donates free holes to the InAs core. Moreover, the off-current in field-effect devices can be reduced up to 6 orders of magnitude. The effect of shielding critically depends on the thickness of the InP capping layer and the dopant concentration in the shell.
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Affiliation(s)
- H-Y Li
- Philips Research Laboratories, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands
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Bakkers EPAM, Roest AL, Marsman AW, Jenneskens LW, de Jong-van Steensel LI, Kelly JJ, Vanmaekelbergh D. Characterization of Photoinduced Electron Tunneling in Gold/SAM/Q-CdSe Systems by Time-Resolved Photoelectrochemistry. J Phys Chem B 2000. [DOI: 10.1021/jp000286u] [Citation(s) in RCA: 70] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- E. P. A. M. Bakkers
- Departments of Chemistry and Physics of Condensed Matter, Physical Organic Chemistry, and Interfaces Chemistry, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
| | - A. L. Roest
- Departments of Chemistry and Physics of Condensed Matter, Physical Organic Chemistry, and Interfaces Chemistry, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
| | - A. W. Marsman
- Departments of Chemistry and Physics of Condensed Matter, Physical Organic Chemistry, and Interfaces Chemistry, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
| | - L. W. Jenneskens
- Departments of Chemistry and Physics of Condensed Matter, Physical Organic Chemistry, and Interfaces Chemistry, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
| | - L. I. de Jong-van Steensel
- Departments of Chemistry and Physics of Condensed Matter, Physical Organic Chemistry, and Interfaces Chemistry, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
| | - J. J. Kelly
- Departments of Chemistry and Physics of Condensed Matter, Physical Organic Chemistry, and Interfaces Chemistry, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
| | - D. Vanmaekelbergh
- Departments of Chemistry and Physics of Condensed Matter, Physical Organic Chemistry, and Interfaces Chemistry, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
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33
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Affiliation(s)
- E. P. A. M. Bakkers
- Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
| | - E. Reitsma
- Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
| | - J. J. Kelly
- Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
| | - D. Vanmaekelbergh
- Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
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