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Core/Shell Nanowires

Semiconductor nanowires can consist of different materials.

However, in most cases semiconductor nanowire are made of only a single material, e.g. InAs. Here, the transport properties can be tuned by means of doping. However, by making use of axial or radial heterostructures the nanowire properties can be tailored in a much more versatile fashion. By inserting two semiconductor barriers with a larger band gap in axial direction quantum dot structures can be realized [1]. By covering the nanowire radially by a higher band gap material the surface can be passivated and the surface scattering of the carrier can be reduced [2].

Schematic illustration of a core/shell nanowireSchematic illustration of a core/shell nanowire. The magnetic field (arrow) is oriented along the nanowire axis.
Source: Sebastian Heedt, PGI-9

In Jülich nanowires comprising a radial heterostructure are grown by means of molecular beam epitaxy and metal organic vapor phase epitaxy. The first type of structure can be viewed as a rolled up 2-dimensional electron gas in an AlGaAs/GaAs heterostructure [3]. Here, the GaAs core is surrounded by an AlGaAs barrier layer. The carriers in the GaAs core are supplied by a dopant layer in the AlGaAs shell.

In a different concept of the isolating GaAs core is covered by a conductive InAs shell [4]. Here, the GaAs core can even be removed completely by means of selective wet chemical etching [5].

InAs Shell of a Core/Shell-NanowireElectron microscopy image of nanowires where the GaAs core was removed by wet chemical etching.
Source: Fabian Haas, PGI-9



On the GaAs/InAs core/shell nanowires magnetotransport measurements were performed [6]. By combining the low band gap shell material with a large band gap core material a tube-like conductor is formed. If at low temperatures a magnetic field is applied along the wire axis very regular oscillations are observed. These oscillations originate from electron interference effects. Particularly at low temperatures the wave properties of electrons are visible. The superposition of these electrons waves lead to interference phenomena, i.e. an enhancement (constructive interference) or a cancelation (destructive interference). By applying a magnetic field the interference pattern measured by the conductance can be shifted periodically.
Electron interference effects might be used for switching purpose in future nano-scaled devices. An advantage would be the superior energy efficiency compared to conventional devices.

GaAs/InAs Core/Shell NanowireGaAs/InAs Core/Shell nanowire grown by molecular beam epitaxy
Source: Torsten Rieger, Dr. Mihail Ion Lepsa, PGI-9

Magnetoconductance Oscillations Magnetoconductance Oscillations: The conductance is plotted in units of the conductance quantum. The oscillation period corresponds to a magnetic flux quantum.
Source: Önder Gül, PGI-9

References

[1] Thelander, C.; Agarwal, P.; Brongersma, S.; Eymery, J.; Feiner, L.; Forchel, A.; Scheffler, M.; Riess, W.; Ohlsson, B.; Gösele, U. & Samuelson, L.
Nanowire-based one-dimensional electronics
Materials Today, 2006, 9, 28-35

[2] van Tilburg, J. W. W.; Algra, R. E.; Immink, W. G. G.; Verheijen, M.; Bakkers, E. P. A. M. & Kouwenhoven, L. P.
Surface passivated InAs/InP core/shell nanowires
Semiconductor Science and Technology, 2010, 25, 024011

[3] S. Wirths, M. Mikulics, P. Heintzmann, A. Winden, K. Weis, Ch. Volk, K. Sladek, N. Demarina, H. Hardtdegen, D. Grützmacher, Th. Schäpers,
Preparation of Ohmic contacts to GaAs/AlGaAs-core/shell-nanowires
Applied Physics Letters, 100 (2012) 4, 042103

[4] T. Rieger, M. Luysberg, Th. Schäpers, D. Grützmacher, and M. I. Lepsa,
Molecular Beam Epitaxy Growth of GaAs/InAs Core−Shell Nanowires and Fabrication of InAs Nanotubes
Nano Lett. 12 (2012) 5559−5564 (dx.doi.org/10.1021/nl302502b)

[5] F. Haas, K. Sladek, A. Winden, M. von der Ahe, T. E. Weirich, T. Rieger, H. Lüth, D Grützmacher, Th Schäpers and H Hardtdegen,
Nanoimprint and selective-area MOVPE for growth of GaAs/InAs core/shell nanowires,
Nanotechnology 24 (2013) 085603 (http://dx.doi.org/10.1088/0957-4484/24/8/085603)

[6] C. Blömers, T. Rieger, P. Zellekens, F. Haas, M. I. Lepsa, H. Hardtdegen, Ö, Gül, N. Demarina, D Grützmacher, H Lüth, and Th Schäpers,
Realization of nanoscaled tubular conductors by means of GaAs/InAs core/shell nanowires
Nanotechnology 24 (2013) 035203 (http://dx.doi.org/10.1088/0957-4484/24/3/035203)


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