24 research outputs found
Real structure of lattice matched GaAs-Fe3Si core-shell nanowires
GaAs nanowires and GaAs-Fe3Si core-shell nanowire structures were grown by
molecular-beam epitaxy on oxidized Si(111) substrates and characterized by
transmission electron microscopy (TEM) and X-ray diffraction (XRD). Ga droplets
were formed on the oxide surface, and the semiconducting GaAs nanowires grew
epitaxially via the vapor-liquid-solid mechanism as single-crystals from holes
in the oxide film. We observed two stages of growth of the GaAs nanowires,
first the regular growth and second the residual growth after the Ga supply was
finished. The magnetic Fe3Si shells were deposited in an As-free chamber. They
completely cover the GaAs cores although they consist of small grains.
High-resolution TEM micrographs depict the differently oriented grains in the
Fe3Si shells. Selected area diffraction of electrons and XRD gave further
evidence that the shells are textured and not single crystals. Facetting of the
shells was observed, which lead to thickness inhomogeneities of the shells.Comment: 15 pages, 8 figure
Magnetic properties of GaAs-Fe3Si core-shell nanowires — A comparison of ensemble and single nanowire investigation
On the basis of semiconductor-ferromagnet GaAs-Fe3Si core-shell nanowires (Nws) we compare the facilities of magnetic Nw ensemble measurements by superconducting quantum interference device magnetometry versus investigations on single Nws by magnetic force microscopy and computational micromagnetic modeling. Where a careful analysis of ensemble measurements backed up by transmission electron microscopy gave no insights on the properties of the Nw shells, single Nw investigation turned out to be absolutely essential
Diffraction at GaAs/FeSi core/shell nanowires: the formation of nanofacets
GaAs/FeSi core/shell nanowire structures were fabricated by
molecular-beam epitaxy on oxidized Si(111) substrates and investigated by
synchrotron x-ray diffraction. The surfaces of the FeSi shells exhibit
nanofacets. These facets consist of well pronounced FeSi{111} planes.
Density functional theory reveals that the Si-terminated FeSi{111} surface
has the lowest energy in agreement with the experimental findings. We can
analyze the x-ray diffuse scattering and diffraction of the ensemble of
nanowires avoiding the signal of the substrate and poly-crystalline films
located between the wires. FeSi nanofacets cause streaks in the x-ray
reciprocal space map rotated by an azimuthal angle of 30{\deg} compared with
those of bare GaAs nanowires. In the corresponding TEM micrograph the facets
are revealed only if the incident electron beam is oriented along
[10] in accordance with the x-ray results. Additional maxima in
the x-ray scans indicate the onset of chemical reactions between FeSi
shells and GaAs cores occurring at increased growth temperatures.Comment: 15 pages, 5 figure
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In Situ Transmission Electron Microscopy of Disorder–Order Transition in Epitaxially Stabilized FeGe2
Isothermal crystallization of amorphous Ge deposited on a cubic Fe3Si/GaAs(001) substrate is performed by in situ annealing within a transmission electron microscope. It was found that the formation of epitaxially aligned tetragonal FeGe2 is associated with a disorder–order phase transition mainly consisting of a rearrangement of the Fe/vacancy sublattice from a random distribution to alternating filled and empty layers. Additionally, atomically resolved high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy demonstrated that the vertical lattice spacing of the Ge sublattice reduces across vacancy layers, indicating that strain minimization plays a role in the phase transition process. Crystallization and ordering are both found to proceed layer-by-layer and with square-root-shaped kinetics with a smaller transition rate for the latter