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You can combine two or more materials in a nanowire (NW) to form a
NW-heterostructure.
Most generally, both materials will have
different lattice parameters. When they grow
one on top of the other, the lattice deforms. The
resulting deformation will depend on the
geometry of the heterostructures.
For GaN/AlN heterostructures the differences are
explained in the following figure:
Raman scattering is an optical technique that provides information
about the frequency of the
vibrational modes of the material. Since the frequency of
the modes change with strain,
Raman scattering results may provide information of strain in NW heterostructures.
In this example Raman scattering experiments
were performed on GaN/AlN nanowire
radial heterostructures. It
was possible to
distinguish between heterostructures
under elastic
and plastic deformation.
For this experiment the samples, consisting of
GaN/AlN axial NW-heterostructures grown on silicon
By MBE at CEA (Grenoble) were illuminated with
a 515 nm line Ar+
laser. A 100x microscope objective
was used to focus the light on the sample and
collect
the scattered light to the spectrometer.
In the case of core-shell NW heterostructures,
stran is only important along the NW axis. Under
these
conditions Raman scattering provides numerical
information of strain in the samples.
 Here left
we can see the evolution of the E2h phonon
Mode of GaN as
the AlN shell thickness increases. The
mode indicates that tensile strain
increases in the NWs
until a shell thickness of 3 nm. After that
strain is
partially relaxed. Transmission Electron
Micrographs
showed that this relaxation correlates
with nanowires
that are bent due to the anisotropic
deposition of the
AlN shell around the GaN
core.
K. Hestroffer
et al., Nanotechnology 21,415702 (2010).
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