6,607 research outputs found
Local oxidation of Ga[Al]As heterostructures with modulated tip-sample voltages
Nanolithography based on local oxidation with a scanning force microscope has
been performed on an undoped GaAs wafer and a Ga[Al]As heterostructure with an
undoped GaAs cap layer and a shallow two-dimensional electron gas. The oxide
growth and the resulting electronic properties of the patterned structures are
compared for constant and modulated voltage applied to the conductive tip of
the scanning force microscope. All the lithography has been performed in
non-contact mode. Modulating the applied voltage enhances the aspect ratio of
the oxide lines, which significantly strengthens the insulating properties of
the lines on GaAs. In addition, the oxidation process is found to be more
reliable and reproducible. Using this technique, a quantum point contact and a
quantum wire have been defined and the electronic stability, the confinement
potential and the electrical tunability are demonstrated to be similar to the
oxidation with constant voltage.Comment: 7 pages, 7 figures, accepted by J. Appl. Phy
Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene
We present Raman spectroscopy measurements on single- and few-layer graphene
flakes. Using a scanning confocal approach we collect spectral data with
spatial resolution, which allows us to directly compare Raman images with
scanning force micrographs. Single-layer graphene can be distinguished from
double- and few-layer by the width of the D' line: the single peak for
single-layer graphene splits into different peaks for the double-layer. These
findings are explained using the double-resonant Raman model based on ab-initio
calculations of the electronic structure and of the phonon dispersion. We
investigate the D line intensity and find no defects within the flake. A finite
D line response originating from the edges can be attributed either to defects
or to the breakdown of translational symmetry
Raman imaging of doping domains in graphene on SiO2
We present spatially resolved Raman images of the G and 2D lines of
single-layer graphene flakes. The spatial fluctuations of G and 2D lines are
correlated and are thus shown to be affiliated with local doping domains. We
investigate the position of the 2D line -- the most significant Raman peak to
identify single-layer graphene -- as a function of charging up to |n|~4 10^12
cm^-2. Contrary to the G line which exhibits a strong and symmetric stiffening
with respect to electron and hole-doping, the 2D line shows a weak and slightly
asymmetric stiffening for low doping. Additionally, the line width of the 2D
line is, in contrast to the G line, doping-independent making this quantity a
reliable measure for identifying single-layer graphene
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