PTB has for the first time
made graphene visible on gallium arsenide - A successful combination of two
unique electronic materials.
 | | The normally practically invisible single-carbon-atom layers can be made visible under a normal light (optical) microscope, if the support (layer) is designed as an anti-reflection filter. Single-layer graphene was identified inside the markings. (Image: PTB)
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It is the marriage of two top candidates for the electronics of the future,
both excentric and extremely interesting: Graphene, one of the partners, is
an extremely thin fellow and besides, very young. Not until 2004 was it possible
to specifically produce and investigate the single layer of carbon atoms. Its
electronic properties are remarkable, because, among other things, its electrons
can move so tremendously fast. It is a perfect partner for gallium arsenide,
the semiconductor that allows tailoring of its electrical properties and which
is the starting material of the fastest electrical and opto-electronic components.
Besides, it is possible to produce gallium arsenide with an atomic-layer-smooth
surface; this should suit well as a support for graphene. Scientists of the
Physikalisch-Technische Bundesanstalt (PTB) have now, with the aid of a special
design, succeeded in making graphene visible on gallium arsenide. Previously
it has only been possible on silicon oxide. Now that they are able to view with
a light optical microscope the graphene layer, which is thinner than one thousandth
of a light wavelength, the researchers want to measure the electrical properties
of their new material combination. As experts for precision measurements, the
PTB physicists are thus especially well equipped to do this.
They use the principle of the anti-reflective layer: If on a material one superimposes
a very thin, nearly transparent layer of another material, then the reflectivity
of the lower layer changes clearly visibly. In order to make their lower layer
of gallium arsenide (plus graphene atomic layer) visible, the PTB physicists
chose aluminium arsenide (AlAs). However, it is so similar to gallium arsenide
(GaAs) in its optical properties that they had to employ a few tricks: They
vapour-coated not only one, but rather several wafer-thin layers. "Thus,
even with optically similar materials it is possible, in a manner of speaking,
to 'grow' interference effects", Dr. Franz-Josef Ahlers, the responsible
department head at PTB, explained. "This principle is known from optical
interference filters. We have adapted it for our purposes".
First of all, he and his colleagues calculated the optical properties of different
GaAs and AlAs layers and optimized the layer sequence such that they could expect
a sufficiently good detectability of graphene. Following this recipe, they got
down to action and with the molecular beam epitaxial facility of PTB accurately
produced a corresponding GaAs/AlAs crystal atom layer. Then in the same procedure
as with silicon oxide, it was overlaid with graphite fragments. "Different
from silicon but as predicted by the calculation, although single carbon layers
are no longer visible at all wavelengths of visible light, it is, however, possible,
e.g. with a simple green filter, to limit the wavelength range such that the
graphene is easily visible", explained Ahlers.
In the photo, all lighter areas of the dark GaAs are covered with graphene.
From the degree of lightening it is possible to conclude the number of individual
layers of atoms. The marked areas are 'real', that is, single-layer graphene.
But next to them, there are also two, three and multiple layers of carbon atoms,
which also have interesting properties. This arrangement was confirmed again
with another method, Raman spectroscopy.
Following such a simple identification with a normal light optical microscope,
the further steps in the manufacture of electrical components from graphene
surfaces are now possible without any difficulty. Thus the PTB scientists can
now begin to accurately measure the electrical properties of the new material
combination.
Posted September 16th, 2009
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