It is the marriage of two top candidates for the electronics
of the future, both eccentric 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".
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. Credit: (PTB)
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.
The original publication: Graphene on Gallium Arsenide:
Engineering the visibility. M. Friedemann, K. Pierz, R. Stosch, F. J. Ahlers. Applied
Physics Letters, Appl. Phys. Lett. 95, DOI: 10.1063/1.3224910, http://link.aip.org/link/?APL/95/102103
Physikalisch-Technische
Bundesanstalt (PTB)
