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NPEG - NanoPatterned Epitaxial Graphene
Originally posted on sciy.org by Ron Anastasia on Fri 25 May 2007 11:43 AM PDT
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Graphene
Clusters
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Currently Under Construction 3/8/06
Motivation:
There has been much excitement in recent
years over the properties of carbon nanotubes. Carbon nanotubes are
essentially a single sheet of graphite (graphene) rolled up to form a
tube. Nanotubes are found metallic or semiconducting depending on the
orientation of the rolling up. Metallic nanotubes
display quantized ballistic conduction at room temperature, that is
there is essentially no scattering for electrons propagating along the
tube on micrometer length, while a resistance is present a each
metal-nanotube interface with a theoretical minimum value of 6kOhm. The
electronic band gap of the the semiconducting nanotubes varies
approximately as the inverse of the nanotube diameter and their
conductance can be controlled by applying an electrostatic gate. Simple
nanonotube transistors and interconnected logic gates have been
demonstrated. These exceptional properties makes carbon nanotubes an
attractive material candidate for applications in electronics where the
limitations of conventional Si-based devices is foreseen to impede the
exponential growth in computing power. However nanotube-based
electronics faces challenges for large scale integration with the
questions of metallic vs.
semiconductiong nanotube selection, positionning and the metal-nanotube
high quantum resistance contact. In fact it was recognized early on the electronic properties
of nanotubes stem from the properties of a single graphene layer and
its unusual band structure. Graphene is a zero gap semi-conductor with
only two bands crossing at the Fermi level. The particularity comes
from their linear dispersion relation (so called Dirac particle) :
the energy is proportional to momentum whereas in a normal system
energy rises as the square of the momentum. It is therefore anticipated
(and it has been shown theoretically) that most of the electronic
properties of nanotubes are shared by other low-dimensional graphitic
structures.
In particular theoretically studies show that graphene ribbons may be
either metallic or semiconducting depending on the crystallographic
direction of the ribbon axis. Similarly to nanotubes, the
semiconducting band gap is determined by the ribbon width, with similar
size dependent magnitude. Because of the similarities in the band structures we expect
that patterned graphene also will have nanotube-like transport
properties, which includes coherent transport, ballistic transport at
room temperature, and high current capabilities.
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From K. Nakada, M. Fujita, G. Dresselhaus,
and M. Dresselhaus, Phys Rev. B 54, 17954 (1996)
| Differences in Graphene Ribbon production, determined by crystallographic orientation.
Our goal is to investigate the fundamental properties of graphene layers as a coherent bi-dimensional electron gas and
demonstrate the capability of graphene as a material for electronics. Since the begining of this project (2001),
we have recognized the potential of nanographitic objects and
have developped a traditional top-bottom approach closely related to the current silicon based
nanoelectronics so that the road map toward large-scale integration is essentially built-in.
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Epitaxially Grown Graphene Layers: |
AFM images of the surface of Silicon Carbide before and after hydrogen etching. |
The graphene layers are grown epitaxially on single crystal Silicon Carbide substrate.
A pass of hydrogen etching dramatically improves the surface quality
making it suitable for growth of graphene layers by thermal desorbtion
of the Silicon.
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Thin layers of graphene can be patterned using standard
lithographic methods. As a demonstration, we fabricated a first attempt
at a NPEG based field effect transistor. The pattern and an AFM image of
the device is shown below.
The gates of the device are made from a conductive coating applied to a 100nm
thick aluminum oxide layer. The conductance as a function of the gate
voltage is shown below. While the leakage for this "device" is large - it
clearly demonstrates the potential of NPEG for use in electronics.
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| A major difference between planar graphene ribbons and is the presence of
dangling bonds on a graphitic ribbon (which are closed in a nanotube).
Normally these bonds are terminated with hydrogen, but we envision
to bind donor / acceptor atoms to them providing a way to modify the
electronic properties of the graphene ribbon.
Low Energy Electron Diffraction (LEED) patterns show the epitaxial
growth of graphene on SiC, for successively higher temperature in UHV.
The graphene diffraction spots are rotated 30 degree with the SiC
spots. The satellites are due to surface reconstruction. |
Shown below are the stages that SiC samples undergo during production
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If it can be drawn, it can be patterened. Examples of our sub-micron lithographic methods are shown below.
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A Diced Wafer of Sic (upper left), SiC sample during Graphitization (bottom),
SiC sample preparred for transport measurements in a cryostat.
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Electrostatic Force Micrsoscope Picture of a patterened sample, with various potentials
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See our publications page for more information
related to this research. This project was funded by a grant from NIRT.
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