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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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NPEG - NanoPatterned Epitaxial Graphene

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.

 

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.

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.
 
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.

 
 
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.

 

Sample Patterning:

Shown below are the stages that SiC samples undergo during production
If it can be drawn, it can be patterened. Examples of our sub-micron lithographic methods are shown below.
  

Sample Packaging:

A Diced Wafer of Sic (upper left), SiC sample during Graphitization (bottom), SiC sample preparred for transport measurements in a cryostat.
Electrostatic Force Micrsoscope Picture of a patterened sample, with various potentials
  
  

Transport Measurements:

The Magnetoconductance plot of a Hall Bar Structure
Magnetoresistance vs. B-Field plot, showing our average sample characteristics.
  
  
Shubnikov de Haas oscillations, illustrating the Quantum Transport ability of our samples.
Coherent transport and reproducible fluctuations consistantly reinforce our production methods.

Conductance as a function of gate voltage for a sample at 4K. The top gate is only partially effective due to the open geometry. Nevertheless, a 2% change in conductance is observed.

  
  
See our publications page for more information related to this research. This project was funded by a grant from NIRT.



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