What is Epitaxial Graphene?
Epitaxial graphene is basically a single layer of graphite, a hexagonal array of carbon atoms extending over two dimensions endlessly; a two-dimensional honeycomb arrangement of carbon (sp2) atoms which is analogous to the layered structure of graphite.
Graphene in Layman's Terms:
Graphene has been known for decades in many forms (exfoliated, epitaxial, isolated) and a number of its properties were measured or inferred from related materials, like graphite and carbon nanotubes. Yet, only recently was its potential as an electronic material recognized. Epitaxial graphene on silicon carbide has played a pivotal role in this development: it was the first to be proposed as a platform for graphene-based electronics; the first measurements on graphene monolayers were made on epitaxial graphene; and the graphene-electronic band structure was first measured on EG. The epitaxial graphene program, initiated in 2001 at the Georgia Institute of Technology, has spearheaded graphene-based electronics and developed methods to produce electronic grade epitaxial graphene. The Georgia Tech program demonstrated many of graphene's fundamental and technologically important properties, including coherence and quantum confinement effects,chemical modification, electrostatic gating and large-scale integration. Currently, epitaxial graphene stands at the forefront of materials that may succeed (not replace!) silicon. In contrast to other candidate graphene-based materials, epitaxial graphene is produced in a simple, high-temperature annealing step on single-crystal silicon carbide, which itself is an important electronic material. Subsequent epitaxial graphene processing is straightforward and compatible with microelectronics procedures.
Why Epitaxial Graphene?
Silicon has been the material of choice for electronic devices since the 1960s. However, within the next ten years, fundamental property limitations of silicon will inhibit the ability to fabricate operational devices and circuits due to continuing device size reduction. The ability to reduce device size and thus pack more and more devices on a chip has allowed adherence to Moore’s Law and therefore has facilitated the phenomenal progression of the silicon-based semiconductor industry.
Epitaxial graphene (EG) has extraordinary electronic properties that offer the possibility of greatly enhanced speed and performance relative to silicon; this material may serve as the successor to silicon in integrated circuits and microelectronic devices.
How is Epitaxial Graphene Grown?
The Georgia Tech MRSEC focuses on the growth of EG (single and multiple layers) on single crystal silicon carbide substrates. This approach offers the advantage that high quality layers can be grown on large area substrates. In addition, processes similar to those employed for silicon-based device manufacture can be used for the fabrication of epitaxial graphene devices and circuits. In contrast, much of the worldwide effort has concentrated on exfoliated epitaxial graphene, where epitaxial graphene flakes are obtained by peeling layers from graphite. Although devices can be constructed on such sheets, the ability to fabricate large area arrays of high quality devices and hence establish consistency with the well-established silicon-based technology is severely restricted using exfoliated epitaxial graphene techniques.
What is the history of epitaxial graphene work at Tech?
In this decade of research on graphene, methods have been developed to grow monolayer and multilayer epitaxial graphene (MEG) on the Si- and C-face of hexagonal silicon carbide with of up to 100 graphene sheets. The Georgia Tech Confinement Controlled Sublimation method can produce uniform epitaxial graphene layers with remarkable electronic properties. The monolayer films have high mobilities and exhibit the half integer quantum Hall effect. Surprisingly, the properties of MEG are more closely related to monolayer graphene rather than graphite, as a result of an unusual rotational stacking of the graphene; the electronic band structure of MEG is composed of Dirac cones like electronically decoupled graphene layers. The charge carriers are chiral and exhibit a non-trivial Berry's phase. Weak anti-localization andcoherence and quantum confinement have been demonstrated. Landau level spectroscopy further exhibits record-breaking room temperature mobilities and well resolved Landau levels below 1 T, indicating extremely low carrier densities and good homogeneity of the material.
Large-scale electronic device patterning are being developed. Integration of 10,000 transistors has been demonstrated. Nanoscale structures (as narrow as 10 nm) are grown directly at specified positions on templated SiC substrate prepared using scalable photolithography and microelectronics processing. This avoids any damaging lithographic post-processing and smooth ribbons are obtained.
The excitement actually started more than a decade ago, when people studied carbon nanotubes and saw that they had really great electronic properties. A whole community evolved out of that, looking specifically at the properties of carbon nanotubes. From that evolved the idea that maybe carbon nanotubes were only a specific form of epitaxial graphene and that two-dimensional epitaxial graphene might also be a useful electronic material. In 2001, Professor de Heer thought that perhaps two-dimensional epitaxial graphene, the basis of carbon nanotubes, could be used as an electronic material, and that’s how the epitaxial graphene work project began.
Originally while he was performing calculations on carbon nanotubes, he began to look at what would happen if there was no epitaxial graphene tube, but instead, an epitaxial graphene ribbon. He found that many of the properties that carbon nanotubes have, on paper at least, were also there in epitaxial graphene ribbons. It was a theoretical calculation yet a very simple one. Later he realized that he was not the first one to make this calculation, but discovered that if a two-dimensional epitaxial graphene sheet was grown and then cut into shapes, it could be a new electronic material.