Surface order, thermal stability of surfaces to the formation of extended defects and 2D growth
Prof. Conrad has been actively involved in the area of Surface Physics for more than 20 years. He specializes in the study of surface order, thermal stability of surfaces to the formation of extended defects and 2D growth.
Past work has included his discovery of the infinite order surface roughening transition predicted as early as the 1950's, demonstration of anomalous surface anharmonicity, stress induced self assembly of Pd islands, surface faceting transitions caused by both atomic step structure and step-step interactions. Prof. Conrad’s current research area is the study of epitaxial graphene grown on SiC. He is a member of the Georgia Tech Graphene Lab. Experimental studies include, LEED, Surface X-ray Diffraction, Low Energy Electron Microscopy, Photoemission electron microscopy.
Epitaxial graphene is a viable candidate for an all-carbon post-CMOS electronics revolution. The advantage of graphene over CNTs for electronics resides in its planar 2D structure that enables circuit design with standard lithography techniques. This allows graphene to be cut with different shapes and selected edge direction. By selecting the correct ribbon edge direction it would be possible to design metallic or semiconductor graphene ribbons (analogous to helicity in CNTs).
While mechanically exfoliated graphene sheets are being studies, our group focuses on producing graphene by high temperature sublimation of Si from SiC substrates. Once grown, the films are lithographically patterned and metal contacts applied to make electronic devices. Graphene produced this way is referred to as epitaxial graphene (EG). While mechanically exfoliated graphene flakes have been used to study a variety of fundamental graphene properties, EG is the only viable a approach to scalable electronics as recognized by the 2007 International Technology Semiconducting Roadmap.
|AFM image of graphene grown on the C-face of SiC||
x-ray reflectivity data from multilayer graphene grown on the C-face of SiC. Fits are for different structural models of the of the graphene/SiC interface.
Our role in this research has been to understand the relationship between structure and transport properties of EG on both polar faces of SiC. The similarity of EG’s (or for that matter, exfoliated graphene’s) transport properties with the transport properties of a theoretically isolated graphene sheet is remarkable, considering that graphene/substrate interactions should influence the 2D Dirac electrons responsible for graphene’s unusual properties. We have searched for a structural explanation of why epitaxial graphene behaves like an isolated graphene sheet. For a review of epitaxial graphene see Ref. 8.
We have been able to show that on both polar face of SiC, a buffer layer forms between the substrate and the graphene film that isolates the graphene from further interactions with the substrate. Even more interesting is how thick multilayer graphene films grown on the C-face of SiC behave electronically like isolated graphene sheets and not like bulk graphite. By forming a high number of roational faults in the multilayer graphene stack, adjacent planes electronically decouple (see figure below). This is results is extremely important because the graphene film thickness does not need to be precisely controlled and because buried layers are protected from chemical processing during device manufacturing.
- Ultrathin Epitaxial Graphite: 2D Electron Gas Properties and a Route toward Graphene-based Nanoelectronics, C. Berger, Z. Song, T. Li, X. Li, A.Y. Ogbazghi, R. Feng, Z. Dai, A.N. Marchenkov, E.H. Conrad, P.N. First, and W.A. de Heer, J. of Phys. Chem. B 108, 19912 (2004).
- Electronic confinement and coherence in patterned epitaxial graphene, C. Berger, Z. Song; X. Li; X. Wu; N. Brown, C. Naud, D. Mayou, T. Li; J. Hass, A.N. Marchenkov, E.H. Conrad, P.N. First, W.A. de Heer, Science 312, 1191 (2006)
- Highly-ordered graphene for two dimensional electronics, J. Hass, R. Feng, T. Li, X. Li, Z. Zong, W. A. de Heer, P. N. First, E. H. Conrad, C. A. Jeffrey, C. Berger, Appl. Phys. Lett. 89, 143106 (2006).
- Epitaxial graphene, W. A. de Heer,C. Berger, X. Wu, P. N. First, E. H. Conrad, X. Li, T. Li, M. Sprinkle, J. Hass, M. L. Sadowski, M. Potemski, and G. Martinez, Solid State Comm. 143, 92 (2007).
- The structural properties of the multi-layer graphene/4H-SiC(000-1) system as determined by Surface X-ray Diffraction, J. Hass, R. Feng, J.E. Millán-Otoya, X. Li, M. Sprinkle, P. N. First, C. Berger, W. A. de Heer, E. H. Conrad, Phys. Rev. B 75, 214109 (2007).
- Electronic structure of epitaxial graphene layers on SiC: effects of the substrate, F. Varchon, R. Feng, J. Hass, X. Li, B. Ngoc Nguyen, C. Naud, P. Mallet, J.-Y. Veuillen, C. Berger, E.H. Conrad and L. Magaud. Phys. Rev. Lett. 99, 126805 (2007).
- Why multilayer graphene grown on the SiC(000-1) C-face behaves like a single sheet of graphene, J. Hass, F. Varchon, J.E. Millán-Otoya, M. Sprinkle, N. Sharma, W.A. de Heer, C. Berger, P.N. First, L. Magaud and E.H. Conrad, Phy. Rev. Lett. 100, 125504 (2008).
- The growth and morphology of epitaxial multilayer graphene, J. Hass, W.A. de Heer and E. H. Conrad, J. Phys.: Condens. Matt. 20, 323202 (2008).