Graphene on SiC wafers for high performant RF transistors GRAPHIC RF
Experimentelle Physik der kondensierten Materie
Zusammenfassung der Projektergebnisse
Epitaxial graphene grown on SiC results in two distinct physical systems depending on whether the growth is on the C- or Si-face of SiC. On the one hand, the C-face growth results is a novel system of mutually rotated graphene layers, and serves as a starting point for the investigation of the physics of stacking order in few layer graphene. The Si-face physics, in contrust, i.s governed by the electronic properties of an intermediate buffer layer that serves as a prototypical example of the perturbation of single layer graphene by a substrate interaction. We consider in what follows each of these two aspects of the project in turn. The rich physics associated with the interlayer degree of freedom in few layer graphenes is manifest in perhaps no system more strikingly than the graphene twist bilayer. 'I'his system, consisting of two mutually rotated layers of graphene, exhibits a qualitatively changing of electronic structure as a function of the twist angle and has since its introduction in the context of multilayer graphene epitaxially grown on the surface of SiC (000I), attracted sustained attention from both theory and experiment. The aim of this project was to elucidate the electronic structure of the twist bilayer at both the fundamental level of the ideal system, as well as in various applied contexts such as twist bilayer flakes and the intercalation of the twist bilayer. The main project results concerning graphene grown on the C-face of SiC and as a prototype of this system the graphene twist bilayer have resulted in an almost complete understanding of the twist bilayer electronic structure. In particular, the “lattice paradox" that the twist angle of the bilayer should, intuitively, determine all electronic properties yet by itself does not fix the real or reciprocal space lattices has been resolved. The very dilferent electronic structures at large angles (a decoupling of the two constituent layers) and small angles (very strong coupling) have been treated in a single theoretical approach, unifying previously disparate theories of the twist bilaycr. The small angle limit has been shown to have a very rich and interesting Fermiology, and the existence of massive quasiparticles near the Dirac point (in contrast to the massless quasiparticles found in single layer graphene, or the twist bilayer at large twist angles). The twist geometry of graphene layers lias also been studied in other more applied physical contexts, in particular graphene twist flakes were investigated with the rather striking finding that only one moire unit cell is required to reproduce the low energy spectrum of the extended twist bilayer. Returning to the “birth place” of the graphene twist bilayer, the electronic structure of graphene twist stacks of up to hundreds of layers was studied where it was found that the twist stack Hamiltonian decouples into a direct sum of twist bilayer Hamiltonians, each with an interlayer coupling given by a Toeplitz eigenvalue. Finally the twist bilayer has been explored from the point of view of an intercalation scaffold, i.e. as providing a natural energy landscape for the intercalation of impurity atoms. In addition to the sustained investigation of the C-face of SiC and twist bilayer the project investigated the electronic properties of graphene grown on the Si-face of SiC. Here the physics is that of a perturbed single layer graphene, rather than the strongly altered electronic properties that can be found in the twist bilayer at small angles. Understanding of how the electronic properties of the first graphene epilayer are altered by the presence of the buffer layer formed the central theme of the first funding period and resulted in an understanding of the low energy electronic structure of the epilayer. In particular of the doping and gap opening of the epilayer induced by the buffer layer. On the basis of this understanding the trausport properties of graphene on the Si-face of SiC, and the pronounced reduction in the all-important charge carrier mobility of this system as compared to ideal free-standing graphene, were explained.
Projektbezogene Publikationen (Auswahl)
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“Electron spectrum of epitaxial graphene monolayers.” Phys. Rev. B 82:121416(R) (2010)
O. Pankratov, S. Hensel, and M. Bockstedte
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“Electronic structure of turbostratic graphene.” Phys. Rev. D, 81:165105, (2010)
S. Shallcross, S. Sharma, E. Kandelaki, and O. A. Pankratov
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"Electronic structure of graphene twist stacks.” Phys. Rev. B, 83:153402, (2011)
S. Shallcross, S. Sharma, W. Landgraf, and O. Pankratov
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“Buffer layer limited conductivity in epitaxial graphene on the Si face of SiC." Phys. Rev. B, 86:125426. (2012)
N. Ray, S. Shallcross, S. Hensel, and O. Pankratov
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“Buffer layer limited conductivity in epitaxial graphene on the Si face of SiC.” Phys. Rev. B, 86:155432, (2012)
O. Pankratov, S. Hensel, P. Götzfried, and M. Bockstedte
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“Electronic structure of twisted graphene flakes.” Phys. Rev. B, 87:075433, (2013)
W. Landgraf, S. Shallcross, K. Türschmann, D. Weckbecker, and O. Pankratov
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“Emergent momentum scale, localization, and van hove singularities in the graphene twist bilayer." Phys. Rev. B, 87:245403 (2013
S. Shallcross, S. Sharma and O. Pankratov
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“Band-gap engineering with a twist: Formation of intercalant superlattices in twisted graphene bilayers.” Phys. Rev. B, 91:205412, (2015)
Franz Symalla, Sam Shallcross, Igor Beljakov, Karin Fink, Wolfgang Wenzel, and Velimir Meded
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"Electron-phonon scattering and in-plane electric conductivity in twisted bilayer graphene.’’ Phys. Rev. B 94, 245403 – Published 2 December 2016
N. Ray. D. Weckbecker, S. Sharma, O. Pankratov, and S. Shallcross
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"Low energy theory of the graphene twist bilayer.”, Phys. Rev. B, 93:035452 (2016)
D. Weckbecker, S. Shallcross, M. Fleischmanu, N. Ray, S. Sharma, and O. Pankratov
