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Register a new userGrowth of Large Domain Epitaxial Graphene on the C-Face of SiC
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Growth of epitaxial graphene on the C-face of SiC has been investigated. Using a confinement controlled sublimation (CCS) method, we have achieved well controlled growth and been able to observe propagation of uniform monolayer graphene. Surface patterns uncover two important aspects of the growth, i.e. carbon diffusion and stoichiometric requirement. Moreover, a new stepdown growth mode has been discovered. Via this mode, monolayer graphene domains can have an area of hundreds of square micrometers, while, most importantly, step bunching is avoided and the initial uniformly stepped SiC surface is preserved. The stepdown growth provides a possible route towards uniform epitaxial graphene in wafer size without compromising the initial flat surface morphology of SiC.
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Epitaxial graphene layers were grown on the C-face of 4H- and 6H-SiC using an argon-mediated growth process. Variations in growth temperature and pressure were found to dramatically affect the morphological properties of the layers. The presence of argon during growth slowed the rate of graphene formation on the C-face and led to the observation of islanding. The similarity in the morphology of the islands and continuous films indicated that island nucleation and coalescence is the growth mechanism for C-face graphene.
An in vacuo thermal desorption process has been accomplished to form epitaxial graphene (EG) on 4H- and 6H-SiC substrates using a commercial chemical vapor deposition reactor. Correlation of growth conditions and the morphology and electrical properties of EG are described. Raman spectra of EG on Si-face samples were dominated by monolayer thickness. This approach was used to grow EG on 50 mm SiC wafers that were subsequently fabricated into field effect transistors with fmax of 14 GHz.
We demonstrate hydrogen assisted growth of high quality epitaxial graphene on the C-face of 4H-SiC. Compared with the conventional thermal decomposition technique, the size of the growth domain by this method is substantially increased and the thickness variation is reduced. Based on the morphology of epitaxial graphene, the role of hydrogen is revealed. It is found that hydrogen acts as a carbon etchant. It suppresses the defect formation and nucleation of graphene. It also improves the kinetics of carbon atoms via hydrocarbon species. These effects lead to increase of the domain size and the structure quality. The consequent capping effect results in smooth surface morphology and suppression of multilayer growth. Our method provides a viable route to fine tune the growth kinetics of epitaxial graphene on SiC.
The maximum oscillation frequency (fmax) quantifies the practical upper bound for useful circuit operation. We report here an fmax of 70 GHz in transistors using epitaxial graphene grown on the C-face of SiC. This is a significant improvement over Si-face epitaxial graphene used in the prior high frequency transistor studies, exemplifying the superior electronics potential of C-face epitaxial graphene. Careful transistor design using a high {kappa} dielectric T-gate and self-aligned contacts, further contributed to the record-breaking fmax.
We review progress in developing epitaxial graphene as a material for carbon electronics. In particular, improvements in epitaxial graphene growth, interface control and the understanding of multilayer epitaxial graphenes electronic properties are discussed. Although graphene grown on both polar faces of SiC is addressed, our discussions will focus on graphene grown on the (000-1) C-face of SiC. The unique properties of C-face multilayer epitaxial graphene have become apparent. These films behave electronically like a stack of nearly independent graphene sheets rather than a thin Bernal-stacked graphite sample. The origin of multilayer graphenes electronic behavior is its unique highly-ordered stacking of non-Bernal rotated graphene planes. While these rotations do not significantly affect the inter-layer interactions, they do break the stacking symmetry of graphite. It is this broken symmetry that causes each sheet to behave like an isolated graphene plane.
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