On the rotational alignment of graphene domains grown on Ge(110) andGe(111)
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On the rotational alignment of graphene domains grown on Ge(110) and Ge(111) P.C. Rogge, Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, CA 94720, USA; Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA M.E. Foster, Sandia National Laboratories, Livermore, CA 94550, USA J.M. Wofford, Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, CA 94720, USA; Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA† K.F. McCarty, and N.C. Bartelt, Sandia National Laboratories, Livermore, CA 94550, USA O.D. Dubon, Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, CA 94720, USA; Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Address all correspondence to P. C. Rogge at [email protected] (Received 27 June 2015; accepted 31 August 2015)
Abstract We have used low-energy electron diffraction and microscopy to compare the growth of graphene on hydrogen-free Ge(111) and Ge(110) from an atomic carbon flux. Growth on Ge(110) leads to significantly better rotational alignment of graphene domains with the substrate. To explain the poor rotational alignment on Ge(111), we have investigated experimentally and theoretically how the adatom reconstructions of Ge interact with graphene. We find that the ordering transition of the Ge(111) adatom reconstruction is not significantly perturbed by graphene. Density functional theory calculations show that graphene on reconstructed Ge(110) has large-amplitude corrugations, whereas it is remarkably flat on reconstructed Ge(111). We argue that the absence of corrugations prevents graphene islands from locking into a preferred orientation.
Introduction Because of its intrinsic two-dimensional (2D) character, graphene interacts weakly with the substrates on which it is grown. Consequently, graphene domains tend to nucleate in random orientations, producing polycrystalline films.[1–5] Much has been learned about the factors that control domain orientation, particularly on metal substrates. For example, it has been shown that a single, energetically preferred orientation exists on Ir(111), where the preferred graphene orientation is dictated by the amplitude of the moiré corrugation of the graphene film induced by film-substrate interactions.[6] Recently, germanium, a group IV semiconductor, has emerged as a viable substrate for graphene epitaxy.[7–9] High quality epitaxial growth on semiconductor wafers is very attractive because it presents a path toward production by existing very large scale integration processes. Graphene growth on Ge(110) by chemical vapor deposition (CVD) has been shown to exhibit considerably improved rotational alignment compared with growth on Ge(111).[7] Here, we clarify the reasons for this improvement. Due to the directional nature of covalent bonds, semiconductor surfaces undergo distinctive reconstructions to mi
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