| AVS 63rd International Symposium & Exhibition | |
| 2D Materials Focus Topic | Wednesday Sessions |
| Session 2D+NS-WeA |
| Session: | Nanostructures including Heterostructures made of 2D Materials |
| Presenter: | Alex Summerfield, University of Nottingham, UK |
| Authors: | A. Summerfield, University of Nottingham, UK A. Davies, University of Nottingham, UK T.S. Cheng, University of Nottingham, UK V.V. Korolkov, University of Nottingham, UK Y. Cho, University of Nottingham, UK C.J. Mellor, University of Nottingham, UK E.F. Smith, University of Nottingham, UK C.T. Foxon, University of Nottingham, UK A.N. Khlobystov, University of Nottingham, UK K. Watanabe, National Institute for Materials Science (NIMS), Japan T. Taniguchi, National Institute for Materials Science (NIMS), Japan L. Eaves, University of Nottingham, UK S.V. Novikov, University of Nottingham, UK P. Beton, University of Nottingham, UK |
| Correspondent: | Click to Email |
To scale up the production of graphene-hexagonal boron nitride (hBN) heterostructure devices, direct epitaxial growth of these materials will be necessary. As an alternative to commonly used techniques such as the exfoliation of graphene/hBN flakes or growth using chemical vapour deposition we have investigated high-temperature molecular beam epitaxy (HT-MBE) in order to produce high-quality graphene and hBN monolayers.
We show that graphene grown using HT-MBE on hBN surfaces form continuous domains with dimensions of order 20 μm, and exhibits moiré patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moiré patterns are observed using atomic force microscopy (AFM) and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moiré periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moiré period reveal split and shifted G and 2D peaks confirming the presence of strain.
We also demonstrate the epitaxial growth of high-quality hBN atomic layers on graphite using plasma-assisted HT-MBE. AFM reveals mono- and few-layer island growth, while conductive AFM measurements show that the grown hBN has a resistivity which increases exponentially with layer thickness comparable with exfoliated hBN samples. Furthermore, X-Ray photoelectron spectroscopy, Raman and spectroscopic ellipsometry confirm the formation of sp2-bonded hBN with a band gap of 5.87 eV. Hexagonal moiré patterns of 15-17 nm are also observed on the hBN surface, suggesting that the grown layers may be strained due to the lattice mismatch with the graphite surface.
Our work demonstrates a new approach to the growth of epitaxial graphene/hBN and provides a route to the production of vertical superlattice structures for use in future devices.