| AVS 65th International Symposium & Exhibition | |
| 2D Materials Focus Topic | Friday Sessions |
| Session 2D+EM+MN+NS-FrM |
| Session: | Nanostructures including Heterostructures and Patterning of 2D Materials |
| Presenter: | Daniel J. Pennachio, University of California at Santa Barbara |
| Authors: | D.J. Pennachio, University of California at Santa Barbara N.S. Wilson, University of California at Santa Barbara E.C. Young, University of California at Santa Barbara A.P. McFadden, University of California at Santa Barbara T.L. Brown-Heft, University of California at Santa Barbara K.M. Daniels, U.S. Naval Research Laboratory R.L. Myers-Ward, U.S. Naval Research Laboratory D.K. Gaskill, U.S. Naval Research Laboratory C.R. Eddy, Jr., U.S. Naval Research Laboratory C.J. Palmstrøm, University of California at Santa Barbara |
| Correspondent: | Click to Email |
Despite the prevalent use of hexagonal boron nitride (hBN) in 2D devices as a gate dielectric, tunnel barrier, or substrate, the quality of hBN thin films are typically lacking relative to flakes exfoliated from bulk crystals. To address the challenges of hBN epitaxy, this work studies the growth of hBN on single-crystal epitaxial graphene on SiC(0001) via plasma-enhanced chemical beam epitaxy (PE-CBE). As PE-CBE is conducted in an ultra-high vacuum environment, hBN nucleation, composition, and morphology were able to be examined using a combination of in-situ, in-vacuo, and ex-situ characterization techniques to gain insight into the formation of high-quality hBN films and hBN/graphene heterostructures.
It was found that utilization of high growth temperature (>1400°C) and nitrogen plasma flux (5×10-6 Torr background pressure) resulted in improved multilayer hBN film morphology over lower temperature (1300°C) depositions and CBE growths without nitrogen plasma flux. PE-CBE also produced more stoichiometric films than CBE without plasma at temperatures above 1400°C, as determined by in-vacuo X-ray photoelectron spectroscopy (XPS). In-situ reflection high energy electron diffraction (RHEED) showed streaky diffraction patterns persisting throughout several nanometers of PE-CBE hBN growth, indicative of a smooth, epitaxial film. Crystallinity and epitaxial arrangement of hBN nuclei were examined by in-vacuo and ex-situ scanning probe microscopy (SPM). Scanning probe spectroscopy provided information on the electrical properties of the hBN films relative to bulk values.
The epitaxial alignment of the hBN/graphene/SiC(0001) heterostructure was studied by RHEED and by comparing nuclei edge alignment, as measured with SPM or scanning electron microscopy, to the substrate lattice orientation. It was found that the rotational alignment of the hBN nuclei depended on the substrate surface morphology. Nuclei on the (6√3×6√3)R30° SiC surface reconstruction, a graphene-like buffer layer, aligned directly to the buffer layer, while hBN nuclei on 4° off-cut epitaxial graphene substrates showed preferential alignment to substrate macrosteps rather than the graphene lattice. These ~25nm high macrosteps were then examined by cross-sectional transmission electron microscopy (TEM), which showed that the epitaxial graphene and hBN conformally blanketed the macrostep facets despite the macrostep’s effect on nuclei orientation. The macrostep-directed nucleation outlined in this work provides a potential route to controlling the hBN/graphene rotational alignment during van der Waals epitaxy, an important variable for modulating electronic properties in this 2D system.