Epitaxial co-deposition growth of CaGe 2 films by molecular beam epitaxy for large area germanane
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Patrick M. Odenthal Department of Physics and Astronomy, University of California, Riverside, California 92521
Adam S. Ahmed Department of Physics, The Ohio State University, Columbus, Ohio 43210
Walid Amamou Department of Physics and Astronomy, University of California, Riverside, California 92521
Joshua E. Goldbergera) Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
Roland K. Kawakamib) Department of Physics, The Ohio State University, Columbus, Ohio 43210; and Department of Physics and Astronomy, University of California, Riverside, California 92521 (Received 17 September 2013; accepted 30 December 2013)
Two-dimensional crystals are an important class of materials for novel physics, chemistry, and engineering. Germanane (GeH), the germanium-based analogue of graphane (CH), is of particular interest due to its direct band gap and spin–orbit coupling. Here, we report the successful co-deposition growth of CaGe2 films on Ge(111) substrates by molecular beam epitaxy and their subsequent conversion to germanane by immersion in hydrochloric acid. We find that the growth of CaGe2 occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize in situ reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 °C. Finally, the CaGe2 films are immersed in hydrochloric acid to convert the films to germanane. Auger electron spectroscopy of the resulting film indicates the removal of Ca, and RHEED patterns indicate a single-crystal film with an in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. Ex situ atomic force microscopy shows that the grain size of the germanane is on the order of a few micrometers, being primarily limited by terraces induced by the miscut of the Ge substrate. Thus, optimization of the substrate could lead to the long-term goal of large area germanane films. I. INTRODUCTION
The discovery and mechanical exfoliation of single layer graphene has led to a revolution in nanoscale materials science by enabling a plethora of new physics and chemistry in two dimensions (2D) that is not possible in conventional three-dimensional materials.1–3 Beyond graphene, there is great interest in exploring 2D materials that offer properties not found in graphene, such as a native band gap, strong spin–orbit coupling, and strong exciton confinement.4 Of particular interest is germanane (GeH), a)
This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org_jmr_policy. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.2 410
J. Mater. Res., Vol. 29, No. 3, Feb 14, 2014
http://journals.cambridge.org
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the germanium-based analogue of graphane (CH), which has recently been synthesi
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