Epitaxial Co-Deposition Growth of CaGe$_{2}$ Films by Molecular Beam Epitaxy for Large Area Germanane

Here, we report the successful co-deposition growth of CaGe$_{2}$ 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 CaGe$_{2}$ occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize \textit{in situ} reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 $^{\circ}$C. Finally, the CaGe$_{2}$ 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 in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. \textit{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.