  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 (MBE) 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 {\deg}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 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 (AFM) 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.
