  Epitaxial growth of Fe-based superconductors such as Co-doped SrFe$_2$As$_2$
(SrFe$_2$As$_2$:Co) was reported recently, but has still insufficient
properties for device application because they have rough surfaces and are
decomposed by reactions with water vapor in an ambient atmosphere. This letter
reports that epitaxial films of Co-doped BaFe$_2$As$_2$ grown at 700 oC show
the onset superconducting transition tempearture of 20 K. The transition is
sharper than those observed on the SrFe$_2$As$_2$:Co films, which would
originate from their improved crystallinity. These films also have
atomically-flat surfaces with steps-and-terraces structures and exhibit
chemical stability against exposure to water vapor.
