  The Pd, and Pt based ABO2 delafossites are a unique class of layered,
triangular oxides with 2D electronic structure and a large conductivity that
rivals the noble metals. Here, we report successful growth of the metallic
delafossite PdCoO2 by molecular beam epitaxy (MBE). The key challenge is
controlling the oxidation of Pd in the MBE environment where phase-segregation
is driven by the reduction of PdCoO2 to cobalt oxide and metallic palladium.
This is overcome by combining low temperature (300 {\deg}C) atomic
layer-by-layer MBE growth in the presence of reactive atomic oxygen with a
post-growth high-temperature anneal. Thickness dependence (5-265 nm) reveals
that in the thin regime (<75 nm), the resistivity scales inversely with
thickness, likely dominated by surface scattering; for thicker films the
resistivity approaches the values reported for the best bulk-crystals at room
temperature, but the low temperature resistivity is limited by structural
twins. This work shows that the combination of MBE growth and a post-growth
anneal provides a route to creating high quality films in this interesting
family of layered, triangular oxides.
