Growth of metallic delafossite PdCo O 2 by molecular beam epitaxy

The Pd- and Pt-based $AB{\mathrm{O}}_{2}$ delafossites are a unique class of layered, triangular oxides with two-dimensional electronic structure and a large conductivity that rivals the noble metals. Here, we report successful growth of the metallic delafossite $\mathrm{PdCo}{\mathrm{O}}_{2}$ 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 $\mathrm{PdCo}{\mathrm{O}}_{2}$ to cobalt oxide and metallic palladium. This is overcome by combining low-temperature (300 \ifmmode^\circ\else\textdegree\fi{}C) atomic layer-by-layer MBE growth in the presence of reactive atomic oxygen with a postgrowth 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 postgrowth anneal provides a route to creating high-quality films in this interesting family of layered, triangular oxides.