Sputter-grown c-axis-oriented PdCoO2 thin films

Metallic delafossites, ABO2 (A = Pd or Pt), are layered oxides that are as conductive as elemental metals. The high conductivity and surface polarity make metallic delafossites fascinating electrode materials for heterostructure devices. Here, we report the successful growth of c-axis-oriented PdCoO2 thin films on Al2O3 (001) substrates by magnetron sputtering that is widely used in industries. The observation of the PdCoO2 thin films through scanning transmission electron microscopy revealed layered crystal structures. A sharp interface exhibiting a layer stacking sequence of Pd/CoO2/Al2O3 was observed clearly, similar to the interfaces obtained with other growth methods such as pulsed laser deposition and molecular beam epitaxy. This layer stacking is particularly interesting because it can induce a high work function at the interface. The in-plane resistivity of the as-grown PdCoO2 thin film was 73 μΩ cm at room temperature, which decreased to 11 μΩ cm after post-annealing. The residual resistivity ratio of the annealed thin films was approximately 2.9. The impurity phases of PdOx were observed using x-ray diffraction and scanning transmission electron microscopy. The sputtering deposition of c-axis-oriented thin films could lead to the practical application of the polar surface of PdCoO2 in semiconductor devices.