Chemical-state evolution and phase stability of epitaxial PtCoO 2 thin films probed by XPS, HAXPES, and in situ XRD

Epitaxial PtCoO 2 thin films were grown on Al 2 O 3 ( 0001 ) substrates by physical vapor deposition under back-end-of-line compatible conditions, followed by postdeposition annealing to induce crystallization. While annealing-driven structural evolution in delafossite oxides has been reported, the corresponding chemical-state evolution across crystallization and decomposition boundaries remains insufficiently understood. Here, x-ray photoelectron spectroscopy, hard x-ray photoelectron spectroscopy (HAXPES), and x-ray diffraction are combined to track chemical states and phase stability in PtCoO 2 thin films as a function of thermal processing. This multimodal approach resolves the spectroscopic fingerprints of the metallic delafossite phase, unambiguously identifying formal P t 1 + and C o 3 + valence states. Bulk-sensitive HAXPES, reported here for PtCoO 2 , mitigates surface artifacts and enables direct correlation between bulk chemistry and structural order. Annealing at 800 ∘ C yields phase-pure, epitaxial PtCoO 2 , whereas higher temperatures induce decomposition into metallic Pt and cobalt oxide phases, defining a narrow thermal stability window. Highly crystalline films down to 4.5 nm thickness exhibit low in-plane resistivities consistent with the metallic delafossite phase. These results establish a chemically specific framework for assessing phase formation and stability in PtCoO 2 thin films and demonstrate the utility of combined photoelectron spectroscopy and diffraction for complex oxide metals.