Oxidation of polycrystalline copper films – Pressure and temperature dependence

Highlights: • Temperature and pressure-dependent oxidation study of Cu thin films • Identification of optimal preparation conditions for Cu₂O and CuO • Development of a p-T phase diagram for the Cu-O system - Abstract: Oxidation of polycrystalline Cu films is investigated in a wide range of temperatures and O₂ pressures. The composition of the resulting oxide is derived from transmission and absorption spectra, analyzed with the transfer-matrix and a modified Tauc method, as well as from photoelectron spectroscopy. No Cu oxidation occurs at oxygen pressures below 1 mbar, indicative for a pressure-dependent barrier. At higher pressures, either cuprous or cupric oxide forms at oxidation temperatures below 460 K and above 500 K, respectively, while both phases coexist at intermediate temperatures. Oxide films of unknown stoichiometry, possibly Cu₄O₃, are revealed in the pressure range between 1 and 10 mbar. Based on these results, a formation diagram for copper oxides is developed and compared to respective phase diagrams in the literature.