Phase formation and electrical properties of reactively sputtered Fe_1-x O thin films

Wustite, Fe1-x O, is a crucial phase for the transition to CO2-free steel manufacturing as well as promising for electrochemical applications such as water splitting and ammonia synthesis. To study the effect of interfaces in these applications, thin-film model systems with defined interfaces are ideal. Previous studies lack a description of the growth mechanism to obtain Fe1-x O thin films. Here, we investigate the phase formation of metastable Fe1-x O during reactive magnetron sputtering while systematically varying the O-2/Ar flow ratio from 1.8% to 7.2% and the pressure-distance product between 3.5 and 7.2 Pa cm. If bulk diffusion is minimized, thin films containing 96 vol.% wustite and 4 vol.% Fe as impurity phase were achieved. Therefore, the wustite phase formation appears to be surface diffusion dominated. To reveal the influence of impurity phases in wustite on the electrical resistivity, systematic electrical resistivity measurements while cooling in situ were performed for the first time. The electrical resistivity was lower than that of single crystals of the respective iron oxides. This is attributed to the formation of Fe-rich layers at the substrate-film interface, which serve as additional conduction paths.