Scale-bridging characterization of iron oxide thin films obtained by reactive magnetron sputtering

Iron oxides are promising candidates for sustainable energy applications due to being low cost, earth abundant and environmentally friendly. However, precise control over microstructure including phase formation remains a critical challenge for optimizing their functional properties. In this study, we investigated the influence of cathode power and substrate temperature on the microstructure and phase formation of iron oxide thin films by reactive DC magnetron sputtering. Two series were deposited at cathode power of 100 W (series I) and 250 W (series II) with variating substrate temperature, from no intentional heating up to 600 °C. We employed a scale-bridging characterization approach, combining X-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning-/transmission electron microscopy (STEM/TEM) with advanced techniques including energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS) and four-dimensional STEM (4D-STEM). We showed, that by carefully tuning the deposition parameters, iron oxide films can be synthesized with phases ranging from pure Fe₃O₄ to pure α-Fe₂O₃, as well as the formation of metastable nano-sized γ-Fe₂O₃ grains. At high sputtering rate the substrate temperature stabilized different phase from α-Fe₂O₃ to Fe₃O₄ with increasing temperature. At low sputtering rates, elevated temperatures enables grain growth and the formation of a bimodal grain-sized α-Fe₂O₃ film. With the help of EELS analysis we identified the metastable γ-Fe₂O₃ phase. Our findings highlight the critical role of deposition parameters for controling oxide phase and microstructure formation of iron oxide thin films during reactive sputtering. This offers a parthway to tailor their properties for applications like solar water splitting or oxygen evolution reactions.