MBE Stabilization of Metastable FeO on 4H-SiC: Pathways to Wüstite Phase Formation 

 

Unlike other rocksalt-structure metal monoxides (e.g., MgO, NiO, CoO, etc.), iron (II) oxide (FeO), or wüstite, is thermodynamically unstable at standard ambient conditions, making thin film production difficult. To date, there are very few reports of FeO epilayers, and only a maximum of 8 nm as-grown thickness has been achieved (or nearly 20 nm with post-deposition reductive annealing); thicker films are found to decompose into thermodynamically stable metallic Fe and/or magnetite Fe3O4. Conversely, we report here on the work on the growth of epitaxially-stabilized FeO thin films on small-mismatch 4H-SiC substrates using molecular beam epitaxy (MBE). This effort has thus far yielded pure wüstite epilayers of up to nearly 200 nm, more than 20 times thicker than any previously reported, with no post-processing required. The as-grown, single-crystal films exhibit a small, consistent rhombohedral distortion, with lattice parameters

𝑎𝐹𝑒𝑂 = 4.306 ± 0.004 ÅaFeO = 4.306 ± 0.004 Å and 𝛼 = 90.78° ± 0.06°𝛼 = 90.78° ± 0.06°, seemingly regardless of thickness. Preliminary SQUID magnetometry measurements confirm the characteristic antiferromagnetic behavior, but with an apparent small shift in Neel temperature, potentially related to the epitaxial strain. Magnetic measurements, as well as other optical, electronic, and magnetic properties, are currently being conducted against a wider range of film thicknesses and growth conditions, and the results will be discussed at the conference.