Epitaxially-stabilized growth of wüstite FeO on 4H-SiC

Abstract

Iron(II) monoxide (FeO) is thermodynamically stable in the halite (wüstite) structure only at elevated temperatures in a typically non-stoichiometric, Fe-deficient, Fe${}_{1-z}$O form that tends to phase separate and/or transform into metallic $\alpha$-Fe and magnetite Fe${}_3$O${}_4$ at ambient conditions. Here we report on the successful growth of up to 180 nm thick $(111)$-oriented FeO heteroepitaxial films on slightly lattice-matched 4H-SiC$(0001)$ using molecular beam epitaxy (MBE). The films have flat, terraced surfaces with tall multi-layer steps. X-ray diffraction (XRD), high-resolution scanning transmission electron microscopy (S/TEM), energy-dispersive X-ray spectroscopy (EDS), and core-level electron energy loss spectroscopy (EELS) collectively confirm the epilayer as phase-pure wüstite FeO, with atomically sharp FeO/SiC interfaces. The films are found to exhibit a slight misfit strain-induced rhombohedral distortion that does not appear to vary over the range of thicknesses examined. These results demonstrate the power of epitaxial stabilization for integrating a thermodynamically unstable, yet functionally interesting material with a commercially available and technologically important semiconductor platform.