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

TL;DR

The paper addresses the challenge of stabilizing FeO, the thermodynamically unstable wüstite phase, by using epitaxial stabilization on lattice-matched 4H-SiC(0001) substrates. It employs molecular beam epitaxy with careful pre-growth substrate conditioning and rapid quenching to grow phase-pure FeO(111) films up to 180 nm thick and near-epitaxial alignment with the SiC substrate. Comprehensive characterization by XRD, reciprocal-space mapping, S/TEM, EDS, and EELS confirms a coherent FeO/SiC interface, a slight rhombohedral distortion due to tensile strain, and absence of Fe3O4, supporting true wüstite stabilization. The results demonstrate a viable route to integrate thermodynamically unstable oxides with technologically important semiconductors, potentially enabling novel oxide–semiconductor devices and antiferromagnetic functionalities.

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 $α$-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.

Figures (8)

• Figure 1: Schematic of the proposed FeO/SiC heteroepitaxial system as viewed in the (a) FeO$[111]$ / SiC$[0001]$, (b) $[\bar{2}11]$ / $[1\bar{1}00]$, and (c) $[\bar{1}01]$ / $[11\bar{2}0]$ directions. The interfacial coincidence lattice matching is also indicated in (a).
• Figure 2: RHEED images captured of the (a) bare SiC$(0001)$ surface immediately prior to growth initiation and (b) after 1 minute of FeO growth. Both images were taken along the SiC$[11\bar{2}0]$ direction at 750$^\circ$C.
• Figure 3: (a) AFM image of an 80 nm FeO thin film (Sample C) grown on SiC$(0001)$. (b) SEM image of a 180 nm thick FeO film (Sample D) grown on SiC$(0001)$.
• Figure 4: (a) A representative long-range $\theta-2\theta$ XRD scan of Sample C, taken along the SiC$[11\bar{2}0]$ zone, indicating wüstite FeO as the only non-substrate phase present. (b) HRXRD scans collected from Samples A, B, C, and D around the $(0004)$ SiC peak. Of any potential iron oxide phases, only the near-matched $(111)$ FeO is observed in all samples.
• Figure 5: Representative XRD reciprocal space maps, collected from Sample C, taken around the (a) symmetric $(0004)$SiC / $(111)$FeO, (b) asymmetric $(10\bar{1}7)$SiC / $(311)$FeO, and (c) asymmetric $(10\bar{1}9)$SiC / $(331)$FeO diffraction peaks.