Impact of Lattice Mismatch and Stoichiometry on the Structure and Bandgap of (Fe,Cr)2O3 Epitaxial Thin Films
The structural properties of high-quality epitaxial (Fe1-xCrx)2O3 thin films are investigated across the composition range. Epitaxial films are deposited on a-Al2O3(0001) substrates by oxygen-plasma-assisted molecular beam epitaxy. Corundum (Fe1-xCrx)2O3 supercells relaxed by density functional theory confirm that the non-linear behavior of the bulk lattice parameters originates in the magnetic structure of the alloy films. High-resolution x-ray diffraction reveals the degree of epitaxial strain relaxation in the films, with Cr-rich films remaining partially strained to the Al2O3 substrate. For intermediate compositions, a lattice expansion and non-Poisson-like tetragonal distortion are found. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal a columnar grain structure in the films, with uniform mixing of cations on the nanometer scale. Oxygen non-stoichiometry is quantified by non-Rutherford resonant elastic scattering measurements utilizing 3.04 MeV He+. Intermediate-composition films are found to be slightly over-stoichiometric, resulting in the observed lattice expansion. Cr-rich films, in contrast, appear to be slightly oxygen deficient. A model is proposed to explain these results based on the energetics of oxygen defect formation and rate of oxygen diffusion in the corundum lattice. Compressive biaxial strain is found to reduce the bandgap of epitaxial Cr2O3 relative to the bulk value. The relationships which are elucidated between epitaxial film structure and optical properties can be applied to bandgap optimization in the (Fe,Cr)2O3 system.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1129313
- Report Number(s):
- PNNL-SA-99168; 48144; 44701; KC0203020
- Journal Information:
- Journal of Physics: Condensed Matter, 26(13):Article No. 135005, Journal Name: Journal of Physics: Condensed Matter, 26(13):Article No. 135005
- Country of Publication:
- United States
- Language:
- English
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