PNNL

Abstract

Alpha-Fe2O3(0001) films of thickness equal to ~7 nm and ~70 nm were epitaxially grown on alpha-Al2O3(0001) by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interfaces were characterized using high resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and x-ray diffraction (XRD). The interface exhibited coherent regions separated by equally-spaced misfit dislocations. When imaged from the [2110] direction, the dislocation spacing is 7.0 +- 1.1 nm for the 70 nm thick speciman, and 7.2 +- 0.1 nm for the 7 nm thick speciman. When imaged from the [0110] direction, the disslocation spacing is 4.5 +- 0.1 nm for the 7 nm thick speciman. The experimentally observed dislocation spacings are appproximately consistent with those calculated from the lattice mismatch between alpha-Al2O3 and alpha-Fe2O3, implying that the lattice mismatch is accomodated mainly be interface misfit dislocations above the critical thickness, which is less than 7 nm. This conclusion is also corroborated by the measured reidual strainof ~0.5% determined from x-ray diffraction for the 70 nm film . EELS analysis reveals that the Fe L2,3-edge shows no measurable chemical shift relative to the L2,3-edge of structural Fe3?, indicating complete oxidation of Fe in the as-grown film.