Preparation and characterization of epitaxial iron oxide films
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Preparation and characterization of epitaxial iron oxide films Y. Gao, Y. J. Kim, and S. A. Chambers P.O. Box 999, MS K8-93, Pacific Northwest National Laboratory, Richland, Washington 99352 (Received 13 January 1997; accepted 30 July 1997)
Well-ordered, pure-phase epitaxial films of FeO, Fe3 O4 , and g –Fe2 O3 were prepared on MgO(001) by oxygen-plasma-assisted MBE. The stoichiometries of these thin films were controlled by varying the growth rate and oxygen partial pressure. Selective growth of g –Fe2 O3 and a –Fe2 O3 was achieved by controlling the growth conditions in conjunction with the choice of appropriate substrates. Growth of the iron oxide epitaxial films on MgO at >350 ±C is accompanied by significant Mg outdiffusion. The FeO(001) film surface exhibits a (2 3 2) reconstruction, which is accompanied by a significant 31 amount pof Fe ±in the surface region. Fe3 O4 (001) has been found to reconstruct to a p s 2 3 2dR45 structure. g –Fe2 O3 (001) film surface is unreconstructed.
I. INTRODUCTION
One success of thin-film technology is the ability to tailor film properties through the selection of appropriate deposition parameters. These parameters determine the nucleation and growth of the film on the chosen substrate, which in turn determine the structure and microstructure of the film. Homogeneous nucleation is desired to make fine grain polycrystalline films with designed properties, whereas heterogeneous nucleation gives rise to epitaxial films. Though polycrystalline films have been used in many technological applications today, epitaxial growth can offer a great opportunity to advance both science and technology of materials.1 It provides a means to the synthesis of new materials with specific, desirable properties. Successful epitaxial growth requires proper selection of deposition techniques, substrate materials, and growth conditions. Molecular beam epitaxy (MBE) has been shown to produce superior-quality epitaxial thin films of simple oxides: MgO,2 TiO2 ,3 and Fe3 O4 ,4 – 6 and to have the ability to control the film quality for ternary oxides.7 However, magnetron sputtering, pulsed laser ablation, and metal-organic chemical vapor deposition are the preferred techniques for epitaxial growth of complex oxides such as high Tc superconductors and oxide ferroelectrics. Selection of proper substrates is essential for successful epitaxial growth. When a thin film is grown on top of a slightly mismatched substrate, the film may grow coherently with the substrate, absorbing the lattice mismatch by an elastic strain, until a critical layer thickness (tc ) is reached. Above tc the growth is semicoherent and the strain is relaxed by forming misfit dislocations. The crystal quality of the epitaxial film decreases with increasing mismatch until the mismatch is so large that epitaxial growth becomes impossible. Epitaxial growth usually requires a relatively high growth temperature and low growth rate. J. Mater. Res., Vol. 13, No. 7, Jul 1998
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