Oxidation of epitaxial Fe films monitored by x-ray reflectivity
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We have studied the oxidation of thin epitaxial Fe(100) films on MgO(lOO) with and without an Au(100) protecting cap by x-ray reflectivity measurements. The oxidation was carried out under atmospheric conditions between 20 °C and 200 °C. The results are compared to the oxidation of Fe(110) oriented films on Ai2O3(H20) substrates with an A u ( l l l ) cap. Auger electron spectroscopy before and after oxidation was carried out for sublimentary chemical information of the surface. For the uncovered Fe films we observe smoothly growing oxide films at the surface during oxidation at elevated temperatures. As expected, the Au(100) cap serves as an effective shield against oxidation, while the A u ( l l l ) cap, surprisingly, does not. In the case of Au/Fe/Al 2 O 3 , we find Fe 2 O 3 formation at the surface of the Au layer at 200 °C. The different behavior of Au(100) and A u ( l l l ) is discussed in terms of stacking faults and/or domain structure occurring in the latter case during epitaxial growth.
I. INTRODUCTION Thin metal oxide layers are of fundamental interest in very different fields of science and technology. Starting from heterogeneous catalysis, where thin oxide films are required as model systems,1'2 metal oxide layers play an important role in semiconductor device fabrication and Josephson tunnel junction barriers.3 Protective layers and optical coatings are also realized on the basis of thin metal oxide films. Therefore, much work has been devoted in the past on the study of the formation of oxide films on various metal substrates. On the other hand, it is also important to know how to protect pure metal thin films from oxidation. Effective oxide protection has an important impact on technological areas as far apart as metal mirrors in optical astronomy and magnetic thin films for information storage. In all these cases the structural properties of the oxide layer, including its film thickness, chemical composition, homogeneity, and roughness, are instrumental for the understanding and control of the oxidation process. A nondestructive method to detect these properties at once is the reflectivity of hard x-rays.4 The power and sensitivity of this technique on the monolayer scale have been demonstrated by many examples in the recent years5"8 and have been verified by different other methods like Rutherford backscattering9 or electron microscopy studies.10'11 The sensitivity of x-ray reflectivity to oxidation reactions has also been demonstrated in the case of oxidation of Ni/Al bilayer interfaces.12 In this study we are concerned with oxide layers in excess of one monolayer. A number of studies have reported the formation of divalent ferrous oxide FeO, 13 or trivalent ferric oxide Fe2O3 in addition to mixed di884
J. Mater. Res., Vol. 9, No. 4, Apr 1994
valent and trivalent Fe 3 O 4 . 14 Furthermore, it was recently shown that FeO forms at low temperatures, while at higher temperatures above 300 K and in a high oxygen atmosphere Fe3O4 prevails.13 Various photoemission studies of Fe oxidation provided diff
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