Effect of ion bombardment during the low-mobility growth of metallic superlattices
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I. INTRODUCTION Metallic superlattices involving quite dissimilar metals have interesting physical properties and potential technological applications. '~12 Except for a few special cases, most of the metallic superlattices reported have been grown at room temperature or below, where surface diffusion is not active, and as a consequence the growth has limited perfection. X-ray studies have shown that the crystalline structure is coherent in the growth direction over many periods, but has a coherency in the growth plane limited to 100 to 300 A.12 The defects in the films associated with the lowtemperature growth13 provide a ready means to accommodate the lattice mismatch between the constituents. Even so, the appreciable width of the x-ray peaks for most systems shows that the coherency is limited in extent. As the period is reduced, the coherent crystalline superlattice structure disappears, usually giving way to an amorphous phase, but still with a strong modulation in chemical composition. In a study of Mo/Fe, Nb/Fe, and W/Fe superlattices using x-ray diffraction and Mossbauer spectroscopy,10 the amorphous phase and the crystalline phase were found to coexist over a certain periodicity range. Narrow superlattice peaks were observed in this range because the boundaries between the mixed phases assisted in accommodating the mismatch strains. One way to modify the structures of the films without increasing the temperature is to bombard the growing film with energetic ions.14 This removes defects by enhancing surface diffusion, by knock-on cascades, by thermal spike effects, and by stress-induced plastic flow.15 Eltoukhy and Greene"" 8 have shown that by varying the bias voltage in the rf sputtering of InSbGaSb multilayers that the ion bombardment produced 856
J. Mater. Res. 3 (5), Sep/Oct 1988
http://journals.cambridge.org
mixing of the interfaces due to enhanced diffusion, provided the substrate temperature was high enough to promote diffusion. We report here an x-ray diffraction study of Mo/Fe superlattices in the periodicity range of 9 to 30 A, in which the films have been deposited at low temperatures under varying degrees of ion bombardment. The effects of ion bombardment on features such as the appearance of the crystalline phase as the period is increased, the coherency of the crystalline phase, and the mixing between the layers were observed and interpreted. There are indications that the intensity of the reflections do increase substantially with some ion bombardment, suggesting that this may assist in making better x-ray optical components. II. EXPERIMENTAL DETAILS The films were deposited at a nominal 300 K substrate temperature using argon gas pressures of 0.2 and 1 Pa. The sources utilized the unbalanced magnetron design19 to achieve high ion fluxes, and the iron target had an annular gap to break the magnetic circuit.20 The deposition rates were determined for the two sources by measurement of the weight of films on Al foils that were scanned under the guns 500 times at a known speed. All the sample
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