Gold-Nickel Multilayer Films: Structure-Property Correlations
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GOLD-NICKEL MULTILAYER FILMS: STRUCTURE-PROPERTY CORRELATIONS
S. R. NUTT,* K A.GREEN,* S. P. BAKER,** W. D. NIX,** and A.JANKOWSKI*** *Brown University, Box D,Providence, RI 02912 "**Department of Materials Science and Engineering, Bldg. 550, Stanford University, Stanford, CA 94305 ***Lawrence Livermore National Lab, P.O. Box 808, L-350, Livermore, CA 94550
ABSTRACT Gold-nickel multilayer films with periods of 1.2 - 4.6 nm were deposited on silicon substrates by magnetron sputtering, and plan and cross-sectional specimens were examined by transmission electron microscopy. The cross-sectional specimens revealed well-defined layering and columnar growth features that extended through the film thickness. Dark striations extending normal to the layers were attributed to diffraction contrast from defect strain fields. Diffraction patterns showed that the films were highly textured and that short-period films had a single fcc structure, while long-period films separated into three fcc structures. High-resolution images of the layer interfaces showed local regions of epitaxy partitioned by regions of disorder. Indentation tests using a Nanoindenter, a depth-sensing indentation device, were performed to measure the elastic modulus and hardness of the films. No modulus enhancement was detected, and a small variation in hardness was measured.
INTRODUCTION Artificially layered bi-metallic thin films can exhibit a remarkable enhancement in elastic modulus known as the 'supermodulus effect.' The supermodulus effect is the enhancement in elastic modulus (by a factor of two or more) observed in highly textured compositionally modulated bi-metallic films in which the wavelength or bilayer thickness is about 2 nm. This phenomenon was first reported by Hilliard and co-workers in 1977 [1], and has subsequently been confirmed by other groups for several different bimetal systems [2-3]. Asatisfactory explanation of the phenomenon has been elusive, although two theories have been proposed. One theory Is based on singularities inthe electronic energy caused by interactions between the Fermi surface and the Brillouin zones associated with the composition modulation, and another theory is based on higher order elastic effects arising from the large coherency strains associated with the epitaxial layers [3]. Both of these theoretical approaches have been recently
reviewed [3-4]. Many of the systems which exhibit a supermodulus effect are fcc metals in which the lattice parameters of the two components are widely different. The films are generally deposited in vacuum, and the resulting structure is a highly textured strained layer superlattice inwhich the layers are alternately in tension and compression. The biaxial alternation between compressive and tensile stresses can be described as a strain wave, since the component with the larger lattice parameter is compressed, and the component with the smaller lattice parameter Is stretched. However, the details of how the strain is accommodated within the layers is presently unknown. Our app
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