Structural Characterization of Multilayers Using X-ray Diffraction
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Structural Characterization of Multilayers Using X-ray Diffraction B.M. CLEMENSI, J.A. BAINt, A.P. PAYNEt, T.C. HUFNAGELt, AND S.M. BRENNAN't tDepartment of Materials Science and Engineering, Stanford University, Stanford, CA 94305-2205 tStanford Synchrotron Radiation Laboratory, Stanford, CA 94309 ABSTRACT Structural deviations from ideal layering of bulk constituents can have dramatic effects on the properties of multilayered materials. We discuss three examples of the use of x-ray diffraction in non-standard geometries to examine these effects. In Mo/Ni multilayers, we use asymmetric diffraction and grazing incidence x-ray scattering (GIXS) to deduce the strain and intermixing. We find that coherency stresses between the BCC Mo and FCC Ni planes play a major role. In the Fe/Cr system, we use rocking curves and asymmetric scans about the small angle superlattice lines to investigate the nature and extent of layer roughness. We find that conformal roughness dominates our best samples, while non-conformal roughness increases with sputter deposition pressure. In the Gd/Co system, we use in-situ GIXS to investigate amorphization reaction during deposition. We find a strong diffusional asymmetry and rapid reaction during growth. INTRODUCTION Multilayer materials have generated considerable interest due to possible applications in many technological areas (see for example Ref.[1]). Many of the interesting properties of these materials are related to structural phenomena which result from the artificially imposed atomic scale composition modulation. These structural effects include; interface reactions[2], epitaxial stabilization of metastable phases, and unusual levels of strain or defect concentrations[3, 4, 5]. In addition, multilayers fall prey to local deviations from the ideal layer structure - interface roughness and layer thickness fluctuations can also affect the observed physical properties[6]. Determination of the underlying causes of physical properties must start with a thorough investigation of these structural effects.
a) Figure 1: Schematic of structural phenomena in multilayer materials, a) Perfect interfaces with each material taking on its bulk form. b) Perfect interface, but with each material strained to match the in-plane lattice parameter. c) Inhomogeneous strain within each layer. d) Conformal interface roughness. e) Non-conformal interface roughness resulting in layer thickness fluctuations. f) Intermixing or reaction at the interface.
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Among the physical properties of multilayers most sensitive to structural imperfections is the scattering of x-rays[7, 8]. While this has been depressing for those interested in multilayers as x-ray optical elements, it is beneficial to those interested in structural determination. This paper presents several examples of the use of this sensitivity for structural investigations in multilayer materials. We start with a description of the physical effects manifesting themselves in symmetric reflection diffraction, which
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