Elastic strains and enhanced stress relaxation effects induced by ion irradiation in W(110)/Cu(111) multilayers: Comparative EXAFS and x-ray diffraction studies

A characterization of the local structure in Cu/W superlattices (period $\Lambda =20\mathrm{}$ nm) prepared by ion beam sputtering with average composition ${\mathrm{Cu}}_{50}{\mathrm{W}}_{50}$ has been performed by x-ray absorption spectroscopy (EXAFS) and x-ray diffraction (XRD). Copper and tungsten are immiscible and so, sharp concentration modulation is obtained. The elasticity analysis of the EXAFS data evidences the existence of large strains due to compressive stress in the growth planes of W sublayers. The stress level determinated from EXAFS agrees fairly well with the one deduced from XRD while lattice parameters obtained from EXAFS are smaller than those measured with XRD. This discrepancy can be attributed to the inherent differences between short-range and long-range order methods for probing the material, but also to the limited accuracy of the EXAFS method for determining lattice parameters. However, EXAFS is well suited to the study of the disorder inside each elemental components of these superlattices. After an irradiation with a low Kr ions dose ${(10\mathrm{}}^{13}\hspace{0.5em}{\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^2),$ the W sublayers are partially relaxed to a stress level equal to one half of its original value while the structure of the Cu remains unchanged. The combination of the EXAFS and XRD techniques is a unique approach to achieve a complete characterization of the structural properties of strained nanometric layers as those considered in the present paper.