Anisotropic strain-stress state and intermixing in epitaxial Mo(110)/Ni(111) multilayers: An x-ray diffraction study

The present study deals with the analysis of elastic strains and stresses in high-quality heteroepitaxial Mo/Ni superlattices with periods Lambda lying in the range 4.8-27.6 nm. The strain-stress state in this lattice-mismatched system grown under energetic deposition conditions (ion beam sputtering) is rather complex, resulting from three contributions: (i) intrinsic (growth) stress due to atomic peening, (ii) coherency stresses of opposite sign in the two elemental layers due to the observed Nishiyama-Wassermann epitaxial relationship Ni[110](111)Mo[001](110), and (iii) interfacial mixing. The measurement of the lattice parameters of Mo and Ni sublayers in various crystallographic directions was performed by x-ray diffraction, using the sin(2) Sigma method adapted for epitaxial layers. A large anisotropy of elastic strain and associated in-plane coherency stresses is revealed in the Mo sublayers, while for Ni sublayers no such behavior could be detected due to the superimposition of growth variants with threefold symmetry. Postgrowth ion irradiation with Ar ions at very low dose (similar to 0.2 dpa) was employed as a powerful tool to modify the intrinsic stress, thus providing additional data to be implemented in a triaxial strain-stress model, which enabled us to separate the different stress sources (intrinsic and coherency stresses) as well as to quantify the intermixing occurring during growth. This model, which has been successfully applied previously to Mo thin films, yields in the case of multilayer systems to the determination of the "stress-free and defect-free" lattice parameter, a(0), i.e., solely linked to chemical mixing. The linear dependence of a(0) with Lambda observed in both sublayers reveals an interface-mediated chemical mixing mechanism, the extent of this interfacial mixing being much more pronounced in Mo sublayers than in Ni ones.