Intrinsic stress and alloying effect in Mo/Ni superlattices: a comparison between ion beam sputtering and thermal evaporation

Epitaxial Mo(110)/Ni(111) superlattices were grown on (1120) single-crystal sapphiresubstrates, by ion beam sputtering (IBS) and thermal evaporation (TE), in order to investigate the role of deposited energy on the interfacial mixing process observed in Mo sublayers. To separate intermixing and growth stress contributions, a careful and detailed characterization of the stress/strain state of both samples was performed by X-ray Diffraction (XRD). Non-equal biaxial coherency stresses are observed in both samples. For the IBS specimen, an additional source of stress, of hydrostatic type, due to growth-induced point defects, is present, resulting in a triaxial stress state. The use of ion irradiation to achieve a controlled stress relaxation can provide additional data and, as shown elsewhere, allows to obtain the stress-free lattice parameter a0 solely linked to chemical effects. For the TE sample, a standard biaxial analysis gives a0. In both samples, the a0 value is lower than the bulk lattice parameter, due to the presence of intermixed Mo(Ni) layers. However, the intermixing is larger in the sputtered Mo sublayers than in the thermal evaporated ones, putting forward the prime role of energy and/or momentum transfer occurring during energetic bombardment.