Defect-interface interactions

Concepts for the design of advanced nanoscale PVD multilayer protective thin films

Ion beam induced surface and interface engineering

Nitride-based coatings are nowadays widely studied both from fundamental and technological point of views due to their unique physical and mechanical properties. Among the binary nitrides, TiN is the most stable thermodynamically and has been widely used due to the combination of its covalent and metal-like characteristics. Coatings produced by Physical Vapor Deposition (PVD) techniques generally exhibit a crystallographic texture, which in turn may strongly affect their properties, such as hardness, wear resistance, or diffusion barrier properties in microelectronic devices. Therefore great efforts have been made in recent years to understand the underlying mechanisms governing texture development in nitride thin films. In particular, the issue of stress build-up during PVD growth and its possible interplay with film preferred orientation is essential to address. We present a brief overview of stress and preferred orientation in nitride-based thin films, either in the form of single-, multi-layered or nanocomposite coatings. X-ray Diffraction (XRD) was used in the standard θ –2 θ configuration to study the texture development with film thickness, while the sin 2 ψ method combined with linear elasticity theory was employed to determine the complete strain/stress state. XRD measurements were made in the framework of the crystallite group method, which is of prime importance in thin films exhibiting a mixed texture, as it enables to selectively measure the elastic strain in a given subset of grains. For PVD films grown with energetic particles, the appropriate modeling requires the use of a triaxial stress tensor, including a hydrostatic stress component to take into account the local distortions induced by growth-defects. This approach enables us to determine the ‘stress-free and defect-free lattice parameter’, a 0 , solely linked to chemical effect. Illustrations will be given for fiber-textured TiN and ZrN films deposited on Si substrates, epitaxial TiN layers as well as epitaxial TiN sub-layers in TiN/Cu multilayers grown on (001) MgO single crystal substrates. Ternary TiN-based coatings, either in the form of solid solutions or nanocomposites will be also investigated.

Stress and preferred orientation in nitride-based PVD coatings

The authors present a van der Waals epitaxy of high-quality ultrathin nanoplates of topological insulator Bi(2)Se(3) on a pristine graphene substrate using a simple vapor-phase deposition method. Sub-10-nm-thick nanoplates of layered Bi(2)Se(3) with defined orientations can be epitaxially grown on a few-layer pristine graphene substrate. We show the evolution of Raman spectra with the number of Bi(2)Se(3) layers on few-layer graphene. Bi(2)Se(3) nanoplates with a thickness of three quintuple-layers (3-QL) exhibit the strongest Raman intensity. Strain effects in the Bi(2)Se(3)/graphene nanoplate heterostructures is also studied by Raman spectroscopy. 1-QL and 2-QL Bi(2)Se(3) nanoplates experience tensile stress, consistent with compressive stress in single-layer and bilayer graphene substrates. Our results suggest an approach for the synthesis of epitaxial heterostructures that consist of an ultrathin topological insulator and graphene, which may be a new direction for electronic and spintronic applications.

Epitaxial heterostructures of ultrathin topological insulator nanoplate and graphene.

Multilayer coatings with layer thicknesses in the nanometer range have been shown to exhibit original mechanical properties: these include pronounced softening of some elastic moduli, large increase in yield strength and hardening effect. Such original properties are related to the high interface density and reduced grain sizes associated with these systems. In the first part of this article, the elastic and plastic properties of metal/metal, metal/nitride and nitride/nitride nanoscale multilayers are briefly outlined. The origin of large compressive stress commonly observed in PVD films grown with energetic particles is then presented and illustrations given for the case of epitaxial multilayers for which different stress sources exist. The second part focuses on recent experimental results obtained for three multilayered systems, Mo/Ni, TiN/Cu and ZrN/W, corresponding to different crystal structure and lattice mismatch combinations. Two issues will be addressed: the interdependence between interfacial mixing and elastic softening in Mo/Ni superlattices and the mechanical behaviour of TiN/Cu and ZrN/W multilayers combining ‘hard’ and ‘soft’ materials. These multilayers were grown epitaxially on either MgO (001) or Al2O3(112¯0) substrates by a dual ion beam sputtering technique. Low-angle and high-angle X-ray diffraction experiments as well as Transmission Electron Microscopy observations were used to characterize the microstructure and crystalline orientation, structure of interfaces and type of growth defects. Elastic properties were studied by Brillouin light scattering and hardness values determined from nanoindentation tests. For selected samples, a combined FIB-TEM technique was implemented to image the deformed nanolaminates beneath the indentor.

Stress, interfacial effects and mechanical properties of nanoscale multilayered coatings

On the origin of the metastable β-Ta phase stabilization in tantalum sputtered thin films

Stress and microstructure evolution during growth of magnetron-sputtered low-mobility metal films: Influence of the nucleation conditions

Real-time stress evolution during early growth stages of sputter-deposited metal films: Influence of adatom mobility

Mossbauer spectroscopy (57Fe) shows evidence for mixing effects induced by electronic energy deposition in nanoscale Fe/Si multilayers irradiated with swift heavy ions. A decrease in the mixing efficiency with electronic stopping power is reported; a threshold is found, under which iron environment modifications no longer occur. The kinetics of Fe–Si phase formation after irradiation suggests the existence of three regimes: (i) for high excitation levels, a magnetic amorphous phase is formed directly in the wake of the incoming ion and an almost complete mixing is reached at low fluence (1013 U/cm2); (ii) for low excitation levels, a paramagnetic Si-rich amorphous phase is favored at the interface while crystalline iron subsists at high fluences; (iii) for intermediate excitation levels, saturation effects are observed and the formation rate of both magnetic and paramagnetic phases points to direct mixing in the ion wake but with a reduced track length in comparison to U irradiation. The measured interfac...

C. Jaouen

Papers

Stress, interfacial effects and mechanical properties of nanoscale multilayered coatings

On the origin of the metastable β-Ta phase stabilization in tantalum sputtered thin films

Stress and microstructure evolution during growth of magnetron-sputtered low-mobility metal films: Influence of the nucleation conditions

Real-time stress evolution during early growth stages of sputter-deposited metal films: Influence of adatom mobility

High electronic excitations and ion beam mixing effects in high energy ion irradiated Fe/Si multilayers