科研之友
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摘要
We report results of a detailed systematic computational analysis of strain relaxation mechanisms and the associated defect dynamics in ultrathin, i.e., a few nanometers thick, Cu films subjected to a broad range of biaxial tensile strains. The analysis is based on isothermal-isostrain molecular-dynamics simulations of the response of Cu films that are oriented normal to the [111] crystallographic direction using an embedded-atom-method parametrization for Cu and multimillion-atom slab supercells. Our analysis reveals five regimes in the thin film's mechanical response with increasing strain. Within the considered strain range, after an elastic response up to a biaxial strain level epsilon=5.5%, the strain in the metallic thin film is relaxed by plastic deformation. At low levels of the applied biaxial strain above the yield strain (epsilon similar to 6%), threading dislocation nucleation at the surface of the thin film in conjunction with vacancy cluster formation in the film leads eventually to the formation of voids that extend across the thickness of the film. For 6%<epsilon < 8%, dislocations are emitted uniformly from the thin-film surface, inhibiting the nucleation of voids. For epsilon >= 8%, in addition to nucleation of dislocations from the film surface, dislocation loops are generated in the bulk of the film and grow to intersect the thin-film surface. For epsilon >= 10%, a high density of point defects in the film leads to nucleation of Frank partial dislocations that dissociate to form stacking fault tetrahedra. In addition, dislocation-dislocation interactions due to the high dislocation density lead to the formation of Lomer-Cottrell dislocation locks and complex stable dislocation junctions that act as obstacles to dislocation glide. As a result of these defect mechanisms, nanoscale domains are formed in the crystalline film with an average domain size of 1.5 nm and low-angle misorientations.
- 出版日期2008-6-15
