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Results are reported of a systematic atomic-scale computational analysis of strain relaxation mechanisms and the associated defect dynamics in nanometer-scale thin or ultrathin Cu films that are subjected to a broad range of biaxial tensile strains. The films contain pre-existing voids and the film planes are oriented normal to the [formula] crystallographic direction. The analysis is based on isothermal-isostrain molecular-dynamics simulations according to an embedded-atom-method parameterization for Cu and employing multimillion-atom slab supercells. In addition to an initial elastic response for an applied biaxial strain level [variant_greek_epsilon]<2%, our analysis reveals three regimes in the thin-film mechanical response as [variant_greek_epsilon] increases. For 2%≤[variant_greek_epsilon]≤6%, biaxial strain relaxation is dominated by emission and propagation of dislocations (plastic flow) from the surface of the void accompanied by ductile void growth. For 6%<[variant_greek_epsilon]<10%, the biaxial strain in the thin film is relaxed by both ductile void growth and emission of dislocations from the surfaces of the thin film. For [variant_greek_epsilon]≥10%, strain relaxation is dominated by dislocation emission from the surfaces of the thin film, leading to a structural transformation from the face-centered cubic to a hexagonal close-packed phase. The defect nucleation mechanisms and the high-strain response of the thin films are found to be significantly different from those observed in <111>-oriented Cu thin films [M. R. Gungor and D. Maroudas, J. Appl. Phys. 97, 113527 (2005); M. R. Gungor and D. Maroudas, Appl. Phys. Lett. 87, 171913 (2005)]. [ABSTRACT FROM AUTHOR]

We report results of large-scale molecular-dynamics simulations of dynamic deformation under biaxial tensile strain of prestrained single-crystalline nanometer-scale-thick face-centered cubic (fcc) copper films. Our results show that stacking faults, which are abundantly present in fcc metals, may play a significant role in the dissociation, cross slip, and eventual annihilation of dislocations in small-volume structures of fcc metals. The underlying mechanisms are mediated by interactions within and between extended dislocations that lead to annihilation of Shockley partial dislocations or formation of perfect dislocations. Our findings demonstrate dislocation starvation in small-volume structures with ultrathin film geometry, governed by a mechanism other than dislocation escape to free surfaces, and underline the significant role of geometry in determining the mechanical response of metallic small-volume structures. [ABSTRACT FROM AUTHOR]

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 [variant_greek_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 ([variant_greek_epsilon]∼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%<[variant_greek_epsilon]<8%, dislocations are emitted uniformly from the thin-film surface, inhibiting the nucleation of voids. For [variant_greek_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 [variant_greek_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. [ABSTRACT FROM AUTHOR]

We report a molecular-dynamics study of the mechanical response to dynamic biaxial tensile straining of nanometer-scale-thick Al, Cu, and Ni films. We find that the mechanical behavior of such films of face-centered cubic metals with moderate-to-high propensity for stacking-fault formation (Cu and Ni) is significantly different from those where such propensity is low (Al). The plastic strain rate in Cu and Ni films is greater than that in Al ones, leading to an extended easy-glide stage in Cu and Ni but not in Al films. These differences arise due to the different dislocation annihilation mechanisms in the two film categories. [ABSTRACT FROM AUTHOR]

We report an analysis of void nucleation as a relaxation mechanism in freestanding biaxially strained ultrathin films of face-centered cubic metals based on large-scale molecular-dynamics simulations. Above a critical strain level, multiple threading dislocations are emitted from the film surface. The surface step traces formed by gliding dislocations on intersecting and on adjacent parallel glide planes lead to formation and growth of surface pits and grooves, while vacancies form due to gliding of jogged dislocations and dislocation intersections. Coalescence of the surface pits with vacancy clusters is the precursor to the formation of a larger void extending across the film. [ABSTRACT FROM AUTHOR]

We report the results of a theoretical analysis for the kinetics of strain relaxation in Si1-xGex thin films grown epitaxially on Si(100) substrates. The analysis is based on a properly parametrized dislocation mean-field theoretical model describing plastic deformation dynamics due to threading dislocation propagation and addresses strain relaxation kinetics during both epitaxial growth and thermal annealing, including post-implantation annealing. Theoretical predictions for strain relaxation as a function of film thickness in Si0.80Ge0.20/Si(100) samples annealed after epitaxial growth either unimplanted or after He ion implantation are in excellent agreement with experimental measurements [J. Cai et al., J. Appl. Phys. 95, 5347 (2004)]. [ABSTRACT FROM AUTHOR]