Large-scale molecular dynamics simulations of glancing angle deposition

Abstract

Using a computationally efficient method, we have carried out large-scale molecular dynamics simulations of Cu/Cu(100) growth up to 20 monolayers (ML) for deposition angles ranging from 50° to 85° and for both random and fixed azimuthal angles. A variety of quantities including the porosity, roughness, lateral correlation length, average grain size, strain, and defect concentration are used to characterize the thin-film morphology. For large deposition angles (θ≥80°), we find well-defined columnar growth while for smaller angles, columnar growth has not yet set in. In addition, for θ=70°−85°, the thin-film porosity and columnar tilt angles (for fixed azimuthal angle ϕ) are in reasonable agreement with experiments. For both random and fixed ϕ, the number of grains, average grain-size, and number of surface atoms belonging to (111) facets increase rapidly with deposition angle. As a result, twin facet formation and budding occur in our simulations, in good agreement with experiments. In good qualitative agreement with recent experimental observations, we also find that the average strain is initially compressive but becomes tensile after the onset of columnar growth. Our simulations also reveal that for large deposition angles a variety of unexpected and complex dynamical processes play a key role in determining the evolution of the surface morphology and microstructure. In particular, due to the existence of deposition-induced events, the vacancy density remains very small, even though the defect density is relatively large and increases with deposition angle. In addition, large-scale re-arrangement events as well as thermal (elastic) vibrations lead to large-amplitude oscillations in the columnar growth regime. These oscillations play a key role in promoting rapid coalescence via additional large-scale collective motion, thus, significantly enhancing the coarsening process.