Two-dimensional non-equilibrium molecular dynamics simulations were used to model the structure of thin films which were grown onto monocrystalline Lennard-Jones substrates. It was found that the main microstructural features which developed within these films were single vacancies, and small voids which tended to be slightly elongated and aligned in the growth direction. Both the void volume and the mean surface roughness of the films were decreasing functions of the substrate temperature and deposition kinetic energy. The voids were shown to form as a result of both surface roughness and shadowing effects. The attraction between deposited atoms, and the sides of surface depressions, led to the formation of out-growths from the side-walls of the surface depressions. These out-growths shadowed the open void beneath them and continued to grow across the voids, via interaction with the deposited atoms, until a continuous bridge formed that closed off the void. As this bridging mechanism left behind a surface depression above the closed-off void, new voids tended to form above it. This led to an alignment of voids along the film-growth direction. The spacing of the resultant void tracks was related to the wavelength of the surface roughness. Increasing the temperature and the deposited kinetic energy enhanced the surface mobility. This led to an increase in the wavelength of the surface roughness, and thus to an increase in the spacing between void tracks. Edge dislocations tended to form within voids, as a natural consequence of the void-bridging process. However, non-dislocated voids were also observed.

R.W.Smith, D.J.Srolovitz: Journal of Applied Physics, 1996, 79[3], 1448-57