The low-temperature growth and relaxation of misfitting films are analyzed on the basis of two-dimensional molecular dynamics simulations using Lennard-Jones potentials. The temporal evolution of the surface morphology and the mechanisms for misfit dislocation nucleation and stress relaxation are monitored. Pseudomorphic film growth is observed up to a critical thickness. In some cases, the formation of voids within the film relaxes some of the stress. At the critical thickness, dislocations nucleate and relax most of the misfit. The critical thickness increases with decreasing lattice mismatch and depends on the sign of the misfit. The critical thickness of compressively strained films is smaller than that of films with the same magnitude of misfit, but in tension. The mechanism of dislocation nucleation is different in tension and compression and, in all cases, is associated with the roughness of the film surface. In the compressive misfit case, dislocations nucleate by squeezing-out an atom at the base of surface depressions. In the tensile misfit case, however, the nucleation of misfit dislocations involves the concerted motion of a relatively large number of atoms, leading to insertion of an extra lattice ͑plane͒ row into an already continuous film. These results show that the critical thickness depends intimately on the film morphology which, in turn, is determined as an integral part of the film growth process.

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Two-dimensional, nonequilibrium molecular dynamics simulations have been applied to study the structure of thin films grown on single-crystal Lennard-Jones substrates. The principal microstructural features to develop within these films are single vacancies and small voids which tend to be slightly elongated and to be aligned in the growth direction. Both the void volume and the mean surface roughness of the films are found to be decreasing functions of substrate temperature and deposition kinetic energy. Voids are shown to form as a consequence of both surface roughness and shadowing effects. The attraction between deposited atoms and the sides of surface depressions lead to the formation of outgrowths from the sidewalls of the surface depression. These outgrowths shadow the open void beneath them and continue to grow across the voids by interaction with the depositing atoms until a continuous bridge is formed that closes off the void. Since this bridging mechanism leaves behind a surface depression above the closed-off void, new voids tend to form above it. This leads to the alignment of voids along the film growth direction. The spacing of the resultant void tracks is correlated with the wavelength of the surface roughness. Increasing temperature and deposition kinetic energy enhancing surface mobility leads to an increase in the wavelength of the surface roughness and hence an increase in the spacing between void tracks. Edge dislocations tend to form within voids as a natural consequence of the void bridging process, however nondislocated voids are also observed.

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