STRESS-RELAXATION AND MISFIT DISLOCATION NUCLEATION IN THE GROWTH OF MISFITTING FILMS - A MOLECULAR-DYNAMICS SIMULATION STUDY

The low-temperature growth and relaxation of misfitting films are anal yzed on the basis of two-dimensional molecular,dynamics simulations us ing Lennard-Jones potentials. The temporal evolution of the surface mo rphology and the mechanisms for misfit dislocation nucleation and stre ss 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, disloc ations nucleate and relax most of the misfit. The critical thickness i ncreases with decreasing lattice mismatch and depends on the sign of t he misfit. The critical thickness of compressively strained films is s maller than that of films with the same magnitude of misfit, but in te nsion. The mechanism of dislocation nucleation is different in tension and compression and, in all cases, is associated with the roughness o f the film surface. In the compressive misfit case, dislocations nucle ate by squeezing-out an atom at the base of surface depressions. In th e tensile misfit case, however, the nucleation of misfit dislocations involves the concerted motion of a relatively large number of atoms, l eading to insertion of an extra lattice (plane) row into an already co ntinuous 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. (C) 1998 American Institute of Physics.