Conditions for formation of misfit dislocation in epitaxial films — a molecular dynamics study

Molecular dynamics simulations for the formation of misfit dislocation in compre ssive epitaxial aluminum films have been carried out. The potential in an embed ded atom method (EAM) is employed. The results show that, in long relaxation at 500K, the films with a perfect surface remain dislocation-free in thickness ran ge of 9—80 atomic layers, which corresponds to 3—40 times of its thermodynami c critical thickness. However, with the presence of small boss or pit of one-at om high and three-atom wide on surface, misfit dislocations form readily in film s of 15 atomic layer thick. In dynamic growth of a preset 9 atomic layer thick film, under deposition, the surface develops significant roughness naturally, l eading to rapid formation of misfit dislocation. The dislocations formed under the three conditions are all complete edge dislocations, with their Burgers vec tors parallel to the axis of misfit. Analysis revealed that, under compression , the micro-boss induces squeezing-out of atoms beside, leading to nucleation of a dislocation, and the micro-pit is directly reshaped to a nucleus of dislocat ion semi-loop.