NUCLEATION OF A 60-DEGREES GLIDE DISLOCATION IN 2-DIMENSIONAL OR 3-DIMENSIONAL GROWTH OF EPILAYERS

The nucleation of a 60-degrees glide dislocation in epitaxial thin films is analyzed. Nucleation from either a free surface or from a threading dislocation is considered. In this work, the presence of either infinitely long steps in a two-dimensional film or finite size steps in a two-dimensional or a three-dimensional film is proposed to reduce the activation energy associated with dislocation nucleation. The activation energy is reduced below 40 kT, a value predicted by standard nucleation theory for favorable nucleation, if the step is partially eliminated upon nucleation of a dislocation. However, when the coherency stress is small due to smaller lattice mismatch parameter, the activation energy can not be reduced below 40 kT. In addition, the activation energy is found to be a function of the size of the step but not that of the epilayer. Bowing of a threading dislocation by nucleation of kinks and propagation of the kinks along the dislocation is also analyzed using small angle approximation of the bow out. The activation energy associated with the bow out is again found to be independent of the size of the epilayer thickness. The critical size of the epilayer can be defined if the coherency strain energy remaining in the film is released in the form of thermal energy that helps to overcome the activation energy for nucleation of a dislocation.