Molecular Dynamics Simulations Study Of Formation And Control Of Crystal Defects In Thin Films

Crystal defects in thin films mostly form in the atomic deposition processes, while current experimental means do not have enough resolution power in both space and time to analyze any processes at atomistic scale. Using three dimensional molecular dynamics method, and taking FCC metals as prototypes, we have carried out atomistic simulation studies of the formation mechanisms of thin film crystal defects and their control. EAM many-body potentials are adopted to calculate the inter-atomic forces. The work conducted mainly include establishing the molecular dynamics model, the simulation methods and codes for deposition of thin films; fundamental studies of the atomistic deposition; the structures, formation mechanisms and possible control methods of misfit dislocations, twins, and a stress-driven growth texture in <111> epitaxial films; the possibility of growing high quality and easy-to-peel-off epitaxial films on nano pillar crystals arrays. The results show that(1) An epitaxial film can maintain dislocation-free if its surface maintain flat at atomic scale, even when it is tens times thicker than its critical thickness. This structure is, however, very sensitive to disturbances. A single atomic layer variation on surface or merely thermal disturbances at elevated temperatures will trigger formation of the misfit dislocations.(2) For the epitaxial films with negative mismatch, the misfit dislocation nucleates in a local surface fusion-crystallization process. The local fusion begins with a surface step triggered squeeze-out of an inverse mini-tetrahedron. The dislocation formed is a perfect edge dislocation with its Burgers vector paralleled to the misfit and its sliding plane paralleled to the interface. Occasionally, the dislocation appears in form of Shockley type extended dislocation.(3) For the epitaxial films with positive mismatch, two kinds of structures and nucleation mechanisms have been identified for the misfit dislocations, depending onmagnitude of the mismatch f_x. When f_x ≥0.05, they are similar to those in the films with negative mismatch, except that the local fusion is originated from a cross-surface glide; When f_x ≤0.04, the misfit dislocation forms directly as a result of a partial...