Grain Boundary Propagation and Epitaxy on (111) Surfaces of FCC Substrates: A Kinetic Monte Carlo Study with Lennard-Jones and Iridium Potentials

A Kinetic Monte Carlo (KMC) method was developed to model homoepitaxy and grain boundary propagation on a (111) surface. Barrier energies were calculated using the Nudged Elastic Band (NEB) technique. A recently reported inertial relaxation technique named FIRE (the Fast Inertial Relaxation Engine) was used to relax the NEB images. Both the Lennard-Jones potential and a Sutton-Chen Iridium potential were used and compared. A doubly-refined lattice mesh was developed to incorporate atoms in Face-Centred-Cubic (FCC) and Hexagonal-Close-Packed (HCP) sites as well as atoms in decorated row sites (i.e. supported by 4 atoms). A look-up table was developed to identify hops in the KMC algorithm. The KMC results show that a small difference in energy barriers between FCC and HCP sites on the substrate can cause a substantial bias in the direction of grain boundary propagation. We also investigated the effect of the geometry of the grain boundary on its propagation, as well as the atomistic processes involved in grain boundary propagation and the merger of grain boundaries. Our deposition simulations produced islands with loosely triangular envelopes, where FCC islands are rotated 180° with respect to HCP islands. The results are similar to scanning tunneling microscopy (STM) images of Iridium deposition, although lack of computing power forced us to use a high deposition rate and this caused some differences.