Evolution to equilibrium of the shape of an island formed by the aggregation of adsorbed atoms

Abstract

It is possible to heat up a surface on which islands have been prepared and cause them to change shape. In this paper we use kinetic Monte Carlo simulations to study this process. The rate constants used in the model are of the order of magnitude encountered in metal-on-metal systems. The atomic motions take place on a square lattice. We find that the evolution of an island’s shape is controlled by the ability of the atoms located at single-atom kinks and at the corners of larger kinks to leave their places. If the temperature is so low that the kink atoms are immobile, any island shape achieved during growth is stable. If the kink atoms can move, but the corner ones cannot, the edges that are not parallel to the directions of the square lattice are unstable and will disappear. The island reaches a stable shape in which the perimeter consists of many edges parallel to the crystal directions. At a higher temperature, at which the corner atoms are mobile, the island will reach the equilibrium shape. Its evolution has three kinetic stages. In the earliest the diagonal edges disappear quickly and the shape evolves to a superposition of several rectangles. In the second stage this many-rectangle shape evolves towards one rectangle. In the third, the rectangle evolves, at a much slower pace, into a square. We discuss in detail the mechanism of these changes.