At sufficiently high temperature, epitaxial growth is expected to occur by the flow of atomic steps: atoms deposited on flat terraces on the surface of a crystal diffuse across the surface until they encounter steps. At steps they are incorporated at low energy kink sites causing the steps to advance. Since the utility of thin films grown in this way often depends on the roughness of the film surface, much work has been devoted to characterizing the morphology of steps as they flow. In single component systems the basic processes governing the morphology of moving steps is increasingly well understood. However, in heteroepitaxy, new types of cooperative behavior can arise, e. g., related to the coupling between surface alloying and step motion.
In this work we report the discovery of a striking 100 nm length-scale patterned growth mode during the deposition of palladium on ruthenium. After an initial stage were Pd starts to decorate the Ru step edges, the growth continues only from a few well separated locations leading to a snake-like motion of islands. We have combined real-time imaging using low-energy electron microscopy (LEEM) with atomically resolved information gained from scanning tunneling microscopy (STM) coupled to ab-initio calculations of the relevant surface processes. We propose that the labyrinthine growth mode arises from slow alloying at step edges. The alloying in turn decreases the step edge attachment rate, so only step sections that move fast enough, and thus remain un-alloyed, can grow by the attachment of additional Pd. |