Ostwald ripening on surfaces when mass conservation is violated: Spatial cluster patterns

The dominant mechanisms of late stage cluster growth on surfaces is determined by the external flux of material impinging on the substrate during clustering. When the flux is zero, i.e. under mass conservation conditions, Ostwald ripening dominates cluster growth and a narrow, unimodal cluster size distribution is predicted. When instead the external flux is sufficiently large, coalescence is the dominant growth mechanism and a very broad bimodal distribution is predicted. We present experimental results on the clustering of Sn on Si(111) when mass conservation of the clustered material is violated but with the flux in a regime where we expect a transition from ripening to coalescence. Observations include (i) an unexpected narrowing of the cluster size distribution and (ii) spatial correlation of clusters which prominently feature groups of small clusters and isolated large clusters. Possible mechanisms for these observations are discussed based on cluster-cluster correlations and perturbations of the Ostwald ripening formalism.