Competing growth mechanisms of Ge/Si(001) coherent clusters

The growth of Ge three-dimensional coherent clusters on Si(001) during gas source molecular-beam epitaxy and post-deposition anneals has been investigated using in situ elevated-temperature scanning tunneling microscopy. By monitoring the growth of individual so-called “hut” clusters, this technique allowed us to separate various factors that may affect the final size distribution of entire cluster ensembles. It has been found that during the course of epitaxy the hut clusters grow by nucleation and growth of deposited material on the cluster facets; however, the low growth rate ((Formula presented) where (Formula presented)), and the large scatter in absolute rate constants indicate diffusion-limited, rather than interface-limited mass transport, although both the facet-nucleation step and the wetting-layer defects inhibit the cluster growth. The strain-induced energy barriers at the cluster bases prevent material addition, and thus growth of large clusters, facilitating the domination of growth mechanisms other than Ostwald ripening at temperatures below 700 K, leading to symmetric or positively skewed cluster-size distribution functions. The tendency towards negatively skewed and bimodal distributions at higher temperatures signaled the contribution of the ripening in accord with the Lifshitz-Slyozov-Wagner theory. Raising the temperature above 770 K leads to a gradual replacement of the huts by the 〈110〉-based macroscopic clusters.