Real-time coarsening dynamics of Ge/Si (100) nanostructures

The coarsening dynamics of GeSi (100) nanostructures were monitored using real-time, elevated temperature scanning tunneling microscopy (STM). Gas-source molecular beam epitaxy from digermane onto Si(100) was used to produce mixed hut and pyramid cluster ensembles. The width of the most elongated rectangular-based hut clusters was always less than the side length of square-based pyramid clusters for the growth conditions employed. This suggests that pyramid elongation to form hut clusters occurred at early growth stages for some smaller clusters. A previously unidentified coarsening mechanism was characterized during growth temperature annealing and was interpreted using atomistic elastic modeling. Pyramid clusters were more stable than narrow hut clusters with larger volumes. These larger volume huts decayed by reducing their length at a constant width, finally becoming small pyramids. These small pyramids are less stable than those that never elongated to form huts and consequently dissolve. The decrease in the total island volume identified using STM was attributed to large (diameter 1 μm), low chemical potential clusters. These clusters depleted the surface of Ge adatoms as evidenced by denuded zones devoid of smaller clusters observed for growth and annealing at higher temperatures. These results suggest that a preferred island size for pyramids and huts does not exist.