Low area density Ge/Si(100) hut cluster ensembles are stable during days-long growth temperature anneals. Real-time scanning tunneling microscopy shows that all islands grow slowly at a decreasing rate throughout the anneal. Island growth depletes the Ge supersaturation that, in turn, reduces the island growth rate. A mean-field facet nucleation and growth model quantitatively predicts the observed growth rate. It shows that Ostwald ripening is kinetically suppressed for Ge supersaturations high enough to support a critical nucleus size less than the smallest facet.

The coarsening dynamics of Ge∕Si(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>1μm), 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.

No abstract

We have investigated the morphological evolution of islanded Ge/Si(100) samples formed by > 3 monolayer (ML) Ge deposition. Ge was deposited onto Si(100) surfaces cleaned by flash desorption of the native oxide at rates near 1/2 ML per minute. Growths were performed in an ultra-high vacuum system with a base pressure of < 10−9 Torr. Substrate temperature during growth was 500 °C. Post-deposition processing ranged from no additional treatment to 1 hour at 560 °C anneals. Samples removed from the growth chamber were processed using standard transmission electron microscopy (TEM) specimen preparation techniques and characterized using plan-view TEM. Micrographs were computer analyzed to generate island size distributions (histograms of island size). These size distributions fall into general classes. First, samples with only coherent Ge islands exhibit relatively narrow size distributions. Secondly samples with both coherent and incoherent islands presented bi-modal size distributions with coherent islands populating the smaller radii. These results will be discussed in the context of a model which includes elastic as well as surface and interface energies as driving forces for ripening.

Coarsening dynamics of Ge/Si(100) nanostructures were monitored using real-time, elevated temperature scanning tunneling microscopy. Gas-source molecular beam epitaxy from digermane at 0.2 ML / min onto Si(100) at temperatures near 500 °C produced mixed hut and pyramid cluster ensembles. The width of the most elongated rectangular-based hut clusters is always less than the side length of square-based pyramid clusters for island ensembles grown using these conditions. This suggests that pyramid elongation to form hut clusters occurred at early growth stages for some smaller clusters. Growth temperature annealing revealed that pyramid clusters are more stable than narrow hut clusters with larger volumes. These larger volume huts decay by reducing their length at constant width, finally becoming small pyramids. These pyramids, which are smaller than those which never elongated to form huts, are less stable and consequently dissolve. Atomistic elastic modeling confirms that hut clusters less efficiently store elastic energy than pyramid clusters which explains our observations. Large (> 1µm diameter), low chemical potential clusters deplete the surface of Ge adatoms as evidenced by the existence of denuded zones devoid of smaller clusters. These large clusters are responsible for the decrease in total island volume in the STM field of view during the anneal.