Real-Time Coarsening Dynamics of Ge/Si(100) Nanostructures Using Elevated Temperature Scanning Tunneling Microscopy.
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Real-Time Coarsening Dynamics of Ge/Si(100) Nanostructures Using Elevated Temperature Scanning Tunneling Microscopy. Michael R. Mckay1, Jeff Drucker2 and John Shumway2 1 Science and Engineering of Materials, Arizona State University Tempe, AZ 85287-1704 2 Physics and Astronomy, Arizona State University Tempe, AZ 85287 ABSTRACT 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. INTRODUCTION Heteroepitaxy of self-assembled Ge quantum dots on Si(100) has been studied extensively over the past several years because of their potential technological applications as well as for the fundamental insights they provide into nanocrystal growth. The Ge/Si(100) system follows the Stranski-Krastanov growth mode. Ge initially wets the Si surface, due to its lower surface energy, and grows pseudomorphically to a thickness of approximately three monolayers (1ML=6.78 x 1014 atoms/cm2). Due to the 4.2% lattice mismatch between Ge (aGe=5.65 Å) and Si (aSi=5.43 Å), elastic energy linearly increases during layer growth. Beyond three monolayers, it becomes energetically favorable to from 3-D islands, reducing elastic energy at the cost of increased surface energy. The first faceted, 3-D islands to form are square based pyramids [1], with side length s, and rectangular based huts [2], with length l and width w and basal aspect ratio l/w (hereafter, aspect ratio). Both are bound by {105} facets, which have contact angles of 11.3° with the substrate. Huts result from a kinetic instability due to the larger energy barrier to nucleate additional atomic planes on the large facet [3,4]. Similarly, it has been found that the hut length is kinetically limited, because the energy barrier to nucleate additional atomic planes on the
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