  Structural models of growing Ge hut clusters---pyramids and wedges---are
proposed on the basis of data of recent STM investigations of nucleation and
growth of Ge huts on the Si(001) surface in the process of molecular beam
epitaxy. It is shown that extension of a hut base along <110> directions goes
non-uniformly during the cluster growth regardless of its shape. Growing
pyramids, starting from the second monolayer, pass through cyclic formation of
slightly asymmetrical and symmetrical clusters, with symmetrical ones appearing
after addition of every fourth terrace. We suppose that only symmetrical
configurations of pyramids composed by 2, 6, 10, 14, etc. monolayers over the
wetting layer are stable. This might explain less stability of pyramids in
comparison with wedges in dense arrays obtained at low Ge deposition
temperatures.
  Possible nucleation processes of pyramids and wedges on wetting layer patches
from identical embryos composed by 8 dimers through formation of 1 monolayer
high 16-dimer nuclei different only in their symmetry is discussed. Schematics
of these processes are presented for both species of huts.
  It is concluded from precise STM measurements that top layers of WL patches
are relaxed when huts nucleate on them.

Materials Science;Mesoscale and Nanoscale Physics