On atomic structure of Ge huts growing on the Ge/Si(001) wetting layer
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.