  We report a direct observation of Ge hut nucleation on Si(001) during UHV
MBE. The study was carried out using a UHV instrument coupling MBE chamber and
STM which enables the sample study on atomic level at any stage of treatment.
Si wafers were deoxidized by annealing at 925C. Ge was deposited by electron
beam evaporation; the coverage was varied from 3 to 14 \AA; the wafer
temperature was 360C. The nuclei of pyramids and wedges were observed on the
(MxN) wetting layer (WL) patches and found to have different structures. The
atomic models of nuclei of both hut species have been built as well as the
models of the growing clusters. The growth of huts of each species has been
demonstrated to follow generic scenarios. The formation of the second atomic
layer of the wedge results in rearrangement of its first layer. Its ridge
structure does not repeat the nucleus. The pyramid grows without phase
transitions. The structure of its vertex copies the nucleus. Transitions
between hut species are impossible. The wedges contain point defects in the
upper corners of the triangular faces and have preferential growth directions
along the ridges. The derived structure of the {105} facet turned out to follow
the PD model. Further growth of hut arrays results in domination of wedges, the
density of pyramids exponentially drops. The heights of wedges are limited; no
limitation of pyramid heights is observed. The second generation of huts arises
at coverages >10 \AA; new huts occupy the whole WL.
