  Morphological investigations and classification of Ge hut clusters forming
the arrays of quantum dots on the Si(001) surface at low temperatures in the
process of the ultrahigh vacuum molecular beam epitaxy have been carried out
using in situ scanning tunnelling microscopy. Two main species of Ge hut
clusters composing the arrays - pyramidal and wedge-shaped ones - have been
found to have different atomic structures. The inference is made that shape
transitions between pyramids and wedges are impossible. The nucleation
probabilities of pyramids and wedges equal 1/2 at the initial stage of the
array formation. The wedges become the dominating species as the amount of the
deposited germanium is increased. A fraction and a density of the pyramids in
the arrays are rapidly decreased with the growth of Ge coverage.
  The derivative types of the clusters - obelisks (or truncated wedges) and
accreted wedges - have been revealed and investigated for the first time, they
have been found to start dominating at high Ge coverages. The obelisks
originate from the wedges as a result of their height limitation and further
growth of trapezoid facets. The apexes of the obelisks are formed by sets of
the parallel (001) ridges.
  The uniformity of the cluster arrays have been evidenced to be controlled by
the length of the wedge-like clusters. At low growth temperatures (360C)
nucleation of new clusters is observed during the array growth at all values of
Ge coverage except for a particular point at which the arrays are more uniform
than at higher or lower coverages. At higher (530C) temperatures cluster
nucleation has not been observed after the initial stage of the array
formation.
