  Difference in nucleation of Ge quantum dots during Ge deposition at low (<
600C) and high (> 600C) temperatures on the Si(001) surface is studied by high
resolution scanning tunneling microscopy. Two process resulting in appearance
of {105}-faceted clusters on the Ge wetting layer have been observed at high
temperatures: Pyramids have been observed to nucleate via the previously
described formation of strictly determined structures, resembling blossoms,
composed by 16 dimers grouped in pairs and chains of 4 dimes on tops of the
wetting layer M x N patches, each on top of a separate single patch, just like
it goes on at low temperatures; an alternative process consists in faceting of
shapeless heaps of excess Ge atoms which arise in the vicinity of strong sinks
of adatoms, such as pits or steps. The latter process has never been observed
at low temperatures; it is typical only for the high-temperature deposition
mode.
