An initial phase of Ge hut array formation at low temperature on Si(001)

We report a direct STM observation of Ge hut array nucleation on the Si(001) surface during ultrahigh vacuum molecular-beam epitaxy at 360C. Nuclei of pyramids and wedges have been observed on the wetting layer MxN patches starting from the coverage of about 5.1 \r{A} (~3.6 ML). Further development of hut arrays consists in simultaneous growth of the formerly appeared clusters and nucleation of new ones resulting in gradual rise of hut number density with increasing surface coverage. Huts nucleate reconstructing the patch surface from the usual c(4x2) or p(2x2) structure to one of two recently described formations composed by epitaxially oriented Ge dimer pairs and chains of four dimers.