Classification of Ge hut clusters in arrays formed by molecular beam epitaxy at low temperatures on the Si(001) surface

Ge hut clusters forming quantum dot arrays on the Si(001) surface in the process of low-temperature ultrahigh-vacuum molecular beam epitaxy are morphologicaly investigated and classified using in situ scanning tunnelling microscopy. It is found that two main Ge hut cluster types — pyramidal and wedge-shaped — have different atomic structures, and it is concluded that shape transitions between the two are impossible. Derivative cluster species — obelisks (or truncated wedges) and accreted wedges — are revealed and investigated for the first time and shown to start dominating at high Ge coverages. The uniformity of cluster arrays is shown to be controlled by the scatter in the length of wedge-like clusters. At low growth temperatures (360$^{\circ}$C), cluster nucleation during the growth of the array is observed for 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 temperatures (530$^{\circ}$C), no cluster nucleation is observed after the initial formation of the array.