Formation and ordering effects of C-induced Ge dots grown on Si (001) by molecular beam epitaxy

The deposition of sub-monolayer coverages of C on Si (001) prior to Ge growth leads to the formation of small, irregularly shaped Ge islands well below the critical thickness of Ge on Si. We studied the nucleation of these Ge dots on Si and the ordering of these dots in stacks of dot layers by in-situ scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). It is found that island formation already starts at a sub-monolayer deposition of Ge on these C covered Si surfaces. Ge islands 2–3 nm wide with a height of a few monolayers are obtained by STM after the deposition of 0.1 monolayer of C and 0.5 monolayer of Ge. Apparently the Stranski–Krastanov mode of growth, typical for the formation of Ge hut clusters on Si, does not occur. Instead a Volmer–Weber type of growth is responsible for the island formation. It is noteworthy that no wetting layer is observed for these small C-induced Ge dots. TEM investigations of multiple dot layers containing 10–20 nm wide C-induced Ge dots reveal vertical alignment of the dots for Si barriers of less than 10 nm.