Nanoscale pit formation at 2D Ge layers on Si: influence of energy and entropy.

The structural stability of two-dimensional (2D) SiGe nanostructures is studied by scanning tunneling microscopy. The formation of pits with a diameter of 2-30 nm in one atomic layer thick Ge stripes is observed. The unanticipated pit formation occurs due to an energetically driven motion of the Ge atoms out of the Ge stripe towards the Si terminated step edge followed by an entropy driven GeSi intermixing at the step edge. Using conditions where the pits coalesce results in the formation of freestanding 8 nm wide GeSi wires on Si(111).