Pathway for the strain-driven two-dimensional to three-dimensional transition during growth of Ge on Si(001)

The two-dimensional (2D) to three-dimensional (3D) morphological transition in strained Ge layers grown on Si(001) is investigated using scanning tunneling microscopy. The initial step takes place via the formation of 2D islands which evolve into small (similar or equal to 180 Angstrom) 3D islands with a height to base diameter ratio of similar or equal to0.04, much smaller than the 0.1 aspect ratio of (105)-faceted pyramids which had previously been assumed to be the initial 3D islands. The "prepyramid" Ge islands have rounded bases with steps oriented along (110) and exist only over a narrow range of Ge coverages, 3.5-3.9 monolayers.