Surface diffusion limited nucleation of Ge dots on the Si(001) surface

The formation of Ge islands during MBE growth is a spontaneous process and these islands, i.e. dots, are usually randomly arranged. In order to implement these nanoscaled islands into device applications, ordering of epitaxial dots is a crucial step. We report a study on the MBE growth of Ge islands on Si(001) substrates, containing 〈110〉-oriented square and long stripe type patterns defined by anisotropic wet etching of Si, in order to provide more understanding of how surface diffusion of Ge atoms would influence the formation of Ge islands on various types of surfaces. It has been found that there were preferential nucleation sites for Ge islands along the bottom edges of the Si ridges. The Ge islands at the edge positions were larger than those formed on the free surface and they could be regularly spaced. Due to the consumption of Ge at the bottom edges of ridge patterns, the density of Ge dots on the free surface varied between ≈3×108 and ≈1×109 cm−2 when changing the spatial separation between two adjacent Si ridges (2–100 μm). A Ge mean diffusion length of ≈7.5 μm has been determined for Ge growth at 700 °C.