Self-assembled germanium nanostructures formed using electron-beam annealing

Germanium nanostructures have been fabricated on silicon substrates using ion-beam sputtering growth followed by an ex-situ annealing step. The substrates are not heated during growth, resulting in a post-growth deposited layer ∼225 nm thick with a surface which has no evidence of nanostructure formation. Following annealing at temperatures of 400–700 °C dramatic nanostructuring is observed at the surface. For temperatures below 600 °C atomic force microscopy analysis reveals dense arrays of nanostructures with heights typically around 5–30 nm. Increased feature size and surface roughening is observed for samples annealed above 600 °C, with a broadened size distribution centred at 450 nm. This is assigned to intermixing at the Si/Ge interface, which reduces the stress in the layer, allowing larger features to form.