Strain relaxation during the initial stages of growth in Ge/Si(001)

Abstract

Grazing-incidence x-ray diffraction has been utilized to give a direct measure of the lateral strain distribution during in situ molecular-beam epitaxy deposition of Ge onto Si(001). The results demonstrate that the critical thickness for strain relaxation is 3--4 monolayers (ML), which coincides with that at which islanding is observed. Strain relief is gradual and depends strongly on growth conditions and annealing procedure. At a coverage of \ensuremath{\sim}10 ML the strain distribution exhibits two components, one of which is almost fully relaxed and the other having a range of lattice spacings intermediate between those for bulk Si and Ge. Concurrent specular reflectivity measurements have been used to monitor intermixing at the interface, showing that the island formation onset is at 3--4 ML, with a rapid increase in island height beyond a coverage of 6 ML. Comparison with electron-microscopy results indicates that strain relaxation is intimately related to islanding on the Ge surface.