Microstructure and strain relief of Ge films grown layer by layer on Si(001)

We have studied the microstructure of Ge films grown layer by layer on Si (001) surfaces. The growth mode was changed from a Stranski-Krastanov mode (layer by layer for 3 monolayers, followed by islanding) to a layer-by-layer growth mode by passivation of the surface with 1 monolayer of arsenic. This change in growth morphology results in drastic changes in the mechanism of strain relief. Unlike films grown on bare Si, these films remain pseudomorphically strained up to a thickness of about 10 monolayers. At a film thickness of 12 monolayers, we observe the catastrophic formation of strain-induced defects. These consist of several 111 planes tilted perpendicular to the substrate. The defects are ssV-shaped and, consequently, relieve the misfit progressively as the film grows. At a film thickness of 50 monolayers, we observe that the ssV-shaped defects serve as nucleation sites for dislocations that climb down into the Si substrate. These dislocations then glide through the film to relieve the misfit in previously undefected areas. Thus, the misfit is relieved partly by ssV-shaped defects located in the Ge layer and partly by edge dislocations located in the Si substrate. For thick films, we observe that most of the ssV-shaped defects have been covered by Ge oriented epitaxially with the substrate, but they have also generated twins and stacking faults that extend throughout the whole film. This work has fundamental implications for the understanding of strain relief during ``normal'' growth. Indeed, it demonstrates that the so-called critical thickness has to take into account the formation energy of the strain-relieving defects (in general, dislocations), and not only the energy to move the defects, as has generally been done up to now.