Ge-on-Si films obtained by epitaxial growing: edge dislocations and their participation in plastic relaxation

Pure edge 90° misfit dislocations (MDs) are the most effective linear defects that combine the substrate and the film with different lattice parameters. A system consisting of a nonstressed film and a substrate approaches the perfect case in terms of the structural transition from one lattice parameter to the other if imperfections in the form of an ordered network of edge MDs are located exclusively at the interface, while threading dislocations are practically absent. The path to this perfect case goes through studying the possibilities of creating such an ordered network of edge MDs. The mechanism of formation of edge MDs proposed previously by Kvam et al (1990 J. Mater. Res. 5 1900) is discussed. This mechanism involves induced formation of a complementary pair of 60° MDs whose coalescence at the interface creates an edge MD. Some publications are presented, which demonstrate on the basis of experimental data that this mechanism under certain conditions can be the basic mechanism responsible for plastic relaxation of Ge-on-Si films. A cardinal method for decreasing the number of defects at the initial stages of growth of Ge/Si heterosystems is a set of procedures that allow a specified number of MDs to be inserted into the stressed film earlier than conditions of spontaneous nucleation of MDs from the film surface in the 2D–3D transition occur. When the low-temperature/high-temperature strategy of growth is used, the low-temperature GeSi seed layer tuned with respect to the growth temperature, composition and thickness can serve as a source of 60° dislocations, which facilitate earlier formation of edge MDs at the initial stage of plastic relaxation of the GeSi or Ge main layer. Results of some recent publications that report reaching high structural perfection of thin (∼1 µm and less) Ge-on-Si films are discussed. The proposed explanation of these results is based on postulates of controlled insertion of MDs and formation of edge MDs by the model of induced nucleation. (topical review)