High Perfection Approaches to Si-based Devices through Strained Layer Epitaxy

1. Introduction In developing materials and processes for VLSI, the demands of high manufacturing yield make it necessary to consider approaches which are inherently capable of creating structures without defects. For example, the success of the well-known Si/SiGe/Si HBT relies on pseudomorphic strained layers below critical thickness to avoid misfit dislocations. Similarly, since the role of strain was first made clear in altering the band alignment in Si/SiGe heterostructures [1], there has been a nearly 20-year search for technological approaches to achieve virtual substrates – layers with in-plane lattice constants different than those of the Si substrates, on top of which pseudomorphic layers could be grown with low defect densities. The usual approaches for such virtual substrates involve growing a " relaxed " layer such as SiGe by lattice-mis-matched epitaxy. The mismatch must be taken up by misfit dislocations, and their inevitable interaction leads to threading defects which reduce the quality of such layers. Typical densities in modern approaches are still ~10 5 cm-2 .