Strain relaxed buffers (SRB) act as "Virtual Substrates" (VS) on which to grow material of a different lattice constant to the original substrate, which is usually Si  for compatibility with the silicon industry and because Si substrates are available cheaply and to high quality. These SRB platforms can either support strained or relaxed layers, but should themselves be relaxed so as not to generate crystal imperfections in the overlayer during subsequent growth or processing. Key measures to the quality of a virtual substrate are the threading dislocation density (TDD) and the surface roughness. At Warwick we have developed SiGe VSs over many years and hold a number of patents in the design of ultra-low defect platforms.
The image below shows a terrace graded VS, in which the Ge concentration is increased from nothing to 50% in a series of steps with terraces of constant compostion material between. This design allows relaxation within the layer, but limits the generation of threading dislocatons that reach the surface to below 105cm-2 - two orders of magnitude lower than posible in a standard linearly graded structure, and with no pile up of surface cross-hatch. Terrace graded VSs can be fabricated to support the relaxed growth of all SiGe compositions up to pure Ge.
For applications where the terraced grading approach produces too thick a structure, thin VSs can also be fabricated using low temperature growth of highly dislocated seed layers followed by thermal annealling.
This EPSRC funded project concerns the development of novel and radical approaches to the development of SiGe virtual substrates of unprecedented quality, with extremely low defect density and smooth surface topography.