TABLE OF CONTENTS In the following, we discuss the present status of growth of strained Si on relaxed SiGe buffer layers on Si using various techniques. It is now known that the problem of high threading dislocation densities in relaxed layers may be avoided by using a series of low mismatched interfaces and increasing the Ge concentration in steps (step grading) or linearly with a relatively high growth temperature [11, 140 143]. Because of gradual increase of the lattice mismatch in such a buffer, the misfit dislocation network is distributed over the range of compositional grading rather than being concentrated at the interface to the Si substrate. The greatly improved buffer quality via the compositional grading lowers the threading dislocation density by three orders of magnitude and results in a much improved electron mobility at low temperatures. Strained layer epitaxial growth on patterned substrates has been attempted [144] which can reduce epilayer threading dislocation densities by up to two orders of magnitude. Powell et al. [145] have proposed a method for producing an almost dislocation-free relaxed SiGe buffer layer on thin silicon-on-insulator (SOI) substrates. In this growth process, the SiGe epitaxial layer relaxes without the generation of threading dislocations within the SiGe layer rather in the bottom ultrathin SOI substrate with a superficial Si thickness less than the SiGe layer thickness.
Experimental studies for the last few years on strained SiGe materials have resulted in a significant progress in the understanding of strain relaxation kinetics and optimization...
