The study of relaxation in asymmetrically strained Si_1-x Ge _x Si superlattices

Asymmetrically strained Si/SiGe superlattices consisting of 12 nm Si/4 nm Si_0.65Ge_0.35 have been grown in Si(001) by molecular beam epitaxy (MBE) and studied as a function of thermal treatments. Results indicate that initially, the interdiffusion is very rapid and non-linear, and at later annealing stages a steady-state interdiffusion is attained. Raman spectroscopy has been used to determine the Ge content and the strain independently, and to show that in the very early annealing stages, strain relaxation occurs predominantly by interdiffusion. This is supported by transmission electron microscopy (TEM) which indicates that less than 10% of the initial strain relaxation is caused by dislocation formation. In addition, a low temperature relaxation has been observed which may be related to misoriented SiGe crystallites at the superlattice/substrate interface, and increased in size with annealing at 631‡ C.