Raman study of strain relaxation in Ge on Si

Strain in thin Ge layers grown by molecular beam epitaxy on (100) Si is measured by a Raman technique. When the average Ge thickness is 7 monoatomic layers (ML), Raman results show that the layer is almost coherent to the Si lattice. The strain begins to decrease at an average thickness of 10 ML, i.e., the critical thickness of dislocation generation is 10 ML. On the other hand, the relaxation begins at a thickness of 5 ML, according to reflection high-energy electron diffraction observation during the growth. This initial stage relaxation is due to deformation of islands and not due to dislocation formation. Raman results for thicker layers show that with increasing layer thickness, the misfit strain decreases gradually but more rapidly than predicted by the theory of Matthews and Blakeslee .