Accommodation of lattice mismatch in Gex

We present evidence that the critical thickness for the appearance of misfit defects in a given material and heteroepitaxial structure is not simply a function of lattice mismatch. We report substantial differences in the relaxation of mismatch stress in Ge0 .5 Si0 .5 /Si superlattices grown at different temperatures on ( 100) Si substrates. Samples have been analyzed by x-ray diffraction, channeled Rutherford backscattering, and transmission electron microscopy. While a superlattice grown at 365 oc demonstrates a high degree of elastic strain, with a dislocation density < 10 em, structures grown at higher temperatures show increasing numbers of structural defects, with densities reaching 2 X 10 em 2 at a growth temperature of 530 OC. Our results suggest that it is possible to freeze a lattice-mismatched structure in a highly strained metastable state. Thus it is not surprising that experimentally observed critical thicknesses are rarely in agreement with those predicted by equilibrium theories.