Effect of compressive and tensile strain on misfit dislocation injection in SiGe epitaxial layers

The relaxation behavior of short-period Si/Ge superlattices and SixGe1−x alloy layers under compressive and tensile strain field is compared experimentally by means of transmission electron microscopy as well as theoretically on the basis of a half-loop dislocation nucleation model. It was found that misfit dislocations in tensily strained layers grown on Ge(001) substrates are imperfect and of the 90° Shockley type provided some critical misfit fc is exceeded. Subsequent nucleation and glide of these partial dislocations on adjacent {111} glide planes leads to the formation of stacking faults and microtwins. In the low misfit regime (f<fc), the nucleation of 60° perfect dislocations is energetically favorable. In contrast, misfit dislocations in layers which experience a compressive strain field within the (001) growth plane are generally of the 60° type. In this case the critical thickness for coherent growth is found to be substantially enlarged with respect to the inverse strain situation where microtwin formation occurs.