Dislocation blocking in elastically anisotropic semiconductor thin films

One strategy for decreasing the density of threading dislocations penetrating the surface of a heteroepitaxial semiconductor film is that in which the greater mechanical stiffness of a dislocation blocking layer acts to reduce the Peach-Koehler image forces acting on the leading segment of the half loop generated by dislocation multiplication sources at the heteroepitaxial interface situated below the blocking layer. Reducing the Peach-Koehler force, drawing the half loop to the film surface, helps prevent the two threading arms of the half loop from becoming threading dislocations once the half loop penetrates the film surface. The calculation of the Peach-Koehler force employs an analytical continuation formalism using anisotropic elasticity theory for treating dislocation image forces generated by three heteroepitaxial interfaces corresponding to the top and bottom interfaces of the blocking layer and the film surface. The system used in this calculation is that of a Ge film grown on a (001) Si substrate, using a SiGe blocking layer just below the critical thickness for dislocation generation. It is found that the dislocation blocking is favored by thinner blocking layers of greater mechanical stiffness, rather than thicker blocking layers of moderate mechanical stiffness. Specifically, for the blocking layers of composition Si0.2Ge0.8, Si0.3Ge0.7, and Si0.4Ge0.6, of thickness 50, 18, and 10 nm, respectively, it is the thinnest (and mechanically stiffest) layer (Si0.4Ge0.6, 10 nm) that brings about the greatest reduction in the Peach-Koehler force, drawing the leading segment of the half loop to the surface of the film.