STRAIN RELAXATION IN LATTICE-MISMATCHED EPITAXY

Growth of lattice mismatched layers epitaxially on top of each other is a challenge for today's semiconductor industry. For strained layer devices, it requires that the strained layers are not relaxed through the formation of misfit dislocations. For other device applications, such as GaAs on Si, it may be desirable that the overlayers are completely relaxed and all the misfit defects are confined to the lower interfaces. This article investigates the theories on strain relaxation mechanisms in face-centered-cubic (f.c.c.) systems. The tensile layers are found to initially relax through the formation of partial dislocations, at a critical thickness about half the thickness predicted by the conventional model based on the formation of perfect misfit dislocations. This poses a stringent condition on the maximum thickness useful for tensile layer devices. We also explain the difficulties involved in preparing thick strained layers with few threading dislocations and microtwins.