MECHANISM OF FORMATION OF 60-DEGREES AND 90-DEGREES MISFIT DISLOCATIONS IN SEMICONDUCTOR HETEROSTRUCTURES

A proposal is made and experimental evidence provided for a mechanism of formation of 60-degrees glissile (a/2)<101>{111} (mixed) and 90-degrees (a/2)<110>{001} (edge) misfit dislocations in semiconductor heterostructures such as Ge/Si and GaAs/Si. With increasing thickness of the epilayer the nature of dislocations was found to change from being predominantly 60-degrees to mostly 90-degrees. Calculations of atomic structures of dislocations using Stillinger-Weber interatomic potentials were done to show that it is energetically favorable for a 60-degrees core to transform into a 90-degrees core. Thus the strained layer system relaxes its energy by minimizing short-range core distortions as well as long-range elastic displacements. A distinguishing feature of the present mechanism is that the second 60-degrees dislocation is nucleated near the surface in the appropriate glide plane to meet (primarily via a glide process and partly by a climb process) and react with the first 60-degrees dislocation which is already at the interface. High resolution transmission electron microscopy studies have been performed to rationalize and confirm the proposed mechanism.