Dislocations in strained-layer semiconductor heterostructures

Abstract

The reliability of semiconductor devices depends upon the stability of the constituent materials. Strained-layer semiconductor structures contain a layer whose lattice constant differs from the surrounding layers, resulting in a misfit strain. As dislocations are the main failure mechanism in semiconductor lasers, it is essential to establish whether the stability of these structures is affected by the lattice mismatch and the possibility of relaxation by the formation of misfit dislocations. In this thesis, dislocations in strained-layer semiconductor structures are investigated. Relaxation of strained-layer GaAs/InxGa1-xAs/GaAs heterostructures through the formation of misfit dislocations is found to occur in stages. Firstly, generation and elongation of misfit dislocations on threading dislocations during molecular beam epitaxy (MBE) growth have been demonstrated with increasing strained-layer thickness. The onset of this stage has been shown to occur at strained-layer thicknesses below those predicted by the Matthews-Blakeslee (M-B) model. The second stage of relaxation is marked by the formation of a network of 60° misfit dislocations. A third stage of relaxation has been discovered, in which pure edge (i.e. 90°) misfit dislocations are formed in addition to the existing network of 60° misfit dislocations. Different mechanisms are found to be responsible for the three stages of relaxation. The M-B model describes the transformation of threading dislocations to generate 60° misfit dislocations. As this affects individual dislocations, it results in only local relaxation. A separate mechanism, which remains unclear but is not dependent on the interactions of dislocations, dominates the formation of a 60° misfit dislocation network. The edge dislocations responsible for further relaxation of the structures are produced by vacancy-producing jogs. These protrude from pre-existing 60° dislocations and trail edge dislocation pairs as they climb. It is shown here that as-grown structures with strained-layer thicknesses below the theoretical prediction can be relaxed by the formation of a 60° dislocation network during post-growth thermal processing and bending. Thus the first two stages of relaxation are dependent on both the strained-layer thickness during fabrication, and on the temperature and applied stress of as-grown structures. Finally, dislocation motion in strained layer structures has been shown to be slower than that in unstrained structures. Thus the misfit stress acts as an additional resistant force to the displacement of atoms in non-misfit dislocations, even though it drives the formation of misfit dislocations.