Dislocation arrangements in gaas/ga1−xlnxas multilayers grown on (001), (111) and (112) substrates

Abstract

GaAs/Ga1−xInxAs strained layer superlattices with well-widths of 7 nm, barrier widths of 14 nm and periods of 10 to 30 have been examined by transmission electron microscopy in plan view and in cross-section for (001), (111) and (112) substrates. Individual layers are below the critical thickness while the overall SLS's are above the critical thickness for dislocation generation. (001) substrates give rise to square grids of 60° dislocations lying parallel to 〈110〉 directions with inclined 1/2 〈101〉 Burgers vectors, resulting from dislocation motion on 8 slip systems. (111) substrates give triangular networks of 60° dislocations lying parallel to 〈110〉 directions resulting from motion on 6 slip systems. (112) substrates have two sets of primary dislocations lying along (132) directions, along with secondary 60° dislocations lying along [110]. Long Lomer-Cottrell dislocations with Burgers vectors lying parallel to the substrate/SLS interface are occasionally observed for (001) substrates, while short segments are observed for (112) substrates; these are formed by reaction between conjugate dislocations. These dislocations arrangements are discussed in terms of the resolved shear stresses resulting from epitaxy for the various substrates orientations. Dislocation densities are much less than those required for complete strain relief. This is analyzed in terms of the reduction in the stress acting on dislocations from partial strain relief, along with a friction stress due to a combination of the Peierls stress and solution hardening from the In substitution. A friction stress ∼10-3μ is required to explain the observed dislocation densities (μ, is the shear modulus).