Interface Dislocation Structures in Indium

A new method of thin section preparation of III -V semiconductors and multilayers for transmission electron microscopy (TEM) is presented which exhibits considerable advantages over conventional methods such as ion beam and jet thinning. GaAs thin films and multilayers of GaAs/In _ xGa_{1-x} As/GaAs are grown over an etch release layer of AlAs on GaAs substrates by molecular beam epitaxy (MBE). Planar TEM sections prepared by selective etching from these samples show improved ability to image film morphology and dislocation arrangements, and the resulting large, thin, electron-transparent areas facilitate dislocation density measurements and detection of spatial variations. Avoidance of radiation effects and wedge shaping, both common to ion milled samples, allows this method to be used for the preparation of uniform thickness standards for EDS analysis or lattice imaging. This method was used to investigate the interfacial dislocation morphology in MBE-grown GaAs/In_ xGa_{1-x}As/GaAs thin films (60-300 nm, x = 0.15-0.40). The dislocation structures change with increasing x from orthogonal arrays, with dislocations lying in the < 011 > directions, to semiorthogonal and random tangled arrays, with a reduced preference for crystallographic orientation. The change in morphology with misfit is correlated to a change in growth habit from 2D to 3D. For a given x value the interfacial dislocation density increases with increasing thickness. A marked asymmetry in the dislocation density in the (001) and (011) directions is observed in some of the films exhibiting orthogonal arrays for specific values of In_ xGa_ {1-x}As film thickness above the critical value. The measured dislocation densities are generally inadequate to fully relax the interfacial strain, implying a significant partitioning of strain between elastic and plastic components. In 300-nm thick films, dislocation elongation (consistent with glide models) and a pinning reaction were directly observed in the electron microscope. Weak-beam dislocation density measurements led to an estimate of critical thickness at x = 0.25 and curves of strain relaxation with thickness. (211) oriented samples are shown to have double the dislocation density of (100) samples. Stereo-imaging is used to provide dislocation density information with depth.