Transmission electron microscopy (TEM) has been used to study the electron diffraction contrast mechanisms and extended defect structures associated with coherently strained, Si-based superlattice heterostructures. A series of germanium, arsenic and boronalloyed Si multilayers have been grown by molecular beam epitaxy (MBE) on (100) oriented substrates and prepared for TEM study in cross-section and plan-view geometries. The thinning of coherently strained layers in cross-section for electron microscopy results in stress relaxation effects near the thin foil surfaces which can give rise to significant strain field contrast in otherwise perfect layers. Experimental diffraction contrast images of Ge_xSi_1-x/Si heterostructures exhibit anomalous surface relaxation strain field contrast which can be successfully predicted for all foil thicknesses using a Fourier series elasticity solution and for relatively thick foils using a simple line force solution. It was found that the visibility of compositional and doping layer superlattices depends solely on structure factor differences between the layers and surface relaxation effects have a negligible effect on layer visibility. Furthermore, structure factor differences give rise to additional contrast effects namely, thickness extinction fringe shifts and δ-fringe contrast at tilted interfaces. In the case of relatively low concentration epitaxial layers, which exhibit negligible atomic number contrast, the existence of atomic misfit between impurity and matrix atoms (eg. B in Si) can produce significant contrast due to atomic displacement-dependent electron scattering effects.

At sufficiently high compositions, the strain energy associated with misfitting epitaxial layers will exceed a critical value whereby interfacial misfit dislocations are generated to relieve elastic strain. An equilibrium model has been developed which successfully describes the stability limits of Ge_xSi_1-x/Si strained layer heterostructurcs in the absence of kinetic barriers to plastic flow and quantitatively predicts the detailed dislocation relaxation mechanisms observed by TEM in cross-section. Plastic relaxation in (100) oriented heterostructurcs occurs principally by the generation of 60* a/2<110> type misfit dislocations which are shown to nucleate from heterogeneous sources associated with:​ (i) interfacial carbon/oxygcn precipitation and (ii) atomic surface stcps/lcdgcs both during MBE-growth and upon post-growth annealing of metastablc heterostructurcs. Alternative strain relaxation mechanisms have also been studied and include:​ (i) atomic interdiffusion between superlaltice layers at annealing temperatures above -800°C and (ii) the formation of "pagoda" defects which appear as a direct consequence of elastic strain relief at interfacial β-SiC precipitates