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Transmission electron micrographs of an heterostructure taken under diffraction conditions. The substrate is a bulk GaN crystal with a dislocation density below . (a) Plan-view bright-field image showing dislocation half loops bowing out of a pit, along directions, following the sixfold symmetry of the epilayer. (b) Dark-field cross-section image of the heterostructure interface, which is slightly inclined in this projection. Dislocation half loops “d” at the interface are observed in the vicinity of a surface pit “p.”
Plan-view dark-field TEM image of an heterostructure taken under diffraction condition. Two types of misfit dislocations are observed. One is the radial-shape half loops originating from the surface pit in the middle. The other is a set of straight dislocations that cross the whole field of the image. Dislocation line segments parallel to (along the vertical direction) are out of contrast (i.e., . Their Burgers vector is perpendicular to and therefore to , implying that they are all pure edge dislocations of the misfit type. These include two sets of radial shape dislocation half loops (propagating horizontally) and one set of straight line misfit dislocations (oriented vertically).
Schematic diagrams illustrating the generation process of dislocation half loops at the heterointerface, originating from a surface pit. (a) The dislocation half loop consists of three sections , , and , with a Burgers vector . (b) Strained pseudomorphic state before critical thickness. (c) Relaxed state with surface ledge and misfit dislocation after critical thickness is reached.
Two mechanisms for strain relaxation that have been observed in the system. One involves the introduction of dislocations by slip on the basal plane starting from the lateral surfaces of pits at threading dislocations (right portion of figure). The other involves the introduction of misfit dislocations from the top surface via inclined planes (by slip or punch-out mechanism).
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