(Color online) Cross-section HAADF STEM images with the growth direction pointing up. (a) The GaN template, InGaN quantum well structure, and p-GaN top layers with voids present. (b) The InGaN quantum wells with voids present. (c) and (d) Two typical pyramid voids. The apparent fringes in the bottom right of (a) are Moiré fringes between the square STEM scan and the underlying crystal lattice.
(Color online) Simultaneously acquired (a) HAADF and (b) ABF cross-section STEM images of GaN with the  growth direction pointing up in the image, showing Ga polarity. A model of the Ga polarity atomic structure is shown in the inset of (b), with the larger green atoms being Ga and the smaller blue atoms being N.
(Color online) plan-view images. (a) TEM image showing Fresnel contrast from the embedded voids and central dislocation contrast. (b) STEM image showing Fresnel contrast from the embedded voids and central dislocation contrast. (c) and (d) Typical higher magnification STEM images of the embedded voids with the facets labeled in (c). (e) High magnification STEM image of an open core dislocation cap.
Schematic diagrams showing the structure and facets of the hexagonal-based pyramid voids with (a) a dislocation cap and (b) an open core dislocation cap. The dislocation in (a) has some edge character, and the dislocation in (b) has none.
(Color online) A STEM EELS spectrum image of a pyramid void. (a) HAADF STEM image of the sample area where the spectrum image was collected. (b) Simultaneous STEM image taken during the spectrum image. (c)-(e) are (c) nitrogen K, (d) gallium L, and (e) carbon K edge EELS intensity maps after background subtraction. (f) Horizontally integrated line profile of the red box in (e) showing the vertical carbon distribution across the void.
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