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Structural investigations of organometallic vapor phase epitaxy grown α-GaN films using high-resolution transmission electron microscopy and scanning force microscopy have revealed the presence of tunnel-like defects with 35–500 Å radii that are aligned along the growth direction of the crystal and penetrate the entire epilayer. These defects, which are termed ‘‘nanopipes,’’ terminate on the free surface of the film at the centers of hexagonal growth hillocks and form craters with 600–1000 Å radii. Either one or two pairs of monolayer-height spiral steps were observed to emerge from the surface craters which allowed us to conclude that nanopipes are the open cores of screw dislocations. The measured dimensions of the defects are compared to Frank’s theory for the open-core dislocation.
Qian et al. (Mon,) studied this question.