Large-angle convergent-beam electron diffraction methods showed that hollow tubes, 5 to 25nm in diameter, which had been observed in α-GaN (grown onto sapphire by means of metalorganic chemical vapor deposition) contained screw dislocations with a Burgers vector magnitude of 0.52nm. It was noted that these coreless dislocations were not in equilibrium, but could arise via the trapping of dislocations at pinholes in the early stages of growth. Although the pinholes could contain a, c and c + a dislocations, it was suggested that only those which contained c dislocations could survive and generate nano-pipes of constant cross-section along the [00▪1] axis; as observed experimentally.

D.Cherns W.T.Young, J.W.Steeds, F.A.Ponce, S.Nakamura: Journal of Crystal Growth, 1997, 178, 201-6