A formation mechanism was proposed for so-called nanotube defects. It was shown that 2 related types of defect were formed: nanotubes and pinholes. Both began with V-shaped facets on {10•1} polar planes. Slow growth on these polar planes, and impurity poisoning of growth steps, were suggested to be responsible for the initiation of these defects. The formation of a nanotube appeared to require the nucleation of a pinhole. The latter resulted from slow growth on polar planes, thus resulting in the observed V-shaped features; with about 60º between the arms. This value was in good agreement with the angle between two {10•1} planes. It was possible that the formation of nanotubes and large pinholes could be eliminated by reducing the impurity levels below some critical value, which was expected to depend upon the growth rate and the choice of substrate orientation.

Z.Liliental-Weber, Y.Chen, S.Ruvimov, J.Washburn: Physical Review Letters, 1997, 79[15], 2835-8