This article deals with a plan view and cross section transmission ele
ctron microscopy study of columnar defects in GaN films epitaxially gr
own on sapphire (0001). They are identified as open-core (0001) Burger
s vector dislocations. Their behavior along the film thickness is desc
ribed: it alternates from open core sections (nanopipes) to closed cor
e sections. This alternating behavior is observed in the first 0.5 mu
m close to the interface with sapphire. (C) 1997 American Institute of
Physics.