Long propagating nanopipes in thick HVPE-GaN layers, studied by means of transmission electron microscopy, were analyzed. The nanopipes appeared to have the characteristics and behavior of screw dislocations. They propagated with a constant diameter along tens of μm, and a pit in the shape of an inverted pyramid formed when the top surface of the layer was reached. The generation of the nanopipes was explained in terms of their relationship to the growth mechanism of the HVPE-GaN material, and to the kinetics of screw dislocations in the early stages of the growth of highly strained material. The nanopipes appeared to promote crystal growth when the substrate surface did not provide a sufficient nucleation base for subsequent HVPE growth. In this case they mediated the surface morphology terminating surface steps by hexagonal pits and acted as the centers of growth spirals. When the substrate surface provided a sufficient number of nucleation sites, the step flow mechanism prevailed and no growth spirals associated with nanopipes were observed.

Nanopipes and their Relationship to the Growth Mode in Thick HVPE-GaN Layers. E.Valcheva, T.Paskova, B.Monemar: Journal of Crystal Growth, 2003, 255[1-2], 19-26