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Using a density-functional-based tight-binding method we investigate the stability of various vacancy clusters up to a size of 17 vacancies. Additionally, we compute the positron lifetimes for the most stable structures to compare them to experimental data. A simple bond-counting model is extended to take into account the formation of new bonds. This yields a very good agreement with the explicitly calculated formation energies of the relaxed structures for V 6 to V 14 . The structures, where the vacancies form closed rings, such as V 6 and V 10 , are especially stable against dissociation. For these structures, the calculated dissociation energies are in agreement with experimentally determined annealing temperatures and the calculated positron lifetimes are consistent with measurements.

Local density-functional methods are used to examine the behavior of O and O-related defect complexes on the walls of nanopipes in GaN. We find that O has a tendency to segregate to the (101̄0) surface and identify the gallium vacancy surrounded by three oxygen impurities [VGa-(ON)3] to be a particularly stable and electrically inert complex. We suggest that during Stranski-Krastanow growth, when interisland spaces shrink, these defects reach a critical concentration beyond which further growth is prevented and nanopipes are formed. © 1998 American Institute of Physics

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