The formation of nanopipes caused by donor impurities in GaN: A theoretical study for the case of oxygen

Local density-functional methods are used to examine the behaviour of O and O-related defect complexes at {1010}-type surfaces in GaN. We find that O has a tendency to segregate to the (1010) surface and we identify the gallium vacancy surrounded by three oxygen impurities (VGa-(ON)3) to be a particularly stable and electrically inert complex. We suggest that these complexes impede growth at the walls of the nanopipes preventing them from growing in. Also, other donor-related defect complexes, in particular gallium vacancies surrounded by three silicon atoms as second nearest neighbours, are expected to have the same effect