Abstract
We present density-functional theory based studies for several types of line defects in both hexagonal and cubic GaN. {10-10} type surfaces play an important role in hexagonal GaN since similar configurations occur at open-core screw dislocations and nanopipes as well as at the core of threading edge dislocations. Except for full-core screw dislocations which possess heavily strained bonds all investigated stoichiometric extended defects in hexagonal GaN do not induce deep acceptor states in the band-gap and thus cannot be responsible for the yellow luminescence. However, electrically active point defects in particular gallium vacancies and oxygen related defect complexes are found to be trapped at the stress field of the dislocations. Preliminary calculations for cubic GaN find the ideal stoichiometric 60°-dislocations to be electrically active. As in hexagonal material, vacancies and impurities like oxygen are likely to be trapped at the dislocation core.