A comprehensive first-principles investigation was made of single and multiple gallium and nitrogen vacancies in gallium nitride nanowires. Nanowires were considered in the [00•1] growth direction, with diameters of 9.5 and 15.9Å, and the stability of multiple vacancies was investigated for a wide range of configurations in order to determine the preferred spatial distribution. The influence of saturating the dangling bonds at the edge of the nanowires was also investigated. For one, two, and three nitrogen vacancies, it was found that the most favourable configuration was with the vacancies at the edge of the nanowires. It was also found that, for multiple (two and three) nitrogen vacancies, the vacancies preferred to cluster together rather than remain well separated. For one and two gallium vacancies, the preferred vacancy location was also at the edge of the nanowires, with clustering favoured for two vacancies. For the band structure of unsaturated nanowires (with and without vacancies), states in the band-gap were observed that could be attributed to edge states. The latter were removed when the dangling bonds at the edge of the nanowires were saturated with hydrogen. For unsaturated wires, the natures of the single Ga and N vacancy states was similar to those in the bulk; acting as a triple acceptor and single donor, respectively. The position of the vacancy states relative to the two regions of edge-induced states was similar to their location relative to the conduction band minimum and valence band maximum in the bulk; suggesting that any conductivity arising from the vacancies would be confined to the outer region of the nanowires. The two- and three-nitrogen vacancy complexes introduced additional states into the band gap; acting as double and triple donors respectively, while the two-gallium vacancy complex reconstructed to an N3-like structure and introduced several fully occupied and unoccupied singlet states into the band-gap. For the defect-induced states of the gallium vacancy in the saturated wire versus bulk GaN, a similar result was found with regard to the number, location and occupation of the defect states. The states in the wire were slightly deeper in the band-gap. For a nitrogen vacancy in the saturated wire versus one in the bulk, a similar behaviour was found in terms of the number and occupation of the defect states. However, the higher-lying singly occupied state was closer to the conduction band in the wire, and the location of the fully occupied singlet state was below the valence band maximum in bulk GaN, and above it in the saturated nanowire. With regard to the formation energy of gallium and nitrogen vacancies, it was found that the latter were significantly more stable than the former, and were expected to be the major defect in GaN nanowires.

Atomic and Electronic Structure of Single and Multiple Vacancies in GaN Nanowires from First Principles. D.J.Carter, C.Stampfl: Physical Review B, 2009, 79[19], 195302